Microstructure and electrical–optical properties of cesium tungsten oxides synthesized by solvothermal reaction followed by ammonia annealing

Graphene has been studied intensively in opto-electronics, and its transport properties are well established. However, efforts to induce intrinsic optical properties are still in progress. Herein, we report the production of micron-sized sheets by interconnecting graphene quantum dots (GQDs), which are termed ‘GQD solid sheets’, with intrinsic absorption and emission properties. Since a GQD solid sheet is an interconnected QD system, it possesses the optical properties of GQDs. Metal atoms that interconnect the GQDs in the bottom-up hydrothermal growth process, induce the semiconducting behaviour in the GQD solid sheets. X-ray absorption measurements and quantum chemical calculations provide clear evidence for the metal-mediated growth process. The as-grown graphene quantum dot solids undergo a Forster Resonance Energy Transfer (FRET) interaction with GQDs to exhibit an unconventional 36% photoluminescence (PL) quantum yield in the blue region at 440 nm. A high-magnitude photocurrent was also induced in graphene quantum dot solid sheets by the energy transfer process.

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Graphene Quantum Dot Solid Sheets: Strong blue-light-emitting & photocurrent-producing band-gap-opened nanostructures.

electrochromic properties of n-doped tungsten oxide thin films prepared by reactive dc-pulsed sputtering

The contribution of curvature dependent sputtering and mass redistribution to ion-induced self-organized formation of periodic surface nanopatterns is revisited. Ion incidence angle-dependent curvature coefficients and ripple wavelengths are calculated from 3-dimensional collision cascade data obtained from binary collision Monte Carlo simulations. Significant modifications concerning mass redistribution compared to the model of Carter and Vishnyakov and also models based on crater functions are introduced. Furthermore, I find that curvature-dependent erosion is the dominating contribution to pattern formation, except for very low-energy irradiation of a light matrix with heavy ions. The major modifications regarding mass redistribution and ion-induced viscous flow are related to the ion incidence angle-dependent thickness of the irradiated layer. A smaller modification concerns the relaxation of inward-directed mass redistribution. Ion-induced viscous flow in the surface layer also depends on the layer thickness and is thus strongly angle dependent. Simulation results are presented and compared to a variety of published experimental results. The simulations show that in most cases curvature-dependent erosion is the dominant contribution to surface instability and ripple pattern formation and also determines the pattern orientation transition. The simulations predict the occurrence of perpendicular ripple patterns at larger ion incidence angles, in agreement with experimental observations. Mass redistribution causes stabilization of the surface at near-normal ion incidence angles and dominates pattern formation only at very low ion energies.

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Surface instability and pattern formation by ion-induced erosion and mass redistribution

Silver incorporated tungsten oxide thin films are prepared by RF magnetron sputtering technique. The effect of silver incorporation in micro structure evolution, phase enhancement, band gap tuning and other optical properties are investigated using techniques such as x-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, and UV-Visible spectroscopy. Effect of silver addition in phase formation and band gap tuning of tungsten oxide thin films are investigated. It is found that the texturing and phase formation improves with enhancement in silver content. It is also found that as the silver incorporation enhances the thickness of the films increases at the same time the strain in the film decreases. Even without annealing the desired phase can be achieved by doping with silver. A broad band centered at the wavelength 437 nm is observed in the absorption spectra of tungsten oxide films of higher silver incorporation and this can be attributed to surface plasmon resonance of silver atoms present in the tungsten oxide matrix. The transmittance of the films is decreased with increase in silver content which can be due to increase in film thickness, enhancement of scattering, and absorption of light caused by the increase of grain size, surface roughness and porosity of films and enhanced absorption due to surface plasmon resonance of silver. It is found that silver can act as the seed for the growth of tungsten oxide grains and found that the grain size increases with silver content which in turn decreases the band gap of tungsten oxide from 3.14 eV to 2.70 eV.

