carbon nanotubes for nanorobotics - michigan state...

ISSN:1748 0132 © Elsevier Ltd 2007DECEMBER 2007 | VOLUME 2 | NUMBER 612

Carbon nanotubes for nanorobotics

The well-defined geometry, exceptional mechanical properties, and extraordinary electrical characteristics of carbon nanotubes qualify them for structuring nanoelectronic circuits, nanoelectromechanical systems, and nanorobotic systems. Relative displacements between the atomically smooth, nested shells in multiwalled carbon nanotubes can be used as robust nanoscale motion enabling mechanisms for applications such as bearings, switches, gigahertz oscillators, shuttles, memories, syringes, and actuators. The hollow structures of carbon nanotubes can serve as containers, conduits, pipettes, and coaxial cables for storing mass and charge, or for transport. Not only can nanotubes serve as building blocks for more complex structures, tools, sensors, and actuators, but they can also be used as fundamental components for future nanorobots. We review the technological progress on carbon nanotubes related to nanorobotics.

Lixin Dong, Arunkumar Subramanian, and Bradley J. Nelson*Institute of Robotics and Intelligent Systems, ETH Zurich, CH-8092 Zurich, Switzerland

*E-mail: [email protected]

Nanorobotics is the emerging field of robotics at the nanometer

scale. It includes robots that are nanoscale in size, i.e. nanorobots

(that have yet to be realized), and large robots capable of

manipulating objects that have nanoscale dimensions with

nanometer resolution, i.e. nanorobotic manipulators. Although

visionaries have foreseen a nanorobotic future of molecular

manufacturing1–3 and nanomedicine4,5, the form nanorobots will

take and what tasks they will actually perform remain unclear.

However, it is clear that nanotechnology is progressing toward the

construction of structures, tools, sensors, actuators, and systems

smaller than 100 nm that will extend our ability to explore

the nanoworld from perception, cognition, and manipulation

perspectives.

Shrinking device size to nanoscale dimensions presents many

fascinating opportunities, such as manipulating nano-objects with

nanotools, measuring mass in femtogram ranges, sensing forces at

piconewton scales, and inducing gigahertz motion, among other

possibilities waiting to be discovered. While we still understand little

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Carbon nanotubes for nanorobotics REVIEW

DECEMBER 2007 | VOLUME 2 | NUMBER 6 13

a bout how to implement ‘advanced features’, such as intelligence,

replication, and atom-by-atom mechanochemical synthesis at the

nanometer scale, early developments that have founded the field of

nanorobotics include the invention of new tools such as scanning

tunneling microscopes (STMs)6 and the synthesis of atomically well-

defined new nanomaterials such as carbon nanotubes (CNTs)7.

CNT-based building blocks for nanorobotic systemsThe well-defined geometry, exceptional mechanical properties, and

extraordinary electric characteristics, among other outstanding

physical properties of CNTs7 (see Table 1) qualify them for potential

applications8 in nanoelectronic circuits, nanoelectromechanical

systems (NEMS), and nanorobotic systems. For nanorobotics, some

of the most important characteristics of nanotubes include their

nanometer diameter7,9, large aspect ratio (10–1000), terapascal-

scale Young’s modulus10–17, excellent elasticity18,19, ultrasmall

interlayer friction20–22, excellent field-emission properties23,24, various

electric conductivities25–27, high thermal conductivity28–30, high

current carrying capability with essentially no heating, sensitivity of

conductance to various physical or chemical changes31–33, and charge-

induced bond-length change34. CNTs can serve in nanorobotic systems

as structural elements, tools, sensors, and actuators. As shown in Fig. 1,

starting from as-grown CNTs, nanotubes can be assembled into more

complex structures using bottom-up approaches or engineered to

achieve secondary building blocks using top-down approaches.

Fig. 1 CNT-based building blocks. Starting from (a) as-grown CNTs, nanostructures can be created by the bottom-up approaches of (b–d) assembling, (e) filling, or (f) decorating them, or in a top-down fashion by (g–i) engineering their shells/caps.

