ufom ultraprecision machining (upm) ultrahigh vacuum ... · u ufom microﬂuidic optomechanics...

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uFOM

▶Microfluidic Optomechanics

Ultrahigh Vacuum Chemical VaporDeposition (UHVCVD)

▶Chemical Vapor Deposition (CVD)

Ultralarge Strain Elasticity

▶ Superelasticity and the Shape Memory Effect

Ultra-precision Finishing

▶Ultraprecision Surfaces and Structures withNanometer Accuracy by Ion Beam and PlasmaJet Technologies

Ultra-precision Machining

▶Ultraprecision Surfaces and Structures withNanometer Accuracy by Ion Beam and PlasmaJet Technologies

# Springer Science+Business Media Dordrecht 2016B. Bhushan (ed.), Encyclopedia of Nanotechnology,DOI 10.1007/978-94-017-9780-1

Ultraprecision Machining (UPM)

Suhas S. JoshiDepartment of Mechanical Engineering, IndianInstitute of Technology Bombay, Mumbai,Maharashtra, India

Synonyms

Nanomachining; Nano-mechanical machining

Definition

Ultraprecision machining refers to the ultimateability of a manufacturing process whereinprocessing of a material at its lowest scale thatis, at the atomic scale, is achieved. It is known thatthe lattice distances between two atoms are of theorder of 0.2–0.4 nm; therefore, the ultraprecisionmachining refers to processing or removal actionsof a manufacturing process in the vicinity of 1 nm.The process is also referred as “atomic bit”processing. To remove or process atomic bits,extremely large energy density is required,which is equivalent to the atomic bonding energy.The conventional cutting tools neither have highstrength to sustain high specific cutting energy norhave hardness to sustain the tool wear. Therefore,ultraprecision machining refers to use of singlecrystal diamond (SCD) tools for ultrafine cutting

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4254 Ultraprecision Machining (UPM)

or very fine abrasives for lapping or polishing.It may also refer to use of high-energy elementaryparticles like photons, electrons, ions, and reactiveatoms to undertake removal [1].

Historical Perspective

In early 1970s, Japanese researcher N. Taniguchiillustrated historical evolution of precisionin manufacturing through “Taniguchi curves”[1]. It was realized that it takes about 20 yearsto improve precision by one decimal point;see Table 1 depicting the progression ofprecision.

He extrapolated the curves further to postulatethat the resolution of machining processes wouldreach to 1 nm in the year 2000 [1]. True to theexpectations, today, a number of ultraprecisionmachining processes are available.

Applications

Ultraprecision processing is required in the man-ufacture of high-precision block gages, diamondindenters and tools, 3D metallic mirrors, etc., inthe mechanical domain; Si wafers, ICs memory,thin film, ULSI devices in electronics field; andoptical flats, diffraction gratings, mirrors, andaspherical lenses in the optical field.

Types of UPM Processes

The ultraprecision machining processes can bebroadly classified into three categories [1]:

1. Nanomechanical processing: These processesuse either SCD tools to perform diamond turn-ing or very fine abrasives in the bonded or looseform to perform nanogrinding, nanolapping, ornanopolishing. The other processes in this

Ultraprecision Machining (UPM), Table 1 Evolution of u

Year 1900 1920

Resolution (mm) >10 5

category include progressive mechanochemicalpolishing for Si wafers, pitch polishing foraspherical lenses.

2. Nanophysical processing: It involves use ofhigh-energy elementary particles like photons,electrons, ions, and reactive atoms to performdirect ablation of substrate or carry outlithography.

3. Nanochemical or electrochemical processing:These processes involve use of chemical orelectrochemical principles to effect materialremoval. They are chemical milling, photo-chemical machining, and other processes.

In the following sections, principles ofnanometric removal in ultraprecision machiningprocesses are elucidated along with theirapplications.

Nanomechanical Processing

In these processes, the processing unit, defined asthe size of chip generated in a single stroke of tool,should be of the order of single or subatomicmagnitude.

Principle of Ultraprecision RemovalThe removal of atomic bits in nanomechanicalprocesses requires specific cutting energy of vary-ing degree as the scale of the process decreasesand cutting tools with certain sharpness and hard-ness as discussed below:

Size-Effect ImplicationsAs the processing scale reduces from simple ten-sion testing to grinding, where submicrometricchips are generated, the specific shear or breakingenergy becomes extremely large. This effect iscalled as size effect. See Table 2 showingprocessing scales and shear energies [2].

Extending this trend further, when the machin-ing scale reaches an atomic-bit, the specific

ltraprecision

1940 1960 1980 2000

0.5 0.05 0.005 0.001

Ultraprecision Machining (UPM) 4255

shear energy reaches atomic bonding energy [1]or the theoretical shear strength of a defect-freematerial given by

tth ¼ G

2p(1)

Ultraprecision Machining (UPM), Table 2 Size-effectin processes as a function of chip thickness

ProcessProcessscale

Chipthickness(mm)

Resistingshear stress(N/mm2)

Tensiontest

Multicrystalgrain

300–500 300

Turning Subcrystalgrain

40–50 500

Precisionmilling

Subcrystalgrain

5–10 1,000

Grinding Atomicclusterregion

0.5–1 10,000

0.001µm 0.1µm

Atomic cluster

processing

Point defects range1nm –0.1µm

Scale of crystalli

Spe

cific

she

ar e

nerg

y (J

/Cm

3 )

Atomic scale

processing

Atomic lattice range

< 0.1nm

Sub-duc

proce

Dislorange0.1µm10 µm

10

102

103

104

105

1 µm

Vacancy

Lattice atom

0.2-0.4 nmInterstitial atom

Fixed dislocation

1 µ

Movable

dislocation

Precipitate

Ultraprecision Machining (UPM), Fig. 1 Defect distributi

For carbon steel with the modulus of rigidity,G = 82 GPa, the theoretical shear strength isgiven by

pthCarbonSteel ¼ 13GPa (2)

The reason for increasing the specific shear energyis attributed to the kind of defects available in thecrystalline structure at that processing scale. Atthe atomic bit scale (0.001 mm), the energyrequired is comparable to the theoretical shearstrength (105 J/cm3) [1] (Fig. 1). At the atomiccluster scale, deformation of a material cantake place only with the help of point defects(Fig. 1) and requires energy of the order of103–104 J/cm3 [1].

Nevertheless, at some higher scale (0.1–10 mm),called as subcrystal grain size, 2D or 3D defects inthe crystalline structure cause a significant reduc-tion in the specific shear energy to 102–103 J/cm3

Multi-grainDuctile/brittle

processing

Crystal grain void

range> 10µm

ne structure defects

Movable dislocation

Grain boundary

1 µm

Micro-crack

10µm

grain tile ssing

Sub-grainbrittle

processing

cation Microcrack

-range0.1µm-10 µm

Cavity/crack

m

on and scale of specific shear energy required

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Ultraprecision Machining (UPM), Table 3 Salient fea-tures of an ultraprecision machining system

Machine elements/purpose/types Main features

Machine base

� Provides thermal andmechanical stability,dampingcharacteristics

� Made of cast iron,natural or epoxygranite, polymerconcrete

Work spindle

� Spindle motion errorssignificantly affectsurface quality andaccuracy of machinedfeatures

� Use aerostatic orhydrostatic, recentgrooved air bearings

� Both the spindle typeshave high rotationalaccuracy androtational speeds

� Aerostatic spindles arefor low/medium loads,hydrostatic bearingstake heavy loads

Drives

� Slide drives providestiffness, acceleration,speed, smoothness ofmotion, accuracy, andrepeatability

� Spindle drives areusually AC/DCmotors

� Slides are usuallyprovided with linearmotor or friction drives

� Servo drives are usedin contouringoperations

� Small and precisemotions of tools fortool positioning andfine motion areachieved bypiezoelectric actuators

Controls

� Controls are requiredfor linear and rotarydrives, limiting,position, and timeswitches, sensors

� They also controlthermal, geometrical,and tool setting errors

� A multiaxes CNCcontrollers are used

� PC-based controls areused more recently

� Feedback controlshave a resolution ofnm or sub-nm

Measurement and inspection systems

� Provides rapid andaccurate positioningof cutting tool towardswork surface

� It also monitors thetool wear condition

� Online measurementand errorcompensation

� Laser interferometerfor tool position control


(Fig. 1) [1]. In the crystal grain size range,(>10 mm), all kinds of dislocations and defects atgrain boundaries lower the processing energy fur-ther to 101–102 J/cm3 [1]. Unlike the case of theductile (metallic) materials discussed above, thebrittle materials have a network of microcracks inplace of the network of dislocations (Fig. 1). In theductile materials, below the point defect scale(1 nm to 0.1 mm) (Fig. 1), and in the brittle mate-rials below the microcracks scale (0.1–10 mm)(Fig. 1), the specific shearing energy is equal tothe theoretical shear strength of the work material.

Cutting Tool Material and Geometry ImplicationsThe cutting tool edge in ultraprecision machiningbeing subjected to extremely high cutting pressureshould withstand not only the pressure but also thewear. In addition, the edge should have highdegree of sharpness to perform atomic bitremoval. The SCD tools, possessing the desiredproperties, are the most suitable tool materials forultraprecision machining. However, their limita-tion stems from the achievable cutting edge sharp-ness, which normally is limited to hundreds ofnanometers. Therefore, in UPM, several atomiclayers or atomic clusters are processed thanatomic bits, primarily on softer metallic materialslike Al and Cu. On the other hand, on hardenedsteels, and other brittle materials, SCD toolsundergo rapid wear, thereby necessitating the useof diamond abrasive process like nanogrinding.

Ultraprecision Machining SystemsIt uses time-tested principles of precision engi-neering along with (1) the recent advancementsin the control, feedback, and drive systems,(2) CAD/FEM for designing, followed by(3) fine-tuned assembly as well as system integra-tion. This combination results in a machine that isthermally stable, highly reliable, and flexible andat the same time faster in response [3]. Severalfeatures of an ultraprecision machine are summa-rized in Table 3.

Ultraprecision Machines and ProcessesA number of ultraprecision machines and pro-cesses have evolved using nanomechanicalremoval principles. They are discussed below.

Single Point Diamond TurningIt is a nanomechanical processing machine thatwas developed primarily for 3D shaping and sur-face finishing of soft metals such as copper andaluminum and polymers. The machine with thefeatures listed in Table 3 uses very sharp SCDtools to perform ultraprecision cutting. The chipsformed during the process are extremely thin,

ba

Workpiece

Single point diamond tool

X-Table

Y-Table

Grinding wheel

X-Table

Y-Table

Workpiece

UltraprecisionMachining (UPM),Fig. 2 Typical schematicsof (a) Single-point diamondturning (b) Diamondgrinding machine


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which indicates ductile deformation of the mate-rial during machining. At the same time, the pro-cess leaves extremely thin degenerated layer ofthe work surface as the process initiates shear slipusing dislocations (Fig. 1).

Applications of the process include fabrication ofmetallic mirrors, precision component fabrication inlaser, and space and optics fields. A typical diamondturning lathe has T-axis configuration (Fig. 2a). Themain machine slide moves along x-axis, and thecutting tool is moves along Z-direction.

NanogrindingSingle-point diamond turning machine is usuallyprovided with add-on grinding attachment(Fig. 2b) for nanogrinding. This way, harder workmaterials can also be processed on the machineusing the grinding attachment. However, itincreases the stiffness requirement of the machine.In nanogrinding process, shear slip is generateddue to very low (tens of nm) depth of cut (Fig. 1).This causes crackless ductile failure of hard/brittlematerials under the abrasive grains in the diamondgrinding wheels. Since the working stress on theabrasives is extremely high, only fine-grained dia-mond abrasives can be used for this process. Theprocess can achieve mirror like finish on hard andbrittle materials like glasses and ceramics. Its otherapplications involve grinding of mould inserts,glass lenses, and aspherical lenses.

Electrolytic In-process Dressing (ELID) GrindingIt is a type of nanogrinding process in whichmetal-bonded diamond grinding wheels are

used. The process uses an electrolytic solutionas a grinding liquid. Upon application of volt-age, anodic dissolution of bonding metal fromthe grinding wheel occurs. This ensures thatworn abrasive particles on the grinding wheelare removed quickly (a wheel dressing action),thereby ensuring continuous availability ofsharp particles for the grinding process. Thesharp particles initiate shear slip at low depthof cut (Fig. 3) causing crackless materialremoval. The process therefore is capable ofachieving mirror like surface finish on glassesand ceramics.

Nanolapping and NanopolishingUnlike nanogrinding, which uses fixed abrasives,these processes use free abrasives to achievemirror-like finish. The processes involve atomiccluster processing.

Nanopolishing involves sliding of soft abra-sives between a soft polishing pad and work sur-face (Fig. 3a). The abrasives are usually soft anddull; they include Fe2O3, Cr2O3, CeO2, or MgO[1]. The abrasives get embedded in the soft padand originate shear slip at the point defects toeffect removal [1] (Fig. 3a). This ensures smoothpolishing of hard metallic surfaces.

The nanolapping on the other hand uses amedium to hard pad besides extremely sharp andhard abrasives such as diamond, CBN, SiC, SiO2,or B4C [1]. The fine and hard abrasives initiatecracking at the point defects to effect removal [1](Fig. 3b). This helps generate mirror-like finishingon hard and brittle materials.

ba

Nano-cracking layer

Sharp edged and hard abrasives

Hard workpiece

Soft polishing plate

Hard workpiece

Point defect Nano-shear slip

Soft and dull abrasive particles

Lapping plate

UltraprecisionMachining (UPM),Fig. 3 (a, b)Nanopolishing andnanolapping

100Chemical action ratio

Arb

itrar

y un

its

Polishing rate Depth of damaged layer Flatness

0 00 100

100

Ultraprecision Machining (UPM), Fig. 4 Generalprocessing characteristics of P-MAC


P-MACThe progressive mechanical–chemical (P-MAC)polishing is a hybrid nanopolishing process thatuses mechanical as well as chemical action toeffect removal to achieve mirror-like finish anddamage-free polishing of Si wafers. The processuses ultrafine SiO2 abrasives (size ~10 nm)suspended in an alkaline (pH ~10) solutionalong with an artificial leather foam to performpolishing in four stages. The abrasive particle sizeand the polishing pressure and the stock removalare gradually reduced after each stage [1].

In P-MAC, the greater the chemical action, thehigher is the polishing efficiency and the lower isthe damaged layer [1] (Fig. 4). However, theincreased chemical action reduces control overaccuracy (flatness), Fig. 4 [1].

Aspherical Lens PolishingAspherical profiles are required at the edges of thelenses to ensure that light beams from the lensedges converge at the focal point. Therefore, theaspherical lens polishing involves polishing overfree-form surfaces. The process is often requiredfor polishing of highly accurate X-ray optics andlenses in commercial cameras.

It involves gradual removal using a pitchpolishing tool that has very small contact area ascompared to the dimensions of the workpiece.The polishing rate is governed by Preston’s ruleof thumb [1].

h ¼ a � v � Dt (3)

The pitch polishing tool acts as a pad and supportsthe abrasives. It remains solid at room temperaturebut flows with time when heated or under pres-sure. The composition of the pitch material isusually proprietary of various manufacturers butcontains materials like tar, oil, wood, paraffinwax, shellac, and so on. The properties ofpitch material include [1] viscosity in the rangeof 107–109 Pa � s), softening point, (55–7 �C),penetration hardness (60–80 by Shore D). In thisprocess, the surface roughness tends to improve asthe penetration number of the pitch increases, i.e.,as the pitch becomes softer. It is seen that CeO2

and ZrO2 give better surface finish than super harddiamond abrasives.


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Nanophysical Processes

The principle of removal in these processesinvolves photothermal or photochemical interac-tions with the work surface. The photothermalinteraction causes melting and vaporization ofwork material due to energy of photon or laserbeams. On the other hand, the photochemicalinteraction involves use of photon energy tobreak bond between work elements [1]. It isunderstood that usually laser beams with metallicmaterials perform removal action by thephotothermal principle. However, when thepolymeric materials interact with laser beams,photochemical reaction takes place. With theceramics, the interaction is partly by thephotothermal and partly by photochemicalreactions.

The photon beams owing to their wavelengthof the order of 10–0.1 mm and the photon energyof the order of 0.1–10 eV cause thermal energytransfer that is smaller than the atomic bondingenergy of materials. Hence, the photon beams arenot suitable for nanometric material removal.Similarly, an electron beam due to small size(2.8 � 10�6 nm), mass (9 � 10�31 kg), and lowenergy of several hundred kilovolts penetratedeep (several micrometers) into the work surface.Therefore, the electron beams are also not suitablefor nanometric material removal.

Unlike the electrons, most of the ions interferewith the surface atoms of a workpiece because theirmean diameter is ~0.1 nm and the mean atomicdistance is ~0.3 nm. Consequently, the projectedenergized ions frequently collide with the nuclei ofatoms of the workpiece and knock out or sputterthe surface atoms. Hence, ion beams are mostsuitable for atomic bit removal. The process isalso called as ion sputter machining. In the process,the electrically accelerated inert-gas ions such asAr ions with average energy of 10 keV (whichcorresponds to a speed of ~200 km/s) are unidirec-tionally oriented and projected on to the workpiecesurface in a high vacuum (1.3 � 10�4 Pa)[1]. Hence, they adhere firmly on the target surface.A beam of focused ions can also be used for cuttingof extremely hard materials like diamond or sharp-ening of SCD tools.

Nanochemical or ElectrochemicalProcessing

The chemical reactions inherently involve atomicbit processing. A chemical reaction is a change inthe atomic combination of reacting molecules, inwhich the atomic bonding of a reacting moleculeis broken and a new molecule is generated.

In the process, a chemically reactive gas orliquid is applied to a specified position on thesolid surface of the workpiece. The reacted mole-cules are removed or diffused into the surroundingreacting gas or liquid. If the reacted molecules areinsoluble or not in vapor form, chemically reactivedeposition occurs on the workpiece surface. But ifthe reacted or reagent molecules diffuse into thesurface layers of the workpiece and react with theatoms or molecules there, they perform a chemi-cally reactive surface treatment [1]. The dimen-sional accuracy obtainable in chemical reactions isin the nanometer range when the processing condi-tions are stable. In addition, the following aspectsare necessary for nanometric processing [1]:

1. In-process measurement and feedback controlof position of the processing point and controlof the processed volume (area) are necessary.However, it is difficult to realize these in prac-tice. In the photoresist method, control of theprocessing point position or area is achieved bythe patterned mask.

2. The control of the processing volume or depthcan be done only by adjusting the processingtime and flow rate of etchants.

In the electrochemical process, the basic reac-tion is the same, but activation energy for the reac-tion is given by electric field potential and differsfrom the ordinary activation energy of the chemicalreactions based on the thermal potential energy.

Summary

Ultraprecision machining uses mechanical, physi-cal, chemical, or electrochemical sources of energyfor effecting material removal to the nanometricscale. Grain size, load applied on the grains, and

4260 Ultraprecision Surfaces and Structures with Nanometer Accuracy by Ion Beam and Plasma Jet Technologies

the type of work material govern removal innanomechanical processes. Size of the energybeam particle governs the processing resolution innanophysical processes. In-process measurementand feedback control are essential for thenanochemical or electrochemical processes.

Cross-References

▶Electrochemical Machining (ECM)▶Nanochannels for Nanofluidics: FabricationAspects

▶Nanotechnology

References

1. Taniguchi, N. (ed.): Nanotechnology – IntegratedProcessing Systems for Ultra-precision and Ultra-fineProducts. Oxford University Press, New Delhi (2008)

2. Baker, W.R., Marshall, E.R., Shaw, M.C.: The sizeeffect in metal cutting. Trans. ASME 74, 61 (1952)

3. Lou, X., Cheng, K., Webb, D., Wardle, F.: Design ofultra-precision machine tools with applications to man-ufacture of miniature and micro components. J. Mater.Process. Technol. 167, 515–528 (2005)

Ultraprecision Surfaces andStructures with Nanometer Accuracyby Ion Beam and Plasma JetTechnologies

Thomas Arnold1, Thomas Franz2, Frank Frost1

and Axel Schindler21Leibniz-Institute of Surface Modification,Leipzig, Germany2NTG Neue Technologien GmbH & Co. KG,Gelnhausen, Germany

Synonyms

Chemical-assisted ion beam etching (CAIBE);Chemical vapor machining (CVM); Ion beametching (IBE); Ion beam figuring (IBF); Ionbeam smoothing (IBS); Plasma-assisted chemicaletching (PACE); Plasma jet machining (PJM);Rapid atomic processing (RAP); Reactive ion

beam etching (RIBE); Surface figuring; Surfacepatterning; Ultra-precision finishing; Ultra-precision machining

Definition

Ultra-precision surfaces and structures with nano-meter accuracy comprise surface shapes and/orsurface structures with the highest achievableprocessing accuracy of nanometer or evenpicometer rms range. Root mean square (rms) isa value calculated from differences of the surfacemeasuring data and the desired (designed) surfaceshape data. Its value is a measure of how far onaverage the error is from zero. The ultimate accu-racy of surface shapes and structures is a prereq-uisite for their physical functioning in many oftoday’s fields of science, technology, and productslike electronics, microsystems, and mainly inadvanced optics. Examples range from small mil-limeter size microscope lenses for deep ultraviolet(DUV) light to large telescope mirrors of up tometers in size. In high-performance optics, theextremely low rms values have to be realized forpart or the complete surface spatial wavelengthspectrum from the full aperture down to themicrometer. In special cases, the processing accu-racy must be within few atomic layers for spatialwavelength up to some micrometers. In general,ultra-precision surface machining requires and isdone in repeated loops in turns with adequatesurface measurements. Advanced measuringequipment of different kinds for the coverage ofmore than eight orders of magnitude of spatialwavelength is necessary for it.

Ultra-precision surfaces are fabricated bymultistep processing chains starting from coarseshaping like grinding up to final fine shaping likepolishing. Ion beam and plasma jet techniquesbesides others, such as magnetorheologicalfinishing (MRF), chemical vapor machining(CVM), and elastic emission machining (EEM),are highly deterministic. They belong to the classof finishing tools that are favorable to achieve theultimate accuracy.

Ion beams and plasma jets or plasma-assistedtechniques, respectively, base upon physical

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and/or chemical interaction on atomic levelbetween atomic particles out of the ion and plasmatool, respectively, with surfaces. Generally, suchprocesses are adaptable for different machining inwide application ranges. The capability to setappropriate parameters together with their tightcontrollability adequate to the required processingcondition qualify them as key tools to achieve thedesired ultra-precision of surfaces and structures.

High-vacuum ion beam processing uses accel-erated mostly inert ions of some hundred eV toabout 2 keV (i) to remove atoms from a surface byatomic impact – the so-called ion or atomsputtering; (ii) to deposit films by secondary ionbeam sputter deposition or to assist film deposi-tion; (iii) to create nanostructures by complexion-induced self-assembling effects; or (iv) tosmooth surfaces. Combined with additionalchemical active species by adding respectivegases to the ion source, chemical reaction withsurface atoms is exploited to enhance removalrates and/or etching selectivity between two mate-rials like an adhesive mask and the underlyingglass substrate. This is called reactive ion beametching (RIBE). A second version is chemical-assisted ion beam etching (CAIBE). In this casethe source is operated by an inert gas and thereactive gas is directed to the surface processed.

Plasma jets or torches are free-burning electri-cally excited gas discharges under atmospheric orrough vacuum conditions using mostly inert gaseslike argon. By admixture of adequate gases asprecursor, chemical reactive atomic particles likeradicals are produced in the plasma. Dependingon the application, different gas types are used.They can (i) chemically react with surface atomsto form volatile compounds, thus removing mate-rial by dry etching, or (ii) dissociate to deposit thinfilms. Just the transfers of energy out of theplasma to the surface can (iii) smooth roughsurfaces.

Ion beam techniques especially ion beam fig-uring (IBF) is well established in optics produc-tion today, whereas plasma jet techniques likeplasma jet machining (PJM), plasma-assistedchemical etching (PACE), and rapid atomicprocessing (RAP) are at the doorstep to enter inproduction. Nevertheless, presently both principles

are improved further by intense research and devel-opment for higher performance, productivity, andnew applications.

