Juneho In

Single Crystalline β-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.

Vertical Epitaxial Co5Ge7 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.

Polymorph-Tuned Synthesis of α- and β-Bi2O3 Nanowires and Determination of Their Growth Direction from Polarized Raman Single Nanowire Microscopy

We report polymorph-tuned synthesis of α- and β-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 β) 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 β- 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 β-Bi(2)O(3) nanowires with a very small diameter, the polarized Raman single nanowire spectrum was strongly influenced by the shape effect.

Composition-Tuned ConSi 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.

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.

Topotaxial Fabrication of Vertical AuxAg1–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.

Three-dimensionally kinked high-conducting CoGe nanowire growth induced by rotational twinning

We have synthesized single-crystalline horizontal and free-standing monoclinic CoGe nanowire (NW) arrays on high-k dielectric Y-stabilized ZrO2 (110) substrates via a chemical vapor transport process without using any catalysts. Horizontal NWs are grown epitaxially on the substrate. Three-dimensionally (3D)-kinked NWs are grown from the tip of the horizontal NWs homoepitaxially initiated by rotational twinning. Electrical measurements show that both horizontal and 3D-kinked CoGe NWs have low resistivity. The 3D-kinked NWs as well as free-standing metallic CoGe NWs integrated on Y-stabilized ZrO2 substrates could find applications as effective on-chip interconnects and nanoelectrodes for highly integrated nanoelectronic devices and as platforms for fuel cells and as efficient catalysts.

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.

Vertical epitaxial Co 5 Ge 7 nanowires and nanobelts arrays on a thin graphitic layer for flexible FED

We have successfully synthesized vertically aligned single-crystalline Co<inf>5</inf>Ge<inf>7</inf> NWs on a curved thin graphite layer as well as on a very thin graphite layer. Vertical Co<inf>5</inf>Ge<inf>7</inf> nanobelts are also synthesized on a graphite substrate. The vertical Co<inf>5</inf>Ge<inf>7</inf> nanobelt arrays or Co<inf>5</inf>Ge<inf>7</inf> NW arrays have been selectively grown on a graphite substrate depending on the experimental conditions. As grown Co<inf>5</inf>Ge<inf>7</inf> NW arrays satisfy most requirements for the ideal emitters of FEDs, such as sharp tips, superb electrical conductivity, thermal and chemical stability, and vertical alignment to the substrate. The vertical Co<inf>5</inf>Ge<inf>7</inf> NW arrays showed very efficient FE properties comparable to those of CNTs because of their optimum morphology. The NW emitter arrays on the curved graphite layer could be used for future flexible FEDs. Since the crystal structure of thin HOPG is close to that of graphene layers, our results could be extended to the epitaxial growth of Co<inf>5</inf>Ge<inf>7</inf> NWs and nanobelts on graphene.