Polarization-dependent electrolyte electroreflectance study of Cu2ZnSiS4 and Cu2ZnSiSe4 single crystals

Polarization-dependent electrolyte electroreflectance (EER) measurements were carried out on the oriented Cu₂ZnSiS₄ and Cu₂ZnSiSe₄ single crystals at room temperature. Thin blade single crystals of Cu₂ZnSiS₄ and Cu₂ZnSiSe₄ were grown by chemical vapor transport technique using iodine as a transport agent. Laue pattern normal to the basal plane of the as-grown crystal revealed the formation of orthorhombic structure with the normal along [2 1 0] and the c axis parallel to the long edge of the crystal platelet. The polarized EER spectra in the vicinity of the direct band edge of Cu₂ZnSiS₄ displayed distinct structures associated with transitions from two upper-most valence bands to the conduction band minimum at Γ point. In the E⊥c configuration, the feature designated as E_A ∼ 3.345 eV was detected and for Е‖c, only E_B ∼ 3.432 eV appeared. For Cu₂ZnSiSe₄, three features denoted as E_A, E_B, and E_C at around 2.348, 2.406 and 2.605 eV, respectively, were recorded for E⊥c polarization, whereas in the Е‖c, only E_B and E_C were the allowed transitions. Based on the experimental observations and a recent band-structure calculation by Chen et al. [Phys. Rev. B 82 (2010) 195203], plausible band structures near the direct band edge of Cu₂ZnSiS₄ and Cu₂ZnSiSe₄ have been proposed.     

Syntheses,structures, and luminescent properties of cadmium (II) complexes: 3D supramolecular [Cd(phen)(NO3)(NO2)(H2O)]n and Cd(phen)2(NO3)(NO2) constructed by π–π stacking interactions

Two novel complexes [Cd(phen)(NO₃)(NO₂)(H₂O)]_n (1) and Cd(phen)₂(NO₃)(NO₂) (2) (phen = 1,10-phenanthroline) have been synthesized by the reductive reaction of metal source Cd(NO₃)₂·4H₂O with phen and benzidine in the mixed solution of DMF, ethanol and water. Crystal structures of complexes 1 and 2 were determined by single-crystal X-ray diffraction. Complex 1 is a one-dimensional (1D) zig-zag infinite chain in which (phen)Cd^(II) units were bridged by two O atoms of NO₂⁻. The three-dimensional (3D) supramolecular structure of 1 is constructed through hydrogen-bond and aromatic π–π stacking interactions between adjacent metal–organic polymeric coordination chains. Complex 2 is a mononuclear structure, and self-assembled through π–π stacking interactions to form a three-dimensional (3D) supramolecular structure. The complex 1 exhibits luminescent property in the near-UV at room temperature in solutions of DMSO and DMF with the emission energy following the order DMF < DMSO, which might be ascribed to the presence of a highly polarized ground state. Complexes 1 and 2 have blue-purple luminescence at room temperature in the solid state. The blue-purple luminescence of the complexes is due to π* → π transition of phen.     

Synthesis,X-ray structure and luminescent properties of a new 1D terbium(III) coordination polymer based on 2,4-dichlorophenoxyacetate and 4,4′-bipyridine

A new 1D terbium coordination polymer, {[Tb(2,4-dcpa)₃(H₂O)₂]·(4,4′-bpy)_1.5(H₂O)₂}_n (1) [2,4,-dcpa = 2,4-dichlorophenoxyacetate, 4,4′-bpy = 4,4′-bipyridine], was prepared by hydrothermal synthesis and characterized by IR spectroscopy, elemental analysis, thermogravimetric analysis (TGA) and single-crystal X-ray diffraction. Complex 1 exhibits an infinite chain structure with a {Tb₂(2,4-dcpa)₆(H₂O)₄} dimeric repeat unit. Each Tb³⁺ ion is nine-coordinated to two water molecules, one monodentate carboxylate group and four bridging carboxylate groups in which the carboxylate groups are bonded to the terbium ion in one modes: the chelating–bridging tridentate. Three-dimensional fluorescence spectra of 1 were detected at room temperature under the excitation and the emission wavelengths in the range of 420–750 nm with the interval of 5 nm. The lifetime of 1 in solid-state at room temperature up to 1.132 ms. On the other hand, poor luminescence efficiency has been noted for the Tb³⁺-2,4-dcpa complex.     

