ELECTRONIC, LINEAR AND NON-LINEAR OPTICAL PROPERTIES OF TRANSITION METAL DI-CHALCOGENIDES

Abstract

A theoretical study of the electronic and optical properties of some transition metal dichalcogenide compounds (TMDC's) is presented in this thesis. The TMDC's (TX2) are formed by transtion metals (T) belonging to IVB, VB, and VIB groups of the periodic table and a chalcogen (X=S, Se, Te). The IVB and VIB group compounds exhibit semiconducting or insulating behavior whereas the VB group compounds tend to be metallic and are reportedly superconducting at low temperatures. These compounds show a variety of interesting physical properties [175]. The TMDC's can exist in various crystallographic structures. They have highly anisotropic physical properties arising from their layered structure with covalent bonding within layers and weak van der Waals bonding between the layers. Consequently the TMDC's behave as two dimensional solids and can be intercalated with foreign atoms and molecules leading to significant and dramatic changes in their electronic properties. A notable example is lithium intercalated MoS2 which is used in lithium batteries [56]. In the past few decades the study of electronic and optical properties of TMDC's has been actively pursued due to their potential for technological applications, such as photovoltaic, optoelectronic applications and electrochemical devices [121,122], phase transitions [85], and solid state lubricants [1271. We have used the full potential linear augmented plane wave (FPLAPW) method to determine the electronic properties of the TMDC's. Development in linear methods for solving the band structure problem during the last two decades has totally erased the limitations that were present in other contemporary band structure techniques. The linear methods have been used to calculate a wide range of electronic properties [31,134,159,165], magneto-optical Kerr effect [53], Fermi surface in disordered alloys [126], superconductivity in La3X compounds [130], vibrational properties of phonons in random alloys [1], vibrational spectra of small hydrogenated silicon clusters [57], and linear and nonlinear optical properties [128,129,139,142,143]. In the LAPW (linear augmented plane wave) method the unit cell is partitioned into non-overlapping atomic spheres centered on atomic sites and an interstitial region. For the construction of the basis function the muffin-tin approximation (MTA) is used. In the interstitial region, plane waves form the complete basis set and inside the atomic sphere, the solutions for a spherically symmetric potential are atomic basis functions. The LAPW's form the basis for expanding the crystalline orbitals (Bloch states). The MTA works reasonably well in highly coordinated systems, such as fcc metals. For covalently bonded solids or layered structures the MTA is a poor approximation and leads to discrepancies with experiments. The more general treatment of the potential, such as provided by a full potential (FP) method has none of the drawbacks of the atomic sphere approximation (ASA) and MTA based methods. In full potential methods the potential and charge density are expanded into lattice harmonics inside each atomic sphere and as a Fourier series in the interstitial region. The FPLAPW calculations have proven to be one of the most accurate methods for the computation of the electronic structure of solids within density functional theory (DFT). The foundation of the DFT was laid by Hohenberg and Kohn [58]