N型掺杂ZnSe/BeTeⅡ型量子阱中空间间接带电激子跃迁发光的直接证据

本文研究了N型掺杂ZnSe/BeTe/ZnSeⅡ型量子阱空间间接发光谱的外加电场依赖性.实验结果表明,其发光谱只显示了一个线性偏振度较低的发光峰.这是由于掺杂电子屏蔽了Ⅱ型量子阱中的内秉电场,并使得两个ZnSe阱层具有相同的势.同时该发光谱具有反玻耳兹曼(inverse-Boltzmann)分布,并且线型和线性偏振度在...     

Evidence for energy band tailing and localization of states in highly disordered crystalline GeTe

Substantial absorption coefficient ‘tails’ at energies below the absorption edge have been observed in the crystalline Ge_1?x Te_x system. For part of this ‘tail’ region the absorption coefficient is found to vary as the photon energy squared, and the extent of this behavior to depend on the stoichiometry of the sample, while near the band edge a more suitable variation is αE = B(E ? E₀)². At very low energies an exponential variation α = const · exp (E/Δ), with Δ ≈ 0.1 eV (3.8 kT), is more appropriate. This behavior can be understood only if localized states (α = kE²) and band edge tailing (α = const · exp (E/Δ), features normally associated with the Mott model of the amorphous state, are present in the band structure of the crystalline material. The variation of the extent of the α = kE² region with stoichiometry supports this model, as the depth of localization, being comparable to the strength of the fluctuation potential due to the disorder, depends on the amount of disorder present in the lattice.     

Luminescence of crystals of divalent tungstates

Crystals of divalent tungstates are characterized by two main luminescence spectral ranges: a short-wavelength (blue) luminescence band in the range 390–420 nm and a group (often two groups) of longer wavelength (green) bands in the range 480–520 nm. For crystals of calcium, strontium, barium, cadmium, magnesium, zinc, and lead tungstates, it is shown that the wavelength corresponding to the maximum of the blue luminescence band (λ_max) correlates with the melting temperature (T_m) of these compounds. The position of the blue luminescence band is the same (in the range 510–530 nm) for crystals with different divalent cations. Annealing in vacuum and electron irradiation decrease the intensity of both blue and green luminescence bands but do not change the ratio of their maximum intensities. This circumstance suggests that vacancies serve as luminescence quenchers to a greater extent rather than facilitate the formation of emission centers responsible for a particular luminescence band.     

UV,violet and blue‐green luminescence from RF sputter deposited ZnO:Al thin films

The mechanism of ultraviolet (UV), violet and blue green emission from ZnO:Al (AZO) thin films deposited at different radio frequency (r.f.) powers on glass substrates was investigated. The structure and surface morphology of AZO films have also been observed. The optical transmittance spectra shows more than 80% transmittance in the visible region and the band gap is found to be directly allowed. From the photoluminescence measurement, intense UV and blue green luminescence is obtained for the samples deposited at higher sputtering powers. The mechanism of luminescence suggests that UV luminescence of AZO thin film is related to the transition from near band edge to the valence band and the concentration of antisite oxide (O_zn) increases with increase in r.f. power which in turn increases the intensity of green band emission while the violet PL is due to the defect level transition in the grain boundaries of AZO films. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The first observation of dislocation blocking in pure metal without external stress

Self-blocking of c + a edge dislocations at second-order pyramidal slip in magnesium single crystals whose axis is parallel to the c axis has been found. Self-blocking is confirmed by the dislocation extension along a selected direction in the absence of external stress. Transmission electron microscopy (TEM) images of (c + a) dislocations extended in the $\left\langle {1\bar 100} \right\rangle$ directions are obtained. A model of a two-valley potential relief in Mg is proposed: the Peierls relief determines the second-order slip of (c + a) dislocation in the pyramid plane, while the immersion in deep valleys is related to splitting of the c + a edge segment (or the spread of its core) in the basal plane.     

