Laser Ablation Creates Nanowires

Michael D. Wheeler

An Nd:YAG laser is one tool in a new technique that creates nanometer-scale "wires" for atomic microscopy and submicron electronics.
Chemistry Professor Charles M. Lieber and graduate student Alfredo M. Morales of Harvard University took a novel approach to producing these wires: They combined laser ablation and vapor-liquid-solid growth to create single-crystal silicon and germanium nanowires. Not only did the nanowires have an extremely small diameter -- as small as 3 nm -- but they were longer than 1 µm.
The technique involves using a pulsed frequency-doubled Nd:YAG laser emitting at 532 nm. The laser vaporizes a silicon target containing a catalyst -- in this case, iron.
The team put the target in a quartz furnace tube heated to 1200 °C, monitoring the temperature and pressure. When the laser ablated the target, the reaction caused a dense, hot vapor of silicon and iron. The vapor condensed into small clusters while a stream of gas carried it into a cooling chamber. Throughout the process, Morales and Lieber adjusted the furnace temperature, ensuring that the silicon-iron cluster remained in a liquid state.
Nanowire growth began after the liquid became supersaturated with silicon, causing silicon-iron nanodroplets to ride atop a growing nanowire. The growth terminated when the nanowire passed out of the hot reaction zone and into the cold chamber.