Large-area vertically aligned silicon nanowire (Si NW) arrays were synthesized with a controlled length (0.3 ˜ 9 μm) by the chemical etching of n-type silicon substrates. Upon their excitation using a fs Ti-sapphire laser pulse (800 nm), their THz emission intensity exhibits strong dependence on their length; the intensity increases sharply up to a length of 3 μm and then decreases slightly, due to the complete absorption of the optical pump power. The Raman scattering spectrum exhibits the same behavior as that of the THz emission. We suggest that the field enhancement by localized surface plasmons induces more efficient THz emission or Raman scattering for the longer Si NWs. The photocurrent measured in a photoelectrochemical cell showed consistently the length dependence with a maximum value at the length of 5 μm.

Electron tomography and high-resolution transmission electron microscopy were used to characterize the unique 3-dimensional (3D) structures of twinned Zn3P2 (tetragonal) and InAs (zinc blende) nanowires synthesized by the vapor transport method. The Zn3P2 nanowires adopt a unique superlattice structure that consists of twinned octahedral slice segments having alternating orientations along the axial [111] direction of a pseudocubic unit cell. The apices of the octahedral slice segment are indexed as six equivalent <112> directions at the [111] zone axis. At each 30 degrees turn, the straight and zigzagged morphologies appear repeatedly at the <112> and <011> zone axes, respectively. The 3D structure of the twinned Zn3P2 nanowires is virtually the same as that of the twinned InAs nanowires. In addition, we analyzed the 3D structure of zigzagged CdO (rock salt) nanowires and found that they include hexahedral segments, whose six apices are matched to the <011> directions, linked along the [111] axial direction. We also analyzed the unique 3D structure of rutile TiO2 (tetragonal) nanobelts; at each 90 degree turn, the straight morphology appears repeatedly, while the in-between twisted form appears at the [011] zone axis. We suggest that the TiO2 nanobelts consist of twinned octahedral slices whose six apices are indexed by the <011>/<001> directions with the axial [010] direction.

High-density TiO2-CdS and ZnO-CdS core-shell nanocable arrays were synthesized on large-area Ti substrates. The CdS layers were deposited on the pre-grown vertically-aligned TiO2 (rutile) and ZnO nanowire arrays, with a controlled thickness (10~50 nm), using the vapor transport method. The ZnO-CdS nanocables consisted of single-crystalline wurtzite CdS shells whose [001] direction was aligned along the [001] wire axis of the wurtzite ZnO core, which is distinctive from the polycrystalline shell of the TiO2-CdS nanocables. We fabricated the photoelectrochemical cell using the ZnO-CdS photoelectrode exhibits much more efficient hydrogen generation than that using the TiO2-CdS one.

Single-crystalline rock-salt PbS nanowires (NWs) were synthesized using three different routes; the solvothermal, chemical vapor transport, and gas-phase substitution reaction of pre-grown CdS NWs. They were uniformly grown with the [100] or [110], [112] direction in a controlled manner. In the solvothermal growth, the oriented attachment of the octylamine (OA) ligands enables the NWs to be produced with a controlled morphology and growth direction. As the concentration of OA increases, the growth direction evolves from the [100] to the higher surface-energy [110] and [112] directions. In the synthesis involving chemical vapor transport and the substitution reaction, the use of a lower growth temperature causes the higher surface-energy growth direction to change from [100] to [110]. We fabricated field effect transistors using single PbS NW, which showed intrinsic p-type semiconductor characteristics for all three routes. For the PbS NW with a thinner oxide layer, the carrier mobility was measured to be as high as 10 cm2V−1s−1.

Electron tomography and high-resolution transmission electron microscopy were used to characterize the unique three-dimensional structures of helical or zigzagged GaN, ZnGa2O4 and Zn2SnO4 nanowires. The helical GaN nanowires adopt a helical structure that consists of six equivalent <0-111> growth directions with the axial [0001] direction. The ZnGa2O4 nanosprings have four equivalent <011> growth directions with the [001] axial direction. The zigzagged Zn2SnO4 nanowires consisted of linked rhombohedrons structure having the side edges matched to the <011> direction, and the [111] axial direction.

We synthesized Ge and Ge1-xMx (M = Mn, Co, and Fe, x ≤ 0.2) nanowires using thermal vapour transport method. All nanowires consisted of single-crystalline Ge nanocrystals grown uniformly with the [111] direction. High-resolution X-ray diffraction pattern shows no cluster formation for all Ge1-xMx nanowires. The Mn and Fe doping decreases the lattice constant, but not Co doping. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurement revealed that the Mn2+ and Fe2+ ions preferentially occupy the tetrahedral sites, substituting for Ge. We suggest that the Mn or Fe ions produce dopant-acceptor hybridization with host defects in p-type Ge, but not Co ions. The magnetic moment of Mn2+ ions reaches a maximum for x = ∼ 0.1, which is much larger than that of the Fe2+ ions. The magnetization measurement also confirms the room-temperature ferromagnetism of Mn-doped Ge nanowires, which is maximized at x = ∼ 0.1. We conclude that the Mn ions are most efficiently doped into the Ge nanowires to form a ferromagnetic semiconductor.

