Point Defect Sites Research Articles

Atomic Gradient Structure Alters Electronic Structure in 3D across the Bulk and Enhances Photoactivity

AbstractUnderstanding structure–function relationships enables the design of materials with tailored functionalities. The long‐standing challenge is to design materials with high active volume to improve efficiency. Tailored grain boundaries and lattice defects are traditionally used to tune the electronic structure near interfaces or defects to promote electron and hole separation. However, the active volume of point defect sites or interfaces in these traditional photocatalysts is extremely low. This study reports a structure with continuous atomic positional deformation across the bulk, altering the electronic structure in 3D and creating a significantly higher active volume. Such a structure in anatase is obtained and tuned by phase transformation during the heating process. Transmission electron microscopy and density functional theory results reveal that atomic deformations result in continuous band bending across the particles, facilitating electron–hole separation, inhibiting their recombination, and inducing dramatically enhanced photoactivity. These findings enable a different materials design paradigm that can potentially be harnessed for a broad range of applications.

Spatially resolved photoluminescence brightening in individual single-walled carbon nanotubes

Single-molecule microscopy was used to image photoluminescence (PL) brightening of individual sodium-dodecyl-sulfate (SDS)-wrapped single-walled carbon nanotubes (SWCNTs) upon the addition of dithiothreitol (DTT). PL enhancement varied for each nanotube (NT), with some brightening by 16% and others by a factor of about 7. Interestingly, NTs that displayed lower initial QY values showed the largest increases in PL enhancement. SDS-SWCNTs longer than the diffraction limit were studied in order to spatially resolve the brightening phenomenon. Quite unexpectedly, a uniform, single-step PL brightening along the NT was consistently observed, suggesting that the PL enhancement is the result of a non-localized process. The even PL brightening seen over SWCNTs that are micrometers long implies that single point defect sites, which are known to be largely responsible for exciton nonradiative decay, play no significant role in the brightening process. Interestingly, affixing the SWCNT strongly to the substrate surface mitigated the PL brightening response, consistent with a hypothesis that surfactant reorganization upon the addition of DTT is responsible for exciton PL brightening.

Prevalence of oxygen defects in an in-plane anisotropic transition metal dichalcogenide

Atomic scale defects in semiconductors enable their technological applications and realization of novel quantum states. Using scanning tunneling microscopy and spectroscopy complemented by ab-initio calculations we determine the nature of defects in the anisotropic van der Waals layered semiconductor ReS$_2$. We demonstrate the in-plane anisotropy of the lattice by directly visualizing chains of rhenium atoms forming diamond-shaped clusters. Using scanning tunneling spectroscopy we measure the semiconducting gap in the density of states. We reveal the presence of lattice defects and by comparison of their topographic and spectroscopic signatures with ab initio calculations we determine their origin as oxygen atoms absorbed at lattice point defect sites. These results provide an atomic-scale view into the semiconducting transition metal dichalcogenides, paving the way toward understanding and engineering their properties.

Electrical Transport and Thermoelectric Properties of SnSe-SnTe Solid Solution.

SnSe is considered as a promising thermoelectric (TE) material since the discovery of the record figure of merit (ZT) of 2.6 at 926 K in single crystal SnSe. It is, however, difficult to use single crystal SnSe for practical applications due to the poor mechanical properties and the difficulty and cost of fabricating a single crystal. It is highly desirable to improve the properties of polycrystalline SnSe whose TE properties are still not near to that of single crystal SnSe. In this study, in order to control the TE properties of polycrystalline SnSe, polycrystalline SnSe–SnTe solid solutions were fabricated, and the effect of the solid solution on the electrical transport and TE properties was investigated. The SnSe1−xTex samples were fabricated using mechanical alloying and spark plasma sintering. X-ray diffraction (XRD) analyses revealed that the solubility limit of Te in SnSe1−xTex is somewhere between x = 0.3 and 0.5. With increasing Te content, the electrical conductivity was increased due to the increase of carrier concentration, while the lattice thermal conductivity was suppressed by the increased amount of phonon scattering. The change of carrier concentration and electrical conductivity is explained using the measured band gap energy and the calculated band structure. The change of thermal conductivity is explained using the change of lattice thermal conductivity from the increased amount of phonon scattering at the point defect sites. A ZT of ~0.78 was obtained at 823 K from SnSe0.7Te0.3, which is an ~11% improvement compared to that of SnSe.

