Optics of atomically thin films and van der Waals heterostructures made from two-dimensional semiconductors

This thesis discusses optical investigations of two-dimensional metal-chalcogenide semiconductor materials and their heterostructures. Topics include a study of continuous wave (cw) and time-resolved photoluminescence (PL) of GaTe and GaSe thin films. Based on experimental evidence, we propose a model explaining the strong PL intensity decrease for thin films as a result of non-radiative carrier escape via surface states. We investigate the stability of thin films of InSe and GaSe using a combination of PL and Raman spectroscopies. By comparing signal intensities in films exposed to ambient conditions for up to 100 hours, we find notable degradation in GaSe and high stability of InSe. We continue our study with the investigation of optical properties of light emitting diodes (LED) made of van der Waals (vdW) heterostructures comprising graphene as transparent contacts, hexagonal boron nitride as tunnel barriers and transition metal dichalcogenides (TMDC), MoS2 and WS2, as the semiconductor active regions. Single and multiple 'quantum well' structures were fabricated with an aim to enhance the external quantum efficiency (EQE) under electrical injection. We also present PL characterisation of LEDs based on vdW heterostructures comprising WSe2 and MoSe2 as active layers. Temperature dependent experiments show unusual enhancement of the EQE with temperature in WSe2 in contrast to MoSe2, where both electroluminescence and PL are reduced with temperature. A theoretical approach to explain this behaviour is proposed, which is based on the strong spin-orbit interaction present in both materials.