Binding and decomposition of organic dye molecules on ZnO crystals

Prism surfaces of ZnO crystals are prepared by various pretreatments as heating in ultrahigh vacuum (UHV), annealing in oxygen at pressures up to 10⁵ Pa, argon bombardment, cleavage in UHV. An organic dye is deposited by sublimation in UHV and the exposure (10¹²−10¹⁵ cm⁻² is recorded by means of a vibrating quartz. Adsorption and desorption studies including thermal desorption spectroscopy (TDS) are performed with variation of crystal temperature during deposition and of coverage. In these experiments the spectral distribution of absorption is used for the recording of coverage down to about 10¹² cm⁻², for the observation of ordering processes within the dye layer and for the detection of dye decomposition. By the various treatments the catalytic activity of the ZnO surface for decomposition of the dye molecules at temperatures above 350 K can be increased or decreased in a wide range. Possible sources of the activity are discussed. Molecules bound to a clean inactive ZnO surface desorb above 600 K. In contrast they come off already at 350 K if they are bound only to other molecules. A sticking coefficient is derived as a function of coverage at 470 K and compared with calculations after several adsorption models. The best fit is obtained by a precursor state model.     

New experimental results for electron transport in weak accumulation layers of ZnO crystals

Previous Hall measurements on (0001) and (0001&#x0304;) faces of ZnO have shown a Hall mobility oscillating as a function of Hall surface electron density in the range between N_SH=10⁶ and 10¹¹ cm^?2. Here we report on new results obtained by a field effect arrangement for free surfaces in UHV. With donors from H exposure or by illumination weak accumulation layers (n_sh <10¹¹ cm^?2) are established. The field effect shows oscillations in surface conductivity as a function of gate voltage. Also the combination of a field effect with a Hall effect measurement reveals discrete values of Hall surface electron density n_sh. Various pretreatments do not change the periodicity of these oscillations. Necessary preconditions are a temperature below 130 K, a Hall surface electron density below 3 × 10¹² cm^?2 and a source-drain field of a few V/cm. A model regarding impurity levels in the space charge layer relates the results of the field effect measurements to the results of the Hall effect measurements.     

Charge transfer between a superconductor and a hopping insulator

We develop a theory of the low-temperature charge transfer between a superconductor and a hopping insulator. We show that the charge transfer is governed by the coherent two-electron-Cooper pair conversion process time-reversal reflection, where electrons tunnel into a superconductor from the localized states in the hopping insulator located near the interface, and calculate the corresponding interface resistance. A specific feature of this problem is the interplay between the time-reversal reflection at the interface and transport through the percolation cluster. To allow for this interplay, we have generalized the connectivity criterion of the percolation theory to include surface effects. We show that the time-reversal interface resistance is accessible experimentally, and that in mesoscopic structures it can exceed the bulk hopping resistance.     

Charge transfer excited states of hydroxyl ion impurities in alkali halide crystals

A theoretical investigation of the u.v. absorption anisotropy factor g and the transition energies of OH^? molecule in alkali halide crystals is presented in terms of a charge transfer model and the free OH^? ground wave-function. Comparison of calculated and observed transition energies for OH^? in KBr and NaCl shows reasonable agreement between theory and experiment. For KCl:OH^?, a reasonable result for the transition energy is also obtained through a confinement of the free K4s function used for the one-electron transfer function. The model fails, however, to explain the value of about 0·3 for the anisotropy factor which is observed in a number of host crystals.     

Energy transfer between fluorescent dyes in photonic crystals

Energy transfer from fluorescein (Fl) to Rhodamine B (RhB) in the opal photonic crystals has been investigated by photoluminescence. The results show that the energy transfer can be enhanced effectively by photonic bandgaps. When the fluorescence emission wavelength of donor Fl overlaps the photonic bandgap the fluorescence intensity of the donor is suppressed, while the fluorescence intensity of acceptor RhB is obviously enhanced. This enhancement can be attributed to the inhibition of radiative emission of the donor in the photonic crystals.     

Charge relaxation and instability in barrier layers

It is shown experimentally that the order of magnitude of the relaxation time of a physical barrier layer is determined by the product of the Maxwell relaxation time and the corresponding exponent. The experiments deal with dielectric dispersion in ferrites and nickel oxide. It is shown that near the relaxation maximum these inhomogeneous semiconductors can be considered as quasihomogeneous. A new treatment of a number of instabilities in barrier layers is presented: low frequency voltage fluctuations in p—n junctions and Schottky barriers, barrier instability, etc.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 20–23, June, 1981.     

