Tuning Electronic Properties Of Janus MoSSe Nanomaterials By Strain Or External Electric Field

Janus MoSSe is a new type of two-dimensional(2D)material,which has attracted wide attention because of its broken symmetrical structure and unique electronic and optical properties.Related exp erimental studies show that structural symmetry breakage causes Janus MoSSe to have intrinsic vertical piezoelectric effect,Rashba spin orbit interaction enhancement,and optically active vertical dipole.These excellent characteristics make Janus MoSSe in optoelectronic devices,lithium ions Many aspects such as batteries,catalysts and spintronic devices have great application prospects.The calculations are performed by using density functional theory(DFT)implemented in SIESTA code.The physical properties of zigzag Janus MoSSe nanoribbon and Janus MoSSe nanotubes are systematically investigated with the effects of strain and external electric field,providing theoretical guidance for the development of new electronic devices and applications based on Janus MoSSe nanoribbons and nanotube materials.The thesis is composed of five chapters,including the followings:Chapter 1 introduces the research background of this thesis,including the two-dimensional nanomaterials and recent research progress of Janus MoSSe nanomaterials.Chapter 2 briefly introduces the first-principles and density functional theory.In C hapter 3,zigzag Janus MoSSe nanoribbons are systematically studied by first principles,and the electronic structure and magnetic properties of the nanoribbons are tuned with strain and external electric field.With the energy comparison,it determines that the ground state of the zigzag Janus MoSSe nanoribbons is ferromagnetic.After the analysis of the band structures,it reveals that the zigzag Janus MoSSe nanoribbons are a ferromagnetic metal.Taking the Z₇-JMoSSe NR structure as an example.Its magnetic moment is 0.99?B/cell.The spin density distribution is mainly located on the two edges of the nanoribbon.Strain affects the magnetic moment of the nanoribbon.In addition,when the strain is 14%,the magnetic moment is maximum,which is 1.72?B/cell.The spin density distribution can only appear on the left edge(the edge with S and Se atoms)by the regulation of strain.O n the other hand,app lied the external electric field,as the applied electric field increases,the magnetic moment will also increase.When the applied electric field is less than-0.2 V/?,the phenomenon is reversed,and the spin density is distributed on the left edge of the nanoribbon.Analysis of the total energy of the nanoribbons shows that the structure is reversible under strain and an external electric field.Through the combination of these two tunable tools,an improved modulation response of the magnetic moment can be induced.In Chapter 4,the effects of different nanotube diameters,outer-layer atoms,chirality,and strain on the electronic structure of Janus MoSSe nanotubes are studied systematically by first principles.Armchair nanotubes are mainly indirect semiconductors,while zigzag nanotubes are mainly direct semiconductors.When the diameter of the nanotube is smaller,the corresponding band gap is narrower.The nanotubes with S outer layers have narrower band gap than the nanotubes with Se outer layer.When the diameter of the nanotube is very small,the nanotube with S outer layer will become a semi-metal or a conductor.For MoSSe nanotube with Se outer layer,its electronic structure is similar to that of Mo S₂ nanotube,while MoSSe nanotube with S outer layer has different electronic properties from Mo S₂ nanotube.When strain is applied to a nanotube,as the strain increases,the band gap of most nanotubes increases first and then decreases,especially the nanotube with the Se outer layer.Additionally,with the applied strain,the change of the band gap of MoSSe nanotube with Se outer layer is also similar to that of the corresponding Mo S₂ nanotube,but the band gap of of MoSSe nanotube with Se outer layer will be slightly larger.By controlling the strain,some MoSSe nanotubes also undergo a transition from semiconductor to metal conductor.Chapter 5 summarizes the research works of this thesis and gives some outlooks for Janus MoSSe materials.