Defects In Monolayer Research Articles

Modulation of Carbon Nanotube Electronic Structure by Grain Boundary Defects in RbI on Au(111).

The electronic properties of single-walled carbon nanotubes (SWCNTs) are known to be highly sensitive to environmental effects. Here, we use scanning tunneling microscopy and spectroscopy to investigate the electronic properties of SWCNTs deposited on RbI monolayer films grown on Au(111). We find that grain boundary defects in RbI monolayers cause the appearance of spatially confined localized states in the SWCNTs. Our density functional theory calculations show that grain boundary defects in RbI/Au(111) produce a stabilizing electrostatic potential caused by reduced coordination of iodine atoms at the RbI grain boundary. The presented results may offer insights into the performance of devices involving transport through SWCNTs subjected to external electrostatic disorder.

Exploring the adsorption characteristics of toxic CO gas on pristine, defective, and transition metal-doped I-AsP monolayer

Utilizing first-principles calculations based on density functional theory, the adsorption characteristics of toxic carbon monoxide (CO) gas adsorbed on pristine, defective, and transition metal (TM) atoms (Sc, Mn, Fe, Ni, Cu, and Zn)-doped blue arsenic-phosphorene (I-AsP) are investigated to design potential sensing platforms. The pristine and defective monolayers exhibited weak CO gas sensing performance due to weak adsorption and unaltered electronic properties. In contrast, TM-doped monolayers exhibited strong interactions and enhanced the adsorption sensitivity due to their chemisorption behavior towards CO molecule. Furthermore, electronic properties obtained through the spin density of states analysis show significant changes in the adsorption of CO molecules on the doped I-AsP monolayer, influencing electrical conductivity. The significant change in the work function of system monolayers after CO adsorption confirmed that the TM-doped I-AsP monolayer is sensitive to CO gas. Therefore, the TM-doped I-AsP monolayer possesses strong sensitivity to CO gas, among which the Sc and Cu-doped monolayers emerge as promising candidates for toxic CO gas detection.

Potential application of a defective NiCl2 monolayer as a toxic gas sensor: a computational study

An experimentally feasible defective NiCl2 monolayer is used as a substrate in this paper, and its feasibility as a gas sensor for detecting O2 and CO gases under different conditions is demonstrated by density functional theory calculations.

First-principles study of NO adsorption on defective hexagonal boron nitride monolayer

This study explored the potential of hexagonal boron nitride (h-BN) as a support for selective catalytic reduction (SCR) which means converting NOx to N2 and H2O with the aid of a catalyst. Specifically, we investigated the adsorption behavior of a NO molecule on a defective h-BN monolayer using density functional theory. Our results indicated that the initial configuration of N and O atoms over vacancy defects plays a key role in determining whether the NO molecule is adsorbed on the h-BN monolayer via covalent or ionic bonding. In the case of mono-vacancy, the ON configuration of the NO gas resulted in lower total energy (Etotal) compared with the NO configuration. Furthermore, electron localization function and Bader charge analysis, as well as projected density of states, revealed that the ON configuration over B or N vacancy resulted in chemisorption by hybridizing p-orbitals. Furthermore, the di-vacancy system led to an even lower Etotal compared with the mono-vacancy case. In particular, both N and O atoms were adsorbed chemically to the edge of vacancy defects, irrespective of the NO configurations above the di-vacancy. Our calculations revealed that the presence of vacancy defects contributed to improving the adsorption of the NO molecule to the h-BN monolayer.

