Synthesis of germanium nanowires in supercritical fluid reactor

Abstract

Relatively high intrinsic carrier concentration (2.4 X 1013 cm-3) and carrier mobility (3,800 cm2/Vos for electron and 1,820 cm2/Vos for hole respectively) of germanium (Ge) make germanium nanowires (Ge NWs) an attractive candidate as a charge transport channel in transistors, sensors, and optoelectronic devices. Ge NWs grow when Ge atoms diffused into Au seed particles supersaturate and precipitate on the particle surface at a temperature above the eutectic point of Ge and Au (361 ℃). Typically, the reaction takes place in a vapor phase, and thus the number of seed particles tethered on a substrate limits the amount of Ge NWs. A large-scale synthesis of Ge NWs is possible by using dense fluid as a reaction medium. In this study, we used a supercritical fluid reactor to replace the conventional vapor-phase reactors, and successfully synthesized Ge NWs, with diameter ranging from 15 nm to 100 nm and length of several um using diphenylgermane (DPG) as germanium precursor. In order to attain supercritical fluid, anhydrous hexane was heated and pressurized above its critical point (234.5 ℃ and 3020 kPa) in a plug-flow reactor. The optimum reaction temperature ranged from 425 ℃ to 450 ℃, while the pressure did not cause substantial changes in final product as long as it is above the critical point. Below 400 ℃, amorphous Ge particles were synthesized instead of nanowires because decomposition of DPG was too slow to form Au-Ge liquid alloy. Dodecanethiol-protected gold nanocrystals, average 2 nm in diameter acted not only as the nucleation site of Ge adatom but also as catalyst for the growth of nanowires. Depending on reaction conditions, Ge NWs exhibited widely different morphologies, ranging from straight to wavy to necklace-like. The crystallographic growth direction of Ge NWs also relates to the morphologies. The straight NWs show <111> direction is predominant whereas we rarely observed <110> growth direction. On the other hand, the wavy NWs show no preferred...