Single crystalline ß-Ag2Te nanowire as a new topological insulator.

A recent theoretical study suggested that Ag(2)Te is a topological insulator with a highly anisotropic Dirac cone. Novel physics in the topological insulators with an anisotropic Dirac cone is anticipated due to the violation of rotational invariance. From magnetoresistance (MR) measurements of Ag(2)Te nanowires (NWs), we have observed Aharanov-Bohm (AB) oscillation, which is attributed to the quantum interference of electron phase around the perimeter of the NW. Angle and temperature dependences of the AB oscillation indicate the existence of conducting surface states in the NWs, confirming that Ag(2)Te is a topological insulator. For Ag(2)Te nanoplates (NPLs), we have observed high carrier mobility exceeding 22,000 cm(2)/(V s) and pronounced Shubnikov-de Haas (SdH) oscillation. From the SdH oscillation, we have obtained Fermi state parameters of the Ag(2)Te NPLs, which can provide valuable information on Ag(2)Te. Understanding the basic physics of the topological insulator with an anisotropic Dirac cone could lead to new applications in nanoelectronics and spintronics.

Topotaxial fabrication of vertical Aux Ag1-x nanowire arrays: plasmon-active in the blue region and corrosion resistant.

Topotaxial growth of Au(x) Ag(1-x) alloy nanowires (NWs) by postepitaxial deposition of Ag vapor on Au NWs and investigation of their plasmonic properties are reported. Ag vapor is supplied onto the epitaxially grown Au NWs, topotaxially turning them into Au(x) Ag(1-x) alloy NWs. The original geometries and alignments of the Au nanostructures are well preserved, while the composition of the alloy NWs is controlled by varying the Ag vapor supply time. The Au(0.5) Ag(0.5) NWs show high surface-enhanced Raman scattering (SERS) activity comparable to that of Ag NWs as well as highly increased oxidation resistance. The plasmon-active wavelength range of the Au(0.5) Ag(0.5) NW is significantly extended to the blue region compared to Au NWs. The Au(x) Ag(1-x) alloy NWs that have plasmonic activity in the blue region in addition to high corrosion resistance will make a superb material for practical plasmonic devices including SERS sensors and optical nanoantennas.

Polymorph-tuned synthesis of α- and ß-Bi2O3 nanowires and determination of their growth direction from polarized Raman single nanowire microscopy.

We report polymorph-tuned synthesis of α- and ß-Bi(2)O(3) nanowires and their single nanowire micro-Raman study. The single crystalline Bi(2)O(3) nanowires in different phases (α and ß) were selectively synthesized by adjusting the heating temperature of Bi precursor in a vapor transport process. No catalyst was employed. Furthermore, at an identical precursor evaporation temperature, α- and ß- phase Bi(2)O(3) nanowires were simultaneously synthesized along the temperature gradient at a substrate. The growth direction of α-Bi(2)O(3) nanowires was revealed by polarized Raman single nanowire spectra. For thin ß-Bi(2)O(3) nanowires with a very small diameter, the polarized Raman single nanowire spectrum was strongly influenced by the shape effect.

Diffusion-driven crystal structure transformation: synthesis of Heusler alloy Fe3Si nanowires.

We report fabrication of Heusler alloy Fe(3)Si nanowires by a diffusion-driven crystal structure transformation method from paramagnetic FeSi nanowires. Magnetic measurements of the Fe(3)Si nanowire ensemble show high-temperature ferromagnetic properties with T(c) >> 370 K. This methodology is also successfully applied to Co(2)Si nanowires in order to obtain metal-rich nanowires (Co) as another evidence of the structural transformation process. Our newly developed nanowire crystal transformation method would be valuable as a general method to fabricate metal-rich silicide nanowires that are otherwise difficult to synthesize.

In situ TEM observation of heterogeneous phase transition of a constrained single-crystalline Ag2Te nanowire.

Laterally epitaxial single crystalline Ag2Te nanowires (NWs) are synthesized on sapphire substrates by the vapor transport method. We observed the phase transitions of these Ag2Te NWs via in situ transmission electron microscopy (TEM) after covering them with Pt layers. The constrained NW shows phase transition from monoclinic to a body-centered cubic (bcc) structure near the interfaces, which is ascribed to the thermal stress caused by differences in the thermal expansion coefficients. Furthermore, we observed the nucleation and growth of bcc phase penetrating into the face-centered cubic matrix at 200 °C by high-resolution TEM in real time. Our results would provide valuable insight into how compressive stresses imposed by overlayers affect behaviors of nanodevices.

