eld scanning optical microscopy nanoprobes

Nanotechnol Rev 1 (2012): 313–338 © 2012 by Walter de Gruyter • Berlin • Boston. DOI 10.1515/ntrev-2012-0027

Near-fi eld scanning optical microscopy nanoprobes

Monika Fleischer

Institute for Applied Physics and Center for Light-Matter Interaction , Sensors and Analytics (LISA + ), Eberhard Karls Universit ä t T ü bingen, Auf der Morgenstelle 10, 72076 T ü bingen , Germany , e-mail: monika.fl [email protected]

Abstract

Near-fi eld scanning optical microscopy (NSOM) is a pow-erful method for the optical imaging of surfaces with a resolution down to the nanometer scale. By focusing an external electromagnetic fi eld to the subwavelength aper-ture or apex of a sharp tip, the diffraction limit is avoided and a near-fi eld spot with a size on the order of the aper-ture or tip diameter can be created. This point light source is used for scanning a sample surface and recording the signal emitted from the small surface area that interacts with the near fi eld of the probe. In tip-enhanced Raman spectroscopy, such a tip confi guration can be used as well to record a full spectrum at each image point, from which chemically specifi c spectral images of the surface can be extracted. In either case, the contrast and resolution of the images depend critically on the properties of the NSOM probe used in the experiment. In this review, an overview of eligible tip properties and different approaches for tai-loring specifi cally engineered NSOM probes is given from a fabrication point of view.

Keywords: nanofabrication; nano-optics; near-fi eld scanning optical microscopy; scanning probe microscopy; tip-enhanced Raman spectroscopy.

1. Overview

Near-fi eld scanning optical microscopy (NSOM) using visi-ble light was fi rst demonstrated in the 1980s. With this novel optical microscopy method, optical resolution of a frac-tion of the applied excitation wavelength can be achieved, where the resolution mostly depends on the tip dimensions and tip-sample distance. The method is based on an illumi-nated probe tip that is scanned across a sample surface with a distance of a few nanometers. The distance is feedback controlled. The force feedback is recorded together with the optical signal from the interaction volume near the probe tip. This way, the surface topography is imaged simultane-ously with the optical properties of the surface, leading to complementary information, which can be subsequently

correlated [1] . In the case of tip-enhanced Raman spectros-copy (TERS) measurements, where a spectrum is recorded at each sample position, chemical mapping of the surface is performed, which can be analyzed for several constituents in parallel by subsequent fi ltering for characteristic Raman lines. The topographic and optical resolution depends on the sharpness and shape of the probe tip. The image contrast and signal-to-noise ratio depend on tip properties such as the material, intensity of the electric near-fi eld near the tip, scattering by the bulk of the probe, effi ciency of the cou-pling, plasmon resonance frequency, polarization, etc. For this reason, processes are required for the high-yield fabri-cation of high-quality NSOM probes with reproducible geo-metric parameters and optical properties. Two basic types of probes can be distinguished. With aperture probes, light is introduced to or collected from the surface (or both) through a small aperture at the tip apex, whereas apertureless probes are used in an external illumination and collection confi gura-tion. Aperture probes have been created by covering dielec-tric probes with a thin metal layer, leaving a small opening at the tip. For high-resolution apertureless NSOM, metal wire tips and metalized fi bers are commonly used. Over the last decade or two, a number of groups started engineer-ing well-defi ned NSOM nanoprobes consisting of a single plasmonic nanostructure positioned at a probe tip. These probes typically address specifi c challenges for high-quality image formation. They are optimized for ultrahigh resolu-tion, high-intensity imaging, particular polarizations, and/or specifi c resonance wavelengths. In this review, essential design considerations are summarized. Main strategies that have been applied for the fabrication of NSOM nanoprobes to date are presented. The respective advantages and limita-tions of the different concepts are discussed in view of the properties of the fabricated nanostructures.

2. Introduction

In the age of nanotechnology, gaining images and information on the nanoscale is a vital part of science, research, and appli-cation development. The quest for understanding matter and processes on ever-smaller length scales has led to the devel-opment of a variety of technologically advanced techniques that are used for obtaining complementary information on the nanoscale.

Among the most commonly used methods are scanning electron microscopy (SEM) or scanning ion microscopy (SIM) and transmission electron microscopy (TEM) [2] . In SEM/SIM, focused beams of high-energy electrons or ions are used for imaging surfaces, for example by means of

314 M. Fleischer: Near-fi eld scanning optical microscopy nanoprobes

secondary or backscattered electrons, with a resolution down to the nanometer scale and some material contrast. In TEM, high-energy electrons are detected after transmission through ultrathin layers of material. They can give information on the crystal orientation of the material and the morphology down to the atomic scale. In the quest for chemical information on the microscale and nanoscale, micro-Raman or energy-dispersive X-ray spectroscopy (EDX) with a resolution of about 1 μ m [3, 4] or X-ray photoelectron emission micro-scopy (X-PEEM) with a resolution down to 10 nm may be applied [5] . X-PEEM is a high-resolution technique yielding chemical information on surface composition and bonding structures, which, however, may require synchrotron radia-tion for analytical purposes.

Optical imaging techniques have been continually refi ned to fi nd methods that beat the diffraction limit of light. This has led from confocal imaging, where the detected light is passed through a pinhole to enhance contrast, to techniques such as stimulated emission depletion (STED) and photo-activated localization microscopy (PALM) or stochastic opti-cal reconstruction microscopy (STORM), which allow for gaining fl uorescence information with subdiffraction limited resolution [6] .

In scanning probe microscopy (SPM), a sharp tip is raster scanned across a surface to gain information on the nanoscale via the tip-sample interaction. A whole variety of SPM meth-ods has been developed based on different types of interaction [7 – 9] . Atomic force microscopy (AFM) uses the force or dis-tance feedback of a cantilever that is defl ected by following the surface to gain high-resolution information about topo-graphy, local friction, and, to a degree, surface chemistry. As the chemical information is indirectly deduced from the force interaction via model calculations, it is an intricate method prone to artifacts. Topographic imaging may achieve a single-nanometer resolution, depending on the sharpness of the can-tilever tip. In analogy, scanning tunneling microscopy uses a sharp conducting cantilever tip on conducting substrates while measuring the tunneling current. This way, surfaces may be imaged down to atomic resolution.

NSOM is another member of the SPM family. In NSOM, the surface is locally optically excited, and the optical signal from the small interaction volume between the tip and the surface is monitored. NSOM thus enables optical imaging on the subwavelength scale, below the resolution limit of far-fi eld optics [1] . Optically active probes may also be applied to gain spectral, and thus chemical, information from the surface through TERS. TERS unites the advantages of micro-Raman (direct chemical information) and X-PEEM (high-resolution chemical mapping). At the same time, it incorporates the advantages of AFM because it is controlled via force feed-back, such that atomic force interactions and topographical and spectral information are gathered simultaneously.

The possibility of near-fi eld imaging was predicted by E.H. Synge in 1928 [10] , initially applied for microwave and infrared studies (cf., [11, 12] ), and fi rst realized for visible light in 1984 [13, 14] . An overview of the history of NSOM can be found in [15] . Two general approaches for the scan-ning probe design exist. The fi rst experiments were realized

with aperture probes. Here the sample is illuminated through a nanoaperture at the tip apex, which is illuminated from the back and the signal is detected in the far fi eld, or the sam-ple is illuminated in far-fi eld illumination and the signal is collected through an aperture at the tip [16] , or illumination and collection take place through the probe [17] . The inten-sity in this case is limited by the throughput of the aperture. As visi ble radiation can penetrate to a fi nite depth into the metal, the virtual aperture diameter is increased, such that the potential resolution is limited to several tens of nanometers [18] . The resolution can be increased using apertureless scan-ning probes. In apertureless NSOM, a metallic tip acts as an optical antenna for incident light. Collective oscillations of the free electron density (plasmons) are excited at the metal-dielectric interface of the tip, which are accompanied by an evanescent electric near-fi eld with high fi eld intensity near the tip apex [1, 19 – 22] . For extended metal structures, the surface plasmon polaritons (SPPs) travel along the surface. For nanostructures with dimensions on the order of 100 nm, they are confi ned to the structure as localized surface plas-mon polaritons (LSPPs). These usually exhibit a well-defi ned plasmon resonance frequency that depends on the size, shape, material, and dielectric environment of the nanostructure. The near-fi eld concentrated at the tip is used as a scanning probe, locally probing light-matter interaction at a surface. The reso-lution of the optical imaging roughly corresponds to the tip diameter. The solid probes are externally illuminated from the side, through an objective from below, or via a parabolic mirror [23 – 25] . The light is scattered by the probe, and the signal is detected in the far-fi eld. Using this technique, resolu-tions down to few nanometers have been demonstrated (e.g., [26 – 29] ). Different NSOM implementations are discussed in [30 – 35] .

TERS imaging is a special type of NSOM, and a pow-erful tool for chemical analysis on the nanoscale [12, 28, 36 – 41] . In TERS, the strongly localized evanescent near-fi eld at the tip of the scanning probe interacts with the top layer of a sample, across which the plasmonic probe is scanned. At each tip position/image pixel, a spectrum is recorded. Inelastic scattering processes lead to a unique spectral fi ngerprint of the chemical composition within the subdiffraction-limited region below the tip. Simultaneous mappings of the topography and the chemical composition of the surface can thus be obtained. The observed Raman intensity is strongly enhanced by the near-fi eld interaction. The signal enhancement can reach the fourth power of the local electric fi eld enhancement due to the antenna effect of the tip. By fi ltering for characteristic lines, chemical imag-ing maps can be displayed with sub-100-nm resolution. In [29] for example, carbon nanotubes were imaged by TERS with 10- to 20-nm resolution. As multiple element informa-tion is contained within the spectra, the elemental or com-pound distribution of several different constituents can be extracted from a single scan. For material science, chemical science, and engineering purposes, the modifi cations of the surface resulting from interactions or surface treatment can be monitored, and the interface can thus be optimized by surface engineering.

M. Fleischer: Near-fi eld scanning optical microscopy nanoprobes 315

The ultrahigh-resolution NSOM and TERS techniques have been used in research laboratories for some time now but have not yet found broad distribution for industrial applications. One crucial point in this context is the role of the scanning probe. The quality of imaging depends very sensitively on the qual-ity of the scanning probe. The sharpness of the tip determines the spatial resolution of both force and optical measurements. The shape of the metallic tip determines the polarization and resonance wavelength of the electric near-fi eld. Large near-fi eld enhancement is expected when the tip is resonant with the external illuminating fi eld. Therefore, control of the tip resonance is desirable. Contrast, which relates the total signal measured when the tip is in contact to the signal measured when the tip is withdrawn, is the key parameter controlling one ’ s ability to image with the tip and depends on tip charac-teristics such as roughness, shape, radius, and material under the plasmonic structure [42] . The shaft behind the tip may lead to background. The overall probe design is essential for the signal-to-noise ratio, the feedback loop distance control, and the ease of implementation. Also, scanning probes need to be replaced on a regular basis and therefore should be manufac-turable with high yield and reproducibility at affordable cost.