Vertically-aligned Mn (10%)-doped Fe3O4 (Fe2.7Mn0.3O4) nanowire arrays were produced by the reduction/substitution of pre-grown Fe2O3 nanowires. These nanowires were ferromagnetic with a Verwey temperature of 129 K. X-ray magnetic circular dichroism measurements revealed that the Mn2+ ions preferentially occupy the tetrahedral sites, substituting for the Fe3+ ions. We observed that the Mn substitution decreases the magnetization, but increases the electrical conductivity. We developed highly sensitive gas sensors using these nanowire arrays, operating at room temperature, whose sensitivity showed a correlation with their bond strength of diatomic/triatomic molecules. Based on the fact that the sensitivity was highest toward water vapor, an excellent-performance humidity sensor was fabricated.

Novel Mn-incorporated ZnSe and CdSe 1-dimensionl nanostructures; straight nanowires, zigzagged nanobelts, and nanohooks, were first synthesized using chemical vapor deposition method. The Mn content reaches up to 40%. They all consisted of single-crystalline wurtzite structure for all Mn content. The structure has been thoroughly investigated by high-resolution transmission electron microscopy images as well as energy-dispersive X-ray fluorescence spectroscopy. The X-ray diffraction pattern confirms the formation of the wurtzite structure, even for 40% Mn incorporation. The lattice constants of Mn-doped ZnSe and CdSe 1-D nanostructures are expanded and reduced, respectively, by the Mn doping. The Mn2+ emission at 2.1 eV, originating from the d-d (4T1 → 6A1) transition, confirms the effective paramagnetic Mn2+ doping at tetrahedral coordinate sites. These Mn-incorporated nanostructures exhibit a paramagnetic behavior.

We report Mn-doped GaN nanowires exhibiting ferromagnetism even at room temperature. The growth of single-crystalline wurtzite structured GaN nanowires doped homogeneously with about 5 atomic % Mn was achieved by chemical vapor deposition using the reaction of Ga/GaN/MnCl2 with NH3. The ferromagnetic hysteresis at 5 and 300 K and the temperature-dependent magnetization curves suggest the Curie temperature around 300 K. Negative magnetoresistance of individual nanowires was observed at the temperatures below 100 K.

Two longitudinal superlattice structures of In2O3(ZnO)4 and In2O3(ZnO)5 nanowires were exclusively produced by thermal evaporation method. The diameter is periodically modulated in the range of 50-90 nm. They consist of one In-O layer and five (or six) layered Zn-O slabs stacked alternately perpendicular to the long axis, with a modulation period of 1.65 (or 1.9) nm. These superlattice nanowires were doped with 6-8 % Sn. X-ray diffraction pattern reveals the structural defects of wurtzite ZnO crystals due to the In/Sn incorporation. High-resolution X-ray photoelectron spectrum suggests that In/Sn withdraw the electrons from Zn, and enhance the number of dangling-bond O 2p states, resulting in the reduction of band gap. Photoluminescence exhibit the peak shift of near band edge emission to the lower energy as the In/Sn content increases.

The Si nanowires were synthesized using a novel catalytic thermal reaction under Ar flow. The average diameter is in the range of 50 ∼ 100 nm. They consist of defect-free single-crystalline cubic structure with the [111] growth direction. The thickness of amorphous oxide outer layers was controllable by growth conditions or surface treatment. In order to protect the oxidation, the Si nanowires were coated with boron nitride layer by the reaction of boron oxide mixture with NH3.

Gallium oxide (Ga2O3) and indium oxide (In2O3) nanostructures were synthesized by chemical vapor deposition (CVD). Ga2O3 nanowires were synthesized using Ga/Ga2O3 mixture and O2. The diameter of the nanowires is 30–80 nm with an average value of 50 nm. They are consisted of single-crystalline monoclinic crystal. While the nanowires grown without catalyst exhibit a significant planar defect, the nanowires grown with nickel catalytic nanoparticles are almost defect-free. The growth direction of the nanowires grown without the catalyst is uniformly [010]. In contrast, the nanowires grown with the catalyst have random growth direction. X-ray diffraction, Raman spectroscopy, and photoluminescence are well correlated with the structural characteristics of the nanowires. The result provides an evidence for the catalyst effect in controlling the structure of nanowires. In2O3 nanostructures were also synthesized in a controlled manner by selecting the catalyst. The reactants were In and In/In2O3 mixture. The nanowires were produced using catalytic Au nanoparticles and Ga. But the unique bifurcated-structure nanobelts were instead grown without Ga. The nanowires have uniform [100] growth direction with rectangular cross-section. We converted the In2O3 nanowires to In2O3-Ga2O3 nanostructures.

