R.C. Junghare, G.C. Patil
Visvesvaraya National Institute of Technology Nagpur,
India
Keywords: molybdenum disulfide, density functional theory, Band structure, Hamiltonian
Summary:
Due to its finite energy gap and comparable electron mobility with graphene, transition metal dichalcogenides, particularly molybdenum disulfide (MoS2) has shown immense potential for the nanodevices. However, to implement the MoS2 based 1-diemensional (1-D) and 2-diemensional(2D) sub-7 nm nano-devices, simple and accurate model which explores the band structure and electronic properties of the MoS2 is strongly needed. To investigate the electronic properties of mono-layer to multi-layer structures, ab-initio calculations have been used by various research groups. Although the reported ab-initio calculations based on the density functional theory (DFT) computes the valuable information about the electronic properties, due to computational limitations for large atom systems, the reported ab-initio approaches are not suitable for 1D and 2D device structures. To overcome this problem, tight binding approximations have been used to simulate the 1 D and 2 D device structures consisting of 10 to 10^6 number of atoms in the channel. Although the tight binding approximation has been used by the researchers for investigating the electronic properties, the ab-initio DFT calculations along with the tight binding approximation have not been reported for the MoS2 based 1 D and 2 D device structures. In this work, by performing the ab-initio DFT calculations, the Wannier tight binding approach has been used to generate the accurate Hamiltonian. The generated Hamiltonian can be further used for the transport calculation performed by the non equilibrium green function (NEGF) for MoS2 based 1 D and 2 D device structures. The DFT computations have been performed by using Quantum ESPRESSO, which uses self consistent plane wave pseudo potential total energy method. Further, generalized gradient approximation (GGA) proposed by Perdew and Wang (PW91) has been used for exchange correlation potential. The lattice constants considered for MoS2 are a = 3.16 Å and c = 12.294 Å which are fully relaxed until the energy convergence of 10^-9 eV is reached. Further, wave function and charge density cut-offs of 70 Ryd and 300 Ryd have been used respectively. In addition, the optimized structure (coordinates) have been used to perform self consistent calculations with a Monkhorst-Pack 4 × 4 × 1 k-mesh followed by the non-self consistent calculations for band structures and density of states(DOS). Further the tight binding approximation of electronic band structure is accomplished by transforming plane wave basis into maximally localized wave functions (MLWF). The open source Wannier-90 tool implements this transformation to get tight binding Hamiltonian from plane wave DFT calculation performed in quantum ESPRESSO. In order to perform Wannier unitary transformation, 11 orbitals have been considered which are 7 highest valence bands and 4 lowest conduction bands consisting of d-orbitals of metal atom and p-orbitals for chalcogen atom. The remaining orbitals are excluded from the calculation as 11 Wannier functions have been used in unitary transformation. The d/p orbitals are selected as initial projections so that Wannier functions are closer to localize atomic orbitals.