Fabrication and magnetic properties of metallic nanowires via AAO templates

Metallic (Ni, Co, Cu and Fe) nanowires were fabricated by electrodeposition into anodic aluminum oxide (AAO) template. In this work, we have studied the effect of the electrode potential on the microstructure and magnetic properties of nanowires. Transmission electron microscopy (TEM) results showed that the metal nanowires were single-crystal. Cu and Ni nanowires had the same orientation along the [2 2 0] direction, while Co had a preferred orientation along the [1 0 0] direction. Fe nanowires had a preferred orientation along the [2 0 0] direction. The growth mechanisms are probably due to the competition growth of the adjacent grains and the confinement of growth in the nano-sized hole of the AAO template. Results showed that single crystal or highly textured nanowires had better magnetic properties compared with that of polycrystal nanowires in terms of coercivity and squareness.     

Effect of sputtered films on morphology of vertical aligned ZnO nanowires

Vertical aligned ZnO nanowires were grown by MOCVD technique on silicon substrate using ZnO and AlN thin films as seed layers. The shape of nanostructures was greatly influenced by the under laying surface. Vertical nanopencils were observed on ZnO/Si, whereas the nanowires on both sapphire and AlN/Si substrate have the similar aspect ratio. XRD patterns suggest that the nanostructures have good crystallinity. High-resolution transmission electron microscopy (HRTEM) confirmed the single crystalline growth of the ZnO nanowires along [0 0 1] direction. Room-temperature photoluminescence (PL) spectra of ZnO nanowires on AlN/Si clearly show a band-edge luminescence accompanied with a visible emission. More interestingly, no visible emission for the nanopencils on ZnO/Si substrates, were observed.     

Electronic, elastic, optical properties of rutile TiO2 under pressure: A DFT study

The electronic, elastic constants and optical properties of rutile TiO₂ have been investigated using first principle pseudopotential method within generalized gradient approximation (GGA) proposed by Perdew-Burke-Ernzerhof (PBE). The calculated volume, bulk modulus and pressure derivative of bulk modulus are in good agreement with previous experimental and computational results. An underestimated band gap (1.970 eV) along with the higher density of states and expanded energy bands around the fermi level is obtained. Calculated elastic constants satisfying the Born stability criteria suggest that rutile TiO₂ is mechanically stable under higher hydrostatic pressure. The acoustic wave speeds in [1 0 0], [0 1 0], [0 0 1], [1 1 0] and [45° to [1 0 0] and [0 0 1]] directions are predicted using the investigated elastic constants. The dielectric constant is identified with respect to electronic band structure and is utilized to derive the other optical properties like refractive index, energy loss function, reflectivity and absorption. The effect of hydrostatic pressure (0-70 GPa) is described for listed properties. Our investigated results are in good accord with the existing theoretical and experimental results.     

Cr掺杂ZnO纳米线的电子结构和磁性

采用自旋极化密度泛函理论系统研究了Cr掺杂ZnO纳米线的电学、磁学以及光学属性.计算结果显示,Cr原子沿[0001]方向替代ZnO纳米线中的Zn原子时体系一般呈现铁磁耦合,沿[1010]和[0110]方向替代Zn原子时体系呈现反铁磁耦合,且磁性耦合状态在费米能级附近出现了明显的自旋劈裂现象,发生了强烈的Cr 3d和O 2p杂化效应.自旋态密度计算结果显示,磁矩主要来源于Cr原子未成对3d态电子的贡献,磁矩的大小与Cr原子的电子排布有关.光学性质计算结果显示,Cr掺杂ZnO纳米线在远紫外和近紫外都具有明显的吸收峰,吸收峰发生了明显的红移.这些结果都表明Cr掺杂ZnO纳米线也许是一种很有前途的稀磁半导体材料. 关键词： ZnO 纳米线 第一性原理 磁性     

Fabrication of ZnO nanostructures of various dimensions using patterned substrates

ZnO nanostructures were grown on silicon, porous silicon, ZnO/Si and AlN/Si substrates by low-temperature aqueous synthesis method. The shape of nanostructures greatly depends on the underlying surface. Scattered ZnO nanorods were observed on silicon substrate, whereas aligned ZnO nanowires were obtained by introducing sputtered ZnO film as a seed layer. Furthermore, both the combination of nanorods and the bunch of nanowires were found on porous silicon substrates, whereas platelet-like morphology was observed on AlN/Si substrates. XRD patterns suggest the crystalline nature of aqueous-grown ZnO nanostructures and high-resolution transmission electron microscopy images confirm the single-crystalline growth of the ZnO nanorods along [0 0 1] direction. Room-temperature photoluminescence characterization clearly shows a band-edge luminescence along with a visible luminescence in the yellow spectral range.     

