An ultraviolet selective photodetector based on a nanocrystalline TiO2 photoelectrochemical cell

Solar ultraviolet (UV) radiation causes a range of skin disorders as well as affecting vision and the immune system. It also inhibits development of plants and animals. UV radiation monitoring is used routinely in some locations in order to alert the population to harmful solar radiation levels. There is ongoing research to develop UV-selective-sensors [1–3]. A personal, inexpensive and simple UV-selective-sensor would be desirable to measure UV intensity exposure. A prototype of such a detector has been developed and evaluated in our laboratory. It comprises a sealed two-electrode photoelectrochemical cell (PEC) based on nanocrystalline TiO2. This abundant semiconducting oxide, which is innocuous and very sta-ble, is the subject of intense study at present due to its application in dye sensitized solar cells (DSSC) [4]. Since TiO2 has a wide band gap (EG = 3.0 eV for rutile and EG = 3.2 eV for anatase), it is inher-ently UV-selective, so that UV filters are not required. This further reduces the cost of the proposed photodetector in comparison with conventional silicon detectors. The PEC is a semiconductor–electrolyte device that generates a photovoltage when it is illuminated and a corresponding photocur-rent if the external circuit is closed. The device does not require external bias, and the short circuit current is generally a linear function of illumination intensity. This greatly simplifies the elec-trical circuit needed when using the PEC as a photodetector. DSSC technology, which is based on a PEC containing nanocrystalline TiO2 sensitized with a ruthenium dye, holds out the promise of solar cells that are significantly cheaper than traditional silicon solar cells. The UV-sensor proposed in this paper relies on the cre-ation of electron–hole pairs in the TiO2 by UV radiation, so that it would be even cheaper than a DSSC since no sensitizer dye is needed. Although TiO2 has been reported as a suitable material for UV sensing [3], to the best of our knowledge, the PEC configuration described in the present paper is a new approach. In the present study, a novel double-layer TiO2 structure has been investigated. Fabrication is based on a simple and inexpensive technique for nanostructured TiO2 deposition using microwave-activated chemical bath deposition (MW-CBD) that has been reported recently [5]. The highly transparent TiO2 (anatase) films obtained are densely packed, and they adhere very well to the transparent oxide (TCO) substrate [6]. These compact layers have been studied as contacting layers in double-layer TiO2 structures for DSSC since improvement of electron extraction at the TiO2–TCO interface is expected [7]. Here we compare devices incorporating a single mesoporous nanocrystalline TiO2 structure with devices based on a double structure in which a MW-CBD film is situated between the TCO and the mesoporous nanocrystalline TiO2 layer. Besides improving electron extraction, this film could also help to block recombination of electrons transferred to the TCO with oxidized species in the electrolyte, as has been reported in the case of DSSC for compact TiO2 films obtained by other deposition tech-niques [8,9]. The two types of UV-selective sensors were characterized in detail. The current voltage characteristics, spectral response, inten-sity dependence of short circuit current and response times were measured and analyzed in order to evaluate the potential of sealed mesoporous TiO2-based photoelectrochemical cells (PEC) as low cost personal UV-photodetectors.

