Vacancy Point Research Articles

Understanding the role of defects in influencing the thermoelectric properties of SnSe

As compared to single crystals, polycrystalline SnSe shows a considerable decline in its ZT value. Optimization of carrier concentration by the way of chemical doping is useful but creates point defect and vacancies that are often overlooked. Here we study polycrystalline Sn0.95M0.05Se (M = Co, Ni, In) with an aim to understand the role of defects. The overall crystal structure and microstructure of SnSe is not much affected with substitution as evident from X-ray diffraction and scanning electron microscopy study. Rietveld refinement confirms the single phase nature of the all compositions and provides unit cell parameters. Analysis of the stoichiometry reveals the presence of cation vacancies. Optical spectroscopy indicates a degradation of the in-direct gap and Urbach band tail-width fitting confirms the presence of localized states within the gap. Electrical resistivity and Seebeck coefficient are adversely affected by defects, but thermal conductivity decreases by almost 50% of SnSe value.

Effects of different valence states of Mo and point vacancies on magneto-optical properties of ZnO

Experimental studies have been conducted to characterize the physical properties of Mo-doped ZnO. However, they have not considered the effects of different valence states and point vacancies on the magneto-optical performance of the system. The present study adopted the plane-wave ultrasoft pseudopotential + U method within the framework of spin density functional theory. The formation energy, electronic structure, and optical properties of ZnO systems with coexisting different valence states of Mo and point vacancies were calculated. Results showed that the formation energy of all doping systems was negative under Zn-rich and O-rich conditions, but the formation energy was relatively lower under O-rich conditions than under Zn-rich conditions. Compared with Mo6+ and Mo5+ when the concentration of O vacancies or Zn vacancies was the same, Mo6+-doped ZnO had the lowest formation energy, was the easiest to dope, and had the most stable system structure. Zn34Mo5+O36 and Zn34Mo6+O36 systems exhibited ferromagnetism, and their Curie temperature was above room temperature. The Zn34Mo6+O36 system had the largest magnetic moment, which is beneficial for designing and preparing new types of diluted magnetic semiconductors. The band gap widths of the Zn34Mo5+O36 (3.04 eV) and Zn34Mo6+O36 (2.92 eV) systems were both narrower than that of the pure ZnO system. By comparison, the band gap widths of the Zn35Mo5+O35 and Zn35Mo6+O35 systems were 4.07 and 3.94 eV, respectively. Although the band gap width widened, under light excitation conditions, the valence band electrons could absorb the photons of lower energy to transition to the impurity energy level, and then transition further from the impurity energy level to the conduction band. During the hierarchical transition, visible light and infrared light effects still occurred. The range of photon energy response of all doping systems expanded, and the absorption spectrum red-shifted in the light-absorption edge within the low-energy (0–3.2 eV) range. In particular, the Zn35Mo6+O35 system exhibited the strongest polarization ability, photoelectric field strength, and charge-binding ability in the low-energy region. Moreover, the dielectric and absorption peaks of the imaginary part of the dielectric function were relatively large, and the red-shift of the absorption spectrum was relatively substantial. Therefore, the Zn35Mo6+O35 system is useful in designing and preparing novel ZnO-based photocatalysts.

First-principles study of the effect of Mn and point vacancies with different valence states on the magnetic properties of ZnO