The following properties are aimed for in high-end NSOM and TERS probe design:

Sharp tip/small localized region of near-fi eld enhancement • for high-resolution imaging High intensity enhancement factor • Broadband excitation • Reproducible geometric parameters and optical properties • Engineered polarization properties suitable for the system • under investigation Low background • Predefi ned plasmon resonance wavelength adaptable to • different excitation wavelengths High-yield fabrication • Robust, cost-effective batch fabrication process • Easy integration with the microscope/tip holder •

Until today, the lack of reproducible scanning probes is a main bottleneck for NSOM and TERS applications. Very few probes that combine the above properties are commer-cially available. Mostly, freshly prepared individually etched gold wires or dielectric tips with a thin metal fi lm are used in experiments. The quest for optimized probes has gone on for over a decade, but for most of the existing prototype designs, no corresponding cost-effective robust production processes have been introduced yet.

3. Experimental and instrumental methodology

NSOM probes are typically based on either cantilevers or wires/fi bers. Cantilever probes are mounted in a near-fi eld microscope in analogy to AFM cantilevers. The distance between probe and sample is controlled by a feedback loop. Wire or fi ber probes are typically glued to a tuning fork, which is used for distance control, e.g., in shear force mode. For aperture probes, the sample is mostly either illuminated

from the far fi eld, while the signal is collected through the fi ber, or light is focused to a near-fi eld spot near the aperture through the fi ber, while detection is performed in the far-fi eld. For apertureless NSOM, the probe is illuminated through an objective from below, under a high angle from the side [23, 24] , or from all sides via a parabolic mirror [25] .

For the fabrication of NSOM probes, standard methods known from nanotechnology are utilized. For aperture probes and apertureless wire and fi ber probes, wet chemical etching, thin fi lm metallization, annealing, and sometimes focused ion beam (FIB) milling are applied. Nanoprobes based on a single engineered nanostructure can be created by bottom-up or by top-down fabrication. Bottom-up methods may be used to directly synthesize a metal particle at the desired location or to produce tips by growing a nanowire below a plasmonic particle in a chemical vapor deposition (CVD) process. Alternatively, nanostructures are directly grown by electron beam- or ion beam-induced deposition (EBID and IBID, respectively) of a noble metal. Inversely, optical align-ment or capillary forces can be used for capturing an existing nanoparticle, such as a colloidal nanosphere or nanorod, and chemically bonding it to a probe tip. Top-down methods use subtractive methods, such as gallium FIB milling, or electron beam lithography and subsequent argon ion beam milling for shaping a continuous metal layer on a probe into the envis-aged nanostructure.

Tip preparation preferentially takes place in a clean room environment. This way, the nanoprobes are protected from dust particles. The light-sensitive processes need to be per-formed in a clean room with yellow light illumination.

Wet chemical etching requires a beaker with the appropri-ate acid [e.g., hydrochloric acid (HCl) for gold (Au) wires or hydrofl uoric acid (HF) for glass fi bers], a suitable holder for positioning the wire or fi ber, and potentially a counter-electrode and a current meter if the tips are electrochemically etched.

Thin fi lm evaporation of adhesion layers and plasmonic metal layers is performed by thermal or electron beam evapo-ration in a high-vacuum chamber. For thermal evaporation, metal pellets are placed in a boat made from a high-melting-point material, which is heated by applying several amperes of current to evaporate the metal from the boat. For elec-tron beam evaporation, the metal target is placed in a cru-cible. Evaporation takes place by hitting the target with a high-energy electron beam via defl ection in a magnetic fi eld. Alternatively, thin fi lms can be applied by sputter coating, which leads to better three-dimensional coating of the high-aspect-ratio tips. In this case, metal atoms are ejected from the sputter target under bombardment by the atoms or ions of a sputtering medium. The thickness of the homogeneous fi lm is monitored in situ using a crystal oscillator.

Annealing is performed by heating in an annealing oven. At suffi ciently high temperatures, dewetting of metallic thin fi lms sets in. The fi lms break up into islands with a relatively broad size distribution on the order of several tens to hun-dreds of nanometers.

The chemical bonding of colloidal gold particles to probe tips can be achieved by immersing a glass tip in a solution of


molecules that bind to the glass via one functional group and to gold via a second one. When the functionalized tip is sub-sequently brought into contact with nanoparticles dispersed in a suspension or on a substrate, a single gold nanoparticle can be attached.

For CVD, the sample is placed in a reactor, for example, in a quartz glass tube or chamber. The sample is heated to several hundred degrees centigrade and a fl ow of the reactive gases containing the material to be deposited is introduced to the chamber. Using metal particles as catalysts, nano-wires or nanotubes can be grown on top of (base growth) or below (tip growth) such a particle. Their position can be predetermined by specifi cally structuring or positioning the catalyst.

For focused EBID or focused IBID, a sample is placed in the evacuated chamber of a scanning electron microscope, an FIB machine, or a dual beam system. A precursor gas is intro-duced to the sample surface via a heated gas injection nozzle. The precursor contains the elements that are to be deposited. An electron or ion beam is focused onto the sample. At the point of incidence, the precursor molecules are decomposed by the energy of the secondary electrons created in the sam-ple. A solid layer is locally deposited on the sample, which grows into pillars for increasing exposure doses, while the residual volatile component is removed by pumping [43] . These methods allow for the fl exible positioning of structures and for direct writing of high-aspect-ratio pillars or high-resolution nanostructures defi ned via a pattern generator. On the downside, the deposited material is typically strongly carbon contaminated.

Electron beam lithography takes place in the evacuated chamber of a scanning electron microscope with a pattern generator or a dedicated electron beam writer. A sample is spin-coated with a thin homogenous layer of electron beam resist. The envisaged nanopattern is programmed and sent to the pattern generator. By defl ecting the electron beam, which is focused to a diameter of a few nanometers, the pattern is written in the resist. After wet chemical development of the sample, only the resist areas that were exposed to the electron beam remain on the sample for a negative resist, while only the exposed areas are removed for a positive resist. Resist nanostructures can be used as etch masks for the underlying material. Alternatively, nanostructured holes in the resist can be used as a mask for metallic thin fi lm deposition. After the residual resist is removed in the solvent in a liftoff process, metallic nanostructures remain on the surface. These methods are powerful and well-developed techniques, but they depend on homogeneous resist layers as the starting point, which are diffi cult to achieve on high-topography surfaces such as scan-ning probes.

The principle of FIB milling is similar to electron beam lithography. In this case, a beam of gallium ions is focused to a diameter of few nanometers. The ion beam is used for locally removing material from the sample surface with potential sub-10-nm precision. The shapes that are thus cre-ated can again be predefi ned via a pattern generator. This method offers high three-dimensional fl exibility for creating structures on the nanoscale in a subtractive process. Some

gallium doping and amorphization is observed in the vicinity of the exposed surfaces.

Argon ion milling, as applied in the presented work, is a mechanical dry etching process. A collimated beam of argon ions with a beam waist of several millimeters to several cen-timeters is created in a plasma source and accelerated toward the sample surface with an acceleration voltage of typically several 100 V to few kV. Material is removed from the sam-ple surface by energy transfer under the impact of the argon ions. The etch rate depends on the surface topography due to an angle-dependent sputtering yield [44, 45] . A surface of several square centimeters can be homogeneously etched in this way. Here, the process is mostly used for transferring the shape of etch masks into an underlying metal layer.

The experimental methodology of nanotechnology has seen immense progress over the last few decades. With the above methods, it now offers a powerful selection of tech-niques that can be applied for the specifi c engineering of plas-monic nanostructure-based NSOM nanoprobes, which could not otherwise be achieved before.

4. Key developments

Since the introduction of NSOM, considerable effort has gone into the development of particularly favorable nanoprobe geometries. The main idea is to create a single strongly local-ized, high-intensity near-fi eld spot near the apex of a scan-ning probe. The probe can be either a cantilever or a fi ber or wire probe. As aperture probes suffer from limited throughput and resolution and extended apertureless probes may lead to considerable background, alternative realizations of the near-fi eld spot by means of individual nanostructures at the probe tips have been developed. These plasmonic nanostructures should act as effi cient optical antennas, coupling the energy of the incident electromagnetic fi eld into the nanostructure. Nanostructures with various localized plasmon resonance fre-quencies, shapes, and tip radii were fabricated. The purity of the metal may depend on the fabrication method. Depending on the antenna orientation, the plasmonic nanostructures are selectively excited by electric fi eld components that are ori-ented either parallel or vertical to the tip axis. Correspondingly, the electric near-fi eld is oriented in different directions, mak-ing the tips best suited for different samples, depending on the respective orientation of the transition dipole moments of objects at the sample surface. Particularly high near-fi eld enhancement was achieved by coupling two nanostructures across a gap, for example, in a bowtie confi guration [46] . At the same time, resolution is typically decreased for planar structures at a probe tip. Each nanoprobe concept thus has its particular advantages and disadvantages. The choice of a well-suited nanoprobe critically depends on the demands and peculiarity of the sample under investigation and the setup as well (e.g., the excitation wavelength). In the following, several characteristic types of nanoprobe geometries and fab-rication processes are elucidated.

