Electronic band engineering of Transition metal dichalcogenides: First Principles Calculation

Author

Nikesh Maharjan, Southern Illinois University CarbondaleFollow

Date of Award

5-1-2015

Degree Name

Master of Science

Department

Physics

First Advisor

Jayasekera, Thusari

Abstract

Based on first principles Density Functional Theory calculations, we have investigated for possible paths for engineering electronic band structure of Transition Metal Dichalco- genides (TMDs). We have considered two approaches which have shown to be promising for engineering electronic bands of TMDs: substitutional chemical doping and heterostruc- turing. All the calculations are done using first principles Density Functional Theory as it is implemented in Quantum Espresso package. Two possible substitutional doping meth- ods for MoS2 are considered in our calculations; cation doping where Mo is substituted by metal atoms and anion doping where Nitrogen and halogen group atoms take the posi- tion of S-sites. We observe the n-type characteristics for halogen group doping and p-type characteristics for Nitrogen group doping at S site. Similarly, we observe these bipolar characteristics when substituted by the transition metal elements (4d elements in the peri- odic table) at Mo site. Our results on doping monolayer MoS2 are in agreement with those results obtained by Dolui et al. for similar systems. Our work is extended to explore the effect of substitutional doping in bilayer MoS2. We observe the promising bipolar char- acteristics on doping while the magnitude of the band gap decreases upon the controlled S-site doping with F and As. In the second part, we considered two types of heterostructuring; Van der Waals heterostructures, and lateral heterostructures. In Van der Waals heterostructures, a direct band gap is observed with a physical separation of charges into two layers from orbital isosurface plots. We present a brief overview of the folding of energy bands in supercell approach. Using heterogeneous supercell approach, we studied the electronic properties of a mixed system of MoS2 -WS2 . The separation of the charges into the two sections shows that our MoS2 -WS2 in-plane heterostructure shows a potential for a pn junction. These systematic studies of the doped and heterostructures of TMDs can be useful for device applications.