Photoinduced anomalous current changes in some organometallic materials

Photoinduced changes have been investigated by monitoring electrical measurements in some organometallic materials such as ferrocene derivatives, namely, ferrocenecarboxylic acid [Fc(COOH)], acetylferrocene [Fc(COCH₃)], ferrocenecarbaldehyde [Fc(CHO)], hydroxymethylferrocene [Fc(CH₂OH)]. The investigations were performed in dry nitrogen gas atmosphere at different constant temperatures. Powdery materials were used in sandwich type of cell configuration with constant d.c. bias voltage (27 V) and were exposed to polychromatic light source. Measurements performed in Fc(COCH₃) and Fc(CH₂OH) with a fixed intensity of exciting light source have shown a drastic change (usual to unusual) in the current vs. time profile. The similar measurement conditions in case of Fc(COOH) and Fc(CHO) showed the unusual/anomalous current vs. time profiles in their entire temperature range of study. Anomalous changes that have been observed in the plot of the photocurrent vs. reciprocal of temperature for each material were completely absent in their corresponding dark current behaviour. The photoinduced changes were almost reversible along with some fluctuations/instabilities in equilibrium current measured after switching off the light source in a particular temperature range. The observed anomalous changes in photocurrent with time and cell temperature have been explained. The effects of various functional groups substituted into ferrocene-unit have been discussed.     

Observation of persistent photoconductivity at room temperature in ferrocene-doped poly(methyl methacrylate) thin films containing chloroform molecules

Persistent photoconductivity has been observed at room temperature (300 K) in solution cast ferrocene-doped poly(methyl methacrylate) thin films, prepared from chloroform and the mixture of solvents chloroform and benzene. The maximum photoinduced current, the buildup time and decay time of persistent photocurrent increases with the increasing amount of chloroform present in the film as well as the duration of photoexcitation. The buildup and decay kinetics have been studied. Our results indicate that the photooxidation of ferrocene in the presence of chloroform in the film leads to a photocurrent persistent for a longer period at room temperature.     

Reordering kinetics of ion-disordered Ni Al

A study of the reordering kinetics of ion-irradiated Ni₃Al is presented. The development of long-range order during annealing treatments is measured by quantitative electron diffractometry. According to the temperature dependence of the observed kinetics, the reaction is dominated by non-equilibrium vacancies. As a consequence, the kinetics can be calibrated versus the absolute number of atomic jumps necessary to establish the observed degree of order. In order to analyse the experimental data, Monte Carlo simulations are performed. It is shown that, beside the temperature dependence of the driving force, the ordering efficiency of the vacancy jumps itself is temperature dependent due to different mobilities of the atomic species. Received 6 December 1999 and Received in final form 23 June 2000     

Modelling the thermal pattern of a statue or column exposed to periodic lighting conditions

Summary In the case of monuments, the thermal effects due to periodic lighting conditions in the long run may be very dangerous. In this article the thermal response of a column or a statue cylindrically shaped and subjected to periodic lighting conditions is studied. Two sources are considered: in the first the intensity is time dependent according to a sinusoid (i.e. diffuse solar radiation from a skylight or window or partially closed outdoor environments); in the second according to a square-wave law (i.e. spot-light switched on and off). The model shows some characteristic features as a function of a dimensionless parameter μ, expressed as the ratio between the radius of the cylinder and its penetration depth, which takes into account the thermal properties of the substance and the lighting period. For high values of μ, the amplitude is poorly dependent on azimuth. The shadowed part is practically insensitive to the azimuth variation. For small values of μ the amplitudes of the thermal wave are very large, causing enhanced thermohydrometric cycles on the bodies. The phase lags between the different point depths increase with μ. The phase lags behave differently in three zones of the cylinder: i) the part facing the light, ii) the two transition zones, iii) the shadowed part. For small lighting durations and small μ the switching on or off of artificial light causes sudden and dangerous variations of temperature. Some hours after the switching off, all the surface of a cylinder with small μ reaches a homogeneous temperature. The mathematical analysis of the thermal behaviour of a cylinder shows how a physical simulation can be made, by changing radius, substance and lighting period, separately or together. Paper presented at the IV Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 22–24, 1987, Rome.     

