Vacancy Defect Clusters Research Articles

High performance and highly selective sensing of triethylamine sensors based on Cu-doped MoO3 nanobelts

Developing high-performance triethylamine (TEA) gas sensors are vital for human health. Defect engineering is a common method to enhance sensor performance. In this work, Cu-doped MoO3 nanobelts with different Cu/Mo molar ratios were synthesized by a low-cost solvothermal method. The 7.5CM sensor (molar ratio of Cu/Mo=7.5%) exhibited a response value of 135 to 10 ppm TEA at 240 °C. The sensor also showed excellent selectivity and sub-ppm detection limit (12.5 ppb). Based on X-ray photoelectron spectroscopy (XPS) and positron annihilation lifetime spectroscopy (PALS) analysis, the enhanced gas sensing performance and mechanism of Cu-doped MoO3 were discussed. PALS results indicated that higher V′MoVo··Vo·· vacancy defect clusters content on the surface of Cu-doped MoO3. The enhanced gas sensing performance was attributed to the increased concentration of vacancies/vacancy clusters on the surface of MoO3 nanobelts due to Cu doping, which facilitated the formation of surface chemisorbed oxygen and provided more active sites for the reaction of TEA. Additionally, CuMoO4 nanocrystals formed in MoO3 nanobelts can provide p-n heterojunctions on the surface, which generates potential barrier, enabling a wide variation range for the resistance of the sensor. This work provided a simple and effective method for the application of defect engineering in gas sensing materials.

Structural and electronic properties and optical absorption of oxygen vacancy cluster defects in KDP crystals: hybrid density functional theory investigation

The electron transition between oxygen vacancies and cationic vacancies shortens the band gap of KDP crystals.

Theoretical calculation and experimental research for structural stability and electronic properties induced by certain cluster defects in ADP crystal

Ammonium dihydrogen phosphate (NH4H2PO4, ADP) is an excellent nonlinear optical crystal and has had wide application. It was the same type of potassium dihydrogen phosphate (KH2PO4, KDP) with a similar crystal structure. The difference in structure and properties of ADP crystal in the presence of defects, especially cluster defects, was one of the important issues of concern in the research. In this work, first-principles calculation, coupled with HSE06 functional and the van der Waals-Wannier function method, was applied to investigate the structural stability and electronic properties induced by oxygen vacancy cluster defects and FeP2-+VO2+ cluster defects. And some spectra experiments, such as Raman spectroscopy, the Fourier transform infrared spectroscopy and the ultraviolet absorption spectroscopy, were also applied to investigate the detailed influence for ADP crystal doped with different Fe3+ concentration, which was grown with the “point-seed” rapid growth method. Combined with the theoretical results and the spectra tests, it confirmed that the structural changes in ADP crystal caused by oxygen vacancy cluster defect and FeP2-+VO2+ cluster defect were smaller than that in KDP crystal, mainly due to the restriction of hydrogen bonds and NH4+ group. With the increase of defect concentration, the microstructure stress could also damage the crystal structure due to the microscopic stress induced by Fe3+. The defect states moved towards right from 1.1 eV to 6.6 eV with the concentration of oxygen vacancy increasing. Similarly, the defect state composed of Fe 3d and O 2p states induced by FeP2- defect also moved to the conduction band minimum. The absorption peaks around 220-350 nm induced by FeP2- defect and FeP2-+VO2+ cluster defect were along the xy plane. It provided a good suggestion based on the calculation that it was very important to minimize defects or control cluster defect concentration during crystal growth.

Microstructure Modulation of Zn Doped VO2(B) Nanorods with Improved Electrochemical Properties towards High Performance Aqueous Batteries

Vanadium dioxide with monoclinic structure is theoretically a promising layered cathode material for aqueous metal-ion batteries due to its excellent specific capacity. However, its poor cycling stability limits its application as an electrode material. In this study, a series of Zn-doped VO2 (V1−xZnxO2) nanorods were successfully fabricated by the technology of one-step hydrothermal synthesis. The XRD result indicated that there was a slight lattice distortion caused by doped Zn2+ with a larger ion radius. The positron lifetime spectrum showed that there were vacancy cluster defects in all the samples. The electrochemical measurement demonstrated the enhancement of the specific capacitance of V1−xZnxO2 electrodes compared with the undoped sample. In addition, the discharge capacitance of the sample remained around 86% after 1000 charge/discharge cycles. This work proves that Zn2+ doping is a valid tactic for the application of nano-VO2(B) in energy storage electrode materials.