Figure 1 shows a schematic overview of the most common tip confi gurations. Figure 1A and B depicts aperture probes,


in which light is guided through a metal-coated dielectric probe for excitation, and/or optical signal from the surface is collected through the tip. In Figure 1A, an aperture is located near the tip apex. The evanescent near-fi eld spot depends on the aperture size and shape. Different geometries have been demonstrated, from circular apertures to inverse bowtie shapes or coaxial ring shapes. Aluminum (Al) metalized dielectric probes with circular apertures are the most commonly used confi guration for aperture NSOM. Figure 1B shows a varia-tion of the circular aperture tips, where the plateau at the tip containing the hole-aperture is locally metalized with gold, leading to fi eld enhancement by extraordinary transmission of light. Figure 1C – L depicts apertureless scanning probes that are fabricated by different techniques. Figure 1C and D shows the most common types of apertureless probes. The near-fi eld spot near the apex is created by using SPPs and the lightning rod effect. Plasmons are adiabatically compressed toward the tip of the conically shaped probe, leading to a high energy density near the tip apex [47] . Figure 1C shows a massive probe made from a sharpened metal wire. Figure 1D consists of a sharp dielectric tip that is coated by a thin plas-monic metal layer. To undergo a transition from excitation via SPPs to excitation via confi ned LSPPs, type 1D probes may be thermally annealed such that the metal layer decom-poses into nonuniform islands with a size on the order of 10 to 100 nm. Tips 1E to 1L are all based on the concept of sin-gle nanostructures with well-defi ned geometrical parameters that are specifi cally crafted at the probe tip. In Figure 1E, a spherical or rod-shaped colloidal metal particle is picked up

and chemically attached to a functionalized dielectric tip in a microscope setup. Figure 1F schematically displays a metal particle at the tip of a nanowire. The nanowire is grown below the particle in a self-aligned process using the metal particle as catalyst. Instead of picking up particles, colloidal nanopar-ticles may be attached to hollow probes by exploiting capil-lary forces for trapping a single particle at the tip opening, as seen in Figure 1G. Moving away from bottom-up methods based on colloidal particles, FIB milling offers a powerful technique for top-down crafting of predefi ned geometries from extended metal fi lms with nanometer precision. So far, FIB has, for example, been applied to create vertical nanorod antennas (mostly on aperture probes) (Figure 1H) or lateral confi gurations of bowtie antennas on plateaus at the tips of apertureless probes (Figure 1I). The method of focused EBID or IBID, in contrast, has been used for both additive and sub-tractive approaches. Using a precursor that contains gold or silver (Ag), nanopillars of a plasmonic metal can be directly deposited on the tip with high precision. This metal, however, typically contains a high percentage of carbon, thus deterio-rating the optical properties (Figure 1J). To use the strength of the process in producing conical nanostructures and at the same time maintain high-purity metal layers, in some cases, the nanostructure was subsequently selectively metalized (Figure 1K). Instead of using the locally deposited material as the base of the nanostructure, it can also be used as a mask for an underlying metal layer. When the mask and surround-ing metal are removed by argon ion milling, individual well-defi ned metallic nanostructures remain at the probe apex, as

Figure 1 Overview of different types of NSOM probes. Aperture probe tips: (A) metalized (typically with aluminum) dielectric tips with circular, bowtie-, or ring-shaped apertures, (B) metalized dielectric tip with hole-aperture in a gold metal disc. Apertureless probe tips: (C) sharp metal wire tip, (D) sharp dielectric tip with thin metal coating, (E) colloidal metal particle attached to a scanning probe, (F) nanowire tip grown below a metal particle, (G) colloidal nanorod attached to a hollow tip by capillary force, (H) vertical nanorod (mostly on aperture probes) and (I) bowtie antenna crafted at the tip by focused ion beam milling, (J) metal tip grown by EBID or IBID, (K) metal-coated tip grown by EBID or IBID, (L) metal nanostructure or nanocone defi ned by induced deposition mask lithography. (Sketches courtesy of B. Schr ö ppel, NMI Reutlingen, Germany, and M. Fleischer.)


seen in Figure 1L. Combinations of the different approaches may be possible as well. In the following, examples of imple-mentations of the different tip confi gurations are presented.

4.1. Aperture probes

In NSOM using aperture probes, light is guided through a metal-coated sharp-tipped glass fi ber and introduced to the surface via a subwavelength aperture acting as the near-fi eld probe or collected from the surface via the same nano-aperture while the sample is illuminated from the far fi eld [13, 14, 48] . Alternatively, hollow metal-coated cantilever pyramids with a subwavelength aperture at the apex can be used for excitation or collection. Evanescent fi elds are pro-duced near the apex, which contain high spatial-frequency Fourier components that can be converted into propagating waves if a sample structure is brought close enough to the aperture [49] . The sample is illuminated very locally; thus, the imaging is essentially background-free, which makes the aperture probes excellently suited for imaging low-intensity objects. On the downside, the reproducible fabrication of such probes remains diffi cult. The spatial resolution suffers from a practical lower limit of about 30 nm because the opti-cal radiation penetrates into the metal coating at the aper-ture, thus increasing the effective aperture size [18] . Light throughput diminishes very strongly with decreasing aper-ture diameter and cone angle of the probe. Also, the metal fi lm, which has a thickness of about 100 nm to prevent light leakage, increases the diameter of the aperture probes, lead-ing to poor topographical resolution, and restricts the optical imaging to fl at surfaces [50] . For an overview of light propa-gation in near-fi eld probes, see [1] .

4.1.1. Aperture in metal coating Standard aperture probes consist either in glass fi bers coated with a metal layer to prevent light leakage, or in hollow cantilever pyramids. In either case, light is transmitted or collected through an aperture on the order of 100 nm at the fi ber or cantilever tip apex. The probes are typically metalized with an aluminum coating by electron beam evaporation or sputter coating. Apertures can be added by simply applying pressure to or repeated scanning of the probe, using a shadowing scheme, aperture punching, wet or solid-state electrolysis, evanescent fi eld-induced corrosion, or, in the most defi ned fashion, FIB milling. The fabrication of fi ber-based aperture probes is outlined in [17, 48, 51 – 59] as well as in [60] and reviewed in [16] and in references therein. The schematics and an SEM image of an aperture fi ber probe can be seen in Figure 2 . The fabrication of metal-coated cantilever pyramid arrays with nanoscale apertures at the pyramid apex is detailed in [61 – 66] . Roughly triangular apertures for higher resolution are discussed in [67] .

4.1.2. Shaped aperture in metal coating Light transmission through standard aluminum-coated aperture fi bers suffers from low throughput through the subwavelength aperture in combination with a low damage threshold. This drawback may be alleviated by engineering the nanoaperture.

A BAluminum

Aluminum vapor

Glass

Figure 2 Aluminum-coated fi ber probe prepared by pulling and evaporation. (A) Schematics and (B) SEM image of the aperture (scale bar = 300 nm). Reprinted with permission from [16] . Copyright 2000, American Institute of Physics.

Figure 3 Bowtie-shaped nanoaperture at a fi ber probe apex. Reprinted with permission from [70] . Copyright 2010, Optical Society of America.

Strong near-fi eld enhancement can be created within a nanoaperture acting as an antenna. In planar metal fi lms, it was shown that the transmission through a single aperture was enhanced by reshaping the aperture, for example, to “ negative ” antenna shapes such as bowtie-shaped apertures [68] or C-shaped apertures [69] . The concept of shaped apertures was implemented with NSOM probes in [70] (see Figure 3 ).

4.1.3. Aperture for extraordinary transmission Extraordinary transmission through nanoholes has been harnessed by researchers ever since it was reported in [71] . By carefully cutting a fi ber probe at the chosen diameter of the mode cutoff and etching a nanoaperture into a metal fi lm locally deposited onto the end face, intensity enhancement by extraordinary transmission can be observed, offering improved throughput and effi cient coupling through the aperture. This concept combines the idea of aperture probes, where light is guided through the waveguide of the tip, and resonant coupling with the nanostructured plateau. Such nanoprobes were demonstrated in [72] . An optical fi ber is coated with 220 nm of aluminum to prevent light leakage.


The fi ber end is removed by FIB milling up to the diameter corresponding to the cutoff of the TM 11 mode. The end face is then coated by a thin gold fi lm. A subwavelength aperture is fi nally added at the center of the gold disc by FIB milling, as can be seen in Figure 4 . These probes were shown to allow for high-intensity imaging, whereas the LSPP resonance could be tuned from 600 to 900 nm. The transmission was improved by roughly a factor of 100, and the damage threshold was increased compared with conventional aperture probes. On the downside, due to the planar plateau around the aperture, the achievable spatial resolution is limited.

4.1.4. Coaxial tip with ring-shaped aperture Apart from shaped single apertures at the tip of the metal coating, coaxial tips with ring-shaped apertures were demonstrated as well, cf. [60, 73 – 76] . An example is shown in Figure 5 [76] . To obtain such coaxial tips, commercial silicon nitride (Si 3 N 4 ) contact mode AFM cantilever pyramids were coated with a titanium (Ti) adhesion layer and a 120-nm gold layer. The ring structure with a diameter varying between 40 and 160 nm and a narrow gap on the order of 15 nm was shaped in the metal by focused gallium ion beam milling. The ring apertures show distinct

Figure 4 Cutoff tip of an aluminum-coated fi ber with a gold-coated end face and FIB-cut aperture (scale bar = 500 nm). Reprinted with permission from [72] . Copyright 2011, American Chemical Society.

20 nm

120 nm

SiN tip65 nm

Au

Figure 5 Schematics and SEM image of coaxial probe tip with ring-shaped aperture. Reprinted with permission from [76] . Copyright 2011, American Chemical Society; left image courtesy of A. Weber-Bargioni, The Molecular Foundry, Berkeley, USA.

optical resonances in the visible spectral range, which can be tuned, for example, via the ring diameter. The resonance was thus adapted to the excitation laser wavelength. The coaxial probes were successfully applied for hyperspectral Raman imaging of carbon nanotubes on a dielectric substrate with ∼ 20 nm resolution and integration times down to 50 ms per pixel. As an advantage of this geometry, high near-fi eld enhancement is obtained due to antenna coupling across the narrow gap of the ring. Thus, the necessity of gap-mode imaging across a small distance of a few nanometers to a metallic substrate is avoided, which usually reduces the choice of surface to ultrathin analyte layers. The coaxial geometry offers the potential for implementing different polarization schemes. The circular metal-insulator-metal slit was suggested to be relatively broadband and to grant effi cient coupling to far-fi eld radiation [77] . By back-illumination through the probe, low background imaging should be possible.

4.1.5. Campanile probe Figure 6 shows the schematic outline of a “ campanile ” aperture probe, which is presented in [78] . At the tip of an optical fi ber, a tapered waveguide consisting in two nanostructured plane capacitors with a nanogap is created. This design was used for photoluminescence imaging of InP nanowires, where both excitation and collection were achieved through the optical fi ber, thus precluding the need for transparent substrates. Such probes allow for low background by excitation and collection through the fi ber, strong fi eld enhancement at the nanogap, and broadband operation.

4.1.6. Tip-on-aperture probes In an effort to combine the almost background free near-fi eld imaging demonstrated with aperture probes with the high-resolution, high-intensity imaging provided by apertureless antenna probes, tip-on-aperture probes were developed, as outlined in [79, 80] . In this approach, a metallic nanorod is positioned next to the edge of the aperture of a standard aperture probe. This antenna is excited by the near fi eld created at the rim of the aperture, which is oriented parallel to the nanorod axis if the correct orientation of the linear input polarization is chosen. The intense near-fi eld spots at the nanorod tip that are created this way are used for imaging. The much less intense, wider spot from the aperture, which is removed from the sample surface by the length of the nanorod, is visible in images as a weak halo.


1 μm

Figure 6 “ Campanile ” probe tip schematics and SEM image. Images courtesy of A. Weber-Bargioni [78] .

