A First-Principles Study On The Surface Adsorption And The Surface-Interface Regulation Of Two-Dimensional Materials

Since graphene was successfully exfoliated of in 2004,due to its excellent properties in thermal,electrical,optical and mechanical aspects,it has aroused the research hotspot of two-dimensional materials.In addition to graphene and its derivatives,a variety of new two-dimensional materials have been successfully synthesized experimentally or theoretically predicted to be stable at room temperature,for example,transition metal dichalcogenides（TMDs）,hexagonal boron nitride（h-BN）,two-dimensional elemental materials（black phosphorus、selenium、arsenic、antimony、tellurium,etc.）,ⅢA-ⅥA compounds（In Se、In₂Se₃、Ga S、Ga Se,etc.）,ⅣA-ⅥA compounds（Ge S、Ge Se、Sn S、Sn Se,etc.）,graphite carbonitride（g-C₃N₄）,MXenes,organic-inorganic hybrid perovskite,etc.Two-dimensional materials have atomic thickness,large specific surface area,significant quantum confinement effects and surface-interface properties,and have important application prospects in the fields of energy,sensing,catalysis and optoelectronics.By applying external control parameters,the physical properties of two-dimensional materials can be further regulated and the application range can be expanded.Among them,the high specific surface area and weak layer interaction of two-dimensional materials provide scientific feasibility for external strategies such as surface adsorption and surface interface regulation,which is conducive to the optimization and design of new high-performance two-dimensional materials.The atomic coordination environment,carrier concentration and mobility,band structure and electrical conductivity of two-dimensional materials can be controlled effectively by designing surface adsorption,surface modification and surface defect types.Two-dimensional materials with specific crystal structure,phase composition and layer spacing can be modulated by interface control methods such as constructing heterostructures or superlattices,changing the stacking mode between layers,controlling the number of material layers,selecting different growth substrates and forming moirésuperlattices with different angles,etc.Based on this,this thesis adopts surface adsorption and surface-interface means to carry out systematic optimization design of physical properties of several typical two-dimensional materials,such as TMDs,MXenes,organic-inorganic hybrid perovskite nanocrystalline films,and obtains the following research results:1.Developing the scripting program for two-dimensional materials surface adsorption based on first-principle high-throughput calculations.Currently,some progress has been made in the field of two-dimensional material surface adsorption based on first-principle calculations,which can calculate the adsorption energy of different atoms,molecules and groups on the surface of materials more accurately.However,the potential energy surface of the interaction between the surface and adsorption molecules is very complicated,so it is difficult to obtain the most stable surface adsorption configuration quickly and accurately.Therefore,we independently developed a high throughput calculation method for two-dimensional material surface adsorption.According to the characteristics of two-dimensional material system,the surface adsorption structure is constructed by the program.And based on JAMIP（Jilin Artificial-intelligence aided Materials-design Integrated Package,http://jamip-code.com）,a high throughput computing software independently developed by our research group,to achieve the high-throughput calculation of adsorption structures and subsequent data extraction and analysis.Among them,surface adsorption methods mainly include:two-dimensional material surface atom identification,surface site triangulation and potential adsorption site acquisition,batch introduction of surface adsorption molecules,adsorption structure model construction,removal of repeated adsorption structures,high throughput calculation of adsorption energy and batch data processing,and other functions to achieve rapid screening of adsorption structures.This method provides the tool support for the study of two-dimensional material surface adsorption,so as to improve the research efficiency of related work.2.Revealing the mechanism of ultra-sensitive NO2 detection by MXene-based gas sensor.Based on the above surface adsorption method and first-principle calculations,the mechanism of excellent sensitivity and high selectivity of the synthesized Ti₃C₂T_x/Ti O₂sensor material to NO₂ gas was explained successfully.Combined with experimental characterization methods,the first-principles high throughput calculation of surface adsorption energy shows that,the in-situ oxidation of Ti O₂ on