Overview

N. Taniguchi introduced the term “nanotechnol-ogy” in a keynote paper of the International Con-ference on Production Engineering (ICPE) inTokyo in 1974. This time nanotechnology wasinitially proposed to provide precise target accu-racy for fabrication processes that involve ultra-precision surface finishing [1, 2]. In the frame ofdevelopments of high technologies like semicon-ductor, laser, x-ray, synchrotron radiation, spacetelescopes, and advanced military technologiesbeginning about 1980 worldwide, big efforts hadbeen started for the development of deterministicmachining technologies to meet the steadilygrowing demands of extreme high-quality sur-faces mainly for optics. In high-performanceoptics, the extremely low rms values have to berealized for the complete surface spatial wave-length spectrum from the full aperture down tomicrometer or for parts of it. Only these extremeaccuracies near the physical limits for processingof (i) macroscopic surface shapes like lenses forelectronic circuit lithography or meter-sized giantspace telescope mirrors and of (ii) sub-nanometeruniformity of thin film thickness, for example, ofmass storage hard disks, enable the functionalityof those components in modern equipment andproducts. The same applies for dimensional con-trol of micro- or nanostructures on wafer surfacesfor electronics or micromechanics and micro-optics as used, for example, in miniaturized cam-era systems for smartphones.

For ultra-precision surfaces, there is a wide vari-ety of shapes fromflat to spherical and aspherical orrecently even so-called free forms on a similarlywide diversity of work sizes. Materials range frommonocrystalline silicon overmany types of glass ormetal up to very hard and brittle ceramics. Binaryand 3D micro- and nanostructures on flat andmore and more on curved surfaces are needed, forexample, for diffractive lenses or antireflectionenhancement.


Different surface shaping and finishing tech-nologies targeting ultra-precision quality havebeen developed. Besides ion beam and plasmajet techniques described here, there are SPDT(single-point diamond turning), CCP (computer-controlled polishing), chemical mechanicalpolishing (CMP), MRF, RAP, CVM, EEM, andsome others. Summarizing overviews onadvanced deterministic surface machining andfinishing technologies have been published veryearly at the beginning of the ongoing developmentand recently [3–7].

Ion beams and plasmas including recentlydeveloped atmospheric plasma jets with their dif-ferent but well-defined physical and/or chemicalcharacteristics for their interaction with materialsurfaces are the main tools to achieve ultra-precision accuracies. The heavy atomic particlesinvolved in these technologies are ions, atoms,molecules, and radicals. The capability to tightlycontrol their parameters like mass, kinetic energy,excitation, density, charge state, incidence angleto the surface, chemical reactivity, and others inwide ranges allows tailoring of respective particletools for the individual processing of surfaces toachieve the demanding surface quality. Automa-tion, long-term stability, and flexibility of ionbeam and plasma jet sources combined withadvanced surface processing algorithms and reli-able machinery including computer control makethese technologies efficient for production up tohigh volumes. Further, in high-end optics fabrica-tion ion beam figuring (IBF) and ion beamsmoothing (IBS) are able to overcome physicalconstraints of the conventional full lap and smalltool abrasive polishing processes. For ultra-precision ion beam and plasma jet processing,there are two basic principles: (i) material removalas sputtering and/or dry etching and (ii) materialdeposition.

Surface patterning includes techniques for pre-paring, patterning, and structuring surfaces, fromthe arrangement of single molecules to the macro-scale, for example, through etching, electrochemi-cal patterning, and film deposition. Patterning andstructuring in most cases are a combination of amask, binary, or 3D that is defining the pattern andone or more subsequent etching steps to transfer

the pattern onto the substrate or a film. Mask struc-turing is performed by lithography. Photolithogra-phy, e-beam lithography, laser writing, laserholographic interference lithography, and ionbeam lithography are used. Main etching tech-niques are the ion-based ones, RIE (reactive ionetching) or RIBE/CAIBE, due to their most impor-tant characteristic of being anisotropic (no maskunder-etching) and highly selective for differentmaterials (e.g., mask and substrate). Both tech-niques comprise physical sputtering and chemicalreaction between reactive species and surfaceatoms to remove material.

Not included in this survey of ultra-precisionsurface processing of ion beams and plasma jets isthe special field of focused ion beam techniques.The physical and or chemical ion-induced pro-cesses are more or less the same, but the techniquesimilar to electron beam microscopy is very dif-ferent. Applications are in the field of fabricatingprototype nano- and microstructures and mainlyon mask repair, basic research, and sample prep-aration for surface and structure analytics anddepth profiling as well. Recently reviews havebeen published [8–10].

Figure 1 shows schematically the usage ofdifferent ion beam and plasma jet techniqueswith respect to the spatial wavelength over morethan eight orders of magnitude.

The main technology driver in this field wasand is still lithography for semiconductor technol-ogy. Advanced deep and extreme UV lithography(DUVL and EUVL) use 193 nm light wavelengthand EUV soft x-rays of 13.4 nm, respectively[11]. Advanced high end aspheric lenses and mir-rors riqire demanding surface precision in the sub-nanometer range of rms surface shape errorsacross the entire spatial wavelength range withrespect to the mathematical design. This has tobe realized from the full aperture range downto micrometer spatial roughness in series produc-tion on all surfaces of projection optics for lithog-raphy for instance. Figure 2 shows measuredpower spectral density (PSD) curves of an EUVmirror substrate surface. PSD function representsthe surface error amplitude distribution overthe spatial wavelength. The measurements wereperformed over the entire spatial wavelength

Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 1 Scheme of summarizing of ultra-precision ion beam and plasma jet methods for large sur-face processing represented with respect to the spatialwavelength. Some typical target parameters are given(black). “Subtractive” means material removal, “additive”material deposition; HSFR, MSFR, and LSFR high, mid,and low spatial frequency roughness, respectively, IBE,IBF, IBP, IBS, IBAD, and IBSD ion beam etching, ion

beam figuring, ion beam planarization, ion beam smooth-ing, ion beam-assisted deposition, and ion beam secondarydeposition, respectively, PJM, PJP, and PJD plasma jetmachining, plasma jet polishing, and plasma jet deposition,respectively, PACE plasma-assisted chemical etching,CVM chemical vapor machining, RAP rapid atomicprocessing, RIBE reactive ion beam etching, CAIBEchemical-assisted ion beam etching, RIE reactive ion etch-ing, FIB focused ion beam, GLAD glancing angle deposi-tion (by IBSD)


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using different equipment according to theirappropriate spatial resolutions. From nanometerto micrometer spatial range, atomic force micros-copy (AFM, blue curves) from micrometer tomillimeter white light interference microscopy(WLI, black curves) and from millimeter to fullaperture (green curve) high-resolution interferom-etry (IFM) was used. The red curve is a polyno-mial fit comprising the individual PSDs. The threecolored areas represent the impact of the differentspatial wavelength surface error ranges on theoptical performance of the imaging EUV opticfor the 13.4 nm light wavelength. The figures arethe rms value specs for EUVoptics. Shape errors

from full aperture up to 1 mm wavelength (LSFR)cause image aberrations. MSFR from 1 mm to1 mm leads to field of view light scatteringresulting in flare or blurring of the image, andHSFR (<1 mm spatial wavelength) causes wide-angle light scatter lowering the image contrast.The overall rms value of the surface s is relatedto the integral of the PSD over a considered part orthe entire spectral wavelength (formula insert).

As Fig. 2 exemplarily shows, surface topologymeasurement is the crucial point for final ultra-precision accuracy. This is necessary not onlyfor the final steps but for all phases of theprocessing chain. Because generally surfaces

Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 2 Power spectral density functions (PSD)of a well-finished EUVoptic surface calculated from indi-vidual 3D surface topology measurements [12]. For HSFR,AFM measurements (blue curves) with two scan areas, forMSFR white light interference microscopy (black curves)with two magnifications and for figure large area

interferometry (green curve) have been used. The redcurve is the polynomial fit of all measured data. The threecolored areas “figure,” “MSFR,” and “HSFR” representthe different impact of surface errors of the spatial wave-length ranges on the optical performance of the surface.The indicated figures are the error specs for EUVL optics(Courtesy of U. Dinger, Carl Zeiss SMT GmbH,Oberkochen, Germany)


must be measured for all parts of the spatial wave-length spectrum, different measuring equipment isnecessary since each measuring principle is band-width limitedwith respect to the spatial wavelengthspectrum as shown in Fig. 2. Therefore, the devel-opment of ultra-precision surface processing ischaracterized by alternating stimulation betweenmeasurement development and technology perfor-mance, which is still going on [13, 14]. In Table 1major requirements for surface shape and profilemeasurements together with some status quo fig-ures for advanced optics are given.

Ion Beam Technologies

Ion Beam FiguringAt the beginning of the 1980s, worldwide effortshad been started for the development of determin-istic ultra-precision machining technologies tomeet the steadily growing demands of extreme

high-quality optics surfaces. This time digitalinterferometry was in an early stage of its devel-opment with very limited performance especiallyconcerning its spatial resolution because CCDcameras had pixel figures of only 32 � 32. Nev-ertheless, ion beam figuring for polishing errorcorrection of lenses could be demonstrated.Besides scientists from Arizona State University,USA, German researchers in Leipzig in coopera-tion with Carl Zeiss from Jena obtained early IBFresult in 1989 as shown in Fig. 3.

In the middle of the 1990s, IBF brought thebreakthrough for efficient volume production inDUV lithography optics due to its high processingdeterminism connected with the capability toovercome physical constraints of the conventionalfull lap and small tool abrasive polishing pro-cesses as it is their unstable tool behavior at theedges of the optic.

Using multistep scanning schemes combinedwith dwell time algorithms, processing by


sub-aperture ion beams of different sizes results inan efficient correction of low and part ofmid-spatial frequency surface errors down to thesub-nanometer amplitude level with almost no orminimum surface or subsurface damage, respec-tively, due to the gentle beam surface interaction.Figuring involves specified scanning paths, likemeander, spiral, and cycloid, with a variation ofthe scan speed according to the calculated dwell

Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Table 1 Surface shape and profilemeasurement requirements for ultra-precision surfaceprocessing mainly for optics

Requirement Influenced by

Currentspecificationrms [nm]

Repeatability Temperaturefluctuations,coherent noise

0.2 . . . 0.6

Reproducibility Handling 0.5 . . . 1.0

Accuracy Calibration 0.7 . . . 1.5

Precision Deviation from: (i) an ideal surface or(ii) a given reference

Ultraprecision Surfacesand Structures withNanometer Accuracy byIon Beam and Plasma JetTechnologies,Fig. 3 Historical earlyresult on IBF:Interferograms (taken witha 32 � 32 pixel CCDcamera) of an 80 mmdiameter convex lens beforeand after IBF obtained byresearchers in Leipzig,Germany 1989

times. In a standard procedure the pathway of theion beam spot on the surfaces follows a sphericalbowed meander along the lines of latitude. Thepath velocity of the beam spot on the work surfaceresults from a dwell time distribution proportionalto the desired material removal depth distribution.Typically IBF processing software involve thecalculation of the dwell times (deconvolutionbetween the error 3D topology to be removedand the 3D removal function of the ion beamtool) for a given topology to be removed and thesimulation of the removal process (convolution).Main standard procedures for both are fast Fouriermethods. At given local speeds, dwell times t, andthe material removal function r, the dwell timeprocedure can be expressed as follows:

h ¼ t � r þ e (1)

where h stands for the heights of the removedmaterial and the symbol * denotes the convolutionoperator. Matrix notation yields

hij ¼Xk

Xl

ti�k, j�lrkl þ eij; (2)

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where eij denotes a certain amount of error. Thesummation extends over all values of the materialremoval function rkl, which are assumed to benonzero. The amount eij is composed of errorsresulting from height measurements of the topol-ogy, fluctuations in the removal function, anderrors coming from speed and location of themotion system. The calculation of the dwelltimes and the arising final topology are based forplanar and spherical surfaces onmodified iterationmethods selectable according to Van Cittert,Lucy-Richardson, Wiener, and regularized filter[15] and for aspherical surfaces or local depen-dency removal function on enhanced Gold’smethod [15]. The relevance of the dwell timeswith respect to acceleration and speed limits ofthe motion system is examined, and if necessary, itis immediately corrected by additional smoothingprocedure calculations in order to get a final topol-ogy with highest quality at a minimum in etchingtime. The iteration is stopped if (i) the rms valuefalls below the targeting data of the residual errortopology, (ii) the maximum acceleration of themotion system is exceeded despite theabovementioned numerical smoothing procedure,and (iii) the maximum number of iterations isachieved or no better dwell times leading tosmaller rms values are calculable. The materialremoval function of the ion beam can either begenerated as a data file of the etching ratematrix or by parameters of superimposedtwo-dimensional Gaussians. The parameters ofthe material removal function are externally fore-seeable by a corresponding nonlinear parameterfit of the superimposed function to the measuredvalues of a footprint, grove, or meander etching.

The assumption of modeling the removal pro-cess by using an isotropic removal function andetching proportional to the dwell times enables theapplication independent of the physical removalprocess. The removal function of the ion beam isestimated experimentally by etching the samematerial as for the figuring and measuring theetched topology by interferometry together withthe etching time. Ion beam figuring methods havebeen reviewed recently [16]. Recently IBF tech-nology development is aimed to extend surfacecorrection of millimeter spatial size range down to

below nanometer dimension in height to meet thedemanding requirements of the MSFR especiallyfor aspheres or free forms for lithography DUV,EUV, and synchrotron optics. High spatial resolu-tion in deterministic figuring of such tiny surfaceerror features tight technical specs reqirementshave to be fulfilled: (i) for the ion beam tool withbeam size, stability, and linear etching behaviorwith time, (ii) for the surface figure measuringwith the sufficient spatial resolution, (iii) for theadjustment of the beam tool with respect to thework, and (iv) for the adjustment of the measureddata matrix to the physical work surface. Improve-ments of all these items have been realized alreadyand are still under development to make themmature for production.

For the correction of smaller spatial error fea-tures, for instance, an 8 mm FWHM high-current-density beam of a compact 13.56 MHz RF sourceis reduced by diaphragms of different sizes to2 mm, 1 mm, and 0.5 mm FWHM (Fig. 4).Figure 5 shows exemplarily the polishing errorcorrection of small spherical optics down torms <1 nm.

Ion Beam Figuring PlantIBF high-vacuum plants for workpiece sizes frommillimeter up to 2–3 m in diameter are todayavailable on the market. The main componentsof the systems are (i) a stainless steel processingchamber with a base vacuum of 10�4 Pa, (ii) acomputer-controlled precision 5-axis (x, y, z +orthogonal rotational axes A and B) or 3-axis(x, y z) system for the scanning path movementof the ion source, (iii) an RF-ion source with anautomated Faraday cup system for measuringbeam profile and beam position, (iv) a workpiecehandling system, (v) a vacuum load-lock chamberfor small- and medium-sized optics up to about70 cm in diameter enabling uninterrupted opera-tion of the high-vacuum and fast workpiecechange, (vi) power supply, and (vii) a processgas handling system. The machines are computercontrolled and operate fully automatic includingworkpiece data handling and process modeling. 3-axis systems are preferred as they are more stablein their dynamic behavior compared to the 5-axissystems. In contrast motion to 5-axis in the case of

Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 4 RF-type ion source (13.56 MHz) forIBF; left upper corner: ion source in operation with an Arion beam; right upper corner: same ion source equippedwith a frame for beam shaping diaphragms made fromgraphite and hosting inside a hot filament electron emittingneutralizer for avoiding charging up insulating workpieces

by the positively charged ions. In the middle row, threediaphragms with different hole diameters are shown andbelow interferometrically measured removal spot sizes(footprints) of the beamwithout and with the beam shapingapertures, respectively. Also given are the respective workdistance between diaphragm front side and workpiece sur-face and the beam sizes at full width at half maximum(FWHM)


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3-axis motion system, the ion beam centerlinecannot be directed perpendicular to each point ofthe curved workpiece surface; instead, the beamdirection is parallel to the z-axis and the source inthis case is maintained in a constant distance to allsurface points. Thus, the removal spot size on theone hand and the atomic sputter yield on the otherchange depending on the curvature of the surfacecausing variation of the incidence angle of the ionbeam. This behavior is now fully included in theIBF processing software for calculating the dwelltime matrix to control the ion beam scan pathmotion. Figure 6 shows state-of-the-art IBF plantsfor high-end optics production.

The IBF processing simulation module on thebase of the measured ion beam removal functionand the measured initial surface figure date calcu-lates a dwell time of the ion gun for each point ofthe surface to get the desired final surface figure.After the checking of the simulation results by theoperator, they are fully automatically converted

into motion data for the multi-axis system and theion gun, respectively.

Ion Beam SourcesIon energies for ion beam etching and figuring arein the range of about 400 eVup to 2 keV. Differenttypes of ion sources hot filament (Kaufman type),RF type (inductively coupled 13.56 MHz), andECR type (ECR: electron cyclotron resonance)energized by 2.45 GHz microwave are in use.The ion beam is extracted by a high-voltage-powered multigrid ion extraction system forminga compact ion beam out of the individual beamletsfrom the grid holes. Different grid designs allowvariable beam shapes, divergent, parallel, andconvergent. With the help of the grid voltages,the beam shape can be adjusted within somelimits. For IBF technology mainly Kaufman-typeand RF-type sources are used. The RF-type ionsource (Fig. 4) is preferred for IBF productionpurposes due to lower machine downtime by

Convex mirror: RoC= + 35,032 mm, ∅ = 12 mm / 10,6 mm,

Concave mirror: RoC= − 100, 116 mm, ∅ = 63 mm / 52 mm,

a b

d ec

IBF:

IBF:

2 Runs:

2 Runs:

PV: 18,9 nm RMS: 4,0 nm

PV: 173,3 nm RMS: 22,5 nm

PV: 6,2 nm RMS: 0,8 nm

PV: 22,1 nm RMS: 4,2 nm PV: 4,6 nm RMS: 0,5 nm

FWHM: 1,0 mm

FWHM: 2,2 mm

1.: 75 min,2.: 75 min

1.: 125 min,2.: 79 min

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00 2 4 6 8 10 12

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Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 5 Interferograms of IBF with small spotion beam for ULE® mirror substrates of a Schwarzschildoptic for EUV application; (a) convex mirror initial sur-face, (b) result after 2 IBF runs using a 1.0 mm FWHMbeam size, (c) concave mirror, initial surface, (d) after 1st

run and (e) final result after 2nd run. For the concavemirror, a 2.2 mm FWHM beam size was used. The IBFmachining times are given together with the interferomet-rically measured shape of both beam sizes. RoC radius ofcurvature, the second diameter value is the opticallyactive area


maintenance (no hot filaments exchange aftertheir breaking). Sources are available for differentbeam sizes in the range of about 5 mm to 200 mmFWHM of the near-Gaussian beam shape withreasonable working distances by means of differ-ent extraction grid designs. Additional dia-phragms mounted in front of the source are usedto reduce beam sizes down to ~0.5 mm FWHM.Hot filament or so-called plasma bridge neutral-izer mounted outside of the beam area ejects elec-trons and compensates the positive charge of theion beam. This is necessary for a stable processingof insulating materials.

Ion Beam Erosion Self-AssembledNanostructures and SmoothingStringently deterministic IBF finishing technolo-gies with macroscopic beam tool sizes asdescribed above are now well established in

production for shape correction and figuring ofhigh-performance optics. In contrast, ion beam-induced smoothing of spatially micrometer andnanometer features is strongly coupled to atomis-tic processes that are characteristic for the muchsmaller spatial length scales and are less determin-istic. Low-energy ion beam techniques (ionenergy <2 keV) have been developed to improvethe microscopic surface roughness of solid sur-faces on a nanometer to some ten micrometerspatial scale to achieve ultrasmooth surfaceswith rms roughness values <0.2 nm. Twoprocessing schemes ion beam direct smoothingand smoothing with planarization or sacrificiallayers are demonstrated [17]. In ion beam directsmoothing (IBS), favorable relaxation mecha-nisms in the development of surface topographyduring the ion beam erosion are exploited. For thesecond method, the smoothing effect results from

Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 6 State-of-the-art ion beam plants for IBFand partly for IBS (5-axis motion system necessary, as inplant (a)); (a) plant is for high-throughput production ofworkpieces up to 100 mm in diameter and (b) IBF plant for

large mirrors up to 1.5 m in diameter, up to 520 mmthickness including mechanical frame of the assembly oflarge mirrors, and up to 1000 kg weight. On the left of thephotograph, the mechanical interface including slide sys-tem for mounting the mirrors is shown (Courtesy NTGGmbH, Gelnhausen, Germany; www.ntg.de)


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the removal of a planarizing layer deposited on thesubstrate (IBP – ion beam planarization). Bothsmoothing techniques were investigated for awide range of various materials. In Fig. 7 ionbeam smoothing working principles are shownschematically.

Generally, the surface topography evolutionon the nanometer and micrometer spatial scaleduring low-energy ion beam sputtering is rathercomplex. A multitude of topographies can resultfrom the surface erosion due to different rough-ening and smoothing mechanisms. Besides theactual removal of material, the surface erosionprocess often gives rise to a pronounced topog-raphy evolution, generally accomplished by akinetic roughening of the surface and the gener-ation of so-called self-assembling nanostructures[17]. Figure 8 shows 3D structures in the range of100 nm spatial dimensions: a moth eye as a resultof the evolution of life (Fig. 8a–c) and fabricatedby self-assembling effects by Ar ion beam ero-sion of an InP semiconductor wafer surface(Fig. 8d) [19].

One goal of ongoing intensive research anddevelopment is to get insight in the physics of

the surface atomic processes responsible for suchstructures. The second one follows the vision toexploit the ion-induced self-assembling surfaceeffects to develop techniques to generate nano-structured surfaces efficiently without costlymask techniques. Theoretical and experimentalresearch during the last several years broughtmuch progress for the understanding of theion-induced nanoscale surface topography evolu-tion by self-assembling effects. Several atomic-level effects have been identified. They are allconnected with stimulated motion of surfaceatoms along the surface. Surfaces can be smoothedvia thermally driven surface diffusion at sufficienttemperatures under ion bombardment-relateddefect production. Ion-induced directed or randomfluxes of recoil atoms moving parallel to the sur-face are able to compensate the curvature-dependent sputtering or can contribute to ballisticsurface diffusion process. For amorphous materialsor ion beam amorphized surface regions, defect-mediated ion-enhanced viscous flow may alsooccur. The individual conditions of material, ionspecies, energy, angle of incidence, and surfacetemperature or the participation of third-party

http://www.ntg.de/

Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 8 Scanning electron microscopy picturesof nanometer-sized 3D structured surfaces; (a–c) a motheye with three different magnifications; (c) nanostructures

are generated by the evolution of life to enhance theantireflection of the eye surface for improved night visionof the insect; (d) similar 3D nano-patterned Si wafer sur-face fabricated by atomic-scale self-assembling due to Arion beam erosion [19]

Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 7 Ion beam-assisted methods for surface

smoothing. The planarization technique was described firstby L. F. Johnson and K. A. Ingersoll [18]


particles (intentionally or unintentionally added tothe ion beam processing), to mention the mostimportant ones, determines the resulting surfacestructure.

However, dependent on the same physicaleffects, also favorable ion beam erosion condi-tions can be realized where surface smoothingdominates the topography evolution on the

Rq = 2.15 nm Rq = 0.81 nm Rq = 0.54 nm 8 nm

0 nm

750 nm

PS

D [n

m4 ]

spatial frequency f [nm−1]

SiC as polished

IBS step#1

IBS step#2

750 nm

106

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10−1

10−110−210−3

750 nm

a b c

d

Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 9 Example for ion beam direct smoothingof a polished SiC surface (Ar+, Eion = 800 eV, jion = 250mAcm�2, simultaneous sample rotation). AFM imagesshow (a) the initial as polished surface, (b) after IBS

step1 (ion incidence angle 70�) and (c) after IBS step2(ion incidence angle 0�). The rms roughness was reducedfrom Rq = 2.15 nm to Rq = 0.54 nm. (d) shows the cal-culated power spectral density curves from the three AFMdata [20]


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microscopic spatial scale. This is illustrated inFig. 9 for ion beam direct smoothing of hard-to-polish polycrystalline SiC. Starting with amechanically polished surface (Rq = 2.15 nm,Fig. 9a) after Ar+ IBS (Eion = 800 eV, jion = 250mA cm�2) at 70� incidence under simultaneoussample rotation (Fig. 9b), 0.7 nm rms wasachieved, and after a second IBS step at 0�,0.54 nm rms (Fig. 9c). The PSD functions inFig. 9d show for the first IBS step a uniformreduction of the roughness over the whole spatialfrequency spectrum, whereas for spatial frequen-cies <0.002 nm�1, the smoothing drops down.This indicates a long spatial wavelength limit forefficient ion beam direct smoothing owing to thelimited spatial range of the related atomic surface

transport processes mentioned above, which areresponsible also for this kind of smoothing.