Hybrid light-emitting diodes based on ZnO nanowires

ZnO is a wide band-gap semiconductor that has several desirable properties for optoelectronic devices. With its large exciton binding energy of ~60 meV, ZnO is a promising candidate for high stability, room-temperature luminescent and lasing devices [1]. Ultraviolet light-emitting diodes (LEDs) based on ZnO homojunctions had been reported [2,3], while preparing stable p-type ZnO is still a challenge. An alternative way is to use other p-type semiconductors, ether inorganic or organic, to form heterojunctions with the naturally n-type ZnO. The crystal structure of wurtzite ZnO can be described as Zn and O atomic layers alternately stacked along the [0001] direction. Because of the fastest growth rate over the polar (0001) facet, ZnO crystals tend to grow into one-dimensional structures, such as nanowires and nanobelts. Since the first report of ZnO nanobelts in 2001 [4], ZnO nanostructures have been particularly studied for their potential applications in nano-sized devices. Various growth methods have been developed for growing ZnO nanostructures, such as chemical vapor deposition (CVD), Metal-organic CVD (MOCVD), aqueous growth and electrodeposition [5]. Based on the successful synthesis of ZnO nanowires/nanorods, various types of hybrid light-emitting diodes (LEDs) were made. Inorganic p-type semiconductors, such as GaN, Si and SiC, have been used as substrates to grown ZnO nanorods/nanowires for making LEDs. GaN is an ideal material that matches ZnO not only in the crystal structure but also in the energy band levels. However, to prepare Mg-doped p-GaN films via epitaxial growth is still costly. In comparison, the organic semiconductors are inexpensive and have many options to select, for a large variety of p-type polymer or small-molecule semiconductors are now commercially available. The organic semiconductor has the limitation of durability and environmental stability. Many polymer semiconductors are susceptible to damage by humidity or mere exposure to oxygen in the air. Also the carrier mobilities of polymer semiconductors are generally lower than the inorganic semiconductors. However, the combination of polymer semiconductors and ZnO nanostructures opens the way for making flexible LEDs. There are few reports on the hybrid LEDs based on ZnO/polymer heterojunctions, some of them showed the characteristic UV electroluminescence (EL) of ZnO. This chapter reports recent progress of the hybrid LEDs based on ZnO nanowires and other inorganic/organic semiconductors. We provide an overview of the ZnO-nanowire-based hybrid LEDs from the perspectives of the device configuration, growth methods of ZnO nanowires and the selection of p-type semiconductors. Also the device performances and remaining issues are presented.

Photocatalysis on supported gold and silver nanoparticles under ultraviolet and visible light irradiation

Studies of the optical properties and catalytic capabilities of noble metal nanoparticles (NPs), such as gold (Au) and silver (Ag), have formed the basis for the very recent fast expansion of the field of green photocatalysis: photocatalysis utilizing visible and ultraviolet light, a major part of the solar spectrum. The reason for this growth is the recognition that the localised surface plasmon resonance (LSPR) effect of Au NPs and Ag NPs can couple the light flux to the conduction electrons of metal NPs, and the excited electrons and enhanced electric fields in close proximity to the NPs can contribute to converting the solar energy to chemical energy by photon-driven photocatalytic reactions. Previously the LSPR effect of noble metal NPs was utilized almost exclusively to improve the performance of semiconductor photocatalysts (for example, TiO2 and Ag halides), but recently, a conceptual breakthrough was made: studies on light driven reactions catalysed by NPs of Au or Ag on photocatalytically inactive supports (insulating solids with a very wide band gap) have demonstrated that these materials are a class of efficient photocatalysts working by mechanisms distinct from those of semiconducting photocatalysts. There are several reasons for the significant photocatalytic activity of Au and Ag NPs. (1) The conduction electrons of the particles gain the irradiation energy, resulting in high energy electrons at the NP surface which is desirable for activating molecules on the particles for chemical reactions. (2) In such a photocatalysis system, both light harvesting and the catalysing reaction take place on the nanoparticle, and so charge transfer between the NPs and support is not a prerequisite. (3) The density of the conduction electrons at the NP surface is much higher than that at the surface of any semiconductor, and these electrons can drive the reactions on the catalysts. (4) The metal NPs have much better affinity than semiconductors to many reactants, especially organic molecules. Recent progress in photocatalysis using Au and Ag NPs on insulator supports is reviewed. We focus on the mechanism differences between insulator and semiconductor-supported Au and Ag NPs when applied in photocatalytic processes, and the influence of important factors, light intensity and wavelength, in particular estimations of light irradiation contribution, by calculating the apparent activation energies of photo reactions and thermal reactions.

Zinc oxide quantum dot nanostructures

Zinc oxide (ZnO) is one of the most intensely studied wide band gap semiconductors due to its many desirable properties. This project established new techniques for investigating the hydrodynamic properties of ZnO nanoparticles, their assembly into useful photonic structures, and their multiphoton absorption coefficients for excitation with visible or infrared light rather than ultraviolet light. The methods developed are also applicable to a wide range of nanoparticle samples.