Crystal structures and magnetic properties of iron (III)-based phosphates: Na4NiFe(PO4)3 and Na2Ni2Fe(PO4)3

Crystal structures from two new phosphates Na₄NiFe(PO₄)₃ (I) and Na₂Ni₂Fe(PO₄)₃ (II) have been determined by single crystal X-ray diffraction analysis. Compound (I) crystallizes in a rhombohedral system (S. G: R-3c, Z = 6, a = 8.7350(9) Å, c = 21.643(4) Å, R₁ = 0.041, wR₂=0.120). Compound (II) crystallizes in a monoclinic system (S. G: C2/c, Z = 4, a = 11.729(7) Å, b = 12.433(5) Å, c = 6.431(2) Å, β = 113.66(4)°, R₁ = 0.043, wR₂=0.111). The three-dimensional structure of (I) is closely related to the Nasicon structural type, consisting of corner sharing [(Ni/Fe)O₆] octahedra and [PO₄] tetrahedra forming [NiFe(PO₄)₃]⁴⁺ units which align in chains along the c-axis. The Na⁺ cations fill up trigonal antiprismatic sites within these chains. The crystal structure of (II) belongs to the alluaudite type. Its open framework results from [Ni₂O₁₀] units of edge-sharing [NiO₆] octahedra, which alternate with [FeO₆] octahedra that form infinite chains. Coordination of these chains yields two distinct tunnels in which site Na⁺.The magnetization data of compound (I) reveal antiferromagnetic (AFM) interactions by the onset of deviations from a Curie-Weiss behaviour at low temperature as confirmed by Mössbauer measurements performed at 4.2 K. The corresponding temperature dependence of the reciprocal susceptibility χ⁻¹ follows a typical Curie-Weiss behaviour for T > 105 K. A canted AFM state is proposed for compound (II) below 46 K with a field-induced magnetic transition at H ≈ 19 kOe, revealed in the hysteresis loop measured at 5 K. This transition is most probably associated with a spin-flop transition.     

Synthesis, crystal and band structures, and optical properties of a new framework mercury pnictides: [Hg4As2](InBr3.5As0.5) with tridymite topology

A new framework compound, [Hg₄As₂](InBr_3.5As_0.5) (1), has been prepared by the solid-state reaction of Hg₂Br₂ with elemental In and As at 450 °C. Compound 1 crystallizes in the space group P₆₃/mmc of the Hexagonal system with two formula units in a cell: a = b = 7.7408(6) Å, c = 12.5350(19) Å, V = 650.47(12) Å³. The crystal structure of 1 features a novel 3D framework, [Hg₄As₂]²⁺ with tridymite topology. The optical properties were investigated in terms of the diffuse reflectance and infrared spectra. The electronic band structure along with density of states (DOS) calculated by DFT method indicates that the present compound is semiconductor, and the optical absorption is mainly originated from the charge transitions from Br-4p and As-4p states to Hg-6s and In-5p states.     

Influence of cluster composition on NLO properties of neutral cubane-like heterothiometallic clusters

A series of new candidates as nonlinear optical materials, tetra-nuclear heterobimetallic clusters [MOS₃M₃′Y(PPh₃)₃] (M = Mo, M′ = Ag, Y = Br 1; M = W, M′ = Ag, Y = I 2; M = Mo, M′ = Cu, Y = I 3; M = W, M′ = Cu, Y = I 4), have been synthesized by newly developed ligand-redistribution reaction for third-order nonlinear optical (NLO) studies. Single-crystal X-ray diffraction shows that clusters [MoX(μ₃-S)₃(μ₃-Br)(AgPPh₃)₃] 1 and [WX(μ₃-S)₃(μ₃-I)(CuPPh₃)₃] 4 (X = O_0.5S_0.51, O 4) adopt an isomorphous neutral cubane-like skeleton. Their optical nonlinearities were measured by Z-scan technique with an 8 ns pulsed laser at 532 nm. These clusters were found to exhibit effective nonlinear absorption, self-focusing effects and large optical limiting capabilities. The effective NLO susceptibilities χ⁽³⁾ and the corresponding second-order hyperpolarizabilities γ of these clusters are also reported. The influence of cluster composition on NLO properties has been discussed accordingly.     