Orbital mechanism of the circular photogalvanic effect in quantum wells

It is shown that the free-carrier (Drude) absorption of circularly polarized radiation in quantum well structures leads to an electric current flow. The photocurrent reverses its direction upon switching the light helicity. A pure orbital mechanism of such a circular photogalvanic effect is proposed that is based on the interference of different pathways contributing to light absorption. Calculation shows that the magnitude of the helicity-dependent photocurrent in n-doped quantum well structures corresponds to recent experimental observations. The text was submitted by the author in English.     

Temperature dependent local Cu-O displacements from underdoped to overdoped La-Sr-Cu-O superconductor

We have studied doping evolution of the temperature dependent local Cu-O displacements in the La_2-x Sr _x CuO₄ superconductor by polarized Cu K-edge extended X-ray absorption fine structure (EXAFS) measurements. While temperature dependent Debye-Waller factor of the Cu-O bonds, measuring the local Cu-O displacements, shows an anomalous increase at low temperature for the underdoped single crystals, we do not find such a dependence for the case of the overdoped system. The results, which are discussed in the light of recent angle resolved photoemission measurements, provide an evidence for some important correlation between the doping dependent electron-lattice interaction, the charge inhomogeneities and the local Cu-O displacements in the copper oxide superconductors.Received: 9 July 2003, Published online: 19 November 2003PACS: 74.72.Dn La-based cuprates - 61.10.Ht X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc. - 74.81.-g Inhomogeneous superconductors and superconducting systems     

Specific features of photo-and electrical conduction in manganese germanium garnets

The specific features of photo-and electrical conduction in manganese germanium garnet crystals are investigated in the temperature range 4.2–370 K for the first time. Under exposure of samples with ohmic contacts to visible light, the photocurrent in these samples is observed only at high temperatures. The characteristic times of the photocurrent rise differ from those of photocurrent relaxation after the light is switched off. The inference is made that the photo-and electrical conduction is determined by the electrical recharging of manganese ions. The generation and transport of charge carriers are controlled by centers with electrical inhomogeneities and shallow attachment levels.     

Some photoelectric properties of p-GaSe monocrystals

The thermostimulated conductivity, static characteristics, and kinetics of photoconductivity in undoped GaSe monocrystals with p-type conductivity are studied over the temperature range 80–350K. Photocurrent as a function of intensity and wavelength of exciting light, and relaxation curves are analyzed. Insignificant temperature and infrared photocurrent extinction was observed. The major parameters of photosensitive centers are determined. It is demonstrated that the experimentally observed principles of photoconductivity in gallium selenide can be explained within the framework of the two-center recombination model, as realized for many wide-zone photoconductors [1–3].Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 66–70, January, 1976.     

The conformation of adsorbed polyacrylamide and derived polymers

We have studied the adsorption of different polyacrylamide derivatives in water on freshly cleaved mica surfaces. Using the potential of measuring force versus distance profiles with an atomic force microscope (AFM), we investigate the conformation and the loop distribution of adsorbed chains. In particular, we explain that (i) the loop distribution of the polyacrylamide homopolymer can be described from scaling laws arguments, (ii) the loop distribution of statistical copolymers should be a signature of the distribution of the monomers along the chain backbone, and (iii) hydrophobically modified polyacrylamide copolymers form surface gels whose growing kinetics can be studied by AFM. Our results also show that the co-operative effect of H-bonding can give much larger adhesion that ionic association.     

