Surface Structural Defects Research Articles

Study on Structure and Magnetization of Spinel Ferrites

NiFe2O4, ZnFe2O4 and Ni0.64Zn0.36Fe2O4 were prepared by a chemical co-precipitation method. The positive ferrite ZnFe2O4 usually exhibits antiferromagnetism. The magnetization of Ni0.64Zn0.36Fe2O4 is much larger than that of NiFe2O4, which can be explained by doping with nonmagnetic cations or cations with weak magnetization in an inverse spinal ferrite if these cations occupy A sites, the magnetic moment of Fe3+ ions at the B sites would be oriented in the same direction. Experimental values found for the molecular magnetic moments of NiFe2O4 and Ni0.64Zn0.36Fe2O4 were lower than their theoretic values because surface structure defects resulted in a magnetic spin glass structure.

Influence of post-treatment on the corrosion resistance of PEO coated AM50B and AM60B Mg alloys

Magnesium alloys are highly reactive and require protection for their successful applications. In this work, coatings of 10 μm and 25 μm nominal thicknesses were prepared by Plasma Electrolytic Oxidation on AM50B and AM60B alloys. The effect of subsequent post-PEO treatments such as alkaline phosphate, alkaline silicate and sol–gel sealing was studied on coatings with 10 μm thickness. The microstructure and composition of the PEO coatings and post-treated coatings were analysed by Scanning Electron Microscope and Energy Dispersive X-ray Spectroscopy. Phase analysis of the coatings was carried out using X-ray Diffraction method. Potentiodynamic polarization method was used to evaluate the corrosion behaviour of these coatings in 3.5% NaCl solution. Electrochemical corrosion results indicated that the sealed PEO coatings of 10 μm thickness showed better corrosion resistance than the unsealed PEO coatings of 25 μm thickness. The structural surface defects of the PEO coatings were sealed by post-treatments resulting in decreases in surface roughness and open pores. Sol–gel post-treatment provided the most effective sealing to the PEO coatings.

Spectroscopic ellipsometry of SrTiO3 crystals applied to antiferrodistortive surface phase transition

AbstractThis work is devoted to the ellipsometric study of antiferrodistortive (AFD) $O_{h}^{1} $ → $D_{4h}^{18} $ cubic‐to‐tetragonal phase transition (PT) of SrTiO3 surface. Strong influence of surface defect structure on magnitude and temperature evolutions of surface refractive index related to PT was found and investigated. It is shown that even small surface imperfections result in enhancement and strong changes of the surface refractive index when approaching the temperature of PT. This effect is caused by emergence and evolutions in the surface of the structural changes corresponding to order parameter at the temperatures sufficiently higher than transition temperature in the bulk. In the case of structurally perfect crystal surface, the features of the temperature dependence of surface refractive index appeared to be very small and visible at the temperatures a little smaller than transition temperature for bulk that agrees well with predictions of Kaganov–Omel'yanchuk theory.

Effect of Ridge Growth on Wafer Bowing and Light Extraction Efficiency of Vertical GaN-Based Light-Emitting Diodes

In this paper we report on the selective area growth (SAG) of vertical GaN-based light-emitting diodes (LEDs) by low-pressure metal-organic chemical vapor deposition (MOCVD). SAG, under optimized growth conditions, leads to ridge-shaped epilayers with a smooth top surface, devoid of any surface defect structures. The final ridge-shaped vertical LED structures, after the removal of the sapphire substrate by laser lift-off (LLO), exhibit a smaller bowing effect than conventional vertical LED structures. The suppression of lateral strain in the epilayers is responsible for the smaller bowing effect because of the reduction in lateral dimensions. Consequently, the use of SAG LEDs can achieve a 21% higher light output power than conventional vertical LEDs, indicating a significant improvement in light extraction efficiency due to the light guiding pathways offered by the ridge-shaped geometry of the LED structures.