It is challenging to experimentally modulate the ferromagnetic properties of ZnO:Mn by point vacancies. Moreover, the magnetic source and magnetic mechanism of ZnO containing doped Mn and point vacancies, both with different valence states, have not been fully understood; using first principles can theoretically analyse this problem systematically. Therefore, in this study, the effects of different valence states of Mn and point vacancies on the magnetic properties of ZnO are calculated by first principles, revealing that ZnO:Mn has a lower formation energy and a more stable structure under oxygen-rich conditions than under zinc-rich conditions. Under oxygen-rich conditions, less Zn34Mn22+O35:VO0/1+/2+ and more Zn34Mn22+O36, Zn34Mn23+/4+O35:VO°, Zn33Mn22+O35:VZn0/1−/2−, and Zn33Mn23+/4+O36:VZn° are formed, facilitating the formation of a stable structure for the latter system compared to the others. The Zn33Mn22+O35:VZn° system not only possesses the lowest formation energy, but also the largest magnetic moment (10μB), indicating that a Zn vacancy allows the doping system to possess a high magnetic moment and also enhances the stability of the system. Therefore, Zn33Mn22+O35:VZn° is the most valuable of the systems studied. Analysis of the magnetic source and magnetic mechanism for this system revealed that the system possesses the magnetism and room temperature ferromagnetism, presents the characteristics of half-metallization, and that its magnetism is mainly derived from the itinerant electron and double exchange between O2p and Mn3d using holes as the medium. This is of great value for the design and preparation of new dilute magnetic semiconductors with high magnetic moments and high-Curie-temperature hole polarisation from 100 % injection sources.

Catalytic activity for hydrogen evolution reaction in square phase Janus MoSSe monolayer: A first-principles study

Based on density-functional theory, the catalytic property for hydrogen evolution reaction (HER) of square phase Janus MoSSe monolayer is systematically investigated. Similar to 2H-phase transition metal dichalcogenides, the basal plane of pristine 1S- MX 2 and 1S- MXY ( M : Mo; X/Y : S/Se) are catalytically inert. By introducing vacancy defects, the catalytic performance of the basal plane can be activated. The calculated Gibbs free energy differences (Δ G H ) of non-Janus 1S-MoS 2 and 1S-MoSe 2 with point vacancies reduce to 35 and −105 meV, respectively. Due to the atomic asymmetry of Janus structure, Δ G H of 1S-MoSSe monolayer are −53 and −28 meV for S- and Se-vacancy defect, respectively. Qualitatively, the catalytic activity of 1S-MoSSe and 1S-MoS 2 with S vacancy are comparable while 1S-MoSSe is much better than 1S-MoSe 2 with Se vacancy. Additionally, Δ G H of 1S-MoSSe with Se-vacancy keeps within ±50 meV under a −2%–2% biaxial strain, demonstrating high robustness of catalytic activity against strain. The results indicate the potential of square phase Janus MoSSe monolayer as an option for monolayer HER catalyst. • Basal planes of pristine square phase non-Janus and Janus TMD monolayers are catalytically inert. • Introduction of vacancy defects can activate the basal plane of 1S-MoSSe monolayer. • Δ G H of 1S-MoSSe_VSe and 1S-MoSSe_VS are −0.028 eV and −0.053 eV by DFT calculations, respectively. • Introduction of tensile (compressive) biaxial strain reduces (enlarges) the value of Δ G H . • Within the strain range from −2% to 2%, Δ G H of 1S-MoSSe_VSe keeps in the catalytic window.

Electron Microscopy Studies of Local Structural Modulations in Zeolite Crystals.

Zeolites are widely used in catalysis, gas separation, ion exchange, etc. due to their superior physicochemical properties, which are closely related to specific features of their framework structures. Although more than two hundred different framework types have been recognized, it is of great interest to explore from a crystallographic perspective, the atomic positions, surface terminations, pore connectivity and structural defects that deviate from the ideal framework structures, namely local structural modulation. In this article, we review different types of local modulations in zeolite frameworks using various techniques, especially electron microscopy (EM). The most recent advances in resolving structural information at the atomic level with aberration corrected EM are also presented, commencing a new era of gaining atomic structural information, not only for all tetrahedral atoms including point vacancies in framework but also for extra‐framework cations and surface terminations.

