Presence Of Point Defects Research Articles

Investigations on the morphological evolution of zinc oxide nanostructures and their optical properties

The morphological evolution of ZnO nanostructures grown under various argon sputtering pressures and substrate temperatures has been systematically investigated. A surface phase diagram depicting compact two-dimensional thin films to various types of one-dimensional structures as a function of pressure and temperature is constructed. Structural studies of ZnO nanostructures reveal the preferential orientation along the c-axis of a hexagonal wurtzite crystal structure with a highly crystalline nature. However, the as-grown ZnO nanostructures slightly experience compressive strain, which increases with the argon sputtering pressure. Exciton recombination and point defects have been extensively analyzed by temperature-dependent photoluminescence spectroscopy. The observation of donor-bound exciton emission and its replicas reveals the high optical quality of the nanostructures. Nevertheless, the observed visible emission indicates the presence of point defects which significantly depend on the deposition pressure. The characteristic visible emission bands at 2.28 and 2.02 eV provide strong evidence for the existence of oxygen vacancies and interstitial sites in ZnO nanostructures irrespective of growth temperature and pressure.

Electronic and elemental properties of the Cu2ZnSn(S,Se)4 surface and grain boundaries

X-ray and femtosecond UV photoelectron spectroscopy, secondary ion mass spectrometry and photoluminescence imaging were used to investigate the electronic and elemental properties of the CZTS,Se surface and its oxides. Oxide removal reveals a very Cu poor and Zn rich surface relative to bulk composition. O and Na are observed at the surface and throughout the bulk. Upward bending of the valence bands indicates the presence of negative charge in the surface region and the Fermi level is found near the band gap center. The presence of point defects and the impact of these findings on grain boundary properties will be described.

Effect of chirality and point defect on resonant characterization of single-walled boron nitride nanotube-based mass sensor

In this article, the resonant behavior of a single-walled boron nitride nanotube (SWBNNT)-based mass sensor has been analyzed considering the chiral atomic structures and the presence of point defect along the length of the nanotube. The single atom vacancies and divacancy (BN divacancy) are considered as point defects. The resonant frequency-based analysis has been performed considering a finite element model of a cantilevered single-walled boron nitride nanotube, based on molecular structural mechanics. The analysis has been performed considering the different chiral atomic structures of a single-walled boron nitride nanotube, having a chiral angle of between 0° (zigzag) and 30° (armchair). Also, along with the effect of chirality, the presence of a point defect along the length of nanotube has been analyzed as a poor atomic structure of a single-walled boron nitride nanotube. The three different positions of point defect along the length of nanotube have been considered. The obtained results indicate that, for the particular size of nanotube, as the chiral angle decreases, the atomic structure of nanotube become more closely packed and ultimately effects the resonant behavior of nanotube. The presence of a point defect and its different positions along the length of nanotube effects the overall resonant behavior of a single-walled boron nitride nanotube, which is attributed due to change in the structural stiffness of the nanotube. The reported results can be used to identify the different types of considered point defects as well as its position along the length of nanotube. The present simulation approach is found to be very effectual, incorporates the different atomic structures of a single-walled boron nitride nanotube and simulates the different boundary conditions.

A Scanning Tunneling Microscopy Study of Ultrathin Film Rutile TiO2(110) Supported on W(100)-O(2 × 1)

Ultrathin films of rutile TiO2(110) have been grown on a W(100)-O(2 × 1) surface and characterized with a combination of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). LEED shows the presence of two orthogonal rotational domains of rutile TiO2(110). In line with this, STM images reveal that the rutile TiO2 grows as discrete islands and can be aligned in either of the principal directions of the underlying substrate W(100)-O(2 × 1). High-resolution STM images reveal atomic-scale rows and the presence of point defects on the rutile islands that are characteristic of the native rutile TiO2(110)-(1 × 1).

Mechanical behavior of FCC single crystals at finite temperatures in the presence of point defects

Molecular dynamics simulations have been performed to study the effect of temperature and the presence of various types of point defects on the mechanical response of copper. Nanoindentation and uniaxial tests have been modeled to compare different loading conditions. Anisotropy effects have been also added to the study by applying the uniaxial load in various crystallographic orientations. The yield stress in all cases decreases with increased temperature for both perfect crystals and crystals with point defects. However, the temperature sensitivity of the yield stress is significantly lowered by the presence of defects in simulated nanoindentation and compression tests, while the combined effect of point defects and elevated temperature on the yield in tension is less than in the other loading conditions. The deformation mechanism which causes this effect is described, and points towards possible explanations for previously observed variations in nanomechancial testing between common experimental methods.

