Theoretical Study Of The Optoelectronic Properties Of New Type 2DEG Materials:Multilayer Graphene And Monolayer MoS₂

Traditional two dimensional electron gas materials have played a significant role in promoting the development of science and technology over the past few decades.Since the discovery of graphene,the researches of new type of atomic thin two dimensional ma-terials have received a great attention.The investigation of new type of two dimensional electron gas material’s optical property can enrich the understanding of it and explore the potential applications.In this thesis,we conduct a theoretical study on the optoelectron-ic of multilayer graphene and monolayer MoS2(ML-MoS2)systems.The layers number and stacking order can affect the electronic structure and optical property of graphene system.ML-MoS2 was successful fabricated in a few years ago.The presence of a strong intrinsic spin-orbit coupling(SOC)and the curious valley contrasting selective rules have led to the proposal that ML-MoS2 can be an interesting material for valleytronics and spintronics.The artificial manipulation of SOC in ML-MoS2 can be realized through the R.ashba effect and the optical property is also affected.At present,plasmonics is one of the most promising areas of scientific research.Iin this thesis,we use the balance equa-tion approach and Kubo-Greenwood formula to calculate the optical conductivity.The dynamical dielectric function and plasmon property are calculated under Random Phase Approximation(RPA).The main contents and results of this thesis are as follows.(1)The optical conductivity and transmission coefficient of monolayer and multilayer graphene systems are examined and compared.The universal optical conductivity σ0N=Nπe2/2(2h)for N layer graphene systems in the visible region are verified.For N ≥ 3 layer graphene,the mini-gap induced absorption edges can be observed in odd layers AB-stacked multilayer graphene,where the number and position of the absorption edges are decided by the layers number.Meanwhile,we can observe optical absorption windows for those graphene systems in the infrared to terahertz(THz)bandwidth(0.2-150 THz).The depth and width of the absorption window can be tuned not only via varying temperature and electron density but also by changing the number of graphene layers and the stacking order.This study has laid a theoretical foundation for the application of multilayer graphene with different layers and stacking orders in optical and optoelectronic devices working in infrared and THz bandwidths.(2)We investigate the effect of Rashba SOC on the optoelectronic property of n-and p-type ML-MoS2.The optical conductivity is calculated within the Kubo-Greenwood formalism.We find that the spin-flip transitions enabled by the Rashba effect result in a wide absorption window in low frequency optical spectrum.We find that the position,width,and shape of the absorption peak or absorption window can be tuned by varying temperature,carrier density and the strength of Rashba SOC parameter.This study provides a theoretical basis for the application of ML-M0S2 in frequency tunable devices working in infrared and THz range.(3)The interband optical conductivity of ML-MoS2 with and without Rashba effect are calculated with the four band and two band hamiltonian modes,respectively.When the Rashba parameter approaches to zero,the results of four band model are identical to the two band model.The Rashba effect will increase the number of optical absorption channels and affects the optical conductivity curve near the band gap,which indicates that Rashba effect can be used to adjust the light absorption near the band gap spectrum range.(4)We investigate the dynamical dielectric function and plasmons of ML-MoS2 un-der RPA with and without the circularly polarized optical pumping.The plasmons of the multi-component ML-MoS2 system has been calculated with the massive Dirac fermion(MDF)and two dimensional parabolic band(2DPB)models.The charge plasmon mode in ML-MoS2 is similar to a traditional 2D electron gas.More interesting,novel weakly damped linear acoustic plasmons could be observed in a multi-component ML-MoS2 sys-tem.This study further elucidates the physical mechanism of different plasmon modes in ML-MoS2 system.(5)Considering a ML-MoS2 layer placed on a Sio2 wafer,we calculate the intrinsic optical phonon and substrate surface optical phonon coupled plasmon-phonon modes in ML-MoS2.The dispersion relation of the coupled plasmon-phonon modes can be effectively tuned by carrier density.This study shows that surface optical phonon in the substrate would have an impact on plasmon property of ML-MoS2.The study in this thesis shows that multilayer graphene and ML-MoS2 systems have excellent optoelectronic properties and can be applied as frequency tunable optical and optoelectronic devices working in infrared to terahertz bandwidth.