Growth of single crystal nanowires in supercritical silicon solution from tethered gold particles on a silicon substrate

Molecularly tethered gold (Au) nanocrystals with average diameter of 7 nm were used to grow single-crystal silicon (Si) nanowires on a substrate through a supercritical fluid-liquid-solid (sc-FLS) mechanism. Si wires were obtained by degrading diphenylsilane (DPS) in cyclohexane heated and pressurized well above its critical point (T-c = 281 degreesC; P-c = 4.1 MPa) and the Si/Au eutectic temperature (363 degreesC). Under these conditions, silicon dissolves in the gold nanocrystals and subsequently crystallizes in the form of wires. The nanowire diameter reflects the gold nanocrystal diameter, ranging from 5 to 30 nm, with lengths of several micrometers. Both batch and flow reactors were used. The flow reactor minimized the undesirable deposition of Si particulates formed in the bulk solution. Scanning electron microscopy (SEM) images of Si nanowires grown at a series of temperatures, reactor residence times, and Si precursor concentrations reveal that the wire growth kinetics influence nanowire morphology significantly and can be controlled effectively using a supercritical fluid flow reactor. A kinetic analysis of the process explains the dependence of the nanowire morphology on the reaction conditions qualitatively.