Hardness of Crystals with NaCl Structure and the Significance of the GILMAN-CHIN Parameter

The microhardness of PbTe, PbSe, EuS and EuSe has been determined. The microhardness vs lattice constant plot for chalcogenides is a curve similar to the one for alkali halides. The Gilman-Chin parameter (H/C₄₄) has been calculated for a large number of crytals with NaCl structure. There is no unique value representing the entire group of crystals, the values ranging from 0.01 to 0.20. The Gilman-Chin parameter broadly correlates with the effective ionic charge (e) indicating that it is a measure of the ionicity. Empirically, it is observed that the product (H/C₄₄) · (e)² is approximately constant.     

Microstructure and optical properties of (Pb,Ca)TiO3 films grown on Si(1 0 0) substrates by a simple sol-gel process

Amorphous and polycrystalline (Pb_0.76Ca_0.24)TiO₃ (PCT) thin films deposited on an Si(1 0 0) substrate have been prepared by a simple sol-gel process. The microstructure and surface morphologies of the thin films have been studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The polycrystalline PCT film on the Si(1 0 0) substrate has a tetragonal perovskite structure with grain size from 60 to 110 nm. AFM reveals smooth surfaces and root mean square (rms) roughness of 0.17 and 4.4 nm for amorphous and polycrystalline films, respectively. The refractive index n and extinction coefficient k of the amorphous and polycrystalline thin films was obtained by spectroscopic ellipsometry as a function of the photon energy in the range from 2.0 to 5.4 eV. The maximum n and direct bandgap energies of amorphous and polycrystalline thin films were 2.66 and 4.11 eV, 2.64 and 3.84 eV, respectively.     

Synthesis and characterization of PbSe and PbSe/PbS core–shell colloidal nanocrystals

A new colloidal procedure, for the synthesis of PbSe and PbSe/PbS core–shell semiconductor nanocrystals (NCs), is reported. The synthesis includes the reaction between tributyl-phosphine selenium and lead 2-ethyl-hexanoate, under inert gas at room temperature. High-resolution transmission electron microscopy (HRTEM), X-ray energy dispersive spectroscopy (EDS), absorption and photoluminescence (PL) spectroscopy were used to characterize the samples. The EDS and HRTEM confirmed the existence of rock-salt cubic structures of the PbSe. Furthermore, the HRTEM showed the formation of PbSe/PbS core–shell structures, with epitaxial shell layer with thickness varying between 1 and 4 ML. The absorption spectra of these materials exhibited distinct transitions, related to the e1–h1, e2–h2 and e2–h3 and e2–h1 transitions. These bands are blue shifted with decrease in the NCs diameter. However, the e1–h1 is slightly red shifted with increase in the PbS shell thickness. The last effect can be due to an electronic mixing of the PbSe and PbS conduction states. The PL measurements showed a substantial increase of the exciton emission in the core–shell structures, arising by an efficient chemical passivation of the PbSe core.     

Microwave-assisted green synthesis of SnO2 nanoparticles and their optical and photocatalytic properties

Abstract

In this study, SnO₂ has been synthesized using Andrographis Paniculata (A. Paniculata) by microwave-assisted method. It is also characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). It exhibits the tetragonal structure with average crystallite size of 27?nm. The stretching vibration peak of Sn-O-Sn is 670?cm^?1. SEM reveals the formation of nanoparticles. The bandgap was estimated by the tauc’s relation as 3.52?eV from the UV-Visible (UV-Vis) spectra. Biosynthesized SnO₂ nanoparticles show excellent photocatalytic activity against Congo red dye under sunlight irradiation.     

Semiconducting TiO2SiO2 glasses (II): Optical properties

The refractive index, optical absorption coefficient α and the thermomodulated absorption dα/dT have been measured on 70% TiO₂?30% SiO₂ glasses with up to 8% Ti³⁺. The direct absorption data show intense ligand field absorption at photon energies hν = 1.9 and 3.0 eV, arising from Ti³⁺ in a distorted octahedral environment. In the bandgap region at 3.5 eV α obey αhν ～ (hν ? E_g)²; it is qualitatively different from the bandgap absorption in crystalline TiO₂. The da/dT spectra show peaks in the bandgap region and at 1.1 eV in the near IR. This last peak is attributed to absorption by small polarons, and its line shape is compared with theoretical predictions.     

