Photoelectric Properties Of Graphene And Two-dimensional Transition Metal Sulfides

Since successfully prepared in laboratory,graphene has become a hotpot in scientific research and thus promoted the rapid development of two-dimension materials（2D materials）,among which the two-dimension transition metal sulfides are one of the most concerned kind.As one kind of newly emerging materials,two-dimension transition metal dichalcogenides exhibit rich physical characteristics,and are expected to be widely used in various new nano optoelectronic devices,due to their atomic single-layer thickness,excellent electrical properties and the possibility of obtaining a direct band gap when changing from bulk materials to single-layer materials.Based on above,several main research aspects in this paper are as follows:1.Firstly,the Schr?dinger equations of graphene and typical two-dimension transition metal sulfides（Monolayer Mo S₂,WS₂,WSe₂ and Mo Se₂,i.e.TMDs）are solved through the effective Hamiltonians in the low energy region,and then the intrinsic energy and corresponding wave function are obtained.Besides,their photoelectric responses are obtained by solving a series of models based on the electronic structures.The photon-excited carriers and emission of graphene are obtained based on the mass-balance equation and the charge number conservation equation,which are derived from Boltzmann equation.The analytical results of photon excited carrier density and photon emission coefficient are achieved self-consistently in terahertz radiation fields.The dielectric function of monolayer TMDs system under the condition with the spin-orbit coupling is solved by random phase approximation（RPA）.The electron screening length and dispersion relation of plasmon are obtained.At the same time,the semi classical Boltzmann equation is used to solve the electric transport properties at low temperature of monolayer TMDs.2.A graphene-based metamaterial structure is proposed.By solving Maxwell’s equations,it is concluded that a dual tunable coupled terahertz surface plasmon is achieved in this graphene metamaterial.Coupled SPPs show very interesting characteristics,including the optical mode and the acoustic mode.Importantly,SPPs in optical mode can be effectively excited without specific coupling devices,which agrees with recent experiments.In addition,the optical SPPs can be easily tuned by the gate voltage.3.How the anisotropic medium environment affects the plasma characteristics of the two-dimension electron fluid is calculated.And how the plasmon of the proposed model changes with the anisotropy of the cover layer and the direction of the wave vector of the incident light is explored by solving Maxwell’s equations and Poisson’s equations.The results show that the plasma dispersion relationship in this theoretical model is anisotropic.Based on the dielectric function of monolayer TMDs and Maxwell equations,the dispersion relations of the SPPs of monolayer TMDs with different carrier densities on different substrates are obtained.It is found that the frequency of the SPPs can reach gigahertz（GHz）,which depends on the carrier densities and the type of substrates.