| Molybdenum Disulfide(Mo S2) as a novel two-dimensional layered semiconductor materials, because of its unique molecular structure, considerable band gap and other excellent properties, has caused extensive research among international fields of materials. Based on the bright future of Mo S2 research, we have made systemic calculations of the intrinsic Mo S2, vacancy system, rare earth elements doped, phonon spectra and other research directions by the Materials Studio software. The results of the study are as follows:1. Optimized calculation results of single-layer Mo S2(6×6×1) Supercell show that single-layer Mo S2 is direct band-gap semiconductor with 1.726 e V band-gap. The band gap width is slightly smaller than the experimental result. Optical absorption spectrum indicates that single-layer Mo S2 has a strong absorptive performance from visible to ultraviolet region. It demonstrates that monolayer Mo S2 has potential applications for the fabrication of opto-electronic devices, especially the ultraviolet detector. Phonon dispersion curves do not have degenerated parts because of the D3 h symmetry of single-layer Mo S2. Raman active modes in the Brillouin zone center of two optical branches LO2 and TO2 has LO-TO splitting nearly 1.9cm-1.2. We investigated the vacancy behavior in single layer Mo S2 on the basis of the intrinsic Mo S2 study. Both of Mo vacancy and doubles S vacancies have the characteristics of point defect. At the same time, S vacancy can be formed more easily. Combining electronic distribution and analysis of DOS indicates that defect levels in band-gap may be acceptor levels. The DOS of the conduction band in the high-energy region of Mo and S vacancy defect systems is significantly smaller than intrinsic Mo S2. We attribute this consequence to the confinement of defect levels. Additionally, the DOS of the conduction band, whatever Mo vacancy or S vacancy system, emerges several orders of magnitude decreased.3. It is known that rare earth elements with rich 5d, 4f valence electrons and long life of the excited states. In this calculation part, we selected the La, Ce, Nd as impurity atom to replace a Mo atom of single-layer MoS2. The lattice parameter, band structure,density of states and electron density difference is obtained, respectively. Comparisons with intrinsic Mo S2 indicate that the lattice distortion of La-doped is maximum, but Nd-doped is minimum. Analysis show that the difference of lattice distortion among three kinds of impurities caused by the difference covalent radius(La>Ce>Nd). La-doped induces acceptor levels in the band gap, but Ce-doped and Nd-doped often form coexistence of donor and acceptor levels. Electron density difference of three different kinds of doped systems shows that rare earth dopant changes electron distribution in a single layer Mo S2, especially Nd-doped which exist 4f electron.4. In the last section, we applied GGA method and GGA+U method which on the basis of GGA but introduces the repulsion between d-state electrons as a correction for the calculation of magnetic properties of La-doped monolayer Mo S2. The results demonstrate that GGA+U method makes the moment of magnetism, band gap and the degree of lattice distortion bigger than the consequences of GGA method, which corresponds with the analysis of basic theory. Considering that only Mo and La have d-state electrons in La-doped system, but the distance between Mo and La is larger than 3?. So that the interaction is weak among d-state orbits of Mo and La which lead slight change of lattice structure and band structure, but GGA+U method may be more nearly to the actual situation in the S substituted by La supercell. |
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