Spin-dependent electron scattering in cobalt interconnects

We use state-of-the-art ab initio simulation methods to study fundamental electron scattering mechanisms in cobalt conductors for applications in advanced interconnect technology. In particular, we consider electron scattering at intrinsic defect sites, twin grain boundaries, and at the Co/metal/Co interfaces present in vertical interconnects. Effective resistivity values and reflection coefficients are calculated in each case. The explicit treatment of electron spin results in distinct majority and minority spin behavior, with majority spin states exhibiting less scattering than minority states. Our results indicate that grain boundary scattering dominates over scattering at intrinsic point defect sites. Vertical resistance calculations indicate that Co vias can have substantially lower resistance than corresponding Cu vias at comparable dimensions. These results help build a fundamental understanding of electron scattering mechanisms in non-Cu conductors for advanced interconnect applications.

Fission gas in thoria

The fission gases Xe and Kr, formed during normal reactor operation, are known to degrade fuel performance, particularly at high burn-up. Using first-principles density functional theory together with a dispersion correction (DFT + D), in ThO2 we calculate the energetics of neutral and charged point defects, the di-vacancy (DV), different neutral tri-vacancies (NTV), the charged tetravacancy (CTV) defect cluster geometries and their interaction with Xe and Kr. The most favourable incorporation point defect site for Xe or Kr in defective ThO2 is the fully charged thorium vacancy. The lowest energy NTV in larger supercells of ThO2 is NTV3, however, a single Xe atom is most stable when accommodated within a NTV1. The di-vacancy (DV) is a significantly less favoured incorporation site than the NTV1 but the CTV offers about the same incorporation energy. Incorporation of a second gas atom in a NTV is a high energy process and more unfavourable than accommodation within an existing Th vacancy. The bi-NTV (BNTV) cluster geometry studied will accommodate one or two gas atoms with low incorporation energies but the addition of a third gas atom incurs a high energy penalty. The tri-NTV cluster (TNTV) forms a larger space which accommodates three gas atoms but again there is a penalty to accommodate a fourth gas atom. By considering the energy to form the defect sites, solution energies were generated showing that in ThO2−x the most favourable solution equilibrium site is the NTV1 while in ThO2 it is the DV.

Large-area nanopatterned graphene for ultrasensitive gas sensing

Chemical vapor deposited (CVD) graphene is nanopatterned using a spherical block copolymer etch mask. The use of spherical rather than cylindrical block copolymers allows homogeneous patterning of cm-scale areas without any substrate surface treatment. Raman spectroscopy was used to study the controlled generation of point defects in the graphene lattice with increasing etching time, confirming that alongside the nanomesh patterning, the nanopatterned CVD graphene presents a high defect density between the mesh holes. The nanopatterned samples showed sensitivities for NO2 of more than one order of magnitude higher than for non-patterned graphene. NO2 concentrations as low as 300 ppt were detected with an ultimate detection limit of tens of ppt. This is the smallest value reported so far for non-UV illuminated graphene chemiresistive NO2 gas sensors. The dramatic improvement in the gas sensitivity is believed to be due to the high adsorption site density, thanks to the combination of edge sites and point defect sites. This work opens the possibility of large area fabrication of nanopatterned graphene with extremely high densities of adsorption sites for sensing applications.

