Nanocrystalline ZnS dispersed in polymer electrolyte (PEO :NH4I): Preparation and electrical conductivity measurements

Nano-composites of a polymer electrolyte PEO:NH₄I (80:20) have been prepared by dispersing nano-size ZnS crystallites in it. The measured band gap of dispersed ZnS is 3.9 eV and its particle size as estimated from the XRD linewidths is 11 nm. Detailed I–V and polarisation studies show that the composite polymer film is a mixed (ionic+electronic) conductor and that the dispersoid ZnS is n-type. The total electrical conductivity Vs composition studies show two peaks at the ZnS concentrations of 4 and 10wt% which can be qualitatively explained on the basis of two-percolation threshold model.     

炔醇类缓蚀剂对铁在硫酸溶液中保护作用的光电化学研究

在含不同缓蚀剂的0．5mol/L H2SO4的电解质溶液中,用光电化学方法研究了氧化铁钝化膜的电子传递过程;缓蚀剂的加入在金属表面形成的沉积膜是非光电化学活性的,基本上对光电流的瞬态性质不造成影响;在硫酸溶液中测得两个明显的光电流峰,其禁带宽度值Eg^dir=3.7eV(对应300nm处的弱峰）,Eg^ind=1.95eV(380到500nm范围的强峰）,加入不同缓蚀剂后测得Eg^ind值略有减小;测定电位阶跃下发生的电流瞬态引起电荷量的改变,表明在恒定电位下延长钝化时间会减少膜中羟量和水的含量,而缓蚀剂的加入减少了膜中OH^-和H2O的含量,说明缓蚀剂沉积膜与钝化膜之间存在着化学作用,造成了钝化膜表面附近的化学组成发生变化。     

Electronic properties of C-doped boron nitride nanotubes studied by first-principles calculations

The geometry and electronic structure of C-doped BNNTs are investigated using the hybrid Heyd–Scuseria–Ernzerhof. The van Hove singularity (vHs) peaks split in density of states (DOS) The impurity states decrease the bandgap.

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Elastic band structures of two-dimensional solid phononic crystal with negative Poisson's ratios

In this paper, the elastic band structures of two-dimensional solid phononic crystals (PCs) with both negative and positive Poisson's ratios are investigated based on the finite difference domain method. Systems with different combinations of mass density ratio and shear modulus ratio, filling fractions and lattices are considered. The numerical results show that for the PCs with both large mass density ratio and shear modulus ratio, the first bandgap becomes narrower with its upper edge becoming lower as Poisson's ratio of the scatterers decreases from −0.1 to −0.9. Generally, introducing the material with a negative Poisson's ratio for scatterers will make this bandgap lower and narrower. For the PCs with large mass density ratio and small shear modulus ratio, the first bandgap becomes wider with Poisson's ratio of the scatterers decreasing and that of the host increasing. It is easy to obtain a wide low-frequency bandgap by embedding scatterers with a negative Poisson's ratio into the host with a positive Poisson's ratio. The PCs with large filling fractions are more sensitive to the variations of Poisson's ratios. Use of negative Poisson's ratio provides us a way of tuning bandgaps.     

Synthesis and electronic properties of bis-styryl substituted trimeric aryleneethynylenes. Comparison of cruciforms with iso-cruciforms

Four 1,2-distyryl-4,5-bis(phenylethynyl)benzenes are described. A two step synthesis, a combination of Horner and Sonogashira methods is utilized. The targets and their para isomers were examined by UV-vis and fluorescence spectroscopies as well as by cyclic voltammetry. They show solvatochromic behavior and are easily oxidized if the styryl units carry dialkylamino substitutents. Single crystal structures of three derivatives have been obtained.     

