Using seed particle composition to control structural and optical properties of GaN nanowires

The morphology, structure, and optical properties of gallium nitride (GaN) nanowires grown using metal-organic chemical vapor deposition (MOCVD) on r-plane sapphire using gold and nickel seed particles were investigated. We found that different seed particles result in different growth rates and densities of structural defects in MOCVD-grown GaN nanowires. Ni-seeded GaN nanowires grow faster than Au-seeded ones, and they do not contain the basal plane stacking faults that are observed in Au-seeded GaN nanowires. We propose that stacking fault formation is related to the supersaturation and surface energies in different types of seed particles. Room temperature photoluminescence studies revealed a blue-shifted peak in Au-seeded GaN nanowires compared to the GaN near-bandgap emission. The blue-shifted peak evolves as a function of the growth time and originates from the nanowire base, likely due to strain and Al diffusion from the substrate. Our results demonstrate that seed particle composition has a direct impact on the growth, structure, and optical properties of GaN nanowires and reveal some general requirements for seed particle selection for the growth of compound semiconductor nanowires.     

A comparative study of the effect of gold seed particle preparation method on nanowire growth

Highly controlled particle-assisted growth of semiconductor nanowires has been performed for many years, and a number of novel nanowire-based devices have been demonstrated. Full control of the epitaxial growth is required to optimize the performance of devices, and gold seed particles are known to provide the most controlled growth. Successful nanowire growth from gold particles generated and deposited by various different methods has been reported, but no investigation has yet been performed to compare the effects of gold particle generation and deposition methods on nanowire growth. In this article we present a direct comparative study of the effect of the gold particle creation and deposition methods on nanowire growth characteristics and nanowire crystal structure, and investigate the limitations of the different generation and deposition methods used.      

Formation mechanism and optical properties of InAs quantum dots on the surface of GaAs nanowires

Formation mechanism and optical properties of InAs quantum dots (QDs) on the surface of GaAs nanowires (NWs) were investigated. This NW-QDs hybrid structure was fabricated by Au-catalyzed metal organic chemical vapor deposition. We found that the formation and distribution of QDs were strongly influenced by the deposition time of InAs as well as the diameter of GaAs NWs. A model based on the adatom diffusion mechanism was proposed to describe the evolution process of the QDs. Photoluminescence emission from the InAs QDs with a peak wavelength of 940 nm was observed at room temperature. The structure also exhibits a decoupling feature that QDs act as gain medium, while NW acts as Fabry-Perot cavity. This hybrid structure could serve as an important element in high-performance NW-based optoelectronic devices, such as near-infrared lasers, optical detectors, and solar cells.     

The influence of reagents on the preparation of Cu nanowires by tetradecylamine-assisted hydrothermal method

Copper (Cu) nanowires are inexpensive conducting nanomaterials intensively explored for transparent conducting electrodes and other applications. Here, Cu nanowires with approximately 40-nm diameter and a few hundreds of micrometers in length were selectively and facilely synthesized by a tetradecylamine (TDA)-assisted hydrothermal method. The Cu nanowires were highly flexible and were not oxidized by oxygen in air because of TDA’s effective coating on the Cu nanowires, which was confirmed by SEM observation and FT-IR spectrum. Moreover, the Cu nanowires tended to self-assemble into close-packed bundles due to hydrophobic–hydrophobic interactions between alkyl chains of TDA. The roles of the reagents in the preparation process were investigated systematically. First, a proper concentration of TDA was essential to high-quality Cu nanowires and TDA had two effects: (1) TDA molecules could coordinate with copper cations to form Cu(II)-complex, which was then reduced to Cu by glucose; (2) In the growth mechanism of Cu nanowires, the newly formed side surfaces, {100} facets, was stabilized through chemical interactions with the nitrogen atom of TDA (capping agent). With regards to Cu source, when using cupric chloride, cupric nitride, cupric acetate, and cupric bromide, Cu nanomaterials with a variety of shapes such as nanowires, nanoparticles, hollow spheres, and nanoflakes could be obtained. Among these Cu sources, cupric chloride was a proper selection for the preparation of Cu nanowires. About reductant agents, glucose could be replaced by other reductant agents such as VC. The UV–Vis absorption spectrum showed that the Cu nanowires had an absorption peak at 580 nm and a slightly higher transmittance in the visible region. These Cu nanowires were expected to find widespread use in the applications such as fabrication of transparent electrodes for flexible electronics and display devices. This TDA-assisted hydrothermal method could be expanded to preparation of different types of Cu nanostructures.     

