2D materials and their hetero bilayer systems for optoelectronic applications

In the pursuit of the materials systems compatible with the paradigm shift in the future optoelectronics, immense effort is devoted to exploring the potential of 2D materials. Motivated by this, a comprehensive study of 2D TMD and their hetero bilayer systems for optoelectronic applications is presented in this thesis. Monolayer MoS2 and MoSe2 have been chosen by keeping the transition metal (Mo) the same while changing the dichalcogenides from S to Se, so we could investigate their interaction by structural and optical characterizations. Single layer MoS2 and MoSe2 have direct bandgaps of ~ 1.82 eV and ~ 1.55 eV, respectively. Monolayer MoS2 is grown both by using the ALD-assisted CVD method on SiO2/Si substrates and conventional CVD method on glass substrates. For MoSe2, SiO2/Si substrates coated with NaCl solution as are used in the conventional CVD system. The growth parameters are carefully optimized for high coverage of high-quality and large area monolayers. Subsequently, MoS2/MoSe2 hetero bilayer system is formed by PMMA-assisted wet transfer method. The Monolayer TMDs and their hetero bilayer system are systematically characterized by Raman, PL, and AFM spectroscopies. The hetero bilayer system is further characterized by temperature dependent µRaman spectroscopy by varying the temperature between 83 K and 483K. Change in the Raman mode's peak position and FWHM with respect to temperature in each of the materials has been analyzed. We observed characteristic softening of the MoS2 and MoSe2 Raman modes with an increase in temperature, and the temperature coefficient of the materials is larger in the hetero bilayer system than in the isolated region. Keywords: 2D materials, TMD, MoO3, MoS2, MoSe2, ALD, CVD, Optoelectronic