Atomic Vacancy Defects Research Articles

Threefold Impact on Reaction Coordinate Mapping of Zirconium Based Tri‐Chalcogenide Pseudo‐Monolayers as Emerging Ultrathin Catalysts for Water Splitting

AbstractThe continuous quest of finding optimum catalysts for photo‐electrocatalytic (PEC) water splitting has been revolving around 2D ultrathin materials because of the fact of exploiting both the surface area. In this work, a threefold impact is envisaged route to enhance the activity both in terms of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in the emerging Zirconium Tri‐sulfides (ZrS3) based pseudo‐monolayers. Due to the unique and distinct electronic properties of the tri‐chalcogenide systems as compared to the di‐chalcogenide counterpart of Zirconium, one could afford to explore the implications of not only single‐atom functionalization and vacancy defect, but also the external strain to expedite the bifunctional catalytic activity in this promising material. Rigorous electronic structure calculations have been performed for the mentioned threefold effect on not only the band‐edge alignment, but going beyond to it by further constructing the reaction coordinate mapping corresponding to HER and OER mechanism. The repercussion of external strain in addition to the single atom functionalization and vacancy defect on the adsorption free energies of the prime intermediates of HER and OER reaction has been further correlated with the work function variation in this pseudo‐monolayer system.

Effect of Atomic Vacancy Defect in h – BN Monolayer on Electronic Properties: A DFT Study

Abstract h-BN is a 2D material which also has considerable development potential in several fields. 2D materials such as h-BN are basically insulating materials but have unique properties and it would be good if they could be developed, especially to meet future energy needs or in the development of electronic devices. Therefore, the properties of h-BN are certainly needed to support this, one of which is the magnetic properties of h-BN. In this research, we modified the h-BN structure by providing defects in the form of vacancy defects by removing atoms in the h-BN layer. We modeled a monolayer h-BN with a supercell size of 4 x 4 x 1, and our calculations were carried out with Quantum ESPRESSO, which is based on density functional theory. The results we obtained showed the emergence of magnetic moment values in h-BN due to defects in the form of atomic vacancies. Furthermore, this research successfully illustrates how vacancy defects in h-BN can modify the band structure, leading to changes in electron behavior around the Fermi energy level.

Endowing 1T'-ReS2 Nanosheets with Sonopiezoelectric Property by Theoretical-Guided Vacancy-Manipulated Peierls Distortion for Tumor Ferroptosis Therapy.

Sonopiezoelectric therapy harnesses piezoelectric materials to efficiently generate destructive reactive oxygen species when exposed to ultrasound. This innovative approach shows promise for tumor treatment by combining precise targeting of tumor sites through noninvasive ultrasound control with high reactive oxygen species generation capabilities via the piezoelectric effect. This study utilizes a theoretical-guided method to manipulate atomic vacancy defects and regulate the Peierls distortion in 1T'-ReS2 nanosheets, thereby imparting them with sonopiezoelectric properties not inherent to the original material. Furthermore, the plentiful unsaturated sites of ReS2 nanosheets endow them with excellent catalase- and peroxidase-mimicking activities. The reactive oxygen species generation by the engineered ReS2 nanosheets also leads to the depletion of glutathione. These capabilities are leveraged for tumor ferroptosis therapy via the classical pathway involving the 7-member 11-glutathione-GPX4 signaling axis, alongside the downregulation of dihydroorotate dehydrogenase and ferritin levels and the upregulation of fatty acid CoA ligase 4 expression. This showcases the innovative approach and potential applications of employing 1T'-ReS2 nanosheets in cancer treatment through theoretical design and materials engineering.