Synthesis of Ga-riched zinc gallate nanowires by reactive evaporation and the cathodoluminescence properties of individual nanowires

A facile Au-catalytic reactive evaporation method was developed to synthesize cubic zinc gallate (ZnGa₂O₄) nanowires through the reaction of Ga₂O₃ film and ZnO vapor at high temperature. The ZnGa₂O₄ nanowires are well crystalline and the length is up to tens of micrometers. The growth process follows typical vapor-liquid-solid (VLS) mechanism. All the cathodoluminescence (CL) spectra of individual nanowires reveal a strong, broad and asymmetric blue emission band centered at ca. 460 nm. It is thought that the excess Ga³⁺ is the main reason for the unusual blue emission properties.     

A facile method for morphological control of MgZnO nanostructures on GaAs substrates and their optical properties

This research reports on morphological changes depending on the growth temperature in MgZnO nanostructures grown on GaAs substrates by metalorganic chemical vapor deposition as well as the investigation of their optical properties. As the growth temperature increased, the morphology of the MgZnO nanostructure changed from one-dimensional nanowires (480 degrees C) to pseudo-two-dimensional nanowalls (500 degrees C) to pyramid-shaped structures (520 degrees C). Among these structures, the nanowalls exhibited the best optical properties due to the large active surface area and high crystalline quality.     

Non-linear optical properties of zinc oxide nanowires

High quality zinc oxide (ZnO) nanowires were grown on n-type Si (100) using vapor-liquid-solid process. We obtained the photoluminescence spectra of ZnO nanowires based on nonlinear optical process using an ultrashort wavelength femtosecond laser as a pumping source. The spectra shows the second harmonic generation phenomenon, as well as the exciton-exciton collision peak at 388 nm and the green emission peak at 515 nm caused by oxygen vacancy. A laser emission peak near 392 nm was observed when pump intensity surpassed 52 mJ/cm2 and a sharp peak about 0.5 nm wide emerged when the energy intensity reached 700 mJ/cm2. We attribute this excitation process to a two-photon absorption process enhanced by Rabi oscillation.     

Phenomenological studies of optical properties of Cu nanowires

We report on the experimental observation of surface plasmon resonance in Cu nanowires fabricated by shadow deposition method. When the incident light is polarized perpendicular to the wire axes, plasmon maxima appeared at about 2.3 eV in the absorption spectra. Plasmon resonance appeared at lower photon energy when the incident light is polarized parallel to the wire axes. Resonance peaks move to lower energy when the nanowire widths are increased. We have found that finite-difference time-domain (FDTD) simulation gives better results than Maxwell–Garnett model in explaining the relation between the light polarization and the energies of the observed absorption maxima.     

Phenomenological studies of optical properties of Cu nanowires

We report on the experimental observation of surface plasmon resonance in Cu nanowires fabricated by shadow deposition method. When the incident light is polarized perpendicular to the wire axes, plasmon maxima appeared at about 2.3 eV in the absorption spectra. Plasmon resonance appeared at lower photon energy when the incident light is polarized parallel to the wire axes. Resonance peaks move to lower energy when the nanowire widths are increased. We have found that finite-difference time-domain (FDTD) simulation gives better results than Maxwell-Garnett model in explaining the relation between the light polarization and the energies of the observed absorption maxima.     

Fabrication and optical properties of Sb-doped CdS nanowires

Sb-doped CdS nanowires with an average diameter of 30 nm and lengths of up to 20-30 μm are fabricated by chemical vapor approach. The as-synthesized products have a single-crystal phase and grow along the <011> direction. The growth of Sb-doped CdS nanowires is suggested for Quasi-vapor-solid mechanism (QVSM). In particular, the PL spectra show enhancing emission peaks that strongly shift to long wavelength (up to 55 nm redshift) with a doping Sb where Sb-doped CdS nanowires are found to be responsible for the different characteristics. The PL mechanism is explained in detail.     

