Anomalous photoluminescence thermal quenching of sandwiched single layer MoS₂

We report an unusual thermal quenching of the micro-photoluminescence (μ-PL) intensity for a sandwiched single-layer (SL) MoS₂. For this study, MoS₂ layers were chemical vapor deposited on molecular beam epitaxial grown In_0.15Al_0.85N lattice matched templates. Later, to accomplish air-stable sandwiched SL-MoS₂, a thin In_0.15Al_0.85N cap layer was deposited on the MoS₂/In_0.15Al_0.85N heterostructure. We confirm that the sandwiched MoS₂ is a single layer from optical and structural analyses using μ-Raman spectroscopy and scanning transmission electron microscopy, respectively. By using high-resolution X-ray photoelectron spectroscopy, no structural phase transition of MoS₂ is noticed. The recombination processes of bound and free excitons were analyzed by the power-dependent μ-PL studies at 77 K and room temperature (RT). The temperature-dependent micro photoluminescence (TDPL) measurements were carried out in the temperature range of 77 - 400 K. As temperature increases, a significant red-shift is observed for the free-exciton PL peak, revealing the delocalization of carriers. Further, we observe unconventional negative thermal quenching behavior, the enhancement of the μ-PL intensity with increasing temperatures up to 300K, which is explained by carrier hopping transitions that take place between shallow localized states to the band-edges. Thus, this study renders a fundamental insight into understanding the anomalous thermal quenching of μ-PL intensity of sandwiched SL-MoS₂.