Therefore, ion beam direct smoothing is lessefficient for spatially larger roughness features.Alternatively, a modified planarization techniquecan be used for the reduction of surface roughnessat lower spatial frequencies. In ion beamplanarization (IBP), the rough surface is spin orspray coated by a suited sacrificial layer with alow viscosity (e.g., photoresist) to level out thesurface roughness. Subsequently the sacrificiallayer is removed by ion beam etching at theplanarization angle. This angle is defined as thelocal ion beam incidence angle where the removalrates (sputter yields) of the sacrificial layer andthe underlying substrate are nearly identical.


Thus, the smooth surface of the sacrificial layer istransferred into the substrate. Using this tech-nique, roughness components with spatial wave-length up to some tens of microns can be reduced.In Fig. 10 NiP surface smoothing by IBP com-bined with IBS steps after single-point diamondturning is shown. The efficient removal of thediamond tool marks is evident from the AFMpictures in Fig. 10a for different resolutions.Also shown in the right upper corner are the etchrates dependent on the ion incidence angle for NiPand resist type AZ1450J used for the planarizingfilm, indicating ~30� as the planarization angle inthis case. The resist was spray coated with a thick-ness of ~80 nm. The PSD in Fig. 10b comprisesmeasurements by AFM and optical profilometryand clarifies the range of spatial wavelengthwhere smoothing operates is up to some 10 mm.

In Fig. 11 a sequence of ion beam smoothingsteps and thin film Si deposition to reduce the ionbeam etched-related roughness enhancementresulting from IBF for polishing error correctionof Zerodur® is shown. First, an IBF shape correc-tion was performed. Zerodur® as a glassyceramics in contrast to glass, fused silica, or Sihas a nonuniform chemical and structural compo-sition leading to different ion sputter removal ofthe material constituents. Therefore, during lBFsurface roughness is enhanced above the EUVLspec of the HSFR (<0.15 nm rms) (left in Fig. 11).In a second step, the substrate is coated with a thina-Si layer by IBSD. The surface roughness of theSi film surface is reduced (middle in Fig. 11). Nextan IBP step and finally an IBS step were applied(right in Fig. 11). After optimization loops wereperformed, an HSFR of <0.15 nm rms could beachieved. One has to exercise care in doing suchtechnical trick not to remove the Si layercompletely at any point. This would cause againroughness enhancement on Zerodur. The proce-dure all at all does not destroy the initiallyachieved shape accuracy by IBF due to the mini-mum film thicknesses applied and hence the min-imum and deterministic material removal for thesmoothing. The error values given correspond tothe standard deviations obtained from at least tenmeasurements on a two-inch substrate. The PSDgraph shows the effective surface smoothing for

spatial frequencies that are >f = 1 � 10�4 nm�1

corresponding to a long wavelength limit in therange of some 10 mm.

Three examples of ion beam-related surfacesmoothing of very different materials und surfaceconditions shown here demonstrate conclusivelythe high application potential for fabrication ofultrasmooth surfaces. Meanwhile, the effectswere demonstrated for a wide variety of materialsand different applications. At present develop-ments are going on to qualify ion beam equipmentand processing schemes for making ion beamsmoothing ready for an efficient production tech-nology. It will definitely supplement existing tech-nologies for such cases where they come totechnical or performance limits.

Finally, a further machining method with highsmoothing potential, the gas cluster ion beamtechnique [22], exists. Here clusters of up tosome 1000 atoms are formed when high-pressuregas flows through a tight nozzle. Then they areionized and accelerated up to some 10 keV. Thegas cluster ions break when they hit a surface, andevery individual atom gets only a small part of thekinetic energy in the range of some 10 eV. Theirpreferred motion is parallel to the surface; thus,they transfer momentum to top most surfaceatoms and smooth, and additionally they generateonly minimum or no subsurface damage.

RIBE Pattern Transfer

In modern microsystems and micro-optics tech-nologies, there is a growing need of micro- andnano-patterned surfaces. Besides binary struc-tures, 3D-shaped complex patterns for a varietyof materials are required, e.g., refractive microlens arrays or diffractive blazed Fresnel lenses.Advanced 3D mask techniques like dose depen-dent e-beam lithography, laser direct writinglithography, grey tone mask, and holographicinterference lithography have been developed.The majority of substrates are plane wafers butalso thick curved quartz glass substrates, forexample, for molds to fabricate polymer imagingblazed diffraction gratings by embossing must beprocessed. The standard procedure for micro- and

Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 10 Removal of tool marks and surfacesmoothing on NiP surfaces by ion beam planarization. (a)Sequence of AFMmeasurements using different scan areasafter single-point diamond turning (upper row) and after

2� IBP + 1� IBS (lower row), (b) PSD calculated fromthe shown AFM and additional white light interferencemicroscope measurements indicating efficient smoothingfor spatial frequencies >10�4 nm�1 (<10 mm) spatialwavelength [21]


U

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thetext

[21]



nano-patterning is the transfer of masks by reac-tive plasma or ion-based etching. In 3D case themask is sacrificial and the transfer can be eitherproportional or altering the shape. Reactive ionetching (RIE) as inductively coupled plasma etch-ing (ICPE) or capacitively coupled plasma (CCP)is the mainly used process due to its high perfor-mance for etch rates and material selectivities [23,24]. Ion beam techniques IBE, RIBE, and CAIBEoffer the advantage of separating the workpiecefrom the plasma. This allows additional parame-ters with respect to RIE. Thus, the transfer etchingbehavior can be adjusted beneficially: angle ofparticle incidence, independent control of processparameters, sample manipulation duringprocessing (rotation), and co-deposition of othermaterials featuring, e.g., to etch profiles ofpredetermined sidewall angles. Figure 12a showsthe schemes for IBE, RIBE, and CAIBE, Fig. 12bshows a RIBE-etched blazed grating on a thickquartz substrate, and Fig. 12c shows AFM mea-surements of profiles of the resist mask and theblazed grating RIBE transferred into fused silica.Figure 13 shows examples of tailoring of profilestructure during RIBE by variation of ion beamincidence angle and process gas mixture.

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Plasma Jet Technologies

PJM: Deep Aspherization and Polishing ErrorCorrectionEarly developments of plasma-assisted chemicaletching (PACE) originate from Bollinger andZarowin [25]. A variety of atmospheric plasmajet (APJ) tools with different removal rates andtool sizes have been developed covering differentapplications by material removal but also includ-ing thin film deposition. In advanced high-techoptics, there is a growing demand for precisionaspherical and free-form optical elements withlarge deformations from standard flat, cylindrical,or spherical shapes. Plasma Jet machining (PJM)is a noncontact, locally dry-etching method thatcan be beneficially used in the fabrication of suchelements. The working principle is pure chemicalreaction between reactive species originatedwithin the plasma and the surface atoms. In the

case of silicon or silicon compounds, fluorineradicals or atoms are used to build together withSi atoms the volatile molecule SiF4. This escapesand the etching proceeds. Figure 14 shows theplasma jet source operating scheme and the sim-plified chemical surface reaction of F with Si andfused silica together with an atmospheric plasmajet source in operation machining a concave fusedsilica cylinder lens.

Preferred precursors admixed to the plasmainert source feed gas (e.g., He or Ar) are thestandard process gases usually used in siliconwafer technology like CF4, SF6, and the like. Forfused silica and ULE®, material removal rates ofup to 50 mm3/min are easily achieved. Besidessuch high-rate PJM with removal rate perfor-mances comparable to grinding, very low removalrates can be achieved as in the case of IBF byreducing RF power, reactive gas concentration, orgas flow. Also wide changes in the size of theremoval function are possible by changing thediameter of the nozzle of the plasma jet source.Thus, large parts can be as efficiently processed toachieve deep aspheres as the correction of spatialmillimeter size error features with nanometerdeviation can be performed on up to large sur-faces. The processing algorithm is mostly thesame dwell time principle as described above forIBF. Primary advantages of the plasma-assistedchemical methods are the following: (i) there arenearly no limitations arising from local curvaturesof the parts to be machined, and most importantly(ii) no subsurface damages of the material aregenerated during the pure chemical machiningprocess. Therefore, plasma-assisted chemicaletching leads to machined surfaces withunchanged material characteristics equal to thevolume. This is in beneficial contrast to the sub-surface damaged layers originating from mechan-ical or chemomechanical machining (e.g.,grinding, lapping, polishing, and others) wherethe processing modifies up to some 10 mm thicksurface layers by mechanical forces and/or theincorporation of contaminants from polishingslurry. Even the bombardment of low-energyheavy ions during IBF leads to comparable lowmaterial modification within some 10 nm from thesurface. PJM can also completely remove all such

900

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Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 12 Schemes of IBE, RIBE, and CAIBEwith variation of ion beam incidence angle and workpiece

rotation (a), RIBE-etched blazed grating on a thick quartzsubstrate (b), and measured profiles of the resist mask andthe blazed grating RIBE transferred into fused silica (c),(Fig. 12b) (Courtesy Carl Zeiss Jena GmbH)


damage layers. This reduces the polishing effortafterwards largely. Shape conserving postpolishing after PJM is usually necessary forultra-precision surfaces in cases with severalmicrometer plasma jet removals because the spon-taneous chemical etching, which starts preferredat the statistical distributed microscopic surfacedefects with their weakened atomic bindingforces, results in a slight roughness enhancement.

Mid-spatial surface error features are typicalfor modern CNC coarse machining (grinding

and lapping), and if existing on a surface, theyare hard to remove. They originate mainly fromunavoidable residual noise or vibrations from themachine mechanics transferred to the tool that isin more or less hard contact with the workpiecesurface. Plasma-assisted high-rate removal tech-nology is contactless as is ion beam, and therefore,it can a priori avoid or at least minimize such hardto remove features.

On the other hand, there are some critical pointsfor at least high-rate plasma jet machining that

Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 14 (a) Scheme of an atmospheric plasmajet source with a coaxial electrode configuration typical forthe use of 2.45 GHzmicrowave or standard RF frequenciesplasma excitation, indicated are simplified chemical reac-tions for Si and SiO2 etching by plasma generated

F radicals with surface atoms; (b) plasma jet sourcemachining of a fused silica concave cylinder lens, inthis case an additional shielding aperture in front of theplasma source prevents the contact of the microwaveexcited primary plasma with the surface in favor of aradical beam [26]

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Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 13 Resist grating pattern transfer intoquartz by CHF3/O2/Ar-RIBE: tunable blaze, anti-blazeangle, and length ratio of a sawtooth structure by variation

of the ion beam incidence angle but with constant profiledepth; (a) only Ar-IBE (selectivity quartz/resist ~1) and(b) CHF3/O2-RIBE with variation of the flow rate ratio ofprocess gases CHF3/O2 which allows to alter the profiledepth in situ (selectivity ~3)


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should be considered and that indeed slows downthe progress to implement PJM into production:(i) the etching process is very sensitive to surfacestructural defects and contaminant chemical impu-rities, which means that perfect cleaning of surfacesto remove any traces of grease or other contaminat-ing materials that will inhibit chemical reaction bythe plasma-generated reactive species is crucial forplasma-assisted chemical etching; (ii) in contrast tosemiconductor industry, hazardous chemicals likeF radicals that are produced in the plasma jet are notcommon in standard optics fabrication; and (iii)plasma etching rate is proportional to two indepen-dent parameters: (a) linear to the flow density of thereactive species on the surface and (b) exponentialto the surface temperature in the reaction zone,which causes principally a nonlinear etching rate.The chemical reaction is characterized by an acti-vation energy that causes an exponential Arrheniusbehavior of the etching rate on temperature. Theplasma jet delivers both the flow of radicals and thethermal energy for the reaction. Therefore, in theusual case of dwell time controlled figuring, theremoval is nonlinear with respect to the dwelltime in contrast to IBF. Especially for materialswith rather low thermal conductivity as fused silica(quartz glass), the local surface temperature in thecontact point of the jet and the extension of thereaction zone around this point are strongly depen-dent on the scan velocity of the jet. Additionally,superimposed to this is the heating up of the wholeworkpiece with proceeding machining time. For areliable technology, measures must be taken tolinearize the removal rate with dwell time. Recentlya first attempt was made to compensate for thenonlinearity of the fast local heating effects fromthe dwell time variation and of the slowly changingtemperature distribution within the whole work-piece. The procedure calculates the dynamicalheat flow from the jet throughout the workpieceincluding the geometrical and thermal data of thesample and the relevant data from the jet and itsdwell time-related motion across the surface in aFEM model [27]. In the simple case of a flat quartzcylinder workpiece used to demonstrate the feasi-bility of the procedure and to check it experimen-tally, an accuracy improvement from 10 % residualerror from the design shape without nonlinearity

compensation to 1 % with the compensation wasachieved.

In addition to deep aspherization, also deter-ministic surface machining up to sub-mm spatialwavelength range with nanometer depth accuracycan be performed by PJM [25]. In such cases mainPJM source processing parameters, namely, elec-trical power and reactive precursor gas flow, willbe downsized to reduce the etching rate by severalorders of magnitude. The size of the removalfunction can be reduced up to about 0.5 mmFWHM by using appropriate diameter pipe elec-trodes. Figure 15 shows results of the manufactur-ing of a fused silica cylindrical off-axis asphereusing a two-step PJM, (i) a high-rate PJM foraspherization and (ii) a low-rate high spatial reso-lution figure error correction. For the low etchingrate regime of PJM, nonlinear effects as describeddo not occur.

PJP: Plasma Jet Polishing Demonstration

A third atmospheric plasma jet (APJ) processingtechnique in optical surface fabrication is smooth-ing of, e.g., ground surfaces with nearly no mate-rial removal and hence with minimum changingof the surface figure. Furthermore, due to thepossible small size of the plasma jet tool, all APJtechniques can be performed very locally. Thisgives the opportunity either to polish small sub-areas or small-sized and strongly curved parts.Now, the idea for the future is to establish a cost-efficient two-step process chain consisting of pre-cision grinding and plasma jet polishing for themanufacturing of aspheres and free forms. Ifhigher shape accuracy is necessary, the plasmajet-based surface error correction should beincluded just by switching the plasma jet processfrom smoothing to etching. Figure 16 showsexamples of plasma jet polishing of fine groundedfused silica surfaces [28].

Outlook

Ion beam etching-based surface finishing is a ver-satile technology with a high degree of

Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 15 PJM – nanometer figure errorcorrection of a fused silica plano-elliptical mirror blankfor synchrotron radiation optics, (a) 2D surface designprofile of the optic, (b) and (c) stitching interferometer

topology measurements, (b) initial surface after high-ratePJM figuring and post polishing of clear aperture of130 � 25 mm2, (b) after two plasma jet correction cycleswith a 1 mm FWHM PJ-tool, (d) center profile lines of themeasurement of (b) and (c) [26]


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predictability due to the high stability of state-of-the-art ion sources and the acquired knowledge ofthe physics of beam surface interaction. The inde-pendent control of the ion energy and the ioncurrent density over wide ranges and the possibleadditional use of chemical reactive species incombination with physical sputter removal allowsolving tasks in a wide variety of applications ofultra-precision surface figure and patterns.

The developed techniques, the productivemachines, and the finishing results show that ionbeam etching technologies are advanced and pow-erful tools for present and future surfaceprocessing and finishing in ultra-precision tech-nology. Ongoing developments on ion beamfinishing technology will concentrate on furtherimprovement of accuracy, reliability, new appli-cations, and materials. Presently higher spatialresolution figuring is a challenging target just asthe introduction of the ion beam smoothing tech-niques in production chains. New supplementaryapproaches will possibly further improve the per-formance of the technologies. One example ispulse width modulation ion sources combinedwith standard continuous beam to extend the

dynamic range of the dwell times by a controlledreduction of the average ion beam power. Thus,two orders of magnitude and more dwell timereduction should be possible with an apparentbenefit for the removal of very small amounts ofmaterial as in cases of small beam sizes. Further,locally controlled beam switching on and off insitu during dwell time processing allows figuringof separate areas of a surface, leaving other partsunaffected. This allows to generate differentshapes on one surface, enabling multifunctionalultra-precision surfaces. Another new approach ision beam figuring by dwell time-controlled depo-sition of materials using ion beam sputter deposi-tion [29] instead of removal. Targets of ongoingdevelopment of pattern transfer are experimentaldata-backed process modeling for future adaptiveRIBE processing to enhance versatility and reli-ability on the one hand and sub-aperture RIBE byscanning large surfaces like IBF technique on theother.

During the last years new research fieldswith high technological potential for precisionsurface processing emerged in ion beam and inplasma jet techniques. Related keywords are ion

Ultraprecision Surfaces and Structures with Nano-meter Accuracy by Ion Beam and Plasma Jet Tech-nologies, Fig. 16 Demonstration of the efficientpolishing effect by a plasma jet. (a) Plasma polished partof Yb-doped SiO2. (b) Laser profiler measurement dataalong track “A –B” shown in a) Initial: Ra = 443.0 nm;plasma jet polished: Ra = 1.3 nm (white light interferom-eter, magnification 20�); black curve: raw data; red and

green curve, low pass filtered data. (c) Plasma polishing ofa fine ground fused silica wafer (~100 nm rms), polishedarea Ø 60 mm (~5 nm rms). (d) Microwave (2.45 GHz)-powered atmospheric plasma jet source operating in cwmode for plasma polishing and (e) AFMmeasurement of aplasma jet polished fine ground fused silica surface show-ing a roughness of 0.3 nm rms on a measuring area of5 mm � 5 mm [28]


beam-induced self-organized nanostructures, ionbeam-assisted growth of sculptured thin films,damage-free plasma jet figuring, and plasma jetsacrificial oxidation [30], to mention some of theactivities. Several research groups in the world

from academia and industry are focused on thesefields on both basic and technology-orientedresearch and development for further progress inatomic beam-related technologies for ultra-precision surface processing.


Cross-References

▶Atomic Force Microscopy▶Dry Etching Processes▶EUV Lithography▶Nanotechnology▶Optical Techniques for NanostructureCharacterization

▶ Self-Assembly of Nanostructures▶Ultraprecision Machining (UPM)

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https://en.wikipedia.org/wiki/Richardson%E2%80%93Lucy_deconvolution

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22. Yamada, I.: Historical milestones and future prospectsof cluster ion beam technology. Appl. Surf. Sci. 310,77–88 (2014). doi:10.1016/j.apsusc.2014.03.147

23. Laermer, F., Franssila, S., Sainiemi, L., Kolari, K.:Deep reactive Ion etching. In: Handbook of SiliconBased MEMS Materials and Technologies,pp. 349–374. Elsevier (2010). http://linkinghub.elsevier.com/retrieve/pii/B9780815515944000231

24. Vawter, G.A.: Ion beam etching of compound semi-conductors. In: Shul, R.J., Pearton, S.J. (eds.) Hand-book of Advanced Plasma Processing Techniques,p. 507 ff. Springer Science & Business Media (2000)

25. Bollinger, L.D., Zarowin, Ch.B.: Rapid,nonmechanical, damage-free figuring of opticalsurfaces using plasma-assisted chemical etching(PACE): part I experimental results. In: Arnold, J.B.,Parks, R.E. (eds.)Proceedings of SPIE, Advances inFabrication and Metrology for Optics and LargeOptics, vol. 966, pp. 82–90. (1989). doi:10.1117/12.948052

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Ultrashort Carbon Nanotubes

Lesa A. Tran and Lon J. WilsonDepartment of Chemistry, The RichardE. Smalley Institute for Nanoscale Science andTechnology, Rice University, Houston, TX, USA

Synonyms

US-tubes

Definition

Ultrashort carbon nanotubes (US-tubes) are20–100 nm segments of single-walled carbonnanotubes (SWNTs).

Overview

In recent years, single-walled carbon nanotubes(SWNTs) have been extensively studied for theiruse in biomedical applications. Owing to theirpeculiar physical and chemical properties,SWNTs have become a widely used platform forthe design of medical therapeutic and diagnosticagents [1, 2]. Nevertheless, the biocompatibilityand biodistribution of SWNTs still remain uncer-tain and are under current investigation. The ideallength of SWNTs for biomedical applications hasyet to be determined, but it has been suggestedthat discrete, individualized SWNTs of shorter(<300 nm) lengths are more ideal for in vivo use[3]. Therefore, ultrashort carbon nanotubes(US-tubes), which are 20–100 nm segments ofSWNTs, may be best suited for biomedical appli-cations (Fig. 1).

Methodology

Synthesis

Chemical CuttingGenerally, the chemical cutting process ofSWNTs into US-tubes can be viewed as atwo-step procedure: (1) the modification of theSWNT sidewalls with functional groups or defectsites, and (2) the removal of or the cutting at thesesidewall modifications. While the predominantmechanism of chemical SWNT cutting is debat-able, there are two proposed methods under con-sideration: (1) the “fuse burning” mechanism,with carbon etching occurring at the bond-strained tube ends, and (2) defect site propagation,with cutting occurring at the random defect siteson the SWNT walls. Both oxidizing andnonoxidizing methods to chemically cut SWNTshave been explored.

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Single-WalledCarbon Nanotube (SWNT)

> 1 μm

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Ultrashort Carbon Nanotubes, Fig. 1 Illustration of(a) a single-walled carbon nanotube and (b) a collectionof ultrashort carbon nanotubes (with sidewall defects)derived from a single-walled carbon nanotube

Ultrashort Carbon Nanotubes 4283

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Various oxidative processes using differentstrong oxidizing agents (HNO3, H2SO4, andH2O2) have been proposed for the cutting ofSWNTs. However, the efficiency and length dis-tributions of these synthetic methods vary with thetype and concentration of the oxidizing agentsused, the reaction temperature and time, themethod of dispersion during the reaction(refluxing or sonication), and the length and diam-eter of the SWNTs being cut. Although moresevere reaction conditions result in more reactionexposure of SWNTs and, thus, shorter lengths ofresulting US-tubes, less severe reaction condi-tions are preferred in order to minimize amor-phous carbon production.

For instance, it has been shown that 4:1(vol/vol 96 % H2SO4/30 % H2O2) piranha solu-tions are able to attack existing damage sites onthe nanotube sidewalls to chemically shortenSWNTs [4]. While hot (70 �C) piranha solutions

can induce more damage sites, room-temperature(22 �C) solutions can react with existing sidewalldefects but are incapable of initiating further dam-age sites, resulting in minimal carbon loss andslow etch rates (Fig. 2). On the other hand, high-temperature conditions produce increasinglyshorter US-tubes with increasing reaction time,with approximately half of the starting materialdestroyed due to chemical etching. Therefore,room-temperature piranha solutions offer the abil-ity to exploit active damage sites in the sidewallsof the nanotube in a controlled manner without thecounterproductive destruction of the nanotubes.

The aggressive oxidation of SWNTs witholeum (100 % H2SO4 with 20 % SO3) and nitricacid can simultaneously shorten and carboxylatethe SWNTs, rendering water-soluble US-tubesless than 60 nm in length [5]. Purified SWNTsare first dispersed in oleum to disentangle individ-ual SWNTs from their rope-like bundles. Next,the SWNT-oleum dispersion is extensively oxi-dized with strong HNO3 at elevated temperaturesto cut the SWNTs into US-tubes. The oxidation ofthe SWNTs also forms carboxylic acid groups atthe sidewall defects of the SWNTs, makingUS-tubes that are up to 2 wt% soluble in polarorganic solvents, acids, and water.