Sulfur vacancy riched pure 2H phase VS2 for efficient electrocatalytic hydrogen evolution

Vanadium disulfide (VS2), a typical metallic layered transition metal chalcogenide (TMC), has been widely concerned for its high electrical conductivity and reactivity in electrochemical energy storage and conversion applications. The 2H phase VS2 is expected to exhibit high electrocatalytic activity and be more stable for hydrogen evolution reaction (HER) than the 1T phase structure. But at present, there still lacks a facile method to prepare pure 2H phase VS2. Herein, we successfully prepared pure 2H–VS2 through a simple one-step low-temperature hydrothermal method. We have investigated the effect of the hydrothermal reaction conditions (including the proportion of raw materials and hydrothermal reaction temperatures) on the phase composition and microstructure. Under the optimal condition of 140°C-24 h with a vanadium sulfur ratio of 1:5, a nano-flower-like 2H–VS2 material with high crystallinity and rich sulfur vacancy defects was obtained. Leveraging these structural advantages, as a demonstration, the product exhibits excellent electrocatalytic HER activity (with η10 of 181 mV, Tafel slope of 52 mV dec−1, and J0 of 67.9 × 10−3 mA cm−2) and stability (it can continuously produce hydrogen for 20 h at a current density of 50 mA cm−2 with ultra-small increased potential of less than 5 mV). This work provides a new way for constructing atomic vacancy defects in layered TMCs for efficient electrocatalysis and is also expected to promote the research of the basic properties of high phase purity TMCs.

Multi‐Point Collaborative Passivation of Surface Defects for Efficient and Stable Perovskite Solar Cells

AbstractThe inherent defects (lead iodide inversion and iodine vacancy) in perovskites cause non‐radiative recombination and there is also ion migration, decreasing the efficiency and stability of perovskite devices. Eliminating these inherent defects is critical for achieving high‐efficiency perovskite solar cells. Herein, an organic molecule with multiple active sites (4,7‐bromo‐5,6‐fluoro‐2,1,3‐phenylpropyl thiadiazole, M4) is introduced to modify the upper interface of perovskites. When M4 interacts with the perovskite surface, the active bromine (Br) site interacts with lead (Pb) at the surface to repair iodine atomic vacancy defects. The fluorine (F) site of M4 interacts with Pb to correct octahedral crystal lattice distortions and eliminate PbI defects. Additionally, sulfur–iodine (S–I) interactions reduce I–I dimerization and eliminate IPb defects. It is also calculated that the energy level of M4 aligns with the band gap, promoting charge transfer. As a result, the perovskite devices achieve an efficiency of 25.1%, a stabilized power output (SPO) of 25.0%, a voltage of 1.19 V, and a fill factor of 85.2%. The device retains 95% of its initial efficiency after 2000 h of ageing in a nitrogen atmosphere. Thus, multi‐point cooperative passivation of surface defects provides an effective method to improve the efficiency and stability of perovskite solar cells.

Effect of monovacancy defects on anisotropic mechanical behavior of monolayer graphene: A molecular dynamics study

The mechanical properties of monolayer graphene were studied using molecular dynamics (MD) simulation. The simulation results show that the tensile stress in the [010] crystal direction is greater than that in the [110] and [100] directions under uniaxial tensile. This confirms that graphene exhibits anisotropic behavior when subjected to external loading, resulting in changes in the Young's modulus in different directions. The calculated Young's modulus were 764.68 GPa, 532.50 GPa and 697.83 GPa for the [010], [110] and [100] directions, respectively. On this basis, the effect of defects on the mechanical properties was also considered. When there are vacancy defects, the stress concentration is linearly distributed along the Zigzag direction on both sides of the atomic vacancy defects. The effect of crack defects on the mechanical properties of graphene [010] crystal direction is the most significant. With the increase of crack size, the difference in fracture strength and fracture toughness calculated by MD simulation and the Griffith model decreases gradually. The potential energy of the non-defect model was found to be the highest, with the potential energy decreasing as the defect size increases. In addition, it was found that the strain rate also affects the stress-strain behavior of the monolayer graphene. However, when the crack size is large, the difference in the effect of strain rate on the anisotropic tensile fracture stress can be ignored.