Enhanced Gas Adsorption on Graphitic Substrates via Defects and Local Curvature: A Density Functional Theory Study

Using van-der-Waals-corrected density functional theory calculations, we explore the possibility of engineering the local structure and morphology of high-surface-area graphene-derived materials to improve the uptake of methane and carbon dioxide for gas storage and sensing. We test the sensitivity of the gas adsorption energy to the introduction of native point defects, curvature, and the application of strain. The binding energy at topological point defect sites is inversely correlated with the number of missing carbon atoms, causing Stone–Wales defects to show the largest enhancement with respect to pristine graphene (∼20%). Improvements of similar magnitude are observed at concavely curved surfaces in buckled graphene sheets under compressive strain, whereas tensile strain tends to weaken gas binding. Trends for CO2 and CH4 are similar, although CO2 binding is generally stronger by ∼4 to 5 kJ mol–1. However, the differential between the adsorption of CO2 and CH4 is much higher on folded graphene sheet...

Single Atom Microscopy

We show that aberration-corrected scanning transmission electron microscopy operating at low accelerating voltages is able to analyze, simultaneously and with single atom resolution and sensitivity, the local atomic configuration, chemical identities, and optical response at point defect sites in monolayer graphene. Sequential fast-scan annular dark-field (ADF) imaging provides direct visualization of point defect diffusion within the graphene lattice, with all atoms clearly resolved and identified via quantitative image analysis. Summing multiple ADF frames of stationary defects produce images with minimized statistical noise and reduced distortions of atomic positions. Electron energy-loss spectrum imaging of single atoms allows the delocalization of inelastic scattering to be quantified, and full quantum mechanical calculations are able to describe the delocalization effect with good accuracy. These capabilities open new opportunities to probe the defect structure, defect dynamics, and local optical properties in 2D materials with single atom sensitivity.

On the role of oxygen vacancies, aliovalent ions and lattice strain in the in vivo wear behavior of alumina hip joints

We have visualized at the nanometer scale the topological, chemical and mechanical characteristics of long-term in vivo exposed bearing surfaces of femoral heads made of monolithic alumina. Four self-mated alumina retrievals were studied, which were exposed in the human body for relatively long periods of time ranging between 7.7 and 10.7 yrs. Besides conventional morphological features, monitored by atomic force microscopy, the topographic distributions of point defects and lattice strain on the surface of the heads were systematically probed by collecting high spatially and spectrally resolved cathodoluminescence spectra from zones of different wear severity. Three types of optically active point-defect site could be detected: (i) oxygen vacancies; (ii) substitutional (aliovalent) cations; and, (iii) interstitial aluminum cations. These luminescent sites represent the main defects progressively developed in the alumina lattice during exposure in human hip joints. A clear evolution toward (environmentally driven) off-stoichiometry was found with progressing wear. Moreover, the shallow electro-stimulated optical probe also detailed the presence of lattice strain fields (of both elastic and plastic nature) stored in the very neighborhood of the bearing surface. The present spectroscopic characterizations enable substantiating important tribochemical interactions between bearing surfaces and in vivo environment as pivotal parts of progressive events of wear degradation.

Short-period (Si14 / Si0.75 Ge0.25)20 Superlattices for the Growth of High-quality Si0.75 Ge0.25

AbstractIn the present experiment, we have grown 2500-Å thick Si0.75Ge0.25 alloy layers on Si(001) substrate by MBE process using a short-period (Si14/Si0.75Ge0.25)20 superlattice (SL) as buffer layers. In the SL layers, first a layer of 14 monolayers (MLs) of Si (thickness about 20Å) then a thin layer of Si0.75Ge0.25 (thickness 5-6Å) were grown. This Si/(Si0.75Ge0.25) bilayers were repeated for 20 times. The buffer layers were grown at different temperatures from 300-400°C and the alloy layers were then grown at 500°C on the buffer layers. The alloy layer showed low residual strain (about -0.16%) and smooth surface (rms roughness ~15Å) with 300°C grown SL buffer. Low temperature growth of Si in SL layer introduces point defects and low temperature growth of Si1-xGex in SL layer reduces the Ge segregation length, which leads to strained SL layer formation. Strained layers are capable to make barrier for the propagation of threading dislocations and point defect sites can trap the dislocations.

