Lattice expansion in ZnSe quantum dots

Solid ZnSe quantum dots (QDs) have been prepared via chemical route. The QDs have been characterized by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Photoluminescence (PL) and Ultraviolet-Visible Spectroscopy (Uv-Vis). The QD sizes were found to vary from 2.5 to 9.5 nm. The XRD measurement reveals an increase in the interplanar spacing in QDs as compared to their bulk counterpart. This observation is further supported by Rietveld analysis which establishes the formation of single phase zinc blende ZnSe QDs and confirms 3.9% lattice expansion. Calculations based upon the thermodynamical theory yield 8.7% concentration of vacancies due to the lattice expansion. We observe various peaks in the PL spectra which may arise either due to the QD size variations or the defects due to the vacancies.     

Lattice defects and the effect of Melment on the hydration of alite

The hydration of alite has been studied in the presence of different concentrations of the superplasticizer Melment by using various experimental techniques. Heat of hydration and non-evaporable water content determinations show that Melment retards the hydration; pH measurements indicate that in presence of Melment, Ca²⁺ ion dissolution is reduced. Zeta potential measurements give definite proof that Melment molecules are adsorbed at alite surfaces and the retardation of hydration may be due to adsorption. Thermoelectric power measurements prove that the material is an n-type semiconductor, whereas electrical conductivity measurements of solid pellets of C₃S (C = CaO, S = SiO₂) show that the material is an intrinsic semiconductor above 746 K and an extrinsic semiconductor below 746 K. Extrinsic semiconductivity may be due to the presence of defects or impurities in the crystal lattice. The results also show that oxide ion vacancies are created in the crystal lattice and the reactivity of C₃S is related to the defects. Probably Melment molecules are adsorbed at the sites of defects and oxide ion vacancies and hence retard the hydration.     

Investigation on defects in HPHT-grown diamond single crystals

In the diamond single crystals synthesized at high temperature and high pressure using FeNi as catalyst, there are usually supersaturated vacancies and inclusions formed during the diamond crystal growth and rapid cooling from high temperature. Some defects such as prismatic dislocation loops, stacking faults and array of dislocations are closely related to such supersaturated vacancies and inclusions. The supersaturated vacancies agglomerate into discs on the (111) close-packed planes, subsequent collapse of the discs forms the dislocation loops and stacking faults. The thermal internal stresses, which are caused by the difference of thermal contraction between the diamond and the inclusions due to the difference of thermal coefficients between them as the diamond is cooled from high temperature, may be relieved by the formation of array of dislocations. In the present paper, these defects in the diamond single crystals were directly examined by transmission electron microscopy (TEM). The characteristics and formation process of these defects were analyzed briefly.     

衬底温度对脉冲激光沉积在Si衬底上Zn0.95Co0.05O薄膜的影响

在不同的衬底温度下在n型Si(111)衬底上采用脉冲激光沉积的方法生长了(002)择优取向的具有室温铁磁性的Zn0.95Co0.05O薄膜。X射线衍射显示所生长的薄膜呈六方纤锌矿结构。X射线光电子能谱测试表明薄膜中出现的室温铁磁性不是由于Co团簇产生的。发现薄膜生长过程中产生的间隙锌、氧空位以及晶格缺陷对铁磁性有显著的影响。通过改变衬底温度可以控制薄膜中间隙锌、氧空位及晶格缺陷的数量,薄膜的铁磁性同时也可以被明显地改变,这是缺陷与薄膜的室温铁磁性相关的直接证据。     

Investigation of fatigue cracks in aluminium alloys 2024 and 6013 in laboratory air and corrosive environment

Fatigue cracks have been generated in the commercial aluminium alloys AA2024 and AA6013 and analysed with a positron microprobe. This instrument provides laterally resolved positron annihilation measurements, which are sensitive to lattice defects like vacancies and dislocations. These commercial alloys have undergone a solution heat treatment and quenching prior to fatigue testing. Subsequently, they have been aged at room temperature and 190°C for AA2024 and AA6013, respectively. We performed the fatigue crack generation both in air and under the influence of a chemically aggressive environment (artificial seawater). Due to the corrosive environment hydrogen is probably produced at the fresh fractured surface in the vicinity of the crack tip. We discuss the possible implications of in-diffused hydrogen on the produced lattice defects, especially when there is a delayed migration of vacancies in the lattice, due to a reduced mobility.     

Intrinsic defect complexes in CdTe and ZnTe

Radiation defects in CdTe and ZnTe are modeled from first principles. The most important intrinsic defects resulting from cation evaporation or displacement are cation vacancies and tellurium anti-sites, electrically active defects characterized by a low formation energy. The reactions between those two defects are investigated. Since cation vacancy clusters of less than four vacancies are not stable, it is argued that cation vacancy aggregation is not a dominant process in near-equilibrium conditions. In-grown or radiation-induced clusters of four cation vacancies may serve as a nucleation center for tellurium precipitation. The formation energy of these small voids is lower in ZnTe than in CdTe. Additionally, cation-anion divacancies are stable in ZnTe and in p-type CdTe.     