Pd(II)/Pd(IV) redox shuttle to suppress vacancy defects at grain boundaries for efficient kesterite solar cells

Charge loss at grain boundaries of kesterite Cu2ZnSn(S, Se)4 polycrystalline absorbers is an important cause limiting the performance of this emerging thin-film solar cell. Herein, we report a Pd element assisted reaction strategy to suppress atomic vacancy defects in GB regions. The Pd, on one hand in the form of PdSex compounds, can heterogeneously cover the GBs of the absorber film, suppressing Sn and Se volatilization loss and the formation of their vacancy defects (i.e. VSn and VSe), and on the other hand, in the form of Pd(II)/Pd(IV) redox shuttle, can assist the capture and exchange of Se atoms, thus contributing to eliminating the already-existing VSe defects within GBs. These collective effects have effectively reduced charge recombination loss and enhanced p-type characteristics of the kesterite absorber. As a result, high-performance kesterite solar cells with a total-area efficiency of 14.5% (certified at 14.3%) have been achieved.

Impact of Atomic Vacancy Defects on Vibrational Characteristics of Chiral MWCNT for Viruses/Bacteria

In this study the authors investigated the vibrations performance of chiral multi-walled carbon nanotubes (MWCNTs). They examined chiral MWCNTs with attached bacteria positioned at both the tip and middle sections of the nanotubes. The main goal of this research was to create a sensor with the ability to detect and distinguish bacteria or viruses that could potentially adhere to the surfaces of chiral MWCNTs. The authors considered two boundary conditions, fixed-fixed and fixed -free for the chiral MWCNT. They utilized a molecular structural mechanics approach. In this approach the vibrational responses of chiral MWCNT-based nano-biosensors with different diameters of the chiral MWCNTs were examined. The study’s objective was to fill research gaps by introducing an innovative approach for detecting defects in chiral MWCNTs through vibrational analysis. They simulated 432 MWCNT samples to investigate the vibrational performance of defective chiral MWCNTs. The results suggested that the first mode of the vibrational frequency of the chiral MWCNT is particularly significant for sensing applications. It was observed that as the percentage of vacancy defects increased, the natural frequency decreased for both boundary conditions.

Orbital hybridization and defective states of vacancy defects in AlN

Intrinsic atomic defects pose a great effect in wide-bandgap semiconductors owing to the high tendency of forming in-gap defective states. Utilizing density-functional theory, the atomic vacancy defects of Al and N vacancies in hexagonal wurtzite aluminum nitride (AlN) are investigated. We find that the most energetically favorable form of vacancies is predicted to be negatively trivalent charged Al vacancy (VAl−3) in n-type AlN and positively monovalent charged N vacancy (VN+1) in p-type AlN, respectively. N vacancy generally has a low formation energy and could be either an acceptor or a donor: Under p-type conduction, it is a deep donor with a positively monovalent charge of +1, acting as a compensating center for holes; Under n-type conduction, it is a deep acceptor carrying charge of −3, acting as a compensating center for electrons. For the Al vacancy, it tends to be a shallow acceptor in p-type AlN but a high formation energy or a deep acceptor in n-type AlN with low formation energy, most possibly at a negatively trivalent charged state. Our orbital hybridization analysis shows that these vacancies can be facilely generated which are rooted in the antibonding coupling Al3 s-N2p states in valence band. The above finding is independent of the chemical potential of nitrogen and aluminum and holds true under both nitrogen rich and poor conditions. Our work provides a guide to experimental works by providing the energetics and atomic-scale mechanism of doping nature of atomic vacancies in AlN.

Effect of lattice defects on electronic structure and thermoelectric properties of 2D monolayer MoS2

Molybdenum disulfide (MoS2) is considered as a promising economical and non-toxic thermoelectric material due to its low thermal conductivity and high Seebeck coefficient. However, its power factor is not ideal, which limits its energy conversion efficiency in thermoelectric applications. In this paper, the electronic structure of pure and lattice-defect MoS2 2D monolayer is studied by using the first principles method and non-equilibrium Green's function (DFT-NEGF). The effects of atom substitution and vacancy defect on the thermoelectric properties of MoS2 2D monolayer are also investigated. The result shows that the S-vacancy changes the bandgap by introducing donor and acceptor levels, while the substitution of Se and Te atoms slightly increases the bandgap value. In the case of n-type doping, the ZT value of the S atom substituted MoS2 reaches 1.19 at 300 K, while the Te atom substitution MoS2 has a higher ZT value of almost 2.26 at 800 K. The analysis shows that the thermal conductivity of MoS2 decreases due to the presence of S-vacancy, while the atomic substitution of Se and Te increases the power factor of MoS2. This work may provide a new method and theoretical guidance for achieving high performance of thermoelectric devices in the energy industry.

