Designing Low-Dimensional Nanomaterials With Unique Optical And Magnetic Properties Based On First-Principles Methods

With the development of nanotechnology and the miniaturization of devices,the research of low-dimensional nanomaterials is becoming more and more important.The core-shell nano-heterostructure,as zero-dimensional nanomaterials,have many excellent physical and chemical properties,such as photics,electricity,magnetism and catalysis,this is owing to their quantum size effects,surfaces/interface effects and quantum confinement effects.Studies showed that combining the unique properties of precious metal nanoparticle with the properties of semiconductor materials will develop new and versatile materials,especially in the field of optoelectronics,which has become one of the hot research fields of nanomaterials.In addition,with the successful preparation of graphene in experiments and rapid development of spintronics,two-dimensional(2D)magnetic nanomaterials have become ideal materials of the development of next-generation electronic devices,especially two-dimensional intrinsic ferromagnetic semiconductor which contain ferromagnetism,semiconducting and ultra-thin structural features.Incorporating the silicon-based semiconductor technology into 2D ferromagnetic semiconductor can promote the development of chip,enabling chips to integrate computing,storage,communication and information processing functions,while speeding up data transmission,reducing energy consumption and storing data with non-volatile,it is expected to promote the revolutionary changes of information technology.In this thesis,the first-principles calculation method based on density functional theory is used to predict the structural properties,optical,magnetic and electrical properties of zero-dimensional metal-metal oxide semiconductor core-shell heterostructures and two-dimensional magnetic semiconductor nanomaterials.'The main research contents and results are as follows:(1)The Agn@(ZnO)42(n=6-18)core-shell heterostructures are constructed,the calculated results show that Ag13@(ZnO)42 is the most stable core-shell heterostructure comparison with others.The electronic and optical properties of Ag13@(ZnO)42 nanostructures show that Ag as an inner core is embedded in the ZnO shell will improve the absorption strength of ZnO for visible light.In addition,the unoccupied Ag states near Fermi level will get the photoexcited electron from ZnO surface,thereby delaying the recombination of photogenerated electron-hole pairs,then the photocatalytic activity of ZnO is improved.Therefore,Ag13@(ZnO)42 has the good absorption ability for visible light and photocatalytic activity,making it has the potential applications in the photoelectronic fields.(2)The performance of core-shell nanomaterials is closely related to the diversity of their structures.In this work,we built the Ag12@(ZnO)30 core-shell heterostructure which is different from the structures in our previous work,the results show that the Agl2@(ZnO)30 is magnetic materials,the magnetic origin mainly comes from the partial O atoms at the interface between the Ag and ZnO.Compared with the pure ZnO bulk materials,the Ag12@(ZnO)30 system has a strong absorption peak in the visible light region.Therefore,the Ag12@(ZnO)30 will induce slightly magnetic moment and improve the absorption ability of ZnO for visible light,then further promote their application in the photoelectronic field.(3)Two-dimensional CrSiTe3 is an intrinsic half semiconductor,which has been obtained experimentally by mechanical exfoliation.Here,15 kinds of intrinsic defects are induced in monolayer CrSiTe3,respectively.The defect formation energies of all defected systems are investigated by first-principles calculation method,the results show that the Crsi and SiCr antisite defects are preferably generated in monolayer CrSiTe3.More importantly,the CrSi and SiCr antisite defects make the CrSiTe3 become bipolar magnetic semiconductor(BMS),and the BMS properties are robust against the external strain and electrons interference.Furthermore,the defective system and their BMS characters still are remained at or above the room temperature(at 500 K).Therefore,our results show that defect engineering is an efficient way to produce bipolar magnetic semiconductor,it also lays a solid foundation for the development of spintronics devices.(4)To get the intrinsic bipolar magnetic semiconductor,we predicted the structural stability,magnetic and electronic properties of monolayer GdI2.The results show that the 2D GdI2 can be obtained by a mechanical exfoliated method from the bulk structure.The monolayer GdI2 has good lattice and thermodynamic stability.Interestingly,the 2D GdI2 is an intrinsic bipolar magnetic semiconductor with the indirect band gap,in which the different spin conductive channels can be obtained by electrons or holes doping.It is also a promising ferrovalley material with the large spontaneously valley splitting of about 130 meV.And the BMS behaviours and ferrovalley properties are robust against the biaxial strains.Therefore,the monolayer GdI2 has the potential application in both spintronics and valleytronics fields,this will promote the development of quantum information engineering.