Design Of G-C₃N₄-based Semiconductor Heterostructures And Study Of Electronic And Optical Properties

Graphite phase carbon nitriding（g-C₃N₄）has received great attention because of its unique electronic structure,outstanding chemical stability and thermal stability,and has been widely used in photoelectric sensors,bioimaging,photovoltaic solar cells,photocatalysis and other fields.However,the low efficiency caused by the deficiency such as wide band gap and low light absorption cannot be ignored.Therefore,the modification of g-C₃N₄is still one of the key topics in various fields.Compared with bulk material,two-dimensional material has larger specific surface area,more reactive sites,and can realize effective separation of charge and holes.Therefore,two-dimensional materials have attracted much attention in many research fields.The heterojunction is an important means to reduce the band gap and promote the effective separation of space charges.Therefore,the construction of two-dimensional heterostructure is a key way to improve the photocatalytic performance in the fields of optoelectronics,field-effect transistors and photocatalysis..First-principles calculation can effectively design appropriate heterostructures and accurately describe the electronic and optical properties of two-dimensional semiconductor materials.Moreover,Simulation is more effective and economical than experiment in finding new efficient semiconductor materials..In this paper,the electronic and optical properties of g-C₃N₄are systematically studied by first-principles calculations.The essential reasons of the experimental phenomena related to g-C₃N₄were analyzed theoretically,and a new design direction of g-C₃N₄modification was put forward.In order to improve the photocatalytic performance of g-C₃N₄,try by combining g-C₃N₄with blue phosphorus,which has excellent carrier migration ability,to construct heterostructures.At the same time,the regulation of stress on the electronic and optical properties of materials was systematically studied.Based on the stress regulation,a new and efficient photocatalytic water decomposition material was designed and the following results were obtained:（1）Through the study of g-C₃N₄/x BlueP（x=1～3）,it is found that the layer thickness has a great influence on the band gap and photocatalytic performance of g-C₃N₄/BlueP.The increase of the layer number of blue phosphorus leads to the obvious reduction of the band gap,which realizes the visible light response.g-C₃N₄/BlueP has a direct band gap and the most appropriate band gap width（1.68e V）and the band edge position,and has excellent optical absorption performance.The composite structure significantly enhances the light absorption intensity of g-C₃N₄and expands the light absorption range.（2）Based on the g-C₃N₄/BlueP heterojunction,the stress regulation of the electronic structure of the g-C₃N₄/BlueP heterojunction was comprehensively studied by first-principles calculation.The results show that the uniaxial strain can regulate the band gap and change the type of band gap,but it is not enough to change the type of g-C₃N₄/BlueP heterojunction.The type transformation of g-C₃N₄/BlueP heterojunction can be achieved by suitable biaxial strain.The CBM was found to be composed of g-C₃N₄as electron acceptor,while the VBM is contributed by BlueP as electron donor which solves the problem of high electron-hole recombination of type-I heterostructures.The adjustable band gap range and proper band edge position under the action of biaxial strain can satisfy the REDOX（reduction-oxidation）potential of photolysis water,thereby solving the problem of energy level matching.Moreover,the optical property calculations subsequently prove that the g-C₃N₄/BlueP heterostructure has a wide optical response range and good absorbance under the stress,which improve the low solar utilization of single material.（3）By applying different degrees of strain,we found that biaxial compression strain can effectively achieve the change of band gap type and heterojunction type,and improve the photocatalytic performance of materials.The calculated results show that the g-C₃N₄/2BlueP heterojunction has the most suitable band edge position,wide optical response range and good light absorption capacity.By further applying the appropriate biaxial strain,it is found that the electronic structure of g-C₃N₄/2BlueP heterojunction also underwent an important transformation under the appropriate biaxial strain.The calculation results of energy band and density of states show that the electronic structure of the g-C₃N₄/2BlueP heterojunction can be effectively controlled under stress.Appropriate compression stress not only realizes the regulation of band gap,but also significantly promotes the transformation of type-I heterojunction to type-II heterojunction,which has vital significance and broad application prospect in the realization of efficient charge separation and efficient photolysis of water.