Correlation of electrical and optical properties in dually Cd+ and N+ ion-implanted GaAs

Cd in GaAs is an acceptor atom and has the largest atomic diameter among the four commonly-used group-II shallow acceptor impurities (Be, Mg, Zn and Cd). The activation energy of Cd (34.7 meV) is also the largest one in the above four impurities, When Cd is doped by ion implantation, the effects of lattice distortion are expected to be apparently different from those samples ion-implanted by acceptor impurities with smaller atomic diameter. In order to compensate the lattice expansion and simultaneously to adjust the crystal stoichiometry, dual incorporation of Cd and nitrogen (N) was carried out into GaAs, Ion implantation of Cd was made at room temperature, using three energies (400 keV, 210 keV, 110 keV) to establish a flat distribution, The spatial profile of N atoms was adjusted so as to match that of Cd ones, The concentration of Cd and N atoms, [Cd] and [N] varied between 1 x 10(16) cm(-3) and 1 x 10(20) cm(-3). Two type of samples, i.e., solely Cd+ ion-implanted and dually (Cd+ + N+) ion-implanted with [Cd] = [N] were prepared, For characterization, Hall effects and photoluminescence (PL) measurements were performed at room temperature and 2 K, respectively. Hall effects measurements revealed that for dually ion-implanted samples, the highest activation efficiency was similar to 40% for [Cd] (= [N])= 1 x 10(18) cm(-3). PL measurements indicated that [g-g] and [g-g](i) (i = 2, 3, alpha, beta,...), the emissions due to the multiple energy levels of acceptor-acceptor pairs are significantly suppressed by the incorporation of N atoms, For [Cd] = [N] greater than or equal to 1 x 10(19) cm(-3), a moderately deep emission denoted by (Cd, N) is formed at around 1.45-1.41 eV. PL measurements using a Ge detector indicated that (Cd, N) is increasingly red-shifted in energy and its intensity is enhanced with increasing [Cd] = [N], (Cd, N) becomes a dominant emission for [Cd] = [N] = 1 x 10(20) cm(-3). The steep reduction of net hole carrier concentration observed for [Cd]/[N] less than or equal to 1 was ascribed to the formation of (Cd, N) which is presumed to be a novel radiative complex center between acceptor and isoelectronic atoms in GaAs.

Influence of Mn-O-Mn bond angle on the magnetic and electronic properties in YBaMn2O5

The influence of the Mn-O-Mn bond angle on the magnetic and electronic properties of YBaMn2O5 was studied by density functional theory, which was implemented in the CASTEP code. In practical calculation, both G- and A-type antiferromagnetic (AFM) orderings were considered. The calculated results indicated that G-type is more stable than A-type, in agreement with both experiment and previous theoretical study. It is also interesting to note that a transition from G-type to A-type at an Mn-O-Mn angle of ca. 170 degrees was found upon increasing Mn-O-Mn angle. Therefore, the calculation suggested that what is essential to stabilize the G-type AFM state is the reduction of the Mn-O-Mn bond angle. For both magnetic orderings, the compound changes from semiconductor to metal with the increase of Mn-O-Mn angle.

Calculation of the thermal expansion coefficient for Bi2Sr2CaCu2O8

An estimation method of thermal expansion coefficient in term of lattice energy which was developed earlier for simple materials is extended to a complex material of Bi2Sr2CaCu2O8 (Bi-2212). The calculation of the chemical bond property and thermal expansion coefficient of Bi-2212 has been carried out and the theoretical values were in good agreement with the corresponding experimental results. The dependence of the thermal expansion coefficient on the different structures and on the flexible oxidation states of Bi and Cu are investigated. The results indicate that the thermal expansion coefficients of Bi-2212 are insensitive to the low lattice distortion of the average structure and the changes of formal valences of Bi and Cu ions.

Nanocrystalline SnO2 synthesised by means of hydrothermal precipitation

Nanocrystalline SnO2 with different particle sizes has been prepared by means of hydrothermal precipitation. The resulting SnO2 nanometer size powders, which are basically spherical in shape according to TEM, are tetragonal in structure with space group P4/mnm. Calculation shows that the crystallite size of SnO2 increases with increase of the calcination temperature, but that the average crystal lattice distortion rate decreases with increase of crystallite size. The smaller the particle, the bigger the crystal lattice distortion and the slower the crystal growth rate. Weight loss analysis indicates the prepared SnO2 is very slightly impure.

