Theoretical Studies On The Structures And Properties Of Surface Reconstruction And Functionalization Of Diamond And Boron Nitride

As a wide band gap semiconductor material with excellent mechanical,thermal,electrical and optical properties,diamond has broad application prospects in the fields of high-frequency,high-power,high-temperature electronic devices,optoelectronic devices and micro-electro-mechanical systems.Since the discovery of graphene,the discovery,characterization and application of new two-dimensional materials have shown an exponential growth trend.Therefore,the new structures and new properties of two-dimensional diamond have naturally aroused widespread attention,and finding low-dimensional diamond materials with stable structures and novel characteristics has become an important research topic.In addition,the surface properties of materials are widely used in the fields of interface,catalysis and electron emission,so it is of great significance to study the surface structure,stability and electrical properties of diamond material.Cubic boron nitride?c-BN?is also a wide band gap semiconductor with similar crystal structure to diamond.Studying its surface structure and electrical properties can provide a theoretical basis for experimental synthesis and practical applications.In this paper,the structure,stability,and electrical properties of two-dimensional?100?diamond nanofilms depending on the number layer?n?are first studied,and the structural and electrical properties of the nanofilms are modulated by surface hydrogenation and fluorination.Secondly,based on structure search and first-principles calculations,the stable structures,electron affinity and electrical properties of boron-terminated?100?diamond surfaces are systematically studied.Finally,the structural stability of the unreconstructed and reconstructed?111?c-BN surface is studied,and electrical and magnetic properties of the surface structures are further explored through surface hydrogenation and fluorination.We then analyze the adsorption behavior between small nitrogen-based molecules and the surface.The main research contents are summarized as follows:1.Based on the first-principles calculations,the structure,stability and electrical properties of the two-dimensional?100?diamond nanofilms depending on layer number?n?of three types of surface reconstructions?5-MR,5-7-MR and 5/5-7-MR?are obtained.When n>5,the three reconstructed nanofilms have dynamic and thermal stabilities,and maintain the bulk phase characteristics of diamond.The 5-MR and 5-7-MR configurations are indirect band gap semiconductors with band gaps of 1.02?1.40 eV and 0.32?0.55 eV,respectively.However,due to the difference of surface states,the band gap of 5-MR is larger than that of 5-7-MR,and 5/5-7-MR shows metal characteristics.In the band structure of 5-7-MR,the band broadening of adjacent Fermi energy level shows great difference,which shows the one-dimension of hole state transport and the two-dimension of electron state transport.In addition,the arrangement direction of the carbon-carbon double bonds on the upper and lower surfaces changes periodically with the increasing n,resulting in fluctuations of the band gap in a small range.After surface hydrogenation and fluorination of 5-MR,the bulk phase structure of diamond is better maintained.The band gap of fluorinated nanofilm is obviously larger than that of hydrogenated nanofilm.At the same time,due to the change of the electron state on the surface,the band gap obviously depends on the change of the n and gradually decreases with the increasing n,and the nonlinear change originated from the quantum limiting effect.2.The structural and electrical properties of boron-terminated?100?diamond are studied using the structure search method and the first-principles calculations.The particle swarm optimization algorithm was used to obtain the possible boron-terminated surface structures.Then,according to the energy calculations,phonon spectrums and molecular dynamics simulations,four surface structures with stable configurations?0.5 ML,1 ML-?,1 ML-?,and 1 ML-??were obtained.By analyzing the four surface structures,we find that various boron coverages have certain effects on the surface energy stability,electron affinity and electrical properties.From an energy point of view,all four structures can be synthesized experimentally.Since the adsorption of some less electronegative atoms or functional groups on the material surface can effectively reduce the electron affinity of the surface,and we further analyze the change of electron affinity on the different boron-terminated surfaces.According to the calculation results,positive electron affinity?0.24 eV?is formed on 0.5 ML surface,while negative electron affinities??1.27,?1.25,?0.76 eV?are obtained on 1 ML-?,1 ML-?,and 1 ML-? surfaces,indicating that diamond surface can be transformed from positive electron affinity to negative electron affinity by changing the boron coverage.The calculated electron band structures show that the boron-termination introduces the surface states in the band structures.According to the difference in surface band broadening,we find that the surface band broadenings of 1 ML-? and 1 ML-? are larger than 0.5 ML and 1 ML-?,indicating that the effective mass of electrons in 1 ML-? and 1 ML-? are smaller and can be better transported on the surface.3.By means of first-principles calculations,only the reconstructed boron-termination structure is stable in the unreconstructed and reconstructed?111?cubic boron nitride?c-BN?structure.There are three stable structures after surface hydrogenation,but only one stable structure after fluorination.Through the calculated the electron band structures,it is found that the band spin splitting occurred near the Fermi energy level on the reconstructed surface,and the spin charge density is mainly between the surface boron-boron bonds,which is similar to the case of?2×1?reconstructed?111?diamond surface.At the same time,we further studied the structural and electrical properties related to the nitric oxide?NO?and ammonia?NH₃?molecules adsorption on unfunctionalized surface.It can be concluded from the adsorption energy and charge transfer that NO has a strong interaction with the surface.On the other hand,NO adsorption induces asymmetric electron states located near the Fermi energy level,while the relevant energy levels of NH₃ adsorption are located at the deep energy level,indicating that NO adsorption have a great impact on the surface properties.We theoretically predict the structural and electronical properties of two-dimensional?100?nano-films,which can provide a certain theoretical reference for the experimental preparation of ultra-thin diamond films,and help its further application to electronic devices in diamond-related fields.The boron termination can effectively modulate the surface properties of diamond and realize its application in electron emission and other related technologies.By studying the surface reconstruction and functionalization characteristics of c-BN,it can provide some enlightenment for the design of functional materials such as c-BN-based gas sensors in the future.