Radiation damage behavior and mechanism in RAFM steel: Orientation effect

The experiments and molecular dynamics simulations were performed to obtain the orientation effect on the behavior and mechanism of irradiation resistance in the RAFM steel. The irradiation surface morphologies, Frenkel defects, and clusters analysis reveal that the irradiation resistance of different orientations is as follows, (110) > (100) > (112) > (233) > (111) orientation. Additionally, the (233) orientation is easier to form vacancy defect clusters, while the (111) orientation is easy to form a large number of small clusters, which are determined by the clustering rates of interstitial atomic and vacancy clusters. Surface energy, atomic arrangement density, and atomic bonds play a significant role in the orientation effect on the radiation damage resistance. The radiation damage resistance of different orientations increased with the increase of atomic arrangement density and atomic bonding strength, while it decreased with the increase of surface energy.

Ultrafast visualization of phase transitions in nonequilibrium warm dense matter

Intense ultrafast laser excitation brings materials into highly nonequilibrium states with complex solid–liquid phase transitions. These ultrafast processes yield warm dense matter (WDM) conditions characterized by comparable thermal and Fermi energies, and strong ion–ion coupling. Here, we review recent studies employing mega-electron-volt ultrafast electron diffraction to resolve the structural dynamics of nonequilibrium WDM created by femtosecond laser heating of solids. For the study of warm dense gold, the results showed homogeneous melting that occurs within tens of picoseconds at absorbed energy densities of 0.4–1.4 MJ/kg. These results constrained the electron–ion coupling rate and revealed the melting sensitivity to nucleation seeds. For the study of radiation-damaged tungsten, the results showed a melting transition below the melting temperature. Molecular dynamics simulations suggested that this melting behavior is driven by vacancy defect clusters from radiation damage. These studies provided atomic-level insights into the melting behavior of materials under extreme conditions.

Specific cation stoichiometry control of SrMnO3-δ thin films via RHEED oscillations

Cubic heteroepitaxial (001) SrMnO3-δ (SMO) films were grown on SrTiO3 substrates by atomic alternating layer molecular beam epitaxy. Precise control of cation stoichiometry was achieved by in situ reflection high-energy electron diffraction (RHEED) oscillation. During SMO film growth, a correlation between RHEED oscillation features and the cation stoichiometry/monolayer dose was established. In non-stoichiometric films, there were out-of-plane lattice expansions due to off-stoichiometry-dependent defects, including Ruddlesden–Popper SrO planar faults in Sr-rich films and Sr vacancy defect clusters in Mn-rich films.

An Ab Initio Perspective on the Key Vacancy Defects of KMgF3

Perovskite-type KMgF3, which represents a distinct class of multifunctional materials, has drawn immense interest in the advanced technological fields due to its various attractive properties. However, these properties are strongly influenced by the presence of various vacancy point defects in the host crystal structure. This drives us to gain a detailed knowledge of the defect chemistry in KMgF3 by investigating geometry, electronic structure, defect formation energies, and charge transition levels. To achieve reliable results, we have adopted hybrid density functional calculations. To find out the stable growth condition for all possible vacancy defects and vacancy cluster defects in KMgF3, calculations of defect formation energies under different chemical potential range have been carried out. Among all the vacancy defects considered in this study, the calculated formation energy for VMg in a −2 charge state is found to be a more negative value as the Fermi energy level approached toward the conduction band minimum irrespective of the chemical potential range. The present study explored the role of different vacancy defects in creating impurity states in the band gap region by detailed analysis of electronic structure in each case. It has been observed that the impact of anion vacancy and their aggregation on the electronic structure of KMgF3 is much more prominent than that of cation vacancy. The electrical and optical properties of KMgF3 in the presence of various defects have been described on the basis of calculated thermodynamic and optical charge transition levels, respectively. It has been revealed that the cation vacancy defects are not likely to form deep defect states, while anion vacancy and their aggregation act as deep donor defects. The mixed cluster vacancy of K and F pair behaves differently with respect to Mg and F pair in the optical property. The cation vacancies have hardly any contribution to the observed optical spectrum, which is consistent with experimental observation. Thus, an unambiguous and ultimate clarification of the fundamental absorption and emission processes in KMgF3 has been accomplished in the present study.