One nonoxidative process to synthesizeUS-tubes involves the chemical shortening ofSWNTs to lengths of less than 50 nm by a fluori-nation/pyrolysis treatment [6]. Briefly, purifiedSWNTs are exposed to fluorine gas, duringwhich fluorine atoms tend to arrange around thecircumference of the nanotube to form band-likeregions along the nanotube sidewalls. Uponpyrolysis of the fluorinated SWNTs at 1000 �Cin an inert atmosphere, the chemically boundfluorine atoms are driven off in the form of CF4and COF2. This leaves behind chemically cutUS-tubes with partial sidewall defects along thenanotube axis.

Mechanical CuttingThere have been various documented methods tomechanically cut SWNTs into shorter segments,including ultrasonication, grinding, and ball mill-ing. However, these techniques lack length control;suchmechanical methods create extensive sidewall

Ultrashort Carbon Nanotubes, Fig. 2 Atomic force microscopy (AFM) images of (a) purified SWNTs and SWNTsafter piranha treatment at 22 �C for (b) 1 h, (c) 3 h, and (d) 9 h (Reproduced with permission from Ref. [4])


damage and are not able to achieve sub-100 nmlengths. Therefore, more precise mechanical cut-ting techniques have been developed.

Microlithography can be used to cut SWNTsinto US-tubes [7]. This technique involves a pro-tective photoresist polymer layered on top of aSWNT-covered solid substrate in a patternedmanner to protect portions of nanotubes fromlithographic damage and modification. Reactiveion etching using an oxygen plasma is then usedto remove the unprotected sections of the SWNTs.After etching, the newly formed nanotube endsremain, with dangling carboxylic acid and ethergroups. The photoresist polymer can then beremoved, leaving cut segments of SWNTs.Regardless of whether the SWNTs are previously

aligned or properly dispersed prior to cutting,microlithography offers a straightforward methodto shorten SWNTs with a narrow and selectivelength distribution.

Electron beam etching has also been used tocut SWNTs with nanometer precision [8]. Usingelectron energy beams exceeding 100 keV, whichis made possible by currently available electronmicroscopes, partial and complete cuts intoSWNTs can be made with great control withoutthe need of covering portions of SWNTs with aprotective layer. Similar methods have beendeveloped using high voltage-bearing scanningtunneling microscope (STM) tips. Interestingly,upon cutting into a SWNT bundle, the newlyformed tube ends immediately form hemispherical

Ultrashort Carbon Nanotubes, Fig. 3 Transmissionelectron microscopy (TEM) image of the complete cuttingof a SWNT bundle into US-tubes using electron beametching (Reproduced with permission from Ref. [8])


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caps, enhancing the stability of the cut US-tubes(Fig. 3). Another peculiar observation is thatforming a second cut within 10–50 nm from apreexisting cut proves more difficult to perform.This increase in stability is thought to be a result ofthe generation and annealing of vacancy-interstitialatom pairs.

More recently, an ultramicrotome has beenused to cut frozen layers of magnetically alignedSWNT membranes, or Buckypapers [9]. Enoughmechanical control was obtained to create open-ended US-tubes with minimal sidewall damage,typically seen after chemical oxidative methods.When the Buckypaper was cut into 50 nm seg-ments, the mean length was about 87 nm, withabout 60 % of the tubes ranging from 50 to150 nm. However, the large pressure exerted bythe diamond knife tip in its proximity can causedeformation and cracking of the nanotubes.

Individualization of US-tubesLike their SWNT precursors, US-tubes have alarge tendency to bundle together, which rendersit difficult to form single-tube suspensions.Although it is possible for full-length SWNTs todisperse in solution with the aid of polymers andsurfactants, US-tubes cannot disperse as readily inthese same conditions [10]. Because of their highaspect ratios, SWNTs are relatively flexible andcan “peel” more readily from each other uponintroduction to surfactant wrapping. On the other

hand, US-tubes have a much lower aspect ratio,therefore making them more rigid and more diffi-cult for surfactant molecules to insert betweenindividual US-tubes. Therefore, chemical meansto debundle and individualize US-tubes have beenexplored to better disperse US-tubes.

Reduction of US-tubesChemical reduction can be used to individualize,or debundle, US-tubes [10]. Upon sonication withK� metal in THF, the negative charge imparted onthe US-tubes electrostatically overcomes the bun-dling forces to yield individualized US-tubes(Fig. 4). It is thought that the individualization ofUS-tubes via chemical reduction increasesUS-tube dispersion in solution, thus allowing formore even chemical functionalization.

Functionalization of US-tubesThe chemical functionalization of US-tubes isthought to occur at carboxylic acid end groups ofthe sidewall defects. This allows for the attach-ment of various chemical groups that would ren-der the US-tubes more soluble in different solventsystems. One functionalization method utilizes anin situ Bingel-Hirsch (CBr4/DBU) cyclopro-panation to functionalize US-tubes with thewater-soluble malonic acid bis-(3-tert-butoxycarbonylaminopropyl)ester to yield fourto five adducts per nm [10]. Another documentedfunctionalization method involves cyclopro-panation using diazoacetic ester in the presenceof Rh6(CO)16 [11]. This allows for the US-tube tobe readily reactive with primary amines, thusmaking possible the functionalization ofUS-tubes with a number of amino acid deriva-tives, such as water-soluble serine, with 3–19groups per nm. Other biologically relevant moie-ties, such as antioxidants [12] and oligonucleo-tides [13], have been successfully attached ontoUS-tubes and are described below. Alkylation-based reduction can also be performed onUS-tubes to improve nanotube dispersion in non-polar environments [14].

Loading of US-TubesIn addition to being able to chemicallyfunctionalize the sidewalls of US-tubes, the

nmb

a

2.1 nm1.6 nm1.1 nm2.00

−2.00

0

1.000

μm

2.00

Ultrashort CarbonNanotubes, Fig. 4 AFMimage of individualizedUS-tubes after chemicalreduction (Reproduced withpermission from Ref. [10])


hollow interior of the US-tubes can be loaded orfilled with various ions and small molecules, suchas Gd3+ ions [15], molecular I2 [16], and

211AtClradionuclides [17]. This property is especiallyrelevant for drug design, as the carbon structureof the US-tube serves as an encapsulating sheathto protect potentially toxic moieties from theirsurrounding environment. Although the loadingmechanism is not completely clear, it is likely thations and small molecules load into the US-tubesthrough the sidewall defects.

Phenomena

ToxicologyBefore US-tubes are considered for biomedicalapplications, their toxicological profiles must beknown. A recent study documented the toxicol-ogy as well as the acute and subchronic effects of

high doses (50–1000 g/kg bodyweight) of Tween-suspended US-tubes in Swiss mice [3]. Oraladministration of US-tubes did not result indeath or any behavioral abnormalities for up to14 days. Intraperitoneal administration allowedfor the US-tubes to reach systemic circulationand various tissues through the lymphatic system.As opposed to unfunctionalized SWNTs, well-individualized US-tubes had the ability to passthrough the reticuloendothelial system, excretingvia the kidneys and bile ducts. High doses ofUS-tubes can induce strongly adherent granulomaformation on and inside organs due to large(>10 mm) nanotube aggregation into fiber-likestructures. However, small (<2 mm) aggregatesof US-tubes can readily enter various cell typeswithout granuloma formation or any sign of tox-icity. Neither death nor growth or behavioralabnormalities were observed for the animalsused in the experiment.


Applications

Diagnostic Agent Design

Magnetic Resonance Imaging (MRI) ContrastAgentsGadonanotubes, or US-tubes loaded with Gd3+

ions, have been shown to outperform currentlyused clinical T1-weighted MRI contrast agents byapproximately 40-fold [15]. Although the exactloading mechanism is unclear, it is likely that theGd3+ ions enter the US-tubes through the sidewalldefect sites, where they form nanoscale clusters.Because of the lipophilicity of the carbon sidewallsand their short lengths, Gadonanotubes can alsoreadily translocate across cell membrane, allowingfor Gadonanotubes to become an effective T1-weighted cellularMRI contrast agent. It has alreadybeen shown that Gadonanotubes can be readilyinternalized and imaged inside a variety of cells,including mesenchymal stem cells (Fig. 5) [18].

In the absence of Gd3+ ions, the US-tubes alonecan perform as a T2-weighted MRI contrast agent[19]. The US-tubes are approximately three timesmore efficacious than Ferumoxtran®, a clinicallyused Fe3O4-based contrast agent. Although theUS-tubes contain much less (<1 %) iron catalystthan unpurified SWNTs, the US-tubes exhibit amuch higher r2 performance, suggesting that thecarbon nanotube material itself is contributing tothe superparamagnetic nature and that the iron

Ultrashort Carbon Nanotubes, Fig. 5 (a) TEM imagesof a Gadonanotube-labeled mesenchymal stem cell (MSC).Red arrows point to the Gadonanotube aggregates. Yellowarrow points to the ribosomes of the endoplasmicreticulum. (b) T1-weighted MR images at 1.5 T and 25 C

particles are not the only contributing factor tothe superparamagnetic signature of the US-tubes.The shorter lengths and the sidewall defects of theGadonanotubes may also contribute to the highperformance. This also implies thatGadonanotubes can concurrently perform asboth a T1- and T2-weighted MRI contrast agents.Gadonanotubes have previously been used as aT2-weighted contrast agent in vivo, with r2relaxivities as high as 578 mM�1 s�1 at 7 T[20]. This suggests that the Gadonanotubes areuseful T1 positive agents at lower magnetic fieldsand T2 negative agents at higher fields.

Computed Tomography (CT) AgentsMolecular I2 has also been loaded into US-tubesfor CT X-ray imaging contrast agent design[16]. After being functionalized with serinolamide groups via the Bingel-Hirsch reaction, theUS-tubes were filled with gaseous I2 at 100 �C,then washed with ethanol and reduced with NaHto remove excess I2 from the nanotube exterior.A 10 wt% loading of I2 was observed for theUS-tubes. Using XPS and X-ray-induced Augeremission spectroscopy, it was confirmed that themolecular I2 was sequestered within the US-tubes.Micro-CT images were obtained to show that theiodine-loaded US-tubes were more X-ray opaque.These serinol amide-functionalized, I2-loadedUS-tubes are the first water-soluble CT contrastagent derived from SWNT materials.

of (left to right) unlabeled MSCs, Gd-DTPA-labeledMSCs, and Gadonanotube-labeled MSCs at Ti = 500 ms.Scale bar = 1 mm (Adapted with permission fromRef. [18])

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Therapeutic Agent Design

Radionuclide Containment and DeliverySimilar to the loading of Gd3+ ions and I2 moleculesinto US-tubes, 211At� ions can be loaded and oxi-dized into 211AtCl within US-tubes to contain anddeliver the alpha-emitting radionuclides in the nano-tube capsule [17]. Upon oxidizing the 211At� ionswith chloramine-T or N-chlorosuccinimide, theAtCl molecules labeled the US-tubes to a greatextent, with 91.3% by aqueous loading. SubsequentPBS washing and serum exposure removed excessAtCl residing on the exterior or at the sidewalldefects of the nanotubes, but the US-tubes exhibitreasonable stability toward physiological challenge.

Bone Tissue Scaffold CompositesUS-tubes have also been used to dope the highlyporous (poly(propylene fumarate)) polymer forbone tissue scaffold engineering [14]. These com-posites are injectable, thermally cross-linkable,and cytocompatible, making them ideal for tra-becular bone tissue scaffolding biomaterials.While high (75–90 vol.%) porosity is preferredto allow for cell adhesion, proliferation, andvascularized tissue ingrowth, the mechanicalproperties of highly porous polymer scaffoldsbecome compromised. Therefore, US-tubes havebeen added to PPF polymer scaffolds to improvethe mechanical properties. It was found thatdodecylated US-tubes functionalized withdodecyl groups better dispersed into PPF, improv-ing their reinforcing effects. While there was nosignificant difference in porosity, pore size, andpore interconnectivity among US-tube-dopedPPF and PPF alone, the compressive modulus,offset yield strength, and compressive strengthof the dodecylated US-tube nanocompositeswere higher than or similar to that of PPF alone.The US-tube/PPF nanocomposite has also beenshown to be osteoconductive, allowing marrowstromal cells to attach and proliferate readily onthe scaffold.

Gadonanotubes have also been incorporatedinto poly(lactic-co-glycolic acid) (PLGA) poly-mer scaffolds in order to explore the degradationprocess of the scaffold and the biodistributionof the Gadonanotubes upon their release from

the polymer matrix in vivo [20]. T2-weightedMR imaging of the Gadonanotube-PLGAnanocomposites and their degradation processwere obtained in vitro, showing the strong influ-ence of the Gadonanotubes on surrounding waterproton relaxation (Fig. 6). In addition to providingmechanical reinforcement from the US-tubestructure, the Gadonanotube-reinforced PLGAscaffolds may elucidate the mechanism of thebiodegradation of the polymer and the releaseand biodistribution of the Gadonanotubes in vivo.

Free Radical ScavengersUS-tubes have also been functionalized as a scaf-fold for the phenolic antioxidant, butylatedhydroxytoluene (BHT) using DCC/DMAP cou-pling [12]. As expected, the higher amount ofBHT functionalization (either via covalent ornoncovalent interactions) to the carboxylic acidgroups on the oxidized US-tubes corresponded tohigher overall antioxidant activity, with perfor-mances more than 200 times better than that ofthe radioprotective dendritic fullerene, DF-1.Strikingly, PEGylated US-tubes alone withoutany BHT functional groups are extremely effec-tive antioxidants, performing nearly 40 timesgreater than DF-1 in oxygen scavenging ability.Even though the US-tubes have heavilycompromised sidewalls from the oxidative cuttingprocess, US-tubes are still able to sequester oxy-gen radicals due to the sp2-hybridized carbonnanotube framework.

Oligonucleotide DeliveryUS-tubes have also served as delivery vehicles foroligonucleotide (ODN) decoys, which are syn-thetic ODNs containing the DNA-bindingsequence of a transcription factor [13]. BecauseUS-tubes can inherently translocate into cells,US-tubes can efficiently facilitate the intracellulardelivery of ODN decoys into target cells so thatODN decoys can better target transcription factorsof interest for gene therapy. Specifically, US-tubeshave successfully delivered a decoy ODN againstnuclear factor-kB (NF-kB), which is a transcrip-tion factor that regulates genes important inimmune and inflammatory responses. Usingcarbodiimide chemistry, the amino groups at the

ImageSlice

6 mm

1 mm

fedcba

Disc

Ultrashort Carbon Nanotubes, Fig. 6 (a) Schematic ofthe nanocomposites sample arrangement within the MR-I. Representation two-dimensional images through ananocomposite disc after (b) 2 h, (c) 24 h, (d) 3 days,(e) 5 days, and (f) 7 days. The higher (white) pixel

intensities within the dark disc represent regions of higherwater concentration. The high intensity surrounding thedisc is the agarose gel (TE = 15 ms) (Reproduced withpermission from Ref. [20])


50 end of the double-stranded ODN was covalentlycoupled to the carboxylic acid groups of the oxi-dized US-tubes. To test the performance of theUS-tubes as a delivery vehicle, NF-kB-dependentgene expression was significantly downregulatedin monocyte-derived human macrophages incu-bated with ODN-functionalized US-tubes, as com-pared to macrophages incubated with US-tubesfunctionalized with nonspecific ODN decoys.

U

Future Directions

While there have been significant strides made inthe development and application of US-tubes,many challenges still must be addressed. Inorder to produce more uniform US-tubes fromSWNTs, better size control during the cuttingprocedure and size sorting after synthesis mustbe achieved. Additionally, improving the solubil-ity and dispersion of US-tubes in biological mediamust be achieved to fully realize their potential inbiomedical applications.

Taking advantage of the exterior functiona-lization and the interior loading capabilities ofUS-tubes, a nearly limitless number of possibili-ties abound for advanced US-tube-based thera-peutic and diagnostic agent design. US-tubes caneither be used as a nanocapsule, sequestering bio-logically relevant ions or molecules that may beinherently toxic if not sequestered, or as ananovector, delivering various drug payloads orpeptide sequences to target cells. Nevertheless,

much work still needs to be completed to bettercharacterize the acute effects, long-term toxicity,and biodistribution of US-tubes, as well as othertypes of carbon nanostructures, in living systems.

Cross-References

▶Carbon Nanotubes▶ Fullerenes for Drug Delivery

References

1. Kostarelos, K., Bianco, A., Prato, M.: Promises, factsand challenges for carbon nanotubes in imaging andtherapeutics. Nat. Nanotechnol. 4, 627–633 (2009)

2. Liu, Z., Tabakman, S., Welsher, K., Dai, H.: Carbonnanotubes in biology and medicine: in vitro andin vivo detection, imaging and drug delivery. NanoRes. 2, 85–120 (2009)

3. Kolosnjaj-Tabi, J., Hartman, K.B., Boudjemaa, S.,Ananta, J.S., Morgant, G., Szwarc, H., Wilson, L.J.,Moussa, F.: In vivo behavior of large doses of ultra-short and full-length single-walled carbon nanotubesafter oral and intraperitoneal administration to Swissmice. ACS Nano 4(3), 1481–1492 (2010)

4. Ziegler, K.J., Gu, Z., Peng, H., Flor, E.L., Hauge, R.H., Smalley, R.E.: Controlled oxidative cutting ofsingle-walled carbon nanotubes. J. Am. Chem.Soc. 127(5), 1541–1547 (2005)

5. Chen, Z., Kobashi, K., Rauwald, U., Booker, R., Fan,H., Hwang, W.-F., Tour, J.M.: Soluble ultra-shortsingle-walled carbon nanotubes. J. Am. Chem.Soc. 128(32), 10568–10571 (2006)

6. Gu, Z., Peng,H.,Hauge,R.H., Smalley, R.E.,Margrave,J.L.: Cutting single-wall carbon nanotubes throughfluo-rination. Nano Lett. 2(9), 1009–1013 (2002)

http://dx.doi.org/10.1007/978-94-017-9780-1_100136

http://dx.doi.org/10.1007/978-94-017-9780-1_76

4290 Ultrasonic Atomization

7. Lustig, S.R., Boyes, E.D., French, R.H., Gierke, T.D.,Harmer, M.A., Hietpas, P.B., Jagota, A., McLean, R.S., Mitchell, G.P., Onoa, G.B., Sams, K.D.:Lithographically cut single-walled carbon nanotubes:controlling length distribution and introducingend-group functionality. Nano Lett. 3(8), 1007–1012(2003)

8. Banhart, F., Li, J., Terrones, M.: Cutting single-walledcarbon nanotubes with an electron beam: evidence foratom migration inside nanotubes. Small 1(10),953–956 (2005)

9. Wang, S., Liang, Z., Wang, B., Zhang, C., Rahman, Z.:Precise cutting of single-walled carbon nanotubes.Nanotechnology 18(5), 055301 (2007)

10. Ashcroft, J.M., Hartman, K.B., Mackeyev, Y., Hof-mann, C., Pheasant, S., Alemany, L.B., Wilson, L.J.:Functionalization of individual ultra-short single-walled carbon nanotubes. Nanotechnology 17,5033–5037 (2006)

11. Mackeyev, Y., Hartman, K.B., Ananta, J.S., Lee, A.V.,Wilson, L.J.: Catalytic synthesis of amino acid andpeptide derivatized gadonanotubes. J. Am. Chem.Soc. 131, 8342–8343 (2009)

12. Lucente-Schultz, R.M., Moore, V.C., Leonard, A.D.,Price, B.K., Kosynkin, D.V., Lu, M., Partha, R.,Conyers, J.L., Wilson, L.J.: Antioxidant single-walledcarbon nanotubes. J. Am. Chem. Soc. 131, 3934–3941(2009)

13. Crinelli, R., Carloni, E., Menotta, M., Giacomini, E.,Bianchi, M., Ambrosi, G., Giorgi, L., Magnani, M.:Oxidized ultrashort nanotubes as carbon scaffolds forthe construction of cell-penetrating NF-kB decoy mol-ecules. ACS Nano 4(5), 2791–2803 (2010)

14. Shi, X., Sitharaman, B., Pham, Q.P., Liang, F., Wu, K.,Billups, W.E., Wilson, L.J., Mikos, A.G.: Fabricationof porous ultra-short single-walled carbon nanotubenanocomposite scaffold for bone tissue engineering.Biomaterials 28, 4078–4090 (2007)

15. Sitharaman, B., Kissell, K.R., Hartman, K.B., Tran, L.A., Baikalov, A., Rusakova, I., Sun, Y., Khant, H.A.,Ludtke, S.J., Chiu, W., Laus, S., Tóth, É., Helm, L.,Merbach, A.E., Wilson, L.J.: Superparamagneticgadonanotubes are high-performance MRI contrastagents. Chem. Commun. 31, 3915–3917 (2005)

16. Ashcroft, J.M., Hartman, K.B., Kissell, K.R.,Mackeyev, Y., Pheasant, S., Young, S., Van derHeide, P.A.W., Mikos, A.G., Wilson, L.J.: Single-molecule I2@US-tube nanocapsules: a new x-raycontrast-agent design. Adv. Mater. 19, 573–576(2007)

17. Hartman, K.B., Hamlin, D.K., Wilbur, S., Wilson, L.J.: 211AtCl@US-tube nanocapsules: a new concept inradiotherapeutic-agent design. Small 3(9), 1496–1499(2007)

18. Tran, L.A., Krishnamurthy, R., Muthupillai, R.,Cabreira-Hansen, M.G., Willerson, J.T., Perin, E.C.,Wilson, L.J.: Gadonanotubes as magnetic nanolabelsfor stem cell detection. Biomaterials 31(36),9482–9491 (2010)

19. Ananta, J.S., Matson, M.L., Tang, A.M., Mandal, T.,Lin, S., Wong, K., Wong, S.T., Wilson, L.J.: Single-walled carbon nanotube materials as T2-weightedMRI contrast agents. J. Phys. Chem. C 113,19369–19372 (2009)

20. Sitharaman, B., Van Der Zande, M., Ananta, J.S., Shi,X., Veltien, A., Walboomers, X.F., Wilson, L.J.,Mikos, A.G., Heerschap, A., Jansen, J.A.: Magneticresonance imaging studies on gadonantube-reinforcedbiodegradable polymer nanocomposites. J. Biomed.Mater. Res. A 93A(4), 1454–1462 (2010)

Ultrasonic Atomization

▶Acoustic Nanoparticle Synthesis for Applica-tions in Nanomedicine

Ultrasonic Force-AssistedNanomachining

▶Ultrasonic Vibration-Assisted Nanomachining

Ultrasonic Force-RegulatedNanomachining

▶Ultrasonic Vibration-Assisted Nanomachining

Ultrasonic Machining

Suhas S. JoshiDepartment of Mechanical Engineering, IndianInstitute of Technology Bombay, Mumbai,Maharashtra, India

Synonyms

Non-conventional machining; USM; Vibrationassisted machining

http://dx.doi.org/10.1007/978-94-017-9780-1_415

http://dx.doi.org/10.1007/978-94-017-9780-1_415

http://dx.doi.org/10.1007/978-94-017-9780-1_100921

http://dx.doi.org/10.1007/978-94-017-9780-1_100921

http://dx.doi.org/10.1007/978-94-017-9780-1_100807

http://dx.doi.org/10.1007/978-94-017-9780-1_101163

http://dx.doi.org/10.1007/978-94-017-9780-1_101168

http://dx.doi.org/10.1007/978-94-017-9780-1_101168

Ultrasonic Machining 4291

Definition

Ultrasonic machining involves imparting ultra-sonic vibrations (of frequency ~20 kHz) to a toolto effect material removal. The process is moreeffective on materials that have hardness morethan RC 40, but it is used on almost all includingmetallic and nonmetallic materials such asglasses, ceramics, and composites.

Process Variants

The process is configured into two main types(Fig. 1a–b) depending upon how the force istransferred from the vibrating tool to the worksurface. If the transfer of vibration energy isachieved by introducing a liquid containing abra-sive grits in the gap between the tool and worksurface, the process is called “ultrasonic impactgrinding” or simply “ultrasonic machining.” Onthe other hand, if the abrasive grits are not presentin the carrier liquid, then the tool is rotated besidesbeing vibrated, and the process is called as “rotaryultrasonic machining” [1, 2].