Spectroscopic characterization and solubility investigation on the effects of As(V) on mineral structure tooeleite (Fe6(AsO3)4SO4(OH)4·H2O)

Tooeleite is an unique ferric arsenite sulfate mineral, which has the potential significance of directly fixing As(III) as mineral trap. The tooeleite and various precipitates were hydrothermally synthesized under the different of initial As(III)/As(V) molar ratios and characterized by XRD, FTIR, XPS and SEM. The crystallinity of tooeleite decreases with the amount of As(V). The precipitate is free of any crystalline tooeleite at the level of that XRD could detect when the ratio of As(III)/As(V) of 7:3 and more. The characteristic bands of tooeleite are observed in 772, 340, 696 and 304 cm−1, which are assigned to the ν1, ν2, ν3 and ν4 vibrations of AsO33−. These intensities of bands gradually decreases with the presence of As(V) and its increasing. An obviously wide band is observed in 830 cm−1, which is the ν1 vibration of AsO4. The result of XPS reveals that the binding energies of As3d increase from 44.0 eV to 45.5 eV, which indicates that the amount of As(V) in the precipitates increases. The concentrations of arsenic released of these precipitates are 350–650 mg/L. The stability of tooeleite decreases by comparison when the presence of coexisting As(V) ions.

Electronic structure of La1-xSrxCrO3

LaCrO3 is a wide-band-gap insulator which does not evolve to a metallic state even after hole doping. We report electronic structure of this compound and its Sr substituents investigated by photoemission and inverse photoemission spectroscopies in conjunction with various calculations. The results show that LaCrO 3 is close to the Mott-Hubbard insulating regime with a gap of about 2.8 eV. Analysis of Cr 2p core-level spectrum suggests that the intra-atomic Coulomb interaction strength and the charge-transfer energy to be 5.0 and 5.5 eV, respectively, We also estimate the intra-atomic exchange interaction strength and a crystal-field splitting of about 0.7 and 2.0 eV, respectively. Sr substitution leading to hole doping in this system decreases the charge-excitation gap, but never collapses it to give a metallic behavior. The changes in the occupied as well as unoccupied spectral features are discussed in terms of the formation of local Cr4+ configurations arising from strong electron-phonon interactions.

Reduced complexity two stage vector quantization

We address the issue of complexity for vector quantization (VQ) of wide-band speech LSF (line spectrum frequency) parameters. The recently proposed switched split VQ (SSVQ) method provides better rate-distortion (R/D) performance than the traditional split VQ (SVQ) method, even at the requirement of lower computational complexity. but at the expense of much higher memory. We develop the two stage SVQ (TsSVQ) method, by which we gain both the memory and computational advantages and still retain good R/D performance. The proposed TsSVQ method uses a full dimensional quantizer in its first stage for exploiting all the higher dimensional coding advantages and then, uses an SVQ method for quantizing the residual vector in the second stage so as to reduce the complexity. We also develop a transform domain residual coding method in this two stage architecture such that it further reduces the computational complexity. To design an effective residual codebook in the second stage, variance normalization of Voronoi regions is carried out which leads to the design of two new methods, referred to as normalized two stage SVQ (NTsSVQ) and normalized two stage transform domain SVQ (NTsTrSVQ). These two new methods have complimentary strengths and hence, they are combined in a switched VQ mode which leads to the further improvement in R/D performance, but retaining the low complexity requirement. We evaluate the performances of new methods for wide-band speech LSF parameter quantization and show their advantages over established SVQ and SSVQ methods.

A rational approach to the synthesis of one-dimensional acoustic filters

The velocity ratio algorithm developed from a heuristic study of transfer matrix multiplication has been employed to bring out the relative effects of the elements constituting a linear, one-dimensional acoustic filter, the overall dimensions of which are fixed, and synthesize a suitable straight-through configuration for a low-pass, wide-band, non-dissipative acoustic filter. The potential of the foregoing approach in applications to the rational design of practical acoustic filters such as automotive mufflers is indicated.

