Soft X-ray characterization of Zn1−xSnxOy electronic structure for thin film photovoltaics

[[abstract]]Zinc tin oxide (Zn1−xSnxOy) has been proposed as an alternative buffer layer material to the toxic, and light narrow-bandgap CdS layer in CuIn1−x,GaxSe2 thin film solar cell modules. In this present study, synchrotron-based soft X-ray absorption and emission spectroscopies have been employed to probe the densities of states of intrinsic ZnO, Zn1−xSnxOy and SnOx thin films grown by atomic layer deposition. A distinct variation in the bandgap is observed with increasing Sn concentration, which has been confirmed independently by combined ellipsometry-reflectometry measurements. These data correlate directly to the open circuit potentials of corresponding solar cells, indicating that the buffer layer composition is associated with a modification of the band discontinuity at the CIGS interface. Resonantly excited emission spectra, which express the admixture of unoccupied O 2p with Zn 3d, 4s, and 4p states, reveal a strong suppression in the hybridization between the O 2p conduction band and the Zn 3d valence band with increasing Sn concentration.[[notice]]補正完畢[[journaltype]]國外[[incitationindex]]SCI[[booktype]]電子版[[countrycodes]]GB

Thickness dependent magnetic transitions in pristine MgO and ZnO sputtered thin films

We report a systematic study of the thickness dependency of room temperature ferromagnetism in pristine MgO (~100–500 nm) and ZnO (~100–1000 nm) thin films deposited by reactive magnetron sputtering technique under the respective identical controlled optimum oxygen ambience. As far as we know this is the first such report on ferromagnetic pure MgO thin films, a result which should be of significance in understanding the functional aspects of magnetic tunnelling characteristics in devices using MgO dielectrics. From the magnetic characterization we observe a distinct variation in the saturation magnetization (MS) with increasing film thickness. In the case of MgO thin films MS values vary in the range 0.04–1.58 emu/g (i.e. 0.0012–0.046 μB/unit cell) with increasing film thickness showing the highest MS value for the 170 nm thick film. Above this thickness MS is found to decrease and eventually above 420 nm the films show a paramagnetic behaviour followed by the well known diamagnetic property for the bulk (>500 nm). It is obvious that since initially the MS values increase with thickness, there has to be a maximum before the films become diamagnetic at some finite thickness. We also note that the MS values observed for MgO are the highest (more than twice the value observed for ZnO) to be reported for such a defect induced ferromagnetism in a pristine oxide. The origin of ferromagnetic order in both the oxides appears to arise from the respective cat-ion vacancies. The discovery of film thickness dependent ferromagnetic order should be very useful in developing multifunctional devices based on the technologically important materials MgO and ZnO

X‑ray Absorption Spectra of Dissolved Polysulfides in Lithium–Sulfur Batteries from First-Principles

The X-ray absorption spectra (XAS) of lithium polysulfides (Li₂S_<i>x</i>) of various chain lengths (<i>x</i>) dissolved in a model solvent are obtained from first-principles calculations. The spectra exhibit two main absorption features near the sulfur K-edge, which are unambiguously interpreted as a pre-edge near 2471 eV due to the terminal sulfur atoms at either end of the linear polysulfide dianions and a main-edge near 2473 eV due to the (<i>x</i> – 2) internal atoms in the chain, except in the case of Li₂S₂, which only has a low-energy feature. We find an almost linear dependence between the ratio of the peaks and chain length, although the linear dependence is modified by the delocalized, molecular nature of the core-excited states that can span up to six neighboring sulfur atoms. Thus, our results indicate that the ratio of the peak area, and not the peak intensities, should be used when attempting to differentiate the polysulfides from XAS

Developing soft X-ray spectroscopy for in situ characterization of nanocatalysts in catalytic reactions

International audienceUnderstanding the mechanisms of catalytic and reactions calls for in situ/operando spectroscopic characterization. Here we report the developments of in situ reaction cells at the Advanced Light Source for soft X-ray spectroscopic studies of nanoparticle catalysts during the catalytic reactions. The operation of these various cells and their capabilities are illustrated with examples from the studies of Co-based nanocatalysts

X‑ray Absorption Spectra of Dissolved Polysulfides in Lithium–Sulfur Batteries from First-Principles

The X-ray absorption spectra (XAS) of lithium polysulfides (Li₂S_<i>x</i>) of various chain lengths (<i>x</i>) dissolved in a model solvent are obtained from first-principles calculations. The spectra exhibit two main absorption features near the sulfur K-edge, which are unambiguously interpreted as a pre-edge near 2471 eV due to the terminal sulfur atoms at either end of the linear polysulfide dianions and a main-edge near 2473 eV due to the (<i>x</i> – 2) internal atoms in the chain, except in the case of Li₂S₂, which only has a low-energy feature. We find an almost linear dependence between the ratio of the peaks and chain length, although the linear dependence is modified by the delocalized, molecular nature of the core-excited states that can span up to six neighboring sulfur atoms. Thus, our results indicate that the ratio of the peak area, and not the peak intensities, should be used when attempting to differentiate the polysulfides from XAS