Preparation and properties of nanocrystalline Yb2O3

Nanocrystalline Yb2O3 of various particle sizes was prepared using sol-gel method. XRD analysis shows that the prepared nanocrystalline Yb2O3 is cubic in structure with space group Ia3. TEM photographs indicate that Yb2O3 nanoparticles are basically spherical in shape. Calculation of crystallite size indicates that the average crystallite size of Yb2O3 increases with increasing calcination temperature, but the average crystal lattice distortion rate decreases with increasing calcination temperature and crystallite size. This result shows that the smaller the crystallite size, the bigger the crystal lattice distortion, and the worse crystal growth. Solubility test of Yb2O3 in nitric acid shows that the surface activity of Yb2O3 increases with decreasing crystallite size. Fourier Transform Infrared Spectrometer (FTIR) spectra reveal that nanocrystalline Yb2O3 has higher surface activity; than that of ordinary Yb2O3. Absorbance intensity of Yb-O bond of nanocrystalline Yb2O3 is weaker than that of ordinary Yb2O3, and the absorbance of Yb-O bond of nanocrystalline Yb2O3 is small blue-shifted.

Synthesis and properties of cerium oxide nanometer powders by pyrolysis of amorphous citrate

CeO2 nanometer powders of different sizes were prepared at low temperature by pyrolysis of amorphous citrate. XRD patterns show that CeO2 is cubic in structure, space group O-h(5)-F-M3M. TEM indicates that the prepared CeO2 is spherical in shape, and the particle size distribution is in narrow range. It was found that calcination temperature is a more important factor affecting the crystallite size of CeO2 than calcining time, the smaller the particle, the bigger the crystal lattice distortion, the worse the crystal growth. Solubility test of CeO2 in nitric acid reveals that the surface activity of CeO2 decreases with the increasing particle sizes. IR spectra analysis shows that the absorption of Ce-O bond is shifted to higher energy with the decrease of CeO2 particle sizes.

Preparation and properties of rare earth hydroxides and oxides ultrafine powders in alcohol

A series of rare earth hydroxides and oxides ultrafine powders have been prepared by precipitation method using alcohol as dispersive and protective reagent. It was first to find that the crystallite size of cubic rare earth oxides had Lanthanide shrinking effect,but average crystal lattice distortion rate possessed lanthanide swelling effect;the change of diffraction intensity with atomic number presented an inverted W type, and double peaks structure was formed.

EFFECT OF THE CONCENTRATION OF THE ER3+ ION ON THE SPECTRAL INTENSITY PARAMETERS OF ER-YAG CRYSTALS

The absorption spectra of Er:YAG (YAG, yttrium-aluminium-garnet) crystals containing different concentrations of the trivalent erbium ion were measured and the spectral intensity parameters were calculated from these experimental spectra using the Judd-Ofelt model. The results indicate that the phenomenological intensity parameters, OMEGA(lambda) (lambda = 2, 4 and 6), vary as a function of the concentration of the Er3+ ion in the Er:YAG crystal, but no variation in the fluorescence-branching ratios as a function of the concentration of the Er3+ ion is found. An empirical formula is proposed to describe the relationship between the spectral intensity parameters and the Er3+ ion concentration in the Er:YAG crystal. The spectral intensity parameters exhibit a maximum in Er:YAG crystals containing about 1-1.5 at.% Er3+ ion. The effect of the Er3+ ion concentration on the spectral intensity parameters may be attributed to the inhomogeneous lattice distortion in the cell of the Er:YAG crystal caused by the dopant erbium ions.

Systematic studies of the effect of pressure on magnetic and electronic properties of La2/3Ca1/3MnO3 thin films with various thicknesses /

Interest in mixed-valent perovskite manganese oxides of La\-xAxMnO^ (v4-divalent alkaline earth Ca, Sr or Ba), whose unusual properties were discovered nearly a half century ago, has recently been revived. The discovery of the colossal magnetoresistance and pressure effects introduced new questions concerning the complex interplay between lattice structure, magnetism and transport in doped perovskite manganites. In this study, we report our experimental investigations of pressure and magnetic field dependencies of La-i/sCai/sMnOs (LCMO) epitaxial films with various thickness on SrTiO$ substrate. An analysis of film thickness dependency of the resistivity of LCMO epitaxial films under pressure and magnetic field has been performed by taking into account substrate contributions. This verifies the correlation of lattice distortion with magnetic and transport properties. Strong dependencies of Mn — O — Mn bond bending and Mn — O bond stretching with pressure as well as Mn spin alignment with magnetic field, and the lattice distortion induced by the substrate are discussed.