Charge transfer and martensitic nucleation

Summary The microscopic interpretation of phase stability in alloys undergoing thermoelastic martensitic transformation involves a difficult nucleation problem. The strong dependence of the critical temperature for the onset of martensite on composition requires the introduction of nonclassical defective nuclei, of dynamic nature, to obtain reasonable estimates of the energy barrier to nucleation. Experimental data on β-Au−Zn-based alloys are organized in the framework of the atomistic model of the valence electron localization degree. The resulting charge transfer scheme is strictly correlated to elementary surface segregation events driven by elemental surface energy differences within the studied alloys. The connection between martensitic nucleation and charge transfer segregation is discussed, focusing on the physical meaning of the various energy terms entering the model. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Charge transport and accumulation in liquid crystals

Charge carrier formation mechanisms in liquid crystals and their contribution to conductivity in the voltage range 0.1–10 V and field action times of 0–300 sec are compared. Values of accumulated charge are calculated corresponding to the processes of carrier separation within the volume, injection from the electrode, and molecular dissociation under electric field action.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 9–12, July, 1989.     

光子晶体中双通道之间能量的转移

从理论和实验上分析了光子晶体中双通道之间能量的转移.在双通道之间引入复合型光子晶体波导,通过调节复合型波导的内部参数，使其满足一定的条件，就可以实现双通道之间能量的完全转移，从而实现了光的定向传输.在实验中，测得了输出端的透过谱. 关键词： 复合型光子晶体波导 能量转移 定向传输     

Charge transfer between sensing and targeted metal nanoparticles in indirect nanoplasmonic sensors

In indirect nanoplasmonic sensors, the plasmonic metal nanoparticles are adjacent to the material of interest, and the material-related changes of their optical properties are used to probe that material. If the latter itself represents another metal in the form of nanoparticles, its deposition is accompanied by charge transfer to or from the plasmonic nanoparticles in order to equalize the Fermi levels. We estimate the value of the transferred charge and show on the two examples, nanoparticle sintering and hydride formation, that the charge transfer has negligible influence on the probed processes, because the effect of charge transfer is less important than that of nanoparticle surface energy. This further corroborates the non-invasive nature of nanoplasmonic sensors.     

Excitation transfer between components of molecular layers of cyanine compounds

Using the example of layers of symmetric polymethine dyes, we have studied the optical excitation energy transfer processes between nanocomponents of a molecular layer. The fluorescence yields and life-times of excited states of monomeric stereoisomers have been estimated, and the stereoisomerization yields have been determined. We show that the excitation transfer yield between the S ₁ states of monomeric stereoisomers (tens of percent) is considerably higher than the fluorescence yield (several percent) and depends on the structure of terminal groups of the molecule.     

Intrinsic defects in ZnO and GaN crystals

The kinetics of defect formation in the system of a crystal with a nonmetal component in the activated gas phase has been investigated. The data obtained has made it possible to develop physicochemical methods of regulating the defect-formation processes depending on the adsorption—desorption—crystallization equilibrium on the surface of a crystal. A kinetic model of defect formation in the A^IIB^VI and A^IIIB^V compounds is proposed. Results of the kinetic analysis of the intrinsic defects in the ZnO and GaN compounds are presented. The photoluminescence spectra of GaN films annealed in a flow of nonmetal-component radicals (atoms) have been considered. This work was reported at the Vth International Scientific-Technical Conference on Quantum Electronics, November 22–25, 2004, Minsk, Belarus. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 760–765, November–December, 2005.     

Exciton transfer between localized states in ZnO quantum well structures

The dynamics of photocreated excitons in a CdZnO/MgZnO quantum well (QW) was studied by comparing the experimental photoluminescence (PL) data with the results of Monte Carlo simulations of the exciton hopping. The temperature-dependent PL linewidth was found to be in reasonable agreement with the model of exciton hopping, with an additional inhomogeneous broadening (Γ) accounted for. The simulation analysis revealed fluctuations of the band potential to be 20 meV with an additional inhomogeneous broadening of , and a crossover from a non-thermalized to thermalized exciton energy distribution at about 100 K. In addition, a Bose–Einstein distribution like temperature dependence of the exciton energy in the wells was extracted using the data on the PL peak position.     

