Simulation of porous silicon structure formation

A model of porous silicon structure formation is considered. A computer implementation of this model is carried out, and a compact model of the silicon framework is created. A logical-probabilistic approach to simulating the porous silicon structure is described     

The effect of multilayer quantum well structure on the characteristics of OLEDs

Organic light-emitting diodes (OLEDs) with multilayer quantum well (MQW) structure were fabricated, which consisted of alternate organic materials 2-(4-biphenylyl)-5(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) and tris(8-hydroxyquinoline) aluminum (Alq₃). PBD is used as potential barrier layer, Alq₃ used as potential well layer and emitting layer. Compared with double-layer structure, the luminescent characteristics of the devices with MQW structure were investigated. MQW structures conduce to energy transfer between wells and barriers, which is attributed to good overlap and the decrease of the distance between layers. The MQW structures make electrons and holes distribute in different wells and then increase the number of the formation of excitons to further enhance their recombination efficiency. Hence, such device achieves the maximum brightness and efficiency of 3630 cd/m² and 3.28 cd/A, respectively.     

A distributed Bragg reflector porous silicon layer for optical interferometric sensing of organic vapor

In this study, we successfully demonstrated the rapid, sensitive, and reversible sensing of organic vapor using a distributed Bragg reflector (DBR) porous silicon (PS) layer. We fabricated the DBR PS layer on a p⁺-type silicon substrate and investigated its reflectance spectra before, during, and after exposure to the different concentrations of various organic vapors. When the DBR PS layer sample was exposed to methanol, acetone, ethanol, and isopropanol vapors, the maximum reflectance peak promptly shifted toward longer wavelengths by about 4.5, 23.2, 26.0, and 38.2 nm, respectively. We determined that the red-shift in the reflectance spectrum could be attributed to the changes in the refractive index induced by the capillary condensation of the organic vapor within the pores of the DBR PS layer. The DBR PS layer showed excellent sensing ability under the different concentrations and types of organic vapors. In addition, a slight hysteresis of the red-shift was observed during repeated exposure to organic vapors at different concentrations. After removing the organic vapors, the reflectance spectrum promptly returned to its original state.     

A hybrid chemiresistive sensor system for the detection of organic vapors

A five node sensor array, consisting of three films of gold nanoparticles functionalized with p-terphenylthiol, dodecanethiol and mercapto-(triethylene glycol) methyl ether, and films of poly(3-hexylthiophene) and polypyrrole, was integrated into a portable, microprocessor-based system. The system was evaluated for the detection of chloroform, diisopropyl methylphosphonate (DIMP), ethanol, hexane, methanol, and toluene vapors. Direct comparison of the five sensor films with respect to sensitivity, response time and recovery time was made by measurement of the resistance changes upon simultaneous exposure to each analyte. In general, the sensor films responded, with greatest sensitivity, to organic analyte molecules with similar chemical functionality (e.g., polarity). For example, the dodecanethiol-functionalized gold nanoparticle film sensor excelled at detecting hexane, while the mercapto-(triethylene glycol) methyl ether-functionalized nanoparticle film exhibited superb detection of ethanol and chloroform. Although the poly(3-hexylthiophene) film was very sensitive to polar analytes, including DIMP, in many cases it suffered from relatively long recovery times. Following training of the sensor system, successful differentiation and detection of the analytes were realized using a relatively simple algorithm based on “minimization of the squares of differences” method. The ability of the system to optimally differentiate these analytes is considered within the context of principal component analysis, and the effects of long-term sensor drift are discussed.     

硅纳米悬臂梁传感结构压阻特性的试验研究

为了研究拉伸和大形变弯曲共存状态下硅纳米悬臂梁传感结构的压阻特性,采用CMOS工艺制作了硅纳米悬臂梁传感测试结构,结合原子力显微镜和半导体参数测试仪对其电学参数进行了测量,其位移灵敏度高达1.58216×10-4/nm。在电阻相对变化率实验测量和ANSYS有限元平均应力仿真的基础之上,进而提出了一个非线性压阻模型来提取大弯曲硅纳米悬臂梁的一阶和二阶压阻系数。研究结果表明:其一阶压阻系数约为体硅的5倍,该巨压阻效应为利用硅纳米压阻传感结构来实现超高灵敏度的纳米压力传感器提供了可能的途径。研究结果同时也揭示了要获得高的灵敏度和好的可靠性,硅纳米悬臂梁的长度设计需要折衷考虑。     