First charge-transfer complexes between tetrathiafulvalene and 1,2,5-chalcogenadiazole derivatives: Design,synthesis, crystal structures,electronic and electrical properties

The first charge-transfer complexes of tetrathiafulvalene (1) with 1,2,5-chalcogenadiazole derivatives, i.e. with [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (2) and 3,4-dicyano-1,2,5-telluradiazole (3), were designed, prepared in the form of air and thermally stable single crystals and structurally defined by X-ray diffraction as 1·2 and 1·3₂, respectively. Starting compound 2 (effective electron acceptor with potentially broad application in the field) was synthesized by a new efficient one-pot method from 3,4-diamino-1,2,5-oxadiazole and disulfur dichloride. The electronic structure of complexes 1·2 and 1·3₂ and thermodynamics of their formation were studied by means of DFT and QTAIM calculations and UV–Vis spectroscopy. The electrical properties of single crystals of the complexes were investigated revealing semiconductor properties with an activation energy of 0.34 eV for 1·2 and 0.40 eV for 1·3₂. Polycrystalline films of the complexes displayed photoconductive effects with increased conductivity under white-light illumination.     

Fabrication of V3O7·H2O@C core-shell nanostructured composites and the effect of V3O7·H2O and V3O7·H2O@C on decomposition of ammonium perchlorate

V₃O₇·H₂O@C core-shell materials have been synthesized using V₃O₇·H₂O nanobelts as the cores and glucose as the source of carbon via an environmental hydrothermal method. The as-obtained V₃O₇·H₂O@C core-shell materials were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), elemental analysis (EA), Fourier transform infrared spectroscopy (FT-IR) and Raman spectrum. The influences of the reaction temperature, concentration of glucose and reaction time on the morphologies of the samples were respectively discussed in detail. The possible formation mechanism of V₃O₇·H₂O@C was proposed according to our experimental results. Furthermore, the effect of V₃O₇·H₂O and V₃O₇·H₂O@C on the thermal decomposition of ammonium perchlorate (AP) were investigated by thermal gravimetric analyzer (TG) and differential thermal analysis (DTA). The thermal decomposition temperatures of AP in the presence of V₃O₇·H₂O and V₃O₇·H₂O@C were reduced by 70 and 89 °C, respectively, which indicates that V₃O₇·H₂O@C core-shell composites have higher activity than V₃O₇·H₂O.     

Experimental and theoretical calculations on corrosion inhibition of steel in 1 M H2SO4 by crown type polyethers

The corrosion inhibitive efficiencies of two crown type polyethers, namely dibenzo-bis-imino crown ether (C-1) and dibenzo-diaza crown ether (C-2), which are macrocyclic Schiff base and its reduced form (macrocyclic amine), respectively, for the steel in 1 M H₂SO₄ have been investigated by Tafel extrapolation and linear polarization methods. Corrosion and adsorption isotherm parameters were determined from current-potential curves. The studies show that C-1 and C-2 inhibit the corrosion of the steel in H₂SO₄ solution. Semiempirical AM1 method was used for theoretical calculations. The obtained results of these calculations for the compounds were found to be consistent with the experimental findings.     

Fine tuning the HOMO energy levels of polythieno[3,4-b]thiophene derivatives by incorporation of thiophene-3,4-dicarboxylate moiety for photovoltaic applications

To lower the HOMO (highest occupied molecular orbital) energy level of polythieno[3,4-b]thiophene (∼−4.5 eV), a series of ester-functionalized polythieno[3,4-b]thiophene derivatives (P1–P3) were designed and synthesized by Stille cross coupling reaction. The resulting copolymers exhibited broad and strong absorption bands from visible to near infrared region with low optical band gaps of 1.23–1.42 eV. Through cyclic voltammetry measurements, it was found that the HOMO energy levels of the copolymers gradually decreased with increasing the content of the thiophene-3,4-dicarboxylate moiety, i.e. −4.91 eV for P1, −5.00 eV for P2, and −5.11 eV for P3. Preliminary photovoltaic properties of the copolymers blended with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as electron acceptor were investigated. Among the three copolymers, P1 exhibited the best photovoltaic performance with an open circuit voltage (V_oc) of 0.54 V, a short circuit current density (I_sc) of 3.3 mA/cm², a fill factor (FF) of 0.57, and a power conversion efficiency (PCE) of 1.02%. A high V_oc up to 0.71 V was achieved in the solar cell based on a P3:PCBM blend.     