Iron Vacancies Induced Bifunctionality in Ultrathin Feroxyhyte Nanosheets for Overall Water Splitting

Exploring of new catalyst activation principle holds a key to unlock catalytic powers of cheap and earth‐abundant materials for large‐scale applications. In this regard, the vacancy defects have been proven to be effective to initiate catalytic active sites and endow high electrocatalytic activities. However, such electrocatalytically active defects reported to date have been mostly formed by anion vacancies. Herein, it is demonstrated for the first time that iron cation vacancies induce superb water splitting bifunctionality in alkaline media. A simple wet‐chemistry method is developed to grow ultrathin feroxyhyte (δ‐FeOOH) nanosheets with rich Fe vacancies on Ni foam substrate. The theoretical and experimental results confirm that, in contrast to anion vacancies, the formation of rich second neighboring Fe to Fe vacancies in δ‐FeOOH nanosheets can create catalytic active centers for both hydrogen and oxygen evolution reactions. The atomic level insight into the new catalyst activation principle based on metal vacancies is adaptable for developing other transition metal electrocatalysts, including Fe‐based ones.     

Amorphous WO3-x thin films with color characteristics tuned by the oxygen vacancies created during reactive DC sputtering

Tungsten oxides are interesting for a variety of applications due to their versatile optoelectronic charac-teristics,which can be tuned changing the composition and/or the crystalline structure.Coloration due to sub-bandgap absorption is often achieved by ion intercalation or doping in WO3:M films(with M=H+,Li+,Na+,etc.introducing extra electrons),but a more direct way is creating charged oxygen vacancies(Vo+and/or Vo2+)in sub-stoichiometric WO3-x forms.Here,amorphous WO3-x thin films are obtained by reactive DC sputtering of a pure W target,on unheated glass substrates,changing the oxygen to argon pressures ratio.The control of intrinsic defects(oxygen vacancies and tungsten valence states)by the oxy-gen partial pressure allows tuning the morphology,sub-bandgap absorption and carrier density in these WO3-x films,as it is proven by Raman spectroscopy,atomic force microscopy,optical spectrophotometry and Hall effect measurements.     

AC conduction in amorphous thin films of SnO<Subscript>2</Subscript>

Alternating current (a.c.) electrical properties of thermally evaporated amorphous thin films of SnO₂ sandwiched between aluminium electrodes have been investigated for temperature during electrical measurements, film thickness, substrate temperature and post-deposition annealing. The a.c. conductivity, σ(ω), is found to vary with frequency according to the relation σ(ω) ∝ ω^s, indicating a hopping process at low temperature. The conduction is explained by single polaron hopping process as proposed by Elliott. The increase in electrical conductivity with increase in temperature during electrical measurements is ascribed to the increase in the formation and high mobility of doubly ionized oxygen vacancies. The increase in conductivity with increase in film thickness is caused by the increase in interstitial tin, oxygen vacancies and defects produced due to deviation from stoichiometry. The increase in conductivity with increase in substrate and annealing temperature may be due to the formation of singly or doubly ionized oxygen vacancies and tin species of lower oxidation state. Measurements of capacitance C as a function of frequency and temperature show a decrease in C with increasing frequency and increase in C with increasing temperature. The increase in capacitance in the high-temperature low-frequency region is probably due to space charge polarization induced by the increasing number of free carriers as a result of increasing temperature.     

Point defects in metals

This review surveys present knowledge of the nature and behaviour of point defects, especially vacancies. The general theoretical concepts and various experimental observations connected with the formation and mobility of vacancies and their complexes, their effect on the physical properties of metals, are described. The formation of secondary defects by vacancy condensation, the interaction between vacancies and solute atoms and some general ideas about point defects, are also discussed in detail.     

1.8 Influence of defects in the silicon wafer on the properties of silicon oxide films formed by reactive sputtering

It is well known that the properties of SiO₂ films, formed by reactive sputtering in oxygen, sensitively depend upon the details of chemical pretreatment of the Si surface and on the conditions of oxide formation. The influence of bulk defects in the silicon wafer has, however, not been adequately studied. Bombardment of the silicon surface with electrons and ions during the initial stages of the oxide formation is accompanied by extensive interaction with the surface defects, leading to changes in the charge of these defects and properties of the deposited film in general. If vacancies are the dominant defects, then a positive charge is created in the oxide film. If both vacancies and stacking faults are present, then both positive and negative charge can appear in the SiO₂ film.     

Modelling accelerated solid-state diffusion under the action of intensive plastic deformation

A mathematical model of accelerated solid-state diffusion during mechanical alloying (MA) in a binary substitutional system A-B is developed. An individual lamellar particle formed due to fracturing/cold welding during a preliminary stage of MA is considered. Interdiffusion occurs via the vacancy mechanism. During the plastic deformation, jog dragging by moving screw dislocation generates non-equilibrium point defects (vacancies and interstitial atoms), which can diffuse, interact with edge dislocations and recombinate. To evaluate the point defect generation rate, a simple Hirsch-Mott theory is employed. Numerical simulation has been performed for a repeated “deformation-rest” cycle at 100°C using realistic parameter values. The influence of non-equilibrium vacancies on the atomic diffusion is evaluated. It reveals itself via the increase of partial diffusivities of substitutional atoms and through the cross-link terms in the matrix of interdiffusion coefficients. The incoherent phase boundary between parent phases (pure elements) is considered as a sink for non-equilibrium vacancies. Due to interplay of these factors, substantial alloying by solid-state diffusion is observed after a reasonable time of MA (4000 s).     