Microstructural and optical properties of europium-doped zinc oxide nanowires

Single-crystal Eu³⁺-doped wurtzite ZnO micro- and nanowires were synthesized by chemical vapor deposition. The nanostructures grew via a self-catalytic mechanism on the walls of an alumina boat. The structure and properties of the doped ZnO were characterized using X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning and transmission electron microscopy, and photoluminescence (PL) methods. A 10-min synthesis yielded vertically grown nanowires of 50–400 nm in diameter and several micrometers long. The nanowires grew along the ±[0001] direction. The Eu³⁺ concentration in the nanowires was 0.8 at.%. The crystal structure and microstructure of were compared for Eu³⁺-doped and undoped ZnO. PL spectra showed a red shift in emission for Eu³⁺-doped (2.02 eV) compared to undoped ZnO nanowires (2.37 eV) due to Eu³⁺ intraionic transitions. Diffuse reflectance spectra revealed widening of the optical bandgap by 0.12 eV for Eu³⁺-doped compared to undoped ZnO to yield a value of 3.31 eV. Fourier-transform infrared spectra confirmed the presence of europium in the ZnO nanowires.     

Tuning strain relaxation by surface morphology: Surfactant-mediated epitaxy of germanium on silicon

In this paper, we summarize the results on the surfactant-mediated epitaxy (SME) of germanium on (0 0 1) and (1 1 1) silicon substrates. Then, we discuss, how the surfactant-controlled development of micro-facets determines the strain relaxation process. We place particular emphasis on the different types of strain-compensating dislocation networks that form at the Ge/Si(0 0 1) interface in epitaxy with and without Sb as a surfactant. At elevated temperatures, high Sb-coverage promotes the generation of a regular array of edge type misfit dislocations, which allows for abrupt instead of gradual strain relaxation in the initial stage of growth.     

Self-aligned silicon quantum wires on Ag(1 1 0)

Upon deposition of silicon onto the (1 1 0) surface of a silver crystal we have grown massively parallel one-dimensional Si nanowires. They are imaged in scanning tunnelling microscopy as straight, high aspect ratio, nanostructures, all with the same characteristic width of 16 Å, perfectly aligned along the atomic troughs of the bare surface. Low energy electron diffraction confirms the massively parallel assembly of these self-organized nanowires. Photoemission reveals striking quantized states dispersing only along the length of the nanowires, and extremely sharp, two-components, Si 2p core levels. This demonstrates that in the large ensemble each individual nanowire is a well-defined quantum object comprising only two distinct silicon atomic environments. We suggest that this self-assembled array of highly perfect Si nanowires provides a simple, atomically precise, novel template that may impact a wide range of applications.     

Dependence of optical properties of monoclinic MnWO4 on the electric field of incident light

Considering the electric field of incident light along four particular directions [100], [110], [011], and [010], the optical properties of monoclinic MnWO₄ were investigated by the first-principle methods. The calculated electronic structures show that the O 2p states and Mn 3d states dominate the top of the valence bands, while the W 5d and Mn 3d states play a key role in the bottom of the conduction bands. The dielectric function and other optical properties, including absorption coefficient, reflectivity spectra, and energy-loss spectra, were calculated and analyzed. The results predicted the maximum static dielectric function when the electric field of incident light was along the [100] direction; meanwhile the absorption edge was calculated to be consistent with the energy band gap and the values and positions of peaks in absorption coefficient are related with the electric field of light. Otherwise, it is found that the appearance of peaks in the energy-loss spectra is also dependent on the electric field and simultaneously corresponds to the edge of absorption spectra and the peaks' position of reflectivity spectra.     

Electrical conductivity of zigzag carbon nanotubes including Holstein polarons

We report molecular dynamics calculations on the evolution of Co-Au and Ag-Au alloy nanowires stretched along the [100], [110] and [111] crystallographic directions. The strong tendency of chain formation has been found for Ag-Au alloy. On the contrary the Co-Au alloy presents a different breaking pattern. In particular, we have found the formation of tetramer alloy nanowires. Finally, we present the mechanical properties of alloy nanocontacts.     