Adsorption of gas molecules on intrinsic and defective MoSi2N4 monolayer: Gas sensing and functionalization

The adsorption behaviors of CO, NH3, NO, and NO2 on intrinsic and defective MoSi2N4 monolayer, and the electronic properties, magnetic and gas sensing properties are researched by first-principles calculations. The low adsorption energies of gas molecules adsorbed on the intrinsic MoSi2N4 systems demonstrate that physisorption characteristics. The CO and NH3 adsorbed on MoSi2N4 systems exhibit non-magnetic semiconductor behavior, whereas the NO and NO2 adsorbed on MoSi2N4 systems emerge magnetic semiconductor behavior. The recovery time of the CO, NH3, NO, and NO2 adsorbed on MoSi2N4 systems is 1.70 × 10−11, 3.6 × 10−10, 2.60 × 10−11, and 3.21 × 10−10 s, respectively. Meanwhile, the sensitivities are 0.42 (CO), 0.41 (NH3), 0.51 (NO), and 0.27 (NO2) for the intrinsic MoSi2N4, respectively, demonstrating that MoSi2N4 can be an invertible sensor to detect CO, NH3, NO, and NO2 in the environment. Interestingly, the N defect MoSi2N4(VN/MoSi2N4) system displays magnetic semiconductor features, while the NO adsorbed VN/MoSi2N4 system has non-magnetic semiconductor feature. The Si defect MoSi2N4 (VSi/MoSi2N4) system appears magnetic semimetal characteristics, while the NH3 adsorbed VSi/MoSi2N4 system appears nonmagnetic semiconductor characteristics. Compared with the intrinsic MoSi2N4, the defect MoSi2N4 adsorption systems has stronger adsorption energy, suggesting that the stronger capture ability of the defect MoSi2N4 for CO, NH3, NO, and NO2.

Single transition metal anchored on defective boron carbide monolayer for efficient and selective CO2 electrochemical reduction: A theoretical study

Electrochemical reduction of CO2 to produce fuels and industrial chemicals shows significant promise for reducing greenhouse gas emission and mitigation the energy crisis. The exploitation of highly efficient catalysts has long been considered an attractive yet challenging task. Herein, we systematically studied the electrocatalytic performance of a range of transition metal (TM) single atom anchored on defective BC3 monolayer, specifically those with a boron vacancy (VB) and a carbon vacancy (VC), named TM@VB and TM@VC, respectively, as single-atom catalysts (SACs) for CO2 reduction reaction (CO2RR) via density functional theory (DFT). We screened a total of 28 stable SACs by evaluating the formation energy and dissolution potential of metal atoms. Detail reaction mechanisms involved in CO2 reduction process to produce diverse C1 products were considered. Among the 28 candidates, Cu@VB, Mo@VB, Re@VB, Co@VC, and Ir@VC displayed remarkable efficiency in reducing CO2 into HCOOH at nearly zero or extremely low limiting potentials (UL) of -0.01 ∼ 0 V. These SACs exhibit excellent electrocatalytic activity, surpassing most previously reported catalysts. Furthermore, these five SACs effectively suppressed the competing hydrogen evolution reaction, highlighting their high selectivity toward HCOOH. In addition, the volcano relationship between the ∆G*HCOO and UL was established. This work may offer a promising strategy for the research and development of high-efficiency CO2RR electrocatalysts, which has potential implications for addressing environmental and energy challenges.

Effect of biaxial strain and vacancy defects in 2D MoS2 monolayer for the sensing of nitrobenzene: A DFT investigation

Considering the increased release of toxic nitrobenzene (NB) molecules into the atmosphere, proposing a sensor material that could effectively remove it from the environment is essential. In this regard, we have investigated the NB adsorption performance of vacancy-defected (S, Mo and di S) and biaxially strained (−4 % to 4 %) MoS2 monolayers through density functional theory (DFT) simulations. Applying biaxial (tensile and compression) strain to the monolayer significantly increased the adsorption energy and charge transfer to −1.02 eV and 0.09 e, respectively, much greater than in the pristine case. The formation energy calculations verified the stability of (S, Mo, 2S) vacancy-defected MoS2 monolayer. The low formation energy of 3.3 eV suggests the ease of formation of MoS2 monolayer with single S vacancy. Introducing defects and strain improved the conductivity of the monolayer, thereby increasing the NB adsorption ability. The reasonable NB adsorption energy on the modified monolayer was due to the accelerated charge transfer and orbital interaction between the valence S 3p orbitals of MoS2 and 2p orbitals of O in NB. The recovery time of the strained and defective monolayers is the order of only a few seconds at 400 K, which is quite attainable. Therefore, this work put forth a more effective to improve the NB performance of the monolayer and can be a motivation for the further development of high-performance biomolecule sensors.

