(321a) Surface Hydrogen Stabilized Semiconductor Nanowire Synthesis
The 3-D nature of semiconductor nanowires, and nanoscale materials in general, requires new chemical strategies with which to control growth processes and engineer structure. Unfortunately, there is little knowledge of the chemical bonding that underlies nanowire synthesis and, as a result, the ab initio manipulation of nanowire structure and function remains challenging. To this end, we report on a real-time in situ infrared spectroscopic investigation of sidewall hydrogen atoms during germanium nanowire growth via the vapor-liquid-solid (VLS) technique. We find that stable <111> oriented growth is only possible in the presence of adsorbed hydrogen and that these surface species modify taper as well as faceting. Arrays of epitaxially aligned nanowires were synthesized with a Au catalyst at a range of process conditions (210 − 280 °C, 5.0 × 10-5 − 1.5 × 10-4 Torr). Two absorption bands, present near 1980 cm-1 and 1965 cm-1, exhibit intensities that are a function of substrate temperature, precursor pressure, and nanowire elongation time. A series of adsorption studies on substrates with different orientations reveal that these bands result from hydrogen covalently bonded at Ge(100)-like and Ge(111)-like surface sites. Experiments where atomic hydrogen atoms are delivered in situ unambiguously confirm that hydrogen is responsible for these effects and suggest new avenues to rationally manipulate nanowire structure.