Dielectrophoretic transients effects on liquid stability

Effects of dielectrophoretic transients on stability ofn-pentane and methyl methacrylate have been experimentally studied. Dielectrophoretic transients have been found to demonstrate anomalous effects in methyl methacrylate. The effects are observed both on the application of electric field and its subsequent turning off to zero and are equal in magnitude and direction during both the operations. Results have been explained on the basis of the body forces acting on the dielectric in presence of a non-uniform electric field and a temperature gradient. Heater surface thermodynamics has been studied under controlled temperature and electric field gradients. A striking feature of the experiment is the application of a high voltage directly on the heater surface making the body force calculations more realistic.     

Photothermal and electrooptic nonlinearities in ZnSe and ZnTe and their application in bistable devices

In this paper we present results on ZnSe and ZnTe optically bistable elements in connection with electrooptic effects. The behaviour of optical hysteresis under an applied electric field is studied and the electrical characteristics under illumination with monochromatic laser radiation are investigated. Influences of optical bistability on the photocurrent are measured and an attempt is made to explain the unusual behaviour of the current with respect to the absorbed amount of light, namely a switching down in photocurrent when the absorptive switching takes place. Towards this aim we report here for the first time on spatially resolved measurements of photoconductivity in ZnSe to investigate the influence of Schottky contacts on photothermal electrooptic bistability. Furthermore, we report also to our knowledge for the first time on the influence of the 3D-Stank effect on optical bistability.     

Appearance of anomalous phase in <Superscript>4</Superscript>He crystal in the presence of small <Superscript>3</Superscript>He impurity

The kinetics of formation of an anomalous phase from a solution with a small ³He impurity was studied in the temperature range 0.2–0.7 K up to a supersaturation of ～30 mbar. The phase diagram was determined for normal and anomalous growths. It is found that, in the presence of impurities, the formation of a fast-growing state is retarded. This experimental fact indicates that the dissipative processes in fluids have an appreciable effect on the phase formation kinetics. The retardation is, possibly, caused by the direct interaction of an impurity with the crystal surface.     

Frequency-resolved photocurrent studies of a-Se

 Optically modulated photocurrent response of amorphous selenium (a-Se) thin films was measured between 20 K and 295 K, by using the in-quadrature frequency-resolved spectroscopy method. The results show that the modulated photocurrent depends on external parameters such as excitation light intensity and temperature, giving information about recombination kinetics. Received, 28 July 1996 /Accepted: 20 September 1996     

Resistive switching of silicon-rich-oxide featuring high compatibility with CMOS technology for 3D stackable and embedded applications

This paper discusses the resistive switching devices based on highly compatible silicon-rich-oxide, including silicon monoxide (SiO) and SiO _x N _y material, which can be fabricated by low temperature process, and thus fully compatible with the back-end CMOS technology. The demonstrated SiO based RRAM suitable for 3D stackable applications shows repeatable unipolar resistive switching behavior with excellent on/off resistance ratio and good retention performance, but a little bit high switching voltage. The presented silicon-rich silicon-oxynitride RRAM device can effectively reduce the switching voltages (∼1 V) and shows good retention capability under 180°C baking as well as fast speed, giving great potentials for 3D stackable and embedded applications. The switching mechanisms in the studied devices are discussed. The method of switching voltage reduction through nitrogen doping, as a kind of defect engineering, can provide some guidelines for RRAM design.     

探测二茂铁外价轨道(e,2e)反应中的扭曲波效应

利用第三代高效率电子动量谱仪,分别在600和1500 eV两种不同入射电子能量下获得了二茂铁(ferrocene)分子外价轨道的电离能谱和电子动量谱的相关实验结果.并利用非相对论与标量相对论密度泛函方法计算出了二茂铁的重叠型和交错型两种不同构象的理论动量谱.两种构象的外价轨道一一对应,理论电子动量谱基本一样.对二茂铁的外价轨道,在低动量区观测到了强烈的扭曲波效应,这与这些轨道主要由铁原子的3d轨道构成有关.通过相对论和非相对论计算结果的比较,表明相对论效应对于二茂铁的外轨道动量分布几乎没有影响. 关键词： 二茂铁分子 电子动量谱 相对论效应 扭曲波效应     