Electron Microscopy Studies of Local Structural Modulations in Zeolite Crystals

AbstractZeolites are widely used in catalysis, gas separation, ion exchange, etc. due to their superior physicochemical properties, which are closely related to specific features of their framework structures. Although more than two hundred different framework types have been recognized, it is of great interest to explore from a crystallographic perspective, the atomic positions, surface terminations, pore connectivity and structural defects that deviate from the ideal framework structures, namely local structural modulation. In this article, we review different types of local modulations in zeolite frameworks using various techniques, especially electron microscopy (EM). The most recent advances in resolving structural information at the atomic level with aberration corrected EM are also presented, commencing a new era of gaining atomic structural information, not only for all tetrahedral atoms including point vacancies in framework but also for extra‐framework cations and surface terminations.

Atomic Structure and Dynamics of Defects and Grain Boundaries in 2D Pd2Se3 Monolayers

AbstractStructural imperfections of 2D crystals such as point vacancies and grain boundaries (GBs) have considerable impacts on their chemical-physical properties. Here we study the atomic structure and dynamics of defects and GBs in monolayer Pd2Se3 using annular dark field scanning transmission electron microscopy (ADF-STEM). The Pd2Se3 monolayers are reproducibly created by thermally induced phase transformation of few-layered PdSe2 films in an in-situ heating holder in the TEM to promote Se loss. Diverse point vacancies, one-dimensional (1D) defects, GBs and defect ring complexes are directly observed in monolayer Pd2Se3, which show a series of dynamics triggered by electron beam. High mobility of vacancies leads to self-healing of point vacancies by migration to the edge and subsequent edge etching under the beam. Specific defects are stabilized by Se–Se bonds, which shift in a staggered way to buffer strain, forming a wave-like 1D defect. Bond rotations are also observed and play an important role in defect and GB dynamics in Pd2Se3 during vacancy production. The GBs form in a meandering pathway and migrate by a sequence of Se–Se bond rotations without large scale vacancy formation. In the GB corners and tilted GBs, other highly symmetric vacancy defects also occur to adapt to the orientation change. These results give atomic level insights into the defects and GBs in Pd2Se3 2D monolayers.

Cluster dynamics modeling of irradiation growth in single crystal Zr

Irradiation growth of single-crystal Zr is modeled with a reduced set of cluster dynamics equations. Nucleation and growth of basal and prismatic dislocation loops are both accounted for in the model. Equations are developed for the time evolution of single point defects, a limited number of point defect clusters, interstitial and vacancy loop nucleation rates, as well as for the growth of vacancy loops on basal planes and interstitial loops on prismatic planes. Reduction of the usual infinite hierarchy of cluster dynamics equations to the simple set studied here is justified on the physical basis of the stability of small loops once nucleated. This simplified cluster dynamics model with a small number of adjustable parameters avoids the complexity of explicit representation of higher order point defect clusters. The model shows consistency with experimental observations of the following aspects: (1) the growth rates of Zr crystals along the a- and c-axes; (2) the onset dose for breakaway irradiation growth; (3) the saturation dislocation loop densities of vacancy and interstitial loops; (4) the effects of cold work and temperature on irradiation growth.

A phase field model for dislocation climb under irradiation: Formalism and applications to pure bcc iron and ferritic alloys

A phase-field model allowing to describe dislocation climb under irradiation is presented. Whereas previous phase-field models of the literature devoted to this phenomenon only take into account vacancies, our approach includes the effect of self interstitial atoms (SIAs) as required in the context of irradiated metals. Beyond the fact that it rigorously ensures the balance between the quantity of point defects, vacancies and SIAs, absorbed or emitted by the loop and the loop evolution, the present model has several originalities. First, it is capable to quantify the climb rate for systems far from equilibrium, which is commonly the case under irradiation. This required supplemental methodological developments since we clearly show that a mere generalization of existing phase-field models is not satisfactory to tackle this specificity. Secondly, it alleviates the often adopted assumption of perfect sink through the introduction of a kinetic parameter related to the dislocation jog density. A preliminary generic study of dislocations considered as nonperfect sinks leads to nonintuitive results, since the climb rate decreases when the dislocation jog density increases. Thirdly, the possibility to consider different types of interacting microstructural defects in the model allows to show the significant role of the point defect thermal equilibrium fractions on the climb rate in pure bcc iron under irradiation conditions. Finally, the climb model is coupled to the chemical diffusion equations in the same phase-field formalism. For this purpose, a multi-time step algorithm is proposed in order to couple phenomena with different characteristic time scales by several orders of magnitude, namely climb, point defect and chemical diffusion. It allows to study the interaction between the motion of the dislocation and the well-known phenomenon of radiation induced segregation in a Fe–Cr alloy. It is shown that the shape and size of the solute atmosphere can strongly depend on the dislocation motion under irradiation.

