Optical signatures of suppressed carrier localization in encapsulated WSe2 monolayer

Abstract

Low carrier mobility, closely associated with the formation of localized states, is the major bottleneck of utilizing the unique quantum transport properties in transition metal dichalcogenides (TMDCs). Here, we demonstrate an effective method to quantify the localization energy based on the temperature-dependent spectral variation of photoluminescence (PL) in pristine and hexagonal boron nitride (h-BN) encapsulated monolayer (ML) WSe2. Considering the protecting capability of h-BN against contamination and degradation, while not affecting the electronic structure as an insulating dielectric, the localization energy was comparatively extracted out of PL spectra in pristine and encapsulated ML WSe2. In pristine ML WSe2, two distinctive energy traces were resolved with an energy difference of about 17 meV, which was associated with the localized state revealed below 200 K. Clear evidence for the carrier localization was also evident in the integrated PL intensity trace with temperature as the trace from pristine ML clearly deviates from the dark-exciton-like behavior of ML WSe2, violating the spin selection rule of the lowest exciton state. In clear contrast, the temperature dependency of the h-BN encapsulated ML WSe2 in PL spectra matches well with the typical Varshni formula of free excitonic peaks and the integrated intensity trace of thermally populated spin subbands. Our study suggests that the h-BN encapsulation could suppress the carrier localization channels by avoiding surface oxidation due to air exposure and could provide insights into how one could preserve the excitonic features in TMDC materials and devices.