Multiple lamellar {111} twins are observed in GaAs, GaP, and InAs nanowires synthesized by supercritical fluid-liquid-solid (SFLS) and solution-liquid-solid (SLS) approaches. All of these nanowires have zinc blende (cubic) crystal structure and grow predominantly in the 〈111〉 direction. The twins bisect the nanowires perpendicular to their growth direction to give them a "bamboo"-like appearance in TEM images. In contrast, Si and Ge nanowires with 〈111〉 growth direction do not exhibit {111} twins, even though this is a common twin plane with relatively low twin energy in diamond cubic Ge and Si. However, Si and Ge nanowires with 〈112〉 growth directions typically have several {111} twins extending down the length of the nanowires. Here, we present a semiquantitative model that explains the observed twinning in III-V and IV nanowires.

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...

A new GaP nanowire synthesis method is employed to produce a high yield of single crystalline wires. Reaction at 500 °C in supercritical hexane using gold seed particles results in wires that average 20 nm in diameter and tens of micrometers in length. Some wires generate twinning faults orthogonal to the wire growth direction.

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.

Nanotechnology V 1505Supercritical Fluid-Liquid-Solid Synthesis of Gallium Phosphide Nanowires.-Single crystalline GaP nanowires are synthesized with high yields in supercritical hexane by reacting P(SiMe3)3 and (tBu)3Ga in the presence of 7.5-nm-diameter dodecanethiol-stabilized Au nanocrystals (500°C, 37 MPa). The samples are characterized by XRD, SEM, and TEM. The produced GaP nanowires exhibit an average diameter of approximately 20 nm and lengths of up to tens of micrometers. -(DAVIDSON, F. M. I.; WIACEK, R.; KORGEL*, B. A.; Chem. Mater. 17 (2005) 2, 230-233; Dep. Chem. Eng., Univ. Tex., Austin, TX 78712, USA; Eng.) -W. Pewestorf 16-221