Basic differences in these two processes areillustrated in Table 1.

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Basic Equipment

Equipment for ultrasonic machining is availablein various forms, like dedicated machines oras an attachment to other machines as well astable-top machines. Typical machine elementsfor performing both the ultrasonic machining(USM) and rotary ultrasonic machining (RUM)are identical. They include [1, 2]:

1. Power Supply: a power supply that provideselectrical output at ultrasonic frequency

2. Transducer: a transducer that converts high-frequency electrical supply to tool displace-ment at ultrasonic frequency. The transducerswork on magnetostrictive or piezoelectricprinciples. The magnetostrictive effectinvolves application of alternating current toa ferromagnetic coil, which induces vibrations

at the applied frequency in the coil. The phe-nomenon allows transfer of vibrations overwider frequency band of (17–23 kHz). How-ever, it also causes high electrical losses (e.g.,eddy current loss) and has low energy conver-sion efficiency (~50 %). Most of the energylosses appear as heat; hence, these transducersrequire air or water cooling. Consequently,such systems are bulky. On the other hand,the piezoelectric transducers comprise of astack of ceramic discs placed between ahigh-density material base and low-densitymaterial radiating face. The energy conver-sion efficiency can be in the range of(~90–96 %). Therefore, they do not requirecooling.

3. Horn: the horn is referred with variousnames such as tool holder, sonotrode, stub,concentrator, or acoustic coupler. Withappropriate design of horn, the vibrationamplitude can be amplified by as high as600 % over its initial value at the transducersurface of ~0.001–0.1 mm. The increase inthe vibration amplitude is inversely propor-tional to the reduction in the area ratiobetween top and bottom faces of the horn.Usually, the horn is made from Monel, tita-nium 6–4, stainless steel, aluminum, or alu-minum bronze.

4. Tool: the tool is shaped such that it is theinverse of the cavity to be made. The tool isattached to the horn either by brazing or sol-dering. This helps avoid possible fatigue fail-ure or self-loosening of the threaded fasteners.The tool is designed such that the amplitude ofvibrations is maximum at its free end. Massand length of the tool are also important.While the bulky tools absorb ultrasonic vibra-tions, very long tools will flutter during theprocess. Therefore, tools with slendernessratio of less than 20:1 are recommended.Often when the tools are of large cross-sectional area, the tool center faces slurrystarvation. Therefore, the tools with largerperimeter for same cross-sectional area arepreferred. The tools should be designed tohave flutes to aid slurry flow away fromthe cut. Sometimes the tools are relieved

Force

Rotation

Coolant in

Workpiece

λ/2

λ/2

λ/2

Transducer

Transformer

ba

Tool holder

Abrasiveslurry

Ultrasonic Machining, Fig. 1 (a) Ultrasonic machining (b) Rotary ultrasonic machining

Ultrasonic Machining, Table 1 Comparison betweenthe two variants of USM

Parameters USM RUM

Toolmotions

Reciprocation Reciprocation androtation

Tool-workinteraction

No contact Contact

Toolmaterials

Tough and ductilemetals, mild steel,stainless steel

Diamond tools

Tool shape Mirror image ofthe desired cavity

Core drill type tool

Abrasives Diamond, CBN,boron carbide,silicon carbide,aluminum oxide

Diamond particlesimpregnated in thetool

Carrierliquidfunctions

Carrying abrasivegrits and cooling

No abrasive gritshence only cooling

Forcetransfer

Tool vibrationsforce abrasive gritsto impinge onwork surface

Vibrating diamondtool contacts worksurface to causeremoval

Removalmechanism

Indentation andchipping

Indentation andabrasion

Shapesgenerated

Mirrors tool shape Holes and otheraxisymmetricshapes

Size offeaturesgenerated

About 90 mmdiameter and64 mm deep

Maximumdiameter38–50 mm

4292 Ultrasonic Machining

behind the face. In the case of longer tools,availability of center holes for feeding abra-sive slurry is of great advantage as it avoidssidewall friction. The tool materials should be

tough and ductile and should have high wearand fatigue resistance. The tool materials usu-ally include mild steel, stainless steel, brass,Monel, bearing steel, and molybdenum.Softer materials like aluminum or brass mayface significant tool wear.

5. Abrasives: the abrasives transfer the force ofvibrations to the work surface and impinge onthe surface to perform removal operation. Theabrasives material should be harder than thework material, and the size of the abrasivegrits should be the same as that of the ampli-tude of vibrations being used. Abrasives arenormally carried by slurry containing 50 %volume of abrasives and 50 % volume ofwater. The rate of penetration of tool in thework surface increases with increasing abra-sive concentration but reaches a maximum,beyond which the jamming of the abrasivestakes place, thereby the tool penetrationreduces. The abrasives used for the USM pro-cess include diamond, cubic boron nitride,boron carbide, silicon carbide, and aluminumoxide. Boron carbide is the most widely usedabrasive material. It is often used in theprocessing of tungsten carbide, ceramics,minerals, metals, and precious and semipre-cious stones. Silicon carbide is used forlow-density ceramics, glasses, Si, Ge, andmineral stones. Aluminum oxide is used formachining of glasses and sintered or hardpowder components.

Acousticsteaming

Abrasivegrain

Excitation

Workpiece

Crack

Ultrasonic Machining, Fig. 2 Schematic of crack for-mation due to impact in USM


Mechanism of Removal

Typical sequence of events that occurs in ultra-sonic machining includes (Fig. 2) [3]:

1. Indentation by abrasive particles leading togeneration of an inelastic deformation zonearound the particle.

2. At some threshold, deformation induced flowdevelops into a microcrack, termed as a“median” or “lateral” crack.

3. An increase in load further causes steadygrowth of the median crack.

4. During unloading action, the median crackbegins to close inducing formation of lateralvents.

5. Upon complete unloading, the lateral ventscontinue their extension towards specimen sur-face and lead to chipping or forming afragmented section on the work surface.

The critical load for the initiation of the mediancrack is given by [4]

Pc ¼ aK4

IC

H4v

(1)

where, a is a dimensionless factor. The size of themean crack (Cs) is given by [5]

U

Csð Þm ¼ K � P (2)

where m = 1–1.5 and k is a coefficient. Thisindicates that the size of the median crack growswith an increase in the load and a decrease in thefracture toughness of the work material.

A simple model to evaluate material removalrate (MRR) in ultrasonic machining, which givescontribution of direct impact of the abrasive par-ticles on the work surface in total MRR, wasdeveloped by Shaw [6]. The model is primarilybased on the assumption that the work volumeremoved is proportional to the number of particlesmaking impact per cycle and the volume of workmaterial removed per particle impact. The abra-sive grains are assumed to be identical and spher-ical in shape. The MRR therefore is given by [6]

Zw ¼ k � n � f dg � d� �3

2 (3)

where d is the depth of indentation, n is the num-ber of impacting particles per cycle, f frequency ofultrasonic vibrations, and k coefficient.

In the above equation, the depth of indentationis given by

d ¼ 8FsAdgpk1HvC 1þ qð Þ

� �12

(4)

where Fs is static force, A is amplitude of ultra-sonic vibration, k1 is the constant of proportional-ity, Hv is the hardness of workpiece,C concentration of slurry, and q ratio of the hard-ness of the workpiece to that of the tool.

Therefore, substituting Eq. 3 in Eq. 4, we get,MRR for ultrasonic machining as

Zw ¼ knf dg� �3

28FsAdg

pk1HvC 1þ qð Þ� �3

4

(5)

The Shaw’s model is the most widely usedbecause of its simplicity, despite its limitations.It predicts monotonous increase in the MRR withan increase in static force, Fs. However, in prac-tice, this is not so. After a certain increase in thestatic force, the abrasive particles tend to crack,thereby reducing the MRR.

Abrasivegrain

Ultrasonicvibrations

Hammering Abrasion Hammering,abrasion &extraction

Extraction

Rotationalmotion

Rotationalmotion

Tool vibrations

Ultrasonic Machining,Fig. 3 Removalmechanism in RUM


The above mechanism prevails mainly duringmachining of brittle work materials that are moresusceptible to fracture. However, when the workmaterials are ductile, such as metals, the abrasiveshave tendency to get embed in the surface. Theembedded particles work harden the surface, andupon successive impacts, chipping removal fromthe work surface occurs.

Thus, though USM can be used on both brittleand ductile work materials, it is more effective inthe case of former than the latter.

In the RUM, the removal mechanism involveslinear as well as rotational motion to the abrasivesthat are bonded to the tools, (Fig. 3). Therefore,the actions involved include hammering followedby abrasion and extraction [7].

Extensive work on the mechanism of removalshows that besides indentation, multiple mecha-nisms operate in USM process; these include [8]:

1. Mechanical abrasion by hammering of abra-sive particles

2. Microchipping by impact of free abrasives3. Cavitation effects from abrasive slurries4. Chemical action associated with the fluid

employed

Process Parameters

The theoretical as well as experimental investiga-tions on the process have shown that the MRR in

the ultrasonic machining is a function of a largenumber of parameters such as [9] (1) frequency,(2) amplitude, (3) static force, (4) hardness ratio(q) of the work to tool, (5) grain size, and (6) con-centration of abrasive slurry. A pictorial summaryof these effects is presented in Fig. 4a–f.

It is understood that the MRR increases with anincrease in the frequency and the amplitude of ultra-sonic vibrations (Fig. 4a). The MRR increases withan increase in the static force, but after a certainincrease in the static force (Fig. 4b), the abrasiveparticles tend to break thereby reducing the MRR.A decrease in the hardness of workpiece in the ratio(q) causes a rapid reduction in MRR (Fig. 4e). Thisindicates that the brittle work materials aremachined rapidly byUSM.A table showing relativeremoval rate of USM in machining of glass byconsidering it as 100 is presented in Table 2.

The MRR increases in proportion to theincrease in the grain size (Fig. 4c) until the grainsize becomes more than the amplitude of vibra-tions, that is, where the particles tend to fracture.

The MRR increases proportional to theone-fourth power of slurry concentration, i.e., C1/4

(Fig. 4d). However, the increasing trend continuestill concentration reaches 30 %, beyond which anincrease in the concentration does not help. TheMRR decreases significantly with a decrease in theviscosity of the carrier liquid (Fig. 4f).

The machined surface roughness using USMdepends upon grain size of abrasives [8].The smaller the grain size, lower the roughness.

M

R

R

Increasingfrequency

Increasing

Feed force F

MR

R

MR

R

Amplitude A

ba

c d

fe

Mean grain diameter d

MR

R

Actual

Theoretical

MR

R SiC

Abrasive concentration

B4C

MR

R

Work/tool hardness

MR

R

Viscosity (Poise)

Ultrasonic Machining,Fig. 4 (a–f) Variation ofMRR with processparameters in USM

Ultrasonic Machining, Table 2 Relative removal ratesin USM (For f = 16.3 kHz, A = 12.3 mm, mesh size ofabrasives = 100) [9]

Work material Relative removal rate

Glass 100.0

Brass 6.6

Tungsten 4.8

Titanium 4.0

Steel 3.9

Chrome steel 1.4


U

In addition, surface roughness is proportional to{hardness (H)/elastic modulus (E)}n as depictedin Fig. 5a. The work materials that have low H/Eratio are found to give lower surface roughness.

Again, in most of the cases, the RUM processis found to give better surface finish than the USM(Fig. 5a). Nevertheless, the values as low as0.4 mmRa for surface roughness are obtainableby using the process.

The holes made byUSM are oversized and haveform errors [8]. The oversize is maximum at theentry the of hole, and it increases as the length ofthe hole increases. Usually, the upper limit on theoversize is equal to the maximum size of the abra-sives used. The oversize and out of roundness arefound to reduce with increasing the static load, as itsuppresses the lateral vibrations of the tool. Again,the workmaterials with lower H/E ratio again showlower out of roundness (Fig. 5b).

Tool Wear

Tool wear is an important variable in USMthat affects both MRR and hole accuracy [8].

In general, the tool wear occurs along the length(WL) as well as along the diameter (WD) of thetool. It is understood that the tool wear is maxi-mum when the static load is maximum. Also, atthe maximum static load, the MRR is also maxi-mum. Therefore, the maximum static load may beconsidered as the optimum point for the tool wear.The tool wear tends to increase with an increase inthe hardness and size of the abrasives.WL is moreinfluenced by the product of hardness (H) andimpact strength (Ki) of the tool material. On theother hand, the WD is more influenced by thehardness (H) of the tool materials. Therefore,Nimonic 80A, thoriated tungsten, or silver steel

Sur

face

rou

ghne

ss (

µm)

H/E

21.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4

1.28

0.125

0.1

0.075

0.05

0.025

Out

-off-

roun

dnes

s (m

m)

2.36 3.87 8.1 13.25 (*10−4)

CarbideAluminaFerrite Glass Porcelain

Conventional USMRotary USM

Ultrasonic Machining, Fig. 5 Effect of H/E on (a) sur-face roughness for various materials under USM and RUM(b) out of roundness [8]


is highly recommended as tool materials. The toolwear also increases with an increase in the depthof hole drilled and machining time.

A number of times, fatigue failure of thetools is observed in the USM process. Therefore,the tool materials should have high fatiguestrength too.

Recent Trends

The recent developments in ultrasonic machininginclude use of various tools and techniques toimprove performance of the USM process andthe application of ultrasonic vibrations in variousother manufacturing processes to improve theirproductivity.

These developments include (a) use of auto-matic and intelligent machining control systems,(b) use of elliptical vibrations during ultrasonicmachining to improve MRR, (c) use of numericaltools to design the horn for efficient conversion oflongitudinal vibrations to mixed lateral and

longitudinal vibrations thereby maximizing theMRR, and (d) combining the ultrasonic vibrationswith other machining processes, for example, inelectric dischargemachining, where the vibrationshelp better debris disposal.

Summary

• USM is a nonthermal process and is the mostpreferred method for machining of hard andbrittle materials with hardness above 40 HRC.

• Of the two process variants, USM and RUM,the RUM gives better machining performancethan USM; however, the process application islimited to axisymmetric shapes.

• The removal mechanism in the processinvolves mainly indentation, impact, andchipping leading to generation of lateral crackand followed by the materials removal on brit-tle work surfaces.

• On the other hand, on ductile work surfaces,removal involves impact induced work hard-ening followed by chipping.

• In USM, the MRR increases mainly with anincrease in the amplitude, frequency, and thestatic force.

Cross-References

▶Ultraprecision Machining (UPM)

References

1. Tyrrell, W.R.: Ultrasonic Machining, Machining, ASMMetals Handbook, vol. 16. ASM International, MetalsPark (1989)

2. Drozda, T.: Tool and manufacturing engineers hand-book, Chapter 14. In: Randy, G. (ed.) UltrasonicMachining, vol. 1, 4th edn. SME, Michigan (1983)

3. Evans, A.G.: Fracture mechanics determinations. In:Bradt, R.C., et al. (eds.) Fracture Mechanics ofCeramics, vol. 1. Plenum, New York (1974)

4. Lawn, B.R., Evans, A.G.: A model for crack initiationin elastic/plastic indentation fields. J. Mater. Sci. 12,2195–2199 (1977)

5. Lawn, B.R., Evans, A.G., Marshall, D.B.: Elastic/Plas-tic indentation damage in ceramics: the median/radialcrack system. J. Am. Ceram. Soc. 63, 574 (1980)

http://dx.doi.org/10.1007/978-94-017-9780-1_365

Ultrasonic Vibration-Assisted Nanomachining 4297

6. Shaw, M.C.: Ultrasonic grinding. Microtechnic 10(6),257–265 (1956)

7. Pei, Z.J., Ferreira, P.M., Kapoor, S.G., Haselkorn, M.:Rotary ultrasonic machining for face milling ofceramics. Int. J. Mach. Tool Manufact. 35(7),1033–1046 (1995)

8. Thoe, T.B., Aspinwall, D.K., Wise, M.L.H.: Review onultrasonic machining. Int. J. Mach. Tool Manufact.38(4), 239–255 (1998)

9. Ghosh, A., Mallik, A.K.: Ultrasonic Machining (USM),Manufacturing Science. Affiliated East–west Press Pri-vate Limited, New Delhi (1985)

Ultrasonic Vibration-AssistedNanomachining

Jingyan DongEdward P. Fitts Department of Industrial andSystem Engineering, North Carolina StateUniversity, Raleigh, NC, USA

Synonyms

Ultrasonic force-assisted nanomachining; Ultra-sonic force-regulated nanomachining

U

Definition

Ultrasonic vibration-assisted nanomachining isa mechanical machining-based nanofabricationapproach using sharp cantilever tips, in whichcontrolled ultrasonic vibration between thetip and the sample is utilized to regulatethe dimension of the fabricated features andto improve the performance and speed ofnanolithography.

Introduction

Ultrasonic vibration-assisted nanomachining isone of the tip-based nanolithography techniquesthat enable high-rate tunable nanofabricationon polymeric and metallic substrates. The sharp

tip of the microcantilever provides capabilitiesfor low-cost fabrication of polymeric or metallicnanostructures in the sub-50 nm regime andhas potentials for fabricating 3D nanostructures.As a mechanical modification approach withan atomic force microscopy (AFM) tip, theultrasonic vibration-assisted nanomachiningovercomes many limitations from traditionalnanomachining approaches (e.g., nano-scratching and dynamic plowing lithography).One major disadvantage of traditional mechani-cal nanomachining is their low efficiency, sincethe fabricated feature width is mainly determinedby the AFM tip size. Multi-scratching steps haveto be taken to fabricate a large feature. Moreover,poor process controllability is the other disad-vantage of the traditional nanomachining. Innano-scratching, the sharp tip modifies the sam-ple surface by plastic deformation and has beensuccessfully used to produce sub-50 nm featureson many samples (e.g., polymers, silicon, alumi-num, etc.) [1–8]. A large normal force is gener-ally required to push the tip into the sample formechanical modification. This normal forcedepends heavily on properties of the samplematerials and the cantilever used. Control of fea-ture dimension in fabrication for different sam-ples and features is very difficult. In dynamicplowing lithography [9, 10], the cantilever tipoperating at the tapping mode is used in fabrica-tion. The capability to regulate the machiningbehavior is limited by the large tip-sample inter-action at the resonant frequency of the cantilever,since the cantilever oscillation is very sensitive tothe tip-sample gap and set-point force.

Ultrasonic vibration-assisted nanomachiningprovides high-rate tunable fabrication for poly-meric or metallic nano-features. The ultrasonictip-sample z-vibration can reliably regulatethe fabrication depth. A high-frequency in-planecircular tip-sample vibration controls the width ofthe fabricated features in one scan and significantlyimproves the speed of the nanolithography. Theultrasonic tip-sample vibration also reduces thenormal force and friction during nanomachiningand improves the issue of tip wear in traditionaltip-based nanomachining.

http://dx.doi.org/10.1007/978-94-017-9780-1_101159

http://dx.doi.org/10.1007/978-94-017-9780-1_101160

http://dx.doi.org/10.1007/978-94-017-9780-1_101160

4298 Ultrasonic Vibration-Assisted Nanomachining

Schematic of Ultrasonic Vibration-Assisted Nanomachining

In ultrasonic vibration-assisted nanomachining,controlled high-frequency vibration between thecantilever tip and the sample is generated toincrease the speed and controllability of thenanomachining process. The tip-sample vibrationincludes an ultrasonic z-vibration and in-planecircular vibration. The ultrasonic z-vibrationis utilized to regulate fabrication depth andreduce friction in nanomachining. The high-frequency in-plane circular vibration is used tocontrol the width of the fabricated features andto improve the speed of nanolithography. Theschematic illustration of ultrasonic vibration-assisted nanomachining is shown in Fig. 1.

Ultrasonic tip-sample vibration in z-directioneffectively increases the dynamic stiffness of thecantilever and improves the machining perfor-mance. Here the ultrasonic vibration frequencyneeds to be much larger than the resonant fre-quency of the cantilever. When the sample isbrought into contact with the cantilever andvibrated with a frequency f that is below the res-onant frequency fr of the cantilever (f < fr), thecantilever follows the sample vibration. When theultrasonic vibration is applied with its frequencymuch higher than the resonant frequency of thecantilever, the cantilever cannot follow the samplevibration due to its inertia, which makes the tipimmerse into the sample surface on average. Withthe vibration amplitude larger than the initial

Feed Top view

a b

Ultrasonic Vibration-Assisted Nanomachining,Fig. 1 (a) Schematic illustration of vibration-assistednanomachining. (b, c) The top view and side view showthe tip-sample engagement of the virtual tool (when the tip

deflection of the cantilever, in one vibration circle,the tip experiences a contact phase and a detachphase. During contact, the sharp tip is indentedinto the sample since the cantilever is dynamicallyfrozen at ultrasonic frequency. The ultrasonicforce, as an additional force from the tip-sampleinteraction under static condition, has been esti-mated in ultrasonic force microscopy (UFM)[11–14], which is the average of the repulsiveforce that depends on the surface elasticity andvibration amplitude, and is insensitive to the oscil-lation frequency and set-point force. Such ultra-sound excitation has been used in UltrasonicForce Microscopy (UFM) as an imaging techniqueto obtain elastic or subsurface images of the sample.In ultrasonic vibration-assisted nanomachining, theultrasonic tip-sample z-vibration is used to controlthe machining depth. For a chosen sample material,the depth of indentation mostly depends on thevibration amplitude and is insensitive to the oscil-lation frequency, which provides a reliable methodto control the machining depth.

In ultrasonic vibration-assisted nanomachining,besides the ultrasonic z-vibration, a high-frequencycircular tip-sample motion in the xy-plane is uti-lized to achieve better control of the fabricationprocess and to overcome many limitations fromdirect mechanical scratching, such as largetip-sample interaction and slow machining speed.The feature dimension, such as line width, can beregulated by changing the amplitude of the circularvibration/motion. As shown in Fig. 1b, the outeredge of the circular path can be viewed as the edge

Feed

Tip

θ r

Side view

f < fr

f >> fr

c

d

is vibrated with an in-plane circular path) with the sampleduring one rotation cycle. (d) At low frequency, cantileverfollows the sample vibration. At high frequency (f>>fr),cantilever begins to indent into the sample


U

of a virtual tool with a single cutting tooth, and theeffective cutting edge is the edge of the tip that istangent to the virtual tool at that moment. Thediameter of the virtual tool is controlled by theamplitude of vibration. This machining scheme issimilar to the conventional-scale single-toothfly-cutting, in which the rotation of the virtualtool is obtained by the circular motion of the tip.

By rotating the tip in xy-plane with high fre-quency, the tip-sample interaction can be distrib-uted to each rotation cycle. During each cycle,only a thin slice of material defined by feed perrotation is removed. Thus, tip-sample interactionforce can be significantly reduced and the machin-ing speed can be significantly increased. Giventhe feed per rotation f and the rotary speed of thetool N, the machining speed can be expressed asV = fN. Even with a small feed per rotation toreduce the tip-sample interaction in each cycle, ifa high rotary speed is used, a large machiningspeed can still be achieved. For example, assum-ing f is 10 nm and N is 10 kHz, the lithographyspeed can be as large as 100 mm/s, which is verydifficult to be obtained by other mechanical mod-ification approaches with an AFM. Moreover, bychoosing different feed per rotation, the machin-ing force during each rotational cycle can be con-trolled. The process has better controllability toimprove the overall performance (e.g., better fin-ish/topography uniformity) and to achieve moreefficient material removal.

The ultrasonic z-vibration and in-plane rotationfor ultrasonic vibration-assisted nanomachining isproduced by a high-speed xyz nanopositioner as anano-vibrator [15, 16], which is mounted onto theAFM scanner to provide the high-frequency-controlled vibration in xyz directions. The vibra-tion amplitude of the nano-vibrator in xy direc-tions ranges from sub-nm to a few micronsdepending on the command voltage to the piezoactuator, which provides good tunability for thenanomachining process.