Tunable thermal conductivity of single layer MoS2 nanoribbons: an equilibrium molecular dynamics study

Thermal transport in single-layer MoS2 nanoribbons (SLMOSNRs) has been comprehensively studied using equilibrium molecular dynamics (EMD) simulations based on the Green–Kubo formulation. The room-temperature thermal conductivity of a pristine ~ 10 nm × 4 nm zigzag MoS2 nanoribbon is computed to be ~ 117 W m−1 K−1 using the Stillinger–Weber (SW) interatomic potential. The thermal conductivity is also studied as a function of temperature and the dimensions of the sample. The thermal conductivity of SLMOSNRs is found to decrease with increasing temperature due to increased phonon–phonon Umklapp scattering, while it shows the opposite trend as the length is increased. With increasing length, the thermal conductivity initially increases rapidly but gradually less so. The thermal conductivity exhibits a similar trend with increasing sample width. Moreover, the impact of defect engineering, an effective tool for tailoring the thermal transport in single-layer MoS2 by considering various defects, namely point vacancies, bi-vacancies, and edge vacancies, is studied. The results of this study show that the thermal conductivity of SLMOSNRs with defects is significantly reduced compared with their pristine counterparts. The reduction of the thermal conductivity with increasing defect concentration is greater at low than high concentration. To study the underlying mechanism responsible for such characteristics, the phonon density of states (PDOS) of SLMOSNRs is calculated. This study provides a detailed demonstration of how the thermal transport characteristics of MoS2 nanostructures can be tuned, promoting the potential application of MoS2 in thermoelectric and nanoelectronic devices.

Mechanical responses of pristine and defective hexagonal boron-nitride nanosheets: A molecular dynamics investigation

In this work we conducted classical molecular dynamics (MD) simulation to investigate the mechanical characteristics and failure mechanism of hexagonal boron-nitride (h-BN) nanosheets. Pristine and defective structure of h-BN nanosheets were considered under the uniaxial tensile loadings at various temperatures. The defective structure contains three types of the most common initial defects in engineering materials that are known as cracks, notches (with various length/size), and point vacancy defects (with a wide range of concentration). MD simulation results demonstrate a high load-bearing capacity of extremely defective (amorphized) h-BN nanosheets. Our results also reveal that the tensile strength decline by increasing the defect content and temperature as well. Our MD results provide a comprehensive and useful vision concerning the mechanical properties of h-BN nanosheets with/without defects, which is very critical for the designing of nanodevices exploiting the exceptional physics of h-BN.

Native point defects in mono and bilayer phosphorene

We study the stability and electronic properties of intrinsic point defects, vacancy and self-interstitial, in mono- and bi-layer phosphorene. We calculate the formation energies, quasiparticle defect states and charge transition levels (CTLs) of these defects using \textit{ab initio} density functional theory (DFT) and GW approximation to the electron self-energy. Using the DFT + GW two paths formalism for studying interstitial in monolayer phosphorene, we show that with the inclusion of electrostatic corrections CTLs can be calculated reliably. Our calculations show that all the native point defects have low formation energies 0.9-1.6 eV in neutral state. Furthermore, we find that vacancy in phosphorene behaves as an acceptor-like defect which can explain the p-type conductivity in phosphorene. On the other hand, interstitial can show both acceptor- and donor-like behaviour.