Property Item Data Potential applications in nanorobotics

Geometrical

Layers Single/multiple

Structures, probes, grippers/tweezers, scissors

Aspect ratio 10–1000

Diameter ~0.4 nm to >3 nm (SWNTs)

~1.4 nm to >100 nm (MWNTs)

Length Several micrometers (rope up to centimeters)

Mechanical

Young’s modulus ~1 TPa (steel: 0.2 TPa)

Tensile strength 45 GPa (steel: 2 GPa)

Density ~1.33–1.4 g/cm3 (Al: 2.7 g/cm3)

Interlayer friction Ultrasmall Actuators, bearings, syringes, switches, memories

Electronic

Conductivity Metallic/semiconducting Diodes, transistors, switches, logic gates

Current carrying capacity ~1 TA/cm3 (Cu: 1 GA/cm3) Wires/cables

Field emission Activate phosphorus at ~1–3 V Proximity/position sensors

Electromechanical Piezoresistivity Positive/negative Deformation/displacement sensors

Thermal Heat transmission >3 kW/mK (Diamond: 2 kW/mK) Circuits, sensors, thermal actuators

Table 1 Properties of CNTs.

(b)

(a)

(c) (d)

(e)

(h)(g)

(f)

(i)

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REVIEW Carbon nanotubes for nanorobotics

DECEMBER 2007 | VOLUME 2 | NUMBER 614

As-grown CNTs can serve directly as functional elements of

nanodevices. The first example of such a device – a nanotube probe

for an atomic force microscope (AFM) – was demonstrated by Dai et

al.35 for improving the spatial resolution of an AFM and protecting

the tip from ‘tip crash’. In the device, a multiwalled CNT (MWNT)

was manually assembled onto a commercially available Si cantilever.

Further developments improved the construction technique through

direct chemical vapor deposition (CVD)36, controlled assembly16, and

picking up a tube from vertically aligned single-walled CNTs (SWNTs)

grown from planar substrate surfaces37. Nanotube tweezers have been

constructed with two nanotubes on a glass fiber and driven by the

electrostatic interaction between them38.

By assembling as-grown CNTs, more complex structures can

be built. Nanotube intermolecular and intramolecular junctions

are basic elements for such structures. For nanoelectronics, pure

nanotube circuits39–41 created by interconnecting nanotubes of

different diameters and chirality may lead to the next generation of

electronics42–46. Suspended junctions can function as electromechanical

nonvolatile memory47.

Interlayer motion between individual carbon shells, which are

cylindrically nested and suspended by van der Waals forces within

a MWNT, provide exceptional performance as linear and rotary

nanobearings48 with an inherent position feedback capability20–22, and

potentially as gigahertz resonators/oscillators/shuttles49–51, tubular

switches52–54, memories55,56, syringes, linear nano-servomotors with

integrated position sensing57, and rotational elements for use in

NEMS58,59. Apart from their favorable nanomechanical performance,

the variation of CNT resistance with telescoping core motion60

provides a unique electrical mechanism to sense and control their

operation. While prototype devices have proven some of the concepts

previously, the capability to alter the as-synthesized closed-cap

shell structure of MWNTs controllably to form intershell-motion-

based devices in parallel is a significant challenge for their eventual

manufacturability and commercialization.

With their hollow cores and large aspect ratios61,62, CNTs are

possible conduits for nanoscale amounts of various materials. A variety

of materials have been encapsulated by CNTs, such as metals and their

compounds63–65, water61, and fullerenes66. Applications of devices

as templates67, thermometers68, and nano test tubes69 have been

presented. The possibility of delivering encapsulated materials from

carbon shells70 is of great interest because of potential applications as

atomic sources for nanoprototyping, nanoassembly, and injection.

Nanoassembly of CNTsRandom spreading, direct growth71, fluidic self-assembly47, transfer

printing72,73, and dielectrophoretic (DEP) assembly74 have been

used for positioning as-grown nanotubes or other nanostructures

on electrodes for the construction of electronic devices, or NEMS

generally, in some type of regular array.

We have achieved controlled deposition of different nanoscale

forms of carbon on Si chips using DEP-driven, bottom-up assembly

techniques. This type of assembly is compatible with conventional

top-down Si micro- and nanomachining techniques, and hence offers

a powerful tool for batch manufacturing of next-generation NEMS.

Specifically, we have demonstrated the suitability of this technique for

assembling MWNTs, double-walled CNTs (DWNTs), Cu-filled CNTs75,

and CNT coils (Fig. 2).

The process used to build the nanostructure array is as follows.

First, the bottom nanoelectrode layer (15 nm Cr/45 nm Au) is defined

on a Si substrate, which is covered with a 500 nm insulating oxide.