Arsenic Oxide Microcrystals in Anodically Processed GaAs Electrochemical Growth, Spectroscopy, and Morphology

Anodic etching of GaAs wafers in concentrated HCl produces many microcrystals (1–50 μm in size) on the GaAs substrate. The etching process and the nature of the luminescence from these GaAs surfaces have been explored in detail. The etch pits are initiated at point defect sites, not extended defects. High HCl concentration, high current density, as well as long etching times facilitate microcrystal growth. Polarization‐dependent Raman spectroscopy identified the microcrystals and accessed symmetry changes of the GaAs surface induced by the electrochemical processing. Raman signals from quantum‐size effect confined phonons in GaAs were not observed. Far‐field photoluminescence of such samples shows a strong emission band centered around 540 nm. Spatially resolved spectroscopy and imaging (cathodoluminescence and near‐field scanning optical microscopy) unambiguously indicate that the 540 nm emission comes from many weak emitters . A few strong emitters originating from amorphous impurity inclusions are also identified in samples prepared in a macor cell. © 2000 The Electrochemical Society. All rights reserved.

Supercell Model of V-Doped TiO2: Unrestricted Hartree-Fock Calculations

The host crystal space group and the point defect site symmetry group being given the procedure of generation of the imperfect space group in the supercell or periodic defect model is considered. The approach developed is applied to the symmetry analysis of the possible space groups of metal-doped rutile and anatase structures in the supercell model. The results of unrestricted Hartree-Fock calculations of the V-doped rutile electronic structure (an impurity atom charge, valence states, an impurity level position in the energy gap) are given. Supercells of 36, 60, 84 atoms chosen on the base of the symmetry analysis are calculated. The convergence of the results with the supercell increasing is demonstrated.

Supercell Model of V-Doped TiO2: Unrestricted Hartree-Fock Calculations

The host crystal space group and the point defect site symmetry group being given the procedure of generation of the imperfect space group in the supercell or periodic defect model is considered. The approach developed is applied to the symmetry analysis of the possible space groups of metal-doped rutile and anatase structures in the supercell model. The results of unrestricted Hartree-Fock calculations of the V-doped rutile electronic structure (an impurity atom charge, valence states, an impurity level position in the energy gap) are given. Supercells of 36, 60, 84 atoms chosen on the base of the symmetry analysis are calculated. The convergence of the results with the supercell increasing is demonstrated.

Vibrational dynamics associated with multiple elastic configurations in KI: Ag +

A systematic experimental investigation of the fcc lattice-defect system KI: Ag + shows that the thermally activated disappearance of the strengths of the IR and Raman active vibrational modes and the appearance of rf dispersion and frequency-shifted UV electronic transitions are associated with the Ag + ion moving from an on-center to an off-center configuration. The possibility that an impurity/intrinsic-localized-mode complex could dissociate to produce the rapid temperature dependence has been tested but the absence of a concentration dependence for the normalized vibrational mode strengths with temperature appears to rule out this trapping model. A jump-rotational-diffusion model can describe the observed results in terms of two elastic configurations separated from each other by a large energy barrier. The impurity jumps between the configurations with temperature dependent dwell times. The physics behind multiple elastic configurations existing at a single point defect site is still unclear.

Intrinsic Point Defects on a TiO2(110) Surface and Their Reaction with Oxygen: A Scanning Tunneling Microscopy Study

ABSTRACTThe interaction of molecular oxygen, at room temperature, with a reduced TiO2(110) surface has been studied in situ by scanning tunneling microscopy (STM). Oxygen vacancies (point defects) were created on a clean TiO2(110) surface by annealing in ultra-high vacuum and successfully imaged on the atomic scale. These point defect sites were stable under ultrahigh vacuum conditions. During exposure to molecular oxygen, new point defects appear at different locations on the surface although their overall number is reduced. A mechanism for this dynamic healing process is proposed.

