On the origin of blue light emission from Ge-nanocrystals containing a-SiOxfilms

We have studied the photoluminescence of a-Si_xGe_yO_{1 − x − y}films with average Ge-nanocrystal sizes ranging from over 100 nm down to 2 nm. No systematic peak shift of the luminescence bands at 3.0 eV and 2.0 eV with the diameter of the nanocrystals is observed. Comparision with a simplified confinement model shows that quantum size effects cannot explain the blue luminescence. We propose the Ge²₀defect as a likely source for this band, based on considerations about the crystallization process.     

The diamagnetic susceptibility of hydrogenic donor in two-dimensional semiconductors with anisotropic effective mass of carriers

In this study, the effects of quantum confinement and effective mass anisotropy parameter on the diamagnetic susceptibility of a hydrogenic donor placed in GaAs, Si, and Ge quantum wells with infinite confinement potential are investigated in the effective mass and parabolic band approximations by using two and one parameter trial wave functions. It is observed that the diamagnetic susceptibility of a hydrogenic donor in anisotropic quantum wells is essentially equal to the transverse diamagnetic susceptibility part when well widths are larger than L > 100 Å, and the impurity is located at center. Moreover, a two parameter trial wave function model gives higher values of diamagnetic susceptibility, except for χ_z (GaAs).     

High resolution electron energy loss spectroscopy surface studies of hydrogenated detonation nano-diamond spray-deposited films

Nano-diamond films composed of 3–5 nm grains prepared by the detonation method and spray deposited onto silicon substrates were examined by high resolution electron energy loss spectroscopy (HR-EELS), Raman spectroscopy and transmission electron microscopy. The HR-EEL spectrum of the annealed and hydrogenated films displays dominant C–H losses at 360–365 meV; the diamond optical phonon and its overtones. These results suggest that the films reveal well defined hydrogenated diamond surfaces on the nanometric scale. Detailed analysis of the diamond optical phonon overtone revealed a red-shift of the basic C–C vibration by 5 meV. We attribute this shift to a phonon quantum confinement effect detected by HR-EELS spectroscopy.     

Influence of argon ambience on the structural,morphological and optical properties of pulsed laser ablated zinc sulfide thin films

Nanostructured zinc suplhide thin films are successfully deposited on quartz substrates using pulsed laser deposition (PLD) under different argon pressures (0, 5, 10, 15 and 20 Pa). The influence of argon ambience on the microstructural, optical and luminescence properties of zinc sulfide (ZnS) thin films is systematically investigated. The GIXRD data suggests rhombohedral structure for ZnS films prepared under different argon ambience. Self-assembly of grains into well-defined patterns along the y direction is observed in the AFM image of the film deposited under argon pressure 20 Pa. All the films show a blue shift in optical band gap. This can be due to the quantum confinement effect and less widening of conduction and valence band for the films with less thickness and smaller grain size. The PL spectra of the different films are recorded at excitation wavelengths 250 nm and 325 nm and the spectra are interpreted. The PL spectra of the films recorded at excitation wavelength 325 nm show intense yellow emission. The film deposited under an argon pressure of 15 Pa shows the highest PL intensity for excitation wavelength 325 nm. For the PL spectra (excitation at 250 nm), the highest PL intensity is observed for the film prepared under argon free ambience. In our study, 15 Pa is the optimum argon pressure for better crystallinity and intense yellow emission when excited at 325 nm.     

Crossover from strong to weak exciton confinement and third-harmonic generation on one-dimensional quantum dots

We study the effects of exciton confinement on the nonlinear optical susceptibility of one-dimensional quantum dots. We use a direct numerical diagonalization to obtain the eigenenergies and eigenstates of the discretized Hamiltonian representing an electron–hole pair confined by a semiparabolic potential and interacting with each other via a Coulomb potential. Density matrix perturbation theory is used to compute the nonlinear optical susceptibilities due to third-harmonic generation and the corresponding nonlinear corrections to the refractive index and absorption coefficient. These quantities are analyzed as a function of ratio between the confinement length L and the exciton Bohr radius a₀. The Coulomb potential degrades the uniformity of the level separation. We show that this effect promotes the emergence of multiple resonance peaks in the third-harmonic generation spectrum. In the weak confinement regime β = L/a₀ ? 1, the third-order susceptibility is shown to decay as 1/β⁸ due to the prevalence of the hydrogenoid character of the exciton eigenstates.     