The nanoelectrode layer is formed by defining patterns in a bilayer

resist (poly(methyl methacrylate)/poly(methyl 2-acetamidoacrylate),

Fig. 2 Hybrid nanofabrication approach for realizing integrated nanosystems. (a) Nanoarray design. (b) MWNT nanosystem. Insets: schematic and lower magnification scanning electron micrograph (SEM). (c) Direct assembly of sophisticated nanostructures. (d–f) Diverse nanomaterials such as DWNTs, Cu-filled CNTs, and nanocoils.

(b)(a) (c)

(d) (e) (f)

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or PMMA/PMAA) using electron-beam lithography, followed by metal

deposition and lift-off. The electrodes are 300 nm wide and are

separated by 350 nm gaps. Next, carbon-based nanomaterials are

deposited onto the electrodes by coupled ac-dc DEP76,77. For this step,

the nanomaterials are suspended and sonicated in ethanol to insure

homogeneity. The chip is then immersed in a reservoir containing the

suspension and a composite ac-dc electric field is applied with a high

frequency function generator. After about 100 s, the chip is removed

from the reservoir and rinsed in clean ethanol. Finally, it is blown dry

with a nitrogen gun.

This assembly process realizes structures that aid in investigating

and characterizing the electrical and mechanical properties of these

novel nanomaterials. More importantly, the capability of assembling

disparate nanomaterials using the same technique would enable their

integration into complex nanostructures in realizing integrated NEMS

and other nanosystems.

Nanomanipulation is a promising approach for complex

nanoassembly78. Key techniques enable the control of the position

and orientation of the building blocks with nanometer resolution

combined with their connection. Nanorobotic assembly allows for

the construction of more complex structures into prototype NEMS.

Nanotube intermolecular and intramolecular junctions are basic

elements for such assemblies. Although some types of junctions have

been synthesized with chemical methods, there is no evidence yet that

a self-assembly-based approach can provide more complex structures.

In Fig. 3, we show some examples of the nanorobotic assembly

of CNT junctions by emphasizing the connection methods. CNT

junctions have been created using van der Waals forces (Fig. 3a),

electron-beam-induced deposition (EBID)79 (Fig. 3b), bonding through

mechanochemistry (Fig. 3c), and spot welding via Cu encapsulated

inside CNTs (Fig. 3d).

CNT junctions connected by EBID yield stronger junctions than

those connected through van der Waals forces, as shown in Figs. 3a

and 3b. The development of conventional EBID has been limited by

the expensive electron filament used and low productivity. We have

demonstrated a parallel EBID system using CNTs as electron emitters.

To construct stronger junctions without adding extra material,

mechanochemical nanorobotic assembly is an important strategy.

This approach is based on solid-phase chemical reactions, or

mechanosynthesis, which is defined as chemical synthesis controlled

by mechanical systems operating with atomic-scale precision. The

technique enables direct positional selection of reaction sites3. By

picking up atoms with dangling bonds rather than stable atoms, it

is easier to form primary bonds, which provides a simple but strong

connection. Destructive fabrication provides a way to form dangling

bonds at the ends of broken tubes. Some of the dangling bonds may

then bond with neighboring atoms, but generally a few bonds will

remain reactive. A nanotube with dangling bonds at its end will bind

more easily to another to form intramolecular junctions. Fig. 3c shows

such a junction78.

EBID involves high-energy electron beams and needs external

precursors for obtaining conductive deposits, which limits its

applications. Mechanochemical bonding is promising, but not yet

mature. Recently, we have developed a nanorobotic spot welding

technique80 using Cu-filled CNTs for welding nanotubes. The solder is

encapsulated inside the hollow cores of CNTs during their synthesis,

so no external precursors are needed. A bias of just a few volts can

induce the migration of the Cu, making it a cost-effective approach.

Fig. 3d shows a junction welded using this technique. The quality of the

weld is partly determined by the ability to control the mass flow rate

from the tube. An ultrahigh precision deposition of 120 ag/s, has been

realized in our experiment based on electromigration.

Nanorobotic manipulation in three dimensions has opened a

new route for nanoassembly. However, nanomanipulation is still

performed in a serial manner with master-slave control, which is

certainly not a large-scale production technique. Nevertheless, with

advances in the exploration of mesoscopic physics, better control of

the material synthesis, more accurate actuators, and effective tools for

manipulation, high-speed, parallel, and automatic nanoassembly will be

possible.