Study on the properties of piezoelectric materials and manganese-based oxide perovskites /

Perovskite type piezoelectric and manganese oxide materials have gained a lot of attention in the field of device engineering. Lead zirconium titananium oxide (PbZri.iTiiOa or PZT) is a piezoelectric material widely used as sensors and actuators. Miniaturization of PZTbased devices will not only perfect many existing products, but also opens doors to new applications. Lanthanum manganese oxides Lai-iAiMnOa (A-divalent alkaline earth such as Sr, Ca or Ba) have been intensively studied for their colossal magnetoresistance (CMR) properties that make them applicable in memory cells, magnetic and pressure sensors. In this study, we fabricate PZT and LSMO(LCMO) heterostructures on SrTiOa substrates and investigate their temperature dependency of resistivity and magnetization as a function of the thickness of LSMO(LCMO) layer. The microstructure of the samples is analysed through TEM. In another set of samples, we study the effect of application of an electric field across the PZT layer that acts as an external pressure on the manganite layer. This verifies the correlation of lattice distortion with transport and magnetic properties of the CMR materials.

Vibrational behaviour of bcc Cu-based shape memory alloys close to the martensitic transition

Experimental data from ultrasonic and inelastic neutron scattering measurements are analyzed for different families of Cu-based shape-memory alloys. It is shown that the transition occurs at a value, independent of composition and alloy family, of the ratio between the elastic constants associated with the two shears necessary to accomplish the lattice distortion from the bcc to the close-packed structure. The zone boundary frequency of the TA2[110] branch evaluated at the transition point (TM), weakly depends, for each family, on composition. A linear relationship between this frequency and the inverse of the elastic constant C', both quantities evaluated at TM, has been found, in agreement with the prediction of a Landau model proposed for martensitic transformations.