Effects of point vacancy and interstitial H on the carrier activity, separation, and absorption spectrum of ZnO: Li/Na/K

The accurate understanding of Li/Na/K doping and point vacancies in ZnO is difficult. The interstitial H is also neglected in ZnO. In this paper, the generalized gradient approximation plane wave ultrasoft pseudopotential + U method is adopted based on the spin density functional theory to solve the abovementioned problem. The First-principles method is utilized to study the point vacancies and interstitial H on ZnO: Li/Na/K carrier activity, separation, and absorption spectrum. The formation energy of the Zn35MHiO35 (M: Li/Na/K) system is a large positive value regardless of the O- and Zn-rich conditions, indicating the difficulty of Zn35MHiO35 (M: Li/Na/K) system's formation and its poor stability. Compared with the same doping system, the Zn34MHiO36 (M: Li/Na/K) system exhibits smaller formation energy under O-rich conditions and is more stable. The formation energies of different doping systems under O- or Zn-rich conditions are in the following order: Zn35NaHiO35＜Zn35KHiO35＜Zn35LiHiO35. Zn35NaHiO35 and Zn35KHiO35 have negative formation energy and good stability. According to the analysis of the electric dipole moment, effective mass, and absorption spectrum, Zn35NaHiO35 can be separated easily and has the strongest carrier activity, the best absorption spectrum intensity, and the best red shift. These features are relatively advantageous in designing and preparing novel ZnO photocatalysts.

Atomic structure and defect dynamics of monolayer lead iodide nanodisks with epitaxial alignment on graphene

Lead Iodide (PbI2) is a large bandgap 2D layered material that has potential for semiconductor applications. However, atomic level study of PbI2 monolayer has been limited due to challenges in obtaining thin crystals. Here, we use liquid exfoliation to produce monolayer PbI2 nanodisks (30-40 nm in diameter and > 99% monolayer purity) and deposit them onto suspended graphene supports to enable atomic structure study of PbI2. Strong epitaxial alignment of PbI2 monolayers with the underlying graphene lattice occurs, leading to a phase shift from the 1 T to 1 H structure to increase the level of commensuration in the two lattice spacings. The fundamental point vacancy and nanopore structures in PbI2 monolayers are directly imaged, showing rapid vacancy migration and self-healing. These results provide a detailed insight into the atomic structure of monolayer PbI2, and the impact of the strong van der Waals interaction with graphene, which has importance for future applications in optoelectronics.

Pyrolysis mechanisms of graphene oxide revealed by ReaxFF molecular dynamics simulation

Pyrolysis dynamics of graphene oxide (GO) is critical for composite material design, and a few experimental evaluations were conducted so far since extremely high temperature condition is needed. In this work, with help of ReaxFF reactive molecular dynamics simulations, the pyrolysis mechanisms of GO were investigated from two aspects: (1) the structural evolutions of the carbon skeleton and (2) the desorption of the superficial functional groups. At low temperature, desorption of functional groups is dominant, and little point defects or vacancies are introduced to the carbon skeleton. Meanwhile, generation of a number of pyrolysis products was confirmed at higher temperatures. Accordingly, the carbon skeleton is significantly disordered, resulting in an amorphous transformation. Analysis of bonds breaking ratio, hybridization types of carbon atoms and pyrolytic fragments indicated that the restored sp2-hybridized carbons increase with the increasing annealing time, and structure of final products are controlled by hydroxyl and oxygen dangling bonds. Finally, annealing process of GO with different oxygenal group density (R) and OH/O ratio are quantitatively analyzed and found pyrolysis is R and OH/O ratio dependent. These results uncover the pivotal role of oxygen and hydrogen in pyrolysis, and provide a guidance for heat resistant composite designs.