In situ observation of formation of misfit dislocations in pseudomorphic monolayer overgrowth of metals and non-metals

Epitaxial overgrowth in cases of deposit and substrate combinations of PbSe/PbS, Au/Pd and Pd/Au were observed in situ in an electron microscope. It was found that the growth proceeds with the very characteristic features of monolayer overgrowth as implied by the theory of Frank and Van der Merwe; no indication of nucleation was observed and the formation of misfit dislocations was noticed over the whole area of films at certain critical thicknesses of the overgrowths. Points were noted which distinguish the monolayer overgrowth from the case where misfit dislocations are introduced in nucleated islands, to which Matthews has counted the growth processes in these combinations. The importance of climb motion as well as slip motion was stressed as the mechanism of introduction of misfit dislocations. Some results of electron diffraction and electron microscopy of films prepared in separate evaporation units were also quoted. These were to examine the relation between misfit dislocation density and mean lattice parameter and the effect of degree of vacuum on the growth mode and to indicate that the same growth mode takes place for some other combinations of IV–VI compounds, SnTe/PbSe, PbTe/PbS and PbTe/PbSe.     

Optical properties of novel PbS and PbSe quantum-dot-doped alumino-alkali-silicate glasses

Lead sulfide PbS and lead selenide PbSe quantum dots (QDs) were synthesized in novel alumino-alkali-silicate glass. The synthesis of the nanocrystals was stabilized by introduction of two alkaline components. The presence of crystalline phase was confirmed by X-ray diffraction analysis, transmission electron microscopy and optical spectroscopy. For PbS (PbSe) QD-doped glass, the position of the 1S–1S excitonic absorption peak can be managed in the spectral range of 1.5–2.1 μm (for PbS) or 1.8–2.2 μm (for PbSe) by appropriate heat‐treatment mode. The corresponding QD average diameter was found to be 5.8–9.7 nm (for PbS) and 7.5–9.5 nm (for PbSe). The influence of the secondary heat-treatment at the temperatures of 490–525 °C on the PbS QD growth in the glass matrix initially treated at 480 °C was studied in details. The photoluminescence of the PbS-QD-doped glass was observed, it was referred to the radiative recombination of the excitons from the 1S–1S state. The possibility to obtaining narrow 1S–1S absorption lines at the wavelengths longer than 2 μm is discussed.     

Preparation and characterization of monodispersed PbSe nanocubes

Monodispersed PbSe nanocubes were synthesized through a simple hydrothermal method by using Pb²⁺‐EDTA complex and Na₂SeSO₃. Composition and morphology of the samples were characterized by means of XRD, TEM and SEM. The gradually releasing of Pb²⁺ from Pb²⁺‐EDTA complex, and the adopting of PVP can adjust the growth rate in <100> and <111> directions of PbSe. Both Pb²⁺‐EDTA complex and PVP played important roles in the crystal growth of the monodispersed PbSe nanocubes. In addition, UV‐vis and photoluminescence properties were characterized. E_g of the PbSe nanocubes is 1.5 eV approximately, which is much larger than that of bulk PbSe. Under PL excitation at 406 nm, a red emission peak at 696 nm is observed, showing that the synthesized PbSe is luminescent nanomaterials. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigations for Sn diffusion in Pb1−xSnxTe and PbTe

By annealing Pb_1−xSn_xTe and PbTe isothermally in a quartz ampoule Sn diffused from Pb_1−xSn_xTe into PbTe. The profiles obtained have been investigated by means of an electron beam microanalyser, and the coefficients of diffusion have been determined at various temperatures. The diffusion of Sn can be explained by the expressions: D_PbSnTe = 1.5 · 10⁻¹ exp (−1.8 eV/kT) cm² s⁻¹ (0,14 < x < 0,18) D_PbTe = 5,5 · 10⁻⁴ exp (−1.5 eV/kT) cm² s⁻¹. N-type layers are observed at the surface of Pb_1−xSn_xTe specimens.     

Cr content-dependent modification of structural,magnetic properties and bandgap in green synthesized Co–Cr nano-ferrites

Abstract

Honey green synthesis of CoCr_xFe_2?xO₄ (x?=?0.0???1.0) ferrites with increasing Cr content was done by sol-gel auto-combustion method, without any post-preparation thermal treatment. Nanocrystalline nature (grain diameter range between 12.2?nm and 27.9?nm) of the studied samples is confirmed by X-ray diffraction (XRD) data. Observed modification of magnetic properties can be understood in terms of Cr addition induced changes in degree of inversion, modification of disorder affecting A–A, A–B, B–B interaction, re-distribution of cations on A, B sites in the spinel lattice. Grain size (D)-dependent coercivity (H_c) shows that the studied samples lie in the region with overlap between single- and multi-domain regions. Achieved magnetic properties in the studied honey green synthesized samples have significance for self-regulating hyperthermia applications. Results as well reveal modification of bandgap, and would be of use for applications, where minor bandgap tuning is required. Antistructure modeling provides information on the types of surface-active centers.     

