Ultrafast spectroscopic studies of two-dimensional materials and devices

2D materials have garnered significant interest over the past two decades due to their extraordinary properties, including strong quantum confinement, high carrier mobility, and unique light-matter interactions. These attributes make them highly promising for applications in electronics and optoelectronics. Despite substantial progress in large-scale fabrication methods and device optimization, fundamental understanding of carrier dynamics and excitonic behaviour remains limited. Such insights are critical for advancing 2D materials and their integration into next-generation technologies. This thesis investigates the photophysics of excited states—particularly free carriers and excitons—in 2D materials using steady-state and ultrafast spectroscopic techniques. The first two chapters introduce the structural and photophysical principles of widely studied transition metal dichalcogenides (TMDs) and MXenes. Chapter 3 details the spectroscopic methods employed in this research. Chapter 4 investigates hot carrier relaxation in SL TMDs using pump-probe photoluminescence spectroscopy. It reveals the roles of carrier density and excess energy in cooling dynamics and identifies evidence of hot carrier trapping at high energy levels. Chapter 5 focuses on carrier and lattice cooling in metallic MXene thin films, showing that while carrier relaxation occurs within picoseconds, lattice cooling requires hundreds of nanoseconds due to inefficient inter-flake thermal transport. In Chapter 6, steady-state and ultrafast photocurrent spectroscopy are applied to multilayer MoS2 photodetectors, demonstrating band-like photoexcited free carrier transport, and providing evidence of an exciton-to-dark-state transition. Overall, this thesis provides valuable insights into the interactions among free carriers, phonons, and excitons in 2D materials following photoexcitation. The findings contribute to the fundamental understanding of 2D material photophysics and support their development for future technological applications.