A study on photo-generated charges property in highly ordered TiO2 nanotube arrays

In this study TiO₂ nanotube arrays were fabricated by potentiostatic anodization of titanium sheet. The X-ray diffraction (XRD) pattern and field emission scanning electron microscopy (FE-SEM) image indicated the TiO₂ nanotube arrays were of pure anatase form and highly ordered. The properties of the photo-generated charges in the nanotube arrays were investigated by transient photovoltage (TPV) technique and surface photovoltage (SPV) technique based on lock-in amplifier with dc bias, in comparison with the commercial powder derived film. The separation processes of the photo-induced charges in the system of TiO₂ nanotubes on Ti have been demonstrated to be correlated with the incident light intensity, surface trapping states, and the interfacial electric field between Ti and TiO₂. The results also show that the highly ordered nanotube film could generate much stronger SPV responses under external electric field than the commercial powder derived film.     

Deformation Luminescence in gamma-irradiated KCl single crystals

Abstract

Deformation Luminescence(DL) is studied on KCl crystal colored by γ-irradiation. The spectra analysis of DL and thermoluminescence reveals that F-center plays a role as electron donor through the interaction with moving dislocation and V₂-center is a probable luminescence center. A theory is presented for understanding the deformation rate and temperature dependences of DL intensity.     

Surface photovoltage,band-bending and surface states on a-Si : H

We have measured the surface photovoltage (SPV) of intrinsic (i.e., undoped) and phosphorus-doped amorphous Si : H between ?168 and 25°C in the spectral range from 0.5 to 2.5 eV. The a-Si : H was grown in a silane glow discharge. Vibrating Kelvin probe techniques were used for the SPV measurements; Auger spectroscopy was used for monitoring surface cleanliness and chemistry. At all temperatures and for both materials, (1) the SPV was invariably negative, (2) there was no correlation between the spectral, thermal and response-time properties of the SPV and the bulk photoconductivity, and (3) surface treatments such as sputtering and oxygen physisorption strongly affected the SPV but not the photoconductivity. These facts indicated that the SPV was due to the emptying of surface-states via surface transitions, and corresponded to the flattening of bands which, when unilluminated, were bent upwards. Intrinsic material showed a maximum SPV of about 0.2 V. The SPV was characterized at ?168°C by strong electronic isolation between surface-states and valence band (i.e., once light was removed, there was no surface-state refilling or decay of the SPV), slow rise times (～min), saturation at photon fluxes of about 10¹¹/cm² · s, and a SPV spectral threshold occurring at 0.7 eV. At 25°C, all SPV responses were much faster (<0.5 s) and the optical threshold was 0.9 eV. The thermal activation energies associated with the SPV were 0.11 eV for surface-state emptying and 0.22 eV for surface-state refilling. For P-doped material the maximum SPV at ?168°C was 0.3 V and its properties indicated less electronic isolation between surface-states and valence band. There was no SPV at room temperature. Our results are discussed in terms of an energy level scheme which contains a distribution of filled surface states isolated from both conduction and valence bands. The surface-state density is estimated to be about (1?2) × 10¹¹/ cm², a relatively low value which is consistent with the observed lack of Fermi level pinning. In both materials there is a very fast component of the SPV which suggests the presence of additional surface states below the valence band edge.     

Surface photovoltage and photoelectron spectra of GaP

Surface photovoltage (SPV) and photoelectron spectra (PES) of crystalline n-GaP wafers have been studied at 300 K. The magnitude of the surface potential (V_s) decreases in the presence of photons with energy more than the band gap, however the magnitude of V_s increases in the presence of photons with sub band gap energy. The SPV spectrum is helpful in understanding the rigid shift of PES spectra of n-GaP wafers towards higher kinetic energy in the presence of secondary white light from a tungsten lamp.     