Effects of atomic vacancy defects and their evolution mechanisms on the fracture of carbon nanotubes

Effects of atomic vacancy defects and their evolution mechanisms on the fracture of carbon nanotubes

Dual efficiency enhancement in overall water splitting with defect-rich and Ru atom-doped NiFe LDH nanosheets on NiCo2O4 nanowires

The development of layered double hydroxides (LDHs) as bifunctional catalysts for efficient overall water splitting by controlling the composition and structure of the material remains challenging. In this study, defect-rich and Ru-doped NiFe LDH nanosheet shells on NiCo2O4 (NCO) nanowires were successfully synthesized using a hydrothermal method, followed by alkaline etching. The resulting def-Ru-NiFe LDH/NCO (def = defects) catalyst exhibited remarkable catalytic activity with overpotentials of 122 and 225 mV for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, at a current density of 10 mA cm−2. Density functional theory calculations demonstrated that atomic vacancy defects and the rational incorporation of Ru single atoms in def-Ru-NiFe LDH/NCO induced the regulation of the electronic structure and optimized the adsorption energies, thus enhancing the OER and HER performances. Particularly, an electrolyzer utilizing def-Ru-NiFe LDH/NCO exhibited exceptional performance, achieving cell voltages of 1.58 and 1.46 V in 1 M KOH at 25 and 75 °C, respectively. Moreover, the catalyst exhibited outstanding stability during 60 h of operation at a high applied voltage and temperature (75 °C). These results highlight the potential of def-Ru-NiFe LDH/NCO for hydrogen production.

Preparation, thermophysical properties, and thermal shock resistance of Yb2O3-doped YSZ coatings

The yttria-stabilized zirconia (YSZ) coatings were doped with various concentrations of Yb2O3 to build a multi-component solid solution system. The effects of concentration on the phase composition, microstructure, phase stability at high temperatures, thermal conductivity, thermal expansion, fracture toughness, elastic modulus, and other thermophysical properties of the coatings were systematically analyzed. Furthermore, the mechanism through which Yb2O3 doping impacts the thermal shock resistance of YSZ coatings was elucidated. The results reveal that doping with Yb3+, which has a similar radius to Zr4+, allows the YSZ coatings to retain the tetragonal phase structure at low doping concentrations. The Yb2O3 doping can reduce the thermal conductivity of YSZ coatings due to replacement atom and oxygen vacancy defects enhancing phonon scattering and decreasing the mean free path of phonons. The 1YbYSZ coating demonstrates better thermal shock resistance owing to its high CTE and high fracture toughness resulting from its tetragonal phase, high elastic modulus and high grain boundary density. The Yb2O3 doping can form defect clusters and increase the cation diffusion path due to oxygen vacancies, which inhibits the uneven diffusion of Y3+. Therefore, Yb2O3-doped YSZ coatings demonstrate remarkable phase stability at high temperatures of 1700 °C.

Nonmonotonic Effects of Atomic Vacancy Defects on Friction.

