First-principles Investigation On The Optical And Electrical Properties Modulation Of Two-dimensional Materials

Two-dimensional materials have shown great potential in the field of electronics and optoelectronics application,due to their extraordinary physical properties.For devices,like FET,two-dimensional semiconductor materials needs a semiconductor-metal contact electrode,and a Schottky contact is unusually formed between semiconductor and metal.The formation of low-resistance contact is a challenge,which is important to achieve high “on” current,good photoresponse and high-frequency operation.Based on density functional theory,the investigation on the intrinsic properties of two-dimensional materials,doping effect on electronic properties of two-dimensional materials,Schottky barrier formation of two dimensional metallic materials,and doping effect on metal/2D-materials interface.The study is carried out in the following aspects:1.Based on first-principles calculation method,the most stable structures of graphite,MoS2,WS2 and Black phosphorus have been studied.The total energy and lattice constants are obtained.2.Based on the optimizd lattice constants of graphite,MoS2,WS2 and black phosphorus,monolayer structures are built and a stable monolayer material is obtained by atomic relaxation.The energy band structures and density of states(DOS)of graphene,mMoS2,mWS2 and monolayer black phosphorus(mBP)are cacluated using the LDA(local approximation)and the GGA(generalized gradient approximation),for the exchange and correlation potential.The results show that the graphene is a zero band gap half-metal,mMoS2,mWS2 and mBP are direct band gap semiconductors.3.The 3×3×1 mMoS2 supercell was established based on the optimized MoS2 lattice constant.The defects formation energy of S vacancy and Mo vacancy is investigated.The formation energy of S vacancy is smaller than Mo,which means that the defect of S vacancy form is easier to be formed.Accordingly,different defects formation energy due to substitution of halogen-S atom are calculated,and the result is :Ef(F)<Ef(I)<Ef(Cl)<Ef(Br).The band structures and DOS of the stable structures of Halogen atom substitution S doping are studied.The results show that the doping of halogen elements makes the bandgap width becomes larger,and introduce the impurity energy levels in the band gap.4.Based on the lattice constants of two-dimensional materials,metal materials,two-dimensional lattice constant is established in a metal/two-dimensional material interface,including:(1)Mg-graphene,Cu-graphene,Al-graphene,Au-graphene,Pt-graphene,modulation effect on the Fermi energy of graphene metal are studied by using the first principle method based on density functional theory.The results show that Mg,Cu and Al,the Fermi level of graphene belongs to n control,while Au and Pt control on the graphene Fermi level belongs to p regulation,the calculation results and literature reports are consistent;(2)Mg-MoS2,Al-MoS2,Cu-MoS2,Au-MoS2,Ti-MoS2,Ni-MoS2,Pt-MoS2,Pd-MoS2,research shows that Mg,Al,Cu,Au and MoS2 belong to n type monolayer contact,including Schottky Mg,Al,Cu and the barrier layer MoS2 contact formation of lower height,closer to the ohmic contact.Au,Ti,Ni,Pt,Pd,and MoS2 single contact with the formation of the Schottky barrier height is high,is typical of the Schottky contact;(3)Mg-WS2,Cu-WS2,Al-WS2,Ti-WS2,Au-WS2,Ni-WS2,Pt-WS2,Pd-WS2,Mg,Cu and Schottky barrier,Al and single WS2 barrier is relatively lower,close to the ohmic contact,while Au,Ni,Ti,Pt,Pd and WS2 single contact with the formation of the Schottky barrier height is higher,which is a typical Schottky contact;(4)Al-BP,Au-BP,and Al-black phosphorus contact formation potential are lower than Au and the formation of black phosphorus contact barrier,which illustrates that Al may be a better materials to be used for metal electrode than Au.5.Baded on the first principle method,the effect of doping effect of halogen elements on the interface of Au-MoS2 Schottky barrier and Se doping on the interface of Au-BP Schottky barrier are studied.The results show that F and Cl doping can decrease the Schottky barrier height,while Br and I doping can increase it.Based on the results of difference charge and population analysis,the tuning of Schottky barrier height to the influence of the dipole is ascribed by the charge transfer among the interface.Due to the doping effect of Se atoms Au-BP interface,the Schottky barrier height of Au-BP interface is reduced effectively,which with the obitained experimental results.The charge density analysis of the interface in both doping and undoping situation are carried out,which explains the Schottky barrier height reduction on the interface of Au-BP.