Thermoelectric properties of nanocrystalline solid solutions of bismuth and antimony chalcogenides

The samples structure of solid solutions of p- and n-type bismuth and antimony chalcogenides obtained in the form of rods 1.6 mm in diameter by hot extrusion have been studied. These rods are found to consists of nanocrystallites 8?C60 nm in size. In p-type samples, nanocrystallites aggregated into spherical crystallites 50?C100 nm in diameter, whereas nanocrystallites in n-type samples formed disoriented fibers with a transverse cross section of several ten nanometers to several micrometers and a length of a few micrometers to several dozen micrometers. The presence of nanocrystallites of this size in the sample structure reduces the thermal conductivity of bismuth and antimony chalcogenides by 10% but barely changes their resistivity and thermoelectric power. The thermoelectric efficiencies of the p- and n-type chalcogenides are 3.27 × 10^?3 and 2.78 × 10^?3 K^?1, respectively.     

Crystal structures of gold,silver, and sodium chalcogenides: Sphenoidal interpretation

The crystal structures of 13 chalcogenides of Na, Au(I), and Ag(I) in the Na_{2 − n} (Au,Ag) _n (S,Se,Te) series, where 0 ≤ n ≤ 2, are interpreted from unified positions based on the sphenoidal representation. Its essence is in the consideration of the entire crystal space (with packing X atoms in the framework of close-packing, body-centered, or hybrid schemes) as a set of elementary space units (sphenoids). Unified one-dimensional associates of sphenoids, the so-called basic rods, with sets of possible atomic positions in them are selected for all structures. The mutual effect of the dimensional and stoichiometric ratios of all components on the features of filling rod positions is analyzed. New possibilities in the crystallochemical and crystal-geometry analysis of inorganic compounds whose structures are characterized by a relatively uniform distribution of atoms are demonstrated by the example of chalcogenides.     

Influence of silver nanoclusters on formation of PbS quantum dots in glasses

Heat-treatment was used to precipitate PbS quantum dots (QDs) in silicate glasses doped with different amounts of Ag₂O, and the influence of Ag₂O on QDs was investigated. Under given heat-treatment conditions, the absorption coefficients and photoluminescence intensities of PbS QDs increased with the addition of Ag₂O. Ag clusters formed by thermal treatment nucleated formation of PbS QDs in glasses.     

A Polyacetylene via the Cyclopolymerization of 4,10-Bis(diethylmalonate)-1,6,11-dodecatriyne: Synthesis and Characterization

A new class of polyacetylene was synthesized via the cyclopolymerization of 4,10-bis(diethyl malonate)-1,6,11-dodecatriyne by using MoCl₅-EtAlCl₂ catalyst system. The polymerization proceeded in mild homogeneous manner to give 32% yield of polymer. The chemical structure of resulting polymer was characterized by NMR (¹H-, ¹³C-), IR, and UV-visible spectroscopies to have a conjugated backbone system. The polymer showed characteristic wide UV-visible absorption band and PL maximum peak at 530 nm corresponding to the photon energy of 2.34 eV. The cyclic voltammogram of polymer showed the stable electrochemical window in the range of of ?1.50V to +1.50V. From the CV measurements, the HOMO energy level of the polymer was calculated to be 5.55 eV.     

