Executive Summary : | Recently two-dimensional (2D) van der Waals crystals of semiconducting transition metal dichalcogenides (TMDs) have attracted considerable attention for applications in advanced electronic devices owing to an unusually strong Coulomb interaction resulting in the formation of tightly bound excitons. Beyond TMDs, recently another group of layered semiconducting post-transition metal chalcogenides (PTMCs) such as GaS, GaSe, GaTe, SnS₂, InSe with attractive properties of tunable band gaps, high carrier mobility, have drawn the attention as the rapidly expanding family of 2D materials. Since excitons control the optical response, a microscopic understanding of their properties is vital for technological applications in future optoelectronic and photonic devices. Temperature-dependent photoluminescence (PL) study reveal exciton-phonon coupling which bears relevance to exciton relaxation dynamics, homogeneous line widths, and electronic structure. One long-standing question in 2D materials' physics is the effect of 2D confinement on the strength of exciton-phonon coupling and phonon frequency. Further, investigation of the photoconductivity (PC) of 2D materials reveals insights on the electronic properties determined mainly by the intrinsic and impurity point defects, forming local centres (traps) in the forbidden gap. The presence of large spatial fluctuations of the potential energy of the carriers in 2D materials lead to a substantial concentration of strongly localized states resulting in a long recombination time of photo-generated carriers. Therefore, a fundamental understanding of the photophysical processes arising due to strongly bound excitons and defect related trap states in PTMCs is essential for their future electro-optic applications. Though this topic has received considerable experimental and theoretical attention for 2D TMDs but to the best of our knowledge, no experimental study has been reported so far on excitonic physics, and structural defects induced properties of PTMCs. We are proposing to bridge this gap. This proposal intends to investigate on the effect of excitonic-phonon coupling and defects in the PL and PC properties of larger area 2D PTMCs such as GaS, GaInS₂, TlGaS₂, TlInS2 prepared by both mechanical exfoliations from commercially procured single crystals and chemical vapor deposited thin film. An initial study of Raman spectroscopy followed by an investigation of the impact of exciton-phonon coupling on the homogeneous linewidth by measuring temperature dependent PL will be carried out. Another focus is on the investigation of electronic transport properties of PTMCs by studying PC transients under band gap and sub-band gap optical excitations. Therefore, the present proposal bears immense importance by providing a knowledge on the exciton-phonon coupling and localised defects which are relevant for designing optoelectronic devices based on PTMCs. |