Shell engineering of CNTsOpen-ended CNTs have been created by removing the usually

capped ends of MWNTs with acid etching64, saturated current81,

electronic pulse48, or mechanical strain16,82, thus providing access

to the inner core of the nanotube cylinders. Acid etching is effective

for opening nanotube caps but does not expose the inner layers in

a controlled way. Controlled fabrication with saturated current is

potentially a large-scale manufacturing method, whereas electric pulse

and mechanical strain are convenient in situ processes. Mechanical

Fig. 3 CNT junctions. (a) CNTs connected by van der Waals interactions. (b) CNTs joined by EBID. (c) CNTs bonded by a mechanochemical reaction. (d) CNTs welded using Cu.

(b)(a)

(c) (d)

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pulling82 can be used to fabricate telescoping CNTs using nanorobotic

manipulation techniques78. Typical examples of bridged, cantilevered,

and center-supported (with one open end) telescoping MWNTs are

shown in Fig. 4.

Fig. 4a shows a bridged, telescoping MWNT with its left end fixed

on a substrate and its right end fixed to an AFM cantilever. The thin

neck of the bridged MWNT is formed by mechanical pulling, i.e. by

moving the cantilever to the right to break the outer layer(s) of the

MWNT and expose the inner ones, as shown in the inset. Fig. 4b

shows a telescoping MWNT formed by a similar process but the

cantilever is moved until the core is left completely exposed. Similarly,

if the nanotube is not fixed at the end but at the center of the

cantilever, stress will occur only on a partial section. Fig. 4c shows a

center-supported, telescoping MWNT being formed in this way. One

end remains capped, but can be opened if the process shown in Fig. 4b

is repeated.

The primary requirement for simultaneously engineering an array

of CNTs with control over length, location, and number of shells,

while applying a common electrical bias, is that the MWNT-based

nanostructures are created with nearly identical electrical circuit

resistance and heat-transport conditions. This causes uniform Joule

heating across the array and etches all the nanotubes in parallel

to fabricate bearings. In addition, the nanostructure lends itself to

shell-selective alteration of the MWNT geometry. A schematic of

such a nanostructure and the typical array design are shown in Figs.

5a and 5b. The nanostructure consists of a MWNT bridging two

nanoelectrodes at its distal ends while remaining fully suspended and

flat in the region between them. The CNT is sandwiched between

two layers of metal at each end to improve electrical contact. With

this architecture, the metallic contacts serve as heat sinks, and shell

removal as a result of thermal stress occurs in the suspended segment

(Fig. 5a). Also, the nature of contact with lithographically patterned

electrodes ensures that etching proceeds from the outermost shells

inwards.

Robust bearing operation with low friction requires that at least

5–10 outer shells are etched in each CNT in the array59. We find that

the threshold bias for removing these shells in an array is spread over a

300–600 mV range (from VTH-low to VTH-high), which we refer to as the Fig. 4 (a) Bridged, (b) cantilevered, and (c) center-supported telescoping MWNTs.

Fig. 5 Electrically controlled shell engineering of MWNT arrays. (a) Schematic of the MWNT nanostructure. Insets: SEMs of the nanostructure at high and low magnification. (b) Nanoelectrode array design. Inset: lower magnification image showing the entire array. (c) Illustration of an MWNT assembled by floating electrode DEP. The electrode wiring scheme for parallel shell structuring is also shown. Inset: SEM of the fabricated array at a 40° tilt. Scale bar is 50 μm. (d) SEM of a nanobearing shown at a 40° tilt. Inset: the degrees of freedom of the CNT shell structure. (e) MWNT assembled onto five metallic contacts and thinned in the regions between the contacts. The electrode wiring scheme for current-driven engineering is also shown. Inset: array design. (f) Schematic of high-density MWNT rotary motors and independent bearings that can be created by further nanomachining of this structure. (g) Telescoping segments formed with a 220 nm pitch and separated by ~6–10 nm gaps. The arrows point to the intersegment gaps. Image taken at a 40° tilt. (h) Schematic of the core-shell mechanisms formed in (e) with the intersegment gaps exaggerated to reveal the shell structure.