Polyamide‐12/Functionalized Carbon Nanofiber Composites: Evaluation of Thermal and Mechanical Properties

AbstractThis work aims at improving the interfacial bonding between polyamide‐12 and CNFs. CNFs were oxidized and dispersed in polyamide‐12 giving rise to polymer nanocomposites. The oxidation caused an increase in the specific surface area and structural defects of the fibers, as indicated by surface area and Fourier‐transform Raman spectroscopy. The nanocomposites exhibited improved thermal and thermo‐oxidative stabilities. The oxidized nanofibers had marginal effect on the crystallinity and crystallization of the polyamide‐12. An over‐proportional enhancement of stiffness due to the fibers could be achieved. In spite of these improvements the fiber/polymer adhesion should be further improved.magnified image

CO Prefers the Aisle Seat

Carbon monoxide molecules can alter the catalytically important defect structure of platinum surfaces.

Structural changes of SiO2 glass by mechanical milling

Structural changes of ground SiO2 glass were investigated using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy. SiO2 glass samples were ground in a ball mill for up to 500 h. Reduction in the particle size of ground SiO2 glass samples was observed up to ∼ 80 h of milling. After 80 h of milling, the particle size of ground SiO2 glass remained almost constant at approximately 3 μm. The first sharp diffraction peak (FSDP) position (q1) of the XRD pattern for ground SiO2 glass shifted from 1.500 (original) to 1.544 (500 h of milling) A-1. The variation may be attributed to a change of the intermediate-range structure. The FSDP position of the 500 h ground SiO2 glass returned to the original value after annealing at 1000 °C. This annealing behavior includes two components. The variation in q1 for annealing up to ∼ 500 °C is small (0.0087 A-1) and may be attributed to an increase of the Si-O-Si angle. However, for annealing at 500-1000 °C, a large variation was observed (0.0350 A-1) and it may be attributed to a decrease of 3-membered rings. IR spectra of samples milled up to ∼ 80 h revealed sharpening of the ν = 1100 cm-1 band (Si-O stretching mode) and a decrease in the absorption of the ν = 800 cm-1 band (Si-O-Si bending mode). The IR analyses indicate the shrinkage of Si-O-Si linkages by milling. For over 80 h of milling, these variations are small. The particle size remains almost constant for over 80 h of milling, and therefore, only structural changes in a particle would be responsible for the changes in the IR spectra. The Raman spectra exhibited an increase in the luminescence intensity for milling up to ∼ 40 h. The luminescence may result from surface defect structures on SiO2 glass grains, and the increase in the luminescence intensity is consistent with particle size reduction by milling.

Surface Functionalization of WS2 Fullerene-like Nanoparticles

WS(2) belongs to a family of layered metal dichalcogenide compounds that are known to form cylindrical (inorganic nanotubes-INT) and polyhedral nanostructures--onion or nested fullerene-like (IF) particles. The outermost layers of these IF nanoparticles can be peeled under shear stress, thus IF nanoparticles have been studied for their use as solid lubricants. However, the IF nanoparticles tend to agglomerate, presumably because of surface structural defects induced by elastic strain and curvature, a fact that has a deleterious effect on their tribological properties. In the present work, chemical modification of the IF-WS(2) surface with alkyl-silane molecules is reported. The surface-modified IF nanoparticles display improved dispersion in oil-based suspensions. The alkyl-silane coating reduces the IF-WS(2) nanoparticles' tendency to agglomerate and consequently improves the long-term tribological behavior of oil formulated with the IF additive.

Lineshape analysis of Raman scattering from LO and SO phonons in III-V nanowires

Micro-Raman spectroscopy is employed to study the phonon confinement in Au- and Mn-catalyzed GaAs and InAs nanowires. The phonon confinement model is used to fit the LO phonon peaks, which also takes into account the contribution to the asymmetry of the line shape due to the presence of surface optical (SO) phonons and structural defects. This also allows us to determine the correlation lengths in these wires, that is the average distance between defects and the defect density in these nanowires. Influence of these defects on the SO phonon is also investigated. A good agreement between the experimental results and the calculations for the SO phonon mode by using the dielectric continuum model is also obtained.

