The impetus for new methods in nanomaterials chemistry derives from the need for effective and tunable routes to nanostructures with any desired composition, structure, shape, size, and interfacial properties. Nanowires represent an important class of materials, having the characteristics of one-dimensional quantum confinement when smaller than a critical diameter and the potential to electrically connect the components in an integrated nanoscale system. Several general synthetic strategies for nanowires have been developed, including vapor-phase and solution-phase routes.[1] Many different semiconductor materials have been produced using vaporphase routes by the vapor±liquid±solid (VLS) growth mechanism, [1±3] a process that relies on a metal particle to seed and direct semiconductor wire formation. In solution, a variety of nanowire materials have been synthesized without growth catalysts, however the unidirectional morphology of these nanostructures primarily reflects an internal anisotropic crystal structure.[1] Therefore, researchers (including us) have sought to combine VLS growth with solution-phase chemistry to develop a general route to nanowire synthesis that does not depend on crystal structure. [4,5] The challenge of combining VLS with solution-phase chemistry is the required synthetic temperatures that must exceed the seed metal/semiconductor eutectic temperature. In their pioneering work, Buhro and co-workers demonstrated VLStype nanowire synthesis in conventional solvents (called solution±liquid±solid (SLS)) at synthetic temperatures of around 200 C, using metal/semiconductor combinations such as Ga/ GaAs, [4] In/InN, [6] and, most recently, In/GaAs [7] with low eutectic temperatures. Unfortunately, the eutectic temperatures for most metal/semiconductor combinations are greater than 350 C, including the group IV semiconductors, Si and Ge (with Au). These reaction temperatures can be accessed in solution by pressurizing the solvent above its critical point.Recently, we demonstrated Si and Ge nanowire synthesis in supercritical hexane at temperatures ranging from 350 C to 500 C and pressures ranging from 100 atm to 370 atmÐa process we have called supercritical fluid±liquid±solid (SFLS) nanowire growth. [5,8±10] Hypothetically, this process could be applied to any semiconductor/metal combination with eutectic temperatures less than 600 C Ðthe maximum temperature before degradation of most organic solvents. Here, we demonstrate the first SFLS nanowire synthesis of a compound semiconductor material: GaAs. GaAs nanowires were synthesized in supercritical hexane at 500 C and 37 MPa (370 atm) by reacting (tBu) 3 Ga and As-(SiMe 3 ) 3 in the presence of dodecanethiol-stabilized 7 nm Au nanocrystal seeds. High yields of nanowires (60 %) could be obtained with very little particulate formationÐthe GaAs nanowires shown in Figure 1a were taken directly from the reactor without any purification. They were synthesized with 10 mM precursor solution concentration with a 100:1 precursor/gold molar ratio. These pr...

Electrical transport through individual solution-grown GaAs nanowires was measured as a function of temperature. The current–voltage (IV) curves are nonlinear and exhibit space charge limited currents. The IV curves become increasingly nonlinear with decreasing temperature and follow the scaling relationship . This scaling indicates that the space charge limited currents are limited by trapped charge. The characteristic energies of the trap states were estimated from the IV data and found to vary from wire to wire, ranging from 0.024 to 0.11 eV below the band edge. In the low bias region of the IV curves, where the curves were ohmic, the activation energy (related to the Fermi energy) was determined and found to be shifted significantly towards the band edge, which indicates either the presence of a large concentration of impurities, such as Si, in the nanowires or charged surface states.

Colloidal submicrometer-diameter amorphous silicon (a-Si) particles are synthesized with >90% yield by thermal decomposition of trisilane (Si3H8) in supercritical hexane at temperatures ranging from 400 to 500 degrees C and pressures up to 345 bar. A range of synthetic conditions was explored to optimize the quality of the product. Under the appropriate synthetic conditions, the colloids are spherical and unagglomerated. The colloids can be produced with average diameters ranging from 50 to 500 nm by manipulating the precursor concentration, temperature, and pressure. Relatively narrow particle size distributions, as measured by transmission electron microscopy (TEM), with standard deviations about the mean as low as approximately +/-10% could be obtained in some cases. We explored the thermal annealing of the amorphous silicon particles after isolation from the reactor and found that crystallization to diamond structure silicon occurred at temperatures as low as 650 degrees C. The amorphous and crystalline materials were characterized by X-ray diffraction and high resolution scanning and transmission electron microscopy.

The temperature dependence of the field effect mobility was measured for solution-grown single-crystal Ge nanowires. The nanowires were synthesized in hexane from diphenylgermane by the supercritical fluid-liquid-solid process using gold nanocrystals as seeds. The nanowires were chemically treated with isoprene to passivate their surfaces. The electrical properties of individual nanowires were then measured by depositing them on a Si substrate, followed by electrical connection with Pt wires using focused ion beam assisted chemical vapor deposition. The nanowires were positioned over TaN or Au electrodes covered with ZrO2 dielectric that were used as gates to apply external potentials to modulate the conductance. Negative gate potentials increased the Ge nanowire conductance, characteristic of a p-type semiconductor. The temperature-dependent source/drain current-voltage measurements under applied gate potential revealed that the field effect mobility increased with increasing temperature, indicating that the carrier mobility through the nanowire is probably dominated either by a hopping mechanism or by trapped charges in fast surface states.

The electrical properties of Bi 2 S 3 nanowire bundles were investigated. The nanowires were synthesized using a solventless reaction involving a single-source bismuth thiolate precursor and stabilizing organic ligands. For electrical testing, nanowires were dispersed in solution and drop cast onto a substrate with gold contact pads patterned by electron beam lithography techniques (EBL). Electrical connections were made by depositing platinum interconnect lines between the nanowires and the gold pads by focused ion beam (FIB) chemical vapour deposition. Current-voltage (I -V ) curves were measured under nitrogen as a function of temperature. The data revealed activated transport that followed a Meyer-Neldel relationship. Annealing under vacuum decreased the nanowire resistance by nearly four orders of magnitude. The annealed nanowires followed an inverse Meyer-Neldel relationship. Illumination with UV light increased the current, and air exposure decreased the current under constant applied bias. M Supplementary data files are available from stacks.