(b)

(a)

(c)

(d)(b)(a) (c)

(e) (f) (g) (h)

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‘array breakdown spectrum’. Two factors contribute to this spread in

threshold bias. Within a CNT, each shell has a characteristic breakdown

bias that increases from the outer to inner shells83. In addition, the

breakdown bias varies from one nanotube to the other over a small

range as a result of possible differences in contact conditions and CNT

properties such as diameter or chirality.

Since applying a voltage that is much higher than the array

breakdown spectrum causes the destruction of all CNT shells in a single

step, precise determination of the lowest voltage where breakdown

is initiated at one or more CNTs, VTH-low, is important. Some efforts

have reported that shell breakdown in single CNTs occurs at the

current saturation limit associated with backscattering by optical

phonons. Hence, the threshold voltage has been determined by

measuring the current-voltage, I-V, profile of the MWNT84. However,

we have observed such saturation in only two cases in more than

25 experiments involving individual CNTs, CNT arrays, and parallel

transport through multiple segments of single CNTs. This absence

of saturation has also been reported previously85,86, where it was

attributed to a shift in the saturation bias beyond the breakdown

threshold in CNTs with large diameters and short lengths. In arrays, it

also cannot be ruled out that the onset of saturation in some CNTs is

masked by others in the I-V plots.

In the absence of saturation, the only way to determine VTH-low

precisely involves starting with a very low bias (~1.5 V) and

progressively increasing the bias in successive cycles with small

increments until breakdown is initiated. After the initiation of

breakdown, the electric stress is repeatedly applied within the

breakdown spectrum to structure all the CNTs. With more than 20

biasing cycles typically required for structuring arrays, the duration

and voltage profile used for each of these biasing cycles impacts on

process metrics such as yield and cycle time. We find that applying

the breakdown voltages for short times, of the order of tens of

milliseconds, accelerates the shell structuring process without

compromising its controllability. This compares favorably with the

continuous application of the breakdown bias over longer cycle

durations85–87, where we find that the time intervals between shell

removal are on the order of hundreds of seconds.

The voltage profile used within the short duration biasing cycles has

a direct influence on the process yield. A profile that sweeps voltages

from 0 to Vmax provides higher yields than approaches that start with

a high bias, such as constant bias pulses or sweeps from –Vmax to Vmax.

This is demonstrated using results from two CNT arrays, labeled ‘A1’

and ‘A2’74. In array A1, electric stress is applied by sweeping voltages

from –Vmax to +Vmax in 10 mV increments and 100 μs time steps. The

biasing procedure for array A2 is identical to that of A1 except that

voltage sweeps from 0 to Vmax are used. In array A1, 12 CNTs formed

structures suitable for controlled shell engineering. Of these nanotubes,

we were able to form eight bearings (67%). In array A2, eight of the

nine nanotubes formed partly thinned bearings (89%).

We are also able to realize piecewise and parallel modification of

shell structures at different segments along the length of an MWNT.

This is a powerful tool for realizing bearings with more complex

architectures and for forming multiple devices on a single CNT, leading

to ultrahigh-density NEMS. We achieve this with suitable electrode

designs for DEP assembly and current-driven shell etching steps.

A nanotube bridging three electrodes is shown in Fig. 5c, and a SEM

of the array is shown in the inset. By applying a common potential to

the electrodes at distal ends and grounding the central metal contact

(Fig. 5d), we can simultaneously drive currents through both suspended

segments of the CNT and remove its outer walls. A nanotube

engineered using this technique has an architecture that is different

from those illustrated in Fig. 5a. One such CNT and its shell structure,

with possible degrees of freedom, is shown in Fig. 5d. We can realize

higher bearing densities by assembling CNTs onto a larger number of

spatially separated electrodes with appropriate designs. Fig. 5e

shows independent CNT bearings with a 220 nm pitch created at

different locations along the length of a single MWNT bridging five

electrodes. Fig. 5f illustrates how the alternate metal contacts are held

at the same potential to drive currents simultaneously through every

suspended segment and etch outer shells in parallel. The nanostructure

array is shown in the inset. If we break all shells in one step, instead of

partial thinning, we can create five telescoping segments with a

220 nm pitch, separated by ~6–15 nm gaps (Fig. 5g). Each of these

CNT segments is anchored to the metal by only its outer shells. Hence,

we now have bidirectional linear bearings with inner shells capable of

sliding inside the outer housing. A schematic of the nanostructure, with

the intersegment gaps exaggerated to reveal the shell architecture, is

shown in Fig. 5h.