Comparative analysis of sensor responses of thin conducting polymer films to organic solvent vapors

A comparative analysis of responses of thin films of various conducting polymers (polyaniline, polypyrrole and poly-3-methylthiophene) to the vapors of polar as well as low- and nonpolar organic solvents was performed. Conductivity measurements, infra-red and electron paramagnetic resonance spectroscopy were used to characterize responses of the polymer films doped with different dopants. Main physical and chemical factors which define the magnitude of the response to the studied analytes were determined. It was shown that the differences in response of conducting polymer films are conditioned by the influence of chemical structure of the polymer and its dopant, and also by different nature of their doping.     

Optical parameters of calix[4]arene films and their response to volatile organic vapors

The Langmuir-Blodgett (LB) technique was employed to produce thin LB films using an amphiphilic calix-4-resorcinarene onto different substrates such as quartz, gold coated glass and quartz crystals. The characteristics of the calix LB films are assessed by UV-visible, quartz crystal microbalance (QCM) and surface plasmon resonance (SPR) measurements. UV-vis and QCM measurements indicated that this material deposited very well onto the solid substrates with a transfer ratio of >0.95. Using SPR data, the thickness and refractive index of this LB film are determined to be 1.14 nm/deposited layer and 1.6 respectively. The sensing application of calixarene LB films towards volatile organic vapors such as chloroform, benzene, toluene and ethanol vapors is studied by the SPR technique. The response of this LB film to saturated chloroform vapor is much larger than for the other vapors. The response is fast and fully recoverable. It can be proposed that this sensing material deposited onto gold coated glass substrates has a good sensitivity and selectivity for chloroform vapor. This material may also find potential applications in the development of room temperature organic vapor sensing devices.     

Numerical calculation of the reflectance of sub-wavelength structures on silicon nitride for solar cell application

In this study, we calculate the spectral reflectivity of pyramid-shaped silicon nitride (Si₃N₄) sub-wavelength structures (SWS). A multilayer rigorous coupled-wave approach is advanced to investigate the reflection properties of Si₃N₄ SWS. We examine the simulation results for single layer antireflection (SLAR) and double layer antireflection (DLAR) coatings with SWS on Si₃N₄ surface, taking into account effective reflectivity over a range of wavelengths and solar efficiency. The results of our study show that a lowest effective reflectivity of 1.77% can be obtained for the examined Si₃N₄ SWS with the height of etched part of Si₃N₄ and the thickness of non-etched layer of 150 and 70 nm, respectively, which is less than the results of an optimized 80 nm Si₃N₄ SLAR (∼5.41%) and of an optimized DLAR with 80 nm Si₃N₄ and 100 nm magnesium fluoride (∼5.39%). 1% cell efficiency increase is observed for the optimized Si solar cell with Si₃N₄ SWS, compared with the cell with single layer Si₃N₄ antireflection coatings (ARCs); furthermore, compared with DLAR coated solar cell, the increase is about 0.71%. The improvement on the cell efficiency is mainly due to lower reflectance of Si₃N₄ SWS over a wavelength region from 400 to 600 nm that leads to lower short circuit current.     

Calculation of photosensitivity of porous silicon with the cylindrical geometry of pores

The paper theoretically investigates a model of the photosensitivity of porous silicon with cylindrical pores in the condition of homogeneous generation of photocarriers. Dependences of the photoconductivity of a porous semiconductor on the velocity of recombination of nonequilibrium carriers at the surfaces of pores, radius of pores and average distance between them are analyzed. At large velocities of surface recombination the photoconductivity of porous semiconductor is shown to linearly decrease with increasing the pore’s radius. The article is published in the original.     

The structure of electronic states in amorphous silicon

We illustrate the structure and dynamics of electron states in amorphous Si. The nature of the states near the gap at zero temperature is discussed and especially the way the structure of the states changes for energies ranging from midgap into either band tail (Anderson transition). We then study the effect of lattice vibrations on the eigenstates, and find that electronic states near the optical gap can be strongly influenced by thermal modulation of the atomic positions. Finally, we show the structure of generalized Wannier functions for amorphous Si, which are of particular interest for efficient ab initio calculation of electronic properties and forces for first principles dynamic simulation.     