Photoluminescence and optical limiting properties of silicon nanowires

Si nanowires (SiNWs) have been produced by thermal vaporization on Si(111) substrate without catalysts added. The grown SiNWs have been characterized by Raman scattering, SEM, XRD, and electron diffraction and shown to be highly crystalline with only little impurities such as amorphous Si and silicon oxides. Photoluminescence (PL) study has illustrated that the Si band-to-band gap increases from 1.1 eV for bulk Si to 1.56 eV for the as-grown SiNWs due to quantum confinement effect. A strong PL peak at 521 nm (2.37 eV) is attributed to the relaxation of the photon-induced self-trapped state in the form of surface Si-Si dimers, which may also play an important role in optical limiting of SiNWs with 532-nm nanosecond laser pulses. With the observation of optical limiting at 1064 nm, nonlinear scattering is believed to make a dominant contribution to the nonlinear response of SiNWs.     

Synthesis and optical properties of ZnTe single-crystalline nanowires

Single-crystalline ZnTe nanowires with the zincblende structure have been synthesized on silicon (Si) substrates via a vapor phase transport method. The ZnTe (99.99%) powders were used as the source, and 10 nm-thick thermal evaporated gold (Au) film was used as the catalyst. The as-prepared ZnTe nanowires have diameters of 30-80 nm and lengths of more than 10 microm. The products were analyzed by X-ray diffraction, field emission scanning electron microscopy, and high-resolution transmission electron microscopy. Optical properties of these nanowires were investigated by room-temperature Raman scattering spectrum and temperature-dependent photoluminescence measurements. The results show that the as-prepared ZnTe nanowires are of high crystal quality.     

溶胶-凝胶法制备TiO_2薄膜及其光学性能的研究

采用溶胶-凝胶法在K9玻璃上制备了均匀、透明、裂纹较少的纳米TiO2薄膜,以无水乙醇用量、涂层数、煅烧温度为影响因素,设计L9(3)4正交试验,以薄膜的透明度、微观致密程度作为评价标准,讨论了无水乙醇用量、涂层数和煅烧温度对制备TiO2薄膜光学性能的影响。用反射式椭圆偏振光谱仪测试最佳制备工艺下制得的TiO2薄膜的椭偏参数,并用Cauchy模型对椭偏参数进行数据拟合。结果表明,薄膜最优制备工艺参数为无水乙醇用量30 m L、涂层数为2层、煅烧温度为550℃;Cauchy模型能较好的描述溶胶-凝胶薄膜在300~700 nm波段的光学性能;薄膜的折射率和消光系数都有随波长增大而减小的趋势且制备的薄膜具有随着膜层数的增加,折射率增加,而最大峰值透光率、孔隙率减小的规律。     

Low temperature growth and optical properties of ZnO nanowires using an aqueous solution method

ZnO nanowires were grown on indium tin oxide (ITO) coated glass substrates at a low temperature of 90 degrees C using an aqueous solution method. The ZnO seeds were coated on the ITO thin films by using a spin coater. ZnO nanowires were formed in an aqueous solution containing zinc nitrate hexahydrate (Zn(NO3)2 x 6H2O) and hexamethylenetetramine (C6H12N4). The pH value and concentration of the solution play an important role in the growth and morphologies of ZnO nanowires. The size of ZnO naonowires increased as the concentration of the solution increased. It was formed with a top surface of hexagonal and tapered shape at low and high pH values respectively. Additionally, the single crystalline structure and optical property of the ZnO nanowires were investigated using high-resolution transmission electron microscopy and photoluminescence spectroscopy.     

Annealing effects on the structural and optical properties of gallium oxide nanowires

We have prepared gallium oxide nanowires and investigated the effect of thermal annealing on the structural and optical properties. As-prepared and annealed nanowires, respectively, were found to be fully amorphous and amorphous with very small crystallites. Photoluminescence measurements indicated that both as-prepared and annealed nanowires under excitation at 250 nm showed two emission band at 364 and 466 nm but the relative peak intensities of UV and blue emission, respectively, increase and decrease by the thermal annealing.     