Effects of γ rays irradiation on structure and property of TiO2 thin films

TiO₂ thin films were deposited on a glass substrate by the radio frequency magnetron sputtering method, and annealed for 2 h at temperatures of 550°C. Then, ⁶⁰Co γ rays with different doses were used to irradiate the resulting TiO₂ thin films. The surface features of films before and after irradiation were observed by scanning electron microscope (SEM). Simultaneously, the crystal structure and optical properties of films before and after irradiation were studied by X-ray diffraction (XRD), UV–VIS transmission spectrum and Photoluminescence (PL) spectrum, respectively. The SEM analysis shows that the film is smooth with tiny particles on the film surface, and non-crystallization trend was clear after irradiated with γ rays. The XRD results indicated that the structure of the film at the room temperature mainly exists in the form of amorphous and mixed crystal at a sputtering power of 200 W, and non-crystallinity was more obvious after irradiation. Obvious difference can be found for the transmissibility of the irradiated and pre irradiation TiO₂ films by the UV-VIS spectra. The color becomes light yellow, and the new absorption edge also appeared at about 430 nm. PL spectra and photocatalysis experiments indicate that the photocatalysis degradation rate of the TiO₂ films on methylthionine chloride solution irradiated with the maximum dose can be increased to 90%.     

Dependence of the phonon spectrum of a metal on electron temperature in a nonequilibrium electron-phonon system

The dependence of the phonon spectrum of a crystal and the associated thermodynamic functions on electron temperature in the absence of equilibrium between the electrons and the lattice is investigated. The treatment is performed within the Thomas-Fermi approximation for a body-centered cubic crystal at high pressures. Zh. éksp. Teor. Fiz. 115, 231–242 (January 1999)     

An in vitro thermal analysis during different light-activated hydrogen peroxide bleaching

This study measured the critical temperature reaching time and also the variation of temperature in the surface of the cervical region and within the pulp chamber of human teeth submitted to dental bleaching using 35% hydrogen peroxide gel activated by three different light sources. The samples were randomly divided into 3 groups (n = 15), according to the catalyst light source: Halogen Light (HL), High Intensity Diode Laser (DL), and Light Emmited Diode (LED). The results of temperature variation were submitted to the analysis of variance and Tukey test with p < 0.05. The temperature increase (mean value and standard deviation) inside the pulp chamber for the HL group was 6.8 ± 2.8°C; for the DL group was 15.3 ± 8.8°C; and for the LED group was 1.9 ± 1.0°C for. The temperature variation (mean value and standard deviation) on the tooth surface, for the group irradiated with HL was 9.1 ± 2.2°C; for the group irradiated with DL were 25.7 ± 18.9°C; and for the group irradiated with LED were 2.6 ± 1.4°C. The mean temperature increase values were significantly higher for the group irradiated with DL when compared with groups irradiated with HL and LED (p < 0.05). When applying the inferior limits of the interval of confidence of 95%, an application time of 38.7 s was found for HL group, and 4.4 s for DL group. The LED group did not achieve the critical temperatures for pulp or the periodontal, even when irradiated for 360 s. The HL and DL light sources may be used for dental bleaching for a short period of time. The LED source did not heat the target tissues significantly within the parameters used in this study.     

Localization and channeling of light in defect modes of two-dimensional photonic crystals

The spatial distribution of the electromagnetic field in a two-dimensional photonic crystal with a lattice defect is investigated. It is shown that in such a structure the field can be localized in a region smaller than one wavelength in size. The dependence of the spectrum of defect modes on the parameters of a two-dimensional photonic crystal is investigated. The light field at the exit of the photonic crystal possesses properties of a nonradiative mode, making it possible to achieve spatial resolution in the near-field much higher than the radiation wavelength. The possibilities of using this phenomenon in optical near-field microscopy to produce optical memory devices and to increase the efficiency of nonlinear optical interactions are discussed. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 5, 323–328 (10 September 1999)     

Morphology and optical investigations of ZnO pyramids and nanoflakes for optoelectronic applications

Zinc-oxide (ZnO) pyramidal and nanoflakes were grown by electrochemical deposition of Zn(NO₃)₂·6H₂O on n-type Si substrate with different crystallographic orientations and on indium tin oxide (ITO)-coated glass. Various morphological shapes of deposited ZnO nanostructures were observed, which were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The bulk modulus was calculated to determine the material stiffness. Two peaks were observed at room temperature photoluminescence spectrum, i.e., a near-band-edge (NBE) emission in the UV region and a broad deep-level emission (DLE) in the green emission region. The optical properties were calculated to confirm the specific models validity of ZnO nanostructures for optoelectronics. The measured and calculated values show good agreement with other data.     