Effect of silver incorporation in phase formation and band gap tuning of tungsten oxide thin films

This article describes the development of a soft x-ray beamline on a bending magnet source of Indus-2 storage ring (2.5 GeV) and some preliminary results of x-ray absorption spectroscopy (XAS) measurements using the same. The beamline layout is based on a spherical grating monochromator. The beamline is able to accept synchrotron radiation from the bending magnet port BL-1 of the Indus-2 ring with a wide solid angle. The large horizontal and vertical angular acceptance contributes to high photon flux and selective polarization respectively. The complete beamline is tested for ultrahigh vacuum (UHV) ∼ 10−10 mbar. First absorption spectrum was obtained on HOPG graphite foil. Our performance test indicates that modest resolving power has been achieved with adequate photon flux to carry out various absorption experiments.

Development of soft X-ray polarized light beamline on Indus-2 synchrotron radiation source

Nitrogen incorporated tungsten oxide films are deposited on heated quartz substrates (973 K) for various nitrogen background pressures (namely, pN2 = 0.04, 0.08, 0.12, 0.17 and 0.20 mbar) by the pulsed laser deposition technique. The prepared films are characterized by using techniques such as x-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDX), atomic force microscopy (AFM), scanning electron microscopy, micro-Raman spectroscopy, UV–visible spectroscopy and temperature dependent dc electrical resistivity studies. XRD measurements revealed the presence of tungsten sub-oxide and WO3 phases depending on the nitrogen ambient pressure. Nitrogen incorporation in the films is confirmed through EDX analysis. Morphological features consisting of densely packed, void-free grains with a distinct grain boundary are portrayed through the AFM images of the films. The WO3 lattice distortion due to nitrogen incorporation is discussed using micro-Raman spectra of the films. Band gap narrowing, moderate transmittance in the visible range and the very low resistivity exhibited by the films prepared at pN2 = 0.12 mbar are imperative as far as solar cell applications are considered. A comparably higher visible transmittance with a band gap of 3.141 eV and a low resistivity obtained for the N : WO3 films prepared at a nitrogen ambient of 0.17 mbar is among the best reported for the coexistence of transparency in the visible range and conductivity in tungsten oxide films, which make them significant for micro-electronic applications.

Transparent and low resistive nanostructured laser ablated tungsten oxide thin films by nitrogen doping: I. Nitrogen pressure

Crystalline tungsten oxide thin films are obtained by thermal oxidation of metallic W films, deposited on quartz substrates under vacuum conditions by the pulsed laser deposition (PLD) technique. The as-deposited tungsten films are annealed in air at various temperatures, viz. 673, 873 and 1073 K. X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and micro-Raman techniques are employed for the investigation of microstructural characterization of both as-deposited and annealed films. The influence of laser energy and duration of deposition on the microstructural, optical and electrical properties of the films are studied in detail. Among the films investigated, thicker tungsten films (having a thickness of ~500 nm), deposited with laser fluence 4 J cm−2 for 45 min duration and annealed at 873 K, exhibit better physical properties and microstructural features compared to other films.

Influence of post-deposition thermal annealing on the properties of pulsed laser deposited tungsten layers

Generation of self organized nanoripple patterns on Si (100) single crystal surface using low energy Ar ion beam erosion has been studied. Ion energy and ion fluence dependence of the ripple pattern is in general agreement with the reported works. However, it is found that at relatively low fluences, the pattern formation depends upon the direction of the projection of the ion beam on Si surface with respect to its crystallographic orientation. Ripple formation is facilitated if the projection of ion beam on the sample surface is along (010) direction as against (011) direction. For higher ion fluence, when the Si surface layer is fully amorphized, pattern formation is independent of the azimuthal direction of the ion beam. Copyright © 2013 VBRI press.

Satish Potdar

Papers

Development of soft X-ray polarized light beamline on Indus-2 synchrotron radiation source

Transparent and low resistive nanostructured laser ablated tungsten oxide thin films by nitrogen doping: I. Nitrogen pressure

Influence of post-deposition thermal annealing on the properties of pulsed laser deposited tungsten layers

Azimuthal angle dependence of nanoripple formation on Si (100) by low energy ion erosion

Induction cycle influence on physicochemical and optoelectronic properties of nanostructured In2S3 thin films for photocatalytic activity