Pd(111)点缺陷表面乙炔加氢生成乙烯的密度泛函理论研究

本文通过密度泛函理论(DFT)研究Pd(111)表面上点缺陷对乙炔加氢生成乙烯反应的影响。计算结果表明乙炔，乙烯基和乙烯的结合能在Pd(111)和Pd(111)点缺陷表面按照Pd(111) > Pd-fd > Pd-sd的顺序递减。Pd-fd表示点缺陷在第一层，Pd-sd表示点缺陷在第二层。乙炔加氢生成乙烯基的能垒在Pd(111)表面上最大，在Pd-sd表面上最小，而乙烯基加氢生成乙烯的能垒在Pd(111)表面上最大，但是在Pd-fd表面上最小。因此Pd(111)表面上点缺陷的存在提高了乙炔加氢生成乙烯反应的活性。 The hydrogenation of acetylene to ethylene on the Pd(111) surface with point defect is explored by Density Functional Theory (DFT). The results show that the binding energies of acetylene, vinyl and ethylene on the Pd(111) and the Pd(111) with point defect are decreased as the trend of Pd(111) > Pd-fd > Pd-sd. The Pd-fd represents the point defect on the first layer, the Pd-sd represents the point defect on the second layer. The reaction barrier of acetylene hy- drogenation to vinyl is maximum on the Pd(111) surface and minimum on the Pd-sd surface, and the barrier of vinyl hydrogenation to ethylene is also maximum on the Pd(111) surface but minimum on the Pd-fd surface. The result shows that the presence of point defect in the Pd(111) surface promotes the activity of the hydrogenation of acetylene.

A density functional theory analysis for the adsorption of the amine group on graphene and boron nitride nanosheets

In this paper first principles total energy calculations to study the adsorption of amine group (NH2) on graphene (G) and boron nitride (hBN) nanosheets are developed; the density functional theory, within the local density approximation and Perdew-Wang functional was employed. The sheets were modeled with a sufficiently proved CnHm-like cluster with armchair edge. The optimized geometry was obtained following the minimum energy criterion, searching on four positions for each nanosheet: perpendicular to the carbon atom, on the hexagon, inside the hexagon and on the bridge C-C, for the G-amine interaction; and, perpendicular to the B, perpendicular to the N, on the hexagon, and inside the hexagon, for the hBN-amine interaction. A physisorption, with amine parallel to the C-C-C bond with a distance graphene-amine of 2.56 Å, was found. For the case of BN a B-N bond, with bond length equal to 1.56 Å, was found; the amine lies perpendicular to the nanosheet. When the graphene is doped with B and Al atoms a chemisorption with B-N (1.57 Å) and Al-N (1.78 Å) bonds is observed; the bond angle in the amine group is also incremented, 5.5° and 8.1°, respectively. In the presence of point defects (monovacancies) of B in the hBN-amine and C in the G-amine, there exists chemisorption, increasing the reactivity of the sheets.

Generalized index theorem for topological superconductors with Yang-Mills-Higgs couplings

We investigate an index theorem for a Bogoliubov-de Gennes Hamiltonian describing a topological superconductor with Yang-Mills-Higgs couplings in arbitrary dimensions. We find that the index of the Bogoliubov-de Gennes Hamiltonian is determined solely by the asymptotic behavior of the Higgs fields and is independent of the gauge fields. It can be nonvanishing if the dimensionality of the order parameter space is equal to the spatial dimensions. In the presence of point defects, localized zero energy states appear at the defects. The consistency of the index with the existence of zero energy bound states is examined explicitly in a vortex background in two dimensions and in a monopole background in three dimensions.

Photocatalytic water disinfection on oxide semiconductors: Part 1 – basic concepts of TiO2 photocatalysis

Water disinfection (removal of microbial agents) using sunlight is an emerging technology, which has the capacity to address the global shortage of drinking water. Therefore, intensive investigations in many laboratories aim to develop photocatalyst for water disinfection. The research is focused on titanium dioxide (TiO2), which is the most promising candidate for high performance photocatalyst able to address the commercial requirements. The present work (Part 1) considers the effect of defect disorder on semiconducting and photocatalytic properties of TiO2 (rutile) in water disinfection using solar energy. It is shown that photocatalytic properties of TiO2 in water are closely related to the light induced reactivity of TiO2 with water leading to the formation of active species, such as OH*, H2O2 and , which have the capacity to oxidise microorganisms. It is also shown that the ability of TiO2 to form the active radicals is closely associated with the presence of point defects in the TiO2 lattice and the related semiconducting properties. Therefore, photocatalytic properties of TiO2 may be modified in a controlled manner by changes in its defect disorder. Consequently, defect chemistry may be used as the framework in the development of TiO2 with controlled properties that are desired for solar water disinfection. The following work (Part 2) considers the structure of bacteria and their reactivity/photoreactivity with TiO2 in aqueous environments. Both Part 1 and 2 bring together the concepts of TiO2 photocatalysis and the concepts of microbiology in order to derive the theoretical models that are needed for the development of high performance photocatalysts for solar water disinfection.

