Absorption Spectra Of Crystals Research Articles

광전소자 응용을 위한 무극성 6H-SiC 기판의 특성

The present research was focused to investigate the quality of non-polar SiC substrates grown by a conventional PVT method for optoelectronic applications. The half part of the PVT-grown 6H-SiC crystal boules was sliced along a-direction and m-direction to extensively analyze non-polar planes and then remaining part of that was sliced along the basal plane to produce wafers. The non-polar SiC m-plane and a-plane exhibited apparent peaks around 2 theta=<TEX>$120^{\circ}$</TEX>((3-300) plane) and 2 theta=<TEX>$60^{\circ}$</TEX> ((11-20) plane), respectively. FWHM values of m-plane measured along a-direction and c-direction were 60 arc see and 57 arcsec respectively, a-plane measured along m-direction and c-direction were 41 arcsec and 51 arcsec respectively. The typical absorption spectra of SiC crystals indicated that each of SiC crystals were the 6H-SiC with fundamental absorption energy of about 3.04 eV. Non-polar planes contained no micropipe on etched surface. The carrier concentration and mobility of non-polar SiC wafers have estimated by Raman spectrum. It was observed that the carrier mobility is low in the area far from seed crystal with compared to other places.

First-principles calculations of electronic structures and absorption spectra of YAlO3 crystals with F center

The electronic structures and absorption spectra of perfect YAlO3 crystal and YAlO3 crystal containing F center (oxygen vacancy VO2+ catching two electrons) with lattice structure optimized were calculated using density functional theory code CASTEP. The calculated electronic structures of YAlO3 crystal containing F center appear new density of states in forbidden band compared with that of perfect YAlO3 crystal and the calculated absorption spectra of the YAlO3 containing F center along three lattice parameter directions exhibit absorption bands in the range from 200 to 300 nm which are in agreement with experimental values. These new absorption bands are caused by separated F center in YAlO3 crystal.

Spectroscopy of radiation-induced and structural defects in garnet crystals with V and Cr impurity ions

The optical absorption spectra of crystals with garnet structure (A3B2C3O12), containing V or Cr impurity ions, before and after oxidation annealing, γ irradiation, and coactivation with Ca and Mg ions have been experimentally and theoretically investigated. It is shown that V3+ and Cr4+ ions are incorporated into both octahedral and tetrahedral sites of the garnet lattice. The absorption spectra and energy level diagrams of impurity ions are identified.

Can soaked-in scavengers protect metalloprotein active sites from reduction during data collection?

One of the first events taking place when a crystal of a metalloprotein is exposed to X-ray radiation is photoreduction of the metal centres. The oxidation state of a metal cannot always be determined from routine X-ray diffraction experiments alone, but it may have a crucial impact on the metal's environment and on the analysis of the structural data when considering the functional mechanism of a metalloenzyme. Here, UV-Vis microspectrophotometry is used to test the efficacy of selected scavengers in reducing the undesirable photoreduction of the iron and copper centres in myoglobin and azurin, respectively, and X-ray crystallography to assess their capacity of mitigating global and specific radiation damage effects. UV-Vis absorption spectra of native crystals, as well as those soaked in 18 different radioprotectants, show dramatic metal reduction occurring in the first 60 s of irradiation with an X-ray beam from a third-generation synchrotron source. Among the tested radioprotectants only potassium hexacyanoferrate(III) seems to be capable of partially mitigating the rate of metal photoreduction at the concentrations used, but not to a sufficient extent that would allow a complete data set to be recorded from a fully oxidized crystal. On the other hand, analysis of the X-ray crystallographic data confirms ascorbate as an efficient protecting agent against radiation damage, other than metal centre reduction, and suggests further testing of HEPES and 2,3-dichloro-1,4-naphtoquinone as potential scavengers.