Influence of vacancy cluster on the electronic transport properties of silicene sheet

By using non-equilibrium Green's functions (NEGF) and tight-binding method (TB) frameworks, the electronic properties of undefected and defected Silicene sheet (SiSh) in both zigzag (ZSiSh) and armchair (ASiSh) edges are investigated under moderate bias. We have investigated H-type and L-type of vacancy cluster defects for both edges. At first, the formation energy is estimated for each model, and we apperceived that in all cases, an endothermic process is required. In addition, by comprehension the density of state (DOS) of Silicene sheet, we discovered that Silicene sheet is semi-metallic with zero energy band gap. The calculations show that applying any type of defect, results in current reduction and among all the studied defects, H-type defect in zigzag edge and L-type defect in armchair edge are the most effective. I-V, formation energy of defects, transmission spectrum T(E), DOS of electrodes and central scattering region (SR) are studied afterward to apprehend the electronic properties of the perfect and defected cases. Our detections could be used in expanding nanoscale devices.

Size-dependent toughness and strength in defective 3C-SiC nanowires

This paper presents an atomistic understanding of effective toughness and strength in defective 3C-SiC nanowires of different diameters. We consider a set of high-symmetry vacancy defect clusters and employ a combination of density functional theory and molecular dynamics simulations to calculate stress in the nanowires, using an energy-based approach that does not require use of any macroscopic geometric information of the nanowire. Our results suggest that for defect-free nanowires, cracks nucleate from one of the corners of the hexagonal cross section, whereas for defective nanowires—regardless of the size of the defect core—cracks nucleate from the edge of the defect core. With increasing diameter, both strength and toughness increase in defective or defect-free nanowires. Furthermore, defects alter the size-dependent effective toughness and strength of the nanowire: the larger the size of the defect, the stronger the size-dependence of effective toughness and strength. A single vacancy in a 8.0 nm diameter nanowire reduces effective toughness and strength by around 16.5% and 3.4%, respectively. As diameter approaches ∞, effective stiffness approaches the bulk behavior—whereas neither strength nor toughness approaches the behavior of the bulk. This is primarily because of the presence of the surface and associated sustained stress-localization in the nanowire. Effective toughness and strength are, therefore, controlled by the local critical events and not by the macroscopic features of the nanowire. Additionally, both toughness and strength decrease nonlinearly with increasing temperature due to thermal softening of the material—and this thermal softening is, however, weakly dependent on the size of the defective regime.

Surface Defect Dynamics in Organic-Inorganic Hybrid Perovskites: From Mechanism to Interfacial Properties.

Organic-inorganic hybrid perovskites (OHPs) have garnered much attention among the photovoltaic and light-emitting diode research community due to their excellent optoelectronic properties and low-cost fabrication. Defects in perovskites have been proposed to affect device efficiency and stability and to have a potential role in enabling ion migration. In this study, the dynamic behavior and electronic properties of intrinsic defects in CH3NH3PbBr3 (MAPbBr3) were explored at the atomic scale. We use scanning tunneling microscopy to show unambiguously the occurrence of vacancy-assisted transport of individual ions as well as the existence of vacancy defect clusters at the OHP surface. We combine these observations with density functional theory (DFT) calculations to identify the mechanisms for this ion motion and show that ion transport energy barriers, as well as transport mechanisms, at the surface depend on crystal direction. DFT calculations also reveal that vacancy defect clusters can significantly modify the local work function of the perovskite surface, which is then expected to alter interfacial charge transport in a device. Our work provides a microscopic insight into the mechanism of ion migration in OHPs and also delivers the useful information for device improvement from the perspective of interface engineering.

Er3+ induced point defects in ZnO and impact of Li+/Na+/K+ on the vacancy defects in ZnO:Er studied by positron annihilation spectroscopy

The defects species in as-prepared ZnO, Er-doped, and co-doped (with Li/Na/K) nanocrystallites synthesized by combustion method were examined by Positron Annihilation Lifetime (PAL) and Doppler Broadening (DB) spectroscopy. The S and W parameters were derived from positron trapping of valence and core electrons extracted from the central and wing regions of DB spectra to identify the vacancy-type defects. For as-prepared Er-doped ZnO samples, the observed high value of S indicated that the vacancies and vacancy cluster defects were located at the grain surface and two-grain junction. With co-doping of Li/Na/K, the S value was decreased due to the dopant occupying the vacancy clusters. On annealing, the major recovery of such defects was observed with enormous decreasing S for Li co-doped sample. Positron lifetime variations signified the presence of defects compared with bulk ZnO. Er doping in ZnO evidenced a small increase of defects with a gradual decrease when co-doped with Li/Na/K indicating the co-dopant impurity occupied the vacancies in the grain boundary. The first lifetime component of 193 ps owing to the Zn-type vacancies in as-prepared ZnO were increased on Er doping, evidencing the replacement of the Zn2+ by Er3+ ion and thereby created point defects due to charge compensation. On annealing to 800 °C, Li co-doped ZnO:Er resulted to enormous reduction of second lifetime τ2, mean lifetime τave, and S value signifying the Li ion localised in Zn vacancy site. Annealed ZnO:Er + Na/K resulted in considerable intensification in defect concentrations than ZnO:Er + Li due to the vacancy migration to form as clusters. The defect type, defect modification, the influence of Er together with co-doped Li/Na/K in ZnO, and their effect on annealing are elucidated in detail from the positron annihilation characteristics.