Correlation Study of Morphology, Electrical Activation and Contact formation of Ion Implanted 4H-SiC

This paper reports a detailed study of the electrical activation and the surface morphology of 4H-SiC implanted with different doping ions (P for n-type doping and Al for p-type doping) and annealed at high temperature (1650–1700 °C) under different surface conditions (with or without a graphite capping layer). The combined use of atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning capacitance microscopy (SCM) allowed to clarify the crucial role played by the implant damage both in evolution of 4H-SiC surface roughness and in the electrical activation of dopants after annealing. The high density of broken bonds by the implant makes surface atoms highly mobile and a peculiar step bunching on the surface is formed during high temperature annealing. This roughness can be minimized by using a capping layer. Furthermore, residual lattice defects or precipitates were found in high dose implanted layers even after high temperature annealing. Those defects adversely affect the electrical activation, especially in the case of Al implantation. Finally, the electrical properties of Ni and Ti/Al alloy contacts on n-type and p-type implanted regions of 4H-SiC were studied. Ohmic behavior was observed for contacts on the P implanted area, whilst high resistivity was obtained in the Al implanted layer. Results showed a correlation of the electrical behavior of contacts with surface morphology, electrical activation and structural defects in ion-implanted, particularly, Al doped layer of 4H-SiC.

Analysis of Quasi-Periodic Structural Defects in H:LiNbO3 Layers

Quasi-periodic structural defects on X-cut lithium niobate proton exchange waveguides surface were investigated. The structural defects are indicated as parallel bulge strips protruding above the surface up to 70 nm. It was found that under certain conditions proton exchange in pure benzoic and succinic acids leads to the formation of lamellar phase inclusions with crystal lattice deformation from 9.6 · 10 −3 to 22.0 · 10 −3 in waveguide layer, which induce the arising of the surface structural defects. It was proved that interstitial protons play a dominant role in a formation of these phase inclusions.

Dissolution of Biogenic and Synthetic UO2 under Varied Reducing Conditions

The chemical stability of biogenic UO2, a nanoparticulate product of environmental bioremediation, may be impacted by the particles' surface free energy, structural defects, and compositional variability in analogy to abiotic UO(2+x) (0 < or = x < or = 0.25). This study quantifies and compares intrinsic solubility and dissolution rate constants of biogenic nano-UO2 and synthetic bulk UO2.00, taking molecular-scale structure into account. Rates were determined under anoxic conditions as a function of pH and dissolved inorganic carbon in continuous-flow experiments. The dissolution rates of biogenic and synthetic UO2 solids were lowest at near neutral pH and increased with decreasing pH. Similar surface area-normalized rates of biogenic and synthetic UO2 suggest comparable reactive surface site densities. This finding is consistent with the identified structural homology of biogenic UO2 and stoichiometric UO2.00 Compared to carbonate-free anoxic conditions, dissolved inorganic carbon accelerated the dissolution rate of biogenic UO2 by 3 orders of magnitude. This phenomenon suggests continuous surface oxidation of U(IV) to U(VI), with detachment of U(VI) as the rate-determining step in dissolution. Although reducing conditions were maintained throughout the experiments, the UO2 surface can be oxidized by water and radiogenic oxidants. Even in anoxic aquifers, UO2 dissolution may be controlled by surface U(VI) rather than U(IV) phases.