Photochromic effect in Bi12SiO20 crystals doped with molybdenum

The steady-state and photoinduced absorption spectra of Bi12SiO20 crystals doped with molybdenum are investigated. It is demonstrated that the impurity absorption is associated with the Mo6+ and Mo5+ ions occupying the positions close to the tetrahedral Si4+ position. The mechanism of the photochromic effect is proposed. This mechanism involves the change in the charge state of molybdenum impurity ions according to the scheme MoSi6+ + e− → MoSi5+.

Bonding anisotropy in multiferroic TbMnO3 probed by polarization dependent x-ray absorption spectroscopy

O K- and Mn L2,3-edges x-ray absorption spectra of orthorhombic TbMnO3 single crystals show strong polarization dependence, in contrast with results of previous experiments on orthorhombic LaMnO3 and DyMnO3 thin films that show nearly isotropic spectral structure. First-principles calculations reveal that TbMnO3 exhibits a zigzag eg orbital ordering ground state. The highly distorted MnO6 octahedron and orbital ordering in TbMnO3 produce highly anisotropic Mn–O bonding within the ab plane, a frustration of the magnetic ordering, and the formation of complicated incommensurate magnetic structures.

The effects of tableting with potassium bromide on the infrared absorption spectra of indomethacin

The effects of tableting with KBr on the IR absorption spectra of molecular crystals were studied at room temperature. Study substances were mechanically activated mixtures of crystalline indomethacin and the excipient polyethylene glycol at ratios of 1:1 and 1:3. There was a marked widening of absorption bands in the range 2000-500 cm −1 in the IR spectra of mechanically activated mixtures in tablets with KBr. These experimental results are assessed within the framework of possible interactions between study samples and the KBr matrix during tableting and storage as powered mixtures.

Study on infrared absorption spectra of congruent lithium niobate crystals at low temperature

Infrared absorption spectra of congruent lithium niobate crystals are measured at low temperature, and a new peak is found at 3200 cm-1 in the spectra. Results show that this peak has no relationship with hydrogen in the crystals, and that with the increase of the temperature, both the intensity and shape of the peak change in a complex way. Based on these results, we suggest that the new peak results from the electron transfer from small polarons （Nb4+Li） to free polarons （Nb5+Nb）. In addition, we find that this peak can be decomposed into three Gauss peaks, and the three components are attributed to three transfer processes with different energies.

Correlating Ultrafast Function with Structure in Single Crystals of the Photosynthetic Reaction Center

Femtosecond transient absorbance spectroscopy was applied to the study of primary electron transfer in single reaction center crystals from Rhodobacter sphaeroides. Polarized transient absorption spectra of individual crystals are shown to correlate with polarized ground-state absorption spectra and to track cofactor transition moment directions calculated from the crystallographic structure. Electron transfer from the bacteriochlorophyll dimer to the bacteriopheophytin acceptor was found to be multiphasic in crystals and approximately 2-fold slower than in solution. This work demonstrates the ability to resolve ultrafast photosynthetic function in single crystals and allows ultrafast function to be directly correlated with structure.

Photochromism in Cu-and Ag-doped Bi12SiO20 crystals

The equilibrium and photoinduced absorption spectra of copper-and silver-doped Bi12SiO20 crystals are studied. It is demonstrated that the impurity absorption is due to Ag2+, Ag+, Cu3+, Cu2+, and Cu+ ions occupying almost octahedral Bi3 positions. A mechanism of photochromism is suggested, involving changes in the charge states of copper and silver impurity ions according to schemes Cu2+-e → Cu3+ and Ag+-e → Ag2+.

Absorption of Dy3+ and Nd3+ ions in BaR 2F8 single crystals

The Dy3+ absorption and excitation spectra of BaY2F8 and BaYb2F8 single crystals are investigated in the ultraviolet, vacuum ultraviolet, and visible ranges at a temperature of 300 K. These crystals exhibit intense broad absorption bands due to the spin-allowed 4f-5d transitions in the range (56–78) × 10−3 cm−1 and less intense absorption bands that correspond to the spin-forbidden transitions in the range (50–56) × 10−3 cm−1. The Nd3+ absorption spectra of BaY2F8 single crystals are studied in the range (34–82) × 10−3 cm−1 at 300 K for different crystal orientations.