Figure 7 Tip on aperture, fabricated by EBID growth of a nanorod with subsequent metallization. Reprinted with permission from [50] . Copyright 2002, American Institute of Physics.

The concept of a tip-on-aperture was realized in [50] . An optical monomode fi ber was thinned and etched to a tip. The tip was covered by a chromium (Cr) adhesion layer and a 200-nm gold layer, as shown in Figure 7 . The aperture is opened by applying friction via pressing the tip to a surface. The vertical nanorod was grown by focused electron beam-induced con-tamination deposition, focusing an electron beam on the aper-ture and depositing carbon from the residual gas present in the chamber. The rod and tip were then covered with chromium and aluminum by evaporation under an angle of 45 ° . By this shadow evaporation, an elongated slit aperture remains behind the coated rod. With this tip-on-aperture probe, single fl uorescent beads were imaged with a resolution down to 25 nm. Due to the illumination condition through the fi ber, apart from the low background, the probe also offers a spa-tially fi xed, compact, relatively stable illumination source [50] .

The same general concept was applied to cantilever probes in [81] . An aperture at the cantilever pyramid tip is opened by FIB milling. A vertical nanorod tip is then grown in a well-de-fi ned fashion using focused EBID of tungsten from a tungsten hexacarbonyl [W(CO) 6 ] precursor. For imaging purposes, the tip-on-aperture confi guration was coated with a silver layer by electron beam evaporation.

The antenna design was further perfected in [82] . Here, a λ /4 optical monopole antenna was fabricated next to the aper-ture of an aluminum-coated fi ber probe (see Figure 8 ) and modeled via the fi nite integration technique. A heat-pulled single-mode glass fi ber is coated with a chromium adhesion layer and a 150-nm aluminum layer. In two-step FIB milling,

the aluminum is removed at the tip to reveal a ∼ 75- to 100-nm aperture, whilst an aluminum nanorod next to the aperture is spared in the milling. The antenna is driven by the verti-cal local electric fi eld components at the aperture rim. The antenna length and local excitation conditions are engineered for maximum antenna resonance and localized fi eld enhance-ment near the nanorod tip, as investigated in greater detail in [83] . The wavelength of the standing wave charge oscillation in the antenna is blue-shifted for larger antenna diameters, requiring shorter resonant antennas. Mapping of fl uorescent molecules confi rmed an optical resolution of 25 nm.

4.2. Extended apertureless probes

As detailed in [38] , the standard aperture probe technique has some practical drawbacks. The diffi culty to obtain an alumi-num coating that is smooth on the nanometer scale introduces irreproducibility in the probe fabrication and measurements. The fl at end-faces of the aperture probes with the rather large apertures are not suitable for simultaneous high-resolution topographic imaging. The absorption of light in the metal coating causes signifi cant heating, which leads to a low dam-age threshold and poses a problem for biological applica-tions. Also, the light throughput through aperture probes is limited and requires a tradeoff between the aperture size and the optical resolution for suffi cient signal-to-noise ratio. Due to a fi nite penetration depth of the light into the metal coating, the minimum resolution that can be obtained with standard aperture probes is typically limited to several tens of nanome-ters. For these reasons, the technique of apertureless NSOM has been developed, using sharp metallic tips under external far-fi eld illumination for high-resolution imaging. Here, the image resolution is basically only determined by the radius of the tip apex and the tip-sample distance. As a disadvantage of this modifi cation, however, the far-fi eld illumination by a focused laser beam exposes a large area around the tip apex. This leads to a large background scattering signal, which reduces image contrast and may lead to large-area bleaching in fl uorescence imaging [50] . The background may be sup-pressed by modulation techniques [84] or nonlinear processes [38, 39] . The extended metal layer may also be buried below a dielectric layer and only be exposed near the tip [60, 85] . The fabrication of extended massive and surface-metalized apertureless NSOM probes is described in the following. A sharp elongated tip is for example displayed in [86] . For the principle of near-fi eld enhancement at the tip, see [1] .


4.2.1. Metal wires The most common realizations of apertureless NSOM probes are metal tips crafted from thin metallic wires by chemical or electrochemical etching. The quality of the optical imaging is determined by the tip radius, tip aspect ratio/opening angle, and surface roughness. Typically, gold wires are used because of their chemical stability under atmospheric conditions and the large fi eld enhancement due to the excitation of surface plasmons in the visible range [87] .

As an example of chemical etching, a ∼ 100-μm diameter gold wire can be immersed in an aqua regia solution (nitric acid/HCl, ratio 1:3) covered by a several-millimeters-thick isooctane protective layer [88] . The solution forms two phases, in which the immersion depth of the gold wire can be controlled via a micrometric stage. Upon immersion of the wire, a meniscus forms at the phase interface. The meniscus decreases during etching, leading to a self-terminating etch process. The taper geometry is determined by the fall of the meniscus, i.e., the contact angle. After rinsing in deionized (DI) water and ethanol, the tips are inspected by SEM for fur-ther use. With this technique, several wires can be simultane-ously etched. The process yields tip radii of about 50 nm with considerable tip roughness.

Using electrochemical etching, a ∼ 100- μ m-diameter gold wire is introduced in a beaker with HCl diluted 1:1 with DI water for a single-step, DC-voltage etch process [87] . As alternative etching solutions, 25 % HCl [89] , a mixture of HCl and sulfuric acid [90] , a fuming HCl/ethanol solution [91] , or sodium chloride (NaCl) in a 1 % perchloric acid [92] have been used. An AC or DC voltage is applied between the verti-cally oriented gold wire and a counterelectrode, which may consist of a metallic wire loop positioned around the wire [91, 93] or in a carbon rod [87] . The wire is locally etched

1000 nm

Figure 8 λ /4 nanorod antenna on an aperture probe tip fabricated by focused ion beam milling. Reprinted with permission from [82] . Copyright 2007, American Chemical Society.

to a thin constriction, while the etching current is constantly monitored and the process is observed under a microscope. When the wire is fully etched through and the current read-ing reaches 0 A, the lower part of the wire will drop to the bottom of the beaker, while the top half remains etched to a tip. Better results with respect to tip sharpness are obtained if a low threshold current is defi ned, and the voltage is auto-matically switched off as soon as this cutoff is reached [87] . After rinsing in DI water, the wire is inspected in an SEM. It may then be glued to a tuning fork to act as an NSOM probe for imaging purposes. The tip apex typically has a radius of several tens of nanometers, and it can reach diameters on the order of 10 nm. An example of an etched gold wire tip is shown in Figure 9 (cf. [94] ).

It takes well-controlled etching conditions and some expe-rience to reproducibly obtain ultrasharp tips. In some cases therefore, an additional sharpening of the ready etched wire tip by FIB milling is reported [86, 95, 96] . This way, with some instrumental effort, small tip diameters can be reached, however, at the risk of doping the tip region with gallium ions.

Metal wire probes can be fabricated in sequential processes within a reasonable time and with a reasonable yield of sharp tips. The results, however, show noticeable variability from tip to tip. While SPPs can be adiabatically compressed by the tip geometry, no LSPPs localized to the 100-nm scale are involved, which makes adjusting the resonance frequency diffi cult.

4.2.2. Metal-coated tips As an alternative to solid metal tips, SPPs can be excited at the interface of metallic thin fi lms. These may be evaporated onto either fi ber probes or cantilever-based probes. Sharp-tipped glass fi ber probes may be obtained by pulling or by wet etching. In the pulling method, the fi ber is threaded through a metal plate with a pinhole and attached to micromanipulators. While the fi ber is slowly pulled, the metal plate is heated, thus locally heating up the fi ber. The constriction is elongated to the breaking point, whereupon two sharp glass fi ber tips are obtained.

Figure 9 Solid apertureless scanning probe with a tip dia-meter of ∼ 20 – 30 nm etched from a gold wire. Image courtesy of L. Hennemann, D. Zhang/A.J. Meixner Group, University of T ü bingen, T ü bingen, Germany.


For the wet etching of fi ber tips, HF is used (see Figure 10 A). The fi ber is vertically introduced into a beaker of HF, which is covered by a nonmixing phase of toluene. Etching progresses while the liquid evaporates, thus creating a self-limiting process where the tip is etched until it reaches the toluene boundary. The tip sharpness and opening angle can be infl uenced by slowly moving the fi ber up during the process. For cantilever-based probes, ultrasharp cantilever pyramids as routinely used for AFM are commercially available (see Figure 10B).

In either case, the sharp probe tip is coated by a continu-ous thin fi lm, with a thickness of a few tens of nanometers, of gold or silver by thermal evaporation or sputter coating (as seen in Figure 11 ). Metal-coated fi ber tips are glued to a tun-ing fork, while metal-coated cantilevers can directly be used for near-fi eld imaging in a modifi ed AFM setup. Metal-coated dielectric fi ber probes have for example been fabricated and used in [28, 84, 98, 99] , whereas silver-coated AFM tips are applied amongst others in [39, 100 – 102].

The metal-coated tips are not very well-defi ned. The thick-ness of the thin metal fi lm varies locally, and the fi lm thick-ness at the tip depends on the geometry of the underlying tip. With a layer thickness of several tens of nanometers, no ultra-small tip radii can be obtained. In the continuous layer, no LSPPs are created.

To move from extended metal layers to nanostructures with an LSPP resonance, an additional annealing step can be introduced after metallization. When the metal-coated tip is heated up, the metal thin fi lm breaks up into islands with diameters on the order of 100 nm or less, depending on the layer thickness and temperature. Due to surface tension, they contract to metal droplets. As the lateral dimensions of the sharp underlying tip apex are very small, only one or few droplets are formed near the apex of the scanning probe. These droplets will dominate the optical signal for NSOM imaging. On the downside, the nanoscale islands are not very well defi ned in their shape and size distribution. Still, with this method, nanostructures with a defi ned plasmon resonance can be formed at the apex of sharp probes in a straightforward process with few instrumental requirements. The process is fully parallel. Many tips can be metal coated and annealed together in each run.

A

2 μm

B

200 nm

Figure 10 (A) Glass fi ber tip fabricated by wet etching in HF. (Courtesy of J. Fulmes, M. Fleischer/D.P. Kern Group, University of T ü bingen). (B) Commercial Si noncontact AFM probe (purchased from Nanosensors™, Neuchâtel, Switzerland; imaged by the author at the Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA).

4.2.3. Protective coatings and refractive index

modifi cation NSOM nanoprobes are even more easily damaged by mechanical and chemical wear as well as thermal deformation than the harder AFM tips. Gold is a particularly soft metal, whereas silver nanoprobes offer a high enhancement at fi rst but chemically degrade within a matter of days in ambient conditions and may degrade within hours under continuous exposure to a confocal beam [42] .

