nanoscale transport on the tube: nanotechnology
TRANSCRIPT
RESEARCH NEWS
July/August 200410
Researchers at the University of
California at Berkeley and the Lawrence
Berkeley National Laboratory have
synthesized hollow nanocrystals
[Yin et al., Science (2004) 304, 711].
A. Paul Alivisatos and coworkers find
that hollow nanocrystals of cobalt oxide
or chalcogenide form when cobalt
nanoparticles are reacted in solution
with oxygen and either sulfur or
selenium. The nanoscale pores arise
through a mechanism analogous to the
Kirkendall Effect. This effect,
discovered in 1947 by Smigelkas and
Kirkendall, explains how porosity arises
from differential solid-state diffusion
rates. Instead of the interchange of
atoms, this effect demonstrates that
atomic diffusion occurs through
vacancy exchange.
In the case of the Co nanoparticles,
voids begin to form at dislocations or
boundaries. The vacancies diffuse
inward, concentrating at the boundary
rather than the interior of the core.
Diffusion of Co atoms out to the shell
and growth of internal voids results in
the formation of ‘bridges’ between the
core and the shell. The bridges provide
a path for transport of Co atoms to
the outer shell, until the core is
completely consumed. Hollow, ~15 nm
cobalt sulfide nanoparticles were
produced at temperatures as low as
373 K.
Alivisatos and coworkers also
performed Co sulfidation to create
hollow disk-shaped nanoparticles, as
well as oxidation of Fe nanospheres,
and sulfidation of Cd nanospheres. The
process, therefore, could provide a
general route to the synthesis of hollow
nanostructures.
The uses of such structures are wide
ranging, including drug delivery,
nanoelectronics and optics, lightweight
structural materials, and catalysis.
Cordelia Sealy
Creating theperfect poreNANOTECHNOLOGY
Nanoscale transport on the tubeNANOTECHNOLOGY
Researchers at the University of Californiaat Berkeley and the Lawrence BerkeleyNational Laboratory have demonstrated thecontrolled transport of mass, almost atomby atom, along carbon nanotubes [Regan et al., Nature (2004) 428, 924]. The abilityto move such small amounts of materialcould enable new methods for the assemblyof nanoscale devices.Multiwalled carbon nanotubes (MWNTs) aredecorated with In nanocrystals. IndividualMWNTs are then contacted using a W tipmounted on a nanomanipulator and a voltageapplied between the tip and sample holder.The current heats up the system, meltingthe In nanocrystals. Further manipulation ofthe voltage results in the directed transport
of femtogram amounts of In between‘reservoirs’ of molten material. In real-timetransmission electron microscopy (asshown), In particles (on the left) can be seento shrink in size while others (on the right)grow. Reversing the current changes thedirection of mass transport. “We can moveIn almost atom by atom or by as much aspicograms a minute,” says Shaul Aloni. Massis conserved during the process. The diffusive transport of In atoms along theMWNT surface is driven by electromigration.“In atoms, when adsorbed on graphite,become positively charged,” explains Aloni,“so the current in the nanotube causes themto move, but in the opposite direction to theelectrons.” Electromigration on the surfaceof a nanotube has serious implications fornanoelectronics and device fabrication,especially in terms of reliability, he says.The phenomena does, however, open up thepossibility of using MWNTs as nanoscalesoldering irons to deliver controlled amountsof material to desired locations. “One coulduse this phenomenon to make devices. Weare currently trying to exploit this to buildnanoactuators and resonators,” Aloni toldMaterials Today.Jonathan Wood
Atomic whirlpool at the surface MECHANICAL BEHAVIOR/SURFACE SCIENCE
Dislocations terminating on the surface ofcrystalline solids act as sinks for atoms, leading tonanoscale patterning, according to new research atthe University of Illinois [Kodambaka et al., Nature(2004) 429, 49]. The spiral steps that Suneel K.Kodambaka and colleagues observe nucleating andgrowing around dislocation cores are similar tostructures seen during crystal growth or etching.Dislocations climb through solid materials, affectingthe stability of interfaces and nanostructures,mechanical properties, mass and charge transport,and temperature-dependent deformation. But it hasnot been possible to determine where the atomsproduced by the climbing process go, the amountof energy required for their movement, or theireffect on the surface morphology of the material.“This study, for the first time, provides directevidence of the fate of atoms resulting from the
climb of surface-terminated dislocations,” saysKodambaka. Using time-resolved low-energyelectron microscopy (LEEM), the researchersobserved spiral steps on the (111) surface of TiNannealed at 1600 K and 1750 K. The nucleationand growth of the spirals, which occur without anydeposition or evaporation, appear to be thermallydriven. The activation energy is different fromconventional Burton-Cabrera-Frank spiral growthand surface smoothing phenomena such as Ostwaldripening. “The spiral step dynamics strongly suggestthat the cores of surface-terminated dislocationsbehave like an atom ‘whirlpool’, sucking atoms intothe bulk,” says Kodambaka. He attributes theprocess to the migration of point defects betweenthe bulk and the surface along dislocations andexpects that it will be seen in other materials too.Cordelia Sealy
Left to right transport of In on a single MWNT. (Reprinted with
permission of Alex Zettl.)