The experiments of the ultrasonic vibration-assisted nanomachining were performed on a200 nm thick PMMA film, which is spin-coatedon a cleaned silicon substrate. A tapping modecantilever was used in the nanomachining exper-iment. Compared with direct scratching and

dynamic plowing, the ultrasonic vibration-assisted nanomachining process provides a fastermachining speed and greater material removalrate. In Zhang and Dong [15], straight line pat-terns are machined on the PMMA film underdifferent vibrational conditions to characterizethe capability of ultrasonic vibration in regulatingboth machining depths. After an xy-circular vibra-tion is excited with a frequency of 10 kHz andmachining speed of 20 mm/s, even with a smallset-point force of 100 nN, a noticeable cuttingdepth about 1 nm can be achieved, comparedwith direct scratching. With circular xy-vibration,tunable line width can be achieved in one machin-ing path by just changing the xy-vibration ampli-tude, which is controlled by the commandvoltages given to the xy-piezo actuators, asshown in Fig. 2. From the fabrication results inFig. 2a, clearly a large vibrational command pro-duces a wider trench feature, and a small vibrationsignal corresponds to a narrow trench. The linewidth ranges from 35 to 135 nm, as the vibrationalcommands increase from 15 to 45 mV. Since thecutting load is distributed to each small cuttingcycles, good surface finish is obtained; surfaceroughness in the trench along the machiningdirection is less than 1 nm. The machined trenchwidth follows a nearly linear relationship with thexy-piezo driving signal amplitude.

Set-point z-force has been widely used to con-trol the cutting depth in tip-based nanomachining.However, large z-force can bring large frictionand reduce tip life. It was found that ultrasonicz-vibration is able to control the fabrication depthand, at the same time, to reduce friction [15,16]. The effect of ultrasonic z-vibration is shownin Fig. 2b. The machining speed in the tests is setto be 20 mm/s. Without ultrasonic z-vibration,using a small set-point force of 100 nN, the can-tilever can barely make any modification on thesample. After the sample is vibrated at ultrasonicfrequency (2 MHz), noticeable trenches were fab-ricated on the sample surface. The depth of thetrench depends on the vibration amplitude inz-direction. The trench depth is directly controlla-ble by the vibration amplitude in the z-direction,as shown in Fig. 2b. With a larger command toz-piezo, a large trench depth is obtained. With or

0 mV 15 mV 30 mV 45 mV500 nm

35 nm 90 nm 135 nm

500 nm

Feature width

0 V 1 V 3 V 5 V

4 nm 9 nm 1 1 nm0.5 nmFeature depth

a bUltrasonic Vibration-AssistedNanomachining,Fig. 2 (a) Effect ofxy-circular vibration onnanomachining. Largerxy-vibration amplituderesults in large trench widthwith a nearly linearrelationship. (b) Effect ofultrasonic z-vibration onnanomachining.Z-vibration amplitude andtrench depth follow a nearlylinear relationship


without xy-circular vibration, the trench depthfollows an approximately linear relationship withthe z-vibration amplitude, which provides a reli-able method to regulate the cutting depth. Byusing larger z-vibration amplitude, thicker sliceof materials (as thick as 80 nm) can be machinedin one scan with the same machining speed.

Robust Material-Insensitive FeatureDepth Regulation by UltrasonicVibration

It was demonstrated by the nanomachining exper-iments on aluminum and PMMA in Zhanget al. [16] that when the set-point force is used tocontrol fabrication depth (i.e., without ultrasonicz-vibration), a much larger normal set-point forceis required to fabricate features with similar depthon aluminum, compared with the requiredset-point force for PMMA (a few micro-Newtonsfor aluminum vs. a few hundred nN for PMMA).These results are very easy to understand, sincealuminum is a much stiffer material than PMMA.However, when using ultrasonic z-vibration, withthe same small set-point force (200 nN in Fig. 3)and the same vibration amplitude, the ultrasonicvibration-assisted nanomachining producessimilar feature depth on both PMMA and alumi-num, even though these two materials have verydifferent mechanical properties. The fabricationdepth is predominantly controlled by the ampli-tude of the ultrasonic z-vibration and is insensitive

to substrate materials and other process condi-tions, which indicate the great potential of usingultrasonic z-vibration in regulating the featuredepth.

The depth regulation by ultrasonic vibrationcan be explained by the increased dynamic stiff-ness of the vibrating cantilever at the ultrasonicfrequencies. When the sample vibration fre-quency is much higher than the resonant fre-quency of the cantilever, the cantilever cannotfollow the sample vibration due to its inertia. Inother words, the cantilever is dynamically frozenat the ultrasonic frequency, due to its extremelylarge dynamic stiffness. For a rectangular beamcantilever, its static stiffness can be expressed byKs= 3EI/L3, where E is Young’s modulus, I is themoment of inertia, and L is the cantilever length.At an ultrasonic frequency, the effective dynamicstiffness of a cantilever can be expressed byKd = 3EI/b3l3, where l is the wavelength ofultrasonic flexural wave and b is the constant oforder unity. By making Ks = Kd at the resonantfrequency of the cantilever, the constant b can bederived, and the dynamic stiffness can beexpressed as Kd = Ks (L/0.298 l)3 = Ks (o/o0)

3/2,where o0 is the angular resonant frequency of thecantilever. As an example, for a tapping modecantilever with a static stiffness of 48 N/m andresonant frequency of 190 kHz, when it is excitedat the ultrasonic frequency of 3MHz, the effectivedynamic stiffness of 1,350 N/m can be obtained.Compared with the extremely large dynamic stiff-ness of the cantilever, the sample stiffness is

500nm0V 3V 6V 10V

6.3 nm 9.4 nm 13 nmFeature depth on Alunimum

Z-vibration signal

500nm

0.3 nm 5 nm 8 nm 11 nm

0V 3V 6V 10V

Feature depthon PMMA

Z-vibration signal200nN 300nN 350nN 400nN

500nm

0.2 nm 6.5 nm 10 nm 13 nmFeature depth on PMMA

Setpoint force

3 µN

500nm

3.5 µN 4.5 µN4 µN

6 nm 10 nm 12.2 nm3 nm

Setpoint force

Feature depth on Alunimum

a b

c d

Ultrasonic Vibration-AssistedNanomachining,Fig. 3 Comparison ofnanomachining with andwithout z-vibration onaluminum (a, b) andPMMA (c, d). (a, c) Featuredepth controlled byultrasonic z-vibration with asmall set-point force of200nN. (b, d) Feature depthcontrolled directly byset-point force


relatively much smaller for many materials,including aluminum and PMMA. Using Hertzianapproximation, the sample stiffness can be esti-mated as ks = (6RE*2F)1/3, where F is thetip-sample contact force, E* is the effective elas-ticity which can be expressed as

U

1

E� ¼1� n2tEt

þ 1� n2sEs

;

and Et, nt and Es, ns are Young’s modulus andPoisson’s ratio of the tip and sample, respectively.If we assume an elastic contact between the tipand sample surface and a 10 nN contact force, theeffective sample stiffness of PMMA and alumi-num is 24.5 and 153 N/m, respectively. Thedynamic stiffness of the cantilever at ultrasonicfrequency is 10 times larger than the sample stiff-ness of aluminum and more than 50 times largerthan PMMA. Moreover, the above sample stiff-ness is based on the full contact between the

cantilever and the tip. In ultrasonic vibration-assisted nanomachining scheme, since the vibra-tion in the z-direction (MHz) is much faster thanin the xy-plane (�10 KHz), the tip-sample contactin each z-vibration cycle is far less than the fullsphere-plane contact, which will further decreasethe effective sample stiffness.

Simply speaking, at ultrasonic frequency, thecantilever is much stiffer than the sample. Theultrasonic vibration, especially its vibrationamplitude, is not affected by the tip-sample inter-action. During vibration, the tip can easily indentinto relatively soft samples for mechanical modi-fication without much resistance. The tip-sampleengagement or cutting depth is mainly controlledby the vibration amplitude. Therefore, even at asmall set-point normal force, using the samevibration amplitude, ultrasonic vibration-assistednanomachining produces similar feature depthon PMMA and aluminum samples. The fabrica-tion depth is predominantly controlled by the


amplitude of tip-sample ultrasonic z-vibration andis insensitive to substrate materials.

Reduction of Friction and MachiningForce by Ultrasonic Vibration

After applying ultrasonic tip-sample vibration inthe z-direction, a significant reduction of the nor-mal force and lateral force (friction) was observedduring nanomachining [16]. To compare the effectof ultrasonic z-vibration on machining forces, aset of linear patterns were fabricated with depthcontrolled by the amplitude of the ultrasonicz-vibration and by the set-point z-force withoutvibration. Force data is compared for features withsimilar depth and width. The normal force andfriction were acquired by measuring the cantilevernormal deflection (A–B from the photodiodedetector) and torsion deflection (C–D from thephotodiode detector).

For the linear trench features machined onaluminum with depth controlled by ultrasonicz-vibration and by set-point z-force, the acquirednormal and lateral force signals were plottedin Fig. 4. When the feature depth is controlledby the set-point force (no ultrasonic z-vibration),large normal forces have to be applied to fabricatefeatures with the same depth. Ultrasonictip-sample vibration in vertical direction signifi-cantly reduces the normal and lateral forcein nanomachining, and the machining forceonly has a weak dependence on the fabricatedfeature depth. For ultrasonic vibration-assistednanomachining, for feature depth up to about12 nm, normal force and lateral force are kept ata very small value with normal force less than400 mV (compared with 1,400 to 2,300 mV with-out vibration) and lateral friction force less than30 mV (compared with 90 to 228 mV withoutvibration). For example, to fabricate a trenchwith a depth of 12 nm, 10 V z-vibration is requiredfor ultrasonic vibration-assisted nanomachining,or a set-point force of 4,500 nN has to be usedfor nanomachining without z-vibration. Whenusing the ultrasonic z-vibration, the normal forceis 400 mV, about 1/6 of 2,300 mV fornanomachining without ultrasonic z-vibration,

and the lateral force is 28 mV, only 1/8 of230 mV for nanomachining without ultrasonicz-vibration.

The decrease in machining forces by ultrasonicz-vibration can be attributed to the small normalforce required in nanomachining and intermittenttip-sample contact from ultrasonic z-vibration.The reduced normal force improves the applicabil-ity of ultrasonic vibration-assisted nanomachiningto a broad set of materials, especially for hardmaterials. For traditional nanomachining of hardmaterials, custom-designed cantilevers withextremely large stiffness have to be used to providethe required huge normal force. Furthermore, thereduced machining force by ultrasonic vibrationcan potentially reduce tip wear and elongate tiplife for the nanomachining process.

Ultrasonic Vibration-AssistedNanomachining for Nanolithography

Besides fabricating patterns directly on many sub-strates, ultrasonic vibration-assisted nanomachiningcan be used as a nanolithography process to fab-ricating features on polymer films and then trans-fer to other functional substrates. A typicalnanomachining-based nanolithography process[15] is illustrated in Fig. 5. A silicon substrateis evaporated with a thin aluminum layer andthen spin-coated with PMMA film. Then,nanomachining is applied to make features onthe PMMA film. To protect the cantilever tipfrom severe wearing and to elongate its lifetime,the machining depth is set to slightly less than thePMMA thickness to prevent the tip from directcontact with the aluminum layer. The residuePMMA can be removed by oxygen plasma asherto expose the underneath aluminum layer. Afterwet aluminum etching, the pattern is transferredfrom the PMMA to the aluminum layer. The RIEprocess can be further used to transfer the patternfrom the aluminum mask to silicon.

Using nanomachining-based nanolithographyprocess, features with width of 120 nm aremachined on a 35 nm thick PMMA, as shown inFig. 6. The depth of feature on the PMMA film isabout 25 nm. The roughness at the bottom of the

Depth controlled by ultrasonic z-vibration

Friction force

Depth controlled by setpoint force

Depth controlled by setpoint force

Normal force

Depth controlled by ultrasonic z-vibration

Ultrasonic Vibration-Assisted Nanomachining, Fig. 4 Comparison of normal force and friction for nanomachiningon aluminum with depth controlled by z-vibration and by set-point force

Si PMMA Aluminum

AFM Tip

O2 Plasma

CF4 RIE

a

c

e

gh

f

d

b

Ultrasonic Vibration-Assisted Nanomachining,Fig. 5 Fabrication sequences of nanomachining-basedlithography process. (a) Starting from the cleaned siliconsubstrate; (b) aluminum deposition; (c) PMMA spin-coating; (d) machining patterns on PMMA; (e) PMMA

etching in O2 plasma till the aluminum layer surface isexposed; (f) aluminum etching; (g) RIE etching of silicon;(h) aluminum striping and sample cleaning (Reprintedfrom Zhang and Dong [15] with permission)


U

slots is �1 nm. The remaining PMMA in thefeatures after nanomachining is removed in oxy-gen asher. After that, the sample is placed inaluminum etchant to transfer the slot patternsfrom the PMMA to the aluminum layer. Afteraluminum etching, the roughness at the bottomof slots is about 0.7 nm, indicating the exposedsilicon surface. After RIE etching and aluminummask striping, the AFM image shows a cut depthof�30 nm in silicon substrate as shown in Fig. 6fand h. The SEM image of the fabricated linepatterns is shown in Fig. 6h. The patterns can bereliably transferred from features that are machin-ing on PMMA film to silicon substrate.

Future Directions

The ultrasonic vibration-assisted nanomachininghas demonstrated promising potentials forimproving the speed and material removal rateof nanomachining, reducing the machining forceand tip wear, and increasing the controllability ofthe nanomachining processes. Future work aimedat modeling the nanomachining process topredict the feature produced and machiningforce at different process parameters will beimportant to make ultrasonic vibration-assistednanomachining a viable and reliable process fornanofabrication. The application of ultrasonic

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Ultrasonic Vibration-Assisted Nanomachining,Fig. 6 Images of results at different fabrication steps. (a)PMMA surface after nanomachining; (b) PMMA surfaceafter aluminum mask forming etching; (c, c0) silicon sub-strate surface after RIE etching; (d) topography at A–A0;

(e) topography at B–B0; (f) topography at C–C0. (g) Pat-terned image for letters “NCSU” after nanomachining andaluminum etching and (h) after RIE etching. (i) SEMimages of the slot features on silicon substrate (Figure isreprinted from Zhang and Dong [15] with permission)


vibration-assisted nanomachining in the fabrica-tion of 3D nanostructures can also be expected,which is actually a unique capability of mechan-ical machining-based nanofabrication processes.

References

1. Wendel, M., Lorenz, H., Kotthaus, J.P.: Sharpenedelectron beam deposited tips for high resolutionatomic force microscope lithography and imaging.Appl. Phys. Lett. 67, 3732–3734 (1995)

2. Yan, Y., Sun, T., Liang, Y., Dong, S.: Investigation onAFM-based micro/nano-CNC machining system. Int.J. Mach. Tool Manuf. 47, 1651–1659 (2007)

3. Göbel, H., Von Blanckenhagen, P.: Atomic forcemicroscope as a tool for metal surface modification.J. Vacuum Sci. Technol. B. 13, 1247–1251 (1995)

4. Li, X., Wang, X., Xiong, Q., Eklund, P.C.: Top-downstructure and device fabrication using in situnanomachining. Appl. Phys. Lett. 87, 233113 (2005)

5. Yan, Y., Hu, Z., Zhao, X., Sun, T., Dong, S., Li, X.:Top-down nanomechanical machining of three-dimensional nanostructures by atomic force micros-copy. Small 6, 724–728 (2010)

6. Hu, S., Altmeyer, S., Hamidi, A., Spangenberg, B.,Kurz, H.: Novel approach to atomic force lithography.J. Vacuum Sci. Technol. B. 16, 1983–1986 (1998)

7. Gozen, B.A., Ozdoganlar, O.B.: A rotating-tip-basedmechanical nano-manufacturing process. NanoscaleRes. Lett. 5, 1403–1407 (2010)

8. Schmid, S.R., Hector, L.G.: Simulation of asperityplowing in an atomic force microscope. PartII – plowing of aluminum alloys. Wear 215, 257–266(1998)

9. Heyde, M., Rademann, K., Cappella, B., Geuss, M.,Sturm, H., Spangenberg, T., Niehus, H.: Dynamicplowing nanolithography on polymethylmethacrylate

Upconversion Enhancement in Lanthanide-Doped Nanoparticles Using Nanoplasmonics 4305

using an atomic force microscope. Rev. Sci. Instrum.72, 136–141 (2001)

10. Klehn, B., Kunze, U.: Nanolithography with an atomicforce microscope by means of vector-scan controlleddynamic plowing. J. Appl. Phys. 85, 3897–3903(1999)

11. Kolosov, O., Yamanaka, K.: Nonlinear detection ofultrasonic vibrations in an atomic force microscope.Jpn. J. Appl. Phys. 32, 1095–1098 (1993)

12. Yamanaka, K., Ogiso, H., Kolosov, O.: Ultrasonicforce microscopy for nanometer resolution subsurfaceimaging. Appl. Phys. Lett. 64, 178–180 (1994)

13. Shekhawat, G.S., Dravid, V.P.: Nanoscale imaging ofburied structures via scanning near-field ultrasoundholography. Science 310, 89–92 (2005)

14. Cuberes, M.T., Briggs, G.A.D., Kolosov, O.:Nonlinear detection of ultrasonic vibration of AFMcantilevers in and out of contact with the sample.Nanotechnology 12, 53–59 (2001)

15. Zhang, L., Dong, J.: High-rate tunable ultrasonic forceregulated nanomachining lithography with an atomicforce microscope. Nanotechnology 23(8), 085303(2012)

16. Zhang, L., Dong, J., Cohen, P.H.: Material-insensitivefeature depth control and machining force reductionby ultrasonic vibration in AFM-based nanomachining.IEEE Trans. Nanotechnol. 12(5), 743–750 (2013)

Unconventional Computing

▶Molecular Computing

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Upconversion Enhancement inLanthanide-Doped NanoparticlesUsing Nanoplasmonics

Shadi Rohani, Marta Quintanilla,Rafik Naccache, Roberto Morandotti,Luca Razzari and Fiorenzo VetroneInstitut National de la RechercheScientifique – Énergie Matériaux etTélécommunications, Université du Québec,Varennes, QC, Canada

Synonyms

Anti-stokes luminescence; Emission enhance-ment; Nanoplasmonic enhancement

Definition

Upconversion is a process in which the sequen-tial absorption of two (or more) low-energy pho-tons leads to the emission of one photon withhigher energy. The upconversion process canoccur in materials such as lanthanide-dopednanoparticles and can be enhanced by the stronglocal electric field found in the proximity ofmetallic (plasmonic) nanostructures.

Overview

Nanomaterials have attracted considerable atten-tion due to their size-dependent properties,allowing many interesting and sometimes novelfeatures to be observed [1]. Of particular interestare luminescent nanoparticles as they have beentouted to find widespread integration in applica-tions ranging from the realization of displaydevices to bioimaging [2]. One particular classof luminescent nanomaterials that has garneredconsiderable interest is the lanthanide (Ln3+)-doped upconverting nanoparticles (UCNPs).These UCNPs are particularly attractive as theypossess the capability of converting low-energy(long l) light such as near infrared (NIR) intohigh-energy (short l) emission spanning the UV,visible, and even the NIR, in a process known asupconversion [3]. The ability to generate lightcovering the UV-NIR regions of the spectrumvia NIR excitation implies that this material canbe useful in optical applications such as the real-ization of lighting materials, anti-counterfeitingmeasures, displays, and imaging to name afew [4]. However, since upconversion is amultiphoton process, it suffers from low effi-ciency (or low quantum yield which is defined asthe ratio between emitted photons and absorbedphotons) [5] in comparison to other direct excita-tion approaches (i.e., standard fluorescence).Additionally, alternative depopulation routes ofthe excited states of the lanthanides result in lossof energy that otherwise could be used to promoteupconversion emission. Consequently, scientistshave looked at ways to enhance the upconversionemission via several routes including the

http://dx.doi.org/10.1007/978-94-017-9780-1_138

http://dx.doi.org/10.1007/978-94-017-9780-1_100037

http://dx.doi.org/10.1007/978-94-017-9780-1_100313

http://dx.doi.org/10.1007/978-94-017-9780-1_100313

http://dx.doi.org/10.1007/978-94-017-9780-1_100716

4306 Upconversion Enhancement in Lanthanide-Doped Nanoparticles Using Nanoplasmonics

development of novel low-energy phonon hosts,higher crystallinity materials, the preparation ofcore/shell architectures, where a shell passivatesthe luminescent core, and, finally, the exploitationof the synergistic effects that arise due to thecombination of plasmonic nanomaterials withUCNPs. The latter is the subject of this work andis discussed in detail below.

Upconverting NanoparticlesPrior to discussing the synergy between UCNPsand nanoplasmonic materials, we defineupconversion in greater detail. Upconversion is aphysical process that involves the conversion oflower-energy photons into higher-energy photonsusing the real (intermediate) states of an atom.This phenomenon has been observed in transitionmetals and actinides, but mainly in the lanthanideelements. In their trivalent state, Ln3+ ions havevalence electrons in an inner shell with the 4fn

electron configuration that is protected from theenvironment by outer energy levels (shells). Theabundant and unique energy level structuresremain, thus, largely unmodified by a change ofenvironment, and for this reason, they are widelyused as dopants in transparent materials to addnarrow and well-defined emission and absorptionbands in the UV, visible, and NIR ranges[6]. When the lanthanide ions are exposed to alight source with photons of energy equal to theenergy distance between two levels, i.e., theground and an excited state of the ion, the elec-trons can be promoted from the lower to thishigher-energy state. Absorption of another photonfrom the excitation light, and/or energy from elec-trons in neighbor ions, can result in the excitationto higher-energy levels. If the excited electronsthen release their energy and return to the groundstate, the result is the emission of photons featuredby shorter wavelengths (such as UVor visible).

Processes that also account for NIR to visiblelight transformation, such as two-photon absorp-tion, can be observed in other materials. However,it is noteworthy to mention that the excitation ofLn3+-doped UCNPs occurs via real electronicstates featured by defined (relatively long) life-times, in contrast to the case of two-photon

absorption. As a consequence, high-power,ultrafast, expensive laser setups are not requiredfor the efficient excitation of Ln3+-doped UCNPs,which can be achieved by using low-power, read-ily available solid-state laser diodes.

Upconversion is known to occur via three dis-tinct mechanisms, namely, excited state absorp-tion (ESA), energy transfer upconversion (ETU),and photon avalanche (PA). The latter is not com-monly observed in the cases covered by this textand will not be discussed below.

Excited State AbsorptionIn an ESA mechanism, electrons of the groundstate absorb one or more photons, sequentially,from the excitation source. When one electron inthe ground state absorbs one photon, it is excitedinto an intermediate state. Then, after sequentiallyabsorbing a second photon, it can be promoted toa higher excited state, and, upon returning to theground state, it can emit higher-energy photons(Fig. 1a). Although in the simplest case, theenergy levels are equidistant; ESA may beobserved in systems with non-equal energy levelsthrough phonon-mediated processes. Moreover,since this is a single ion process, it is generallyobserved in materials with low Ln3+ dopant con-centrations where the Ln3+-Ln3+ distances are toofar for any interactions between ions to occur.

Energy Transfer UpconversionEnergy transfer upconversion is known to be themost efficient upconversion process in Ln3+-doped nanomaterials [6]. It occurs between twodifferent ions with nearly equally spaced energylevels, with another requirement being the nearbylocalizations of the ions involved in the process(distance typically of the order of one nanometer)so that the transfer of energy can easily occur fromone ion to its neighbor. These two ions are nor-mally called the sensitizer (S) (or donor) and theactivator (A) (or acceptor). The sensitizer is an ionwith a high-absorption cross section and that canabsorb the photons from the excitation sourceefficiently, so electrons are promoted from theirground to an excited state. Subsequently, the sen-sitizer transfers this absorbed energy to the

UpconversionEnhancement inLanthanide-DopedNanoparticles UsingNanoplasmonics,Fig. 1 Schematic diagramof two commonupconversion mechanisms:(a) excited state absorptionand (b) energy transferupconversion


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electrons of the activator, allowing them (afterseveral steps) to reach higher-energy states, fromwhich the electrons will decay by emitting theupconverted luminescence (Fig. 1b).