X-ray photoelectron spectroscopy of silica in theory and experiment

Fourier deconvolved X-ray photoelectron spectroscopy (XPS) valence band spectra obtained from crystalline and amorphous silica, used in conjuction with the results of quantum chemical calculations of the SiO₄ tetrahedral unit and other spectrometric measurements (soft X-ray emission, UV absorption and reflectivity, photoconductivity, photoinjection and energy loss spectroscopy), suggest a reinterpretation of the electronic band structure of silica that is consistent with all the data. A unique method for pinning the Fermi level of insulators to that of a metal calibrant is described, resulting in the ability to obtain absolute binding energies of the electronic levels in wide bandgap insulators. Observe peaks in UV reflectivity and energy loss spectra of silica are all assigned to direct interband transitions, and no excitonic states need be involved to explain the data. Upper and lower limits for the bandgap of dry crystalline (α-quartz) and amorphous (Corning Code 7940 glass) silica are adjusted downward from the 8.9 eV bandgap proposed by DiStefano and Eastman [1] to 7.8–5.55 eV for α-quartz and 7.3–5.05 eV for fused silica, respectively. This in no way compromises the obvious insulating properties of silica in MOS devices, since the conductivity is governed by the high barrier height (～3.8 eV in the case of gold) for metal-insulator electron transfer. The lowered bandgap results from increased low-energy electron density in the valence band, which we ascribe to the 1t₁ molecular orbital predicted by various quantum formalisms, but heretofore not detected experimentally in bulk (thick) silica. Disappearances of this orbital and rearrangement of the non-binding 5t₂ and 1e orbitals in silicas rich in silanols (OH), as may be the case for thin-film silica on Si metals, would increase the bandgap to 8.3 eV, in better agreement with previous determinations.     

Photo-EPR of deformation-produced defects in GaAs

Deformation-produced paramagnetic centres have been studied in GaAs crystals uniaxially compressed at 400° using EPR technique (X band). The effect of light with photon energies from 0.4 to 1.5 eV on the populations of the centres has been investigated. On the basis of the experimental results a model is proposed which locates the energy levels of two of the centres at E_c − 1.05 eV and E_v + 0.75 eV.     

An Iodoargentate Hybrid Coordination Polymer Constructed by Methyl Viologen: Structure,Properties and Theoretical Study

Abstract  

An iodoargentate hybrid coordination polymer induced by MV²⁺, [MV(Ag₂I₄)] _n (1) (MV = methyl viologen, i.e., 1,1-dimethyl-4,4-bipyridinium), has been obtained and structurally determined. In 1, (Ag₂I₄) _n ²⁻ chain exhibits a new type of zigzag-like configuration constructed from edge-sharing of normal and distorted AgI₄ tetrahedra, hydrogen bonds between MV²⁺ cations and (Ag₂I₄) _n ²⁻ chains contributed to the formation of quasi-two-dimensional structure. The band gap of 2.12 eV based on optical absorption spectrum indicated its semiconductor nature. Quantum chemistry calculation with DFT method was carried out to reveal its electronic structure.     

Temperature and excitation intensity tuned photoluminescence in Tl4GaIn3S8 layered single crystals

Photoluminescence spectra of Tl₄GaIn₃S₈ layered crystals grown by Bridgman method have been studied in the wavelength region of 500–780 nm and in the temperature range of 26–130 K with extrinsic excitation source (λ_exc = 532 nm), and at T = 26 K with intrinsic excitation source (λ_exc = 406 nm). Three emission bands A, B and C centered at 514 nm (2.41 eV), 588 nm (2.11 eV) and 686 nm (1.81 eV), respectively, were observed for extrinsic excitation process. Variations in emission spectra have been studied as a function of excitation laser intensity in the 0.9‐183.0 mW cm^–2 range for extrinsic excitation at T = 26 and 50 K. Radiative transitions from the donor levels located at 0.03 and 0.01 eV below the bottom of the conduction band to the acceptor levels located at 0.81 and 0.19 eV above the top of the valence band were proposed to be responsible for the observed A‐ and C‐bands. The anomalous temperature dependence of the B‐band peak energy was explained by configurational coordinate model. From X‐ray powder diffraction and energy dispersive spectroscopic analysis, the monoclinic unit cell parameters and compositional parameters of Tl₄GaIn₃S₈ crystals were determined, respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Energy Band Structure of CuInS2 Crystals

In this work the reflectivity spectra and wave-length derivative reflectivity (WDR) spectra of CuInS₂ crystals have been investigated in the region E ≥ E_g. The n = 1, n = 2 and n = 3 excitonic states are determined and contours of exciton lines n = 1 are calculated. The parameters of excitons and bands have been determined for the region of band gap minimum. The main band gaps are determined for Γ-, N- and T-points of the Brillouin zone.     

Solution-processed small-molecule organic solar cells based on diketopyrrolopyrrole and perlyene derivatives with coplanar structures

ABSTRACT

A series of small-molecules (SMs) based on coplanar perylene unit coupled with diketopyrrolopyrrole chromophoric core exhibit broad absorption in the range of 500–800 nm and low bandgap energise of 1.6–1.7 eV. The power conversion efficiency approximately reach at 0.4% under 1.5 G illumination is achieved for organic solar cells based on a small-molecule bulk heterojunction system consisting of SMs as a donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as an acceptor.