Classification of Mechanoluminescence

On the basis of the processes involved in inducing luminescence, mechanoluminescence (ML) may be classified into different types, such as: (i) piezo-electrification-induced fracto ML, (ii) defective-phase piezo-electrification-induced fracto ML, (iii) charged dislocation electrification-induced fracto ML, (iv) baro diffusion electrification-induced fracto ML, (v) chemically-induced fracto ML, (vi) thermally stimulated fracto ML, (vii) incandescent fracto ML, (viii) stress perturbation-induced fracto ML, (ix) thermal excitation-induced fracto ML, (x) mechanically excited ML, (xi) deep trap-induced fracto ML, (xii) dislocation mechanical interaction-induced plastico ML, (xiii) dislocation electrostatic interaction-induced plastico ML, (xiv) charged dislocation electrification-induced plastico ML, (xv) thermal excitation-induced plastico ML, (xvi) dislocation mechanical interaction-induced elastico ML, (xvii) dislocation electrostatic interaction-induced elastico ML, (xviii) thermal excitation-induced elastico ML, (xix) electrically induced tribo ML, (xx) chemically-induced tribo ML, (xix) electrically induced tribo ML, (xx) chemically-induced tribo ML, and (xxi) thermally-induced tribo ML. It is expected that the processes involving dislocation electrostatic interaction-induced elastico ML and thermal excitation-induced elastico ML may provide suitable ML materials in future.     

Absorption and luminescence in amorphous SixGe1-xO2 films fabricated by SPCVD

Optical absorption and photoluminescence of Ge-doped silica films fabricated by the surface-plasma chemical vapor deposition (SPCVD) are studied in the 2–8 eV spectral band. The deposited on silica substrate films of about 10 μm in thickness are composed as x·GeO₂-(1-x)·SiO₂ with x ranging from 0.02 to 1. It is found that all as‐deposited films do not luminesce under the excitation by a KrF (5 eV) excimer laser, thus indicating lack of oxygen deficient centers (ODCs) in them. After subsequent fusion of silicon containing (x < 1) films by a scanning focused CO₂ laser beam absorption band centered at 5 eV as well as two luminescence bands centered at blue (3.1 eV) and UV (4.3 eV) wavelengths arise, highlighting the formation of the ODCs. The excitation of unfused SPCVD films by an ArF (6.4 eV) excimer laser yields a luminescence spectrum with two bands typical for the ODCs, but with a faster decay kinetics. Intensities of these bands grow up with samples cooling down to a temperature of 80–60 K. Unfused films excited by the ArF laser also demonstrate luminescence due to recombination of a trapped charge resulted from the excitation of localized electron states of the glass network. In the unfused GeO₂ film luminescence related to a self-trapped exciton (STE) typical for GeO₂ crystals with α-quartz structure is observed. The observed STE luminescence can be indicative of the crystalline fraction availability in the film. Whereas GeO₂ crystals are known as not containing twofold coordinated germanium, a polycrystalline inclusion in the SPCVD GeO₂ film serves as a factor explaining the absence of any spectroscopic manifestation of this type of defects in it even after fusion. On the other hand, lack of STE luminescence in other unfused films with x < 1 testifies truly amorphous state of the matter in them.     

X-ray investigations of sulfur-containing fungicide: Structure of the benzoylphenyl(4-tolylazo)methyl 4-tolyl sulfone

Crystal and molecular structure of the of benzoylphenyl(4-tolylazo)methyl 4-tolyl sulfone was determined by x-ray analysis; crystal system, monoclinic; space group P2 ₁/c; a = 12.807(1) Å, b = 17.512(1) Å, c = 11.879(1) Å, β = 111.23(1)°. Conformation of the title compound is constrained by attraction of the positively charged sulfur and β-carbon atoms to the negatively charged oxygen atoms. Characteristic distribution of bond lengths in the central part of a molecule suggests that electrostatic interactions are strengthened by the stereoelectronic donation of electron density from the α-azo N2 towards the sulfonyl S and β-carbonyl C2 atoms.     

Effect of impurities on the optical properties of PbWO<Subscript>4</Subscript> crystals

The effect of excess W and La³⁺, Y³⁺, and Mo⁶⁺ impurities on the luminescence spectra and the thermally stimulated luminescence of PbWO₄ crystals is studied. A high tungsten content (up to 1 at %) in the charge mixture results in a high luminescence intensity in the green spectral region. Lanthanum impurity decreases the transmission in the short-wavelength (k > 29 000 cm^?1) spectral region. Lanthanum and yttrium impurities decrease the luminescence intensity. At low (10² ppm) concentrations, Mo impurity can somewhat increase the light yield. However, the presence of Mo in PbWO₄ shifts the absorption-band edge to longer wavelengths, while an increase in the Mo concentration above 10² ppm decreases the luminescence intensity.     