C3N-supported ⅢA group metal single-atom catalysts with different coordination microenvironments: Electrocatalytic NO reduction to ammonia

Electrochemical NO reduction reaction (NORR) to NH3 is regarded as a low-cost and environmentally friendly method that can simultaneously remove harmful NO and synthesize useful NH3. Using main group metal (MGM) atoms as active centers has the potential to enhance the selectivity of single-atom catalysts (SACs) for NH3. Herein, we selected Al, Ga and In atoms as active centers and designed 12 MGM SACs supported on the defective C3N monolayer with four coordination environments: MGM-C2N, MGM-C3, MGM-C2N2 and MGM-C4. Based on density functional theory (DFT) calculations, the NORR electrocatalytic performance of these 12 SACs were systematically investigated. Detailed studies on the NORR mechanism show that seven of these SACs exhibit excellent activity in reducing NO to NH3, effectively preventing the formation of byproducts (N2O, N2 and H2). Further stability calculations led to the identification of five promising NORR electrocatalysts with both excellent catalytic performance and stability: Al/Ga/In-C2N2 and Ga/In-C4. Moreover, the Bader charge (QMGM) of the MGM atom was used to explain the variation in activity resulting from different coordination environments. This work not only provides valuable insights for future exploration of MGM SACs, but also contributes to a better understanding of the impact of coordination environments on catalytic activity.

Defect engineered magnetism induction and electronic structure modulation in monolayer MoS2

The electronic, magnetic, and optical characteristics of a defective monolayer MoS2 were examined by employing density functional theory (DFT)-based first-principles calculations. The effects of several defects on the electrical, magnetic, and optical properties, including Mo vacancies, MoS3 vacancies, and the substitution of a single Mo atom by two S atoms were studied in this work. Our first-principles calculations revealed that different types of defects produced distinct energy states within the band gap, leading to a band gap reduction after the introduction of various types of defects, which caused a change from semiconducting to metallic behavior. The spin-up and spin-down states were separated in the case of MoS3 vacancy. The total magnetization was ∼ −0.83 μB/cell, and the absolute magnetization was ∼ 1.23 μB/cell. Moreover, spin-up states had a 0.45 eV band gap, whereas spin-down states were metallic. Consequently, it can be promising for spin filter applications. It was disclosed that the broadband part of the electromagnetic spectrum has a high absorption coefficient, which is necessary for applications including impurity detection, photodiodes, and solar cells. Designing spintronic and optoelectronic devices will benefit from the modification of the electrical, optical, and magnetic properties by defect engineering of MoS2 monolayers presented here.

Engineering Vacancies for the Creation of Antisite Defects in Chemical Vapor Deposition Grown Monolayer MoS2 and WS2 via Proton Irradiation.

It is critical to understand the laws of quantum mechanics in transformative technologies for computation and quantum information science applications to enable the ongoing second quantum revolution calls. Recently, spin qubits based on point defects have gained great attention, since these qubits can be initiated, selectively controlled, and read out with high precision at ambient temperature. The major challenge in these systems is controllably generating multiqubit systems while properly coupling the defects. To address this issue, we began by tackling the engineering challenges these systems present and understanding the fundamentals of defects. In this regard, we controllably generate defects in MoS2 and WS2 monolayers and tune their physicochemical properties via proton irradiation. We quantitatively discovered that the proton energy could modulate the defects' density and nature; higher defect densities were seen with lower proton irradiation energies. Three distinct defect types were observed: vacancies, antisites, and adatoms. In particular, the creation and manipulation of antisite defects provides an alternative way to create and pattern spin qubits based on point defects. Our results demonstrate that altering the particle irradiation energy can regulate the formation of defects, which can be utilized to modify the properties of 2D materials and create reliable electronic devices.