Photoreflection and photoconduction spectra of CdS crystals: Excitons in the electric fields of surface states

The exciton photoreflection spectra of CdS crystals are studied. It is found that the form of the exciton photoreflection spectrum is determined by a Stark shift of the exciton energy in the electric field of surface states. The dependences of the exciton photoreflection spectrum on temperature on the intensity and wavelength of the modulating radiation, and on the processes by which the photoreflection signal relaxes is determined. An energy scheme is proposed for the surface states which explains the observed effects of photoinduced changes in the surface field. A correlation is established between the exciton photoreflection spectrum and the form of the fine structure in the photoconductivity. Fiz. Tverd. Tela (St. Petersburg) 40, 875–876 (May 1998)     

Acousto-optical effect in the luminescence excitation spectrum of HgI2

A study of the photoluminescence excitation spectrum in a crystal of mercury diiodide is reported. Each of the two luminescence bands peaking at 543 and 572–575 nm investigated was found to have its own excitation spectrum. The excitation spectrum of the 575-nm line in the long-wavelength doublet band has been observed to be sensitive to strong ultrasound vibrations and preliminary irradiation of the sample by 590-nm light. This line is associated with radiative recombination of photocarriers at intrinsic point defects (vacancies or iodine and mercury interstitials) located close to dislocations. The binding energy of the photosensitive center to a dislocation was estimated from the change in the excitation spectrum. Fiz. Tverd. Tela (St. Petersburg) 41, 1965–1968 (November 1999)     

Mechanoluminescence and submicrorelief of a copper surface

Copper plates are irradiated with pulsed laser light on one side, and the opposite side is investigated with a scanning tunneling microscope. It is found that the submicrorelief of the back surface changes after irradiation. During irradiation it emits a light pulse. It is established that a relation exists between the luminescence intensity and the magnitude of the change in the surface submicrorelief. Zh. Tekh. Fiz. 69, 102–104 (December 1999)     

Photoinduced light absorption by C60 films in the 0.08–4.0-eV spectral range

Spectra of photoinduced light absorption in C₆₀ films at high and low excitations in the temperature range between 15 and 300 K have been measured. In addition to the well-known explanation of photoinduced absorption in terms of optical transitions in the system of photogenerated singlet excitons, triplet excitons, and polarons, changes in the absorption spectrum of the fullerite ground state must be considered. We suggest taking into account the effect of crystal field in explaining the features of the photoinduced absorption spectrum. A feature similar to the inverted luminescence spectrum and ascribed to optical excitation of singlet excitons, which is partially allowed owing to intermolecular interaction, has been detected in spectrum of photoinduced absorption. Zh. éksp. Teor. Fiz. 112, 246–256 (July 1997)     

核壳结构硒化镉/硫化镉/巯基乙酸量子点载流子输运特性

采用水溶液法合成了巯基乙酸(TGA) 包覆的CdSe 量子点. 通过X 射线粉末衍射和高分辨透射显微镜检测结果证实, 合成得到闪锌矿结构CdSe 量子点. 能谱图和傅里叶变换红外光谱图结果说明, 在核CdSe 纳米粒子表面与配体TGA 之间有CdS 壳层结构形成. 利用样品表面光电压(SPV) 谱, 指认CdSe 量子点精细能带结构以及各自对应的激发态特征: 475 nm (2.61 eV) 和400 nm (3.1 eV) 两个波长处的SPV 响应峰分别与CdSe 核和CdS 壳层带-带隙跃迁相对应; 370 nm (3.35 eV) 附近SPV 响应峰可能与TGA 中羰基与巯基或羧基之间发生的n →π^* 跃迁有关. 场诱导表面光电压谱结果证实, 合成的CdSe 量子点具有明显的N 型表面光伏特性, 而上述n→π^* 跃迁则具有P 型表面光伏特性. 荧光光谱谱线均匀增宽以及SPV 响应峰位蓝移, 说明样品具有明显的量子尺寸效应. 结合不同pH 值下合成的CdSe 量子点的SPV 谱和表面光声谱发现, SPV 响应强度与表面光声光谱信号强度变化趋势恰好相反. 上述样品表面光伏效应表明, CdSe 量子点表面和相界面处的精细电子结构以及光生载流子的输运特性均与量子点的尺寸大小存在某种内在联系. 关键词： 硒化镉量子点 光生载流子 表面光电压谱 表面光声光谱     

Annealing effects on the microstructure of sputtered gold layers on oxidized silicon investigated by scanning electron microscopy and scanning probe microscopy

The structure of Au layers deposited by sputtering on oxidized p-type Si(100) substrates is investigated by a combination of scanning electron microscopy and scanning probe microscopy. The effect of the temperature on the grain structure of the layers has been determined, revealing that an annealing temperature of 300° C results in a larger grain size and smoother surfaces but generates some cracks in the film surface. At an annealing temperature of 500° C, further grain growth is observed, but a high density of cracks and voids also results while there is little enhancement regarding the smoothness of the grain surfaces.     

