Study Of Spin Polarization And Photoelectric Properties Of The CdS Low Dimensional Nanostructures

Cadmium sulfide (CdS) is a typical Ⅱ-ⅥI direct band gap semiconductor material, with the band gap energy of around2.42eV at room temperature. CdS is high sensitivity n-type photoconductive device materials and piezoelectric materials, and it is widely used in the field of sensor, light emitting diode, solar cells, optical detector, optical waveguide devices and nonlinear optical devices.Doping is very important for semiconductor material, doping can tailor the physical properties of the material. The researchers have done a lot of research work on hybridizing CdS semiconductor, especially on designing the ferromagnetic half metallic material, which makes the CdS semiconductor have a good prospect in application in the field of spintronics devices. But there are a lot of controversies on the origin of magnetism in dilute magnetic material. The physical properties of the material can also be adjusted by reducing the size or the dimension of the materials. For example, the quantum size effect can change the surface geometry and the electronic structure of CdS if the size or the dimension of Cds is reduced to nanometer magnitude, which has important influence on its magnetic, electrical and optical properties. In2010, the Nobel Physics Prize for pioneering research on Graphene, inspired by Graphene and Boron Nitride, people are beginning to pay attention to other types of graphenelike structure of binary compounds with rich electrical properties. One-dimensional nanostructures (wires, rods, tubes) and one-dimensional semi-conductor nanometer arrays receive extensive attention as well, because they have orderly structures, which are advantageous to the directional transmission of carrier and can improve the separation efficiency and transmission efficiency of electrons and holes. Low-dimensional materials are considered the foundation materials for next generation of nanoscale photoelectric devices. Very recently, the study of CdS has made remarkable achievements in experiment. People have synthesized two-dimensional (2D) CdS nanosheets (CdSNS). which have a graphenelike honeycomb structure, and one- dimensional (1D) CdS nanowires/nanoribbons and quantum dots.On the other hand, first-principles calculations are used widely and successfully in quautum chemistry, condensed matter and materials science. This method is also a powerful tool to study nanometer materials. In this dissertation, firstly, we apply the first principles method to study spin polarization and photoelectric properties of non-magnetic elements doping CdS. Secondly, the electronic band structure can be tailored by changing the surface structure of low dimensional CdS nanomaterials and controlling the hydrogen absorption on the CdS surface. The CdS nanomaterials can be tailored to have a good application value on spintronics devices. The main research contents and results are as follows:(1) The research significance and status at home and abroad of CdS nanomaterials are reviewed. And the significance and research content of the subject are advanced.(2) We introduce the research methods and simulation codes. The idea of density functional theory (DFT), the development of DFT on some specific problems and some commonly simulation pachages based DFT are introduced. At the end of this chapter, we briefly introduce WIEN2K used in our paper.(3) The ferromagnetic materials can be obtained by doping magnetic elements into semiconductor. But no convincing evidence can verify that the observed ferromagnetism is intrinsic. So we chose the non, magnetic metal Cu doping CdS semiconductor for ferromagnetism materials. First,we discuss the feasibility test of Cu doped CdS in the experiment by calculating formation energy. Secondly, we discuss the generalized gradient approximation effects on electronic structure of Cu doped CdS near fermi level, resaerch electronic structure and spin polarization characteristics of Cu doped CdS supercell, and analyse the origin of the ferromagnetism. Finally, we analyse the influence on the electronic structure and optical properties of Cu concentrations in the Cu-doped CdS semiconductor.The calculated results revealed that the moderate formation energy indicates that Cu doped CdS with high concentrations may be easily realized experimentally in S-rich conditions. Due to the d orbitals of Cu hybridized with the p orbitals of S near the Fermi level, Cu-doped CdS systems show half-metallic character with a total magnetic moment of I.Oub per Cd3sCuS36. The room-temperature long-range ferromagnetism is observed, which results from Cu(3d)-S(3p)-Cd-S(3p)-Cu(3d) coupling chain. We find that the d orbitals of Cu are strongly hybridized with the p orbitals of S near the Fermi level, resulting in p and d charge transfer between Cu and S atoms, and enhancing the conductivity of CdS significantly. Along with the increase of Cu concentrations, the materials transit from a semi-metal to a metal, and the first peak of imaginary part of the dielectric function move to the lower energy side (red shift). The static dielectric constant of Cu doped CdS changes obviously. The absorption coefficient is very large in ultraviolet region. There are several absorption peaks in the lower energy comparing with pure CdS. Cu doped CdS is conducive to the absorption of visible light.(4) The single-layer CdS nanosheets (CdSNS) are cut along (0001) direction from the Wurtzite structure. The pristine CdSNS transform to a flat graphitic structure after structures optimization. In the fourth chapter, we mainly discuss the electronic properties of CdSNS, study on the electronic and magnetic properties of hydrogenated CdSNS, and study the effect on the spin polarization and photoelectric properties of C doped CdS.We find that the calculated band structure of the CdSNS exhibits a direct bandgap, which is larger than that in bulk structure. For the structures that are hydrogenated. His easily adsorbed on Cd. Cd-hydro CdSNS are expected to show semimetallic properties with room-temperature ferromagnetism and100%spin polarization. It is noted that the hydrogen-induced magnetic moments are localized in unpaired2p electrons in the unhydrogenated S atoms. C doped CdSNS is also a semimetallic material with magnetic. C-C is antiferromagnetic coupling, but C-C becomes ferromagnetic coupling by the electron injection in C-doped CdSNS. The theory of carrier double exchange can explain the ferromagnetic coupling mechanism between C-C. The conductivity of C-doped CdSNS enhance significantly. The peak of imaginary part of the dielectric function in low energy moves to the lower energy side (red shift). The optical absorption edge of absorption spectrum shift to lower energy. C-doped CdSNS increase the absorption intensity of visible light.(5) CdS nanowires (CdSNWs) are relaxed, and electronic and magnetic properties of hydrogenated CdSNWs are investigated. We find that surface relaxation plays an important role for the hydrogenated CdS NWs and therefore leads to drastic changes of electronic properties. The magnetic properties can be tuned by controlling passivation on surface sites with hydrogen. While hydrogenated nanowires on surface S atoms are nonmagnetic, hydrogenation on surface Cd atoms, and especially a monolayer of H on the surface, results in half-metallic properties with100%spin-polarized carriers, which is helpful for spintronics. The partially filled sulfur3p states hybridized with H s states, which results in the splitting of the energy levels near Fermi level and is the main reason of spin polarization.(6)The major results of the paper are summarized, and the next research works are proposed in the chapter6.Therefore, our works offer a new route toward long-range high room-temperature ferromagnetism of CdS materials in experiment, provide a theory basis for the origin of magnetism, and may motivate potential applications of CdS nanostructures in spintronics and photoelectron devices.