This is an interesting structure with a number of potential

applications. With extremely small lengths (about 210 nm) for the

three segments in the middle, these should exhibit lower friction forces

and faster response times during telescoping core movements than

previously reported53. With the core free to slide in either direction,

the segments can be used as oscillators where neighboring nanotube

segments electrically excite oscillations. We estimate the oscillation

frequency of the three telescoping segments in Fig. 5g to be 1.28 GHz

by supposing an initial 5 nm extrusion using the model derived by

Zheng et al.50,88.

Because shell removal is driven by Joule heating, large structural

defects such as kinks or holes86 can affect the location of shell removal.

However, current transport through MWNTs at length scales of the

order of 100 nm is diffusive85,86, and shell removal is centered on the

midpoint of suspended nanotubes when defects are only minor

(Fig. 5a).

We find that we can controllably alter this midlength electric

breakdown in MWNTs and restrict it to occur predominantly over one

half of the CNT by introducing additional, spatially separated metallic

layers in contact with the current-carrying nanotube89. Fig. 6a

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illustrates a nanotube in contact with two additional metallic regions

that are situated between the biasing electrodes. The additional

metallic regions are held at a floating potential and serve as heat sinks

to remove the Joule heat generated in CNTs. One of the heat sinks is

comprised of two layers of metal, which sandwich the nanotube, and

has a contact area that is greater than at least half of the nanotube’s

surface area. The other heat sink has only one layer of metal

underneath the CNT and, because of the cylindrical CNT surface, makes

only a line contact with the CNT on top of it.

Since the thermal contact resistance and removal of Joule heat

generated in the CNTs through these metallic contacts is dependant

on the contact area, we find that heat dissipation is higher at the

two-layer heat sink than the single-layer heat sink. This results in a

nonuniform temperature profile along the CNT length, with higher

temperatures in the CNT half that is in contact with the single-layer

heat sink. A direct consequence of such a temperature distribution

is that the Joule-heating-induced electric breakdown occurs

predominantly in this half of the CNT. SEM images of nanotubes where

such location-controlled thinning occurs are shown in Figs. 6b–f. In

these images, substantial shell removal occurs in the region between

the red arrowheads. It can be seen that the region of thinning is

confined to only the nanotube half that is in contact with the single-

layer heat sink. Finite-element simulations of the temperature

distribution created in a current-carrying MWNT have confirmed this89.

Electric breakdown can also cause the shrinkage of individual shells

at higher temperatures90,91, generating sharpened tips. Stretching

CNTs under Joule heating and electron-beam radiation has revealed

an even more interesting phenomenon. Huang et al. have observed

superplasticity in individual single-92, double-, and triple-walled CNTs93.

At temperatures above 2000ºC, tensile elongation of SWNTs and

MWNTs can reach up to 280% and 190% with diameter reductions

of 15 times and 90%, respectively. Ding et al.94,95 describe these

observations of plastic relaxation in terms of dislocation theory and

atomistic computer simulations. This phenomenon provides a self-

repairing mechanism to maintain tube perfection, which is particularly

interesting for shell engineering.

Mass delivery from CNTsRecently, novel CNTs filled with single-crystalline Cu nanoneedles have

been synthesized by a thermal CVD method using alkali-modified Cu

catalysts96. Because Cu is a good conductor of heat and electricity

and has a very low binding energy (0.1–0.144 eV/atom) when bound

to carbon, encapsulated Cu inside nanotubes is ideal for many

potential applications. We have investigated the controlled melting

and flow of single-crystalline Cu from CNTs assisted by nanorobotic

manipulation80.

Fig. 7a is a series of time-resolved transmission electron

micrographs (TEMs) taken from video frames showing the flow process.

The Cu core starts to flow inside the carbon shell from the bottom to

the tip of the first bamboo section as the bias voltage reaches 2.5 V.

The entire process continues for about 70 s. The flow rate is 11.6 nm/s

according to the apparent change of length of the Cu core (Fig. 7b).

Accordingly, we have calculated the mass change as shown in Fig. 7c,

and the mass flow rate is determined by fitting the data to the curve.

This yields ~120 ag/s, which is strikingly slow and controllable, and

allows precise delivery of mass at the attogram scale since time-based

control can readily reach sub-second precision.