Synthesis and red-shifted photoluminescence of single-crystalline ZnO nanowires

Local-oriented single-crystalline ZnO nanowires have been synthesized in large scale by a simple microemulsion method in the presence of sulfonate-polystyrene (S-PS) and dodecyl benzene sulfonic acid sodium salt (DBS). The as-prepared product is characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), infrared (IR) spectra and photoluminescence (PL) spectrum. The nanowires exhibit a local congregation and preferentially grow along the [0 0 2] facet. FT-IR spectrum indicates that S-PS is adsorbed on the surface of ZnO nanowires. The PL spectrum shows evidently red-shifted ultraviolet (UV) emission.     

A comparative study of fibre-textured and epitaxial Nb-doped Pb(Zr0.53Ti0.47)O3 thin films on different substrates

Fibre-textured and epitaxial Nb-doped Pb(Zr_0.53Ti_0.47)O₃ (PNZT) thin films were grown on the different substrates by a sol-gel process. The [1 0 0]- and [1 1 1]-fibre-textured polycrystalline PNZT films were obtained on platinized silicon substrates by introducing PbO and TiO₂ seeding layers, while the [0 0 1]- and [1 1 1]-oriented epitaxial PNZT films were formed directly on Nb-doped SrTiO₃ (Nb:STO) single-crystal substrates with (1 0 0) and (1 1 1) surfaces, respectively. The preferential orientation and phase structure of the fibre-textured and epitaxial PNZT films, as well as their influences on the electrical properties were investigated. Higher remnant polarization (P_r) and piezoelectric coefficient (d₃₃) were obtained for the epitaxial PNZT films on Nb:STO substrates than that for the fibre-textured ones on platinized silicon substrates. For both fibre-textured and epitaxial cases, the PNZT films with [1 0 0]/[0 0 1] orientations show higher piezoelectric responses than [1 1 1]-oriented ones, whereas better ferroelectric properties can be obtained in the latter. The intrinsic and extrinsic contributions were discussed to explain the difference in electrical properties for differently oriented fibre-textured and epitaxial PNZT films on different substrates.     

From Si nanowires to porous silicon: the role of excitonic effects

We show that the electronic and optical properties of silicon nanowires, with different size and orientation, are dominated by important many-body effects. The electronic and excitonic gaps, calculated within first principles, agree with the available experimental data. Huge excitonic effects, which depend strongly on wire orientation and size, characterize the optical spectra. Modeling porous silicon as a collection of interacting nanowires, we find an absorption spectrum which is in very good agreement with experimental measurements only when the electron-hole interaction is included.     

Extremely wide band Rayleigh resonant reflector with sharp angular spectra in near infrared wavelength band

In this paper, we propose an extremely broadband Rayleigh resonant reflector with sharp angular spectra operating in near infrared wavelength band, this device consisting of a single germanium resonant grating layer is designed and analyzed by using with the rigorous vector diffraction theory. At the Rayleigh angle, the first diffracted order can be appear from evanescent to a propagating one, thus, a very sharp angular spectrum characteristics can be presented in the device. Based on the guided mode resonant effect, high index material such as silicon and germanium can be designed as wide band reflector, beam splitter and polarizer in near infrared wavelength region. Through connecting Rayleigh phenomena and guided mode resonant effect, we can design a new kind of optical devices with versatile characteristics such as sharp angular spectra and extremely wide reflection band. In this paper, we present a Rayleigh resonant reflector with extremely high reflection (R > 99.5%) for TE polarization light over ∼600 nm wavelength range and sharp angular spectral distribution. In addition, it is shown from our calculations that the high-index nano-layer located adjacent to the substrate is seen to critically affect the resulting spectra of Rayleigh resonant reflector.     