Chemical bath deposition of SnO2 and Cd2SnO4 thin films

A new approach of chemical bath deposition (CBD) of SnO₂ thin films is reported. Films with a 0.2 μm thickness are obtained using the multi-dip deposition approach with a deposition time as little as 8–10 min for each dip. The possibility of fabricating a transparent conducting oxide layer of Cd₂SnO₄ thin films using CBD is investigated through successive layer deposition of CBD-SnO₂ and CBD-CdO films, followed by annealing at different temperatures. High quality films with transmittance exceeding 80% in the visible region are obtained. Annealed CBD-SnO₂ films are orthorhombic, highly stoichiometric, strongly adhesive, and transparent with an optical band gap of ～4.42 eV. Cd₂SnO₄ films with a band gap as high as 3.08 eV; a carrier density as high as 1.7 × 10²⁰ cm^?3; and a resistivity as low as 1.01 × 10^?2 Ω cm are achieved.     

Magnetic anisotropy of epitaxial Fe layers grown on Si(0 0 1)

The magnetic properties of epitaxial iron films up to 80 monolayers (ML) thickness grown on Si(0 0 1) by using a template technique were investigated by means of superconducting quantum interference device and magneto-optic Kerr effect techniques. The thinnest films investigated (∼3 ML) exhibit a composition close to Fe₃Si with a Curie temperature below room temperature (RT) and strong out-of-plane remanent magnetization that reflects the presence of a dominant second order surface anisotropy term. Thicker films (⩾4 ML) are ferromagnetic at RT with remanent magnetization in film-plane and a composition closer to pure Fe with typically 8–10% silicon content. When deposited at normal incidence such films show simple in-plane fourfold anisotropy without uniaxial contribution. The relevant fourth-order effective anisotropy constant K₄^eff was measured versus film thickness and found to change its sign near 18 ML. The origin of this remarkable behavior is investigated by means of a Néel model and mainly traced back to fourth-order surface anisotropy and magneto-elastic effects related to the large biaxial in-plane compressive strain up to 3.5% in the thinnest (⩽25 ML) films.     

Size dependence of ground-state energy of the excitons in spherical Y2O3

The exciton energies of rare earth oxides (Ln₂O₃) have rarely been calculated by the theory. Experimentally, the blue-shift of exciton energy in nanocrystals deviates from the traditional size confinement effect. Herein, the dependence of the ground-state energy of an exciton in Y₂O₃ spheres on particle radius was calculated by using a variational method. In the model, an exciton confined in a sphere surrounded by a dielectric continuum shell was considered. The ground-state energy of exciton comprises kinetic energy, coulomb energy, polarization energy and exciton–phonon interaction energy. The kinetic and coulomb energy were considered by the effective mass and the dielectric continuum and the exciton–phonon interaction energy was given by the intermediate coupling method. The numerical results demonstrate that the present model is roughly consistent with the experimental results. The confinement effect of the kinetic energy is dominant of the blue-shift of the exciton energy in the region of R < 5 nm, while confinement effect of the coulomb energy is dominant of the blue-shift of the exciton energy in the region of R > 5 nm. The polarization energy contributes largely to the exciton energy as the particle size is smaller than ~ 10 nm, while the exciton–phonon interaction energy takes only a little contribution in all the range.     

Thickness dependence of Ic and Jc of LTG-SmBCO coated-conductor on IBAD-MgO tapes