Site related nucleation and growth of hydrides on uranium surfaces

The characteristics of hydride nucleation and growth on certain surfaces of pure uranium and of U-0.1 wt.% Cr samples were studied (under I atm H, at temperatures of 50–75 C) using the hot-stage microscope, microprobe analyzer and atomic force microscope techniques. Different preparation procedures of the samples were applied, in order to check the effects of surface oxidation layer variations on the nucleation and growth characteristics. Four families of hydride nuclei, differing in density, size and growth rates, were observed and classified. The smalles and most dense is in the form of submicron blisters formed instantaneously along mechanical polishing scratches. A larger (1 – 10 μm) blister-like family is formed beneath the oxide at point defect sites (but not discontinuities of the oxide), growing very slowly probably due to the compression of the coating oxide layer. The third family is characterized by preferential nucleation and rapid growth around carbide inclusions due to the discontinuity in the oxide at the carbide/oxide interface. The fourth family is found only on the samples having a thick oxide layer, and is characterized by a rapid growth rate, but is not located around inclusions. In this case, the nuclei probably originate at some other oxide discontinuities, such as twins or grain boundaries.

Failure mechanisms of anodized aluminum parts used in chemical vapor deposition chambers

Anodized aluminum parts used in chemical vapor deposition (CVD) reactor chambers for TEOS (tetra ethyl ortho silicate), passivation, and tungsten process clean chemistries were analyzed using scanning electron microscopy and energy-dispersive spectroscopy x-ray maps. Examination reveals that the primary cause of failure for all anodized Al parts is the same while individual mechanisms vary depending on the process chemistry used. The fundamental reason for failure of anodized aluminum part in CVD reactors is the formation and growth of aluminum fluoride below the anodized film at point defect sites. These point defect sites are imperfections in the anodized layer caused by precipitate particles such as Fe and Si, voids, sharp corners, and cracks due to differential expansion. In the 6061 alloy, the point defect sites act as high energy sites for the plasma fluorine attack. However, such defect sites can be capped off by the formation of a layer of MgF2 below the anodized film. The formation of MgF2 is dependent upon the availability of elemental Mg to diffuse to these point defect sites and also the number of defect sites. Some of the Mg is typically tied up with precipitates like Fe and Si. When free Mg is unavailable to form a barrier layer, subsequent fluorination of the substrate causes formation of AlF3. This results in stresses in the anodized layer that can ultimately lead to delamination of the anodized layer and failure of the part. The 1100 alloy does not contain Mg and thus fluorination results in the direct formation of AlF3 at point defect sites with no protective MgF2 layer present. An enhanced performance of anodized Al parts can be achieved by using pure aluminum alloys with controlled quantities of Mg to form the MgF2 barrier layer.

Point defects in high purity silica induced by high-dose gamma irradiation

The defects induced by high-dose (10 MGy) gamma irradiation (60Co) are studied in various types of high-purity silica glasses [including synthetic crystal (α-quartz)]. While the defects induced by gamma irradiation of up to 1 MGy have been reported to be generated through the bond breaking of manufacturing-method-dependent point defect sites (precursors), such precursor dependency disappears or at least weakens in the defects induced by 10 MGy gamma irradiation. Electron spin resonance, optical absorption, and luminescence investigations suggest that at high-dose irradiation the defects are created mainly by radiolysis or bond breaking, and associated oxygen diffusion occurred at silicon–oxygen bonds other than at point defect sites. Crystalline α-quartz shows much higher radiation resistivity than amorphous silica glasses, suggesting that strained silicon–oxygen bonds are the breaking sites.

Atomic Bonding at Oxide Surfaces

AbstractThe fundamental interactions involved in the bonding of atoms and molecules to metal oxides are discussed. Surface defects play a major role in many of those interactions. Both acid/base and oxidation/reduction reactions occur at metal-oxide surfaces, with the latter dominating at point defect sites. The reaction of metals with oxide surfaces is governed largely by the relative heats of formation of the respective oxides, although surface point defects also play an important role. Preliminary studies of ceramic/ceramic interfaces indicate that interfacial interactions are much weaker than for metal/ceramic interfaces.