Manufacturing Bits: Feb. 26

Vitamin C chips
Using vitamin C, Rice University has developed a process that turns gold nanorods into small gold nanowires.

Nanorods are a type of structure, while nanowires are simply tiny wires. With the technology, Rice is able to produce nanowires with various lengths. These can be used in electronics as well as light-manipulating applications like plasmons. A “plasmon is a quantum of plasma oscillation,” according to Wikipedia.

Researchers from Rice have developed a reversible wet chemical process, which enables tip-selective, one-dimensional growth of gold nanorods into nanowires.

The first step is to develop gold nanorods, which are about 25nm thick. Then, the nanoroads undergo an oxidation process with gold and a surfactant material. In the next step, the nanorods undergo a process of reducing the gold ions on the surface with ascorbic acid. Ascorbic acid or vitamin C serves as a reduction agent.

The process takes an hour, enabling the growth of a micron-long nanowire. Researchers devised structures up to 4 to 5 microns in length.

“There’s no novelty per se in using vitamin C to make gold nanostructures because there are many previous examples,” said Eugene Zubarev, a lab chemist at Rice. “But the slow and controlled reduction achieved by vitamin C is surprisingly suitable for this type of chemistry in producing extra-long nanowires.

“The most valuable feature is that it is truly one-dimensional elongation of nanorods to nanowires,” Zubarev said. “It does not change the diameter, so in principal we can take small rods with an aspect ratio of maybe two or three and elongate them to 100 times the length.”

III-V nanowires
A group of researchers have developed a way of growing better III-V nanowires on silicon.

This technology could one day be used to integrate nano-lasers into chips and improve energy conversion in solar panels, according to the research group, including the École polytechnique fédérale de Lausanne (EPFL), the Massachusetts Institute of Technology (MIT) and the Ioffe Institute.

III-V materials are common in various chips. III-V nanowires would enable faster chips with high mobilities, but the cost and integration issues are challenging. The lattice mismatch between III-V materials and silicon present a problem.

There are several ways to integrate III-V nanowires. Vapor–liquid–solid (VLS) growth is one method. This is where “solid nanowires precipitate from liquid droplets, supersaturated with the vapor phase precursors. The most commonly used external catalyst for the VLS growth is gold, which is unfortunately incompatible with silicon technology,” according to researchers in Nature Communications.

“As an alternative, self-catalyzed (or self-assisted) growth arises as the gold-free VLS method fully compatible with silicon platform,” according to researchers in Nature Communications. “One well-known example of self-catalyzed VLS growth is gallium-assisted growth of GaAs nanowires by molecular beam epitaxy (MBE). Here, a gallium nanodroplet is used instead of gold to gather arsenic precursors to precipitate GaAs underneath. Especially for the growth on silicon, preparation of gallium droplets turns out to be the key for a successful process.”