Surface effects on photoluminescence of single ZnO nanowires

The influence of surface effects on the temperature dependent photoluminescence (PL) spectra from individual ZnO nanowires has been studied. It is found that the surface effects of the nanowire are very important in both ultraviolet (UV) and visible emission. We propose a new luminescence mechanism based on the recombination related to oxygen vacancies to explain the temperature dependent visible emission, which is significantly influenced by the carrier depletion and band bending caused by surface effects. In addition, the observed attenuation of UV emission with increasing temperature is ascribed to the decreasing depletion region and the increasing surface states related nonradiative recombination.     

Chemical role of amines in the colloidal synthesis of CdSe quantum dots and their luminescence properties

The role of organic amines in the colloidal synthesis of CdSe quantum dots (QDs) has been studied. CdSe QDs were synthesized from the source solutions containing 5 vol% of amines having various alkyl chain lengths, stereochemical sizes and electron donation abilities. The role of the additional amines was evaluated on the basis of the photoluminescence (PL) properties such as PL wavelength and intensity of the obtained CdSe QDs. The observed PL spectra were explained by the fact that the amines behaved as capping ligands on the surface of the QDs in the product colloidal solution and complex ligands for cadmium in the source solutions. It was shown that the particle size was controlled by the diffusion process depending on the mass and stereochemical shape of the amines, and the luminescence intensity increased with the increasing electron donation ability and capping density of the amines.     

Structural, Optical and Electrical Properties of ZnS/Porous Silicon Heterostructures

ZnS films are deposited by pulsed laser deposition on porous silicon （PS） substrates formed by electrochemical anodization of p-type （100） silicon wafer. Scanning electron microscope images reveal that the surface of ZnS films is unsmoothed, and there are some cracks in the ZnS films due to the roughness of the PS surface. The x-ray diffraction patterns show that the ZnS films on PS surface are grown in preferring orientation along cubic phase β-ZnS （111） direction. White light emission is obtained by combining the blue-green emission from ZnS films with the orange-red emission from PS layers. Based on the I-V characteristic, the ZnS/PS heterojunction exhibits the rectifying junction behaviour, and an ideality factor n is calculated to be 77 from the I-V plot.     

A novel method for extracting oscillator strength of select rare-earth ion optical transitions in nanostructured dielectric materials

A new technique to obtain the oscillator strength of select rare-earth optical transitions in nanostructured dielectric materials (nanophosphors) is presented. It is based on the experimentally observed nanophosphor lifetime dependence on the embedding medium. A constant oscillator strength and parity-allowed electric dipole transitions of the RE ion emission are assumed. The oscillator strength is obtained from the slope of the 1/τ_ij vs. n(n²+2)² plot, where τ_ij is the radiative lifetime of transition between states i and j, and n is the index of refraction of the embedding medium. The use of the technique is illustrated for the Y ₂SiO₅:Ce nanophosphor.     

Thermodynamics Properties of Mesoscopic Quantum Nanowire Devices

We investigate the thermodynamics properties of mesoscopic quantum nanowire devices, such as the effect of electron-phonon relaxation time, Peltier coefficient, carrier concentration, frequency of this field, and channel width. The influence of time-varying fields on the transport through such device has been taken into consideration. This device is modelled as nanowires connecting to two reservoirs. The two-dimensional electron gas in a GaAs- AlGaAs heterojunction has a Fermi wave length which is a hundred times larger than that in a metal. The results show the oscillatory behaviour of dependence of the thermo power on frequency of the induced field. These results agree with the existing experiments and may be important for electronic nanodevices.     

Luminescence Properties of Nanostructure ZnO-Covered Carbon Fibers Prepared by Thermal Oxidation

We report on ZnO nanosheets and nanorods synthesized by thermal oxidation of zinc films deposited on carbon fiber surfaces. The structure and optical properties are characterized by x-ray diffraction, scanning electron microscopy and photoluminescence spectrum. An orange-red emission around 683 nm is found in the PL spectrum when the sample prepaxs at 400℃ for four hours in air. With annealing temperature increasing from 400℃ to 500℃, the blue shift is observed.     

Inter-Layer Energy Transfer through Wetting-Layer States in Bi-layer InGaAs/GaAs Quantum-Dot Structures with Thick Barriers

The inter-layer energy transfer in a bi-layer InGaAs/GaAs quantum dot structure with a thick GaAs barrier is studied using temperature-dependent photoluminescence. The abnormal enhancement of the photoluminescence of the QDs in the layer with a larger amount of coverage considering the resonant Forster energy transfer between the at 110K is observed, which can be explained by wetting layer states at elevated temperatures.     

