nanoscale transport on the tube: nanotechnology

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.)