Spectroscopic investigation of rare-earth chromium borates RCr3(BO3)4 (R = Nd, Sm)

The optical absorption spectra of single crystals of neodymium and samarium chromium borates are studied. A magnetic phase transition in NdCr3(BO3)4 (at Tc = 8 ± 1 K) and SmCr3(BO3)4 (at Tc = 5 ± 1 K) is observed. The effective magnetic field acting on the Nd3+ ion from the side of the ordered magnetic subsystem of chromium is estimated.

Charge-transfer bands in crystals of alkaline earth fluorides with Eu3+ and Yb3 + 1

The absorption, luminescence, and excitation spectra of CaF2, SrF2, and BaF2 crystals with EuF3 or YbF3 impurity have been investigated in the range 1–12 eV. In all cases, strong wide absorption bands (denoted as CT1) were observed at energies below the 4fn-4fn − 15d absorption threshold of impurity ions. Weaker absorption bands (denoted as CT2) with energies 1.5–2 eV lower than those of the CT1 bands have been found in the spectra of CaF2 and SrF2 crystals with EuF3 or YbF3 impurities. The fine structure of the luminescence spectra of CaF2 crystals with EuF3 impurities has been investigated under excitation in the CT bands. Under excitation in the CT1 band, several Eu centers were observed in the following luminescence spectra: C4v, Oh, and R aggregates. Excitation in the CT2 bands revealed luminescence of only C4v defects.

Circular dichroism spectra of langasite family crystals in the range of electronic transitions of structure defects

The absorption and circular dichroism spectra of langasite family crystals are studied. Wide bands in the range of 285-500 nm that are related to the structure lattice defects are found. For all the crystals, these bands are in approximately the same spectral region, have identical structures, and can be attributed to one type of defects formed by cation vacancies and excess oxygen atoms in the optically active positions.

Electrical and Optical Properties of Porphyrin Single Crystals

In this work, we investigate the transport and optical properties of single crystal 5,10,15,20-tetrakis (4-N-ethylpyridyl) porphyrin salts, [H2TEPyP4+·4I−]. The electrical conductivity exhibits high anisotropy, in that the conductivity along the stacking column, which is equal to 3.2 × 10−8Ω−1· cm−1, is three orders of magnitude larger than that perpendicular to the stacking column. The absorption spectra of the single crystals exhibit a blue shift in the B-band and significant red shifts in the Q-bands in comparison to the solution spectra. Polarized reflectance measurements of the crystals reveal that the anisotropic reflectance is most intense when the light polarization is parallel to the molecular plane. The complex dielectric function, plasma frequency, and optical conductivity associated with each electronic transition are calculated based on analysis with a dielectric-function model.

Growth, etching morphology and spectra of LiAlO2 crystal

Abstractγ‐LiAlO2 single crystal was successfully grown by Czochralski method. The crystal quality was characterized by X‐ray rocking curve and chemical etching. The effects of air‐annealing and vapor transport equilibration (VTE) on the crystal quality, etch pits and absorption spectra of LiAlO2 were also investigated in detail. The results show that the as‐grown crystal has very high quality with the full width at half maximum (FWHM) of 17.7‐22.6 arcsec. Dislocation density in the middle part of the crystal is as low as about 3.0×103 cm–2. The VTE‐treated slice has larger FWHM value, etch pits density and absorption coefficient as compared with those of untreated and air‐annealed slices, which indicates that the crystal quality became inferior after VTE treatment. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Infrared absorption spectra of molecular crystals: Possible evidence for small-polaron formation?

The temperature dependence of the position of the so-called anomalous band peaked at 1650 cm - 1 in the IR-absorption spectrum of crystalline acetanilide (ACN) is theoretically investigated within the small-polaron theory. Its pronounced shift towards the position of the normal band is predicted with the rise of temperature. Interpretation of the IR-absorption spectra in terms of small-polaron model has been critically assessed on the basis of these results.