We have reported SmBa₂Cu₃O_y (SmBCO) films on single crystalline substrates prepared by low-temperature growth (LTG) technique. The LTG-SmBCO films showed high critical current densities in magnetic fields compared with conventional SmBCO films prepared by pulsed laser deposition (PLD) method. In this study, to enhance critical current (I_c) in magnetic field, we fabricated thick LTG-SmBCO films on metal substrates with ion-beam assisted deposition (IBAD)-MgO buffer and estimated the I_c and J_c in magnetic fields.All the SmBCO films showed c-axis orientation and cube-on-cube in-plane texture. T_c of the LTG-SmBCO films were 93.1–93.4 K. J_c and I_c of a 0.5 μm-thick SmBCO film were 3.0 MA/cm² and 150 A/cm-width at 77 K in self-field, respectively. Those of a 2.0 μm-thick film were 1.6 MA/cm² and 284 A/cm-width respectively. Although I_c increased with the film thickness increasing up to 2 μm, the I_c tended to be saturated in 300 A/cm-width. From a cross sectional TEM image of the SmBCO film, we recognized a-axis oriented grains and 45° grains and Cu–O precipitates. Because these undesired grains form dead layers, I_c saturated above a certain thickness. We achieved that I_c in magnetic fields of the LTG-SmBCO films with a thickness of 2.0 μm were 88 A/cm-width at 1 T and 28 A/cm-width at 3 T.     

Texture analysis of RF-sputtered CeO2 buffer layers and superconducting MOD-YBCO films

CeO₂ buffer layers were deposited on YSZ single-crystal substrates using an RF-sputtering method. The development of crystalline textures of sputtered CeO₂ films at different sputtering pressure and their effects on YBCO films, deposited by Metal Organic Deposition (MOD), were investigated. Both CeO₂ and subsequent YBCO films grew well epitaxially. The relative XRD peak intensities of CeO₂ (2 0 0) to substrate YSZ (2 0 0) increased with deposition pressure in the range of 3–5 mTorr and were inversely proportional to the θ–2θ scan FWHM values of CeO₂ (2 0 0). Also, the reaction layers of BaCeO₃ were thicker in the samples with lower CeO₂ (2 0 0) intensities and poor out-of-plane alignment when CeO₂ were deposited at the lower pressure of 3.3 mTorr. It is noted, however, that the superconducting layer grew well epitaxially on these BaCeO₃ layers, possibly due to the epitaxial relation between CeO₂ and YBCO. The superconducting critical currents of MOD-YBCO films showed an increasing tendency as both the Δ2θ (CeO₂) and BaCeO₃ peak intensities decreased.     

Growth and characterization of indium oxide films

In₂O₃ films have been deposited using chemical spray pyrolysis technique at different substrate temperatures that varied in the range, 250–450 °C. The structural and morphological properties of the as-deposited films were studied using X-ray diffractometer and scanning electron microscope as well as atomic force microscope, respectively. The films formed at a temperature of 400 °C showed body-centered cubic structure with a strong (2 2 2) orientation. The structural parameters such as the crystallite size, lattice strain and texture coefficient of the films were also calculated. The films deposited at a temperature of 400 °C showed an optical transmittance of >85% in the visible region. The change of resistivity, mobility, carrier concentration and activation energies with the deposition temperature was studied. The highest figure of merit for the layers grown at 400 °C was 1.09 × 10⁻³ Ω⁻¹.     

The growth and field evaporation of Er and deuterated Er films

The morphological structure of clean and deuterated Er films deposited on W substrates and their removal by field evaporation have been investigated as part of a program directed toward the development of deuterium ion sources for neutron generators. Annealed Er films up to ~ 20 monolayers in thickness deposited on W < 110 > substrates appear pseudomorphic. Thicker annealed films form a hexagonal close-packed < 0001 > orientated over-layer with the Pitsch–Schrader orientation relation. The pseudomorphic and hexagonal close-packed character of the films is retained up to the last atomic layer that forms the film-substrate interface. Deuterated Er films appear polycrystalline. At 77 K in Ar, annealed Er films field evaporate at 2.5 V/Å primarily as Er^2 + and deuterated Er films evaporate at ~ 2.4 V/Å primarily as ErD_x^2 +. Field evaporation of both clean and deuterated Er films shows signs of space charge induced field lowering when film thicknesses exceeding ~ 10 layers were field evaporated using 20 ns duration voltage pulses.     

