 |
|
|
| |
|
|
|
 |
|
Carbon
Nanotube Electrodes |
|
|
|
| We have
constructed functionalized “nanoelectrodes” integrated on scanning
atomic force microscopy (AFM) probes. This is the result of a new method
we have developed for coating individual single-wall carbon nanotubes that
are attached to AFM tips using room temperature plasma-assisted decomposition
of fluorocarbon gases to deposit Teflon-like polymer coatings of controlled
composition and thickness on the nanotubes [Nano Lett. 2004, 4, 1873]. The
figure is an image from a transmission electron microscope of a 5 nm diameter
single walled carbon nanotube attached to a gold AFM tip and coated with
approximately 5 nm of the fluorocarbon polymer. The polymer provides a chemically
inert and electrically insulating outer layer and mechanically stabilizes
the attached nanotube sufficiently to enable imaging in both dry and wet
environments without the need for an intervening adhesive. Electrical pulse
etching of the insulating coating exclusively at the nanotube tip end results
in well-defined, highly conductive nanoelectrodes. For these probes, the
conductive properties of the nanotubes are not affected by the coating.
Some nanoelectrodes behave as rectifying diodes.
|
|
|
| |
|
 |
 |
| |
 |
| |
 |
| |
|
| |
|
| |
|
|
|
|
| We
intend to develop these probes into new classes of sensing and manipulation
tools, with implications for the investigation of intermolecular dynamics,
solid-state physical phenomena at the nanoscale, and the development of
molecular electronics. |
|
|
| |
|
|
Electrowetting
in Carbon Nanotubes |
 |
|
|
| |
We
also demonstrated reversible wetting and filling of open single-wall carbon
nanotubes with mercury by means of electrocapillary pressure originating
from the application of a potential across an individual nanotube in contact
with a mercury drop. Above a threshold voltage, mercury imbibes into the
SWNT core and is also transported along the outer sidewalls. Figure A to
the right shows a segment of a 150 nm long SWNT along the side of a gold-coated
AFM tip. The material with darker contrast in the interior of the nanotube
is entrapped mercury, which has formed a highly curved meniscus with a contact
angle of 150±5o, remarkably close to that of mercury on graphite.
Focusing the TEM electron beam on this material caused it to vanish but
left behind a faint trace of the curved interface (Fig. B, right).
Wetting improves the conductance in both metallic and
semiconducting nanotube probes by decreasing contact resistance and forming
a mercury nanowire inside the nanotube. Molecular dynamics simulations
corroborate the electrocapillarity-driven filling process and provide
estimates for the imbibition speed and electrocapillary pressure.
The figure below shows snapshots from a simulation of
a 15.7 nm long open (20,20) SWNT immersed in mercury. In the equilibrium
state without applied voltage, a non-wetting meniscus forms on the outside
and mercury does not penetrate the open unblocked end. For the same nanotube
1.5 ns after the application of 3.5 V, which is larger than the calculated
threshold of 2.5 V for electrowetting, mercury has filled the core and
wetted the outside walls. In fact, wetting begins immediately after the
potential is turned on and the liquid moves inside as a single front at
a speed of ~13 m/s, while a thin film spreads on the outer wall.
|
|
|
| Electrowetting
in carbon nanotubes may offer opportunities for studies of nanofluidic transport.
It can also be exploited for the formation of continuous nanowires crystallized
in one dimension from low melting point metals (e.g., Ga, In), enabling
the measurement of the intrinsic electrical/magnetic properties of encapsulated
nanowires. Such structures, attached to AFM tips, could serve as robust
nanoelectrode probes with increased current load capacity and enhanced imaging
capabilities. |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