In the energy transfer process between sensi-tizer and acceptor ions, two situations, namely,resonant non-radiative transfer and phonon-assisted non-radiative transfer, can occur. In reso-nant non-radiative transfer, the energy levels ofboth S and A are equally spaced, while, in thephonon-assisted non-radiative transfer, an energymismatch exists between the energy levels of thesensitizer and acceptor ions, so energy from thelattice (quantized in the form of phonons) is nec-essary to fulfill energy conservancy constrains.Figure 2 shows schematic diagrams of the ETUprocesses between two ions. In these images, weobserve resonant non-radiative transfer (Fig. 2a)and phonon-assisted non-radiative transfer(Fig. 2b). Theoretically, most lanthanide ions canundergo the NIR to visible upconversion process;however, relatively efficient upconversion is onlypossible with a few trivalent lanthanide ions (e.g.,Er3+ and Tm3+) under low pump densities. Todate, the Yb3+/Er3+ or Yb3+/Tm3+ dopant ion

combinations have been greatly employed inco-doped nanomaterials. This is due to the factthat their energy levels are reasonably matchedallowing for efficient energy transfer followingNIR excitation.

Interactions Between MetallicNanostructures and UpconvertingNanoparticles

As it was mentioned in the introduction,upconversion processes are intrinsically limitedin terms of quantum yield due to their nonlinearnature. As such, in a two-step process in whichtwo NIR photons are absorbed to emit one greenphoton, as it happens in the very commonupconversion of co-doped Er3+/Yb3+ ions, thequantum yield cannot be higher than 50 %. Inthe same way, blue emission through theupconversion of Tm3+/Yb3+, a three-step process,reduces the theoretical maximum quantum yieldto 33 %. Moreover, as there are additional depop-ulation routes besides upconversion that can affectevery energy state involved in the process, the

UpconversionEnhancement inLanthanide-DopedNanoparticles UsingNanoplasmonics,Fig. 2 Schematic diagramof a resonant (a) and aphonon-assisted (b)upconversion process


quantum yield is typically significantly lower thanthe predicted maximum. The list of alternativedepopulation processes includes radiative andnon-radiative transitions to lower-energy states,as well as additional energy transfer mechanisms,including the quenching triggered by moleculesadsorbed on the surface of nanoparticles, defects,etc. Altogether, the upconversion quantum yieldis normally below 10% and in nanoparticles oftenbelow 1 % (depending on the concentrations ofthe sensitizer and the activator, their nature, thehost material, the size of the nanoparticles, theexcitation power density, etc.) [5].

It has been demonstrated that the plasmonicproperties of metallic nanostructures can beapplied to enhance the emission probabilitiesof emitters (atoms, molecules, or quantum dots)in their close proximity [7]. With regard toupconversion, the fact that the resultant emissionsare due to multistep processes amplifies the pos-sibility for enhancement upon coupling to suitableplasmonic nanostructures. To fully describe theinteraction between metallic nanostructures andUCNPs, two different physical phenomena mustbe considered (Fig. 3). One is the influence of theplasmonic near-field, and the second is thenon-radiative energy transfer between the UCNPand the metallic nanostructures [8, 9]. Prior todelving into the precise nature of the interactions

of luminescent and plasmonic nanostructures, weintroduce the relevant concept of surface plasmonresonance.

Plasmonic Nanomaterials and SurfacePlasmon ResonanceIt has been long known that the properties ofmaterials change once they enter nanoscopicdimensions (in the 1–100 nm range). This can beas a result of confinement (such as limitationsimposed on the movement of electrons), or alter-natively of an increased surface-to-volume ratio.In fact, optical and magnetic properties are depen-dent on the electron motion characteristics of asystem, while, surface properties can gain impor-tance compared to volume properties in a nano-scale scenario [10]. In the case of metallicnanostructures, one of the best examples is relatedto the excitation of localized surface plasmons,resulting from the interaction between the freeelectrons of the metal and the optical electromag-netic field.

A metallic nanoparticle, for example, can bedescribed as a lattice of ionic cores with conduc-tion electrons moving almost freely inside thenanoparticle (the Fermi sea). When it is illumi-nated by a light source, the corresponding electro-magnetic field applies a force on the conductionelectrons at the nanoparticle surface. This results

Upconversion Enhancement in Lanthanide-DopedNanoparticles Using Nanoplasmonics, Fig. 3 Theinteraction between metallic nanostructures (a nanoparticlein this case) and upconverting nanoparticles (UCNPs) is

mainly driven by the plasmonic near-field generated by themetallic particle and by nonresonant energy transferbetween both. Additionally, the metallic nanostructurecan scatter light or release heat


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in a negative charge accumulated on one side ofthe nanoparticle, leaving the other side positivelycharged, thus creating an electric dipole. Thisdipole gives rise to an electric field with oppositedirection to that of the illuminating light and, thus,drives the electrons into returning to the equilib-rium position. The displaced electrons will natu-rally tend to oscillate with a certain frequency thatis known as surface plasmon frequency. If thefrequency of the excitation light matches the fre-quency of the nanoparticle surface plasmon, aresonant effect occurs (called localized surfaceplasmon resonance – localized SPR), whichinvolves the collective oscillations of the electronsin the nanoparticles. This in turn causes anincrease in the kinetic and electrostatic energiesassociated with the electric fields of the dipolewhich is supplemented by the exciting radiation[10]. As a result, the incident light is partiallyextinguished when exciting SPRs.

Noble metal nanostructures have the ability toconfine the radiation well beyond the diffractionlimit. This phenomenon happens when the inci-dent light is at the SPR of the nanostructures. TheSPR is strongly affected by the particle shape andsize. This is clearly exemplified by gold nanorods(GNRs). In this case, since rods are geometricallyanisotropic, the electron oscillation can occurdifferently along the rod’s two main axes,

depending on the polarization of the incidentlight. The charge accumulations will be differentfor the electrons oscillating along the rod longaxis (longitudinal plasmons) and those movingalong the perpendicular direction (transversalplasmons) (Fig. 4a). Accordingly, GNRs showtwo surface plasmon resonances, one strong lon-gitudinal SPR (called LSPR) which is usuallylocated in the NIR region and one transversesurface plasmon resonance (TSPR), which liesin the visible range (Fig. 4b). The LSPR of GNRsstrongly depends on the aspect ratio (length/width) of the GNRs, that is, the higher the aspectratio of the rods, the greater is the red shift asso-ciated to the LSPR, and vice versa (Fig. 4c).The strong LSPR of GNRs in the NIR regionoffers the possibility to use excitation lightof larger wavelength, which is preferable forbiological applications as these wavelengths areless harmful to the biological tissues. This con-trasts to other geometries such as nanosphereswhere only one SPR is observed and its loca-tion can vary depending on the particle radius(Fig. 4d).

Effect of the Plasmonic Near-Fieldon UpconversionThe relaxation rate of each excited state of alanthanide ion, incorporated as a dopant in a

Upconversion Enhancement in Lanthanide-DopedNanoparticles Using Nanoplasmonics, Fig. 4 (a)Schematic of electron accumulation on the surface of aGNR (reproduced with permission from [10]), (b) theabsorption spectra for GNRs showing the TSPR and

LSPR, (c) the absorption spectra for GNRs with differentaspect ratios from 1 to 5 (reproduced with permission from[10]), and (d) the absorption spectra for sphericalnanoparticles with different diameters from 10 to 100 nm(reproduced from [11])


certain host, is characterized by a radiative emis-sion probability (when energy is released in theform of light, i.e., photons), Wrad, and anon-radiative emission probability (when energyis released in the form of heat, i.e., phonons),Wnr.Typically, their sum is inversely related to theexperimental lifetime of the excited state, texp.However, we note that a third term can be addedwhen energy transfer is possible from this state. Inthis case, texp can be defined as:

1

texp¼ Wrad þWnr þWET (1)

where WET is the probability of energy transfer.In general, upconversion processes benefit fromlong lifetimes in the intermediate states and large

radiative emission probabilities in the final emit-ting state. Accordingly, these are the parametersto consider in order to improve the upconversionefficiency. Normally, these values depend onboth the lanthanide and the host (via the crystalfield, the phononic modes, and the presenceof other lanthanides). Thus, in order to modifythese parameters, a change of the host structure isoften necessary. The so-called Purcell’seffect states that the radiative properties of anemitter are dependent on its environment and,therefore, offers an alternative to optimize theupconversion. In the presence of a metallic nano-structure, the plasmonic interaction inducesan increase in the density of states which,according to Fermi’s golden rule, will conse-quently increase the radiative transition rate ofthe ions [7].


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Equation 1 considers the different depopula-tion routes of an energy state. Naturally, absorp-tion must take place prior to depopulation.Therefore, its control constitutes an additionaloption for improving the quantum yield, as theplasmonic electromagnetic field (which isconnected with an inhomogeneous redistributionof the photon energy density in the nanostructuresurroundings) clearly affects this importantparameter.

In order to promote absorption enhancement,the plasmon must be resonant with one (or more)among the absorption lines of the lanthanides,which can then absorb its energy. In upconversionprocesses, the emission intensity, I, is stronglydependent on the excitation power density, P,due to the nonlinearity of this excitation scheme.This dependence follows a power function, I / Pn

, where n is a positive value related to the numberof excitation photons needed to complete theupconversion process, i.e., the number ofupconversion steps [12]. Thus, it is clear that thehigh-photon energy densities achieved in theplasmonic field can strongly affect the intensityemitted by the UCNPs, through the enhancementof the absorption process. In this case, both theabsorption processes originating from an excitedstate (e.g., ESA), as well as stimulated-type emis-sions, should be considered toward an optimizedprocess and this in addition to the standard absorp-tion from the ground state [7, 8].

The different ways in which the plasmonicfield can affect all the steps in an upconversionprocess make the final enhancement larger than ina standard fluorescent particle [13]. In that regard,one further difference between upconversion andstandard fluorescence is that, in upconversion,absorption and emission wavelengths are wellseparated, while this is not normally the case instandard fluorescence. This offers an additionalroute to engineer the upconversion emission,since the anisotropy of certain metallicnanostructures, allowing for more than one SPR,can be used to have enhancement both at theexcitation and at the emission wavelength of theUCNPs [14]. The overall enhancement is notdescribed by a general rule, as it depends oneach particular upconversion process through

parameters such as donor-acceptor distances,nature of the process (ETU vs ESA), and thenumber of upconversion steps.

The Non-Radiative Energy TransferWhen an excited UCNP is in the close environ-ment of a metallic nanostructure, there is a certainprobability of having a non-radiative energy trans-fer from the UCNP to the metallic counterpart,resulting in some energy loss and, as a conse-quence, quenching of the emission. In suchcases, the distance between the emitter or sensi-tizer (the UCNP, S) and the acceptor (the metallicnanostructure, A), RSA, is a critical parameter indefining the process probability. Note that UCNPsare by definition nanoparticles of transparent crys-tals with lanthanides distributed as dopants in thelattice. Consequently, the characteristics of theparticle play a role in determining RSA, which isthus not described by a single value, but rather bya probability distribution that should consider thedimensions, morphology and dopants concentra-tion of the nanoparticle.

The non-radiative energy transfer can happenthrough three different processes: exchange(overlap of the electronic wave functions), multi-polar electric, and multipolar magnetic interac-tions. Due to the nature of the first process, Ln3+

ions must be very close to the metallic nanostruc-ture (few Å) for it to happen, and thus, (although itcan be indeed important at short distances) it isoften negligible in real experimental settings.Magnetic interaction is also relevant at veryshort distances, but only when exchange and elec-tric interaction are otherwise forbidden by therelevant selection rules. Consequently, it isthe multipolar electric interaction, defined by theCoulomb law, which more often accounts forenergy transfer [15].

In many cases, the electric interaction can bedescribed via a dipole-dipole approximation wherethe Förster formalism can be applied [16]. Whenthis approximation is valid, the probability of thenon-radiative energy transfer is related to RSA

through the relation WET ¼ CSA=R6SA , where CSA

is known as the energy transfer microparameter.This relationship clearly shows the strength ofthe dependence of the degree of quenching


on the interparticle distance, so that the processhas a particular relevance when the surface ofthe UCNP is in contact with the metallicnanostructures (in which case, additional interac-tions and multipolar electric terms might berelevant).

However, RSA is not the only quantity thataffects the probability of the process, since themicroparameter CSA contains a dependency onthe refractive index of the environment as wellas on the emission and absorption energies ofsensitizer and activator. Following Förster’smodel, CSA can be defined as

CSA ¼ 3ℏ4c4

4pn4QA

t0

ðLS Eð Þ � LA Eð Þ

E4dE (2)

where the subindex S refers again to the sensitizer,i.e., the ion in the UCNP that provides energy inthe transfer process and A refers to the activator,i.e., the receptor of the energy, which in the pre-sent case is the metallic nanostructure. Accord-ingly, LS and LA are the normalized line shapes ofthe sensitizer’s emission and the activator’sabsorption, respectively, so the integral representsthe overlap between both transitions. QA is theintegrated absorption cross section of the activa-tor, ћ is the Planck constant, c is the speed of lightin vacuum, n is the refractive index of themedium, and, finally, t0 is the radiative lifetimeof the sensitizer in the absence of the activator.Looking at this definition, it becomes clear that fora metallic nanostructure with a high-absorptioncross section and in resonance with at least oneemission line of the lanthanides, the quenchingrelated to the energy transfer can become veryrelevant.

Controlling Enhancement in a Real System:The Key ParametersThe physical phenomena explained thus far arethe main reasons that account for the modificationof the upconversion intensity. Although the emis-sion enhancement of the UCNPs in close proxim-ity to the plasmonic nanostructures is now wellestablished, studies on this matter have reported awide range of enhancement factors or evenquenching in some cases. A comparison between

different approaches and efforts generated to dateshows that upconversion emission enhancementcan be strongly dependent several features thatcan affect any of the two main physical phenom-ena, plasmon mediated enhancement and energytransfer, behind the metallic nanostructure-UCNPinteraction. Here, we describe a few of these fea-tures to illustrate which parameters are morerelevant.

Impact of the Nanostructure ArchitecturePlasmon-induced electromagnetic fields arestrongly dependent on the shape, size, and com-position of the metallic nanostructure. Thus, wideranges of luminescence enhancement factors werefound in previous works due to the interplay ofUCNPs and metallic nanostructures in differentconfigurations. For example, regarding the choiceof material, it has been shown that silvernanoparticles induce a higher emission enhance-ment in comparison to gold nanoparticles with thesame size, since the former possesses strongerplasmonic resonances [17]. On the other hand,for the same composition but different morphol-ogy, recent theoretical studies show that prolate-shaped metallic nanoparticles can lead to muchhigher enhancements than the correspondingspherical nanoparticles (Fig. 5). It was demon-strated by the work of Mertens and Polman thatthe emission enhancement in the proximity of aprolate geometry can be up to two times higher incomparison to the case in which the nanoparticleis a sphere with the same volume [18]. This sig-nificant improvement is due to two effects. Firstly,the coupling between the UCNP emitter and theplasmon mode is more effective in the prolategeometry due to the higher shape-induced fieldenhancement near the sharp tips [18]. Secondly,the drop in quantum efficiency close to theprolate-shaped nanoparticle surface sets in atsmaller separations. For example, the distance tothe metal surface at which the quantum efficiencyreaches half the maximum value is 2.2 nm for aspherical nanoparticle and only 0.8 nm for itsprolate-shaped counterpart. Quenching due to cou-pling with higher-order plasmon modes is thusless effective for prolate-shaped nanoparticles[18]. In a different work by Mertens et al. [19],

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Upconversion Enhancement in Lanthanide-DopedNanoparticles Using Nanoplasmonics, Fig. 5 Lumi-nescence quantum efficiency of a dipole emitter as a func-tion of its position in a plane through the nanoparticle’smajor axis for (a) a 60-nm-diameter Ag sphere and (b) anAg prolate spheroid-shaped nanoparticle with an aspectratio of 2.5 and a volume that is equal to the volume ofthe sphere. The orientation of the source dipole is set asparallel to the z-axis, and the refractive index of the embed-ding medium is 1.5. The emission wavelengths thus

correspond to the maximum radiative decay rate enhance-ment: 500 nm for the sphere and 794 nm for the prolate-shaped nanoparticle. (c) Line traces of the luminescencequantum efficiency along the dashed lines indicated in (a)and (b), taking into account coupling to either all multipolemodes up to l = 100 (solid lines) or to the dipole modeonly (dashed lines) (Reproduced with permission fromMertens et al. [18]. DOI 10.1063/1.3078108. Copyright[2009], AIP publishing LLC)


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the composition and shape of the metal nanopar-ticle are kept constant to address another possiblecontribution: the effect of the size of sphericalplasmonic nanoparticles. It was proven that thereis an optimal diameter in the range of 30–110 nm

(depending on the material properties) for whichmaximum enhancement takes place, defined bythe competition between two different mecha-nisms: emitter-plasmon coupling, which is moreeffective for small spheres, and coupling of


plasmons into radiation, which is more efficientfor large spheres [19].

Emission/Absorption Enhancementin UpconversionAs described earlier, at wavelengths resonant withthe metallic nanostructure plasmon frequency, theinteraction between the incident light and themetal nanostructure results in local electric fieldstrengths that are much higher than the incidentelectric field. This field enhancement is the mainreason for the luminescence enhancement inUCNPs. However, it should be noted that theplasmon resonance of the metallic nanostructurecould be designed to overlap with either theabsorption wavelength of the UCNPs (in theNIR region), with the emission wavelength ofthe UCNPs (visible region), or both. To date,most works have focused on the resonant emis-sion enhancement. For example, in the work bySaboktakin et al. [17], metal nanoparticles of goldor silver were fabricated to match the plasmonresonance with the emission wavelength of theUCNPs (the SPR of gold nanospheres was at534 nm, which is in resonance with the greenemission of Er3+ ions centered at 545 nm), andthe enhancement was found to be 5.2-fold inproximity of Au nanospheres. However, the useof nanostructures such as GNRs that can possesstwo surface plasmons resonant with both theabsorption and emission wavelength is expectedto give rise to even higher enhancement values.GNRs can be tailored to have their longitudinalSPR overlapping with the excitation wavelengthof the UCNPs and their transverse SPRoverlapping with the upconversion emission ofUCNPs. In such structures, the overall enhance-ment mechanism can be a combination of reso-nant enhancement in both absorption andemission, which can lead to an overall increaseof the total enhancement.

Effect of the Excitation Pump PowerAs previously discussed, upconversion efficiencyitself is a function of the excitation power density,I / Pn , and, thus, the enhancement factors fromthe same sample could vary widely depending onthe excitation power. In this equation, n is

presumed as the number of photons needed toexcite the emitting state, also known as the orderof the upconversion process. For example, for atwo-photon upconversion process, the value ofn should be close or equal to 2. However, it istheoretically proven that this relation strictlyrefers to the case of small upconversion rates atlow excitation powers, and an efficientupconversion system will exhibit an intensity-versus-power dependence with lower n [12].

Due to the characteristics of the upconversionemission, a dependence of the plasmonicenhancement on the excitation power becomesintuitive. Its effect on the different contributionsto upconversion enhancement (absorption, emis-sion, and Förster energy transfer between Ln3+

ions) has been studied in the experimental workby Lu et al. [20]. In this work, a silver nanogratingcoated with a spacer of Si3N4 and featured by aplasmon resonance at the excitation wavelengthof the UCNPs was probed at power densitiesbetween 1 and 100 kW/cm2. It was found thatthe described general behavior of the upconvertedluminescence was preserved in the presence of thesurface plasmon, meaning that the upconvertedluminescence intensity depends quadratically onthe excitation power for a weak excitation, andlinearly in the strong excitation limit. Yet, theintensity-versus-power dependency curves wereshifted to lower-power densities, revealing anenhanced local electromagnetic energy densitydue to the surface plasmons. Additionally,through a numerical analysis based on rate equa-tions, it was concluded that the enhancement ofthe upconversion luminescence results from anincrease in absorption and energy transfer rates.The absorption enhancement was found to play adominant role at high-power densities, while bothabsorption and energy transfer enhancement con-tributed in the weak excitation regime (here, theabsorption enhancement was described by Max-well’s equations and energy transfer by Förster’stheory) [20].

Impact of the UCNP-Metallic NanostructureInterparticle DistanceAs described earlier, the local field enhancementcan lead to an increased upconversion emission,

Upconversion Enhancement in Lanthanide-DopedNanoparticles Using Nanoplasmonics,Fig. 6 Upconversion luminescence enhancement of twotypes of UCNPs (NaYF4:Yb

3+, Er3+ and NaYF4:Yb3+,

Tm3+) in close proximity of Au (a) and Ag (b) sphericalnanoparticles with a diameter of 5 nm as a function of thespacer thickness between the two nanostructures. The

green solid lines correspond to the 540 nm emission bandof NaYF4:Yb

3+, Er3+. The green dashed lines correspondto the 650 nm emission band of NaYF4:Yb

3+, Er3+. Theblue solid lines correspond to the 475 nm emission band ofNaYF4:Yb

3+, Tm3+ (Reproduced with permission fromSaboktakin et al. [17])


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which may impose the idea that the closer theUCNPs are to the plasmonic nanostructures,the better is the enhancement. However, thenon-radiative energy transfer from the UCNPs tothe plasmonic nanostructures can happen in veryclose proximity, leading to a decrease of the lumi-nescence quantum yield (quenching). The combi-nation of the plasmonic field enhancement of theupconversion intensity and the non-radiativeenergy transfer from UCNPs to plasmonicnanostructures is such that the luminescenceintensity can be either enhanced or quencheddepending on their relative distance [14]. Conse-quently, this results in the existence of an optimaldistance (dopt) for luminescence enhancement.Anger et al. investigated, both theoretically andexperimentally, the effect of a gold nanoparticleon a fluorescent molecule [9]. They illustrated thata simple change in the relative distance can lead toa luminescence emission widely ranging fromenhancement to quenching, thus proving the exis-tence of a well-defined dopt. In addition, theyshowed that dopt depends on the size of theplasmonic nanoparticle, where, for largernanospheres, dopt was found to be smaller. How-ever, there are reports of either fluorescence

enhancement or fluorescence quenching of theupconversion in the presence of metallic struc-tures. While this can appear somehow confusing,it can be readily explained by the lack of controlover the interparticle distance in the experimentaldesign. In recent years, the study of the effect ofinterparticle distance on the luminescenceenhancement has been extended to upconversion.For instance, in an experimental work bySaboktakin et al., the effect of the distancebetween UCNPs and two different types of metal-lic nanospheres (gold and silver) with 5 nm diam-eters was studied. Preliminary series of controlexperiments in this work showed that, in a randomconfiguration (without a spacer between the twotypes of materials), the luminescence intensitywas greatly reduced. However, by using ananometer-scale oxide layer as the spacer betweenthe nanostructures, the existence of an optimumdistance was revealed and found to be dependenton the metallic nanosphere composition. For goldnanospheres, dopt was equal to 5 nm, while, forsilver nanospheres with the same diameter, doptwas 10 nm (Fig. 6). This could be explainedas the result of a higher per volume extinctioncross section of silver in comparison to gold.

4316 Uptake/Internalization/Sequestration/Biodistribution

Note that the extinction coefficient in metallicnanostructures arises both from absorption andscattering processes; although following Mie’stheory, it is dominated by absorption for smalldiameter nanoparticles (5 nm in this case)[17]. Therefore, the higher absorption cross sec-tion of Ag nanoparticles generates a strongernon-radiative energy transfer (Eq. 2) followed bya stronger quenching effect. Accordingly, a largeroptimal distance for emission enhancement isobserved in this case.