First-Principles Study of Titanium and Lithium Adsorption on Perfect and Defective Hexagonal Boron Nitride Monolayer Under Effects of Charging

Single Titanium (Ti) and Lithium (Li) atoms adsorption on Pristine and defective hexagonal boron nitride (P-h-BN and BV-h-BN) monolayer were employed using Density Functional Theory (DFT) under effect of charging. Obtained data reveal that Li adsorption on P-h-BN is weak, while Ti adsorption on P-h-BN is strong. When Ti and Li atoms interact with P-h-BN surface, Ti and Li generate 4 µB/cell and 1 µB/cell magnetic moments, respectively. The extraction of an electron from the systems leads to a considerable rise in the adsorption energy, notably in the case of Li-P-h-BN. There is a notable decrease in the band gap of Ti-P-h-BN in both the charged states, especially in the electron-added state. Removing an electron from the Li-P-h-BN system results in a non-magnetic state and a significant increase of the band gap to 4.07 eV. Ti-BV-h-BN system shows significantly stronger adsorption energy due to the d-orbitals of the Ti atom. When an electron is added to the systems, the interaction energy between Ti and BV-h-BN decreases, while the interaction energy between Li and BV-h-BN increases. Moreover, removing an electron from Ti-BN-h-BN increases the band gap to 2.29 eV and the disappearance of the magnetic moment.

Revealing defect-bound excitons in WS2 monolayer at room temperature by exploiting the transverse electric polarized wave supported by a Si3N4/Ag heterostructure

Abstract Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers are promising materials for light-emitting devices due to their excellent electric and optical properties. However, defects are inevitably introduced in the fabrication of TMDC monolayers, significantly influencing their emission properties. Although photoluminescence (PL) is considered as an effective tool for investigating the defects in TMDC monolayers. However, the PL from the defect-bound excitons is revealed only at low temperatures. Here, we show that the PL from the defect-bound excitons in a WS2 monolayer can be effectively revealed at room temperature by exploiting the transverse electric polarized wave supported by a Si3N4/Ag heterostructure. It is revealed that the defect-bound excitons in all possible positions of the WS2 monolayer can be effectively excited by the TE wave with significantly enhanced in-plane electric field localized on the surface of the Si3N4 layer. In addition, the emission from defect-bound excitons can propagate to the collection point with small attenuation. More importantly, the exciton dynamics in the WS2 monolayer can be modified by the Si3N4/Ag heterostructure, allowing the simultaneous excitation of neutral excitons, charge excitons (trions), and defect-bound excitons in the WS2 monolayer attached on the Si3N4/Ag heterostructure. We inspect the PL spectra obtained at different positions and find that the relative intensity of defect-bound excitons depends on the collection position. We also examine the dependences of the PL intensity and bandwidth on the excitation power for the three types of excitons. It is found that they exhibit different behaviors from those observed in the optical measurements by using the traditional excitation method. Our findings suggest a new way for exciting and studying the dynamics of multi-excitons at room temperature and indicate the potential applications of the TE wave in probing the defects in TMDC monolayers.

The defective C3N monolayers as high-efficient hydrogen purification membranes: DFT calculations and MD simulations

The design of new high-performance membranes for separation and purification of hydrogen remains highly desirable for industrial applications. Herein, using the density functional theory calculations along with MD simulations, we demonstrated firstly a new 2D membrane based C3N monolayer (D-C3N) with intrinsic pores and then investigated its potential as gas separation membranes for H2 purification. The cohesive energy and ab initio MD simulations confirmed that the D-C3N monolayer is structurally and thermodynamically stable under 1800 K. All considered gas molecules are physisorbed on the D-C3N monolayer with small interaction energy. At room temperature, the D-C3N membrane for H2 gas has high selectivity over other gases such as 1.5 × 103, 1.8 × 105, 8.8 × 109, 1.4 × 1012, and 1.3 × 1019 for H2/N2, H2/CO2, H2/O2, H2/H2O, and H2/CH4 at 300 K, respectively, and the H2 permeance is as high as 2.2 × 10−5 mol·m−2·s−1·Pa−1, exceeding the industrially acceptable value and most of the carbon-based separation membranes. In addition, MD simulations further confirmed that the defective porous C3N monolayer has ideal selectivity and permeation as a promising separation membrane for H2 purification from other gases for industrial applications.