We report the catalytic effect on the synthesis of multiwalled carbon nanotubes (CNTs). The CNTs were grown vertically aligned on the iron (Fe), cobalt (Co), and nickel (Ni) catalytic nanoparticles deposited on alumina substrates by thermal chemical vapor deposition (CVD) of acetylene in the temperature range 900–1000 °C. We also synthesized them on the silicon oxide substrates by pyrolyzing iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), and nickel phthalocyanine (NiPc) at 700–1000 °C. In both syntheses, the CNTs grown using Fe exhibit about 2 times higher growth rate than those using Co and Ni. As the temperature rises from 700 to 1000 °C, the growth rate of CNTs increases by a factor of 45. The Arrhenius plot of growth rates provides the activation energy 30 ± 3 kcal/mol for all three catalysts, which is similar with the diffusion energy of carbon in bulk metal. It suggests that the bulk diffusion of carbon would play a decisive role in the growth of CNTs. The diameter of CNTs is in the range of 20–100 nm, showing an increase with the temperature. As the diameter is below 30 nm, the CNTs usually exhibit a cylindrical structure. The CNTs were intrinsically doped with the nitrogen content 2–6 atomic%. The degree of crystalline perfection of the graphitic sheets increases with the temperature, but depends on the catalyst and the nitrogen content. The graphitic sheets of CNTs grown using Fe are better crystalline than those grown using Co and Ni. As the nitrogen content increases, the degree of crystalline perfection decreases and the structure becomes the bamboolike structure probably due to a release of strains.

Various GaP nanostructures such as nanowires, nanobelts, nanocables, and nanocapsules were synthesized by sublimation of ball-milled powders. They have a single-crystalline zinc blende structure with [111] growth direction. The morphology and structure were controlled by reactant gas, growth time, flow rate, and growth temperature. The size, morphology and properties of the nanostructures were examined by scanning electron microscopy, transmission electron microscopy, electron energy-loss spectroscopy (EELS), electron diffraction, energy dispersive x-ray spectroscopy, powder x-ray diffraction, and Raman spectroscopy using a 514.5 nm argon ion laser. The photoluminescence was carried out using the 458 nm line of an argon ion laser as the excitation source. The GaP nanowires are straight, cylindrical, and smooth in surface, with mean diameter of 40 nm and length up to 300 mm. The nitrogen-doped nanobelts and nanowires were synthesized by ammonia ambient gas. EELS data reveals that the nitrogen doping occurs mainly in the surface region. The PL spectrum shows the typical isoelectronic bound exciton peaks in the range of 2.11∼2.25 e V, suggesting a concentration of (1018 cm-3 nitrogen atoms. We also synthesized two types of GaP nanocables; GaP nanowire sheathed with the amorphous silicon oxide layers and with the graphite layers. The core-shell diameter is under 30 nm and the outerlayer can be removed by acid treatment to produce the 10 nm diameter GaP nanowires. The GaP encapsulated with BCN nanotubes were synthesized under the ammonia flow using the ball-milled carbon-containing boron oxide powders. The number of BCN layers is typically 10∼20.

Various shaped single-crystalline gallium nitride (GaN) nanostructures were produced by chemical vapor deposition method in the temperature range of 900–1200 °C. Scanning electron microscopy, transmission electron microscopy, electron diffraction, x-ray diffraction, electron energy loss spectroscopy, Raman spectroscopy, and photoluminescence were used to investigate the structural and optical properties of the GaN nanostructures. We controlled the GaN nanostructures by the catalyst and temperature. The cylindrical and triangular shaped nanowires were synthesized using iron and gold nanoparticles as catalysts, respectively, in the temperature range of 900 – 1000 °C. We synthesized the nanobelts, nanosaws, and porous nanowires using gallium source/ boron oxide mixture. When the temperature of source was 1100 °C, the nanobelts having a triangle tip were grown. At the temperature higher up to 1200 °C the nanosaws and porous nanowires were formed with a large scale. The cylindrical nanowires have random growth direction, while the triangular nanowires have uniform growth direction [010]. The growth direction of the nanobelts is perpendicular to the [010]. Interestingly, the nanosaws and porous nanowires exhibit the same growth direction [011]. The shift of Raman, XRD, and PL bands from those of bulk was correlated with the strains of the GaN nanostructures.

Bulk-quantity single crystalline wurtzite gallium nitride nanowires with a mean diameter of 25 nm were synthesized on silicon substrate using a catalyst-assisted reaction of gallium and gallium nitride mixture with ammonia. They exhibit a strong and broad photoluminescence in the energy range of 2.9-3.6 eV with no yellow band. X-ray diffraction and Raman scattering data suggest that the nanowires would experience biaxial compressive stresses in the inward radial direction and the induced tensile uniaxial stresses in the wire axis. The blue photoluminescence would originate from the recombination of the bound excitons under the compressive and tensile stresses.