Vertical epitaxial co5 ge7 nanowire and nanobelt arrays on a thin graphitic layer for flexible field emission displays.

Vertically aligned single-crystalline Co5 Ge7 nanowire (NW) and nanobelt arrays are grown on a very thin graphite layer as well as a curved graphite layer with a good epitaxial lattice match. Co5 Ge7 NW arrays, thus grown, show very efficient field emission properties comparable to those of carbon nanotubes and may be used for flexible field emission displays in the future.

Magnetotransport properties and kondo effect observed in a ferromagnetic single-crystalline Fe(1-x)Co(x)Si nanowire.

We report unconventional magnetotransport properties of an individual Fe(1-x)Co(x)Si nanowire. We have studied the dependence of the resistivity on the angle between the directions of the magnetization and electrical current below the Curie temperature (T(C)). The observed anisotropic magnetoresistance (MR) ratio is negative, thereby indicating that the conduction electrons in a minority spin band of the Fe(1-x)Co(x)Si nanowire dominantly contribute to the transport. Unlike typical ferromagnets, positive MR is observed in the overall temperature range. MR curves are linear below T(C) and show a quadratic form above T(C), which can be explained by the change of density of states that arises as the band structures of the Fe(1-x)Co(x)Si nanowire shift under a magnetic field. The temperature dependence of the resistivity curve is sufficiently explained by the Kondo effect. The Kondo temperature of the Fe(1-x)Co(x)Si nanowire is lower than that of the bulk state due to suppression of the Kondo effect. The high single crystallinity of Fe(1-x)Co(x)Si nanowires allowed us to observe and interpret quite subtle variations in the prominent intrinsic transport properties.

Stereoaligned epitaxial growth of single-crystalline platinum nanowires by chemical vapor transport.

Epitaxial Pt nanowire (NW) arrays are synthesized for the first time by a chemical vapor transport method by using a metal halide as a precursor. Here we report that the epitaxial growth direction of NWs can be steered by seed crystal morphology. Octahedral seeds grow into inclined NWs possessing six growth directions, whereas half-octahedral seeds grow into vertical and horizontal NWs. Interfacial energies between the seed material and the substrate are critical in determining the morphology of seed crystals. We also demonstrate that non-SERS-active Pt NWs can show strong surface-enhanced Raman scattering (SERS) spectra by placing them on Ag films. The active SERS observation would help to elucidate platinum-catalyzed chemical reactions.

Composition-tuned Co(n)Si nanowires: location-selective simultaneous growth along temperature gradient.

We report the simultaneous and selective synthesis of single-crystalline Co(n)Si NWs (n = 1-3) and their corresponding crystal structures--simple cubic (CoSi), orthorhombic (Co(2)Si), and face-centered cubic (Co(3)Si)--following a composition change. Co(n)Si NWs were synthesized by placing the sapphire substrates along a temperature gradient. The synthetic process is a successful demonstration of tuning the chemical composition in Co(n)Si NWs. The synthesis and detailed crystal structure of single-crystalline Co(2)Si and Co(3)Si are reported for the first time including the bulk and the nanostructure phases. The electrical and magnetic properties of Co(2)Si NWs are investigated and compared with those of CoSi NWs.

Phase-controlled growth of metastable Fe5Si3 nanowires by a vapor transport method.

We report the synthesis of single-crystalline nanowires (NWs) of metastable Fe5Si3 phase via an iodide vapor transport method. Free-standing Fe5Si3 NWs are grown on a sapphire substrate placed on a Si wafer without the use of any catalyst. The typical size of the Fe5Si3 nanowires is 5-15 microm in length and 100-300 nm in diameter. Synthesis of the metastable phase is induced by composition-dependent nucleation from the gas-phase reaction. Depending on the concentration ratio of FeI2(g) to SiI4(g), different phases of iron silicides are formed. The growth of nanowires is facilitated by the initial nucleation of silicide particles on the substrate and further self-seeded growth of the NWs. The present work not only provides a method for the synthesis of metastable Fe5Si3 nanowires but also suggests that the phase controlled synthesis can be further optimized to produce other metal-rich silicide nanostructures for future spintronic devices.