Computational Modeling, Simulation and Tight–Binding Analysis of Electronic Characteristics of Defective Germanene Nanoribbons for Future Applications of High Sensitivity Ge-Based Nanosensors

In this research, the electronic characteristics of germanene sheet and nanoribbons using the computational modeling, simulation and tight binding approximation are investigated. Our analysis is focused on the pristine sheet of germanene as well as defective monolayer. The obtained results show that applying the Stone–Wales defect into the germanene monolayer changes the energy band structure. The E-k curves around the Dirac point are no longer linear, in which a band gap is opened, and the Fermi velocity is reduced. Furthermore, the main parameters such as density of states, carrier concentration in degenerate and non-degenerate limits, carrier effective mass, conductance and AC conductivity of germanene are analytically modeled with the inclusion of the spin–orbit coupling effect, temperature and ribbon width. Obtained results demonstrate that the inclusion of the spin–orbit coupling makes a small splitting of the energy levels and creating a small band gap. Finally, the Tight binding and computational values are compared with our simulation results and available data, and a rational agreement is reported in terms of trend and value. The findings of this study provide theoretical reference for the design of germanene-based nanosensors and optoelectronic devices.

Theoretical evaluation of gas sensing and capturing characteristics on the point defective diboron dinitride monolayer

Defects can significantly alter the properties of 2D material surfaces, thereby affecting the application results. In this work, the adsorption behaviors of H2S, SO2, NH3, and NO2 gas molecules on rationally defective diboron dinitride (n-BN) monolayers in the presence and absence of an external electric field and moisture condition are studied by the first-principles calculation approach. The results demonstrate that the boron and nitrogen defects are more likely to be generated and both defective monolayers have strong absorption effects on SO2 and NO2 gas molecules with absolute values of adsorption energy above 2.5 eV. And the conductivity change after adsorption of NO2 molecule is particularly pronounced in nitrogen defect monolayer, thus greatly increasing the sensitivity to the gas molecule. Based on the high sensitivity, the defective monolayer could be an ideal one-time scavenger for SO2 and NO2 gas molecules. While the H2S and NH3 molecules could obtain the appropriate adsorption energy and the adjustable recovery time on the defective monolayer under the dry or existent external electric field environment. These results make it possible to design a useable gas device with good gas sensing and capturing characteristics on the defective n-BN monolayer.

Synergistic effects of uniaxial strain and vacancy defect on the mechanical and magnetic properties of the C3N monolayer

Using the first-principles calculations, we study the structural, mechanical, magnetic properties and electronic structures of the pristine and defective C3N monolayers with uniaxial tensile strains. Not only the C3N monolayer with a N vacancy (C3N-N) but also the C3N monolayer with a C vacancy (C3N-C) shows a strain-controlled local lattice deformation. The ultimate strain in the same direction is almost unaffected by missing atomic species. In particular, both C3N-N and C3N-C monolayers may embrace a more significant mechanical anisotropy compared to the pristine C3N monolayer, and show a dependence of mechanical anisotropy on uniaxial tensile strain. With increasing axial strain, the C3N-N monolayer retains metallicity, while the C3N-C monolayer displays a semiconductor-to-metal transition. The spin-polarized calculations indicate a vacancy-induced p-electron magnetism in both C3N-N and C3N-C monolayers, and the strain-tuning of p-electron magnetism is demonstrated. Therefore, manipulating the mechanical performance and magnetism of C3N monolayer through the synergy of vacancy defects and strains greatly broadens the application range of C3N monolayer materials.