Synthesis and characterization of efficient orange-red emitting poly(p-phenylenevinylene) derivatives with 1,3,4-oxadiazole unit

A series of the statistical copolymers of tetradecyloxy-3,5-bis-{2-(4-bromo-phenyl)-[1,3,4]oxadiazole}-benzene (4), 1,4-didecyloxy-2,5-diiodobenzene (5) and 1,4-dodecyloxy-2,5-divinylbenzene (6), for efficient light emitting diode were synthesized. As the feed ratio of compound 4 decreased, the absorption band at 320–360 nm decreased in the UV–vis spectra. In addition, as the composition of compound 4 decreased, the absorption intensity ratio at 1541 cm⁻¹ decreased in the FT-IR spectra. The photoluminescence and electroluminescence emission color of P showed green-yellow color (λ_max = 552 nm). However, the maximum wavelength of photoluminescence and electroluminescence of P1 and P2 were 581 and 583 nm, respectively, which were dramatically red-shifted than that of P. Moreover, these values were almost the same as the values of P3 (580 nm). It is concluded that the energy transfer from high energy oxadiazole segments to low energy alkoxy-PPV segments occurred completely. The efficiency of ITO/P2/Al was 0.0181%, which is the best efficiency among the device based on the copolymers. This is presumably due to that P2 has the best balanced hole- and electron-injection/transporting properties among the copolymers.     

Effect of small oxygen deficiency on the para-coherent transition and 2D-3D crossover in untwinned YBa2СuзО7−δ single crystals

We investigate the influence of the disorientation angle between the direction of a constant magnetic field (up to 15 kOe) and the ab-plane α ≡ ∠(H,ab) to the temperature dependence of the excess conductivity in the temperature interval of the transition to the superconducting state in untwinned YBa₂Cu₃O_7−δ single crystals. We discuss the appearance of low temperature “tails” (para-coherent transitions) on the resistive transitions, corresponding to different phase regimes of the vortex-matter state. At temperatures above the critical temperature, the temperature dependence of the excess para-conductivity is interpreted within the Hikami-Larkin theoretical model of the fluctuation conductivity for layered superconductors.     

New organic semiconductors for thin-film transistors: Synthesis,characterization, and performance of 4H-indeno[1,2-b]thiophene derivatives

Three new hybrid polycyclic aromatic 4H-indeno[1,2-b]thiophene derivatives (DIT-nT) were readily synthesized and characterized. Ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) study shows that the gap energies of these oligothiophene derivatives are dependent on the effective π-conjugation length of the molecules, with decreasing gap energies while the conjugated length is increased. More importantly, the hole injection barrier obtained from UPS and IPES studies decreases significantly at the metal-organic interface as the effective π-conjugation length of the molecules increases. Hence, the hole injection into DIT-2T is expected to be more facile than that into DIT-0T due to the smaller barrier. X-ray diffraction examination revealed that thin films are highly ordered, with the long axes of the molecules nearly perpendicular to the surface. The field-effect transistor (FET) devices based on the three oligomers show good p-type performance with high hole mobilities, and the highest value (0.014 cm² V⁻¹ s⁻¹) was obtained for the longest oligomer DIT-2T.     