A correction method for thermally lowered fission track ages

Abstract

Onset of electric current has been detected on illuminating the near–contact regions of RbAg₄I₅ crystals. The spectral and temperature characteristics of this current have been studied. A method is proposed for studying the spectrum of electronic states in the α-phase of the RbAg₄I₅ crystal. The dependence of the magnitude of the current and its spectral characteristics on the particular region of the RbAg₄I₅ crystal which is exposed to the light was used for the analysis of the variations in the distribution of defects along the length of the sample and the processes which occur near the electrode-(solid electrolyte) junctions.     

ZnSe/ZnS/L-Cys核壳结构量子点光声与表面光伏特性

ZnSe量子点光电子特性的研究对于其微观电子结构探测和应用领域的扩展具有重要的意义.本文结合表面光伏与光声技术以及激光Raman研究了不同回流温度下制备L-半胱氨酸(L-Cys)为配体核壳结构ZnSe量子点的微结构和光声与表面光伏特性.结果发现,具有n-型光伏特性的ZnSe量子点在近紫外到可见光范围内展示出优良的表面光伏性质.尤其在波长为350-550 nm范围内光子能量绝大部分用于产生表面光伏效应,而不是用于无辐射跃迁导致的晶格热振动,同时证实了光声与表面光伏效应之间的能量互补关系.实验指认ZnSe量子点在300-350 nm短波区域出现的光声信号和在1120,1340和1455 cm~(-1)高频区域出现的Raman峰与配体L-Cys的多声子振动模式密切相关.实验结果表明,随着回流温度的降低,ZnSe量子点的平均粒径有减小趋势,这在改善样品的表面效应和小尺寸效应的同时,有利于提高核壳结构ZnSe量子点的光伏转换效率.     

Exciton absorption and optical-gain bands in the presence of laser radiation

The absorption and gain bands of a weak probe signal in the presence of the Bose-Einstein condensation of excitons, which appears under nonequilibrium conditions in a field of coherent laser radiation, are considered. It is shown that the absorption of light is caused by a quantum transition from the ground state of the crystal to the quasiexciton branch of the spectrum. Amplification of the signal occurs as a result of transitions from the quasiexciton branch of the spectrum to the ground state of the crystal. Fiz. Tverd. Tela (St. Petersburg) 40, 924–927 (May 1998)     

Anomalous electron emission spectra and polarization phenomena in a lead magnesium niobate single crystal

The time dependences of the intensity and the energy positions of fine-structure of the spectra of anomalous electron emission from a polarized lead magnesium niobate (PMN) single crystal irradiated with soft x-rays are investigated experimentally. It is shown that the relaxation time of the electret charge can be determined from the graphs of these dependences, and the depth distribution of the potential in the surface layer of the sample can be determined from the profile of the anomalous electron emission spectrum. Estimates of the fields in the surface layers of a polarized PMN single crystal, obtained from the characteristics of the anomalous electron emission spectra, agree with data obtained by electrophysical research techniques. Fiz. Tverd. Tela (St. Petersburg) 39, 1446–1451 (August 1997)     

Wet-chemical treatment and electronic interface properties of silicon solar cell substrates

On textured n-type silicon substrates for solar cell manufacturing, the relation between light trapping behavior, structural imperfections, energetic distribution of interface state densities and interface recombination losses were investigated by applying surface sensitive techniques. The field-modulated surface photovoltage (SPV), in-situ photoluminescence (PL) measurements, total hemispherical UV-NIR-reflectance measurements and electron microscopy (SEM) were employed to yield detailed information on the influence of wet-chemical treatments on preparation induced micro-roughness and electronic properties of polished and textured silicon substrates. It was shown that isotropic as well as anisotropic etching of light trapping structures result in high surface micro-roughness and density of interface states. Removing damaged surface layers in the nm range by wet-chemical treatments, the density of these states and the related interface recombination loss can be reduced. In-situ PL measurements were applied to optimise HF-treatment times aimed at undamaged, oxide-free and hydrogen-terminated substrate surfaces as starting material for subsequent solar cell preparations.