Capped semiconductor nanocrystals for device applications

For device purpose, our main aim is to synthesise material which is chemically and thermally stable, as well as enhancement in luminescence properties followed with matching lattice parameters. This can be achieved by precisely controlling the size of semiconductor nanocrystals which can create an opportunity for producing functional materials with new properties. Here we showed advantages of using both organic and inorganic capping agents. We reported two synthesis routes, one will lead to nanocomposites and other to Core/Shell nanostructures. Our mechanism consists of two stages: core nanoparticle formation and shell growth. Gibbs free energy of hydration of Zn+2 gives more clarity for shell growth over core rather than ion displacement from core. Colloidal films comprising of nanocrystalline CdS/ZnS were fabricated by the dip coating method. A blue shift in energy level at the nanoscale is demonstrated by optical absorption. Electron microscopy studies with an SEM and TEM show a particle size of 10 nm and diffraction patterns show a crystalline nature. Absence of lattice mismatching is one of the important parameter for device fabrication, which is confirmed by Raman spectroscopy. Overall reduction in optical absorption due to blue shift is expected to result in higher performance, especially in short-circuit currents in CdS/CdTe solar cells.     

A general solution to the Schrödinger–Poisson equation for a charged hard wall: Application to potential profile of an AlN/GaN barrier structure

A general, system-independent, formulation of the parabolic Schrödinger–Poisson equation is presented for a charged hard wall in the limit of complete screening by the ground state. It is solved numerically using iteration and asymptotic boundary conditions. The solution gives a simple relation between the band bending and sheet charge density at an interface. Approximative analytical expressions for the potential profile and wave function are developed based on properties of the exact solution. Specific tests of the validity of the assumptions leading to the general solution are made. The assumption of complete screening by the ground state is found be a limitation; however, the general solution provides a fair approximate account of the potential profile when the bulk is doped. The general solution is further used in a simple model for the potential profile of an AlN/GaN barrier structure. The result compares well with the solution of the full Schrödinger–Poisson equation.     

散射体排列方式对二维磁振子晶体带隙结构的影响

本文采用平面波展开法数值计算了由铁圆柱分别按正方排列、三角排列以及六角排列在氧化铕基底材料中构成的二维磁振子晶体带结构,讨论了散射体的不同排列方式对磁振子晶体带隙结构的影响.计算结果表明,三角排列散射体的磁振子晶体能够获得最大的自旋波带隙结构. 关键词： 磁振子晶体 带隙 排列方式     

Synthesis and characterization of Cu doped ZnS nanoparticles using TOPO and SHMP as capping agents

Undoped and Cu²⁺ doped (0.2-0.8%) ZnS nanoparticles have been synthesized through chemical precipitation method. Tri-n-octylphosphine oxide (TOPO) and sodium hexametaphosphate (SHMP) were used as capping agents. The synthesized nanoparticles have been analyzed using X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectrometer (FT-IR), UV-vis spectrometer, photoluminescence (PL) and thermo gravimetric-differential scanning calorimetry (TG-DTA) analysis. The size of the particles is found to be 4-6 nm range. Photoluminescence spectra were recorded for ZnS:Cu²⁺ under the excitation wavelength of 320 nm. The prepared Cu²⁺-doped sample shows efficient PL emission in 470-525 nm region. The capped ZnS:Cu emission intensity is enhanced than the uncapped particles. The doping ions were identified by electron spin resonance (ESR) spectrometer. The phase changes were observed in different temperatures.     

The role of surface terminations on the band structure and optical properties of silicon nanonets

The ab initio calculations are carried out to investigate the effect of hydrogen, oxygen and nitrogen terminations on the properties of the band edge and the values of the band-gap, as well as the oscillator strength of the silicon nanonets (SiNNs). The oxygen functional groups are found to effectively preserve the direct band-gap nature of the SiNNs, and even change the luminescence properties of the silicon nanowires (SiNWs) to the direct band-gap transition. The appreciable oscillator strength of the first direct transition is obtained for the oxygen terminated nanostructure. The study on the electronic states indicates that the variation of the band edge caused by the surface terminations is attributed to the change of the state compositions. These surface modifications are thought to be useful for silicon band-gap engineering in the area of optoelectronics.     

