First-Principles Study Of The Properties Of Defects In Two-Dimensional MoS₂ Under Electron-Beam Irradiation

Two-dimensional materials have become a current research hotspot in the field of physics because of their unique physical properties and potential applications.Among them,Mo S₂with a tunable bandgap and fast responsivity are promising for high perfor-mance electronic and optoelectronic devices,and are a kind of two-dimensional semi-conductor materials that have attracted much attention.Defects in this material can have a significant impact on its electrical and optoelectronic properties.Since the thickness of Mo S₂monolayer is only three atomic layers and its atomic structure can be charac-terized by direct observation through transmission electron microscopy,seveal studies have used transmission electron microscopy to characterize the point defects in it.How-ever,the results obtained from direct observations may present some incomprehensi-ble phenomena.For example,according to calculations of thermodynamic equilibrium state defect properties,Mo_S2（one molybdenum atom replacing a pair of sulfur atoms）is hardly present in Mo S₂monolayer and Mo_S（one molybdenum atom replacing a sul-fur atom）is more likely to form,but transmission electron microscopy characterization shows the opposite.At present,there is a lack of clear explanation for the contradic-tion between theoretical calculations under equilibrium state and experimental results of electron-beam irradiation.Considering that the defects are in a non-equilibrium state under electron-beam irradiation,it is doubtful whether the defect properties predicted based on the equilibrium state can be used to analyze the experimental observations under electron-beam irradiation.In order to solve the above problems,this thesis com-bines the total energy calculation in equilibrium with the simulation of defect struc-ture transition in nonequilibrium,investigates the properties of defects at the intrinsic point under electron-beam irradiation,and analyzes the difference between the defect atomic structure characterized by transmission electron microscopy and the structure of the semiconductor in equilibrium.Meanwhile,the formation process of defects under electron-beam irradiation is investigated based on the real-time time-dependent density functional theory.The specific research and results are as follows:1.The properties of the intrinsic defects in monolayer Mo S₂under equilibrium conditions is systematically calculated based on the accurate calculation of the electri-cal bandgap.The existing studies of the intrinsic defect properties in monolayer Mo S₂are mainly based on the optical bandgap（1.9 e V）measured by photoluminescence spec-troscopy,but the strong exciton effect in monolayer Mo S₂makes the optical bandgap much lower than the electrical bandgap（2.7 e V）,which leads to errors in the defect properties based on the optical bandgap.In this part,the electrical bandgap is used to correct the formation energy,charge transition level,ionization energy and concentra-tion of the major intrinsic defects in monolayer Mo S₂using first-principles calculations.It is found that the sulfur vacancy defect（V_S）has three charge transition levels（0/+）,（-/0）and（-2/-）simultaneously between the forbidden bands,instead of only（-/0）found in previous studies.Among them,the newly discovered（-2/-）charge transition level indicates that V_S^2-can exist stably in the forbidden band and has been experimen-tally confirmed.In addition,the high concentration of intrinsic defects such as sulfur vacancy,sulfur interstitial（S_i）and molybdenum substitution sulfur antisite（Mo_S）in the monolayer Mo S₂at equilibrium are in the neutral state,which makes the concen-tration of intrinsic carriers in the monolayer Mo S₂low and leads to its poor intrinsic conductivity.2.By studying the ionization phenomenon of defects under electron-beam irradi-ation and the structure of ionized defects,the divergence between the experimentally measured defect concentration and the calculated formation energy is explained,reveal-ing the importance of considering the structure of ionized defects when characterizing defects in two-dimensional materials by transmission electron microscopy.The highest concentrations of intrinsic defects in monolayers of Mo S₂prepared by physical vapor deposition are experimentally measured for Mo_S2（one molybdenum atom replaces a pair of sulfur atoms）and Mo_S（one molybdenum atom replaces a sulfur atom）,and the former is more concentrated than the latter.However,the existing first-principle calcu-lation yields that the formation energy of Mo_Sis smaller than that of Mo_S2,and Mo_Sis easier to form than Mo_S2with higher concentration,which contradicts the experimental results.Therefore,in this part,a method for calculating the ionization probability of de-fects is developed to evaluate the possibility of defect ionization under electron-beam irradiation,and the effect of defect ionization on the characterization of Mo_S2and Mo_Sis explored,taking the above disagreement between experiment and theory as the entry point.It is found that when Mo_Sionizes into a positive charge state,it changes from a centrosymmetric structure at the neutral state to an off-center structure,which is very similar to the atomic structure of Mo_S2in the neutral state,while when Mo_S2ionizes,the atomic structure changes less and remains off-center.Identifying Mo_S2and Mo_Sbased on whether they have centrosymmetric features can lead to misconceptions by overestimating the concentration of the former and underestimating that of the latter.3.Based on real-time time-dependent density functional theory,the displacement process of sulfur atoms under electron-beam irradiation was investigated,and the hot carrier cooling assisted atomic displacemnet was found by calculating the displacement threshold energy and comparing it with the conventional first-principles molecular dy-namics.Therefore,in this part,we investigate the variation of the displacement thresh-old energy of sulfur atoms under electron-beam irradiation by using real-time time-dependent density functional theory to address this contradictory phenomenon.It is found that electron-beam irradiation leads to a decrease in the displacement threshold energy of sulfur atoms.Under shallow single-electron excitation and shallow double-electrons excitation,the displacement threshold energy decreases by 1.4 and 2.5 e V,respectively,and under deep single-electron excitation,the hot hole cooling transfers some energy to the sulfur atom and the displacement threshold energy decreases to 4.1e V.Under deep double-electron excitation,the displacement threshold energy decreases to 1.7 e V,which is similar to the 1.5 e V fitted by the experimental data.In addition,sulfur atom motion preceding electronic excitation further reduces the kinetic energy required for sulfur atom displacement to form V_S.In summary,this thesis investigates the properties of two-dimensional Mo S₂in-trinsic point defects under electron-beam irradiation conditions through theoretical cal-culations and simulations in equilibrium and nonequilibrium states,explains the reasons for the discrepancy between the characterization results and theoretical calculations by transmission electron microscopy,and reveals the effect of electron-beam irradiation on the ionization and formation of defects in two-dimensional materials.This work deep-ens the understanding of the properties of the intrinsic point defects of monolayer Mo S₂under the equilibrium state conditions and electron-beam irradiation,and provides theo-retical references to the formation mechanism of defects under electron-beam irradiation and related applications.