Gaussian mixture model based switched split vector quantization of LSF parameters

We address the issue of rate-distortion (R/D) performance optimality of the recently proposed switched split vector quantization (SSVQ) method. The distribution of the source is modeled using Gaussian mixture density and thus, the non-parametric SSVQ is analyzed in a parametric model based framework for achieving optimum R/D performance. Using high rate quantization theory, we derive the optimum bit allocation formulae for the intra-cluster split vector quantizer (SVQ) and the inter-cluster switching. For the wide-band speech line spectrum frequency (LSF) parameter quantization, it is shown that the Gaussian mixture model (GMM) based parametric SSVQ method provides 1 bit/vector advantage over the non-parametric SSVQ method.

Hydrographic observations and model simulation of the Bay of Bengal freshwater plume

Hydrographic observations were taken along two coastal sections and one open ocean section in the Bay of Bengal during the 1999 southwest monsoon, as a part of the Bay of Bengal Monsoon Experiment (BOBMEX). The coastal section in the northwestern Bay of Bengal, which was occupied twice, captured a freshwater plume in its two stages: first when the plume was restricted to the coastal region although separated from the coast, and then when the plume spread offshore. Below the freshwater layer there were indications of an undercurrent. The coastal section in the southern Bay of Bengal was marked by intense coastal upwelling in a 50 km wide band. In regions under the influence of the freshwater plume, the mixed layer was considerably thinner and occasionally led to the formation of a temperature inversion. The mixed layer and isothermal layer were of similar depth for most of the profiles within and outside the freshwater plume and temperature below the mixed layer decreased rapidly till the top of seasonal thermocline. There was no barrier layer even in regions well under the influence of the freshwater plume. The freshwater plume in the open Bay of Bengal does not advect to the south of 16 degrees N during the southwest monsoon. A model of the Indian Ocean, forced by heat, momentum and freshwater fluxes for the year 1999, reproduces the freshwater plume in the Bay of Bengal reasonably well. Model currents as well as the surface circulation calculated as the sum of geostrophic and Ekman drift show a southeastward North Bay Monsoon Current (NBMC) across the Bay, which forms the southern arm of a cyclonic gyre. The NBMC separates the very low salinity waters of the northern Bay from the higher salinities in the south and thus plays an important role in the regulation of near surface stratification. (c) 2007 Elsevier Ltd. All rights reserved.

Hydrographic observations and model simulation of the Bay of Bengal freshwater plume

Hydrographic observations were taken along two coastal sections and one open ocean section in the Bay of Bengal during the 1999 southwest monsoon, as a part of the Bay of Bengal Monsoon Experiment (BOBMEX). The coastal section in the northwestern Bay of Bengal, which was occupied twice, captured a freshwater plume in its two stages: first when the plume was restricted to the coastal region although separated from the coast, and then when the plume spread offshore. Below the freshwater layer there were indications of an undercurrent. The coastal section in the southern Bay of Bengal was marked by intense coastal upwelling in a 50 km wide band. In regions under the influence of the freshwater plume, the mixed layer was considerably thinner and occasionally led to the formation of a temperature inversion. The mixed layer and isothermal layer were of similar depth for most of the profiles within and outside the freshwater plume and temperature below the mixed layer decreased rapidly till the top of seasonal thermocline. There was no barrier layer even in regions well under the influence of the freshwater plume. The freshwater plume in the open Bay of Bengal does not advect to the south of 16 degrees N during the southwest monsoon. A model of the Indian Ocean, forced by heat, momentum and freshwater fluxes for the year 1999, reproduces the freshwater plume in the Bay of Bengal reasonably well. Model currents as well as the surface circulation calculated as the sum of geostrophic and Ekman drift show a southeastward North Bay Monsoon Current (NBMC) across the Bay, which forms the southern arm of a cyclonic gyre. The NBMC separates the very low salinity waters of the northern Bay from the higher salinities in the south and thus plays an important role in the regulation of near surface stratification. (c) 2007 Elsevier Ltd. All rights reserved.