Five-coordinate Co(II) complexes with nitrilotriacetic acid: Crystal structures of Ca[Co(Nta)X] · nH2O (X − = Cl, Br, or NCS)

The synthesis and X-ray diffraction study of three Ca[Co(Nta)X] · nH₂O complexes [X ^? = Cl, n = 2.3 (I); X ^? = Br, n = 2 (II); and X ^? = NCS, n = 2 (III)] are performed. The main structural units of crystals I–III are the [CoX(Nta)]^2? anionic complexes and hydrated Ca²⁺ cations. The anionic complexes have similar structures. The coordination of the Co²⁺ atom in the shape of a trigonal bipyramid is formed by N + 3O atoms of the Nta ^3? ligand and the X ^? anion in the trans position with respect to N. In structures I–III, the Co-O and Co-N bond lengths lie in the ranges 1.998–2.032 and 2.186–2.201 Å, respectively. The Co-X bond lengths are 2.294 (I), 2.436 and 2.445 (II), and 1.982 Å (III). The environments of the Ca²⁺ cations include oxygen atoms of one or two water molecules and six or seven O(Nta) atoms with the coordination number of 9 in I or 8 in II and III. The Ca-O(Nta) bonds form a three-dimensional framework in I or layers in II and III. Water molecules are involved in the hydrogen bonds O(w)-H···O(Nta), O(w)-H···X, and O(w)-H···O(w). Structural data for crystals I–III are deposited with the Cambridge Structural Database (CCDC nos. 287 814–287 816).     

Crystal Structure of Two <Emphasis Type="Italic">V</Emphasis>-shaped Ligands with <Emphasis Type="Italic">N</Emphasis>-Heterocycles

Two V-shaped ligands with N-heterocycles, bis(4-(1H-imidazol-1-yl) phenyl)methanone (1), and bis(4-(1H-benzo[d]imidazol-1-yl)phenyl)methanone (2) have been synthesized and characterized by elemental analyses, IR and 1H NMR spectroscopy. Crystal structures of 1 and 2 have been determined by X-ray diffraction. The crystal of 1 is monoclinic, sp. gr. P2₁/c, Z = 4. The crystal of 2 is orthorhombic, sp. gr. Fdd2, Z = 8. X-ray diffraction analyses show that the V-shaped angles of 1 and 2 are 122.72(15)° and 120.7(4)°, respectively. Intermolecular C–H···O, C–H···N, C–H···π, and π···π interactions link the components into three-dimensional networks in the crystal structures.     

Preparation and characterization of telluride glasses

Chalcogenide bulk glasses Ge₂₀Se_80?xTe_x for x ∈ (0, 10) have been prepared by systematic replacement of Se by Te. Selected glasses have been doped with Er and Pr, and all systems have been characterized by transmission spectroscopy, measurements of dc electrical conductivity and low-temperature photoluminescence. Absorption coefficient has been derived from measured transmittance and estimated reflectance. Both absorption and low-temperature photoluminescence spectra reveal shifts of absorption edge and/or dominant luminescence band to longer wavelength due to Te → Se substitution. Arrhenius plots of dc electrical conductivity, in the temperature range 300–450 K, are characterized by activation energies roughly equal to the half of the optical gap. Arrhenius plots for temperatures below 300 K yield much lower activation energies. The dominant low-temperature luminescence band centered at about half the band gap energy starts to quench above 200 K and a new band appears at 900 nm. The band at 900 nm, due to band to band transitions, overwhelms the spectra at room temperature. Systems doped with Er exhibit a strong luminescence due to ⁴I_13/2 → I_15/2 transition of Er³⁺ ion at 1539 nm, and Pr doped samples exhibit a relatively weak luminescence peak at 1590 nm, which we tentatively assign to ³F₃ → ³H₄ transition of Pr³⁺ ion.     

Synthesis,Characterization, and Crystal Structures of 4,6-Dimethoxy-2-hydroxy-3-methylbenzoic Acid and 2,4-Dimethoxy-6-hydroxy-3-methylbenzoic Acid

A synthetic route involving 4,6-dimethoxy-2-hydroxy-3-methylbenzoic acid (10) and 2,4-dimethoxy-6-hydroxy-3-methylbenzoic acid (11) has been investigated in the process of synthesis of the natural lichen substance namely, crocynol. Crystal structures of 10 and 11 were determined to confirm the ambiguous structures of the two isomers. Crystals 10 are monoclinic, sp. gr. P2₁/m, Z = 2. Crystals 11 are monoclinic, sp. gr. P2₁/c, Z = 4. In the crystal packing of 10, the molecules are arranged into chains along the a-axis formed only by van der Waals interactions, whereas in 11, the molecules are linked by O–H···O and C–H···O hydrogen bonds into chains along the b-axis and further stacked by π···π interactions with the centroid ···centroid distance of 3.6863(8) Å.     

Radiation processes in oxygen-deficient silica glasses: Is ODC(I) a precursor of E′-center?

