Verification of a Distortion in the Microstructure of GaN Detected by EXAFS Using Ab Initio Density Functional Theory Calculations

X-ray absorption fine structure (XAFS) measurements on a series of epitaxially grown GaN samples have shown a distortion in the microstructure of GaN. More specifically the central N atom is 4-fold coordinated but the four Ga atoms are not equidistant. It has been shown that 2.9 to 3.5 of them (depending on the growth conditions) are found in the expected from XRD distance of 1.94 A and the remaining are at a distance longer by approximately 15%. Second derivative calculation of the conformation energy using the Density Functional Theory (DFT) is used to investigate if the symmetric GaN cluster as given by XRD is the most energetically favorable configuration and if not which distorted structure corresponds to the most energetically favorable one. A very good agreement between DFT results and experimental XAFS spectra has been found. Generalization this technique to other dislocated clusters is also discussed

Local environment of Nitrogen in GaN{y}As{1-y} epilayers on GaAs (001) studied using X-ray absorption near edge spectroscopy

X-ray absorption near-edge spectroscopy (XANES) is used to study the N environment in bulk GaN and in GaN{y}As{1-y} epilayers on GaAs (001), for y \~5%. Density-functional optimized structures were used to predict XANES via multiple-scattering theory. We obtain striking agreement for pure GaN. An alloy model with nitrogen pairs on Ga accurately predicts the threshold energy, the width of the XANES ``white line'', and features above threshold, for the given X-ray polarization. The presence of N-pairs may point to a role for molecular N_2 in epitaxial growth kinetics.Comment: Four pages (PRL style) with two figure

Ultrafast Charge Carrier Dynamics in Vanadium-Modified TiO2 Thin Films and Its Relation to Their Photoelectrocatalytic Efficiency for Water Splitting

Light absorption and charge transport in oxide semiconductors can be tuned by the introduction, during deposition, of a small quantity of foreign elements, leading to the improvement of the photoelectrocatalytic performance. In this work, both unmodified and vanadium-modified TiO2 thin films deposited by radio-frequency magnetron sputtering are investigated as photoanodes for photoelectrochemical water splitting. Following a structural characterization by X-ray diffraction, atomic force microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy, photoelectrocatalysis is discussed based on ultrafast transient absorbance spectroscopy measurements. In particular, three different pump wavelengths from UV to the visible range are used (300, 390, and 530 nm) in order to cover the relevant photoactive spectral range of modified TiO2. Incident photon-to-current conversion efficiency spectra show that incorporation of vanadium in TiO2 extends water splitting in the visible range up to approximate to 530 nm, a significant improvement compared to unmodified TiO2 that is active only in the UV range less than or similar to 390 nm. However, transient absorbance spectroscopy clearly reveals that vanadium accelerates electron-hole recombination upon UV irradiation, resulting in a lower photon-to-current conversion efficiency in the UV spectral range with respect to unmodified TiO2. The new photoelectrocatalytic activity in the visible range is attributed to a V-induced introduction of intragap levels at approximate to 2.2 eV below the bottom of the conduction band. This is confirmed by long-living transient signals due to electrons photoexcited into the conduction band after visible light (530 nm) pulses. The remaining holes migrate to the semiconductor-electrolyte interface where they are captured by long-lived traps and eventually promote water oxidation under visible light

Extended X-ray absorption fine structure study of Er bonding in AlNO:Erx films with x <= 3.6%

The structural properties of Er-doped AlNO epilayers grown by radio frequency magnetron sputtering were studied by Extended X-ray Absorption Fine Structure (EXAFS) spectra recorded at the Er L3 edge. The analysis revealed that Er substitutes for Al in all the studied samples and the increase in Er concentration from 0.5 to 3.6 at.% is not accompanied by formation of ErN, Er2O3 or Er clusters. Simultaneously recorded X-ray Absorption Near Edge Structure (XANES) spectra verify that the bonding configuration of Er is similar in all studied samples. The Er-N distance is constant at 2.18-2.19 Å i.e. approximately 15% larger than the Al-N bondlength, revealing that the introduction of Er in the cation sublattice causes considerable local distortion. The Debye-Waller factor, which measures the static disorder, of the second nearest shell of Al neighbors, has a local minimum for the sample containing 1% Er that coincides with the highest photoluminescence efficiency of the sample set

