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February 11, 2002
Volume 80, Number 6
CENEAR 80 6 p. 7
ISSN 0009-2347
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Next level of complexity expected in structures of alternating composition


What kind of designs could you make using a pair of cake decorators--say, one filled with strawberry frosting and the other with blueberry? Or given free reign over the chocolate and vanilla dispensers in a soft-serve ice-cream store, what types of patterns could you draw? One possibility is a long thin line of alternating flavors.

SANDWICH Microscopy methods reveal abrupt interfaces in an InAs/InP (green and orange, respectively) nanowire grown at Lund University.
Scientists have been honing their skills at similar decorating work. Except that the "art" supplies aren't edible--they're semiconductors--and the dimensions are measured in nanometers. Three research groups have just succeeded in preparing continuous nanoscale wires and related structures that are composed of segments of dissimilar materials. The work advances the field to a level of sophistication needed for making complex electronic devices out of nanometer-sized components.

Demonstrations in which semiconductor nanowires or carbon nanotubes are used as the working elements in electronic devices have been rapidly increasing in number and complexity. For example, field-effect transistors (FETs) and various types of sensors and detectors have been made from nanoscale building blocks. More sophisticated devices such as light-emitting diodes (LEDs) and logic circuits constructed from microscopic FETs have also been made.

Judging by today's commercial semiconductor technology, which relies on sandwiches of ultrathin semiconductor layers, nanowire-based devices featuring the next level of complexity and functionality can be expected soon, now that researchers have devised methods for fashioning nanowires in which the composition changes along a wire's length. These "heterostructured" nanowires may lead to resonant tunneling devices, ultrasmall quantum dot lasers, and other advanced equipment.

In one study, Harvard University chemists prepared nanowires of gallium arsenide and gallium phosphide containing up to 21 distinct segments--a nanoscale bar code. The research group, which includes chemistry professor Charles M. Lieber, graduate student Mark S. Gudiksen, postdoctoral associate Lincoln J. Lauhon, and coworkers, also prepared silicon nanowires and indium phosphide nanowires in which some sections of the wires were doped with negative charge carriers (n-doped) and other sections were doped with positive charge carriers (p-doped) [Nature, 415, 617 (2002)].

At Lund University in Sweden, scientists grew indium arsenide nanowires interspersed with segments of indium phosphide [Nano Letters, published Jan. 19 ASAP, http://pubs.acs.org/journals/nalefd]. Electron microscopy analysis shows that the interfaces between segments are characterized by an atomic-scale abruptness, and transport measurements reveal ideal electronic properties at the interfaces. The Lund group includes graduate students Mikael T. Björk and B. Jonas Ohlsson, materials chemistry professor L. Reine Wallenberg, and physics professor Lars Samuelson.

Meanwhile, at the University of California, Berkeley, Peidong Yang, an assistant professor of chemistry, and graduate students Yiying Wu and Rong Fan have synthesized nanowires consisting of silicon and silicon-germanium [Nano Letters, published Jan. 19 ASAP, http://pubs.acs.org/journals/nalefd].

All three research groups make use of synthesis procedures in which nanoclusters of metal--for example, gold--catalyze nanowire growth by serving as nucleation points in a vapor-liquid-solid growth process. The UC Berkeley team grows its two-tone nanowires in a process that combines pulsed-laser ablation and chemical vapor deposition (CVD). Basically, Yang and coworkers use silicon-containing gases to grow silicon nanowires in a CVD apparatus and periodically add germanium to the gas mixture by vaporizing a solid germanium target with a pulsed laser. The germanium supply is switched on and off with each laser pulse.

The Lund group makes use of molecular beam epitaxy methods to modulate the structure of its InAs/InP nanowires. Alternating segments are formed by rapidly switching on and off the supplies of precursor materials. The Harvard researchers use methods similar to the other two groups'.

In addition to miniaturizing existing devices, the recent advances can be expected to bring about "completely new and different applications," Samuelson notes. Examples include single-photon-on-demand devices for applications in quantum optics and quantum cryptography and terahertz-frequency photon emitters.

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