Refraction Light Control by Constructing Output Interface Topography of Metal Waveguide Arrays

We investigate a nanoscale metal waveguide array (MWGA) structure and demonstrate that negative refraction effect exists from the visible to infrared frequency. Our numerical analysis shows that this effect is related to outputinterface of MWGAs. Refraction light would have different directions on the gradient shaped output surface as a result of phase retardation control by waveguide thickness. Finite-difference time-domain analysis shows that more sharp superdiffraction limit imaging can be obtained by constructing convex-like output interface topography.     

Strong violet luminescence from ZnO nanocrystals grown by the low-temperature chemical solution deposition

The luminescence properties of zinc oxide (ZnO) nanocrystals grown from solution are reported. The ZnO nanocrystals were characterized by scanning electron microscopy, X-ray diffraction, cathodo- and photoluminescence (PL) spectroscopy. The ZnO nanocrystals have the same regular cone form with the average sizes of 100-500 nm. Apart from the near-band-edge emission around 381 nm and a weak yellow-orange band around 560-580 nm at 300 K, the PL spectra of the as-prepared ZnO nanocrystals under high-power laser excitation also showed a strong defect-induced violet emission peak in the range of 400 nm. The violet band intensity exhibits superlinear excitation power dependence while the UV emission intensity is saturated at high excitation laser power. With temperature raising the violet peak redshifts and its intensity increases displaying unconventional negative thermal quenching behavior, whereas intensity of the UV and yellow-orange bands decreases. The origin of the observed emission bands is discussed.     

Quantum interference in the Raman scattering from the silicon nanostructures

We report here microscopic process involved in the photo-excited Fano interaction due to nonlinear process in the silicon nanostructures. Photo-excited Raman line-shapes are investigated to reveal the presence of nonlinear Fano interaction in the silicon nanostructures for three different sizes. The Fano interaction is found to be more prominent due to the phase matching between electronic and phonon Raman scatterings for smaller sized nanostructures. Phase matching is achieved by nonlinear process of two-wave mixing in the silicon nanostructures followed by the formation of electron-phonon bound state.     

Growth Mechanism and Optimized Parameters to Synthesize Nafion-115 Nanowire Arrays with Anodic Aluminium Oxide Membranes as Templates

Nation-115 nanowire arrays are synthesized with an extrusion method using AAO membranes as templates. It is indicated that the vacuum treating of AAO templates before surface decoration plays an important role in obtaining high filling rate of the Nation-115 nanowires in the AAO templates, vchile the concentration of Nafion-115 DMSO solutions does not affect the flling rate greatly. The optimized parameters to synthesize the Nation-115 nanowire arrays are studied. The filling rate of the Nadion-115 nanowires in the AAO templates synthesized with the optimized parameters is about 95% discussed to qualitatively explain the experimental results The growth mechanism of Nafion-115 nanowires is     

Growth and characterization of ZnSe/CdSe/ZnSe quantum dots fabricated by using an alternate molecular beam supplying method

ZnSe/CdSe/ZnSe structures inserted CdSe thin layer are fabricated using an alternate molecular beam supply (ALS). Examining the PL peak energy dependence on beam irradiation time in ALS cycle, we studied the initial stage of CdSe growth. When CdSe below the critical thickness is supplied on ZnSe grown on GaAs (1 0 0), two kinds of 2D islands (platelets) appear. We confirmed the alloying of 2D-CdSe islands and 3D-CdSe islands (dots) is prominent under Cd beam irradiation in ALS growth.     

Structural and Optical Properties of Zinc Nitride Films Prepared by Pulsed Filtered Cathodic Vacuum Arc Deposition

Polycrystalline zinc nitride films are deposited on Coming 7059 glass substrates by pulsed filtered cathodic vacuum arc deposition （PFCVAD）. The crystallographic structure is studied by means of x-ray diffraction. These measurements show that all the films are crystallized in the cubic structure, in a preferred orientation along the （332） and （631） directions. Weak XRD signal shows small crystallites distributed in an amorphous tissue. A small improvement of crystallinity is observed with annealing. Optical parameters such as absorption, energy band gap, Urbach tail, extinction coefficients have been determined. The Urbach tail energy is decreased with annealing at 500℃ for one hour. Energy band gap values are found to be increased by annealing.     

Photoluminescence spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532-nm laser radiation and gamma-rays

We have investigated temporal behavior of the photoluminescence (PL) spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532 nm laser radiation and gamma-rays. Under ∼100 W/cm² laser radiation, the PL intensity (I_PL) increases with irradiation time upto about 500 s and thereafter declines linearly. The wavelength of the PL emission (λ_peak) exhibits a blue-shift with exposure time. Upon simultaneous irradiation by 100 W/cm² 532-nm laser, as well as 0.57 and 1.06 MeV gamma-rays, the temporal behaviors of both I_PL and λ_peak are significantly different; I_PL increases to a saturation level, and the magnitude of the blue-shift in λ_peak is reduced. We discuss possible mechanisms underlying these results.     