The accumulation of radiation-induced defects under non-destructive X-ray and destructive cathodoexcitation was studied in pure silica KS-4V glasses possessing an absorption band at 7.6 eV. The correspondence between the existence of this band and the creation of the E′-center by radiation was checked. Detection of induced defects was accomplished by measurement of the luminescence during irradiation, post irradiation afterglow or phosphorescence, induced optical absorption, and thermally stimulated luminescence. In all samples, these observed phenomena associated with charge trapping and recombination on the oxygen-deficient luminescence center. Others centers of luminescence were not significant contributors. In some samples, the intensity of the 7.6 eV absorption band was deliberately increased by treatment in hydrogen at 1200 C for 100 h. The intensity of luminescence in hydrogen-treated samples was smaller because of the known quenching effect of hydrogen on the luminescence of oxygen-deficient centers. The optical absorption method does not reveal an induced absorption band for the E′-center in the hydrogen-free samples with different levels of oxygen deficiency. Therefore, we did not detect the transformation of the defect responsible for the 7.6 eV absorption band or the ODC(I) defect into the E′-center. In the hydrogen-treated sample, the absorption of the E′-center was detected. The E′-centers creation in the hydrogen-treated sample was associated with precursors created by hydrogen treatment (≡Si–O–H and ≡Si–H) in the glass network. The destructive e-beam irradiation reveals an increase with dose of the ODC luminescence intensity in the sample exhibiting a small 7.6 eV band. That means that the corresponding luminescence centers are created. Optical absorption measurements in that case reveal the presence of E′-centers and a broad band at 7.6 eV. A compaction of the irradiated volume was detected. Therefore, we conclude that the E′-center is produced by heavy damage to the glass network or by the presence of precursors.     

Crystal structures of copper(II) and nickel(II) nitrate and chloride complexes with 4-bromo-2-[(2-hydroxyethylimino)-methyl]phenol

The crystal structures of {4-bromo-2-[(2-hydroxyethylimino)-methyl]phenolo}aquacopper(II) nitrate hemihydrate (I), chloro-{4-bromo-2-[(2-hydroxyethylimino)-methyl]phenolo}copper hemihydrate (II), and chloro-{4-bromo-2-[(2-hydroxyethylimino)-methyl]phenolo}aquanickel (III) are determined using X-ray diffraction. Crystals of compound I are formed by cationic complexes, nitrate ions, and solvate water molecules. In the cation, the copper atom coordinates the singly deprotonated molecule of tridentate azomethine and the water molecule. The copper complexes are joined into centrosymmetric dimers by the O _w -H···O hydrogen bonds. The crystal structure of compound II is composed of binuclear copper complexes and solvate water molecules. The copper atom coordinates the O,N,O ligand molecule and the chlorine ion, which fulfills a bridging function. The coordination polyhedron of the metal atom is a distorted tetragonal bipyramid in which the vertex is occupied by the chlorine atom of the neighboring complex in the dimer. Compound III is a centrosymmetric dimer complex. The coordination polyhedra of two nickel atoms related via the inversion center are distorted octahedra shared by the edge.     

Synthesis,antityrosinase activity of curcumin analogues,and crystal structure of (1<Emphasis Type="Italic">E</Emphasis>,4<Emphasis Type="Italic">E</Emphasis>)-1,5-bis(4-ethoxyphenyl)penta-1,4-dien-3-one

Five derivatives of curcumin analogue (R = OCH₂CH₃ (1), R = N(CH₃)₂ (2), R = 2,4,5-OCH₃ (3), R = 2,4,6-OCH₃ (4), and R = 3,4,5-OCH₃ (5)) were synthesized and characterized by ¹H NMR, FT-IR and UV–Vis spectroscopy. The synthesized derivatives were screened for antityrosinase activity, and found that 4 and 5 possess such activity. The crystal structure of 1 was determined by single crystal X-ray diffraction: monoclinic, sp. gr. P2₁/c, a = 17.5728(15) Å, b = 5.9121(5) Å, c = 19.8269(13) Å, β = 121.155(5)°, Z = 4. The molecule 1 is twisted with the dihedral angle between two phenyl rings being 15.68(10)°. In the crystal packing, the molecules 1 are linked into chains by C?H···π interactions and further stacked by π···π interactions with the centroid–centroid distance of 3.9311(13) Å.     

