Abstract
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 In0.15Al0.85N lattice matched templates. Later, to accomplish air-stable sandwiched SL-MoS2, a thin In0.15Al0.85N cap layer was deposited on the MoS2/In0.15Al0.85N 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.
Highlights
Defects in semiconductors are either fatal or vital on the transport and optical emission properties [1,2]
Besides the defect-induced enhancement of the PL intensity, the direct optical transitions associated with the excitons and bound excitons can contribute to an anomalous negative thermal quenching (NTQ) of the PL intensity in semiconductors
The scanning transmission electron microscopy – electron energy loss spectroscopy (STEM-EELS) imaging data sets were acquired to generate the elemental maps of Mo, S, Al and N elements by employing Mo-M (227 eV), S-L (2472 eV), Al-K (1560 eV), and N-K (401 eV), EELS edges, respectively
Summary
Defects in semiconductors are either fatal or vital on the transport and optical emission properties [1,2]. It has been widely reported that the unintentionally formed lattice point defects such as sulfur vacancies and foreign impurities occupied on these vacancy sites can have a remarkable effect on the structural, electrical transport and optical properties of a single layer MoS2 [7,8,9,10,11]. These defects can be formed in the 2D crystal lattice by several ways. The power-dependent and the temperature-dependent μPL studies were performed to understand the recombination processes and the role of defects on the emission properties of the sandwiched SL-MoS2
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