The presence of defects such as vacancies has a significant impact on the frictional properties of 2D materials that are excellent solid lubricants. In this study, we demonstrate that the nonmonotonic effect of Te vacancy defects on the friction of MoTe2 is related to the change in the maximum sliding energy barrier due to the variation in tip position. The experimental results of atomic force microscopy suggest that the friction shows an overall increasing trend with the increase in Te vacancy density, but this variation is nonmonotonic. Molecular dynamics simulations show that the increase in friction force with defect density can be attributed to the large and more sliding energy barriers that the tip has to overcome. Furthermore, the nonmonotonic variation of friction with defect density is dominated by the change of the maximum sliding potential barrier caused by the variation of tip position perpendicular to the sliding direction during the sliding process. Additionally, the uneven charge distribution due to charge transfer occurring at the defect also contributes to the increase in friction. This work shows the mechanism of the effect of Te vacancy defects on the friction of MoTe2, which provides guidance for the modulation of the frictional properties of solid lubricants.

Engineering Cationic Vacancies in Octahedral Sites of Co3O4 for High-Efficiency Oxygen Evolution

The octahedral sites in Co3O4 usually possess better oxygen evolution reaction (OER) activity, which have been identified as the active sites. Atomic cationic vacancy defects can elicit localized electron redistribution to regulate the electronic structure of the host materials. In this paper, Co3O4 nanosheet arrays with cationic vacancies at octahedral sites (Vx-Co3O4) have been synthesized by a facile three-step reaction process. Density functional theory (DFT) computational results further indicate that the cationic vacancies can increase the number of electrons per Co atom and optimize the adsorption energy of the intermediate product. As a result, the optimized V0.3-Co3O4/CC electrode with abundant cationic vacancies displays excellent OER performance in alkaline electrolytes with an overpotential of 240 mV at 10 mA cm–2, Tafel slope of 72.2 mV dec–1, and superior stability, which are much better than those of the pure Co3O4 and commercial IrO2.

Nanoresonator vibrational behaviour analysis of single- and double-layer graphene with atomic vacancy and pinhole defects

Nanosensors and actuators are frequently made of graphene. Any defect in the graphene's manufacturing has an impact on its sensing performance and on its dynamic behaviour. Using a molecular dynamics technique, the influence of pinhole defects and atomic defects on the performance parameters of single-layer graphene sheets (SLGSs) and double-layer graphene sheets (DLGSs) with various boundary conditions and lengths is explored. In contrast to the perfect nanostructure of a graphene sheet, defects are described as holes formed by atomic vacancies. As the number of defects increases, the simulation results show that the presence of defects has the greatest impact on the resonance frequency of SLGSs and DLGSs. The influence of pinhole defect (PD) and atomic vacancy defect (AVD) on armchair, zigzag, and chiral SLGSs and DLGSs was investigated in this article using molecular dynamics simulation. The influence of both types of defects is largest when it is adjacent to the fixed support for all three different types of graphene sheets, i.e. armchair, zigzag, and chiral. The structure of the graphene sheet has been created using ANSYS APDL software. In the structure of the graphene sheet, atomic and pinhole defects have been generated. SLG and DLG sheets are modelled using a space frame structure that is identical to a three-dimensional beam. Dynamic analysis of single-layer and double-layer graphene sheets performed with different lengths using the atomistic finite element method. The interlayer separation in the form of Van der Waals interaction is modelled using characteristic spring element (Combin14). The upper and lower sheets of DLGSs are described as elastic beams connected by a spring element. With atomic vacancy defect for the bridged boundary condition, the highest frequency of 2.86 × 108 Hz was found for zigzag DLG (20 0) and with same boundary condition for pinhole defect 2.79 × 108 Hz frequency achieved. In a single-layer graphene sheet with an atomic vacancy and cantilever boundary condition, the maximum efficiency was 4.13 × 103 Hz for SLG (20 0), while in a pinhole defect, it produced 2.73 × 107 Hz. Moreover, the elastic parameters of beam components are calculated using the mechanical properties of covalent bonds between carbon atoms in the hexagonal lattice. The model has been tested against previous research. The focus of this research is to develop a mechanism for determining how defects affect graphene frequency band in application as nano resonators.

