First-principles Study On The Regulation Of Physical Properties Of Two-dimensional Transition Metal Chalcogenides

Two-dimensional（2D）materials have attracted significant attention since the discovery of graphene.This thesis aims to investigate methods to manipulating the properties of several2D materials for applications in areas including optoelectronics,solar energy harvesting,and photovoltaic devices.The effects of lanthanides（LN）doping and stacking configurations on the structural,electronic,magnetic,and optical properties of Hf Se₂monolayers and Sn Se₂,Sn SSe,and Zr SSe heterobilayers were investigated,respectively.The electronic,optical,and photocatalytic performance of CdX（X=S,Se,and Te）bilayers,as well as the influence of externally applied strain on the properties of these bilayer systems were also systematically studied.The detailed investigations of the different systems are as follows:Firstly,using density functional theory calculations,we investigated the influence of LN doping on the electronic,magnetic,and optical properties of Hf Se₂monolayer.Compared to pristine Hf Se₂,which is non-magnetic with an indirect band gap,the LN-doped Hf Se₂monolayers exhibit significantly altered electronic ground states due to the introduction of defect states.Five of the dopants（La,Nd,Eu,Tm,and Lu）can lead to the formation of metallic ground states while the doped monolayer remained semiconductors when doped with Ce and Pr.Spin-polarized calculations indicate that the ground state of the Nd,Eu,Tm and Pr doped monolayers becomes magnetic with non-zero total magnetic moment which was attributed to the existence of unpaired1)-orbitals.Interestingly,the Ce-doped Hf Se₂monolayer,which has one unpaired 41)orbital,remained non-magnetic as a result of the symmetric spin-up/spin-down states.La and Lu doped Hf Se₂monolayers also retain the non-magnetic ground state due to the lack of 41)orbitals（La）or fully filled 41)orbital（Lu）.More importantly,calculation results show that the absorption abilities of the LN-doped Hf Se₂monolayers with the metallic ground states are strongly enhanced in the infrared region,which suggested potential applications of such systems in solar energy harvesting.These results not only demonstrated the possibility of modulating the electronic,magnetic,and optical properties of the Hf Se₂monolayers via LN doping but also indicated the application potential of such systems for spintronics,nanoelectronics,and optoelectronics.Secondly,first-principles calculations were carried out to study the geometries,elec-tronic structures and optical properties of van der Waals（vd W）heterobilayers consisting of transition metal dichalcogenide（TMDCs）Sn Se₂and Janus TMDCs Zr SSe and Sn SSe.Eight possible configurations Se Sn Se-SSn Se,Se Sn Se-Se Sn S,Se Sn Se-SZr Se,Se Sn Se-Se Zr S,SSn Se-SZr Se,SSn Se-Se Zr S,Se Sn S-SZr Se and Se Sn S-Se Zr S are studied and found to be dynami-cally,thermally,energetically and mechanically stable.Six configurations,（Se Sn Se-SSn Se,Se Sn Se-Se Sn S,Se Sn Se-SZr Se,Se Sn Se-Se Zr S,SSn Se-SZr Se and SSn Se-Se Zr S）have indi-rect band gaps with type-II band alignments,which could enhance carrier lifetime which is an essential feature for potential applications in photovoltaic and nanoelectronics devices.In contrast,Se Sn S-SZr Se and Se Sn S-Se Zr S have indirect band gap with a type-I band align-ment,which provides promising applications for light-emitting devices.The calculated op-tical spectra indicate that all of these heterobilayers exhibit significant absorption spectrum in the visible region,suggests their application potential for optoelectronics.Thirdly,by utilizing hybrid density functional theory,CdX vd W bilayers are reported as possible candidate for visible-light-driven water splitting.This study reveals that Cd S and Cd Se bilayers exhibit a semiconducting nature with a direct band gap of 2.4 e V and 1.93 e V,while the Cd Te bilayer is an indirect semiconductor with a band gap of 1.2 e V.The structural,thermal,and mechanical stabilities of the CdX bilayers were then evaluated by binding en-ergy,ab initio molecular dynamics,and elastic modulus calculations.Cd S and Cd Se bilayers possess suitable band gaps and band edge positions that are favorable to facilitate overall wa-ter splitting.Moreover,CdX bilayers exhibit excellent optical absorption(10⁵cm^-1)and high solar-to-hydrogen conversion efficiencies upto 49.21%,which breaks the conventional theoretical efficiency limit.Furthermore,performing biaxial strain can effectively regulate the band gap and band alignment of CdX bilayers.CdX bilayers have excellent visible light absorption,exhibiting a remarkable red shift and stronger light absorption with tensile strain.This study reveals the potential of CdX bilayers as a new alternative for optoelectronic de-vices and provide new possibilities for designing strain-tunable photovoltaic devices.