Fig. 7d shows time-resolved current versus voltage characteristics

under a constant positive bias of 2.5 V. The current density under 2.5 V

bias when flow occurs is calculated according to the cross-sectional

area as 2.60 × 106–3.07 × 106 A/cm2. This is comparable to the

observed value for electromigration of Fe in CNTs (~7 × 106 A/cm2)70.

The difference may be a result of the lower binding energy of Cu

(0.1–0.144 eV/atom) than that of Fe to carbon shells (0.3 eV/atom)97.

The high current densities employed here will lead to resistive

heating. Temperatures as high as 2000–3000°C have been estimated

according to the change in lattice spacing in electric breakdown

experiments on nonfilled MWNTs86 at a slightly higher bias (3 V)

Fig. 6 Shell engineering. (a) Illustration of an MWNT nanostructure with two additional heat sinks and its electrode wiring scheme. (b–d) SEMs of nanostructures, of the type shown in panel (c), after electric breakdown.Thinning is predominantly confined to the CNT half on the side containing the single-layer heat sink. Images taken at a stage tilt of 40°. Insets in (b),(c): top view of the nanostructures. Inset in (d): top view of the heat-sink region at a higher magnification clearly showing the changes in CNT diameter. (e),(f) SEMs of CNTs showing localized thinning. (g) Typical nanoelectrode array design.

(d)

(b)(a)

(c)

(e)

(f) (g)

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than those used here. We have then correlated the current density, J,

and the mass flow rate, dm/dt, as shown in Fig. 7e. The relation

dm/dt =0.3J2 – 1.6J + 2.1 suggests that a real positive value of dm/dt

(≥43 ag/s) can only be given when the current density J surpasses

2.6 × 106 A/cm2. The existence of this threshold also implies the flow

mechanism is likely to be electromigration70. Under a negative bias,

i.e. when the W probe serves as a cathode, we observe flow in the

opposite direction.

Other possible mechanisms for flow can be excluded. Capillary force

can induce filling/flowing, but the direction should be opposite to the

observed flow, i.e. from the tip to the bottom of the carbon shells.

Thermal expansion can enable flow, but the flow should be isotropic

heading toward both the tip and the bottom of the carbon shells.

A recent investigation has shown that the irradiation of MWNTs can

cause a large pressure buildup within the nanotube core, which can

plastically deform, extrude, and break encapsulated solid material98. In

our experiments, however, no contraction of the carbon shells has been

observed.

Compared with other mass-delivery processes previously

investigated, electrically driven delivery has several interesting aspects:

1. A very low current can induce melting and drive the flow, which is

much more efficient than irradiation-based techniques involving

high-energy electron beams79,98–100, focused ion beams (FIBs)101,

or lasers62. Combined with DEP assembly, it is possible to solder the

tubes onto electrodes for batch fabrication of NEMS.

2. The melting occurs rapidly (at least at the millisecond level), which

is several orders of magnitude faster than using a high-intensity

electron beam or FIB (generally on the order of a minute79,98–101).

3. Because both the rate and direction of mass transport depends

on the external electrical drive, precise control of ultrasmall mass

delivery is possible. Time-based control will allow the delivery of

attograms of mass102.

4. Cu is compatible with conventional semiconductor processing. Our

experiments show that it will also play an important role for scaled-

down systems. Carbon shells provide an effective barrier against

oxidation and consequently ensure long-term stability of the Cu

Fig. 7 Attogram-precision mass delivery for nanorobotic spot welding. (a) Time-resolved TEMs from video frames showing the flow process. The Cu core starts to flow inside the carbon shells from the root to the tip as the bias voltage reaches up to 2.5 V. The process continues for about 70 s. (b) The flow rate is 11.6 nm/s according to the apparent change of length of the Cu core. The W probe is positively biased. (c) Plot of mass change versus time. The mass flow rate from the fitted curve is ~0.12 fg/s. (d) Plot of time-resolved current against voltage characteristics under a constant bias of 2.5 V. The current density under 2.5 V bias as flow occurs is ~2.60 × 106–3.07 × 106 A/cm2. (e) Correlation of the current density, J, and the mass flow rate, dm/dt.

(b)

(a)

(c)

(d) (e)

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core, which also facilitates conservation of the material unlike

conveying mass on the external surface of nanotubes102.