Characterization of ZnO-SnO2 thin film composites prepared by pulsed laser deposition

Thin films of ZnO-SnO₂ composites have been deposited on Si(1 0 0) and glass substrates at 500 °C by pulsed laser ablation using different composite targets with ZnO amount varying between 1 and 50 wt%. The effect of increasing ZnO-content on electrical, optical and structural properties of the ZnO-SnO₂ films has been investigated. X-ray diffraction analysis indicates that the as-deposited ZnO-SnO₂ films can be both crystalline (for ZnO <1 wt%) and amorphous (for ZnO ≥ 10 wt%) in nature. Atomic force microscopy studies of the as-prepared composite films indicate that the surfaces are fairly smooth with rms roughness varying between 3.07 and 2.04 nm. The average optical transmittance of the as-deposited films in the visible range (400-800 nm), decreases from 90% to 72% for increasing ZnO concentration in the film. The band gap energy (E_g) seems to depend on the amount of ZnO addition, with the maximum obtained at 1 wt% ZnO. Assuming that the interband electron transition is direct, the optical band gap has been found to be in the range 3.24-3.69 eV for as-deposited composite films. The lowest electrical resistivity of 7.6 × 10⁻³ Ω cm has been achieved with the 25 wt% ZnO composite film deposited at 500 °C. The photoluminescence spectrum of the composite films shows a decrease in PL intensity with increasing ZnO concentration.     

Electrochemical synthesis of ZnO nanoflowers and nanosheets on porous Si as photoelectric materials

Well-aligned ZnO nanoflowers and nanosheets were synthesized on porous Si (PS) at different applied potentials by electrodeposition approach. The deposits were grown using the optimized program and were characterized by means of cyclic voltammetry (CV), amperometry I-t (I-t), open-circuit potentiometry. X-ray diffraction (XRD) analysis proved a strong preferential orientation (1 0 0) on PS. Scanning electronic microscopy (SEM) observation showed the deposits consist of nanoflowers with uniform grain size of about 100 nm in diameter and nanosheets, which may have potential applications in nanodevices and nanotechnologies. Thus, ZnO grown on PS can be used as photoelectric materials due to its larger photoelectric effect compared to Si wafer according to open-circuit potential (OCP) study. Optical band gap measurements were made on samples using UV-visible spectrophotometer thus giving a band gap of 3.35 eV.     

Strain-induced metal to semiconductor transition in ultra-small diameter single-wall carbon nanotubes

Under a large tensile strain near fracture limit, the band structures of single-wall carbon nanotubes (SWCNTs) with diameter less than 0.5 nm begin a metal to semiconductor transition and these ultra-small SWCNTs can normally maintain their metallicities. The band gap behavior of these SWCNTs intrinsically originates from the long axial direct bond lengths and the severe curvature. The gap opening comes mainly from the transfer of pπ electrons. And the localized π and σ states can result in a lower electrical conductivity. This band gap behavior suggests that it has potential to find applications in nano-electromechanical system.     

一类单调算子的新不动点定理

利用单调迭代法、数学归纳法以及序差距的性质,在半序Banach空间中探究不具有紧性、连续性以及任何凹凸性的单调算子不动点存在以及惟一性问题,得出其新不动点定理,这些结果对相关结论进行了推广,使其适用范围更广,同时将该结论应用于求解Volterra型积分方程组问题中.     

The electronic structure of pentacene revisited

Recently, there have been reports of the valence band photoemission of pentacene films grown on various substrates with particular emphasis on the highest occupied molecular orbital (HOMO) and its dispersion. In various works, evidence for HOMO band dispersion as high as 0.5 eV, even for polycrystalline films, has been presented. In apparent contradiction to these results, we have previously reported a band dispersion of only 50 meV, measured on a well characterised film with a single polymorph and single crystalline orientation, 5A(0 2 2). Here, we first present the two-dimensional momentum distribution of the HOMO of a 5A(0 2 2) film. Then the development of the valence band spectra for films grown at room temperature and low temperature are compared, and we show that morphological aspects can lead to the apparent observation of high HOMO dispersion. Finally, with the aid of the two-dimensional momentum distribution of the HOMO, we show that a reasonably large dispersion (0.25 eV) does indeed exist in 5A(0 2 2).