Electronic Structures Of Low Dimensional Nano Heterojunction:First-principles Study

Low-dimensional nano-heterojunction materials are fundamental materials for developing the nanoscale electronic and optoelectronic devices. They have changed human’s life, from the internet, communication information storage and processing, high-speed computing to aerospace. It is importantly influence on both the high-tech development and our daily life. In this thesis, the first-principles calculations based on the density-functional theory, we have been system investigated on the electromagnetism properties of GaAs/InAs nanowires, monolayer MoS2 and its heterojunctions, and revealed the microscopic process of physical properties of band gap, doping, surface functionalization, magnetic anisotropy energy and magnetic exchange coupling. The main works and innovations of this dissertation are listed as follows:1 We adopted internal and external two mechanisms to manipulate the band structures of wurtzite and zinc-blende GaAs/InAs-core-shell nanowires (NWs) along the [0001] and [111] directions, respectively. Variational geometry size and chemical component are the internal approaches to tune the band structures. The band gaps are nonlinear composition dependence for the core-shell NWs with fixed diameter and linear composition dependence for the NWs with fixed core. Using external uniaxial strain is another alternative approach. We found that the relative band gap decreases evidently with tensile strain, while it gradually increases with increasing compressive strain. The higher the ratio of GaAs composition in the core-shell NWs, the larger the variations of the relative energy. More interestingly, in wurtzite core-shell NWs, we found a critical reflection point, which results from the two competition states between bonding and anti-bonding. Compared with the wurtzite pure GaAs NWs, the reflection point of GaAs/InAs-core-shell NWs2 We investigated p-type electronic structures and the doping mechanism in wurtzite (WZ) and zinc-blende (ZB) GaAs/InAs core-shell nanowires (NWs) along the [0001] and [111] directions, respectively. Comparing the doping in WZ and ZB core-shell NWs, we found it is easier and more stable to realize dopant in WZ NWs. Due to the type I band-offset, p-type doping in the GaAs-core of GaAs-core/InAs-shell for both WZ and ZB NWs makes that the valence band-edge electrons in the InAs-shell can spontaneously transfer to the impurity states, forming one-dimensional hole gas. In particular, this process accompanies with a reverse transition in WZ core-shell nanowire due to the existence of antibonding and bonding states.3 We investigated the effect of surface dangling bond on p-type doping mechanism and the electronic structures in wurtzite (WZ) and zinc-blende (ZB) GaAs/InAs core-shell nanowires (NWs) along the [0001] and [111] directions, respectively. The results of the formation energies show that the surface dangling bond of the In atom is a kind of stable defect. Both in WZ and ZB core-shell NWs, we found it is easier and more stable to realize dopant in the GaAs core. Moreover, the position of Cd impurity plays a key role in the formation of p-type nanowires. The farther the distance between the impurity and the surface dangling In atom, the easier it is to form the p-type characteristic of the nanowires. In particular, it shows an intrinsic behavior when doping the Cd impurity near the surface dangling bond. The surface dangling bonds have an ability to capture the holes from the neighbor doping impurity, resulting in the deactivation of dopants. Meanwhile, the transfer of hole moves the valence band down to the lower energy levels and even can lead to a band anticrossing phenomenon in the conduction band. Our results highlight a new physical coupling between the doped state and surface dangling bonds in GaAs/InAs core-shell NWs, and open a new opportunity for the development of tailoring nanoscale electronic properties.4 Our firstprinciple calculations show that magnetic anisotropy energy (MAE) and the direction of magnetization in the Fe/MoS2 system can be tailored by injecting charge or strain engineering. We demonstrate that these two approaches have a significant influence on the magnitude of the MAE. The hybridization between Fe 3d and Mo 4d electrons changes the occupation of d orbitals around the Fermi level, which leads to the strong dependence of MAE on the external charge injection or strain. MAE increases up to ten times higher than that of the neutral system when injecting charge. More intriguing is that the biaxial strain can affect the easy-axis switching in Fe/MoS2, while the process does not occur in the single Fe monolayer.5 Using density-functional theory calculation including a Hubbard U term, we explore electronic and magnetic properties of transition metal (Fe and Mn) doped MoS2 and MoS2/WS2 heterojunction by charge injection. We find that magnetic coupling is not only related with the layer number, also depends on the selection of doping metal element. Fe(Mn) dopants prefer to the ferromagnetic(anti-ferromagnetic) coupling in monolayer MoS2, and the ferromagnetic and antiferromagnetic states can be exchanged by means of charge injection. Additionally, the magnetic exchange coupling of Fe or Mn dopants in MoS2/WS2 heterojunction is antiferromagnetic interaction, and it can not be changed by injecting charge other than doping in monolayer MoS2.