Exciton-dominant photoluminescence of MoS₂ by a functionalized substrate

Transition metal dichalcogenides (TMDs) have been considered as promising candidates for transparent and flexible optoelectronic devices owing to their large exciton binding energy and strong light-matter interaction. However, monolayer (1L) TMDs exhibited different intensities and spectra of photoluminescence (PL), and the characteristics of their electronic devices also differed in each study. This has been explained in terms of various defects in TMDs, such as vacancies and grain boundaries, and their surroundings, such as dielectric screening and charged impurities, which lead to non-radiative recombination of trions, low quantum yield (QY), and unexpected doping. However, it should be noted that the surface conditions of the substrate are also a critical factor in determining the properties of TMDs located on the substrate. Here, we demonstrate that the optical and electrical properties of 1L MoS₂ are strongly influenced by the functionalized substrate. The PL of 1L MoS₂ placed on the oxygen plasma-treated SiO₂ substrate was highly p-doped owing to the functional groups of -OH on SiO₂, resulting in a strong enhancement of PL by approximately 20 times. The PL QY of 1L MoS₂ on plasma-treated SiO₂ substrate increased by one order of magnitude. Surprisingly, the observed PL spectra show the suppression of non-radiative recombination by trions, thus the exciton-dominant PL led to a prolonged lifetime of MoS₂ on the plasma-treated substrate. The MoS₂ field-effect transistors fabricated on plasma-treated SiO₂ also exhibited a large hysteresis in the transfer curve owing to charge trapping of the functional groups. Our study demonstrates that the functional groups on the substrate strongly affect the characteristics of 1L MoS₂, which provides clues as to why MoS₂ exfoliated on various substrates always exhibited different properties in previous studies.