Insights into the diffusion migration behavior of alloy atoms at the graphene/aluminum interface: First-principles calculations

Based on the density functional theory of first-principles, diffusion migration behavior of alloy atoms in the aluminum matrix and the graphene/aluminum interface are investigated in this paper. By comparing the migration barriers of aluminum atoms at the aluminum matrix and at the graphene/aluminum interface, it is found that aluminum atoms prefer to migrate toward the interface to provide conditions for the formation of the brittle phase Al4C3. Diffusion migration behaviors of 41 alloying atoms at the aluminum matrix and graphene/aluminum interface are calculated, and a group of alloying elements (Sc, Cu, Si, Ni, etc.) that tend to aggregate at the interface are selected. It is also found that Si, Cu, Ni, and Co are more likely to migrate to the interface than Al atoms. Finally, the migration behavior of alloying atoms at the graphene/aluminum interface with a carbon atom vacancy defect are explored, compared to the perfect interface, the migration energy of most alloying atoms at the graphene/aluminum interface with a carbon atom vacancy defect does not vary much, while Sc and B atoms prefer the graphene/aluminum interface with vacancy defect. The results of the theoretical study are expected to provide a theoretical basis for the experimental design of future high-performance graphene/aluminum composites.

Electrical tuning of valley polarization in monolayer transition metal dichalcogenides

The valley degree of freedom in two-dimensional crystalline materials is another promising candidate for designing dissipationless information devices in addition to the spin degree of freedom. The primary challenge for the application of valley degree of freedom is to achieve a significant and controllable valley polarization. Here, we discover an electrical tunable mechanism for generating valley polarization caused by the backscattering induced by Rashba spin-orbit coupling in monolayer nanoribbon of transition-metal dichalcogenides. Since the Rashba spin-orbit coupling in monolayer transition-metal dichalcogenides can be induced by the external electric field, the valley polarization here can be dynamically controlled by means of the external gating. The results of the valley-dependent transport calculations show that a pronounced valley polarization can be generated when the external electric field-induced Rashba spin-orbit coupling is present in the central scattering region of the nanoribbon. Moreover, the efficiency and direction of the valley polarization can be controlled by tuning the Fermi energy and the strength of Rashba spin-orbit coupling. The calculated results also confirm that the valley polarization remains significant in the presence of atomic vacancy defects. This finding opens up a potential possibility for realizing electrical control of the valleytronics devices.

Atomic-Scale Defects Might Determine the Second Harmonic Generation from Plasmonic Graphene Nanostructures.

In this work, we theoretically investigate the impact of the atomic scale lattice imperfections of graphene nanoflakes on their nonlinear response enhanced by the resonance between an incident electromagnetic field and localized plasmon. As a case study, we address the second harmonic generation from graphene plasmonic nanoantennas of different symmetries with missing carbon atom vacancy defects in the honeycomb lattice. Using the many-body time-dependent density matrix approach, we find that one defect in the nanoflake comprising over five thousand carbon atoms can strongly impact the nonlinear hyperpolarizability and override the symmetry constraints. The effect reported here cannot be captured using the relaxation time approximation within the quantum or classical framework. Results obtained in this work have thus important implications for the design of nonlinear graphene devices.

The molecular dynamics study of vacancy defect influence on carbon nanotube performance as drug delivery system

In the current research, we described atomic vacancy defect influence on nanopumping performance of CNT structure with molecular dynamics (MD) method (for the first time). In our simulations, CNT structure is defined in the presence of C20 molecule and Au tips in the nanopumping process. Temperature and potential energy convergence after 1 ns indicated the atomic stability in defined systems, which this result arises from the appropriate settings inside the MD box. Also, the nanopumping process was detected after 0.26 ps for the CNT structure without any vacancy defect. By atomic defect implementation to C-based nanotube, the nano-pumping performance of this tube was disrupted. Numerically, by inserting 1% vacancy to pristine CNT, the nanopumping was detected after 0.29 ps. Furthermore, MD outputs predicted by implementing vacancy defect with a larger 10% ratio, nanopumping process doesn't occur. On the other hand, by nanopumping parameters optimization in defected CNT (with 1% defect), this atomic process (nanopumping) occurs after 0.24 ps.