单层二硫化钼多相性质及相变的第一性原理研究

First-principles study on multiphase property and phase transition of monolayer MoS_2

本文基于密度泛函的第一性原理,并引入范德瓦耳斯力修正,研究了单层二硫化钼2H,1T,ZT三种相的电学性质及相变原理.首先通过结构弛豫确定了三种相的几何结构,能带和态密度计算证实1T相具有金属性质,ZT相具有半导体性质,带隙为0.01 eV.然后结合变形势理论计算了2H和ZT相的迁移率,ZT相的迁移率高达104cm^2·V^(-1)·s^(-1),进一步拓展了单层二硫化钼的应用范围.最后通过对比三种相吸附锂原子结合能,计算2H-1T相变能量曲线,解释了引起二硫化钼相变的原因.本文的研究结果将对单层二硫化钼实验制备表征以及相关光电器件性能分析提供重要参考.

Using first principles calculations within density functional theory, we investigate multiphase property and phase transition of monolayer MoS2. All the quantities are calculated using the Vienna ab initio simulation package. Calcula-tions are performed within the generalized gradient approximation with van der Waals corrections （optimized Perdew-Burke-Ernzerhof-vdW）. The cutoff energy of plane-wave is set to be 400 eV. The atomic plane and its neighboring image are separated by a 15 - vacuum layer. The k-meshes for the structure relaxation and post analysis are 11 ＆#215; 11 ＆#215; 1 and 19 ＆#215; 19 ＆#215; 1 respectively. 〈br〉 Firstly, we obtain the geometry configurations of 2H-MoS2, 1T-MoS2 and ZT-MoS2 phases through structure re-laxing. The lattice constants of 2H-MoS2 are a=3.190 - and b=5.524 -, and total energy is-39.83 eV which means that it is the most stable phase. The lattice constants of 1T-MoS2 are a=3.191 - and b=5.528 -, and total energy is-38.21 eV, which means that it is the most unstable phase. Both 2H-MoS2 and 1T-MoS2 have a three-layer structure with two S layers sandwiching one Mo layer. The difference of 1T-MoS2 from the 2H-MoS2 is the upper S layer shifting. The ZT-MoS2 derives from 1T-MoS2 through lattice distortion. The lattice constants of ZT-MoS2 are a = 3.185 - and b = 5.725 -, and total energy is -38.80 eV. The total energy determines the following stability order of three phases： 2H-MoS2〉ZT-MoS2〉1T-MoS2. Our computed results agree well with the other computed and experimental results. Band structure and density of states confirm that 1T-MoS2 is metallic and ZT-MoS2 is semiconducting. But the bandgap of ZT-MoS2 phase is only 0.01 eV. Then we compute the intrinsic carrier mobility values of 2H-MoS2 and ZT-MoS2 at 300 K with the deformation potential theory. The carrier mobility of 2H-MoS2 is between 100 cm2·V-1·s-1 and 400 cm2·V-1·s-1. Owing to ZT-MoS2 carrier effective mass decreasing obviously, the carrier mobility of ZT phase rises to 104 cm2·V-1·s-1. The great carrier mobility of ZT-MoS2 covers the shortage of 2H-MoS2 and expands the applications of monolayer MoS2. 〈br〉 After obtaining the intrinsic properties of three phases, we investigate the phase transition of monolayer MoS2. Adsorption energy becomes more accurate with van der Waals corrections. Through comparing the adsorption energy, we conclude that the stabilities of Li absorbed on the surfaces of three phases are in the following order： 1T-MoS2〉ZT-MoS2〉2H-MoS2, which is opposite to the stability order of the three phases. It means that 1T-MoS2 absorbs Li more easily than 2H-MoS2. Finally we compute the energy pathways of the phase transition from 2H-MoS2 to 1T-MoS2. Introducing an electron makes the energy barrier of 2H-1T transition change from 1.85 eV to 1.49 eV. Increasing electron concentration reduces the di-culty in producing phase transition. Li intercalation plays the same role as an electron and the energy barrier drops to 1.24 eV. In conclusion, the MoS2 electron concentration change is the key reason for phase transition. The study results may provide guidance for the preparation and characterization of monolayer MoS2.