Growth and optical characterization of single quantum well structure of submonolayer ZnS/ZnTe

Single quantum wells of submonolayer ZnS/ZnTe were grown between ZnTe layers using hot wall epitaxy method with fast-movable substrate configuration. As ZnS well widths decrease from 1 to 0.15 monolayer, the photoluminescence peaks shift to higher energies from 2.049 to 2.306 eV, and the photoluminescence intensities increase. As ZnS well width decrease, the PL spectra show the lower-energy tails and consequently the increased PL FWHMs. This is a result of a convolution of two PL peaks from two-dimensional and zero-dimensional quantum islands, supported by a still lived lower-energy peaks of zero-dimensional quantum islands above 50 K. The energy shift in the power dependence of photoluminescence spectra is proportional to the third root of the excitation density. These behaviors can be described by the formation of submonolayer type-II ZnS/ZnTe quantum well structure, and the coexistence of two-dimensional and one-dimensional islands in ZnS layers.     

Structural, optical, and electronic properties of colloidal CuO nanoparticles formed by using a colloid-thermal synthesis process

Colloidal cupric oxide (CuO) nanoparticles were formed by using a colloid-thermal synthesis process. X-ray diffraction patterns, transmission electron microscopy (TEM) images, high-resolution TEM images, and X-ray energy dispersive spectrometry profiles showed that the colloidal CuO nanoparticles were formed. The optical band-gap energy of CuO nanoparticles at 300 K, as determined from the absorbance spectrum, was 3.63 eV. A photoluminescence spectrum at 300 K showed that a dominant emission peak appeared at the blue region. X-ray photoelectron spectroscopy profiles showed that the O 1s and the Cu 2p peaks corresponding to the CuO nanoparticles were observed.     

Multiple Wavelength-Channels in SPP Waveguides for Optical Communication

Surface plasmon polaritons （SPPs） can be excited, meanwhile some peculiar optical phenomena will appear when light irradiates metal structures under some conditions. Based on photonie band gap theory, in this Letter we present a kind of SPP waveguide with multiple wavelength-channels. By using the Bragg effect and introducing some geometric defect layers into a quasi-periodic metal heterowaveguide, the multiple SPP forbidden bands （SPFBs） in a given waveband can be generated, and the multiple SPP pass bands （SPPBs） with narrow bandwidth in each SPFB can be realized. The SPP propagation in metal heterowaveguide is calculated by FDTD and transfer matrix methods. By selecting appropriate thickness, position and the number of defect layers, two SPPBs can be achieved in the SPFBs around 1.31 and 1.55μm simultaneously.     

Influence of annealing temperature on the photoluminescence properties of ZnO quantum dots

The properties of ZnO quantum dots (QDs) synthesized by the sol-gel process are reported. The primary focus is on investigating the origin of the visible emission from ZnO QDs by the annealing process. The X-ray diffraction results show that ZnO QDs have hexagonal wurtzite structure and the QD diameter estimated from Debye-Scherrer formula is 8.9 nm, which has a good agreement with the results from transmission electron microscopy images and the theoretical calculation based on the Potential Morphing Method. The room-temperature photoluminescence spectra reveal that the ultraviolet excitation band has a red shift. Meanwhile, the main band of the visible emission shifts to the green luminescence band from the yellow luminescence one with the increase of the annealing temperature. A lot of oxygen atoms enter into Zn vacancies and form oxygen antisites with increasing temperature. That is probably the reason for the change of the visible emission band.     

Influence of metallic coatings on the photoluminescence properties of ZnO nanowires

We report on low‐temperature photoluminescence studies of ZnO nanowires coated with thin metallic films. For all analyzed metals (Al, In, Au, Ni, Cu), we find an increased relative intensity of the green deep‐level emission. This is accompanied by a significant reduction of the relative intensity of the surface exciton band. The observed effects are most likely related to the formation of metal induced gap states in the surface region of the ZnO nanowires. A model for the band structure in the surface region of the metal‐coated nanowires is proposed that successfully explains the changes in the photoluminescence spectra after the coating process. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical Properties of Co-BaTiO3/Mg（100） Nano-Composite Films Grown by Pulsed Laser Deposition Method

Co nanoparticles embedded in a BaTiO3 matrix, namely Co-BaTiO3 nano-composite films are grown on Mg（100） single crystal substrates by the pulsed laser deposition （PLD） method at 650℃. Optical properties of the CoBaTiO3 nano-composite films are examined by absorption spectra （AS） and photoluminescence （PL） spectra. The results indicate that the concentration of Co nano-particles strongly influences the electron transition of the Co BaTiO3 nano-composite films. The PL emission band ranging from 1.9 to 2.2eV is reported. The AS and PL spectra suggest that the band gap is in the range of 3.28-3.7eV.