Synthesis, Characterization and Applications of Modified TiO2 Systems

This thesis is divided in to 9 chapters and deals with the modification of TiO2 for various applications include photocatalysis, thermal reaction, photovoltaics and non-linear optics. Chapter 1 involves a brief introduction of the topic of study. An introduction to the applications of modified titania systems in various fields are discussed concisely. Scope and objectives of the present work are also discussed in this chapter. Chapter 2 explains the strategy adopted for the synthesis of metal, nonmetal co-doped TiO2 systems. Hydrothermal technique was employed for the preparation of the co-doped TiO2 system, where Ti[OCH(CH3)2]4, urea and metal nitrates were used as the sources for TiO2, N and metals respectively. In all the co-doped systems, urea to Ti[OCH(CH3)2]4 was taken in a 1:1 molar ratio and varied the concentration of metals. Five different co-doped catalytic systems and for each catalysts, three versions were prepared by varying the concentration of metals. A brief explanation of physico-chemical techniques used for the characterization of the material was also presented in this chapter. This includes X-ray Diffraction (XRD), Raman Spectroscopy, FTIR analysis, Thermo Gravimetric Analysis, Energy Dispersive X-ray Analysis (EDX), Scanning Electron Microscopy(SEM), UV-Visible Diffuse Reflectance Spectroscopy (UV-Vis DRS), Transmission Electron Microscopy (TEM), BET Surface Area Measurements and X-ray Photoelectron Spectroscopy (XPS). Chapter 3 contains the results and discussion of characterization techniques used for analyzing the prepared systems. Characterization is an inevitable part of materials research. Determination of physico-chemical properties of the prepared materials using suitable characterization techniques is very crucial to find its exact field of application. It is clear from the XRD pattern that photocatalytically active anatase phase dominates in the calcined samples with peaks at 2θ values around 25.4°, 38°, 48.1°, 55.2° and 62.7° corresponding to (101), (004), (200), (211) and (204) crystal planes (JCPDS 21-1272) respectively. But in the case of Pr-N-Ti sample, a new peak was observed at 2θ = 30.8° corresponding to the (121) plane of the polymorph brookite. There are no visible peaks corresponding to dopants, which may be due to their low concentration or it is an indication of the better dispersion of impurities in the TiO2. Crystallite size of the sample was calculated from Scherrer equation byusing full width at half maximum (FWHM) of the (101) peak of the anatase phase. Crystallite size of all the co-doped TiO2 was found to be lower than that of bare TiO2 which indicates that the doping of metal ions having higher ionic radius into the lattice of TiO2 causes some lattice distortion which suppress the growth of TiO2 nanoparticles. The structural identity of the prepared system obtained from XRD pattern is further confirmed by Raman spectra measurements. Anatase has six Raman active modes. Band gap of the co-doped system was calculated using Kubelka-Munk equation and that was found to be lower than pure TiO2. Stability of the prepared systems was understood from thermo gravimetric analysis. FT-IR was performed to understand the functional groups as well as to study the surface changes occurred during modification. EDX was used to determine the impurities present in the system. The EDX spectra of all the co-doped samples show signals directly related to the dopants. Spectra of all the co-doped systems contain O and Ti as the main components with low concentrations of doped elements. Morphologies of the prepared systems were obtained from SEM and TEM analysis. Average particle size of the systems was drawn from histogram data. Electronic structures of the samples were identified perfectly from XPS measurements. Chapter 4 describes the photocatalytic degradation of herbicides Atrazine and Metolachlor using metal, non-metal co-doped titania systems. The percentage of degradation was analyzed by HPLC technique. Parameters such as effect of different catalysts, effect of time, effect of catalysts amount and reusability studies were discussed. Chapter 5 deals with the photo-oxidation of some anthracene derivatives by co-doped catalytic systems. These anthracene derivatives come underthe category of polycyclic aromatic hydrocarbons (PAH). Due to the presence of stable benzene rings, most of the PAH show strong inhibition towards biological degradation and the common methods employed for their removal. According to environmental protection agency, most of the PAH are highly toxic in nature. TiO2 photochemistry has been extensively investigated as a method for the catalytic conversion of such organic compounds, highlighting the potential of thereof in the green chemistry. There are actually two methods for the removal of pollutants from the ecosystem. Complete mineralization is the one way to remove pollutants. Conversion of toxic compounds to another compound having toxicity less than the initial starting compound is the second way. Here in this chapter, we are concentrating on the second aspect. The catalysts used were Gd(1wt%)-N-Ti, Pd(1wt%)-N-Ti and Ag(1wt%)-N-Ti. Here we were very successfully converted all the PAH to anthraquinone, a compound having diverse applications in industrial as well as medical fields. Substitution of 10th position of desired PAH by phenyl ring reduces the feasibility of photo reaction and produced 9-hydroxy 9-phenyl anthrone (9H9PA) as an intermediate species. The products were separated and purified by column chromatography using 70:30 hexane/DCM mixtures as the mobile phase and the resultant products were characterized thoroughly by 1H NMR, IR spectroscopy and GCMS analysis. Chapter 6 elucidates the heterogeneous Suzuki coupling reaction by Cu/Pd bimetallic supported on TiO2. Sol-Gel followed by impregnation method was adopted for the synthesis of Cu/Pd-TiO2. The prepared system was characterized by XRD, TG-DTG, SEM, EDX, BET Surface area and XPS. The product was separated and purified by column chromatography using hexane as the mobile phase. Maximum isolated yield of biphenyl of around72% was obtained in DMF using Cu(2wt%)-Pd(4wt%)-Ti as the catalyst. In this reaction, effective solvent, base and catalyst were found to be DMF, K2CO3 and Cu(2wt%)-Pd(4wt%)-Ti respectively. Chapter 7 gives an idea about the photovoltaic (PV) applications of TiO2 based thin films. Due to energy crisis, the whole world is looking for a new sustainable energy source. Harnessing solar energy is one of the most promising ways to tackle this issue. The present dominant photovoltaic (PV) technologies are based on inorganic materials. But the high material, low power conversion efficiency and manufacturing cost limits its popularization. A lot of research has been conducted towards the development of low-cost PV technologies, of which organic photovoltaic (OPV) devices are one of the promising. Here two TiO2 thin films having different thickness were prepared by spin coating technique. The prepared films were characterized by XRD, AFM and conductivity measurements. The thickness of the films was measured by Stylus Profiler. This chapter mainly concentrated on the fabrication of an inverted hetero junction solar cell using conducting polymer MEH-PPV as photo active layer. Here TiO2 was used as the electron transport layer. Thin films of MEH-PPV were also prepared using spin coating technique. Two fullerene derivatives such as PCBM and ICBA were introduced into the device in order to improve the power conversion efficiency. Effective charge transfer between the conducting polymer and ICBA were understood from fluorescence quenching studies. The fabricated Inverted hetero junction exhibited maximum power conversion efficiency of 0.22% with ICBA as the acceptor molecule. Chapter 8 narrates the third order order nonlinear optical properties of bare and noble metal modified TiO2 thin films. Thin films were fabricatedby spray pyrolysis technique. Sol-Gel derived Ti[OCH(CH3)2]4 in CH3CH2OH/CH3COOH was used as the precursor for TiO2. The precursors used for Au, Ag and Pd were the aqueous solutions of HAuCl4, AgNO3 and Pd(NO3)2 respectively. The prepared films were characterized by XRD, SEM and EDX. The nonlinear optical properties of the prepared materials were investigated by Z-Scan technique comprising of Nd-YAG laser (532 nm,7 ns and10 Hz). The non-linear coefficients were obtained by fitting the experimental Z-Scan plot with the theoretical plots. Nonlinear absorption is a phenomenon defined as a nonlinear change (increase or decrease) in absorption with increasing of intensity. This can be mainly divided into two types: saturable absorption (SA) and reverse saturable absorption (RSA). Depending on the pump intensity and on the absorption cross- section at the excitation wavelength, most molecules show non- linear absorption. With increasing intensity, if the excited states show saturation owing to their long lifetimes, the transmission will show SA characteristics. Here absorption decreases with increase of intensity. If, however, the excited state has strong absorption compared with that of the ground state, the transmission will show RSA characteristics. Here in our work most of the materials show SA behavior and some materials exhibited RSA behavior. Both these properties purely depend on the nature of the materials and alignment of energy states within them. Both these SA and RSA have got immense applications in electronic devices. The important results obtained from various studies are presented in chapter 9.