Ytterbium on vicinal Si(1 0 0): Growth and properties of the 2D wetting layer and the Yb silicide phase

The Yb growth on a vicinal Si(1 0 0) surface has been studied by scanning tunneling microscopy and low energy electron diffraction in the coverage range of 1-4 ML. Two different methods of the Yb/Si(1 0 0) interface formation are applied, leading to a remarkable modification of structural and morphological properties of two-dimensional (2D) wetting layer and Yb silicide phase. In particular, the switchover of the 2D layer orientation, similar to the case of the Bi nanolines on Si(1 0 0) [J.H.G. Owen, K. Miki, D.R. Bowler, J. Mater. Sci. 41 (2006) 4568], is observed depending on the growth procedure. Moreover, the structure and morphology of the Yb silicide phase is found to depend critically on the growth conditions, and the ability to grow very long, unidirectional Yb silicide nanowires is demonstrated. The results obtained are discussed in the context of the previous studies of 1D nanowires and 3D islands of rare-earth silicides on Si(1 0 0).     

Anomaly in the field dependence of the anisotropic magnetic susceptiblity of a single-grained α-Mn

A kink was observed near 20 kOe in the field dependence of the magnetization in a single-grained α-Mn without a magnetic hysteresis below the Neel temperature in an accurate magnetization measurement below 80 kOe. It was observed along [1 0 0], [1 1 0] and [1 1 1] crystallographic directions. The field dependence of the magnetization above 40 kOe extrapolates to positive finite magnetization at null field. The kink suggests a weak metamagnetism induced by the external magnetic field. A small but clear anisotropy was observed between the weak-field susceptibilities along [1 0 0] and [1 1 0]/[1 1 1] directions. A previously reported large susceptibility anomaly was not affirmed between 90 and 270 kOe.     

One-dimensional chain structure produced by Ce on vicinal Si(1 0 0)

One-dimensional Ce nanowires have been grown on a single-domain vicinal Si(1 0 0) surface. The growth mode, including the structural and electronic properties as a function of the substrate temperature and Ce coverage, was studied using scanning tunneling microscopy and scanning tunneling spectroscopy. The results show the formation of Ce nanowires along the step edges on the vicinal Si(1 0 0) substrate at 580 °C.     

Catalyst-free synthesis of novel brush-shaped ZnO particles by a simple oxidation in air

Brush-shaped ZnO particles were synthesized by controlling the growth time in the direct melt oxidation process of Al-Zn mixture in air at atmospheric pressure. Particles with two kinds of structures were formed. One was consisted of nanowires grown along [0 0 0 1] direction at the six corners and the center of (0 0 0 1) basal plane on hexagonal ZnO microrod. The other was constructed by nanobelts between the corner-nanowires as well as nanowires at the corners on ZnO microrod. The structural configuration that the nanowires and the nanobelts have a well coherent orientation alignment with the base microrod implies that the brush-shaped ZnO is single crystal. Room temperature PL spectrum of the brush-shaped ZnO particles displayed predominant green emission with a wavelength of 510 nm.     

Quantum conductance of In nanowires on Si(1 1 1) from first principles calculations

The quantum conductance of the paradigmatic quasi-one-dimensional In/Si(1 1 1) surface system is calculated for 4 × 1, 4 × 2 and 8 × 2 surface reconstructions. In agreement with experiment, we find the recently suggested formation of hexagons within the In nanowires [C. Gonzalez, F. Flores, J. Ortega, Phys. Rev. Lett. 96 (2006) 136101] to drastically modify the electron transport along the In chains. In contrast, the formation of trimers barely changes the quantum conductance.     

Structural, optical and photoelectrochemical characterization of CdS nanowire synthesized by chemical bath deposition and wet chemical etching

Nanocrystalline thin films of CdS have been grown onto flexible plastic and titanium substrates by a simple and environmentally benign chemical bath deposition (CBD) method at room temperature. The films consist of clusters of CdS nanoparticles. The clusters of CdS nanoparticles in the films were successfully converted into nanowire (NW) networks using chemical etching process. The possible mechanism of the etching phenomenon is discussed. These films were examined for their structural, surface morphological and optical properties by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and UV-vis spectrophotometry techniques, respectively. Photoelectrochemical (PEC) investigations were carried out using cell configuration as n-CdS/(1 M NaOH + 1 M Na₂S + 1 M S)/C. The film of nanowires was found to be hexagonal in structure with the preferential orientation along the (0 0 2) plane. The nanowires have widths in the range of 50-150 nm and have lengths of the order of a few micrometers. Optical studies reveal that the CdS nanowires have value of band gap 2.48 eV, whereas it is 2.58 eV for nanoparticles of CdS. Finally, we report on the ideality of junction improvement of PEC cells when CdS nanoparticles photoelectrode converted into nanowires photoelectrode.