A study of electron-beam-evaporated MgO films using electron diffraction,optical absorption and cathodoluminescence

Reflection high energy electron diffraction, optical absorption and cathodoluminescence were used to study MgO films deposited onto fused silica, single- crystal silicon and LiF substrates at various temperatures. Results showed that some of the same optical absorption and emission bands observed in X- or UV- irradiated, additively colored or mechanically deformed MgO crystals were observed in evaporated MgO films. The peak positions and the relative peak intensities of the optical absorption and emission bands depended on the substrate temperature during film deposition as well as on the structure of the film. The effect of heating the films in air and vacuum on the optical absorption and emission bands is also discussed.     

Optical properties of heavily doped GaAs nanowires and electroluminescent nanowire structures

We present GaAs electroluminescent nanowire structures fabricated by metal organic vapor phase epitaxy. Electroluminescent structures were realized in both axial pn-junctions in single GaAs nanowires and free-standing nanowire arrays with a pn-junction formed between nanowires and substrate, respectively. The electroluminescence emission peak from single nanowire pn-junctions at 10 K was registered at an energy of around 1.32 eV and shifted to 1.4 eV with an increasing current. The line is attributed to the recombination in the compensated region present in the nanowire due to the memory effect of the vapor-liquid-solid growth mechanism. Arrayed nanowire electroluminescent structures with a pn-junction formed between nanowires and substrate demonstrated at 5 K a strong electroluminescence peak at 1.488 eV and two shoulder peaks at 1.455 and 1.519 eV. The main emission line was attributed to the recombination in the p-doped GaAs. The other two lines correspond to the tunneling-assisted photon emission and band-edge recombination in the abrupt junction, respectively. Electroluminescence spectra are compared with the micro-photoluminescence spectra taken along the single p-, n- and single nanowire pn-junctions to find the origin of the electroluminescence peaks, the distribution of doping species and the sharpness of the junctions.     

Impact of Pb doping on the optical and electrical properties of ZnO nanowires

In this letter, effect of Pb-doping on the electrical and optical properties of the as grown ZnO nanowires (NWs) have been investigated. The microstructural investigations show that the Pb-dopant substituted into wurtzite ZnO nanowires without forming any secondary phase. The amount of contents and valence state of Pb ions has been investigated through energy dispersive spectroscopy and X-ray photospectroscopy. The doped nanowires show a remarkable reduction of 15.3 nm (127.4 meV) in the optical band gap, while an increase amount of deep-level defects transition in visible luminescence. Furthermore, the reduction in the band gap and the presence of deep-level defects induces strong effect in the electrical resistivity of doped NWs, which makes their potential for the fabrication of nanodevices. The possible growth mechanism is also briefly discussed.     

Template assisted electrochemical growth of cobalt nanowires: influence of deposition conditions on structural, optical and magnetic properties

The influence of electrodeposition potential, pH, composition and temperature of the electrolytic bath on the structure of cobalt nanowires arrays electrodeposited into anodic aluminum oxide (AAO) porous membranes is reported. XRD, SEM, and TEM analysis were employed to characterize structural (crystal phase, crystallographic texture, and grain size), and morphological nanowire properties. It was confirmed that at pH 2 the electrodeposition potential has not influence on the preferred crystallographic orientation of the electrochemically grown Co nanowires. At pH 4 the electrodeposition potential controls the growth of cobalt nanowires along some preferential crystallographic planes. The electrolytic pH bath modulates the fcc or hcp phase exhibited by the cobalt nanowires. Single crystalline nanowires with a hcp phase strongly oriented along the (2021) crystallographic plane were obtained at pH 4 and at -1.1 V (vs. Ag/AgCl), a result not previously reported. High electrolytic bath temperatures contributed to improve the single crystalline character of the cobalt nanowires. The presence of chloride anion in the electrolytic bath also influenced on the structural properties of the resulting cobalt nanowires, improving their crystallinity. The optical reflectance of the samples shows a structure in the UV-blue region that can be assigned to the two-dimensional morphology arising in the shape of the almost parallel nanowires. Magnetic measurements showed that different electrodeposition potentials and electrolytic bath pH lead to different magnetic anisotropies on the nanowire array samples.