Nitrogen vacancies in monolayer WSi2N4 for hydrogen evolution reaction: A first-principles study

The recent synthesis of two-dimensional layered WSi2N4 has attracted considerable attention owing to its potential applications in diverse fields. In this study, we use first-principles density functional theory calculations to investigate the catalytic performance of WSi2N4 monolayers with nitrogen (N) vacancies in the hydrogen evolution reaction. The calculated Gibbs free energies of the WSi2N4 monolayers are –0.09 eV and –0.10 eV at the vacancy concentrations of 5.6 % and 12.5 %, respectively, which are close to the zero optimal value. The results indicate that the defective WSi2N4 monolayers can exhibit remarkably high catalytic activity comparable to that of platinum catalysts in the hydrogen evolution reaction. In addition, our electronic structure calculations demonstrate the emergence of spin-polarized states near the Fermi level upon introducing N vacancies, which can be attributed to isotropic Zeeman-type spin-splitting.

Probing Active Sites on Pristine and Defective MnPX3 (X: S and Se) Monolayers for Electrocatalytic Water Splitting.

The state-of-the-art density functional theory approach was used to study the structural and electronic properties of pristine and defective MnPX3 monolayers as well as their activity toward water and hydrogen evolution reaction (HER) catalytic performance. The adsorption behavior of H2O on a pristine MnPX3 structure is of physisorption nature, whereas the adsorption energy is significantly increased for the defective structures. At the same time, the water dissociation process is more energetically favorable, and the reactivity of MnPX3 is determined by the vacancy configuration. Following Nørskov's approach, the HER catalytic performance is evaluated by calculating the hydrogen adsorption free energy on the respective MnPX3 surface. Our calculation results demonstrate that defective 2D MnPX3 with low coordinated P shows significantly higher HER performance compared to the pristine counterpart.

GaN/graphene heterostructures as promising anode materials for Li-ion batteries

In this research, the properties of GaN monolayer, defective GaN monolayer with N vacancies (GaN-VN), and van der Waals (vdW) heterostructures composed of them and graphene (GaN/graphene, GaN-VN/graphene) are systematically investigated using density functional theory (DFT), which includes assessment of the structure's stability, mechanical property, electronic structure analysis, lithium adsorption and diffusion properties, maximum theoretical capacity, and average open-circuit voltage. The calculations show that two-dimensional GaN transforms from semiconductor to metal by forming nitrogen-vacancy defects. This increases lithium adsorption performance while ensuring rapid electron motion in the electrode during lithium adsorption and removal processes. The synergistic effect between the heterostructure layers and the presence of the built-in electric field improves the electron and ion conductivity and lowers the migration energy barriers. In addition, the maximum theoretical capacities of defective GaN-VN, GaN/graphene, and GaN-VN/graphene can reach 970 mAh/g, 884 mAh/g, and 890 mAh/g, respectively, far exceeding those of the conventional graphite anode material (372 mAh/g). The above advantages indicate that defective GaN-VN, GaN/graphene, and GaN-VN/graphene are potential replacements for lithium-ion battery anodes.

Influence of vacancy defects on 2D BeN4 monolayer for NH3 adsorption: a density functional theory investigation

Two-dimensional materials have attracted a great deal of interest in developing nanodevices for gas-sensing applications over the years. The 2D BeN4 monolayer, a recently synthesized single-layered Dirac semimetal, has the potential to function as a gas sensor. This study analyzes the NH3 sensing capacity of the pristine and vacancy-induced BeN4 monolayers using first-principles density functional theory (DFT) calculations. As per the results, the NH3 molecule is physisorbed on the pristine BeN4 via weak Van der Waals interaction with a poor adsorption energy of −0.41 eV and negligible charge transfer. Introducing Be vacancy in BeN4 increased the NH3 adsorption energy to −0.83 eV due to the improved charge transfer (0.044 e) from the defective monolayer to the NH3 molecule. The structural stability, sufficient recovery time (74 s) at room temperature, and superior work function sensitivity promise the potential application of defective BeN4 as an NH3 sensor. This research will be a theoretical groundwork for creating innovative BeN4-based NH3 gas sensors.