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The lattice defects and residual strain in layered perovskite [La.sub.2]Cu[O.sub.4+[delta]] (LCO) thin films, which are grown on LaSrAl[O.sub.4] and SrTi[O.sub.4] substrates, are investigated by using transmission electron microscopy and electron energy loss spectroscopy. The analysis has shown that the strain in LCO films on both the types of substrates are largely relaxed by the formation of misfit dislocation cores, although the small lattice mismatch is accommodated by elastic strain in the LCO film on the LaSrAl[O.sub.4] (LSAO) substrate.

The growth of high quality epitaxial beta-gallium oxide (β-Ga2O3) using a compound source by molecular beam epitaxy has been demonstrated on c-plane sapphire (Al2O3) substrates. The compound source provides oxidized gallium molecules in addition to oxygen when heated from an iridium crucible in a high temperature effusion cell enabling a lower heat of formation for the growth of Ga2O3, resulting in a more efficient growth process. This source also enabled the growth of crystalline β-Ga2O3 without the need for additional oxygen. The influence of the substrate temperatures on the crystal structure and quality, chemical bonding, surface morphology, and optical properties has been systematically evaluated by x-ray diffraction, scanning transmission electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, spectroscopic ellipsometry, and UV-vis spectroscopy. Under optimized growth conditions, all films exhibited pure (201) oriented β-Ga2O3 thin films with six-fold rotational symmetry when grown on a sapphire substrate. The thin films demonstrated significant absorption in the deep-ultraviolet (UV) region with an optical bandgap around 5.0 eV and a refractive index of 1.9. A deep-UV photodetector fabricated on the high quality β-Ga2O3 thin film exhibits high resistance and small dark current (4.25 nA) with expected photoresponse for 254 nm UV light irradiation suggesting that the material grown using the compound source is a potential candidate for deep-ultraviolet photodetectors. [ABSTRACT FROM AUTHOR]

We examine the role of Ti doping in the incommensurately layered thermoelectric oxide material Ca3Co4O9 (CCO). The measured Seebeck coefficient of S=135 lV/K in Ti-doped CCO thin films of composition Ca3Co3.8Ti0.2O9 indicates no significant enhancement of S compared to pristine CCO, thus confirming prior experimental results. Using a combination of aberration-corrected scanning transmission electron microscopy, electron energy-loss spectroscopy and first-principles computations, we determine the atomic and electronic structures of Ti-doped CCO, including the preferred location of Ti dopants and valence states of Ti and Co atoms. Our findings on the structural, electronic, and transport properties of the Ti-doped CCO are discussed in light of the previously published results. [ABSTRACT FROM AUTHOR]

We present a series of electron energy-loss spectroscopy (EELS) studies on niobium (Nb) and its oxides (NbO, NbO2, and Nb2O5) to develop a reliable method for quantifying the oxidation state in mixed niobium oxide thin films. Our approach utilizes a combination of transmission electron microscopy and EELS experiments with density functional theory calculations to distinguish between metallic niobium and the different niobium oxides. More specifically, the differences in the near-edge fine-structure of the Nb M-edge and O K-edge provide sufficient information to determine the valence state of niobium. Based on these observed changes in the core-loss edges, we propose a linear relationship that correlates the peak positions in the Nb M- and O K-edges with the Nb valence state. The methods developed in this paper are also applied to ultrathin niobium oxide films to examine the effects of low-temperature baking on the films' oxidation states. [ABSTRACT FROM AUTHOR]

Temperature dependent photoluminescence (PL) spectroscopy along with structural investigations of luminescent porous Si enable us to experimentally distinguish between the relative contributions of band-to-band and oxide interface mediated electronic transitions responsible for light emission from these nanostructures. Porous Si samples formed using high current densities (J ≥ 80 mA/cm2) have large porosities (P ≥ 85%) and consequently smaller (∼1-6 nm) average crystallite sizes. The PL spectra of these high porosity samples are characterized by multiple peaks. Two dominant peaks-one in the blue regime and one in the yellow/orange regime, along with a very low intensity red/NIR peak, are observed for these samples. The high energy peak position is nearly independent of temperature, whereas the yellow/orange peak red-shifts with increasing temperature. Both the peaks blue shift with ageing and with increasing porosity. The intensity of the blue peak increases whereas the yellow/orange peak decreases with increasing temperature, while the intensity and peak position of the very low intensity red/NIR peak appears to be unaffected by temperature, porosity, and ageing. The low porosity samples (P ≤ 60%) on the other hand exhibit a single PL peak whose intensity decreases and exhibits a very small red spectral shift with increase in temperature. From the variation of intensity and PL peak positions, it is established that both quantum confinement of excitons and oxide related interfacial defect states play dominant role in light emission from porous Si and it is possible to qualitatively distinguish and assign their individual contributions. [ABSTRACT FROM AUTHOR]