Positron Annihilation Studies of Defects in Ti-6Al-4V Subjected to Heat Treatments and Rolling

Abstract Positron lifetime and coincidence Doppler broadening (CDBS) measurements were made on samples of Ti-6Al-4 V alloy subjected to different mechanical and thermal treatments. The results indicated the presence of point defects in moderate concentrations. The as-received sample was initially heat treated at 1343K for 30 minutes and slow-cooled to extract reference values. Even when subjected to heat treatment at 1343K and fast-quenched, the inherent defect structures did not show substantial reconfiguration in size or concentration. Yet in a separate case of heat treatment at the same temperature and duration followed by hot-rolling to 50% deformation, the deformation-induced defects were found retained in it on quenching. Soaking at the elevated temperature did not produce any significant difference. In another sample, when cold-rolling was performed after heat-treatment and fast-quenching, we found the retention followed by room-temperature migration and coalescence of the generated defects with the existing ones. The measured positron lifetimes suggested the existence of defects of smaller sizes and/or dislocations. In another sample of the alloy with identical treatments at 1248K (below the α to β transition), the positron lifetimes were slightly larger in magnitude.

Si passivation for Ge pMOSFETs: Impact of Si cap growth conditions

Ultra thin Si cap growth by Reduced Pressure Chemical Vapor Deposition on relaxed Ge substrates is detailed in this paper for Ge pMOSFET (Metal Oxide Semiconductor Field Effect Transistors) passivation purposes. A cross calibration of different measurement techniques is first proposed to perfectly monitor Si monolayers thickness deposited on Ge substrates. Different characteristics, impacting Ge pMOSFETs device performances, are next detailed for various Si cap growth processes using different Si precursors: DiChloroSilane (DCS), silane and trisilane. The critical Si thickness of plastic relaxation has been determined at 12 monolayers. Presence of point defects has been identified for very low growth temperature as 350 °C. Ge–Si intermixing, caused by a Ge segregation mechanism, is strongly reduced by the use of trisilane as Si precursor at low temperatures.

Unexpected optical response of single ZnO nanowires probed using controllable electrical contacts

Relying on combined electron-beam lithography and lift-off methods Au/Ti bilayer electrical contacts were attached to individual ZnO nanowires (NWs) that were grown by a vapor phase deposition method. Reliable Schottky-type as well as ohmic contacts were obtained depending on whether or not an ion milling process was used. The response of the ZnO NWs to ultraviolet light was found to be sensitive to the type of contacts. The intrinsic electronic properties of the ZnO NWs were studied in a field-effect transistor configuration. The transfer characteristics, including gate threshold voltage, hysteresis and operational mode, were demonstrated to unexpectedly respond to visible light. The origin of this effect could be accounted for by the presence of point defects in the ZnO NWs.

Study of depth-dependent radiation-induced defects using coherent acoustic phonon spectroscopy

ABSTRACTHere we study the effect of radiation-induced point defect distributions on the optical reflectivity signal in GaAs using coherent acoustic phonon spectroscopy. We demonstrate that the presence of point defects significantly modifies the optical response, allowing estimation of the depth-dependent defect distribution in a nondestructive and noninvasive manner. We show that the observed changes are dependent on defect-induced changes to the electronic structure, namely defect-induced band tailing of the direct 1.43eV band edge. This provides a method for subsurface investigations on the complex interaction between different defects species and optoelectronic structure.

Optical and Structural Properties of Pb and Ce Doped ${\hbox {SrHfO}}_{3}$ Powders

We bring into consideration a novel, potentially promising scintillating material, Pb-doped SrHfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . The structural and optical properties of a series of Pb-doped SrHfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> powders prepared by acetate and citrate combustion are investigated and compared to those of undoped and Ce-doped powders with the same composition. Room temperature radioluminescence studies as a function of sintering temperature show that samples sintered up to the highest final temperature (1150°C) display the highest light output. Moreover, Pb-doped SrHfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> shows a higher radioluminescence efficiency with respect to Ce-doped SrHfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , which in turn features a very fast photoluminescence decay time of 15 ns. The presence of point defects and their role in carrier trapping are studied by thermally stimulated luminescence as well as by radioluminescence as a function of temperature.