Sequential split vector quantization of LSF parameters using conditional PDF

A better performing product code vector quantization (VQ) method is proposed for coding the line spectrum frequency (LSF) parameters; the method is referred to as sequential split vector quantization (SeSVQ). The split sub-vectors of the full LSF vector are quantized in sequence and thus uses conditional distribution derived from the previous quantized sub-vectors. Unlike the traditional split vector quantization (SVQ) method, SeSVQ exploits the inter sub-vector correlation and thus provides improved rate-distortion performance, but at the expense of higher memory. We investigate the quantization performance of SeSVQ over traditional SVQ and transform domain split VQ (TrSVQ) methods. Compared to SVQ, SeSVQ saves 1 bit and nearly 3 bits, for telephone-band and wide-band speech coding applications respectively.

Ultrasonic spray pyrolysis of CZTS solar cell absorber layers and characterization studies

CZTS (Copper Zinc Tin Sulphide) is a wide band gap quartnery chalcopyrite which has a band gap of about 1.45 eV and an absorption coefficient of 10(4) cm(-1); thus making it an ideal material to be used as an absorber layer in solar cells. Ultrasonic Spray Pyrolysis is a deposition technique, where the solution is atomized ultrasonically, thereby giving a fine mist having a narrow size distribution which can be used for uniform coatings on substrates. An Ultrasonic Spray Pyrolysis equipment was developed and CZTS absorber layers were successfully grown with this technique on soda lime glass substrates using aqueous solutions. Substrate temperatures ranging from 523 K to 723 K were used to deposit the CZTS layers and these films were characterized using SEM, EDAX and XRD. It was observed that the film crystallized in the kesterite structure and the best crystallites were obtained at 613 K. It was observed that the grain size progressively increased with temperature. The optical band gap of the material was obtained as 1.54 eV.

Seebeck Coefficient in Bulk and Quantum Wells of Non-Parabolic Kane-Type Materials

We present a simplified and quantitative analysis of the Seebeck coefficient in degenerate bulk and quantum well materials whose conduction band electrons obey Kane's non-parabolic energy dispersion relation. We use k.p formalism to include the effect of the overlap function due to the band non-parabolicity in the Seebeck coefficient. We also address the key issues and the conditions in which the Seebeck coefficient in quantum wells should exhibit oscillatory dependency with the film thickness under the acoustic phonon and ionized impurity scattering. The effect of screening length in degenerate bulk and quantum wells has also been generalized for the determination of ionization scattering. The well-known expressions of the Seebeck coefficient in non-degenerate wide band gap materials for both bulk and quantum wells has been obtained as a special case and this provides an indirect proof of our generalized theoretical analysis.

Effect of Grain Size on Structural and Magnetic Properties of CuFe2O4 Nanograins Synthesized by Chemical Co-Precipitation

CuFe2O4 nanograins have been prepared by the chemical co-precipitation technique and calcined in the temperature range of 200-1200 degrees C for 3 h. A wide range of grain sizes has been observed in this sintering temperature range, which has been determined to be 4 to 56 nm. Formation of ferrite has also been confirmed by FTIR measurement through the presence of wide band near 600 and 430 cm(-1) for the samples in the as-dried condition. Systematic variation of wave number has been observed with the variation of the calcination temperature. B-H loops exhibit transition from superparamagnetic to ferrimagnetic state above the calcination temperature of 900 degrees C. Coercivity of the samples at lower calcination temperature of 900 degrees C reduces significantly and tends towards zero coercivity, which is suggestive of superparamagnetic transition for the samples sintered below this temperature. Frequency spectrum of the real and imaginary part of complex initial permeability have been measured for the samples calcined at different temperature, which shows wide range of frequency stability. Curie temperature, T-c has been measured from temperature dependence initial permeability at a fixed frequency of 100 kHz. Although there is small variation of T-c with sintering temperature, the reduction of permeability with temperature drastically reduce for lower sintering temperature, which is in conformity with the change of B-H loops with the variation of sintering temperatures.