Influence of post-radiation deformation on TSL and absorption spectra of irradiated MgO crystals

Thermally stimulated luminescence (TSL) and optical and paramagnetic absorption spectra of MgO and MgO:V2+ have been investigated after quasi-elastic and plastic deformations. Splitting of the low-temperature peak (∼450 K) in TSL curves caused by the deformation in both nominally pure and impure crystals is explained by the formation of an excess of vacancies. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic, Optical, and Electronic Properties of the Phosphide Oxides REZnPO (RE = Y, La–Nd, Sm, Gd, Dy, Ho)

AbstractWell‐shaped yellow to red transparent single crystals of the phosphide oxides REZnPO (RE = Y, La–Nd, Sm, Gd, Dy, Ho) were synthesized from the elements and ZnO in NaCl/KCl fluxes in sealed silica ampoules. Four structures (NdZnPO type, R3m) were refined from single crystal X‐ray diffractometer data: a = 388.5(2), c = 3032(1) pm, wR2 = 0.0380, 360 F2 values for YZnPO, a = 394.6(2), c = 3071(1) pm, wR2 = 0.0587, 226 F2 values for SmZnPO, a = 392.2(2), c = 3056(1) pm, wR2 = 0.0262, 462 F2 values for GdZnPO, and a = 389.33(6), c = 3030.5(4) pm, wR2 = 0.0453, 217 F2 values for DyZnPO each with 14 variables per refinement. The structures are composed of alternate stacks of (RE3+O2−) and (Zn2+P3−) layers with covalent RE–O and ZñP bonding within and weak ionic bonding between the layers. The zinc and oxygen atoms have slightly distorted tetrahedral coordination by atoms of phosphorus and the rare earth element, respectively. According to the electron precise formulation RE3+Zn2+P3−O2−, these pnictide oxides are transparent in visible light. Susceptibility measurements on β‐CeZnPO, β‐PrZnPO, and GdZnPO reveal Curie‐Weiss paramagnetism with experimental magnetic moments of 2.31, 3.60, and 7.72 μB/RE atoms, respectively. β‐CeZnPO and β‐PrZnPO show antiferromagnetic order with Néel temperatures of 7.4 (Ce) and 2.2 (Pr) K. GdZnPO shows no magnetic ordering down to 2 K. Single crystal absorption spectra measured for REZnPO (RE = Y, La, Pr, Nd, Sm, Dy) in the NIR‐Vis region reveal unexpected variations for the optical band gap of these phosphide oxides. For RE = Pr, Nd, Sm, Dy, Ho f‐f electronic transitions with nicely resolved ligand‐field splittings are observed in the range 6000–20000 cm−1. DFT band structure calculations show similarity between the valence bands of tetragonal and rhombohedral REZnPO as they possess mainly P‐3p character. In both cases, the conduction bands have mainly Zn‐4s character, but a significant contribution of RE‐5d occurs in rhombohedral REZnPO, which is responsible for a smaller optical band gap for the latter compounds. Variations of the energy gaps of tetragonal REZnPO can be explained by hybridization of Zn‐4s + RE‐5d + RE‐4f orbitals for the conduction band. DFT volume optimizations of α‐ and β‐PrZnPO show β‐PrZnPO to be more stable by 10.7 kJ mol−1.

First‐principles study of the electronic structures and absorption spectra for the PbMoO4 crystal with lead vacancy

AbstractThe electronic structures and absorption spectra for the perfect PbMoO4 crystal and the crystal containing lead vacancy $ {\rm V}^{2-}_{\rm Pb} $ with lattice structure optimized are calculated using density functional theory code CASTEP. The calculated absorption spectra of the PbMoO4 crystal containing $ {\rm V}^{2-}_{\rm Pb} $ exhibit three absorption bands peaking at 2.0 eV (620 nm), 3.0 eV (413 nm) and 3.3 eV (375 nm), which are in good agreement with experimental values. The theory predicts that the 390 nm, 430 nm and 580 nm absorption bands are related to the existence of $ {\rm V}^{2-}_{\rm Pb} $ in the PbMoO4 crystal. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)