Study of electronic properties of graphene device with vacancy cluster defects: A first principles approach

The discovery of graphene has produced new ideas for the description of several graphene devices with the potential to improve, for examples, solar cells, advanced sensors and communication systems. In the present work we use density functional calculations in order to investigate the electronic properties of zigzag and armchair edge graphene device with H-type and ZZ-type of vacancy cluster defects (these defects are defined and illustrated in the paper). The calculations show that the bond length for relaxed graphene is 1.42 Å while for zigzag graphene with H-type and ZZ-type of defects the bond lengths are about 1.40 Å and 1.38 Å, respectively. Moreover, for the H-type and ZZ-type of defects, the arrangement of atoms suggests nearly sp2 and sp. bonding, respectively. The nonlinear behavior of current-voltage curve for graphene with H-type and ZZ-type of defects is studied and different results are discussed. Furthermore, the transmission probability and density of states for pristine graphene and graphene with H-type and ZZ-type of vacancy defects have been presented. Finally, the armchair graphene devices with H-type and ZZ-type of vacancy defects show the same behavior as the zigzag graphene with H– and ZZ- type of defects.

Modeling of fuel retention and hydrogen-isotope exchange removal from the pre-damaged tungsten material

The fuel retention and hydrogen-isotope exchange removal in the pre-damaged tungsten (W) have been investigated in the present work. The fuel retention code Hydrogen Isotope Inventory Processes Code (HIIPC) has been upgraded to model the isotope-exchange process. Three types of defects are taken into account in the pre-irradiated W, including the dislocation, mono-vacancy, vacancy cluster defects. The SRIM code is employed to calculate the radiation-damage defect distribution. Three phrases are modeled by controlling the source terms by using HIIPC code. (i) deuterium (D) retention phrase, pre-damaged W is irradiated by the D flux; (ii) waiting period, both of the D and hydrogen (H) fluxes are switched off; (iii) isotope exchange phase, the W material is irradiated by H atoms. It is found that enhancing implantation H fluence can reduce the retained D significantly. The simulation also confirms D retention increases with the irradiation damage (displacement per atoms, dpa), until saturation occur. De-trapping energy plays an important role on the D retention, especially for high material temperature. The modeling results are compared with the experiment measurement, showing the good agreement.

Stacking fault tetrahedron induced plasticity in copper single crystal

Stacking fault tetrahedron (SFT) is the most common type of vacancy clustered defects in fcc metals and alloys, and can play an important role in the mechanical properties of metallic materials. In this study, molecular dynamics (MD) simulations were carried out to investigate the incipience of plasticity and the underlying atomic mechanisms in copper single crystals with SFT. Different deformation mechanisms of SFT were reported due to the crystal orientations and loading directions (compression and tension). The results showed that the incipient plasticity in crystals with SFT resulted from the heterogeneous dislocation nucleation from SFT, so the stress required for plastic deformation was less than that needed for perfect single crystals. Three crystal orientations ([1 0 0], [1 1 0] and [1 1 1]) were specified in this study because they can represent most of the typical deformation mechanisms of SFT. MD simulations revealed that the structural transformation of SFT was frequent under the applied loading; a metastable SFT structure and the collapse of SFT were usually observed. The structural transformation resulted in a different reduction of yield stress in compression and tension, and also caused a decreased or reversed compression/tension asymmetry. Compressive stress can result in the unfaulting of Frank loop in some crystal orientations. According to the elastic theory of dislocation, the process of unfaulting was closely related to the size of the dislocation loop and the stacking fault energy.

Microstructures of pure nickel induced by high-current pulsed electron beam irradiation

The characteristics of surface feature and microstructure of the near-surface of nickel were investigated after high current pulsed electron beam (HCPEB) irradiation. Based on a physical model, the temperature profile was simulated. The depth of heat-affected zone, the initial melting position, heating rate and quenching rate were computed. The initial coarse-grained structure on the surface was refined, of which the size is about 70 nm. The TEM observations indicate that dislocation wall, sub-dislocation wall, twin and twin boundary steps were achieved on the surface of material after HCPEB irradiation. Furthermore, huge numbers of vacancy defect clusters including dislocation loops, stacking fault tetrahedras (SFT) and voids were also formed within the sublayer of the irradiated surface. The fact that supersaturation vacancies migrated towards the surface along grain boundaries and/or various dislocation configurations led to the formation of microporous feature on the irradiated surface.