Surface structure and structural point defects of liquid and amorphous aluminosilicatenanoparticles

The surface structure of liquid and amorphous aluminosilicate nanoparticles of compositionAl2O3·2SiO2 has been investigated in a model of different sizes ranging from 2.0 to 5.0 nm withthe Born–Mayer type pair potential under non-periodic boundary conditions.Models have been obtained by cooling from the melts at a constant density of2.6 g cm−3 via molecular dynamics (MD) simulation. The surface structure has been investigated viathe coordination number, bond-angle distributions and structural point defects.Calculations show that surface effects on surface static and thermodynamic properties ofmodels are significant according to the change in the number of Al atoms in the surfacelayers. Evolution of the local environment of oxygen in the surface shell of nanoparticlesupon cooling from the melt toward the glassy state was also found and discussed.In addition, the nanosize dependence of the glass transition temperature waspresented.

STUDY OF ESR, DEFECT STRUCTURE AND SURFACE MORPHOLOGY OF Cu2+ IN CdS CRYSTALS

Cu -doped Cadmium sulfide ( CdS ) films were prepared by chemical bath deposition method. Various quantities of Cu were used for mixing. Surface characterization, structure, morphology and defect structures of the films were studied. The structure of all Cu -doped CdS films was a unique CdS cubic phase. The grain size of CdS was decreased when doped with Cu . The F-type defect in the undoped CdS is clearly confirmed by an ESR signal arising from the hyperfine interaction of electron spin (S=1/2) trapped in sulfur vacancies and neighboring cadmium nuclear spin (I=1/2). In the Cu -doped CdS films, the ESR peaks shift towards low fields as the copper concentration is increased. This is due to the change in crystal field experience by Cu 2+ (3 d 9) ions while the Cd 2+ signal disappears. The Cu 2+ ions in the Cd 1-x Cu x S itself are ESR silent due to a very short relaxation time. The dark resistance increased with increasing amount of Cu concentrations up to about 0.03 M. This 0.03 M Cu -doped CdS sample also possesses the maximum photosensitivity.

Surface exfoliation and defect structures in Si induced by 160 keV He and 110 keV H ion implantation

Cz n-type Si(100) wafers were implanted at room temperature with 160keV He ions at a fluence of 5×1016/cm2 and 110keV H ions at a fluence of 1×1016/cm2, singly or in combination. Surface phenomena and defect microstructures have been studied by various techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM). Surface exfoliation and flaking phenomena were only observed on silicon by successive implantation of He and H ions after subsequent annealing at temperatures above 400°C. The surface phenomena show strong dependence on the thermal budget. At annealing temperatures ranging from 500 to 700°C, craters with size of about 10μm were produced throughout the silicon surface. As increasing temperature to 800°C, most of the implanted layer was sheared, leaving structures like islands on the surface. AFM observations have demonstrated that the implanted layer is mainly transfered at the depth around 960nm, which is quite consistent with the range of the ions. XTEM observations have revealed that the additional low fluence H ion implantation could significantly influence thermal growth of He-cavities, which gives rise to a monolayer of cavities surrounded by a large amount of dislocations and strain. The surface exfoliation effects have been tentatively interpreted in combination of AFM and XTEM results.

The influence of the H2∕Ar ratio on surface morphology and structural defects in homoepitaxial 4H-SiC films grown with methyltrichlorosilane

Characterization of surface morphology and crystal defects is reported for homoepitaxial 4H-SiC films grown at high rates (35–40μm∕h) using methyltrichlorosilane (CH3SiCl3), as single precursor. The ratio of hydrogen to argon (H2∕Ar) in the carrier gas was varied to determine the effect of hydrogen on the surface morphology and the crystalline defects. Due to hydrogen’s reaction with the graphite heater, adjusting the H2∕Ar ratio effectively changed the C∕Si ratio in the gas phase; thereby, influencing surface roughness and dislocation density. Low H2∕Ar ratios of 0.1 and 0.125 produced smooth surfaces without step bunching. Higher H2∕Ar ratios of 0.2 and 0.33 enhanced the conversion of basal plane dislocations into threading edge dislocations and reduced the density of basal plane dislocations to approximately 600cm−2. However, at these H2∕Ar ratios, macrosteps formed on the surface and the roughness increased. Micropipes from substrate dissociated into closed-core threading screw dislocations in the films grown with H2∕Ar ratio in the range of 0.1–0.2. At H2∕Ar ratio of 0.33, micropipes propagated into the film, generating hollow-core threading screw dislocations.