Ca L2,3 edge XANES and Sr K edge EXAFS study of hydroxyapatite and fossil bone apatite

Upon burial, the organic and inorganic components of hard tissues such as bone, teeth, and tusks are subjected to various alterations as a result of interactions with the chemical milieu of soil, groundwater, and presence of microorganisms. In this study, simulation of the Ca L$_{2,3}$-edge X-ray absorption near edge structure (XANES) spectrum of hydroxyapatite, using the CTM4XAS code, reveals that the different symmetry of the two nonequivalent Ca$_{(1)}$ and Ca${(2)}$ sites in the unit cell gives rise to specific spectral features. Moreover, Ca L$_{2,3}$-edge XANES spectroscopy is applied in order to assess variations in fossil bone apatite crystallinity due to heavy bacterial alteration and catastrophic mineral dissolution, compared to well-preserved fossil apatite, fresh bone, and geologic apatite reference samples. Fossilization-induced chemical alterations are investigated by means of Ca L$_{2,3}$-edge XANES and scanning electron microscopy (SEM) and are related to histological evaluation using optical microscopy images. Finally, the variations in the bonding environment of Sr and its preference for substitution in the Ca$_{(1)}$ or Ca$_{(2)}$ sites upon increasing the Sr/Ca ratio is assessed by Sr K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy

Characterization of fossil remains using XRF, XPS and XAFS spectroscopies

Synchrotron radiation micro-X-Ray Fluorescence (m-XRF), X-ray photoelectron (XPS) and X-ray Absorption Fine Structure (XAFS) spectroscopies are applied for the study of paleontological findings. More specifically the costal plate of a gigantic terrestrial turtle Titanochelon bacharidisi and a fossilized coprolite of the cave spotted hyena Crocuta crocuta spelaea are studied. Ca L2,3-edge NEXAFS and Ca 2p XPS are applied for the identification and quantification of apatite and Ca containing minerals. XRF mapping and XAFS are employed for the study of the spatial distribution and speciation of the minerals related to the deposition environment.Fil: Zougrou, I.M.. Aristotle University of Thessalonik; GreciaFil: Katsikini, M.. Aristotle University of Thessalonik; GreciaFil: Pinakidou, F.. Aristotle University of Thessalonik; GreciaFil: Brzhezinskaya, M.. No especifíca;Fil: Papadopoulou, L.. Aristotle University of Thessalonik; GreciaFil: Vlachos, Evangelos. Aristotle University of Thessalonik; Grecia. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Tsoukala, E.. Aristotle University of Thessalonik; GreciaFil: Paloura, E.C.. Aristotle University of Thessalonik; Greci

Detailed Spectroscopic Study of the Role of Br and Sr in Coloured Parts of the Callinectes Sapidus Crab Claw

The exoskeleton of crustaceans consists mainly of calcium carbonate (CaCO$_3$) minerals and in many cases exhibits vivid colouration due to the presence of proteins rich in carotenoid chromophores. The exposure of aquatic animals in sea water results often in the incorporation of trace elements in their exoskeleton. The bonding configuration of Br and Sr trace elements in regions with different staining (white, orange and blue) of the exoskeleton of the Callinectes sapidus in crab claw are systematically investigated by a number of complementary spectroscopic techniques, including X-ray absorption fine structure spectroscopy (EXAFS), X-ray fluorescence, Raman and visible light reflectivity spectroscopies. It is found that Sr substitutes for Ca and the Sr/Ca ratio is constant along the claw. In the orange region that includes the claw fingers, CaCO$_3$ adopts a calcite-like structure, whereas in the blue and white regions, located in the palm of the claw, an aragonite-like structure dominates. On the other hand, Br, present only in the blue and orange stained parts of the claw, is bound to phenyl and/or phenol rings of amino acid residues, most probably to phenylalanine and/or tyrosine, of the chromophore protein