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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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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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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DECEMBER 2007 | VOLUME 2 | NUMBER 620
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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DECEMBER 2007 | VOLUME 2 | NUMBER 6 21
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