Synthesis,Crystal Structure,Antioxidant, and α-Glucosidase Inhibitory Activities of Methoxy-substituted Benzohydrazide Derivatives

Eight methoxy substituted at the benzylidene moiety benzohydrazide derivatives [R = 2-OCH₃ (1), 3-OCH₃ (2), 4-OCH₃ (3), 2,3-(OCH₃)₂ (4), 3,4-(OCH₃)₂ (5), 2,4,5-(OCH₃)₃ (6), 2,4,6-(OCH₃)₃ (7), and 3,4,5-(OCH₃)₃ (8)] were synthesized and characterized by ¹H NMR, FT-IR and UV-Vis spectroscopy. The crystal structure of 4 was determined by single crystal X-ray diffraction (sp. gr. Pbca, Z = 8). The molecule is slightly twisted with the dihedral angle between the two phenyl rings being 9.33(14)°. The methoxy group at the ortho position is twisted [C–O–C–C angle is–109.2(3)°] whereas the other at meta position is co-planar with the attached benzene ring. In the crystal packing, the molecules are linked into two-dimensional network parallel to the (001) plane by O–H···O, O–H···N, and N–H···O hydrogen bonds. Compounds 1–8 were evaluated for an antioxidant and α-glucosidase inhibitory activities and the results suggested that the ?OCH₃ substituent was ineffective for bioactivity enhancement.     

Photoluminescence of undoped and B-doped ZnO in silicate glasses

Photoluminescence of undoped and B-doped ZnO in silicate glasses was investigated by varying the concentration of ZnO (35–50 mol%) and B dopant (0–10 mol%) in the glass matrices. The broad and intense near band edge emissions were observed while the visible light emission was very weak. UV luminescence in all samples was red-shifted relative to the exciton transition in bulk ZnO and enhanced by decreased ZnO concentration due to higher degree of structural integrity and the lower aggregation degree of ZnO. Donor B dopant played the double roles of filling conduction bands to broaden band gap when its concentration was lower than 5 mol%, and emerging with conduction bands to narrow the gap when B dopant exceeded this value.     

Synthesis and structural study of 1-(N,N-diethylamino)-2,2-bis(2-nitrophenylthio)ethene

The synthesis, variable temperature NMR spectra, and crystal structures of two crystalline forms, 2a and 2b, of the enamine 1-(N,N-diethylamino)-2,2-bis(2-nitrophenylthio)ethene have been obtained. Both forms crystallize in the monoclinic space group P2₁/a. The two phases have similar molecular structures but possess different intermolecular C–H······O hydrogen bonding interactions. Both forms exhibit disorder within the NEt₂ fragment at 298 K: sufficient disorder persisted with 2a (orange needles) down to 100 K to make the geometric parameters pertaining to the enamine fragment unreliable. The disorder was effectively eliminated on cooling 2b down (red colored blocks) to 150 K. Cell dimensions for the 2a-phase are at 100 K: a = 11.1030(4) Å, b = 15.1325(7) Å, c = 12.4504(7) Å, β = 114.606(3)°, while for the 2b-phase at 150 K, a = 15.5206(4) Å, b = 7.6958(2) Å, c = 15.7137(3) Å, β = 92.580(7)°. The C–N bond length in the β-form at 150 K of 1.335(3) Å indicates considerable double bond character: the rotational barrier of the C–N bond in CDCl₃ was calculated to be 52.4 kJ mol^?1.     

Kinetics of luminescence of CsI single crystals scavenged by Y3+

CsI single crystals were grown from the melt scavenged by Y³⁺ (YCl₃) addition in 6.7·10⁻⁴–6.7·10⁻³ mol·kg⁻¹ range. The addition of the scavenger amounts comparable with the total concentration of the oxygen‐containing admixtures in molten CsI results in complete destruction of the latter. Because of this, the intensity of the band with a maximum at 2.8 eV in radioluminescence spectra caused by the oxygen‐containing admixtures (anion vacancies) considerably decreases, and the fraction of the slow 2μs‐component corresponding to these admixtures becomes lower than 0.01 (0.007). The addition of larger quantities of YCl₃ leads to the appearance of a wide band with a maximum at 2.8 eV caused by cation vacancies, and the intensity of the slow 2μs‐component increases to 0.02. The maximum ratio of two faster components with the decay constants equal to 7 and 30 ns reaches 0.65:0.33 at Y³⁺ concentration in CsI melt equal to 6.7·10^‐3 mol·kg^‐1, the effective luminescence time of fastest components is ca 14 ns. The dependence of the ‘Fast/Total ratio’ on Y³⁺ concentration passes through its maximum (0.81) corresponding to the equivalence of Y³⁺ and O²⁻ concentrations in the growth CsI melt.