The I-V characteristics of InAs/GaAs quantum dot laser

The I-V characteristics of an InAs/GaAs quantum dot (QD) laser diode have been investigated under both the high and low input current conditions. Under the low current condition, the I-V curve obeys the Shockley equation, from which the forbidden energy gap of the junction can be derived. On the other hand, in the high current range, the I-V characteristics violate the Shockley equation and the device tends to operate as a resistance. In addition, the I-V curve can be used to fit the temperature coefficient of the forward voltage, which is a critical parameter for determining the junction temperature of the laser diode.     

Topology optimization of two-dimensional asymmetrical phononic crystals

The multiple elitist genetic algorithm with the adaptive fuzzy fitness granulation (AFFG) is used to design the phononic crystals with large relative bandgap width (BGW) for combined out-of-plane and in-plane wave modes. Without assumption on the symmetry of the unit-cell, we obtain an asymmetrical phononic crystal with the relative BGW which is quite larger than that of the optimized symmetrical structure. With the help of AFFG, the number of the fitness function evaluations is reduced by over 50% and the procedure converges 5 times faster than the conventional evolutionary algorithm to reach the same final fitness values.     

Dispersion analysis with 45°-rotated augmented supercells and applications in phononic crystal design

The supercell approach can be useful for tailoring dispersion curves of phononic crystals, but the interpretation of the dispersion curves in the supercells can be often intriguing. Supercells formed by integer multiples of an original unit cell along its lattice axes are common, but there are also important situations requiring supercells formed by non-integer multiples of an original unit cell and its rotation with respect to its lattice axes. In these cases, not only dispersion branch folding, but also branch overlapping not found in common supercells, take place, which complicates the correct interpretation of band structures. In this study, we consider 45°-rotated augmented supercells and analyze why and how branch folding and overlapping take place. For the analysis, the relation between the first Brillouin zone of an original cell and that of the corresponding 45°-rotated augmented supercell is investigated. The analysis of the folding and overlapping mechanism found in the dispersion curve of the supercell is also useful for interpreting which branches can be excited over a target wavevector direction. The usefulness of the findings of this supercell-based dispersion analysis is demonstrated in unit cell design problems. Specifically, we show how to interpret correctly the dispersion curves of phononic crystals made of unit cells optimized by bandgap maximizing topology optimization when the optimized unit cells turn out to be 45°-rotated augmented supercells. Conversely, throughout design optimization iterations, the original period of a unit cell that is initially set at the beginning of design optimization can be maintained if branch overlapping is forced not to occur.     

SrAl_2O_4∶Eu,Dy纳米材料与体材料的光电特性研究

用溶胶-凝胶自燃烧法和高温固相法分别制备了纳米和体相SrAl_2O_4:Eu,Dy长余辉磷光体.用X射线衍射对晶体结构进行了表征,用Keithley 2410对材料在有无光照条件下的电流-电压特性进行了分析,同时还测量了样品的真空紫外激发光谱.光照使材料的电流增强,说明至少有部分电子经光照后被激发到导带;纳米材料真空紫外激发光谱发生蓝移,说明纳米材料的禁带宽度要大于体材料的禁带宽度,相同电压下纳米材料的电流小于体材料的电流.     

Study of the bandgap renormalization in Ga-doped ZnO films by means of optical absorption under high pressure and photoelectron spectroscopy

In this paper we investigate the band gap renormalization in heavily Ga-doped ZnO thin films deposited by pulsed laser deposition on C$C$-plane sapphire and mica substrates. Thin films were studied by ultraviolet photoelectron spectroscopy and also by optical measurements under high pressure. The Fermi-level shift, as obtained from ultraviolet photoelectron experiments, exhibits a relatively small and positive shift (about 0.3 eV) with respect to the valence band for increasing electron concentrations up to 10²¹ cm⁻³. The optical gap exhibits a much larger increase (0.6 eV) for the same concentration range. Absorption measurements under pressure show that the pressure coefficient of the optical gap is correlated to the electron concentration in films, decreasing with increasing electron concentration. As a consequence, the contributions of band filling and band renormalization to the optical gap shift can be separated on the basis of the different pressure behavior of the physical parameters involved in each effect.