Vacancy-induced thermal transport in two-dimensional silicon carbide: a reverse non-equilibrium molecular dynamics study.

Because of its impressive electrical, thermal, and mechanical properties, two-dimensional silicon carbide (2D-SiC) has recently gained tremendous attention in the field of nanoelectronics and optoelectronics. Here, we investigated the effects of various types of defects such as bi-, point-, and mixed-vacancies on the thermal conductivity of 2D-SiC using reverse non-equilibrium molecular dynamics simulation. The effects of temperature variation on the thermal conductivity of vacancy-defected 2D-SiC were also studied. A significant reduction of the thermal conductivity was observed when the concentrations of the vacancies were increased. The point vacancy resulted in the thermal conductivity decreasing more quickly as compared to bi vacancy and mixed vacancy defects. Moreover, increasing the temperature of vacancy-defected 2D-SiC further reduced the thermal conductivity due to a strong phonon-vacancy scattering effect. Because of the introduction of vacancy defects in the acoustic phonon density of states (PDOS), a softening behavior in the intensity of the characteristic peaks is perceived, and with increasing temperature, a frequency shrinking is noted in the PDOS curves, both of which contribute to the reduction of the thermal conductivity. Additionally, rapid softening of the phonon transmission spectrum and increase in entropy were obtained for the point vacancy-defected structure, which clearly confirms our findings at different vacancy concentrations as well as for types of vacancies. These findings are very much imperative for realizing heat dissipation in nano- and optoelectronic devices based on 2D-SiC as well as for demonstrating an effective method for modulating 2D-SiC thermal conductivity through defect engineering.

CALANIE: Anisotropic elastic correction to the total energy, to mitigate the effect of periodic boundary conditions

CALANIE (CALculation of ANIsotropic Elastic energy) computer program evaluates the elastic interaction correction to the total energy of a localized object, for example a defect in a material simulated using an ab initio or molecular statics approach, resulting from the use of periodic boundary conditions. The correction, computed using a fully elastically anisotropic Green’s function formalism, arises from the elastic interaction between a defect and its own periodically translated images. The long-range field of elastic displacements produced by the defect is described in the elastic dipole approximation. Applications of the method are illustrated by two case studies, one involving an ab initio investigation of point defects and vacancy migration in FCC gold, and another a molecular statics simulation of a dislocation loop. We explore the convergence of the method as a function of the simulation cell size, and note the significance of taking into account the elastic correction in the limit where the size of the defect is comparable with the size of the simulation cell. Program summaryProgram Title: CALANIE, version 2.0Program Files doi:http://dx.doi.org/10.17632/3h6xffk9h6.1Licensing provisions: Apache License, Version 2.0Programming language: C/C++Nature of problem: Periodic boundary conditions (PBCs) are often used in the context of ab initio and molecular statics atomic scale simulations. A localized defect in a crystalline material, simulated using PBCs, interacts elastically with its own periodically translated images, and this gives rise to a systematic error in the computed defect formation and migration energies. Evaluating the correction to the total energy resulting from effects of elastic interaction between a defect and its periodic images, to alleviate the contribution to the total energy arising from PBCs, is an essential aspect of any accurate total energy calculation performed using PBCs.Solution method: The energy of interaction between a localized defect and its periodically translated images is computed in the linear elasticity approximation. The energy of elastic interaction is expressed analytically in terms of the elastic dipole tensor of the defect and elastic Green’s function. Elements of the dipole tensor are computed as a part of the simulation evaluating the formation energy of the defect. Elastic Green’s function and its first and second derivatives are computed numerically from the elastic constants of the material. The method and the corresponding numerical procedures are implemented in the CALANIE computer program. The program evaluates matrix elements of the elastic dipole tensor of a localized defect and the elastic correction to the total energy arising from the use of periodic boundary conditions.Restrictions: The approach assumes the validity of the linear elasticity approximation. This limits the accuracy of evaluation of the elastic correction, which becomes less precise if the size of the defect is comparable with the size of the simulation cell.Unusual features: An open source code, containing full detail of the relevant theoretical concepts, algorithms and numerical implementation.