It has been shown that damage and degradation can be reduced by coating plasmonic nanoprobes with an ultrathin mechanically and chemically stable protection layer. For exam-ple, 2- to 3-nm-thin silicon oxide (SiO x ) coating layers were applied and simulated [103, 104] . These protective coatings were shown to slow down the rate of contrast decay but were not able to stop the decay. The SiO x layer slightly reduced the contrast compared with unprotected tips [42] . Subsequently, ultrathin Al coatings were applied by atomic layer deposition or physical vapor deposition [42] . The aluminum quickly oxidizes to aluminum oxide (Al 2 O 3 ). Volume expansion occurs during oxidation, thus creating a dense protection layer acting as an effective gas barrier [105] . These hard, optically transparent and chemically stable layers signifi cantly improve the nanostructure stability. The optical contrast of the unmodifi ed tip was roughly maintained. However, as the evanescent near fi eld exponentially decays away from the tip, the intensity will critically depend on the thickness of the protection layer, which acts as a spacer layer. At the same time, the plasmon resonance is systematically shifted to higher frequencies with decreasing effective refrac-tive index of the protection layer.

Corresponding studies were recently performed for the inverse case, where an intermediate dielectric layer of vary-ing thickness was introduced between the silicon (Si) base and the metal coating of a tip [106, 107] . In [106] , the low-refractive-index materials SiO 2 and aluminum fl uoride (AlF 3 ) are added to a silicon probe before silver metallization. The plasmon resonance is blue-shifted, leading to a better overlap with the excitation laser wavelength and thus to higher TERS enhancement factors. Accompanying numerical calculations were performed using fi nite element methods to compare the infl uence of the substrate, looking at silver-coated Si 3 N 4 , glass, and AlF 3 tips. These calculations indicate that the fi eld enhancement for the Ag-coated glass and AlF 3 tip is higher


than that for the Si 3 N 4 tip in the blue-green spectral range. The substrate-dependent width of the resonance plays a consider-able role in Raman enhancement due to the overlap between excitation wavelength and Stokes-shifted scattered light [108] . In [107] , the refractive index is lowered by means of a thermal oxidation step that covers the silicon tip with a lower refractive index oxide in a controlled fashion while maintain-ing a small tip radius. It is thus demonstrated that the plasmon resonance frequency of the extended thin fi lm metal probes is syste matically tuned by varying the effective refractive index of the substrate material. The experimental observations are supported by fi nite difference time domain (FDTD) simula-tions. By changing the dielectric layer thickness below the silver from 0 to ∼ 70 nm, the resonance was tuned over more than 100 nm, thus offering an extra degree of freedom for optical engineering compared with solid metal tips.

4.3. Nanostructured apertureless probes

In 1985, the concept of optical probes for subdiffraction-limited resolution based on optical confi nement by a sub micrometer-sized metal particle was proposed [26] . With a single nanoscopic metal particle of known size and material as a local scatterer, the resolution only depends on the scatterer size and particle-sample distance. Ever since, especially since the early 2000s,

100 nm

Figure 11 Near-fi eld probe coated with a thin silver fi lm forming a 30-nm near-spherical silver bead at the tip. Reprinted with permis-sion from [97] . Copyright 2009, EDP Sciences.

A B

100 nm

10 μm

C

Figure 12 Colloidal gold nanosphere attached to a glass fi ber tip. Reprinted with permission from [49] . Copyright 2001, John Wiley & Sons Inc.

a number of groups have worked on designs for corresponding well-defi ned apertureless particle NSOM probes, mostly based on individual plasmonic nanostructures with controllable pro-perties and well-defi ned LSPP resonances. Such probes consist of individually crafted single- or two-arm metal nanostructures at the tips of dielectric fi bers or cantilever probes. The main strategies are summarized in the following.

4.3.1. Metal spheres attached to dielectric tips by

immersion and picking Metal spheres can be synthesized by colloidal chemistry with very narrow size distributions. Their plasmon resonances can be tuned across the visible spectral range via the sphere diameter, as shown in dark-fi eld scattering spectroscopy experiments [109, 110] . At the same time, the scattered intensity increases roughly with the sphere radius to the fourth power. The ratio of the scattering and absorption cross sections increases with increasing nanoparticle diameter. The lateral extension of the high near-fi eld spot is on the order of the sphere radius [111] .

The fabrication of colloidal NSOM nanoprobes by the functionalization of the tip and immersion in a suspension of gold nanospheres was demonstrated in [112, 113] . A single-mode optical fi ber is aluminum coated with an aperture at the apex and transferred to a silanization apparatus. Reaction with 3-mercaptopropyl-triethoxysilane (3-MPTS) leads to a homogeneous surface layer of propyl-thiol groups. The func-tionalized tips are immersed in a gold colloid suspension. A reaction time of a few minutes on average leads to the attach-ment of a single gold nanosphere, whereas longer reaction times may result in the attachment of several particles. The process is refi ned in [114] , which uses a two-step process of chemical passivation of the tip sidewalls and local removal of the passivation layer at the tip with subsequent functionaliza-tion by silanization. The tips are then again immersed in a gold nanoparticle suspension for local attachment.

The fabrication of colloidal NSOM nanoprobes by selec-tive picking of a particle from a surface was introduced in [49] . Spherical metal particles with diameters of 100 nm were reproducibly mounted at the end of sharp glass fi ber tips with a plateau at the front with a width of a few hun-dred nanometers (see Figure 12 ). For this purpose, colloidal gold particles are dispersed on a cover glass. A dielectric tip is functionalized with a self-assembled monolayer of molecules (polyethylenimine in [50, 115] , 3-MPTS vapor in [116] , and


aminopropyltrimethoxysilane in [117, 118] ) for improved binding of the gold particle upon contact. The functionalized tip is approached to a single particle via shear-force control. The process is monitored in an inverted confocal and scan-ning near-fi eld microscope. The particle is attached at the tip apex by chemical bonding. The successful transfer can be confi rmed by imaging the particle distribution on the surface before and after particle picking (cf. Figure 12A and C). The weakly scattering glass fi ber tip contributes little background signal. In the fi rst experiments, optical resolutions of about 100 nm were obtained [49] .

Variations of this technique were subsequently used on both glass fi ber tips [116 – 120] and cantilever probes [115, 121] . Fiber probes fabricated according to [49] are used in [119] for the controlled enhancement of single-molecule fl u-orescence due to near-fi eld coupling with the gold particle. Detection of the scattering from a single nanoparticle is dem-onstrated in [116] , where a gold nanosphere is attached to the tip of a fi ber axicon microlens by scanning the NSOM tip near the targeted region of an isolated nanoparticle. The single particle plasmon scattering spectrum is evaluated in different solutions. In [117] , local electric fi eld polarization vectors of evanescent standing waves are probed using a nanoparticle fi ber tip. The local fi eld induces a dipole moment in the par-ticle, which then radiates into the far fi eld. Careful charac-terization of the polarizability tensor of each tip is required to reconstruct the local fi eld orientation, which is compared with theoretical predictions. A tapered optical fi ber is com-bined with a metal nanoparticle (80 nm diameter) in [118] and applied to scan a different, smaller metal nanoparticle (30 nm). The nanosphere probe exhibits a low intrinsic degree of polarization dependence and a strongly localized near fi eld. Coupling effects between the two spheres lead to a combined effective resonance. The strong gap fi eld may be exploited in, for example, Raman investigations, where the gold particle is functionalized with the molecules under investigation.

Ref. [115] uses the near-dipolar local fi eld around a 50-nm-diameter colloidal gold nanosphere linked to the end of an AFM tip to image patterned metallic and dielectric surfaces with < 80-nm resolution. The nanosphere is picked up from a sample surface by constantly exciting the tip-sample junction and detecting the backscattered optical signal. The AFM tip is positioned above a particle aggregate, the scattering intensity is maximized in the focus, and the tip is brought into contact while monitoring the third harmonic scattering signal of the tip oscillation frequency. Picking a particle from an aggregate is found to be easier than picking up isolated spheres. Single and multiple pickups may be distinguished by their charac-teristic approach curves. In [121] , a platinum/iridium-coated pyramidal silicon AFM tip carrying an 80-nm gold particle is used to spatially confi ne the near-fi eld scattering for scatter-ing NSOM probing in the mid-infrared spectral regime.

Further nanoparticle probe tips and a review of techniques for attaching microparticles and nanoparticles to probe tips are found in [122 – 124] .

As an advantage of these NSOM particle nanoprobes, the nanostructure at the tip is very well defi ned, leading to better-defi ned plasmon resonances than those of extended metallic

Figure 13 Self-similar colloidal gold nanosphere trimer attached to a glass fi ber tip. Adapted image reprinted with permission from Macmillan Publishers Ltd.: Nature Photonics [125] , Copyright 2011.

probes. Fine-tuning of the resonance via the particle geometry allows for a prior choice of the desired resonance frequency. With colloidal particle tips, one is dealing with a tradeoff between high-intensity imaging for larger metal spheres and ultrahigh-resolution imaging for smaller spheres. This effect was systematically studied in [120] . The interplay between fl uorescence enhancement and spatial resolution was investi-gated on terrylene molecules embedded in ultrathin crystalline fi lms of p -terphenyl. Individual spherical gold nanoparticles with diameters of 40 to 100 nm were attached to dielectric tips. The full width at half maximum of the near-fi eld images of single molecules and the fl uorescence enhancement are evaluated and simulated for the different sphere sizes. Spatial resolution depends on both the particle diameter and the separation between the particle and the imaged molecules. Resolutions beyond 20 nm with single emitter detection even in dense ensembles are demonstrated as well as fl uorescence enhancement up to a factor of 30.

The concept of particle picking can be extended to dimer or trimer probes of stacked self-similar nanospheres at the tip for plasmonic focusing, as shown in [125, 126] (Figure 13 ). With such stacked confi gurations, extremely high near-fi eld enhancement factors can be achieved [127] .

The picking procedure in its current form is a serial pro-cess without the possibility of batch process fabrication of nanoprobes.

4.3.2. Metal particles attached to dielectric tips by

capillary forces The tradeoff between high-intensity antenna function and small tip radius for high-resolution imaging can be minimized by attaching elongated metallic nanorods to the tip apex. The length of the nanorod can be chosen such that it acts as a λ /2 antenna for the incident light,


while the short axis allows for high-resolution ( ∼ 20-nm) imaging. Compared to spherical nanoparticles, gold nanorods exhibit lower damping, sharper resonances, and a stronger local fi eld enhancement factor, whereas the narrower resonances make the rods more sensitive to slight geometric variations [128] . By switching between the distinct resonances of the long and short axis of the rod, polarization-dependent excitation is possible.