Broadband near infrared quantum cutting in Bi–Yb codoped Y2O3 transparent films on crystalline silicon

By a pulsed laser deposition technique the efficient broadband near-infrared downconversion Bi–Yb codoped crystallization Y₂O₃ transparent films have been grown successfully on Si (1 0 0) substrates. Upon excitation of ultraviolet photon varying from 300 to 400 nm, the near infrared quantum cutting has been obtained, which is originated from the transitions of the transition-metal Bi^{3+ 3}P₁ level to Yb^{3+ 2}F_5/2 level. The downconversion quantum efficiency of films is estimated to be 152%. The transparent Y₂O₃ films may have potential application in enhancing the conversion efficiency of crystalline Si solar cells.     

Preparation and characterization of Fe3O4(1 1 1) nanoparticles and thin films on Au(1 1 1)

Fe₃O₄ nanoparticles and thin films were prepared on the Au(1 1 1) surface and characterized using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Fe₃O₄ was formed by annealing α-Fe₂O₃(0 0 0 1) structures on Au(1 1 1) at 750 K in ultrahigh vacuum (UHV) for 60 min. Transformation of the α-Fe₂O₃(0 0 0 1) structures into Fe₃O₄ nanoparticles and thin films was supported by XPS. STM images show that during the growth procedure used, Fe₃O₄ initially appears as nanoparticles at low coverages, and forms thin films at ～2 monolayer equivalents (MLE) of iron. Two types of ordered superstructures were observed on the Fe₃O₄ particles with periodicities of ～50 and ～42 Å, respectively. As the Fe₃O₄ particles form more continuous films, the ～50 Å feature was the predominant superstructure observed. The Fe₃O₄ structures at all coverages show a hexagonal unit cell with a ～3 Å periodicity in the atomically resolved STM images.     

Synthesis of superhydrophobic PTFE-like thin films by self-nanostructuration in a hybrid plasma process

Superhydrophobic poly(tetrafluoro-ethylene) (PTFE) like thin films were grown on silicon wafers using a plasma-based hybrid process consisting on sputtering a carbon target in an Ar/CF₄ atmosphere. The influence of the bias voltage applied to the substrate (V_Bias) as well as of the gas mixture composition (%CF₄) on the chemical composition, the wettability and the morphology of the deposited thin films were evaluated.The chemical composition measured by X-ray Photoelectron Spectroscopy (XPS) has revealed that the F/C atomic ratio is always lower than for conventional PTFE (F/C = 2) and that it decreases when V_Bias increases (from F/C = 1 for V_Bias = ? 100 V to F/C = 0.75 for V_Bias = ? 200 V). This behavior is associated with the preferential sputtering of the fluorine atoms during the plasma-assisted growth of the films. Consecutively, a self-nanostructuration enhanced when increasing V_Bias is observed. As a consequence, the water contact angle (WCA) measurements range from 70° up to 150° depending on (i) the fluorine concentration and (ii) on the magnitude of the nanostructuration. In addition, for the films presenting the highest WCAs, a small hysteresis between the advancing and receding WCAs is observed (< 10°) allowing these films to fulfill completely the requirements of superhydrophobicity. The nanostructuration is probably due to the chemical etching by fluorine atoms of the fluorinated group.In order to get more understanding on the wettability mechanisms of these surfaces, the topography of the films has been evaluated by atomic force microscopy (AFM). The data have revealed, for all films, a dense and regular structure composed by conic objects (A_vH is their average height and A_vD is the average distance between them) for which the dimensions increase with V_Bias. A correlation between A_vH/A_vD, defined as the “morphological ratio”, with the WCA was established. Theoretical evaluations of the WCA using the Wenzel and Cassie equations with, as inputs, the features of the deposited thin film surfaces measured by AFM suggest that the wetting regime is intermediate between these two ideal situations.     

Annealing temperature dependent on structural,optical and electrical properties of indium oxide thin films deposited by electron beam evaporation method

In the present work the influence of annealing temperature on the structural and optical properties of the In₂O₃ films deposited by electron beam evaporation technique in the presence of oxygen was studied. The deposited films were annealed from 350 to 550 °C in air. The chemical compositions of In₂O₃ films were carried out by X-ray photoelectron spectroscopy (XPS). The film structure and surface morphologies were investigated as a function of annealing temperature by X-ray diffraction (XRD) and atomic force microscopy (AFM). The structural studies by XRD reveal that films exhibit preferential orientation along (2 2 2) plane. The refractive index (n), packing density and porosity (%) of films were arrived from transmittance spectral data obtained in the range 250–1000 nm by UV–vis-spectrometer. The optical band gap of In₂O₃ film was observed and found to be varying from 3.67 to 3.85 eV with the annealing temperature.     