Influence of Point Defects on the Free-Radical Scavenging Capability of Single-Walled Carbon Nanotubes

The effect of point defects on the free-radical scavenging activity of armchair and zigzag single-walled carbon nanotubes (SWCNTs), through a radical adduct formation mechanism, has been studied using density functional theory calculations. SWCNTs with different vacancy (V), adatom (AA), and Stone−Wales (SW) defects have been considered, as well as their pristine partners. All the studied reactions were found to be significantly exothermic and exergonic, which supports their viability. The presence of point defects in the carbon lattice of SWCNTs is predicted to increase their free-radical scavenging activity. The AA and V point defects, involving C atoms with dangling bonds, are expected to cause a larger increase on the SWCNTs’ reactivity toward free radicals than the SW and vacancy defects without C atoms with dangling bonds. The studied Stone−Wales point defect shows the largest site-dependent effect on the free-radical scavenging activity of SWCNTs. The presence of nonpolar environments is not expected to change the proposed trends. Characteristic infrared bands in the 3300 and 900−1100 cm−1 regions have been assigned to the νO−H and νC−O vibrations of the OH radical adducts.

Growth direction and morphology of ZnO nanobelts revealed by combiningin situatomic force microscopy and polarized Raman spectroscopy

Control over the morphology and structure of nanostructures is essential for their technological applications, since their physical properties depend significantly on their dimensions, crystallographic structure, and growth direction. A combination of polarized Raman (PR) spectroscopy and atomic force microscopy (AFM) is used to characterize the growth direction, the presence of point defects and the morphology of individual ZnO nanobelts. PR-AFM data reveal two growth modes during the synthesis of ZnO nanobelts by physical vapor deposition. In the thermodynamics-controlled growth mode, nanobelts grow along a direction close to [0001], their morphology is growth-direction dependent, and they exhibit no point defects. In the kinetics-controlled growth mode, nanobelts grow along directions almost perpendicular to [0001], and they exhibit point defects.

Two channels of non-radiative recombination in InGaN/GaN LEDs

Abstract The results of low-frequency noise study in blue InGaN/GaN light emitting diodes (LEDs) allow to suppose that there are two non-radiative recombination channels in InGaN/GaN LEDs: one of them is related to the extended defect system piercing the LED active region and other is related with the presence of point defects. The generation of new defects under current density more than 10 A/cm 2 was found. This process is reversible up to operation current density of about 200 A/cm 2 .

Subtle interplay between native point defects and magnetism in ZnO:Co

Distribution of Co ions and its effect on magnetic properties of ZnO:Co in the presence of native point defects, oxygen vacancy, and interstitial hydrogen, have been investigated using first-principles density functional calculations. The study provides a fundamental theoretical understanding on the correlation between magnetism and the distribution of magnetic ions and the native point defects in the semiconducting host. Results show that Co ions have a strong tendency toward aggregation via oxygen within ab plane in the presence of point defects. The room temperature ferromagnetism observed experimentally in ZnO:Co is mainly dominated by the interstitial hydrogen rather than oxygen vacancy.

Combined polarized Raman and atomic force microscopy: In situ study of point defects and mechanical properties in individual ZnO nanobelts

We present a method, polarized Raman (PR) spectroscopy combined with atomic force microscopy (AFM), to characterize in situ and nondestructively the structure and the physical properties of individual nanostructures. PR-AFM applied to individual ZnO nanobelts reveals the interplay between growth direction, point defects, morphology, and mechanical properties of these nanostructures. In particular, we find that the presence of point defects can decrease the elastic modulus of the nanobelts by one order of magnitude. More generally, PR-AFM can be extended to different types of nanostructures, which can be in as-fabricated devices.

Point defect interaction in tranversally isotropic ferroelectrics

AbstractFunctional materials, especially ferroelectrics are used in many devices like actuators, sensors and electronic devices. Due to high amounts of mechanical and electrical load cycles, fatigue phenomena may occur. This so called electric fatigue causes a decrease of the electromechanical coupling capability. It is assumed, that the ability to switch polarisation states, which is the reason for the ferroelectric effect, is decreased in the presence of point defects. These defects are ionic and electronic charge carriers, which can interact with each other, with microstructural elements in the bulk and with interfaces. Accumulation of defects can primarily lead to degradation, because of the loss of polarisation switchability.The interaction of defects in the bulk is simulated to get a better understanding of the defect accumulation processes. A model based on configurational forces can be used to obtain thermodynamic consistent kinetic laws. The material used is transversally isotropic and modelled with linear electromechanical coupling. The focus is on the influence of this material anisotropy on the defect interaction. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)