The main diffi culty is in attaching a single nanorod to a probe tip with the long axis oriented vertically to the sur-face. One elegant means of attaching a nanoparticle to the apex of a tip was demonstrated using a hollow nanopipette, as shown in Figure 14 [129, 130] . Capillary force is used to attach a metal nanosphere or nanorod to the hollow tip of the pipette. A hollow glass tube pulled to a sharp tip is introduced to a suspension of monodispersed colloidal particles. Water fl ows into the pipette due to capillary forces. The fl ow may be additionally supported by pumping at the back of the pipette. Once a particle has been attracted to the probe tip, the fl ow is interrupted in a self-limiting process and a stable nanoparticle probe is formed [129] . The capturing process is rather time-consuming but has been successfully extended to nanorod probes [128, 130] .

4.3.3. Self-aligned metal particles on nanowires

at cantilever tips The growth of vertically oriented nanowires, especially of different semiconductor nanowires and of carbon nanotubes, by CVD has been demonstrated in many instances. The growth is promoted by metallic catalyst dots, which can be specifi cally placed at the desired nanowire positions. Silicon is introduced, for example, via the precursor gas silane (SiH 4 ) at high temperatures. In the case of tip growth, the wire material diffuses via the catalyst particle while pushing the particle upward. This way, a nanopillar with a metallic nanocrystal on top is grown in a self-aligned fashion. The same principle can be adapted for creating NSOM nanoprobes. A supporting pillar, such as a silicon nanowire, is grown from a colloidal or a lithographically defi ned gold catalyst dot. The particle remains at the tip

200 nm

200 nm

BA

Figure 14 (A) Single nanoparticle attached at a capillary probe tip by capillary fl ow [129] . (B) Single gold nanorod placed in the aperture of a nanoscale quartz pipette [128] . Reprinted with permission from [129] , copyright 2003, American Institute of Physics, and from [130] , copyright 2009, National Academy of Engineering.

of the wire and can support LSPPs, forming a self-aligned plasmonic scanning probe. This concept was described in the patent [131] and is illustrated in Figure 15 . The well-defi ned nanostructure antenna is grown at a cantilever tip. If multiple particle wires are grown, they may be selectively removed by reactive ion etching. The volume of the particle is determined by the predefi ned catalyst particle, allowing some control over the resonance frequency. By placing individual catalyst particles, for example, on fl at cantilever beams and growing NSOM nanoprobes in the described process, batch processing on the wafer level is imaginable. An inherent risk of the process is the danger of particle deformation during growth, changing the plasmon resonance relative to a perfect sphere. The very thin, high-aspect-ratio nanowire probe is very well suited for probing cavities or surfaces with abrupt topography changes. At the same time, they run the risk of breaking. As for the colloidal NSOM probes generated by particle picking discussed before, either high-intensity (larger catalyst particle) or high-resolution (smaller particle) imaging is targeted.

Similar concepts of attaching a single nanotube or nanowire to a silica fi ber taper, where the wire is functionalized with an appropriate nano-optical structure for near-fi eld imaging, are outlined and extended in [132] .

4.3.4. Chemical and photocatalytic deposition In a chemical reaction at the tip of a nanopipette, silver nitrate (AgNO 3 ) may be introduced to the tip through the pipette and reduced to Ag in a glucose solution, which is added by the immersion of the pipette or via a second channel. The size of the Ag particle deposited at the tip can be monitored via the reaction time [124, 133] . The Ag particle may be enlarged into a surrounding gold particle by electroless deposition in a gold solution.

Using the photocatalytic effect of a thin titanium dioxide layer coated onto a cantilever, in [134] a gold nanoparticle was grown on the cantilever tip by local irradiation with UV light in an aqueous solution of tetrachloroauric acid (HAuCl 4 ) (see also review [124] ). Similar photochemical reduction of tetrachloroaureate complexes, leading to the subsequent agglomeration of gold atoms at the particle surface, was dem-onstrated in [135] . The technique was used to exert spectral control over the plasmon resonances of single nanospheres and ellipsoidal particles by well-controlled single-particle growth.

4.3.5. Locally defi ned nanostructures fabricated by

FIB milling FIB milling with a beam of gallium ions with a diameter of a few nanometers is a powerful method for creating high-resolution nanostructures from a continuous fi lm or bulk material. The pattern to be written by the beam is programmed in advance. Three-dimensional patterns can be created with a high resolution. For different NSOM aperture probe designs, a focused gallium ion beam has been used for shaping a predesigned nanostructure from a metal fi lm or a metal coating as shown above (e.g., [76, 82] ).

The same method can be used for engineering metallic antenna structures on apertureless probes, in which LSPPs


are excited. A fi ber tip or AFM cantilever is metal coated, potentially after creating a small plateau at the probe tip. The metal fi lm is then crafted into structures of the desired shape supporting LSPPs. Examples include hollow nano-cones created by cutting a wedge around a metalized tip [136] or bowtie-antennas at the tip of a cantilever probe [137] .

In [136] , a silicon nitride cantilever pyramid is coated with a 20- to 25-nm gold or silver fi lm. A square frame is milled around the tip end, isolating the metal at the tip from the shaft (see Figure 16 ). The metal tip forms a hollow conical shape of about 200 nm in length with a Si 3 N 4 core. The tip radius on the order of 15 to 30 nm almost maintains the original cantilever tip sharpness. Simulations and dark-fi eld scattering spectra confi rm an LSPP resonance in the near-infrared. The

resonance is red-shifted with increasing length of the nano-structure and blue-shifted with increasing metal fi lm thick-ness or with changing from gold to silver. High near-fi eld intensity enhancement > 10 3 at the tip apex due to the light-ning rod effect and plasmon resonances for illumination at the resonance wavelength and the potential for high-resolution imaging are predicted.

In [137] , individual bowtie antennas are crafted at the api-ces of pyramidal Si 3 N 4 AFM cantilever tips from a previously deposited 40-nm aluminum fi lm (see Figure 17 ). The fi lm is removed by FIB milling, leaving a connected bowtie antenna at the tip, which is fi nally separated with a high-resolution FIB cut to defi ne the antenna feed gap of ∼ 50 nm. The struc-ture is slightly overetched, elevating the bowtie above the tip on two Si 3 N 4 pillars. The fi nished probe is used to study the

Au

A

B

C221 nm

100 nm

500 nm

Au

Si3N4

Si3N4

Figure 16 Isolated metal tip by focused ion beam milling. Reprinted with permission from [136] . Copyright 2009, American Institute of Physics.

Si nanowire

200 nm

100 nm

Cantilever

Si tip

Si nanowire

Gold particle

=10.58 nm

=18.66 nm

Figure 15 Gold nanosphere at the tip of a Si nanowire after epitaxial tip growth. Images courtesy of G. Cohen, IBM T.J. Watson Center, Yorktown Heights, NY, USA [131] .


over the positioning of the structure and works on arbitrary substrate topographies, including high-aspect-ratio tips. Nanopillars with tip radii of < 10 nm may be obtained [140] . The deposition rate of the pillar material, however, is sensitive to the coverage of the surface by precursor molecules. It therefore depends on the duration for which the gas fl ow is introduced to the surface before and after applying the focused beam and on the distance of the precursor gas nozzle from the position of the focused beam. Although the pillar height increases linearly with the exposure dose for constant conditions [141] , this dependence may lead to a limited reproducibility of the growth rate as well as dose-dependent pillar height and diameter from exposure to exposure. As surface topography changes the fl ow rates as well, the growth rate on exposed probe tips is strongly reduced compared with planar substrates, which needs to be taken into account for the fabrication of NSOM probes. The plasmonic nanostructure quality is limited by the strong carbon contamination of the metal when EBID or IBID is performed in regular SEM or FIB high-vacuum chambers [142] .

The EBID growth of plasmonic nanoantennas is applied in [113] , where the volatile Au precursor dimethyl-gold-tri-fl uoroacetylacetonate is inserted into the vacuum chamber of a scanning electron microscope for focused EBID on a single-mode fi ber tip. The precursor is locally decomposed due to the interactions with the electron beam. The resulting deposit consists of gold nanoclusters of 2 – 5 nm dispersed in a carbon matrix (87 wt % of gold and 13 wt % of carbon). Ellipsoidal structures with an aspect ratio of about 3.5 are grown, depending on the applied dose determined by the exposure time (Figure 18 ). The tips are tested in illumina-tion mode by imaging the light transmitted through a test grating.

Beam-induced deposition is a serial process that may be performed in a batch process on the wafer level, provided that high precision alignment is ensured.

4.3.7. Metalized EBID- fabricated tips To avoid optical signal deterioration by carbon contamination and plasmon

100 nm 500 nm

Figure 17 Bow-tie antenna on a cantilever pyramid tip created by focused ion milling [137] . Copyright 2005 by the American Physical Society.

photoluminescence and excited-state lifetime of single-semi-conductor quantum dots, where the antenna/quantum dot con-fi guration as a whole forms an effective super-emitter. This bowtie geometry has the advantage that a narrow antenna gap is created directly on the probe, thus avoiding the limitations of gap-mode imaging, where the antenna gap needs to be created between the probe and the sample. The object under investigation may directly interact with this subwavelength feed gap. The design offers very high fi eld enhancement in the coupled structure and a dominant fi eld polarization paral-lel to the surface, which may be benefi cial for imaging objects lying fl at on the substrate. These advantages are counteracted by a wide plateau and the fl at antenna structure at the probe tip, which lead to poor topographic imaging quality and large tip-sample interaction forces.

The method of FIB milling offers excellent control over the (three-dimensional) shape of the plasmonic nanostructure. The process, however, is serial and requires critical align-ment. Treatment by the FIB leads to doping of the plasmonic material with gallium ions and a thin amorphization layer along the cutting line. Sputtering of the original metal layer during alignment and the milling process may lead to reduced or varying fi lm thickness. The contamination of the metal by gallium implantation has been reported to be detrimental for the plasmonic properties [138] . The effect, however, has not been systematically studied yet. NSOM probes created by FIB milling have proved successful for near-fi eld imaging applications.

