Theoretical Research On The Electronic Structure And Magnetic Properties Of Molybdenum Disulfide Nanoribbons Based On First Principles

Enormous research efforts have been focused on the layered transition-metal dichalcogenide MoS₂ because of its distinctive electronic,catalytic,photovoltaic as well as energy storage properties.The bulk MoS₂ crystal is an indirect band gap semiconductor with a band gap of 1.29 eV.The monolayer MoS₂ is easy to be exfoliated from bulk phase due to the weak van der Waals interaction between the layers.After that,monolayer MoS₂?ML-MoS₂?exhibits a direct band gap semiconductor with band gap of?1.8 eV,displaying strong optical absorption,photoluminescence and photoconductivity.With the intense research of MoS₂,scientists have conducted theoretical research and experimental preparation of MoS₂ nanoribbons with quantum-size limiting effect.Owing to the different cutting direction,MoS₂ nanoribbons with both zigzag and armchair edges were produced.Additionally,ML-MoS₂ exhibits outstanding electronic features such as high charge-carrier mobility,tunable charge-carrier types and high current on/off ratios.In this dissertation,we use first-principles to study the optical,electrical,and magnetic properties of MoS₂ nanoribbons with edge passivation and transition metal element doping.Details are as follows:?1?We performed density functional theory study on electronic structure,magnetic properties and stability of zigzag MoS₂ nanoribbons?ZMoS₂NRs?with and without oxygen?O?passivation.The bare ZMoS₂NRs are magnetic metal with ferromagnetic edge states,edge passivation decreases their magnetism because of the decrease of edge unsaturated electrons.Obviously,the electronic structure and magnetic properties of ZMoS₂NRs greatly depend on edge states.When both edges are passivated by O atoms,ZMoS₂NRS are nonmagnetic metals.When either edge is passivated by O atoms,the systems exhibit single-edge ferromagnetism and magnetism concentrates on the opposite edge atoms.Therefore,edge passivation can not only tune the magnetism of ZMoS₂NRS,but also enhance their stability by eliminating dangling bonds.These interesting findings on ZMoS₂NRS may open the possibility of their application in nanodevices and spintronics.?2?The electronic structure,magnetic properties and stability of transition-metals?TM?doped armchair MoS₂ nanoribbons?AMoS₂NRS?with full hydrogen passivation have been investigated by using the density functional theory.The hydrogen passivated AMoS₂NRS are nonmagnetic semiconductor,but TM doping can make AMoS₂NRs display diverse characteristics?such as nonmagnetic metal,magnetic semiconductor,nonmagnetic semiconductor and semi-metal?.The electronic structure analysis shows that the magnetism of TM-doped AMoS₂NRS concentrates on dopant TM and edge Mo atoms,which mainly comes from competition between the exchange splitting and crystal-field splitting.More importantly,Mn-doped AMoS₂NRS may be good candidates for spintronics due to their long-range FM magnetic coupling,reliable Curie temperature and good stability.?3?Finally,we investigate the electronic structure of armchair MoS₂ nanoribbons without and with passivation.The AMoS₂NRS are nonmagnetic semiconductors and exist a transition from indirect to direct band gap with the increasement of ribbon widths.Edge passivation is an effective way to engineer the band structure of AMoS₂NRS because it can vary edge charge distribution.By choosing passivated groups and controlling passivated patterns,we can also tune the band gap of AMoS₂NRS from indirect to direct.As a result,full hydrogen?H?or amidogen?NH2?passivation and partial hydroxyl?OH?passivation can not only tune AMoS₂NRS from the indirect to direct band gap semiconductors,but also enhance the stability of the systems.These interesting finding will open the possibility of their application in optoelectronics.