Band gap engineering of a MoS2 monolayer through transition metal and chalcogen alloying : an ab initio study

Abstract

In this thesis, density functional theory (DFT) calculations are performed to study the transition metal and chalcogen alloying of a molybdenum disul de (MoS2) monolayer for band gap engineering. The e ects of the foreign atoms on the thermodynamic stability, structural and electronic properties of the MoS2 monolayer are investigated. To study these e ects systematically at di erent alloying concentrations, the possible line-ordered con gurations at each concentration are considered. Their energetics, structural and electronic properties are compared with the well-known alloy shapes, random and/or cluster con gurations. For the case of the transition metal alloying, chromium (Cr) atoms are introduced at the molybdenum (Mo) sites. Various unique line-ordered con gurations are considered at each concentration. The most stable ones are identi ed by means of formation energies. The energetics comparison of the line-ordered alloy and the random con gurations generated using special quasirandom structure (SQS) shows that the line-ordered alloy con gurations have relatively low formation energies compared to the random con gurations. The formation energies of all considered con gurations are positive but relatively small, revealing that both shapes of the Cr alloying can be synthesized and co-exist at the same synthesis conditions. For the structural properties, the increase in Cr concentration reduces the lattice constant of the MoS2 system following the Vegard's law. The Cr atoms ne-tune the band gap of a MoS2 monolayer from 1.65 eV to 0.86 eV. Based on the partial density of states and the charge density analysis, the Cr 3d and Mo 4d at the vicinity of the band edges are found to be the main responsible for the reduction of the band gap. For the chalcogen alloying, the in uence of the oxygen (O) and tellurium (Te) atoms are considered. We start with the study of the O alloying in a MoS2 monolayer appearing in di erent shapes: line-ordered, cluster and random. The small calculated formation energy values of the various O alloy con gurations show that this alloying are stable and should be synthesizable under favorable conditions. At high concentration, the O line-ordered alloys seem to be constantly most stable compared to the considered random and cluster alloy con gurations, while the formation energies of all the con gurations are nearly the same at low concentration. Although the O atom has small atomic radii compared to the S atom, their alloying preserve the 2D hexagonal structure of the MoS2 monolayer at each concentration. However, the lattice constant decreases linearly with the increase in O concentration, consistent with Vegard's law. The introduction of O atom in the MoS2 monolayer also ne-tunes the band gap of the MoS2 monolayer with a range of 1.65 eV to 0.98 eV. The band gap reduction is mainly contributed by the Mo 4d and O 2p orbitals at the band edges. We further carried out a thorough systematic study of Te line-ordered alloys in a MoS2 monolayer. The low formation energies of the Te line-ordered alloy con gurations indicate that they are also thermodynamically stable at low concentration. The obtained formation energies for line-ordered alloy con gurations at each concentration compete very well with the random con gurations that are already achieved experimentally. The structural characterization indicates that the lowest energy con guration at each concentration corresponds to the con guration where the Te atom rows are far apart from each other within the supercell. Similar to that of O alloying, the variation of the lattice constant at di erent concentrations obeys Vegard's law, but its values increase with the concentration since Te atom has larger atomic radius than S and O atoms. The Te alloying ne-tunes the band gap ranging between the MoS2 (1.65 eV) and the MoTe2 (1.04 eV) band gap values . In brief, the Cr, O and Te successfully engineer the band of a MoS2 monolayer, and their study should be bene cial for nanotechnological applications.