Germanium nanowires : synthesis, characterization, and utilization
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A supercritical fluid synthesis method was developed for the preparation of single crystal germanium (Ge) nanowires with diameters as small as 4 nanometer and several tens of micrometer in length. Alkanethiol protected gold nanocrystals were used to seed and direct nanowire growth. Nanowire processing and their implementation as building blocks in nanowire based devices requires rigorous control of nanowire surface chemistry, which differs from well-studied monolithic atomically-smooth single crystal substrate surface chemistry due to the nanowire’s high surface area to volume ratio and atomically rough surface. Ge nanowire surface oxidation was studied by Ge 3d x-ray photoelectron spectroscopy. A broad range of solution-phase routes to the Ge nanowire surface passivation were explored including sulfidation, hydride and chloride termination, and organic monolayer passivation. Nanowires with covalently bonded monolayer surface terminations formed via thermally-initiated hydrogermylation reactions with alkenes, alkynes or dienes exhibited excellent chemical stability compared to untreated or etched nanowire surfaces and enabled low contact resistance ohmic electrical contacts to be made to the nanowires. Device characteristics of single Ge nanowire devices fabricated with gold electrical contacts patterned by e-beam lithography were compared with devices prepared using focused e-beam or Ga-beam assisted Pt chemical vapor deposition. These device structures permitted direct investigation of the influence of nanowire surface chemistry, doping, and gate electrode architecture, on device operation. The impact of the surface chemistry on surface state dominated electron transport in single nanowire devices was investigated by room temperature field-effect measurements. The density and relaxation time distribution of electrically active surface states was found to be highly sensitive to the nanowire surface chemistry. Complimentary to the device measurements, fundamental electrical and optical properties were probed via electron energy loss spectroscopy on individual nanowires inside the transmission electron microscope. The volume plasmon energy increased with decreasing diameter for nanowires narrower than 24 nm. Below 24 nm, organic monolayer-coated nanowires also exhibited size-dependent Ge 3d core ionization spectra that shifted to higher energy with reduced diameter that are independent of probe position relative to the surface. In contrast, the Ge 3d edge for surface-oxidized nanowires exhibited a chemically-induced shift when positioned near the surface.