NEMSThe next step along the road to fabricating nanorobots is to fabricate

simpler NEMS. NEMS make it possible to manipulate nanosized objects

with nanosized tools, measure mass in femtogram ranges, sense force

at piconewton scales, and induce gigahertz motion.

As examples, configurations and mechanical models of linear

nanomotors based on bridged, cantilevered, and centrally supported

telescoping MWNTs, and laterally actuated MWNTs are shown in

Fig. 8. Electrostatic force, van der Waals interaction, and the total

intershell sliding resistance force are denoted by Fe, FvdW, and Fr,

respectively.

The motion of the core section can be controlled by the

electrostatic force. Atomic-scale mechanisms, such as interatomic

locking, provide resistance to sliding of the core in the outer tube,

but experiments indicate that the intershell-sliding resistance force

between two neighboring shells of perfect, or nearly perfect, molecular

structure is substantially smaller than the van der Waals restoring

force. The retraction of an extruded core of a MWNT into the outer

shells has been observed experimentally20. The restoring force resulting

from excess van der Waals interaction energies from core extrusion, it

has been realized, drives the core to oscillate with respect to its fully

retracted position because of the small intershell sliding resistance

force. The oscillation frequency can be in the gigahertz range50,88.

The system configuration for a telescoping nanotube with field-

emission position feedback is shown in Fig. 8e. An opened MWNT

is fixed onto an AFM cantilever (acting as the cathode) by EBID on

the right-hand end of the structure. The tube is placed against a

substrate serving as an anode, where G is the interelectrode distance

between the substrate and the AFM cantilever. The protruding length

of the nanotube, l, will change by Δl from its initial length, l0, as the

electrostatic force between the core and the counter-electrode exceeds

the sum of the interlayer friction and van der Waals forces between the

core and the cathode. Hence, the gap between the nanotube tip and

the anode, g, will change whereas G remains constant. Field emission

is measured using the configuration shown in Fig. 8e. Fig. 8f shows a

typical I-V curve of a telescoping nanotube (see the inset micrograph

of Fig. 8f) when G = 1000 nm. Each point represents an average of 100

samples within a 9 s interval. The inset shows the change of emission

current with time at a constant bias of 120 V. An obvious feature of

this I-V curve that differs from conventional ones is the ‘kink’ observed

between 115 V and 135 V. Based on the parallel shell engineering, it is

possible to batch fabricate such devices.

SummaryWe have reviewed technological progress on CNTs related to

nanorobotics. The assembly of large arrays of carbon nanomaterials,

such as MWNTs, DWNTs, Cu-filled CNTs, and CNT coils, onto

nanoelectrodes using DEP of individual nanotubes has been

described. Nanorobotic assembly has been shown to be effective for

interconnecting CNTs. Mechanical strain and electric breakdown have

been demonstrated in shell engineering of CNTs. Site-selective shell

engineering has been realized using electric breakdown with heat-

dissipation modulation using nanomachined heat sinks. Controlled mass

delivery of Cu inside nanotube shells has been realized by applying a

low bias voltage. The mass flow rate has been determined to be

120 ag/s. Nanoscale linear servomotors with integrated position

sensing and other NEMS have been investigated. Whereas these

structures and devices can individually serve as building blocks for

structures, tools, sensors, and actuators, many of them may also be

fundamental components for building nanorobots in the future.

As an emerging field, nanorobotics faces many challenges because

of the extreme scaling effects that must be considered. Problems we

now see, such as how to build more complex systems with smaller

building blocks, calibrate ultrasensitive sensors, and control actuators

with dynamics far faster than the rate of control feedback, are just

the beginning. While the future remains unclear, we can be certain

that nanorobotics is steadily progressing toward the construction

of structures, tools, sensors, actuators, and systems that will extend

our ability to explore the nanoworld from perception, cognition, and

manipulation perspectives.

Fig. 8 Interlayer-motion-based nanodevices. (a) Bridged MWNT prismatic nanomotor. (b) Cantilevered MWNT prismatic nanomotor. (c) Centrally supported MWNT linear nanomotor. (d) Laterally actuated linear nanomotor. (e) Prismatic nanomotor with integrated field-emission position sensing. (f) Typical I-V curve of a telescoping nanotube where the interelectrode gap G is 1000 nm. Inset: change of emission current with time at 120 V.

(b)(a)

(c) (d)

(e) (e)

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