Determining the Co valence, particularly in Co-based nanocatalysts is a longstanding experimental challenge. In this paper, we utilize in situ electron energy-loss spectroscopy and first-principles density functional theory calculations to distinguish between metallic Co, Co3O4, as well as CoO. More specifically, differences in the O K- and Co L-edges are utilized to determine the Co valence in different Co-oxide particles. We will further demonstrate that while the metallic Co L3/L2-ratio equals that of partially reduced Co3O4, the near-edge fine-structure of the metallic Co L-edge exhibits additional features not present in any Co-oxide. The origin of these features will be discussed. Based on our experimental and theoretical results, we will propose a fitting method to distinguish metallic Co from Co-oxides. [ABSTRACT FROM AUTHOR]

We report synthesis and luminescent characteristics of core-shell nanostructures of silicon and silicon oxide having two different morphologies—spherical (nanodot) and rodlike (nanorod), prepared by controlled oxidation of mechanically milled crystalline silicon and by exfoliation of the affected layer of porous silicon. Colloidal suspensions of these nanostructures exhibit intense room temperature photoluminescence (PL), detectable with the unaided eye. PL band peak energies of the colloidal suspensions formed from porous silicon are blue shifted by ∼1 eV compared to the as-prepared films on silicon substrate. In addition, PL spectra of all the colloidal suspensions blueshift with increase in excitation energy but the PL peaks of as-prepared porous silicon are independent of excitation. However, shape of the nanocrystals (spherical or rodlike) is found to have little effect on the emission spectra. These observations are explained in terms discretization of phonon density of states and electronic transitions involving surface defect states and quantum confinement induced widened band states. [ABSTRACT FROM AUTHOR]

Layered perovskite La2CuO4+δ (LCO) thin films were epitaxially grown on SrTiO3 (STO) and LaSrAlO4 substrates by atomic-layer-by-layer molecular beam epitaxy. The lattice defects and residual strain in these films were investigated by means of transmission electron microscopy and electron energy loss spectroscopy. The LCO films showed a high epitaxial quality with flat interfaces and top surfaces. Misfit dislocations with Burgers vector a<010> and shear defects were frequently observed at or near the film/substrate interfaces and in the films, respectively. In one LCO film, grown on STO at the highest temperature, 700 °C, we observed a two layered structure with the top layer being rhombohedral CuLaO2. In addition, stacking faults were observed in the plane views of one LCO film grown on the STO substrate. The residual strains were evaluated from the associated splitting of Bragg reflection. Possible mechanisms of strain relaxations are discussed based on the observed defects. [ABSTRACT FROM AUTHOR]

An erratum is presented to the article "Growth and characterization of β-Ga2O3 thin films by molecular beam epitaxy for deep-UV photodetectors" which appeared in an earlier issue on October 3, 2018.

We examine the role of Ti doping in the incommensurately layered thermoelectric oxide material Ca3Co4O9 (CCO). The measured Seebeck coefficient of S=135 lV/K in Ti-doped CCO thin films of composition Ca3Co3.8Ti0.2O9 indicates no significant enhancement of S compared to pristine CCO, thus confirming prior experimental results. Using a combination of aberration-corrected scanning transmission electron microscopy, electron energy-loss spectroscopy and first-principles computations, we determine the atomic and electronic structures of Ti-doped CCO, including the preferred location of Ti dopants and valence states of Ti and Co atoms. Our findings on the structural, electronic, and transport properties of the Ti-doped CCO are discussed in light of the previously published results. [ABSTRACT FROM AUTHOR]