4.3.6. Tips by direct EBID of metal Instead of crafting a metallic nanopillar at a tip apex using the subtractive approach of FIB milling, metallic pillars can be directly grown at predefi ned positions by focused EBID or focused IBID [43, 139] . As described above, a precursor gas containing a metallic component is locally decomposed by a focused electron or ion beam, whereupon a metal nanopillar is locally grown at this position. Alignment is critical to position the plasmonic structure right at the tip apex. The process offers good control

200 nm

Figure 18 Gold nanoparticles embedded in a carbon matrix at the apex of a fi ber tip by direct EBID. Reprinted with permission from [113] . Copyright 2002, American Institute of Physics.


losses at grain interfaces, long tapered nanotips were grown by EBID and subsequently metalized with a thin silver layer in recent work [140, 143] . EBID was applied to integrate a single conical waveguide within the cavity of a plasmonic crystal patterned on an AFM cantilever beam for better coupling of the incident light into the nanotip and lower background when the cantilever is illuminated from the back (see Figure 19 ) [143, 144] . The two-dimensional dielectric photonic crystal cavity is patterned on a 100-nm-thin Si 3 N 4 membrane in the AFM cantilever beam by FIB milling of air holes. The 2.5- μ m-tall tapered waveguide is placed at the center of the cavity. It is grown by focused EBID of platinum with a strong carbon component. A 30-nm-thin silver fi lm is deposited on the whole device and removed from the photonic crystal surface again. The fi nal waveguide has a base diameter of 300 nm and a tip radius down to < 5 nm, where the tip apex is machined by low-current ion milling. Such long tips support standing waves that evolve along the surface and lead to adiabatic focusing of LSPPs toward the apex. The local electric near-fi eld at the tip is maximized by choosing suitable illumination conditions and tip dimensions for a fi eld maximum near the tip apex [145] . With this photonic-plasmonic device, topographic, chemical, and structural information with a spatial resolution of 7 nm could be obtained.

As the direct writing of plasmonic structures by EBID or IBID, this procedure offers good control over the positioning of the structure, quite sharp tips for high-resolution imaging, and the potential of serial batch processing of nanoprobes in combination with the limited reproducibility of the deposi-tion process. The plasmonic properties are improved by the additional metal thin fi lm, and high-intensity imaging is made possible by the strong coupling and focusing of light.

In recent theoretical work, it was predicted that truncated metallic nanocones butt-coupled to dielectric nanofi bers should form high-throughput and large-bandwidth NSOM probes [146, 147] . The cone angle and cone length need to be individually optimized for maximum enhancement and plas-mon resonance tuning [148] . The conical waveguide shown in Figure 19 combines ultrahigh resolution with a high signal

2 μm

Figure 19 Tapered conical silver-coated EBID grown tip at the center of a photonic crystal cavity. Reprinted with permission from Macmillan Publishers Ltd.: Nature Nanotechnology [143] , copyright 2010.

enhancement, while the single nanocone discussed in the next paragraph combines a geometry-tunable plasmon resonance with an ultrasharp tip.

4.3.8. Metal nanostructure on tip by etch mask

transfer The author and her group established several variations of a process for the fabrication of plasmonic nanocones using a subtractive technique based on etch mask transfer into a metal fi lm. Such metallic nanocones can be produced with a base diameter and height on the order of 100 nm and sharp tips with diameters down to < 10 nm (see Figure 20 ) [45, 141, 149 – 151] .

Planar plasmonic nanostructures are routinely fabricated by electron beam lithography of a positive resist, metal evap-oration, and a subsequent liftoff process, as shown in method A in Figure 21 . Plasmonic nanocones in contrast are obtained by creating a local circular etch mask on a metal thin fi lm and etching both mask and metal in an argon ion milling pro-cess while rotating the sample. The different possible process fl ows for this approach are shown in the schematic overview in Figure 21. The etch mask in method B is fabricated by elec-tron beam lithography of a negative resist. Method C consists of electron beam lithography of a positive resist, deposition of a thin layer of aluminum oxide, and liftoff. In method D, focused EBID of the mask material is chosen, whereas method E applies nanosphere lithography with subsequent size reduc-tion of the polystyrene spheres by oxygen plasma etching. In either case, the local mask pattern is then transferred into the metal by argon ion milling. Milling with the ∼ 2-cm diameter argon ion beam at a perpendicular incidence leads to tilted sidewalls and thus sharp-tipped cones, whereas by tilting the rotating sample with respect to the incident beam, the side-wall slope can be varied up to vertical sidewalls [45] . Method D combines the concept of cone fabrication with the concept of induced deposition mask lithography, where a particular etch mask is deposited by direct EBID of the mask material. This method was initially used by collaborators in [152] to create bowtie antennas on a plateau at the apex of a cantilever tip. Details on the adapted deposition process for nanocone fabrication and on the fi ne tuning of layer thicknesses, etch mask height, and etching times are given in [144] . The advan-tage of this variation compared with methods B, C, or E is

50 nm

Figure 20 Gold nanocone fabricated by etch mask transfer litho-graphy. Image by M. Fleischer.


Figure 21 Strategies pursued for the fabrication of plasmonic nanostructures. Method A, standard liftoff process; method B, electron beam lithography of negative resist on a metal layer and subsequent argon ion milling; original process for nanocone fabrication; method C, electron beam lithography of positive resist on a metal layer, deposition of a transparent Al 2 O 3 mask, liftoff, and subsequent argon ion milling; method D, focused EBID of etch masks on a metal layer and subsequent argon ion milling; process suitable for nonplanar surfaces; method E, self-assembled monolayer of polystyrene beads with subsequent shrinking of the beads and argon ion milling; fully parallel process for larger area coverage. Figure by M. Fleischer.

that it can be applied to extreme topographies, such as the tip of AFM cantilever pyramids. This method was therefore fur-ther pursued for the fabrication of cantilever-based nanocone NSOM probes [153] . The cones can be made from different metals, depending on the choice of thin fi lm material. The mask height only needs to be adapted to the etch rate of the respective metal.

The metallic cones act as optical nanoantennas [151] . The larger cone body is useful for coupling in light, whereas the conical structure is conducive to focusing light near the tip [47] . As localized surface plasmon resonances are excited in the nanostructure, the plasmon resonance frequency can be tuned via the cone geometry. Characterization studies of arrays of cones on planar substrates prove that the height and aspect ratio of the cones can be independently tuned via the original metal layer thickness, the angle of incidence during

etching, and the energy of the argon ion beam during fabrica-tion [44, 45] . The plasmon resonance is infl uenced by these geometric parameters as well as by the dielectric function of the substrate. As an example, Figure 22 shows the depen-dence of the wavelength of maximum scattering intensity on the cone size as determined by single-nanostructure dark-fi eld spectroscopy with a white light source. For gold cones on glass with a closed 50-nm indium tin oxide (ITO) layer, the resonance is red-shifted with respect to cones on glass with only a local thin titanium adhesion layer below the gold.

The cone shape is well suited as the optical antenna at an NSOM scanning probe tip. Due to the effi cient antenna func-tion and little scattering from the bulk of the cantilever probe, potential high-contrast imaging is expected, whereas the sharp tip enables high optical and topographic resolution imaging. The method of placing a single etch mask on a metalized


surface and subsequent back-etching was thus transferred to commercial Si 3 N 4 contact mode and Si noncontact mode AFM probes. An individual gold nanocone with tunable properties is crafted at the apex of the AFM cantilever tip using thin fi lm metallization, local etch mask deposition by focused electron beam deposition, and argon ion milling. For this purpose, the tip of the probe is widened into a plateau with a width of a few hundred nanometers by focused gallium ion beam milling. A metal layer, the thickness of which determines the future cone height, is evaporated on the tip together with a thin adhesion layer. A nanopillar with a height of several hundred nanome-ters and a sub-100-nm diameter is grown at the center of the plateau by EBID. The alignment of the pillar with respect to the plateau is critical. The process fl ow is schematically summarized in Figure 23 A. Focused gallium ion beam mill-ing, EBID, argon ion milling, and visual control of the etch mask and etching process are all performed in a modifi ed Zeiss 1540 XB cross-beam machine (Carl Zeiss NTS GmbH, Oberkochen, Germany) with an added NTI argon ion gun (Nonsequitur Technology, Bend, OR, USA). The fabrication and fi rst tests of a gold nanocone NSOM/TERS probe on an AFM-cantilever pyramid are detailed in [153] (Figure 23B). In this case, Cr is chosen as the adhesion layer and a SiO x

pillar is grown as etch mask on the gold plateau. Transmission electron micrographs demonstrate the polycrystalline nature of the gold cone, which typically consists of a dominant crystalline grain and additional smaller crystals. The result-ing cone probes are used as contact-mode AFM tips to image test samples of narrow resist lines, cadmium sulfi de (CdS) nanorods, and individual carbon nanotubes. Good spatial resolution on the order of the 10-nm tip radius is maintained over several hours of scanning. Single tip-enhanced Raman spectra of dispersed carbon nanotubes are recorded with a low laser power, whereas near-fi eld images and Raman images so far remained elusive due to tip contamination in the contact mode. In a follow-up work, high-aspect ratio noncontact Si AFM cantilever tips are similarly treated. The sharp tip is cut into a plateau with a width of a few hundreds of nanometers by focused ion milling. In this case, the cantilever is sputter-coated with the metal layers for better three-dimensional cov-erage. A titanium adhesion layer, with a thickness of a few nanometers, is followed by a gold fi lm with the thickness of the intended cone height. After performing an additional dose test series for the EBID deposition of carbon, tungsten, and platinum nanopillars as etch masks, carbon is chosen as the optimal mask material for the gold cone fabrication. A

0.2

AB 800

700

600

500

4000 50 100 150 200

0.1

0400 600 800

Wavelength (nm) Cone height (nm)

Gold cones on ITOGold cones on glass

Inte

nsity

(a.u

.)

Wav

elen

gth

max

. int

ensi

ty (n

m)

1000

Figure 22 Dark-fi eld scattering spectroscopy of individual gold nanocones. (A) Resonance curve of a single 90 nm high, 95 nm base dia-meter nanocone. (B) The wavelength of maximum intensity of the dark-fi eld spectra is shifted across the visible spectrum for increasing cone height, with the function depending on the refractive index of the substrate material. Figures by M. Fleischer, measured at the Molecular Foundry, Berkeley, USA.

SiOxAu

A B C

100 nm 100 nm

CrSi

Figure 23 (A) Schematic outline of the process fl ow for the fabrication of cantilever-based gold cone scanning probes: cutting the cantilever tip by focused gallium ion beam milling, metallization of the cantilever with a thin Cr or Ti adhesion layer and a gold layer, the thickness of which corresponds to the envisaged cone height, creating a local etch mask on the plateau by EBID, and removing the etch mask and residual gold by argon ion milling. (B) Gold cone scanning probe on a Si 3 N 4 contact mode cantilever pyramid, sidewall roughness inherent to as-bought cantilevers. (C) Gold cone probe on self-aligned Si pillar on Si noncontact mode cantilever tip. Cone heights are on the order of 100 nm. Images by M. Fleischer, probes fabricated at The Molecular Foundry. Figure 23B adapted with permission from [153] . Copyright 2011, American Chemical Society.


