Experimental Observations And Theoretical Simulation Of Noncentrosymmetric(NCS) Two-dimensional System

Two-dimensional materials with non-centrosymmetric(NCS)structures exhibit unique photoelectric properties,such as piezoelectric properties,ferroelectric properties,nonlinear optics,and thermoelectric properties,which have important application values,such as energy storage and conversion,photoelectric switch,flexible sensor,etc.Therefore,structural design and reasonable preparation of two-dimensional materials with NCS structure have potential application prospects,and this research field is also one of the current research hotspots.This article combines experimental design and theoretical simulation to conduct in-depth research on the non-central symmetric two-dimensional system,and has mainly achieved the following results:1.Based on the concept of molecular configuration and binding site,predict the possible self-assembly configuration of asymmetric organic molecule(Bromomethyl)cyclohexane(abbreviated as BC)on the graphite surface,and perform density functional theory(DFT)calculation on the predicted configuration.The calculation results show that equal amounts of left-handed and right-handed BC molecules are connected by R-ortho-double hydrogen bond(C-H...Br)to form a stable NCS structure,which has the lowest energy among all possible configurations.In order to verify the correctness of the theoretical prediction,the self-assembly behavior of BC molecules on the surface of highly oriented pyrolytic graphite(HOPG)was observed using a scanning tunneling microscope(STM).The experimental results show that the asymmetric BC molecules form the NCS chain structure through hydrogen bonding.,and the STM image of this structure is highly consistent with the theoretically simulated image.2.Scanning tunneling microscope was used to observe that the asymmetric organic molecules BC were co-assembIed with metal ions Ag+,Cu2+,Fe3+ to obtain metal-organic intercalation and coordination self-assembly structures on the surface of HOPG.Combined with the analysis of the high-resolution STM diagram and DFT calculation results,it is found that the Ag+ ions without vacant orbits on outermost layer will fall in the quadrilateral cavity formed by pure chiral BC molecules,forming a centrally symmetric embedded structure with BC molecules;the outermost layer of Cu2+and Fe3+ ions with free orbitals will coordinate with the Br group of the BC molecule to form non-centrosymmetric BC-Cu2+ coordination structure and centrosymmetric BC-Fe3+ coordination structure respectively.3.Through the first-principles calculation method based on DFT theory,predict a thermally stable,monolayer V-group binary compound ?-MXs(?-AsP,?-SbN,?-SbP,?-SbAs,?-BiP)two-dimensional material with NCS structure,and calculated its piezoelectric properties and energy band structure.The calculation results show that ?-MXs are direct band gap semiconductors,and their direct band gap properties are strain stable.In particular,due to the lack of a center of symmetry,these ?-MXs are ferroelectric materials and have excellent piezoelectric properties.Among them,the piezoelectric coefficients of J3-AsP,?-SbP,?-SbAs and P-BiP are an order of magnitude higher than those of commonly used piezoelectric materials(such as quartz,wurtzite gallium nitride,aluminum nitride).Therefore,the combination of its direct band gap characteristics and excellent piezoelectric effect indicates that?-MXs has potential broad application prospects in future optical and electronic devices.