一种基于多孔硅微腔光学特性的气体传感器的研究

当有机物分子吸附到多孔硅表面时,由于有机物分子在多孔硅的孔内的毛细冷凝作用,将引起多孔硅层有效折射率的变化,从而导致多孔硅微腔反射谱吸收峰峰位的变化.本文主要利用Bruggeman介电常数近似理论与传输矩阵的方法,建立了多孔硅微腔的传感模型.使用多孔硅微腔的反射谱实验装置对多孔硅微腔进行了传感实验,结果证明多孔硅微腔传感元件可以实现对有机物蒸汽的检测,且分辨率较高,响应时间和恢复时间短,可重复性好.     

有机溶剂在P型宏多孔硅电化学腐蚀中的应用研究

近年来,多孔硅以其良好的光学、热学、电学以及机械特性使其在微传感器技术领域得到广泛的应用,电化学腐蚀多孔硅的各种方法与原理引起越来越多的关注。研究了P型硅的电化学腐蚀过程中,在腐蚀溶液中使用有机溶剂对多孔硅的制备、速率、成孔机理等方面的影响。研究发现,在分别使用有机溶剂二甲基甲酰胺(DMF)和二甲基亚砜(DMSO)的氢氟酸(HF)腐蚀溶液中,可以制备出孔壁光滑、具有高深宽比的高质量P型宏多孔硅,并发现了一种快速腐蚀P型宏多孔硅的方法,得到高达1900μm/h的腐蚀速率,这有助于提高多孔硅在微传感器批量化生产应用中的效率。在涌流模型基础上,分析了有机溶剂的氧化性和质子(H)提供能力,以及在P型多孔硅快速腐蚀过程中的作用。     

Exploring the relation between spatial structure and wavelength: Implications for sampling reflectance in the field

Recently, the variogram has been used to represent the spatial dependence in remotely sensed data obtained from ground-based, airborne and satellite-borne sensors. The variogram may be used in a variety of techniques such as kriging, cokriging, and conditional simulation and, in particular, optimal sampling design. However, little is known about the relation between spatial variation (summarized by the variogram) and spectral wavelength. Therefore, an investigation was undertaken to determine the relation between spatial dependence and wavelength for two field sites in England: one at Middlebere Heath on the Isle of Purbeck, Dorset, and the other at the Isle of Grain, Kent. At both sites, visible and near-infrared reflectance spectra comprising 252 wavebands were obtained using a Spectron SE-590TM spectroradiometer for 100 observations spaced 1m apart along 100m transects. Variograms were computed for 235 wavebands and these plotted as a three-dimensional surface. The resulting surfaces revealed changes with wavelength not only in the amount, but also in the scales of spatial variation. The spatial variation in all 235 wavelengths was approximately two-dimensional for both case studies. The implication for the design of optimal strategies with which to sample reflectance in the field is that two (and only two) sample spacings are necessary to sample all 235 wavebands.     

Spectral reflectance and leaf internal structure changes of barley plants due to phytoextraction of zinc and cadmium

The focus of our research is to seek spectral signatures that indicate the impact and content of heavy metals in the leaves and canopies of living plants during the process of phytoremediation. Potted plants of barley (Hordeum vulgare) were grown for 5–6 weeks before being subjected to metal treatments of Zn and Cd. Diffuse reflectance spectra (350–2500 nm) of the plant canopies were collected daily using a portable spectroradiometer throughout the treatment period. Foliar structural changes of Zn‐treated plants included a decrease in intercellular space, palisade and epidermal cell size while Cd‐treated plants displayed fewer structural changes in leaf. Spectral analysis revealed that the band ratios at 1110 nm to that at 810 nm might be used as an indicator of the accumulation of certain metals in plant shoots. Normalized Difference Vegetation Index (NDVI) and leaf‐water‐content indices examined as part of our spectral analysis were not able to distinguish plants treated with different metals. Our ratio index R₁₁₁₀/R_810, on the other hand, correlates closely with the magnitude of leaf structural changes. This study suggests that the infrared reflectance spectrum (800–1300 nm) of plant canopy might provide a non‐intrusive monitoring method for the physiological status of plants grown on heavy metal contaminated soil.     

