Electronic And Optical Properties Of Two-dimensional Chalcogenide Layered Nanomaterials And Related Heterostructures From First Principles

The rise of graphene and the subsequent discovery of its excellent mechanical,electronic and optical properties have not only led to an extraordinary amount of interest from scientific community,but also revolutionized the two-dimensional(2D)materials.On one hand,transition metal dichalcogenides(TMDs)as one kind of important 2D nanomaterials have outstanding physical properties as graphene.On the other hand,TMDs make up for the shortcomings of graphene in applications,which are caused by zero band gap and semimetal features of graphene.Previous research has shown that TMDs are important in various applications such as optoelectronics,spintronics,catalysts,chemical and biological sensors,supercapacitors,solar cells,and lithium ion batteries.Besides,it is easy to fabricate different types of heterostructures by TMDs as their specific high surface areas and van der Waal interaction.Therefore,analytical calculations on photoelectrical properties of TMDs and related heterostructures are the focus of theoretical research on 2D nanomaterials in recent years.With respect to the theoretical research on properties of many-body systems,traditional denstiy functional theory(DFT)is the indispensable method for numerical calculation.Because of the existance of van der Waals interactions in 2D nanomaterials,and orbital hybridization plays an important role in electronic and optical properties,we utilize DFT with van der Waal's correction terms and hybrid functional for analytical calculations of many-body systems in order to botain the research results as accurately as possible.Firstly,the electronic and optical dielectric properties of layered transition metal dichalcogenides MX2(M = Zr and Hf;X = S,Se)have been investigated using DFT with van der Waals correction.The band gaps of these materials through Heyd–Scuseria–Ernzerhof hybrid functional are in reasonable agreement with the experimental data.Partial density of state analysis suggests that the metallic atoms play a dominant role in the conduction band and the chalcogenide atoms have the main effect on the valence band.The presence of peaks in the dielectric function spectra mainly result from the transition between first,second,third valence bands and the first conduction bands.What is more,parallel band effect has been observed in monolayer structures, which suggests strong light-matter interactions in these materials.This work not only could provide theoritical explanation on experimental phenomenon of materials,but also promotes the property understanding of these materials and holds potential for the development of optoelectronic devices based on these layered materials.Secondly,the electronic and optical dielectric properties of layered transition metal dichalcogenides MX2(M = Tc,Re;X = S,Se)which belong to flexible and anisotropic materials are investigated by DFT with different van der Waals correction and Heyd-Scuseria-Ernzerhof hybrid functional.The band gaps fall in between 1.70 and 2.12 e V with the d states of transition metal atoms playing an important role in conduction and valence bands.In addition,the appearance of band nesting implies that there are strong light-matter interactions in these materials,indicating they are suitable for photovoltaic and photocatalytic applications.Unlike the traditional 2D materials such as Mo S2,the optical dielectric properties manifest highly in-plane anisotropic in the infrared and visible region,which is suitable for on-chip polarization manipulation with these materials.This work promotes the understanding of flexible and anisotropic response of these materials and their potential applications in new types of optoelectronic devices.In the end,band alignments and heterostructures of IVB-VIA monolayer MX3(M = Zr,Hf;X = S,Se)and VIIB-VIA monolayer MX2(M = Tc,Re;X = S,Se)are calculated by density functional theory with hybrid functionals.The characteristics of the band alignments and the planar-averaged local density of states show that Zr S3,Hf S3,Tc Se2 and Re S2 could be favorable candidates for photocatalytic water splitting.Zr S3,Hf S3 and MX2 with the same structures are able to form type II heterostructures at their interfaces,which could be used for solar energy conversion.The power-conversion efficiency of an MX3 thin-film solar cell is approximately 16–18%,which is higher than those of MX2 thinfilm solar cells.The charge density difference of the heterostructures demonstrates a higher charge accumulation at the interface of MX3 heterostructures than that of MX2 heterostructures.These phenomena show that type II heterostructures formed of MX3 are more stable than those of MX2.