Cross-References

▶Gold Nanorods▶Local Surface Plasmon Resonance (LSPR)▶LSPR in Plasmonic Nanostructures: Theoreti-cal Study with Application to Sensor Design

▶Metal Nanoparticles from First Principles▶ Plasmon Resonance Energy TransferNanospectroscopy

▶ Plasmonic Structures for Solar EnergyHarvesting

▶ Synthesis of Gold Nanoparticles

References

1. Cao, G.: Nanostructures and Nanomaterials: Synthe-sis, Properties and Applications, p. 581. Imperial Col-lege Press, London (2004)

2. Chen, X., Liu, Y., Tu, D.: Lanthanide-Doped Lumi-nescent Nanomaterials: From Fundamentals toBioapplications (Nanomedicine and Nanotoxicology).Springer, Berlin/Heidelberg (2014)

3. Auzel, F.: Upconversion and anti-stokes processeswith f and d ions in solids. Chem. Rev. 104, 139 (2004)

4. Liu, Y., Ai, K., Lu, L.: Designing lanthanide-dopednanocrystals with both up- and down-conversionluminescence for anti-counterfeiting. Nanoscale 3,4804 (2011)

5. Boyer, J.-C., van Veggel, F.C.: Absolute quantumyield measurements of colloidal NaYF4:Er

3+/Yb3+

upconverting nanoparticles. Nanoscale 2, 1417 (2010)6. Liu, G., Jacquier, B. (eds.): Spectroscopic Properties

of Rare Earths in Optical Materials. Springer,Tsinghua (2005)

7. Shvets, G., Tsukerman, I. (eds.): Plasmonics andPlasmonic Metamaterials. Analysis and Applications,vol. 4. World Scientific, Singapore (2012)

8. Fisher, S., Hallermann, F., Eichellkraut, T., Plessen, G.V., Kramer, K.W., Biner, D., Steinkemper, H., Hermle,M., Goldschmidth, J.C.: Plasmon enhancedupconversion luminescence near gold nanoparticles-simulation and analysis of the interactions. Opt.Express 20, 271 (2012)

9. Anger, P., Bharadwaj, P., Novotny, L.: Enhancementand quenching of single-molecule fluorescence. Phys.Rev. Lett. 96, 113002 (2006)

10. Garcia, M.A.: Surface plasmons in metallicnanoparticles: fundamentals and applications.J. Phys. D Appl. Phys. 44, 283001 (2011)

11. http://nanocomposix.com/12. Pollnau, M., Gamelin, D.R., L€uthi, S.R., G€udel, H.U.:

Power dependence of upconversion luminescence inlanthanide and transition-metal-ion systems. Phys.Rev. B: Condens. Matter 61, 3337 (2000)

13. Esteban, R., Laroche, M., Greffet, J.J.: Influence ofmetallic nanoparticles on upconversion processes.J. Appl. Phys. 105, 033107 (2009)

14. Rohani, S.: Coupling of gold nanorods with lantha-nide-doped upconverting nanoparticles for lumines-cence enhancement and nanothermometry. Master’sThesis, INRS-EMT (2015)

15. Dexter, D.L.: A theory of sensitized luminescence insolids. J. Chem. Phys. 21, 836 (1953)

16. Forster, T.: Zwischenmolekulare energiewanderungund fluoreszenz. Ann. Phys. 437, 55 (1948)

17. Saboktakin, M., Ye, X., Oh, S.J., Hong, S.-H.,Fafarman, A.T., Chettiar, U.K., Engheta, N.,Murray, C.B., Kagan, C.R.: Metal-enhancedupconversion luminescence tunable through metalnanoparticle-nanophosphor separation. ACS Nano 6,8758 (2012)

18. Mertens, H., Polman, A.: Strong luminescence quan-tum efficiency enhancement near prolate metalnanoparticles: dipolar versus higher-order modes.J. Appl. Phys. 105, 044302 (2009)

19. Mertens, H., Koenderink, A.F., Polman, A.: Plasmon-enhanced luminescence near noble-metalnanospheres: comparison of exact theory and animproved Gersten and Nitzan model. Phys. Rev.B 76, 115123 (2007)

20. Lu, D., Cho, S.K., Ahn, S., Brun, L., Summers, C.J.,Park, W.: Plasmon enhancement mechanism for theupconversion processes in NaYF4:Yb(3+), Er(3+)nanoparticles: Maxwell versus Forster. ACS Nano 8,7780 (2014)

Uptake/Internalization/Sequestration/Biodistribution

▶Exposure and Toxicity of Metal and OxideNanoparticles to Earthworms

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Use of Nanotechnology in Pregnancy 4317

Use of Nanotechnology in Pregnancy

Achini K. VidanapathiranaAustralian Research Council (ARC) Centre forNanoscale Biophotonics, University of Adelaide,Adelaide, SA, AustraliaHeart Health, South Australian Health andMedical Research Institute, Adelaide, SA,Australia

Synonyms

Chemotherapeutic; Contraception; Drug delivery;Embryotoxic; Fetus; Nanoparticles; Nanoscaletechnologies; Nanotechnology; Nanotoxicity;Placenta; Pregnancy; Teratogenic

Definition

This entry examines the applications of nanotech-nology during pregnancy (current, research, andpotential applications), the unique pregnancy-related characteristics that relate in the applicationof nanoscale materials, and the concerns on mater-nal/fetal well-being.

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Overview

Nanotechnology is being progressivelyresearched, and nanoscale products aremanufactured for various engineering devices,consumer products, diagnostic, and therapeuticapplications to be utilized during differenthuman life stages including pregnancy. Pregnancyis a life stage combined with the unique physio-logical changes and characteristic remodeling ofmaternal tissues supporting the growth and devel-opment of the fetus(es). As a result of these adap-tive mechanisms to support fetal growth, the drugmetabolism, particle distribution, and eliminationkinetics during pregnancy are significantly differ-ent from both males and nonpregnant females.The gestational transformations are primarily

dependent on hormonal changes and are initiatedin the preimplantation stage, progressing to itsmaximum in the third trimester. These physiolog-ical changes evolve into the lactation phase andare partially reversed postpartum. Such alter-ations, when occurring at the molecular levelin conjunction with the changes in tissue perme-ability, become significant particularly whendesigning, developing, and applying nanoscaletechnologies during pregnancy. Three main enti-ties, the mother, the fetus, and the placenta alongwith their physical/functional connections andinterdependency, are the key considerations fornanotechnology use during pregnancy.

The development and application of nanotech-nology becomes crucial during the pregnancyconditions along two different aspects. The posi-tive aspect is that the products incorporating nano-scale technologies can be used from a biomedicalperspective and also in consumer nano-productsduring all stages of pregnancy. The negativeaspect and implications are the reported and pos-tulated detrimental effects of nanoparticle expo-sure during this vulnerable, transforming lifestage.

Constructive Applications of Nanotechnologyin PregnancyThe properties of nanoparticles make them resem-ble biological molecules and are modifiable, mak-ing nanotechnology utilizable during the veryearly stages of pregnancy. Commercially avail-able home pregnancy test is one example wherenanoparticles are currently in use to confirm ges-tation during early pregnancy. These diagnosticconcepts can be further extended to nanoelectro-mechanical systems (lab-on-a-chip devices) fordetailed diagnostics during the different stages ofpregnancy in both the mother and fetus. Theseinclude early detection of fetal congenital abnor-malities by analyzing maternal serum, amnioticfluid, and cord blood. The relatively higher sensi-tivity of the nanoscale analyzing techniques canbe very useful in early pregnancy where minutevolumes (nano- to microscale) of biological mate-rials and tissues are involved. Nanoscale technol-ogies are considered for utilizing in contraception

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4318 Use of Nanotechnology in Pregnancy

by incorporating various binding, transporting,release, and strength-related properties. Researchis currently ongoing for designing intrauterinecontraceptive devices (IUCD) containingnanoparticles to replace the conventional copperIUCD [1]. The applicability of nanotechnologyfor further strengthening the barrier contraceptivedevices is also currently under investigation.

The ability to functionalize nanoparticles andtailor them to support individual tissue-specificneeds makes it a very versatile technology espe-cially in the mid- and later gestational stages. Incircumstances such as in severe preeclampsia andcancer treatment, the benefits of maternal drug/nanoparticle administration outweigh the risks ofminute fetal exposure to nanomaterials [2]. Thesenanoparticulate drug delivery systems offer apotential avenue to deliver diagnostic, therapeu-tic, or theranostic agents to tissues that are notdirectly in contact with the fetus and can be sep-arated by the placental barrier properties. Oneexample that is currently under investigation isthe treatment of ectopic placental tissue or placen-tal tumors by targeted nanoparticle delivery ofchemotherapeutic agents such as doxorubicin. Inthese studies, EnGeneIC Delivery Vehicles(EDVs) which are nanocells that can promotetissue-specific delivery of drugs were loadedwith the chemotherapeutic doxorubicin. Thesenanocells were targeted to the epidermal growthfactor receptor (EGFR) that is very highlyexpressed on the placental surface and wasreported to be able to regress placental cellsin vitro, ex vivo, and in vivo [3].

Potential Negative Consequences of UsingNanotechnology in PregnancyThe occupational, therapeutic, and general envi-ronmental exposure to nanoparticles during thepreconceptional, gestational, and lactation periodsmay impose unknown risks affecting the healthyprogression and outcome of pregnancies. Consid-ering the currently available distribution profilesof different nanoparticles, these adverse effectscan be manifested as alterations in pulmonary,cardiovascular, hepatic, renal, and immunologicalfunctions of the maternal side. The emerging

evidence on selective placental transfer ofnanoparticles raises concerns on their utility inobstetrics and on the possible harmful effects tofetal development. Similarly, detrimental effectsof nanoparticle exposure as postulated with ani-mal studies can influence conception, implanta-tion, placentation, embryogenesis, organogenesis,growth, and development in human pregnancies.These toxicity concerns and their poorly under-stood mechanisms are controversial and may attimes contribute to decelerate the application ofnanotechnology during reproductive life stages.

Key Research Findings

Relatively less number of studies is reported inliterature on the application of nanoparticles,nanoparticle distribution, and toxicity duringpregnancy; hence, literature reviews on nanotech-nology and nanotoxicity relevant to gestationalstages are scarce. Almost all the in vivo studiesare carried out in animal models including zebrafish, mice, and rats.

Distribution of Nanoparticles DuringPregnancy and Placental TransferResearch and the in-depth understanding of thedistribution kinetics of nanoparticles are critical assuch knowledge is essential in both therapeuticsand toxico-pharmacology. Several studies havefocused on the distribution of different types ofnanoparticles during pregnancy. The studies onintravenously administered nonfunctionalizedradiolabeled C60 fullerenes formulated inpolyvinylpyrrolidone (PVP) have reported distri-bution (24 and 48 h post-administration) in bothmaternal and fetal tissues in pregnant rats whenadministered on the fifteenth day of gestation.Radioactivity (~6 %) was also distributed to thereproductive tract, placenta, and fetuses of the preg-nant rats. The relative tissue distribution patternsvaried between the pregnant and lactating femalerats that may be dependent on the physiologicalchanges related to gestation, permeability differ-ences, and structural properties of various maternaltissues. While the majority of the C60 were


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distributed to the maternal liver, there was a limiteddistribution to the maternal lungs during preg-nancy. In contrast, this study reported a higherdistribution in the lungs in the lactating stage [4].

Relatively larger and longer carbon-basednanoparticles such as multiwalled carbonnanotubes (MWCNT) also cross the placentalbarrier as revealed by a study using isotopictracers in mice. Oxidized MWCNT had crossedthe placental barrier and were detectable in thefetal tissues [5]. Similarly, in a study where ratswere periconceptionally exposed with gavages ofzinc oxide nanoparticles, the distribution of zincto mammary tissue in the pregnant rats and theliver and kidney of the pups was reported[6]. Metallic silver nanoparticles, when gavagedto pregnant and lactating rats, were also detectedin the fetal tissues and breast milk [7].

Placenta is the key organ undergoingremodeling throughout the pregnancy, contribut-ing to the systemic changes in pregnancy whilefacilitating nutrient, antibody, and other materialparticle transfers between maternal and fetal tis-sues. It also performs an important barrier func-tion protecting the fetus from the maternal insults.The nanoparticle transfer across the placenta andthe permeability characteristics are emerging asresearch foci in the field of reproductivenanomedicine and nanotoxicology [8]. Studies inpregnant mice exposed to different functionalizedgold nanoparticles have reported placental andfetal accumulation of maternally administerednanomaterials suggesting the placental transferof these materials. This particle transfer acrossthe placenta is influenced by both stage of embry-onic or placental maturation and nanoparticle sur-face composition [9]. The initial in vivo andex vivo studies demonstrate that the ability of thenanoparticles to cross the placental barrier andpassage to the fetus is dependent on the size andthe surface coating of nanoparticles as well as onthe experimental model [10].

The abovementioned studies were done in ani-mal models, and the basic structural differencesand deviations from the human placental functionhave to be considered in interpreting these find-ings as relevant to human exposures. Placenta is a

species-specific organ, and the speculation ofthese findings particularly in relation to particleswith less than 100 nm size range needs to be donewith caution as the differences at cellular/molec-ular level may have a significant influence onthese inferences. Studying the distribution ofnanoparticles in the human placenta is an ethicalchallenge, and the only available research toolsare the ex vivo perfusion of the expelled placentaand in vitro assessments of placental cells [8].Previously reported studies in the human placentahave not observed any transfer of goldnanoparticles to the fetal side of the placentalcirculation [11]. Another study which used theex vivo human placental perfusion model to inves-tigate whether nanoparticles can cross the placen-tal barrier has demonstrated that fluorescentlylabeled polystyrene beads with diameters up240 nm were taken up by the placenta and wereable to cross the placental barrier without affect-ing the viability of the placental explants [12].

Nanotoxicological Studies During PregnancyMost of the researches on pregnancy and nano-technology have focused on the adverse effects ofoccupational, environmental, and medical-relatednanoparticle exposure during pregnancy. Theavailable literature on toxicological studies onpregnancy have focused primarily on pulmonary,intravenous, and oral exposure to differentnanomaterials including silver, gold, silica, iron,cobalt, titanium, cerium oxide, carbon-basednanoparticles, and quantum dots. These studieshave been done in different animal models invarying doses (usually higher doses), sizes, andformulations of nanoparticles. These in vivo expo-sures in mice have resulted in adverse effects inboth fetal and maternal components. Increasedacute cellular inflammation, inhibition of maternalweight gain, and changes in pregnancy hormoneswere reported in pregnant dams [5, 11]. The fetaloutcomes include reduced body weights of pups,changes in gene expression related to develop-ment or function of central nervous system inmale pups, increased susceptibility to allergy inpups, reduced number of Sertoli cells, andalterations in epididymal sperm motility in pups.


C60, quantum dots, silica, and titaniumnanoparticles are reported to be associated withembryological abnormalities, placental abnormal-ities, and early pregnancy loss [11, 13].

The changes observed in these studies in thefetal weight and the placental structure weredependent on the type, size, and surface chemistryon the nanoparticles [13]. The oxidized MWCNTwhich cross the placenta induced gestational ageand parity-dependent abortion rates and alsoresulted in narrowed blood vessels and decreasedthe number of blood vessels in the placenta asassessed by histology [5]. Quantum dots arepotential nanoparticles for imaging which can beused in pregnancy-related diagnostic, imaging,and monitoring progression of diseases/treat-ments. Studies in rats exposed with CdSe/ZnS orCdTe quantum dots on the 6th, 13th, and 18thdays of embryogenesis did not report any directembryotoxic or teratogenic effects. However, theyreported adverse effects on the maternal organismas they induced necrosis in the tissues of theperitoneal cavity. In the same study, spectroscopicand microscopic tissue examination revealed theaccumulation of quantum dots in the placenta inthe absence of penetration to the embryonictissues [14].

Gold nanoparticles are potential drug carriersthat can be coated or formulated differently tomake them biocompatible. In a study with intra-venously administered gold nanoparticles withdifferent surface modifications (ferritin, PEG,and citrate) at 5.5–15.5 days of gestation inmice, it was demonstrated that prior to gestationday 11.5, all tested nanoparticles could be visual-ized and detected in fetal tissues in significantamounts. Subsequently, fetal gold levels declineddramatically after gestation day 11.5. In contrast,gold nanoparticle accumulation in the extraembryonic tissues increased 6–15-fold with ges-tational age. Fetal and extra embryonic accumu-lation of ferritin- and PEG-modified goldnanoparticles was considerably greater thancitrate-capped nanoparticles [9]. These and otherstudies on placental and fetal transfer ofnanoparticles suggest that fetal exposure tonanoparticles and their toxicity in pregnancy is

influenced by both stage of embryonic/placentalmaturation (gestational stage) and physicochemi-cal properties of nanoparticle.

The changes in the fetal growth andabnormities induced by nanoparticle exposureare speculated to be due to several mechanisms.The first mechanism is explained by the placentaltransfer of specific nanomaterials and theirdirected effects on the fetal tissues. The interrup-tions of the placental and uterine blood supply andthe contribution of the maternal cardiovascularsystem during pregnancy in mediating the effectsof nanoparticles are combined as the secondmechanism and are emerging as a significantarea in research. The changes in maternal andfetal microvascular function and the contributionof these have been studied with exposure to tita-nium nanoparticles [15]. In addition, studies usingC60 and multiwalled carbon nanotubes havereported vascular bed, suspension medium, androute of exposure-dependent changes in the vas-cular tissue responses and fetal growth duringpregnancy [16, 17]. The inflammatory responsesin the maternal side (generally following a pulmo-nary exposure) are another mechanism that cancontribute to both maternal and fetal responsesfollowing gestational exposure.

Future Directions for Research

Pregnancy and nanotechnology in combinationinitiate a wide spectrum of research with infinitepossibilities of applications that can be studied.These range from nano-engineering, basic molec-ular mechanisms, preclinical studies in animalmodels or in vitro systems, clinical trials, andtoxicological (teratogenic and embryotoxic)assessments.

Interactions of the Nanoparticles with thePlacentaThe main obstacle in applying nanotechnologyduring pregnancy is the lack of proper understand-ing on the precise nanoparticle interactions withthe placenta. Limited studies focusing on thistopic suggest a considerable uncertainty and


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ambiguity regarding the transplacental depositionand passage of nanoparticles [2]. Our understand-ing on the criteria that determine placental trans-ferability of nanoparticles is inadequate.Designing different nanoscale particles andmanipulating their functional properties canpotentially control the placental function as a bar-rier, filter, or a target to nanoparticles, providedthe properties of placenta that contribute to thesefunctions are properly understood. These are thefirst-line focus areas for future research in thisfield. In order to adequately understandnanomaterial-biokinetics and placental toxicityin early gestation (particularly the first trimesterwhere teratogenic effects are critical), the specialarchitecture of the placenta, the hypoxic condi-tion, the bilayer of villous trophoblast, the plug-ging of spiral arteries, the contribution ofintrauterine glands to nutrition, and the delicateimmunologic situation at the implantation siteneed to be considered [18]. The physicochemicalproperties, functionalizing potential, targetingmechanisms, and radiolabeling ability ofnanoparticles can be used as a research tool tostudy placental transport mechanism in furtherstudies.

Almost all the current reported studies onnanotechnology in pregnancy are in animalmodels, and the speculations of these results tohuman populations are limited by the species-dependent differences in the placental structure,gestational periods, litter size, and particle distri-bution. Descriptive, retrospective studies onexposed human populations and further ex vivoplacental perfusion studies on expelled humanplacentae can be utilized to fill this knowledgegap. The emerging knowledge and applicationsof nanoscale biophotonics expand the possibili-ties of applying these fine imaging techniques tostudy the human placental molecular mecha-nisms that govern the placental functions includ-ing their derangements in conditions such aspreeclampsia.

Utilizing Nanotechnology During PregnancyOne of the significant and ambitious researchquestions is whether these nanoparticles can be

used as modes of drug delivery during pregnancy.This can be in the form of targeted drug delivery tospecific maternal tissues while minimizing thefetal adverse effects during pregnancy. Theexisting knowledge on targeted drug delivery forcancer treatment can be extended and applied tothe pregnant state to selectively deliver drugs tothe placenta and the fetus. These technologies canbe then utilized to diagnose and treat congenitalconditions during the intrauterine life. Intrauterinegene therapy and immunomodulation are fewexamples of the potential applications that couldbe used during pregnancy. Alternatively, theseapplications can be studied and applied in treatingmaternal disease conditions such as progressivecancers, diabetes mellitus, coagulopathies, trans-plant rejection, preeclampsia, and immunologicaldisorders (e.g., asthma, rheumatoid arthritis, sys-temic lupus erythematosus) during pregnancy.These applications include imaging, diagnostics,monitoring, and theranostics for maternal diseasetreatments without affecting the fetal growth[8]. Minimizing fetal side effects using the perme-ability and transfer properties of the placenta andengineering the nanoparticles to suit these barriermechanisms will lay the foundations for suchapplications.

Further areas of research for pregnancy-relatednanotechnology include the management ofsubfertility and diagnosis of preeclampsia usingserum or placental markers. Novel fine nanoscalesensing mechanisms such as nanoscalebiophotonics can be developed for proceduresand functions that need nanoscale sensitivity.These include sensors that can be applied insubfertility and in planned pregnancies. Assistedreproductive procedures such as artificial insemi-nation, in vitro fertilization, and implantationwould benefit from such devices.

Refined Nanotoxicological StudiesThe current nanotoxicological studies are limitedlargely to nonfunctionalized, higher doses ofnanoparticles and are aimed at more mechanisticapproaches. The actual formulations ofnanoparticles that can be utilized in nanomedicineare different and involve tissue-specific

Use of Nanotechnology in Pregnancy, Fig. 1 Thefuture of nanotechnology use during pregnancy. Preg-nancy consists of three critical entities (the mother, fetus,and placenta) and is a unique stage where nanotechnologycan be used for various applications. The fine balancebetween these constructive applications (in green boxes)

and potential maternal or fetal adverse effects (in orangeboxes) needs to be considered with caution before incor-porating nanotechnology in pregnancy (CV cardiovascular,DM diabetes mellitus, IU intrauterine, NP nanoparticles,RES reticuloendothelial system)


functionalization in realistic doses and formula-tions. In addition, the adverse effects ofnanoparticles during the first trimester are rela-tively understudied, but critical [18] and shouldbe a main research focus in the future. This alsoincludes the prepregnancy ovum production(starting as early as the intrauterine stage of thefemale), puberty, fertilization, and implantationstages when the molecular mechanisms can bedisrupted or manipulated by using nanotechnol-ogy. The contribution of nanoparticles on miscar-riages, intrauterine growth restriction, pretermdeliveries, and preeclampsia also need to be fur-ther studied. These conditions are reported withother toxins and inflammatory conditions. Such

adverse consequences need to be excluded beforenanotechnology can be applied to the pregnantstatus. Limited/narrow-scale research and anec-dotal evidence on pregnancy-related complica-tions are reported in pregnant mothers workingin nanoparticle production facilities. These needto be further confirmed with extensive research onexposed populations.

The unique properties of nanoparticles that aretechnically constructive such as high surface-to-volume ratio, protein corona, and its evolutionwithin the biological systems also increase theirpotential for toxicity. These properties make itdifficult to assess the risk of nanomedical or nano-technological products using the current risk


assessment methods particularly during preg-nancy in which the maternal, fetal, and placentalcomponents are functioning in a complex envi-ronment. Pregnancy may be one scenario wherethe nanotechnology itself can be applied to assessthe toxicity of different nanomaterials and otherdrugs.

Nanoscale technologies are very versatile withmany advantageous applications during preg-nancy if used with caution. The fine balancebetween the type, dose, frequency, formulationof nanoparticles, and stage of gestation with therelevant physiological changes will guide to theeffective use of nanotechnology in pregnancy(Fig. 1). The effect of nanoparticle exposure onpregnancy stages need to be addressed beforereleasing any nano-combined diagnostic, thera-peutic, or theranostic applications. As with anyother cosmetic, consumer, or pharmacologicalproduct, teratologic risk assessment is essentialwith nanomedical applications. These includeengineering, mechanistic, preclinical, clinical,and post-marketing monitoring strategies thatneed to be meticulously carried out prior to apply-ing nanotechnology during pregnancy.

U

Cross-References

▶Chemotherapeutic▶Contraception▶Drug Delivery▶Embryotoxic▶ Fetus▶Nanoparticles▶Nanoscale Technologies▶Nanotechnology▶Nanotoxicity▶ Placenta▶ Pregnancy▶Teratogenic

References

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