Microstructural evolution of RPV steels under proton and ion irradiation studied by positron annihilation spectroscopy

The microstructural evolution of reactor pressure vessel (RPV) steels induced by proton and heavy ion irradiation at low temperature (∼373K) has been investigated using positron annihilation spectroscopy (PAS), atom probe tomography (APT), transmission electron microscopy (TEM) and nanoindentation. The PAS results indicated that both proton and heavy ion irradiation produce a large number of matrix defects, which contain small-size defects such as vacancies, vacancy-solute complexes, dislocation loops, and large-size vacancy clusters. In proton irradiated RPV steels, the size and number density of vacancy cluster defects increased rapidly with increasing dose due to the migration and agglomeration of vacancies. In contrast, for Fe ion irradiated steels, high density, larger size vacancy clusters can be easily induced at low dose, showing saturation in PAS response with increasing dose. No clear precipitates, solute-enriched clusters or other forms of solute segregation were observed by APT. Furthermore, dislocation loops were observed by TEM after 1.0dpa, 240keV proton irradiation, and an increase of the average nanoindentation hardness was found. It is suggested that ion irradiation produces many point defects and vacancy cluster defects, which induce the formation of dislocation loops and the increase of nanoindentation hardness.

The microstructures and corrosion properties of polycrystalline copper induced by high-current pulsed electron beam

In order to investigate the corrosion mechanism of pure metal materials containing little impurities, polycrystalline commercial pure (cp) copper was irradiated by high-current pulsed electron beam (HCPEB). The surface microstructures of irradiated samples are characterized by using optical microscopy and transmission electron microscopy (TEM). The corrosion resistance is also investigated by using polarization curves of seawater corrosion and electrode impedance spectroscopy (EIS). The experimental results indicate that the corrosion resistance of cp copper irradiated by 10 pulses is remarkably improved comparing with the original sample. TEM observations suggest that large amount of supersaturated vacancy defects are produced when the material surface is subjected to the HCPEB irradiation. Furthermore, the agglomerations of the vacancy defects cause the formation of the vacancy cluster defects, such as vacancy dislocation loops, the stacking fault tetrahedra (SFTs) and voids. It is suggested that the structural defects on the irradiated surface have some relationships with the corrosion resistance's improvement of the material.

Effects of Vacancy Cluster Defects on Electrical and Thermodynamic Properties of Silicon Crystals

A first-principle plane-wave pseudopotential method based on the density function theory (DFT) was employed to investigate the effects of vacancy cluster (VC) defects on the band structure and thermoelectric properties of silicon (Si) crystals. Simulation results showed that various VC defects changed the energy band and localized electron density distribution of Si crystals and caused the band gap to decrease with increasing VC size. The results can be ascribed to the formation of a defect level produced by the dangling bonds, floating bonds, or high-strain atoms surrounding the VC defects. The appearance of imaginary frequencies in the phonon spectrum of defective Si crystals indicates that the defect-region structure is dynamically unstable and demonstrates phase changes. The phonon dispersion relation and phonon density of state were also investigated using density functional perturbation theory. The obtained Debye temperature (θ D) for a perfect Si crystal had a minimum value of 448 K at T = 42 K and a maximum value of 671 K at the high-temperature limit, which is consistent with the experimental results reported by Flubacher. Moreover, the Debye temperature decreased with increases in the VC size. VC defects had minimal effects on the heat capacity (C v) value when temperatures were below 150 K. As the temperature was higher than 150 K, the heat capacity gradually increased with increasing temperature until it achieved a constant value of 11.8 cal/cell·K. The heat capacity significantly decreased as the VC size increased. For a 2 × 2 × 2 superlattice Si crystal containing a hexagonal ring VC (HRVC10), the heat capacity decreased by approximately 17%.

Vacancy cluster effects on the behaviors of mechanical stretching of Au nanowires

Extensive atomistic simulations of the mechanical deformation of defective nanowires (NWs) were performed using the embedded-atom molecular dynamics modeling approach. The investigation focuses on the coupled effects of various vacancy cluster (VC) defects, operation temperature, and wire cross-sectional area on the mechanical properties and plastic deformations of defective NWs. The stress-strain behaviors show that the elastic modulus is independent of the vacancy cluster defects. Quasi-linear decreasing Young's moduli were observed with increasing operation temperature. For a given operation temperature, NW Young's modulus increased with increasing NW size.