Rietveld analyses and piezoelectric properties of niobium doped bismuth titanate systems

Bi 4Ti 3- xNbxO 12 (BITNb) samples, with × ranging from 0 to 0.40 were obtained using a polymeric precursor solution. Rietveld analyses confirmed that the powders crystallize in an orthorhombic structure free of secondary phases with space group Fmmm. Raman analysis evidenced a sharp increase in the bands intensity located at 129 cm -1 and 190 cm -1 due the lattice distortion in BIT02Nb and BIT04Nb compositions. UV-vis spectra indicated that addition of niobium causes a reduction of defects in the BIT lattice due the suppression of oxygen vacancies located at BO-6 octahedral. Size and morphology of particles as well as electrical behavior of BIT ceramics were affected by addition of donor dopant. Polarization reversal was investigated by applying dc voltage through a conductive tip during the area scanning and was investigated by piezoresponse force microscopy (PFM). PFM measurements revealed a decrease in piezoelectric response with increasing Nb concentration originating from a reduced polarizability along the a-axis. High spontaneous polarization is noted for the less doped sample due the reduction of strain energy and pin charged defects after niobium addition. Copyright © 2010 American Scientific Publishers.

Charge-ordering transition in (TMTTF)2X explored via dilatometry

Charge-ordering phenomena have been highly topical over the past few years. A phase transition towards a charge-ordered state has been observed experimentally in several classes of materials. Among them, many studies have been devoted to the family of quasi-one-dimensional organic charge-transfer salts (TMTTF)2X, where (TMTTF) stands for tetramethyltetrathiafulvalene and X for a monovalent anion (X = PF6, AsF6 and SbF6). However, the relationship between the electron localization phenomena and the role of the lattice distortion in stabilizing the charge-ordering pattern is poorly documented in the literature. Here we present a brief overview of selected literature results, with emphasis placed on recent thermal expansion experiments probing the charge-ordering transition of these salts. © 2013 IOP Publishing Ltd.

Estudo da dopagem no óxido de manganês tipo espinélio nanoestruturado no desempenho de um sensor potenciométrico para íons lítio

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