Optimization of the structure of multifunctional information systems according to the criterion of a required value of the efficiency ratio

A model of the reliability of multifunctional information systems is proposed. Based on this model an integral estimation of the reliability of such systems is obtained, expressed in the form of an efficiency ratio. An algorithm for optimization of a multifunctional information system according to the criterion of a required value of the efficiency ratio of the system is presented. The algorithm is illustrated by an example.     

A BASIC microcomputer program to calculate the secondary structure of proteins from their circular dichroism spectrum

A BASIC program (CDPROT) has been developed to calculate the secondary structure of proteins from their far UV circular dichroism spectrum. This implementation can use different reference spectra, calculated either from model polypeptides or proteins of known tertiary structure. Apart from obtaining the alpha-helical, beta-structure, beta-turns or random percentages which would generate the spectrum of best fit with respect to the experimental measures, CDPROT represents on screen both theoretical and experimental spectra indicating the root-mean-square error. The provision of additional reference spectra by the user is also considered, and another program (STOREREF) performs the editing in an adequate format for CDPROT.     

Atmospheric optical depth and water vapour effects on the angular characteristics of surface reflectance in NOAA AVHRR

The effects of atmospheric optical depth and water vapour content on the bidirectional surface reflectance in channel 1 (visible) and channel 2 (near-infrared) of NOAA AVHRR have been analysed using a coupled surface atmosphere reflectance model. Two different cases of surface: (i) bare soil, and (ii) vegetation cover have been considered. In the case of bare soil, both the amplitude and angular distribution of the bidirectional reflectance of the surface are modified at satellite altitude due to scattering caused by atmospheric molecules and aerosols in the two channels and thereby, the directional properties of the surface are smoothed. Whereas, in the case of lawn, in channel 1, the angular variation of surface reflectance is enlarged together with a large augmentation in reflectance amplitude, and in channel 2, a small reduction in amplitude as well as a variation in angular distribution of reflectances are caused due to scattering particularly over large viewing angles and thereby, the directional variations are smoothed. In channel 1, atmospheric scattering reduces the contrast between the soil and vegetation and is very much significant for medium to high aerosol loadings. Atmospheric water vapour reduces the amplitude of the surface bidirectional reflectance without introducing any significant changes in angular distribution of the surface reflectance for both bare soil and vegetation canopy in channel 2.     

Flux-based methods of characteristics for coupled transport equations in porous media

Mathematical models of transport of radioactive contaminants in flowing groundwater involve large systems of coupled advection dominated transport equations. High-resolution explicit finite volume methods, if applied to advective part of model and combined with appropriate numerical methods for diffusion-dispersion-reaction part, can offer precise and monotone numerical solutions, but they require small time steps. This paper describes Flux-Based Methods Of Characteristics that are extension of explicit finite volume methods, that have no restriction on time steps and that produce numerical solutions with valid discrete minimum and maximum principle. Such particular method was implemented in software package R³T (Retardation, Reaction, Radionuclides and Transport) and it was used successfully to solve large systems of coupled transport equations with different retardation factors for transport.     

Emim-R离子液体气相结构与光谱的密度泛函理论研究

应用B3LYP/6-31G(d)方法研究了气相状态下一系列复杂阴离子BF_4~-、AlCl_4~-、PF_6~-、NO_3~-、CF_3COO~-、CF_3SO_3~-、HSO_4~-及其与阳离子[Emim~ ]形成的离子液体[Emim]BF_4、[Emim]AlCl_4、[Emim]PF_6、[Emim]CF_3COO、[Emim]CF_3SO_3、[Emim] HSO_4、[Emim]NO_3的结构和红外光谱,分析了阴阳离子的结合方式及其相互作用。[Emim]BF_4和[Emim]PF_6的计算结果与前人的计算结果及实验观测值符合得较好。研究发现含有O原子的阴离子与阳离子的相互作用比其它的阴离子都强,其中NO_3最强。