Understanding of electronic and optical properties of ZnS with high concentration of point defects induced by hot pressing process: The first-principles calculations

ZnS is one of the important optical materials for thermal imaging, infrared detecting and laser transmission. Herein, the point defects formation mechanism of hot-pressing ZnS and their influence on optical properties were systematically explored by density functional theory (DFT) to fully understand the experimental observation what the transmittance decreases and the black fogs form with the S vacancies increasing for the hot-pressing ZnS ceramics. Zn (VZn) and S (VS) point vacancies with various amounts of defect concentrations in the range from 0 to 50% were systematically studied to corresponding the experimental results. The result indicates that anion VS are energetically more favorable in regards to cation VZn based on their formation energies. This result explains why only VS were observed in the experiment. Electronic structures show that the band gaps vary from 3.858 eV to 1.569 eV, corresponding to the VS concentration from 0 to 25%. Consequently, the VS also lead to the reflectivity increase and the decrease of the optical transmittance. Continuation of increasing the VS concentrations to 50%, ZnS turns into metallic behavior without a distinctive band gap, so the light fails to pass and maybe cause black fogs formation.

Effect of Li doping and point vacancy on photocatalytic properties and mechanism of ZnO

Controlling the point defect in Li-doped ZnO is difficult. In addition, the absorption spectrum distribution in Li-doped ZnO with O vacancy (VO) has two reports of red- and blue-shift experimental predictions. The photocatalytic properties of Li doping and point defect on ZnO was studied using first-principles method based on generalized gradient approximation (GGA) plane-wave super-soft pseudopotential + U (GGA + U) to solve this problem. Compared with undoped ZnO, ZnO supercell with Li replacing Zn (LiZn) and VO, LiZn and Zn vacancy (VZn), interstitial Li (Lii) and VZn, and ZnO supercell with LiZn, Lii, and VZn have a narrower band gap and a red-shifted absorption spectrum distribution. By contrast, ZnO supercell with Lii and VZn have a wider band gap and a blue-shifted absorption spectrum distribution. The effective mass calculations show that the ZnO supercell with LiZn and VO has the smallest effective mass of carriers and can thus accelerate the separation of electron–hole pairs, thereby improving photocatalytic activity. The analysis of the electric dipole moment showed that ZnO supercell with LiZn and VO has the strongest activity of carries. This condition is advantageous for designing and preparing novel photocatalysts.

Strength analysis of a defective diamondene nanoribbon under uni-axial tension

Due to its unique sp2/sp3 hybrid electron configuration, diamondene with superior physical properties diversifies the allotrope family of carbon and attracts much attention in recent year. Considering the inevitable occurrence of imperfections during its fabricating process under super high compression, influences of point vacancy or Stone-Wales (SW) defects on the tensile strength of a defective diamondene nanoribbon were examined using molecular dynamics method in the present work. Results show that a defective ribbon under tension behaves softening-to-hardening transition owing to abrupt changes of both bond length and bond angle at a critical tensile strain. Point vacancy and SW defects lead to different failure modes of the diamondene ribbon, which can be characterized by shear band originating from defects along ±45° of stretching direction. Especially, for a ribbon with SW-1 defect (by rotating 90° of a bond along stretching direction), it behaves a complicated multi-stage damage process. However, point vacancy produces two separate semi shear bands originating from the defect and later merging to be a whole shear band. After hydrogenation on both surfaces of a pristine ribbon, the softening-to-hardening transition still exists under uni-axial tension along armchair direction. However, the final fracture mode is quite different with that of the pristine counterpart. These characteristics provide guidelines on potential application of nano-devices based on diamondene.