A high-resolution transmission electron microscopy study of defects in γ-Al2O3 nanorods

Surface and planar defect structures of γ-Al2O3 nanorods synthesized by the arc-discharge method were studied by means of high-resolution transmission electron microscopy and image simulation. Our investigation showed that there was a high number density of twins in the nanorods. We suggested a possible configuration of {111} twins in γ-Al2O3, and this model fit our experimental result well. In some nanorods, the ordering of nanotwins gave rise to a local hexagonal-like structure. The twinned nanorods were usually enclosed by {100} and {111} facets, and their growth direction was changed from 〈110〉 into 〈111〉. The surface structures of the nanorods confirmed that the {111}-type surface should be more stable.

Corrosive attack of glass by a pharmaceutical compound

Glass delamination, or the generation of glass flakes, continues to be an unwanted occurrence in the manufacture of parenteral (injectable) solutions and suspensions. In this root cause analysis study, advanced analytical tools including atomic force microscopy, environmental scanning electron microscopy, quantitative image analysis, and dynamic secondary ion mass spectroscopy (D-SIMS) showed significant differences in glass characteristics and performance. By observing the size and spatial arrangement of defects found on the interior surface of vials used as primary packaging for these products, in conjunction with the chemical changes that can occur to the glass because of product contact, a considerable amount of insight can be obtained into this phenomenon. Elemental depth profiling obtained by D-SIMS revealed that the interior vial surface was significantly altered by the presence of the parenteral solution, while another vial (manufactured by another vendor) was not. Although significant chemical changes can occur to the glass, the surface defect structure appears to be the dominant factor controlling the generation of glass flakes.

Charged point defects in semiconductors

Abstract Native point defects control many aspects of semiconductor behavior. Such defects can be electrically charged, both in the bulk and on the surface. This charging can affect numerous defect properties such as structure, thermal diffusion rates, trapping and recombination rates for electrons and holes, and luminescence quenching rates. Charging also introduces new phenomena such as nonthermally photostimulated diffusion, thereby offering distinctive mechanisms for defect engineering. The present work incorporates the first comprehensive account of semiconductor defect charging, identifying correspondences and contrasts between surfaces and the bulk as well as among semiconductor classes (group IV, groups III–V, and metal oxides). For example, small lattice parameters, close-packed unit cells, and basis atoms with large atomic radii all inhibit the formation of ionized interstitials and antisites. The charged defects that exist in III–V and oxide semiconductors can be predicted with surprising accuracy from the chemical potential and oxygen partial pressure of the ambient. The symmetry-lowering relaxations, formation energies, and diffusion mechanisms of bulk and surface defect structures often depend strongly on charge state with similar qualitative behavior, although for a given material surface defects do not typically take on the same configurations or range of stable charge states as their counterparts in the bulk.

Investigation of surface defect structure originating in dislocations in AlGaN/GaN epitaxial layer grown on a Si substrate

Pit arrays forming a network structure were observed by an atomic force microscopy (AFM) on an AlGaN surface of AlGaN/GaN heterostructure on a Si(1 1 1) substrate. In order to clarify the origin of these pit arrays, AlGaN/GaN layers were investigated using a transmission electron microscopy (TEM). As a result, it was confirmed that pit arrays of the surface observed by an AFM represent the surface termination of edge dislocations formed at the small-angle boundaries of slightly misoriented crystal domains. The difference in buffer layer structures formed on Si substrates affected the surface pits arrangement. It was also confirmed that the optimization of a buffer structure on a Si substrate is very effective for the reduction of the pit density.