Fluorescence investigations of pure and mixed evaporated dye layers

Fluorescence spectroscopy is used to investigate energy transfer processes in evaporated layers consisting of several different dyes. In this study films ofN,N-dimethylperylene-3,49,10-bis-dicarboximide (methylperylene pigment, MPP), coevaporated with copper phthalocyanine (CuPc) at varying ratios, and double layers of MPP and CuPc with different thicknesses are investigated. It is shown that energy transfer from MPP to CuPc occurs in both mixed and double layers. The energy transfer leads to a strong quenching of the MPP fluorescence and sensitized CuPc emission in the NIR region. The concentration dependence of the fluorescence quenching in mixed layers can be described by a Stern-Volmer plot. A simple model based on exciton diffusion between MPP molecules toward active quenching centers is used to determine the diffusion length.     

Optical properties of ZnO and ZnO:Ce layers grown by spray pyrolysis

In this paper, zinc oxide (ZnO) and cerium-doped zinc oxide (ZnO:Ce) films were deposited by reactive chemical pulverization spray pyrolysis technique using zinc and cerium chlorides as precursors. The effects of Ce concentration on the structural and optical properties of ZnO thin films were investigated in detail. These films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) measurements. All deposited ZnO layers at the temperature 450 °C are polycrystalline and indicate highly c-axis oriented structure. The dimension of crystallites depends on incorporation of Ce atoms into the ZnO films. The photoluminescence spectra of the films have been studied as a function of the deposition parameters such as doping concentrations and post grows annealing. Photoluminescence spectra were measured at the temperature range from 13 K to 320 K.     

液-液两相液层间传质过程的Rayleigh-Bénard-Marangoni对流特性

传质引发的Rayleigh-Bénard-Marangoni对流(RBM对流)对化工传递过程有着显著影响.但是,已有的相关研究多集中于气-液体系,并且有限的针对液-液体系的相关研究尚缺乏对RBM对流演化及其引发的界面扰动行为的深入分析.因此,本文基于阴影法设计搭建了竖直狭缝内液-液两相液层间传质过程的RBM对流特性可视化实验平台,并实验观测了水-甲苯-丙酮三元体系中丙酮组分扩散传质时出现的RBM对流结构以及其向下层水相主体的发展演变过程,探讨了水相丙酮初始浓度、甲苯相丙酮初始浓度以及甲苯层厚度对RBM对流特性和液-液界面形貌的影响.研究表明:在Rayleigh-Taylor不稳定性作用下,水相上层密度(重力)分层"界面"下凸沉降形成波浪形丘状"界面",并随着"界面"处密度与压力失调的加剧而演变成羽状流;因羽流区"界面"不同浓度梯度引起的传质特性差异,羽状流又可以演变成弱羽状流和强羽状流两种形态;当丙酮浓度梯度增大到一定程度后,近界面处短时间内产生大量RBM对流结构,且结构间相互影响增强而聚并成对流团,并随着传质过程的进行,逐渐演变成独立的强羽状流; RBM对流强度与上下液层丙酮浓度梯度大小呈正相关关系,且液-液界面粗糙度及其非稳态波动随着丙酮浓度梯度的增加而增大.     

Photoinduced charge transfer across the interface between organic molecular crystals and polymers

Photoinduced charge transfer of positive and negative charges across the interface between a single-crystal organic semiconductor and a polymeric insulator is observed in electric field-effect experiments. Immobilization of the transferred charge by deep traps in the polymer results in a shift of the threshold of field-induced conductivity along the semiconductor-polymer interface, which allows for direct measurements of the charge transfer rate. The transfer occurs when the photon energy exceeds the absorption edge of the semiconductor. The direction of the transverse electric field at the interface determines the sign of the transferred charge; the transfer rate is controlled by the field magnitude and light intensity.     

Two-dimensional dye crystals with controllable optical properties

The structure, molecular ordering and optical properties of single crystals of cyanine dyes grown by adsorption from a water subphase to a positively charged lipid monolayer are discussed. These crystals are one monolayer thick, of uniform dimensions between 10 and 100μm (depending on nucleation conditions) and of rectangular shape. Single crystals were studied by transmission electron diffraction and by polarized absorption and emission spectroscopy.

We show that the crystals consist of two rows of densely stacked molecules with two different orientations of the long molecular axes. This leads to two perpendicularly polarized absorption bands. The measured splitting is in accordance with results of extended dipole calculations. The latter were performed for crystals of three slightly different molecules where the angles between the long axes varied between 70° and 100°. The aliphatic tails form a lattice which is epitaxially related to the centered rectangular one of the dyes. It is incommensurate, and the tails are tilted with tilt azimuth either parallel to the a or to the b axis (depending on the type of dye) of the dye lattice.

The procedure allows formation of mixed crystals with structural parameters, depending on composition, between those of the pure compounds. Hence also the optical spectra can be tuned via composition maintaining sharp band edges and emission bands.