In a development that brings the promise of mass production to nanoscale
devices a bit closer, Lawrence Berkeley National Laboratory scientists
have used carbon nanotubes to ferry atoms over microscopic distances.
In their demonstration experiment, which was performed inside a transmission
electron microscope, they used small electrical currents applied to a
carbon nanotube to move indium atoms along the tube from one end to the
other. This work marks another step towards the high-throughput construction
of atomic-scale optical, electronic, and mechanical devices that will
fulfill the promise of nanotechnology.
Developing
methods that can move atoms and nanosized particles to precise locations
has been an ongoing challenge for nanotechnology. Single atom manipulation
was demonstrated as early as 1990 when IBM researchers spelled out
the company logo by positioning 35 xenon atoms with a scanning tunneling
microscope (STM). However, as fabrication tools, both STM and the
related atomic force microscopes (AFM) suffer from a loading deficiency:
although they can manipulate atoms already present, they cannot efficiently
deliver atoms to the work area. Carbon nanotubes, on the other hand,
with their hollow cores and large aspect ratios, are natural candidates
for conduits for nanoscale materials. In this case, the challenge
is in developing methods to achieve controllable, reversible atomic
scale mass transport along the nanotube walls.
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![](conveyor_fig.jpg)
The work here
took advantage of electromigration, a phenomenon of critical importance
to the semiconductor industry. The team thermally evaporated the species
to be transported, indium metal, onto a bundle of carbon nanotubes
at levels so small that it populated the tubes’ surfaces as isolated
indium nanocrystals. The bundle was then placed inside a transmission
electron microscope, where one end of the nanotubes was attached, via
conducting adhesive, to a metal holder and a tungsten tip mounted on
the end of a nanomanipulator made contact with the other end. Voltage
was applied across the tube sending an electrical current through it.
This generated thermal energy that heated and melted the indium particles.
By carefully controlling the electrical current, indium atoms could
be made to move from one indium particle to another further down the
tube. In the example shown in the series of images in the figure, indium
particles shrink and then disappear at the left, while particles to
the right grow. Several seconds later, the newly enlarged particles
also disappear, replaced by others even further to the right, as would
be expected if the indium atoms are indeed moving from left to right.
It was demonstrated that thermally activated, electrically driven indium atoms
could move substantial distances along the nanotube, until all of them pile
up at the end of the nanotubes as a single crystal. Moreover, reversal of the
voltage polarity drives the particles back to their starting point. Repeated
transits back and forth across the tube conserve indium atoms (the indium does
not evaporate), a requirement if the process is to be used to deliver valuable
material to a worksite.
With this successful first step, one might imagine arrays of nano-sized conveyor
belts delivering mass to specific locations atom-by-atom or picking up material
at one site and delivering it to another. In this sense, the work is a demonstration
of a prototype of might be developed into a formidable nanoassembly tool.
A. Zettl (510) 642-4939, Materials Sciences Division (510 486-4755), Berkeley
Lab.
Chris
Regan, Shaul Aloni, Robert Ritchie, Ulrich Dahmen, and Alex Zettl, "Carbon
Nanotubes as Nanoscale Mass Conveyors," Nature 428, 924 (2004).
Some of the equipment used in these studies was purchased with funds provided
by the National Science Foundation.
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