∼ 150- to 300-nm-high carbon pillar is grown by introducing a carbon precursor gas to the cantilever surface through the nozzle of the gas injection system and focusing the electron beam on the gold plateau for ∼ 30 s to decompose the precur-sor. The cantilever tip is placed in an argon ion beam and the etch mask and surrounding gold are etched at perpendicu-lar incidence for about 30 min. In this case, the etch mask is chosen to be higher than actually necessary for removing the gold. First, the gold on the cantilever side walls is removed at a much lower etch rate than the gold on the plateau due to the angle of incidence of the argon ions, and overetching of the plateau is necessary to remove the gold on the side walls as well, in order to avoid scattering from the bulk of the cantilever in the optical experiments. Second, when the substrate is overetched while the emerging gold cone is still protected by a residual etch mask, the cone is elevated on a Si stem above the receding plateau in a self-aligned process (see Figure 23C).

The same concept was adopted in [154] to create self-aligned gold cones on pillars with a height of about 1 μ m as scanning probes on fl at surfaces, in view of fabricating the elevated cones directly on fl at cantilever beams. In this case, a thick dielec-tric spacer layer is introduced between the fl at surface and the plasmonic metal, which is etched together with the metal and oversized etch mask and removed all the way to the substrate. By elevating the cone seamlessly on a pillar in either case, the interaction between the cantilever and the surface will be minimized in the image scans and the danger of touching high-topography substrates with the plateau edges is much reduced.

The main diffi culties of this concept are the limited repro-ducibility of the EBID process and the critical alignment of the mask on the plateau. However, when the overetching of the plateau is realized as described above, the process is rather robust with respect to variation in the EBID because the size and shape of the cone are only determined by the metal thick-ness and etch parameters and a variation in the mask height will merely lead to a higher self-aligned stem below the cone, which makes little experimental difference.

As an advantage of this probe geometry, the process offers excellent control over the positioning, shape, and plasmonic properties of the cone. High topographic and optical spatial resolutions are expected from the sharp tip, whereas the larger body enables high intensity imaging. The plasmon resonance of the probe can be predetermined by choosing the suitable cone geometry. The contamination of the gold is limited because only the mask is fabricated in an EBID process, and the inert gas argon is used for etching. Not least of all, this process opens the possibility of fabricating nanocone NSOM probes in an, albeit serial, batch process on the wafer level for larger-scale manufacturing.

4.4. Alternative designs

4.4.1. Grated and graded probes In [155] , the three-dimensional plasmonic nanofocusing of light with patterned pyramids is investigated. Templates for the fabrication of metallic pyramids on the micrometer scale with an integrated grating are fabricated by chemical etching of silicon with

subsequent FIB patterning. The templates are coated with a metal fi lm, which, after transfer to a carrier material, presents a smooth plasmonic pyramid (see Figure 24 , top left). The template can be reused several times. By changing between symmetrically arranged and asymmetric gratings, the polarization at the tip can be switched between transversal and longitudinal polarization. The pyramid is used for focusing light to the tip apex to create a narrow light spot.

On a related note, scanning probes whose operation is based on nonlocal tip excitation were demonstrated. Excitation takes place via an integrated grating on the probe side at some distance from the tip, which is used for intrinsically almost background-free NSOM microscopy [157–160] . An electrochemically etched gold wire is periodically patterned with lines by FIB milling [157] . The grating is used to effi -ciently couple in light at a suffi cient distance (about 10 μ m) from the probe apex to reduce scattering from the bulk of the tip, while it is positioned within the plasmon propaga-tion length from the tip to allow for SPPs traveling to the apex (Figure 24, bottom). The grating-coupled quasi-planar SPP mode is thus coupled into a spatially confi ned excita-tion, while the probes provide an inherently broad bandwidth [158] . The grating period is determined by the in-plane momentum-conservation condition [160] . Varying the angle of incidence of light onto the grating offers some spectral tuneability. A high resolution, on the order of the 10-nm tip, was demonstrated, and the probe was used for near-infrared TERS [160] .

Recently, silver nanocones were combined with optical gratings for the generation of surface plasmons as well, using three-dimensional lithography techniques as detailed in [144, 156] (Figure 24, top right) for NSOM probes and for superhy-drophobic molecule concentration.

4.4.2. Further design considerations In further recent publications, additional design considerations can be found, for example, with respect to optimizing tip-like structures for excitation with linearly polarized light. Typically, conical tapered probe tips are most effi ciently excited by electric fi eld components that are oriented parallel to the tip axis, thus demanding nontrivial higher-order laser modes, for example the focus of a radially polarized mode, for optimized coupling. By adding controlled asymmetries to the plasmonic structure or by introducing phase shifts or tilting angles for the exciting beam, linearly polarized light may again be used for tip excitation. Excitation of bent tips by light polarized in the horizontal direction was already demonstrated in [38] . Numerical FDTD studies were performed for a linearly polarized Gaussian beam that is aimed at the base of a plasmonic nanocone under different angles using different numerical apertures [161] . SPPs are launched by the tilted beam, and plasmonic standing waves develop along the sides of the high-aspect-ratio cone, leading to a calculated near-fi eld enhancement factor of up to 150 at the cone tip. If a row of nanostructured asymmetric semicircular corrugations is added to a conical tip, simulations with COMSOL Multiphysics indicate that a narrow near-fi eld spot with a full width at half maximum


Far-fieldexcitation

Surfaceplasmon

1 μm

10 μm

3 421

Figure 24 (Top left) Silver pyramid with a grating for plasmonic focusing (scale bar = 1 μ m). Reprinted with permission from [155] . Copyright 2010, American Chemical Society. (Top right) Silver nanocone with three-dimensional grating structure. Reprinted with permission from Macmillan Publishers Ltd.: Nature Photonics [156] , copyright 2011. (Bottom left) Schematic of a grating fabricated on the side of a scanning probe for local excitation of SPPs spatially removed from the imaging tip. Reprinted with permission from [157] . Copyright 2007, American Chemical Society. (Bottom right) SEM image of a gold tip with plasmonic grating fabricated via FIB, superimposed with an optical image of grating coupling and subsequent reradiation of nanofocused SPPs. Adapted with permission from [158] . Copyright 2012, American Chemical Society.

comparable to that obtained by excitation with a radially polarized beam can be achieved under excitation with linearly polarized light [162] . In a publication on asymmetric winged cone tips using the boundary element method, it is shown that asymmetrically adding a plateau to one side of a plasmonic cone leads to more effi cient coupling of far-fi eld radiation into the localized near-fi eld energy density of the tip, as electric fi eld components oriented perpendicular to the cone axis may lead to tip excitation as well [163] . Offset-apertured NSOM probes using a one-dimensional grating and an aperture that is removed to the base of the tip cone or pyramid are numerically investigated in [164] and demonstrated in [165]. Simulations in [166] fi nd that by cutting an optimized slit some distance from the tip for backside illumination, potentially combined with a grating, a very high fi eld enhancement is expected while maintaining a small full width at half maximum of the near-fi eld spot.

In different design concepts, a light-emitting diode is inte-grated at an NSOM probe tip as the light source for near-fi eld imaging applications, which, by further miniaturization, might reach nanoscale dimensions as well (e.g., [167] ).

5. Conclusions and future perspective

NSOM is a powerful method that allows for simultaneous high-resolution imaging of the topography and optical or

chemical properties of a surface. Optical imaging way below the diffraction limit becomes possible that way. The obtain-able information by far surpasses that of pure topography imaging by AFM and yields complementary information to SEM, TEM, and other high-resolution techniques.

NSOM and TERS show a high potential for surface char-acterization (e.g., in material science, biological samples, surface engineering). Still, so far, these techniques remain mostly confi ned to research laboratories due to their non-trivial technical implementation, their relatively low acqui-sition speed, and the fact that data analysis tools, including databases of Raman spectra, are still in their early stages. The diffi culty in producing optimized NSOM and TERS nano-probes that enable high-resolution, high-contrast imaging with high throughput, high yield, and high reproducibility is another reason, which is discussed in depth in this review. Commercially available NSOM/TERS probes are currently mostly limited to either fi ber- or cantilever-based aperture tips with an aluminum coating. First apertureless tips with a metal coating and pioneering single gold or silver particle probes with nanoparticles fi xed in a pipette or on a glass cantilever tip have become available but are not yet widely in use.

With the advance in nanofabrication techniques, more and more refi ned methods with ever better control of feature sizes on the nanoscale become available for shaping pointed opti-cal antennas with the desired properties at the apex of scan-ning probes. The next important step will be to implement


these techniques into batch processes to make affordable nanoprobes according to specifi cation. The probes need to be frequently replaced due to their limited mechanical stability and the danger of contamination by the sample surface. It is vital that comparable results can be obtained independently of the individual nanoprobe.

For NSOM and TERS to take hold outside the research lab-oratory, additional points need to be addressed, such as faster and larger-area scanning, the stability of the setup, and data analysis routines for fast and simple generation of chemical maps of the scanned area, for example.

Altogether, near-fi eld optical imaging offers unique capa-bilities for addressing the mapping of near-fi eld distributions or applying nanospectroscopy to nanophotonic devices [168] . Current knowledge on biological systems may be advanced to unprecedented resolutions using these techniques [169 – 171] . Gaining chemical information on the sub-10-nm scale will offer fascinating new insights into material sciences in areas such as surface damage formation, tribology, corrosion, stress and strain, or energy effi ciency and may be pushed all the way to understanding processes on the molecular level. Creating the optimal scanning probes is one crucial piece of the puzzle.

Acknowledgments

The author would like to thank the authors of the depicted scanning probes for their permission to display their work. Fruitful discussions with D.P. Kern, the members of the author ’ s group, C. Stanciu, the A.J. Meixner/D. Zhang group, researchers at the NMI Reutlingen, and staff members at the Molecular Foundry are gratefully acknowledged as well as fi nancial support by Universit ä t T ü bingen, the Ministry of Science, Research and the Arts Baden-W ü rttemberg, and the European Social Fund. Work at the Molecular Foundry, Lawrence Berkeley National Laboratory, was performed under user proposals 550 and 917 and supported by the Offi ce of Science, Offi ce of Basic Energy Sciences, US Department of Energy under contract DE-AC02-05CH11231.

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Received April 23, 2012; accepted July 8, 2012


Dr. Monika Fleischer received her MSc degree in physics from the University of Sussex (Brighton, UK) in 1999 and her diploma in physics from University of T ü bingen (Ger-many) in 2000. She obtained her PhD in physics in 2006 from University of T ü bingen in the group of Prof. David Wharam. She then joined the

group of Prof. Dieter Kern at University of T ü bingen. She has since established a junior research group at the Institute for Applied Physics and completed her habilitation. Her research interests focus on using nanotechnology and microscopic and spectroscopic techniques for the fabrication, characterization, and application of plasmonic nanostructures. She is a mem-ber of the board of directors of the Center for Light-Matter-Interaction, Sensors and Analytics (LISA + ) at University of T ü bingen. In 2009–2011, she spent several months working as a visiting scientist at the Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley (USA), where she created plasmonic nanocone cantilever probes.

eld scanning optical microscopy nanoprobes

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