Zinc oxide nanoparticles: A study of defect level blue-green emission

Systematic investigation of the structural, morphological and optical properties of hexylamine capped zinc oxide (ZnO) nanoparticles is presented. Optical properties indicate the presence of sufficient amount of surface defects. These defects in highly dispersed ZnO nanoparticles have been studied by annealing the nanoparticles in air at various temperatures and recording the photoluminescence spectra. The annealing temperature was found to strongly influence the UV band edge emission and the blue-green defect level emission (DLE). At low annealing temperatures an increase in UV emission with temperatures is observed and this emission is proposed to be a result of desorption of surface adsorbed water and hydroxyl groups. The DLE of the synthesized sample is likely due to the presence of oxygen vacancies on the surface resulting in green emission.     

Correlation between types of defects/vacancies of Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> nanostructures and their transient photocurrent

Crystalline nanostructures possess defects/vacancies that affect their physical and chemical properties.In this regard,the electronic structure of materials can be effectively regulated through defect engineering;therefore,the correlation between defects/vacancies and the properties of a material has attracted extensive attention.Here,we report the synthesis of Bi2S3 microspheres by nanorod assemblies with exposed {211} facets,and the investigation of the types and concentrations of defects/vacancies by means of positron annihilation spectrometry.Our studies revealed that an increase in the calcined temperature,from 350 to 400 ℃,led the predominant defect/vacancy densities to change from isolated bismuth vacancies (VBi) to septuple Bi3+-sulfur vacancy associates (VBiBiBiSSSS).Furthermore,the concentration of septuple Bi3+-sulfur vacancy associates increased as the calcined temperature was increased from 400 to 450 ℃.The characterized transient photocurrent spectrum demonstrates that the photocurrent values closely correlate with the types and concentrations of the predominant defects/vacancies.Our theoretical computation,through first principles,showed that VBiBiBiSSSS strongly absorbs I2(sol),easily desorbs I-(sol),and enhances the electrocatalytic activity of the nanostructures.     

Computational investigation of elastic properties of bulk and defective ultrahard B<Subscript>6</Subscript>O

Elastic properties of bulk B₆O—an important material with superhard properties have been studied. The influence of O vacancies and light substitutional defects such as C and N have been examined. It has been shown that vacancies in particular exert a pronounced effect on such properties of the material as elastic constants and melting temperature.     

Monte Carlo simulation on the cation diffusion via vacancies in simple spinels

The correlation factors of cations and vacancies for diffusion in simple spinels were calculated by using the Monte Carlo simulation technique. Since the predominant point defects are cationic vacancies at high temperature and especially at high oxygen partial pressures in many spinels, the cations are considered to move primarily via cationic vacancies in this study. Several input parameters including the lattice size, the number of jumps per cation, etc., were optimized to calculate the correlation factors. Then the correlation factors of cations moving via vacancies in simple spinels were calculated in different types of exchange jumps, i.e., diffusion that occurs on tetrahedral or octahedral or both cationic sites. Results were obtained with satisfactory accuracy. The influence of the vacancy concentration on the correlation factors was also analyzed.     

Effects of Fe-ion irradiation and annealing on the optical absorption in sapphire

We have studied the optical absorption in sapphire after Fe-ion irradiation and subsequent vacuum heat treatment. The absorption parameters and the nature of the transitions between localized states and allowed bands have been shown to be influenced by substitutional defects, intrinsic radiation-induced defects, and their complexes. We have evaluated the contributions of individual substitutional defects, their clusters, and complexes of substitutional defects with native vacancies and the effect of the iron oxides forming during annealing on the absorption in sapphire. The nature of the substitutional defect clusters and impurity-native defect complexes has been analyzed in detail.     

Studies of defects in the near-surface region and at interfaces using low energy positron beams

Positron annihilation spectroscopy (PAS) is a powerful probe to study open-volume defects in solids. Its success is due to the propensity of positrons to seek out low-density regions of a solid, such as vacancies and voids, and the emissions of gamma rays from their annihilations that carry information about the local electronic environment. The development of low-energy positron beams allows probing of defects to depths of few microns, and can successfully characterize defects in the near-surface and interface regions of several technologically important systems. This review focuses on recent studies conducted on semiconductor-based systems.     

Detection of surface lattice defects using line scan method

The presence of different kinds of surface lattice defects such as missing atom, interstitial atom, line defects, in graphite single crystal have been identified by using scanning tunneling microscope. These defects cause displacement of atoms from their mean position and lattice strain is introduced. By measuring the displacement of atoms from their mean position. lattice strain has been calculated. It is found that among single point defects, vacancies cause maximum lattice strain. Paper presented at the poster session of MRSI AGM V1, Kharagpur, 1995