Anomalous Vibrational Frequency Shift and Efficient Radiative Sub-bandgap Electronic Transitions in Van der Waals Quasi 2D Post Transition Metal Chalcogenide TlGaS2

Ternary post transition metal chalcogenides are gaining significant interest for their potential optoelectronic applications due to the dual bandgap and highly anisotropic nature. This study utilizes simultaneous vibrational and μ-photoluminescence (PL) spectroscopy to gain understanding of how single-crystalline as well as exfoliated TlGaS₂ behave as the temperature and layer number change. Unprecedently, observed Raman peaks may be assigned to reported group theoretically calculated vibrational modes, which reveal softening and a systematic change in anharmonic characteristics with an increasing temperature. The sub-bandgap electronic transitions exhibit distinct temperature-dependent radiative recombination behaviors owing to donor–acceptor pair (A band) and trapped bound excitons (X_B). The exfoliated flakes reveal an anomalous frequency shift in the Raman modes in contrast to transition metal chalcogenides and almost unaltered μ-PL emission intensity as the thickness is reduced exploring possibilities of advanced semiconductor technologies.