Metal surface nucleated supercritical fluid–solid–solid growth of Si and Ge/SiO_x core–shell nanowires

High yields of both single-crystalline Si and Ge/SiO_x core–shell nanowires were nucleated and grown in metal reactor cells under high-pressure supercritical fluid conditions, without the addition of catalyst particle seeds or a porous template. Nanowire growth was only achieved when the fluid medium of supercritical CO₂ and the organometallic precursors were used in conjunction with a coordinating solvent, trioctylphosphine. The diameter and length of the nanowires are found to be in the ranges of 30 to 60 nm and 1 to 10 µm, respectively. The correlation of nanowire growth with the eutectic binary phase diagrams of the semiconductor–metal and the presence of metal impurities at the base of the synthesized nanowires suggest a supercritical fluid–solid–solid growth mechanism occurring from the reaction cell walls. The nanowires are characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. The electrical characteristics for individually picked nanowires are also investigated by means of mechanical nanoprobing.