Theoretical Study On Applications Of Two-Dimensional Materials In Lithium Ion Battery,Optoelectronics And Spintronics

Since the successful isolation of graphene from graphite in 2004,both experimental and theoretical studies on two-dimensional(2D)materials increased rapidly.Kinds of novel 2D materials have been synthesized.Their applications have touched many areas,such as transistors,ion-batteries,photovoltaic and optoelectronic devices,sensors,and spintronics.Via density functional theory(DFT)methods,this dissertation predicted the possibility of phosphorene and its nanoribbons as lithium ion battery(LIB)anode materials,superhalogen modified 2D organic-inorganic hybrid perovskites as optoelectronic materials,and superhalogen modified graphitic C3N4 as spintronic materials.Additionally,in practical applications,taking the substrates into account,the feasibility of graphitic ZnO as an ideal substrate of graphene was investigated.This dissertation contains six chapters with the main contents discussed in following parts.In the first chapter,the developments of 2D materials are reviewed briefly.2D materials are divided into four species,namely,elemental 2D materials,transition metal dichalcogenides(TMD),transition metal carbides or nitrides(MXenes),and quantum well materials.Their applications and research progresses in LIB anode materials,optoelectronic and spintronic devices are discussed.In the second chapter,non-relativistic approximation,Born-Oppenheimer approximation,and single electron approximation in quantum chemistry are introduced firstly.Then,we discussed the quantum chemical method based on wavefunctions,namely,Hartree-Fock method.We also reviewed the developments and framework of method based on charge density,namely,DFT.At last,we introduced some simulation packages used in this dissertation.In the third chapter,DFT simulations were performed to investigate the possibility of phosphorene and its nanoribbons as anode materials for LIB.Both phosphorene and its nanoribbons are direct bandgap semiconductors,and show semiconductor-to-metal transition upon lithiation with significant charge transfer.Importantly,the computed energy barrier of Li ion diffuses on phosphorene monolayer along zigzag direction is only 0.09 eV,which indicates the possibility of ultrafast charging process in LIB based on phosphorene anode.Furthermore,the fast mobility of Li ion is robust and well-kept in phosphorene nanoribbons.Comparing to other 2D materials,such as graphene and MoS2,energy barrier of lithium ion diffusing on phosphorene is the lowest up to now.In the fourth chapter,halogen atom I in 2D organic-inorganic hybrid perovskites(C4H9NH3)2PbI4 were sequentially replaced with superhalogen molecule BH4,forming(C4H9NH3)2PbI4-x(BH4)x(x= 0-4).The bandgap and exciton binding energy increase monotonically with x.Exciton binding energy of(C4H9NH3)2Pb(BH4)4 approaches the value in monolayer black phosphorus.Lead-free admixtures(C4H9NH3)2MI1-x(BH4)x(M= Sn and Ge;x= 0-4)also show a similar trend.Thus,a combination of quantum confinement and compositional change can be used as an effective strategy to tailor the bandgap and the exciton binding energy of 2D organic-inorganic hybrid perovskites,making them promising candidates for colorful and efficient optoelectronic applications.In the fifth chapter,band alignment method was introduced as a new strategy to design 2D spintronic materials.Taking graphitic C3N4 and superhalogen BH4/BF4 as an example,according to the alignment of their energy levels,BF4 was predicted to be able to induce half-metallic ferromagnetism in graphitic C3N4 sheet.Verified by DFT simulations,a net magnetic moment of 1μB per BF4 is induced in graphitic C3N4 sheet.The magnetic moment distributes evenly on the double-coordinated nitrogen atoms with ferromagnetic coupling.Ab initio dynamics simulations confirmed that the structure of graphitic C3N4 was well protected.In the sixth chapter,considering the practical applications of 2D materials,substrates are inevitable.Taking graphene and graphitic ZnO as an example,the impacts of substrates were investigated.It is found that,regardless of ZnO with or without oxygen vacancies,the intrinsic linear dispersion of graphene was well retained.Comparing to separated graphene and graphitic ZnO,the resultant bilayer structure of graphene and graphitic ZnO layer shows much better optical properties.Moreover,both the band dispersions and Fermi velocities of graphene and graphitic ZnO bilayer are robust towards external electric field.Therefore,our results indicate that graphitic ZnO may be a suitable substrate for graphene in real applications.