Donor binding energies in GaAs quantum wells considering the band nonparabolicity effects and the wavefunction elongation

The donor binding energies in finite GaAs/Ga_xAl_1 − xAs quantum wells have been calculated by considering the confinement of electrons, which increases as the well width increases. The variational solutions have been improved by using a two-parameter trial wavefunction, and by including the conduction band nonparabolicity. It is shown that the method used gives results in agreement with those obtained in the experiments on the effective mass and the donor binding energy, both of which are strongly dependent on the well width.     

AZO/Ag/AZO multilayer films prepared by DC magnetron sputtering for dye-sensitized solar cell application

Ag films were deposited on Al-doped ZnO (AZO) films and coated with AZO to fabricate AZO/Ag/AZO multilayer films by DC magnetron sputtering on glass substrates without heating of glass substrates. The best multilayer films have low sheet resistance of 19.8 Ω/Sq and average transmittance values of 61% in visible region. It was found that the highest figure of merit (F_TC) is 6.9 × 10⁻⁴ Ω⁻¹. For the dye-sensitized solar cell (DSSC) application, the multilayer films were used as transparent conductive electrode (multilayer films/ZnO + Eosin-Y/LiI + I₂/Pt/FTO). The best DSSC based on the multilayer films showed that open circuit voltage (V_oc) of 0.47 V, short circuit current density (J_sc) of 2.24 mA/cm², fill factor (FF) of 0.58 and incident photon-to-current conversion efficiency (η) of 0.61%. It was shown that the AZO/Ag/AZO multilayer films have potential for application in DSSC.     

Room temperature phosphorescence lifetime and quantum yield of erythrosine B and rose bengal in aerobic alkaline aqueous solution

The room-temperature phosphorescence behavior of erythrosine B (ER) and rose bengal (RB) in aerobic aqueous solution at pH 10 (10^?4 M NaOH) is investigated. The samples were excited with sliced second harmonic pulses of a Q-switched Nd:glass laser. A gated photomultiplier tube was used for instantaneous fluorescence signal discrimination and a digital oscilloscope was used for signal recording. For phosphorescence lifetime measurement the oscilloscope response time was adjusted to appropriate time resolution and sensitivity by the ohmic input resistance. In the case of phosphorescence quantum yield determination the gated photomultiplier – oscilloscope arrangement was operated in integration mode using 10 MΩ input resistance. Phosphorescence quantum yield calibration was achieved with erythrosine B and rose bengal doped starch films of known quantum yields. The determined phosphorescence lifetimes (quantum yields) of ER and RB in 0.1 mM NaOH are τ_P=1.92±0.1 μs (?_P=(1.5±0.3)×10^?5) and 2.40±0.1 μs ((5.7±0.9)×10^?5), respectively. The results are discussed in terms of triplet state deactivation by dissolved molecular oxygen.     

Structural and optical properties of Mn-doped ZnO nanocrystalline thin films with the different dopant concentrations

The transparent nanocrystalline thin films of undoped zinc oxide and Mn-doped (Zn_1−xMn_xO) have been deposited on glass substrates via the sol–gel technique using zinc acetate dehydrate and manganese chloride as precursor. The as-deposited films with the different manganese compositions in the range of 2.5–20 at% were pre-heated at 100 °C for 1 h and 200 °C for 2 h, respectively, and then crystallized in air at 560 °C for 2 h. The structural properties and morphologies of the undoped and doped ZnO thin films have been investigated. X-ray diffraction (XRD) spectra, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to examine the morphology and microstructure of the thin films. Optical properties of the thin films were determined by photoluminescence (PL) and UV/Vis spectroscopy. The analyzed results indicates that the obtained films are of good crystal quality and have smooth surfaces, which have a pure hexagonal wurtzite ZnO structure without any Mn related phases. Room temperature photoluminescence is observed for the ZnO and Mn-doped ZnO thin films.