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Cryogenic photoluminescence spectroscopy is a versatile tool to locally probe the defects in diverse material platforms as well as to observe modifications of the underlying electronic band structure in novel two-dimensional quantum materials such as the monolayer transition metal dichalcogenides (TMDs) (e.g. MoS2, WS2, WSe2, and MoSe2) and their heterostructures. These monolayer TMDs feature direct bandgaps and excitons with high binding energies due to quantum confinement which are conducive towards optoelectronic applications. We present our latest results on the characterization of monolayer TMDs and heterostructures based on monolayer TMDs using our newly developed fiber optic-based cryogenic photoluminescence setup in the Quantum Engineered Nano Devices Laboratory (QENDL) at the Naval Information Warfare Center Pacific (NIWC Pacific) towards their future implementation in quantum applications. Specifically, we investigate the temperature dependence of photoluminescence (PL) for Chemical Vapor Deposition (CVD) and Molecular Beam Epitaxy (MBE) grown monolayer TMDs on sapphire (0001) substrates; CVD monolayer WS2-MoS2 heterostructure on sapphire (0001) substrate; CVD monolayer WSe2-MoSe2 heterostructure on sapphire (0001) substrate; CVD monolayer MoS2 on CVD monolayer hexagonal boron nitride (hBN) on SiO2-silicon substrate; and CVD monolayer WS2 on CVD monolayer hBN on sapphire (0001) substrate. We observed a significant temperature dependent direct bandgap red shift in CVD and MBE monolayer MoSe2 on sapphire (0001), MBE monolayer WS2 on sapphire (0001), and MBE monolayer WSe2 on sapphire (0001) substrate. We estimated the exciton binding energy in MBE monolayer WSe2 on sapphire (0001) by fitting the peak PL intensity values to the Arrhenius equation. Furthermore, we observed quite different temperature dependence of PL spectra from the monolayer CVD WS2-MoS2 heterostructure on sapphire (0001) substrate, which suggests the existence of spatial inhomogeneity across the sample. We also observed a temperature dependent PL peak red shift in both monolayer CVD WS2-MoS2 heterostructure on sapphire (0001) and monolayer CVD WSe2-MoSe2 heterostructure on sapphire (0001) substrate. Finally, we observed significant variability in the PL peak wavelength dependence on temperature for the transferred monolayer CVD MoS2 on transferred monolayer CVD hBN on SiO2-silicon substrate as well as for the transferred monolayer CVD WS2 on transferred monolayer CVD hBN on sapphire (0001) substrate.
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