Ti₃C₂T_x surface results in a large number of Ti atom vacancy defects,which is an important reason for the improvement of the gas sensitive property of the material.The binding strength of NO₂ on the surface defect is greatly enhanced compared with that on the surface without defects.This work provides a new idea for the construction of high-performance gas sensor by using MXene’s characteristics of easy oxidation and introduction of defects.3.Demonstrating strong binding strength ofδ-ammonium valeric acid（δ-AVA）and phosphine-oxide-based radical（DPPOO^-）ligands with perovskite surface is the key to improve the luminescence performance and stability of perovskite nanocrystal thin film LEDs.Based on the developed surface adsorption method and first-principle calculations,the effects of different surface ligands’interaction with perovskite surface on the crystallization growth and defect passivation of perovskite were revealed.Combined with experimental characterization methods,the results of surface binding energy calculation from first principles show that,the surface binding energy ofδ-AVA organic ammonium ligand is characterized by an additional hydrogen bond interaction in addition to the ion anchor on perovskite surface.δ-AVA can control both the nucleation and crystal growth of perovskite.The crystal size of the grown perovskite nanocrystal film is more uniform and smaller,and the maximum external quantum efficiency of 24.2%of pure green LEDs is achieved.Secondly,DPPOO^-molecules have stronger interaction with surface defects,which can effectively passivate the surface defects of perovskite,and increase the half-life of the stability of LEDs to 45.6 hours when the initial luminance is 100 cd·m^-2.Experiment combined with theoretical calculations,the mechanism of improving the luminescence performance and stability of perovskite LED was revealed.4.Designing the lateral superlattices（SLs）structure of monolayer transition metal dichalcogenides（TMDs）and revealing the effects of chemical component,interface type,and sub-lattice size of the lateral TMD-SLs on their electronic structures and properties.Based on first-principles high-throughput calculations,we systemically investigated the dependence of electronic structure,bandgap,carrier effective masses,charge density overlap on chemical component,interface type,and sub-lattice size of the lateral TMD-SLs.We found that distinct from the random alloy counterparts,the lateral TMD-SLs exhibit generally type-II band alignment,wider range of bandgap tunability,larger carrier effective masses,and stronger electron-hole charge separation tendency.The bandgap variation with sub-lattice size shows larger bowing parameter for the SLs with heterogeneous anions,by comparison with the homogeneous anion cases.The similar behavior is observed for the SLs with the armchair-type interface,by comparison with the zigzag-type interface cases.We revealed the underlying physical mechanisms responsible for these observations that are attributed to the synergistic interplay among the band offset of sub-lattices,quantum confinement effect,and existing internal strain.This work provides new insights into the design of two-dimensional materials and reveals the laws affecting the electronic properties in two-dimensional superlattices.5.Researching the effect of sulfur vacancy on the electronic structure of monolayer MoS2 and the diffusion barriers of hydrogen species in penetrating monolayer MoS2.The introduction of surface defects as a common means of surface regulation has been widely studied.Due to the lower formation energy in MoS₂,sulfur vacancy is often inevitably introduced in experiments and thus affects the properties of MoS₂.Our first-principle calculations show that the presence of surface sulfur vacancy introduces a defect state in the middle of the band gap of monolayer MoS₂ and decreases the band gap from 1.75 e V to 1.06 e V.The adsorption of hydrogen on the surface of MoS₂-V_Swill further affect its electronic properties,and reduce the defect states in the middle of the band gap.Secondly,the presence of sulfur vacancy on the surface will affect the diffusion barrier of hydrogen species through MoS₂.The calculation results show that the steric hindrance effect in the structure of MoS₂ and the interaction between defects and hydrogen species have major influence on the diffusion of hydrogen species.Specifically,hydrogen atom has a lower diffusion barrier than hydrogen molecule.For MoS₂-V_S structure,hydrogen atom chemically adsorbed on the vacancy will make hydrogen atom’s diffusion barrier larger.The theoretical research deepen the physical understanding of the effect of sulfur vacancy on the electronic properties of MoS₂ materials and the diffusion of small molecules through MoS₂.