Synthesis and third-order nonlinear optical studies of a novel four-coordinated organoboron derivative and a bidentate ligand: The effect of the N → B coordinative bond

The third-order nonlinear optical characterization of a new boronate (2) derived from 4-dimethylaminocinnamaldehyde was performed by third-harmonic generation (THG) at the infrared wavelength of 1550 nm. Compound 2 was prepared from the reaction of diphenylboronic acid and the bidentate ligand (1) and characterization was made through UV, IR, ¹H, ¹¹B, and ¹³C NMR and X-ray diffraction. The THG experiments showed that the N → B coordinative bond in 2 enhanced the second molecular hyperpolarizability of the type γ⁽³⁾(−3ω, ω, ω, ω) by a factor of three with respect to the value exhibited by the ligand 1. On the other hand, Z-scan studies at 800 nm (femtosecond (fs) pulses) also showed that such coordinative bond increased the nonlinear absorption (two-photon absorption (TPA)) in 2 with respect to 1. These studies demonstrate that the N → B coordinative bond facilitates the polarization of the electronic π-system, a situation that optimizes the third-order NLO response. Results on the excited state absorptions in these compounds are also presented.     

Synthesis, crystal structures, and optical properties of NaCdPnS3 (Pn = As, Sb)

Two compounds NaCdPnS₃ (Pn = As (1), Sb (2)) were synthesized as transparent yellow and red platelets by reacting cadmium powder with the molten mixtures of Na₂S/As₂S₃/S and Na₂S/Sb/S at 500 °C. Both compounds are plagued with crystal twinning and acceptable crystal structure refinement could only be obtained by identifying the type of the twinning laws. NaCdAsS₃ (1) crystallizes in the monoclinic space group P2₁/n (no. 14) with a = 5.6561(8), b = 16.5487(15), c = 5.6954(8) Å, β = 90.315(11)°, and Z = 4. NaCdSbS₃ (2) crystallizes in the monoclinic space group C2/c (no. 15) with a = 8.1329(6), b = 8.1296(4), c = 17.2600(13) Å, β = 103.499(6)°, and Z = 8. The structures of both compounds feature a ²_∞[CdPnS₃]⁻ layer composed of [CdS₄] tetrahedra and [AsS₃] or [SbS₃] pyramidal units. Between the ²_∞[CdPnS₃]⁻ (Pn = As, Sb) layers the Na⁺ cations reside in a distorted octahedral environment of S atoms. Compound 1 is characterized with UV/Vis diffuse reflectance spectroscopy and IR and Raman spectra.     

Synthesis of the pyrrolo[3,2-b]pyrrole-based copolymer with enhanced open circuit voltage

A new accepter unit, pyrrolo[3,2-b]pyrrole-2,5-dione, was prepared and utilized for the synthesis of the conjugated polymer containing electron donor–acceptor pair for OPVs. Pyrrolo[3,2-b]pyrrole-2,5-dione unit, regioisomer of the known pyrrolo[3,4-c]pyrrole-1,4-dione, is originated from the structure of stable synthetic pigment. The new conjugated polymer with pyrrolo[3,2-b]pyrrole-2,5-dione, thiophene and carbazole was synthesized using Suzuki polymerization to generate P1. The solid thin film of P1 shows absorption band with maximum peaks at 374 and 548 nm, and the absorption onset at 679 nm, corresponding to band gap of 1.83 eV. The field-effect hole mobility of P1 is 2.2 × 10⁻⁵ cm²/Vs. The device based on the polymer:PCBM (1:2) blend without thermal treatment showed a V_OC of 0.82 V, a J_SC of 6.28 mA/cm², and an FF of 0.39, giving a PCE of 2.00%.     

Analysis of the electrochemical reaction behavior of alloy AZ91 by EIS technique in H3PO4/KOH buffered K2SO4 solutions

The EIS technique was used to analyze the electrochemical reaction behavior of Alloy AZ91 in H₃PO₄/KOH buffered K₂SO₄ solution at pH 7. The corrosion resistance of Alloy AZ91 was directly related with the stability of Al₂O₃ · xH₂O rich part of the composite oxide/hydroxide layer on the alloy surface. The break down of the oxide layer was estimated to occur mainly on the matrix solid solution phase in Alloy AZ91. The mf capacitive loop arose from the relaxation of mass transport in the solid oxide phase in the presence of Al₂O₃ · xH₂O rich part and from Mg⁺ ion concentration within the broken area in the absence of Al₂O₃ · xH₂O rich part in the composite oxide structure on the alloy surface. The lf inductive loop had tendency of disappear when the dissolution rate of the alloy decreased as a result of the formation of the protective oxide layer.