Creating Nanowires That Really Stack Up

2/3/2003 - 2-03w
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Technical Version:
Indium Arsenide Nanowires Spontaneously Organize in Three Dimensions

Scientists at the Naval Research Laboratory have discovered that the right combination of materials can spontaneously form a three-dimensional lattice of "wires" only a few nanometers across. (A nanometer is one billionth of a meter). The nanowires are composed of the compound semiconductor indium arsenide (InAs). They are created when a special crystal is grown containing alternating layers-only a few atoms thick- of InAs and a different semiconductor, gallium antimonide (GaSb).

The striking structure of the nanowire lattice, revealed atom-by-atom using an instrument called a scanning tunneling microscope (STM), was described in December in the scientific journal Applied Physics Letters.

According to the research team, Drs. Brett Nosho and Brian Bennett of the Electronics Science and Technology Division, and Dr. Lloyd Whitman of the Chemistry Division, "we discovered the unusual nanostructure while trying to create new infrared detectors made from a combination of InAs and GaSb." The detectors are under development to help defend the nation against a ballistic missile attack. "Usually," says Dr. Bennett, who led this portion of the study, "we try to make each alternating layer of the two materials as flat and distinct as possible."

The special hybrid crystals are grown by a method called molecular beam epitaxy (MBE). A beam of each type of atom is directed at the surface of a clean GaSb crystal, and shutters blocking each beam control which atoms reach the surface. The scientists discovered that if the arsenic beam is too strong when the InAs films are deposited, something unexpected happens. As described by Drs. Nosho and Whitman, experts in STM, "Instead of making distinct layers, like alternating slices of ham and cheese in a sandwich, each InAs layer spontaneously congealed into long, parallel rows of ribbon-like wires." Each one is about 10 nanometers high, 120 nanometers wide, and many micrometers long.

"The InAs nanowires also stacked up in an interesting way," continue the scientists. When this type of "self-assembly" has been seen before in other materials, they stack up one-atop-the-other. But in this case, "the nanowires stack up like logs, with the next layer of wires aligned between those below."

The scientist could "see" this structure by breaking open the crystal and examining the edge with the powerful STM. "Like slicing through your sandwich to see if the mustard has mixed with the mayonnaise," explains Dr. Whitman. The STM works by scanning a very sharp metal tip less then one nanometer above a surface. Even though the tip does not touch the surface, it is so close that a very small electrical current can cross by a process called quantum mechanical tunneling. By sensing this tiny current, the STM "feels" where each atom is on the surface, and precise images of the individual atoms on a crystal surface can be reconstructed.

The scientists do not yet know whether their nanowire lattice will prove useful, but they suspect the material will have interesting optical and electrical properties. The hard work of looking for those properties has already begun.

X-STM is performed by cleaving a super-lattice sample in situ and imaging the structure in cross-section along the edge of the sample. The three-dimensional structure of the InAs nanowire lattice. (a) Atomic force microscopy image of the growth surface. (b) Illustration of the structure. (c) and (d) X-STM images of the two orthogonal cleavage surfaces. The InAs are the darker layers.
High resolution .tif file High resolution .tif file

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