Influence of fatigue processes on the switching currents in lead titanate and lead zirconate-titanate films

The processes of polarization fatigue in PbTiO₃ and Pb(Zr_0.5Ti_0.5)O₃ films on silicon substrates have been studied by the method of switching in rectangular pulsed fields. Dependences of the switchable polarization, the maximum switching current i _max, and the switching time ??s on the number of switching cycles have been obtained from the data on the switching currents. It has been shown that the mobility of domain walls is retained in the fatigue processes and the observed fatigue is connected with switching off a fraction of the switchable volume due to pinning of reverse domain nuclei by charged defects.     

Enhancement of terahertz coupling efficiency by improved antenna design in GaN/AlGaN high electron mobility transistor detectors

An optimized micro-gated terahertz detector with novel triple resonant antenna is presented.The novel resonant antenna operates at room temperature and shows more than a 700% increase in photocurrent response compared to the conventional bowtie antenna.In finite-difference-time-domain simulations,we found the performance of the self-mixing GaN/AlGaN high electron mobility transistor detector is mainly dependent on the parameters L gs(the gap between the gate and the source/drain antenna) and L w(the gap between the source and drain antenna).With the improved triple resonant antenna,an optimized micrometer-sized AlGaN/GaN high electron mobility transistor detector can achieve a high responsivity of 9.45×102 V/W at a frequency of 903 GHz at room temperature.     

Solvothermal synthesis of AlN

Abstract

Aluminum nitride is an important material due to its physical properties (electronic, thermal conductivity…). Different processes have been used for preparing such a material. The solvothermal synthesis is characterised by the use of a solvent in supercritical conditions in order to improve the reactivity of the precursors and to control the size of the crystalhtes. Using finely divided aluminum particles as precursor, a new process for the preparation of AlN has been optimised versus the thermodynamical parameters: pressure and temperature and the nature of the additive used for improving the kinetics of nitridation.     

A polarization-sensitive,self-powered,broadband and fast Ti3C2Tx MXene photodetector from visible to near-infrared driven by photogalvanic effects

Broadband, self-power, and polarization-sensitivity are desirable qualities for a photodetector. However, currently few photodetectors can fulfill these requirements simultaneously. Here, we propose a Ti₃C₂T_x (MXene) photodetector that is driven by the photogalvanic effect with impressive performances. A polarization-sensitive photocurrent is generated at zero bias under the illumination of linearly polarized laser light of 1064 nm, with an extinction ratio of 1.11. Meanwhile, a fast response with a 32/28 ms rise/decay time and a large on/off switching ratio of 120 are achieved. Besides, a robust zero-bias photocurrent is also generated in the photodetector under the illumination of 940 and 620 nm light, as well as the white light, showing a broadband photoresponse from the near-infrared to visible. Moreover, quantum transport simulations indicate that the photogalvanic effect plays an important role in the generation of the polarized photocurrent at zero bias due to the broken space inversion symmetry of the stacked few-layer Ti₃C₂T_x. Our results shed light on a potential application of the Ti₃C₂T_x–MXene in the low-power photodetection with high performances.     

Mössbauer spectroscopic study of ferrocene derivatives in the adsorption and liquid-crystalline states

Mössbauer spectra of ferrocene derivatives in the solid, adsorption and liquidcrystalline states were measured at temperatures ranging from 78 to 423 K. The peak intensities of the Mössbauer spectra of ferrocene derivatives adsorbed on silica gel decreased markedly with an increase in temperature. The Mössbauer absorption of [4-(4-methoxyphenoxycarbonyl)-phenoxycarbonyl]alkyl 4-ferrocenylbenzoate at 295 K during the cooling process was observed in what was assumed to be the liquid-crystalline state.