Intrinsic and Extrinsic Defect-Related Excitons in TMDCs

Abstract

We investigate an excitonic peak appearing in low-temperature photoluminescence of monolayer transition metal dichalcogenides (TMDCs), which is commonly associated with defects and disorder. First, to uncover the intrinsic origin of defect-related excitons, we study their dependence on gate voltage, excitation power, and temperature in a prototypical TMDC monolayer, MoS2. We show that the entire range of behaviors of defect-related excitons can be understood in terms of a simple model, where neutral excitons are bound to ionized donor levels, likely related to sulphur vacancies, with a density of 7×10 11 cm -2 . Second, to study the extrinsic origin of defect-related excitons, we controllably deposit oxygen molecules in-situ onto the surface of MoS2 kept at cryogenic temperature. We find that in addition to trivial pdoping of 3×10 12 cm -2 , oxygen affects the formation of defect-related excitons by functionalizing the vacancy. Combined, our results uncover the origin of defect-related excitons, suggest a simple and conclusive approach to track the functionalization of TMDCs, benchmark device quality, and pave the way towards exciton engineering in hybrid organicinorganic TMDC devices.

The role of chalcogen vacancies for atomic defect emission in MoS2

“…Vacancies introduce a deep center with sharp optical emission, markedly different from previously observed broad luminescence bands. [4][5][6][7][8] Comparing annealed vs. He-ion treated MoS 2 , we establish that the recently discovered single-photon emitters in He-ion irradiated MoS 2 originate from chalcogen vacancies 3 . The latter can be deterministically created with a precision below 10 nm 9 , underscoring the potential of defect engineering for two-dimensional (quantum-) optoelectronics.…”

“…Nevertheless, there is a surprising lack of consensus about the origin for such broad defect emission. Some studies emphasized radiative recombination at intrinsic point defects as underlying mechanism 4,7 . Other studies highlighted the relevance of molecular adsorbates.…”

How defects influence the photoluminescence of TMDCs

“…Recently, it was reported that the portion of X D gradually increased during the O2 deposition process until oxygen adsorption was saturated ( Fig. 3(b)) [48]. This gas-ambient influence can be elucidated from two aspects.…”

“…Many defective TMDC samples have been reported to present X D peaks. Upon argon/oxygen plasma [46,47], thermal annealing [17,48], α-particle irradiation [16] or electron beam lithography treatment [49], defects can be introduced intentionally. Additionally, low power treatments typically induce chalcogen vacancies because of their lowest formation energy, whereas more complex defects could be introduced via higher energy or longer treatment times.…”

Advances in Liquid‐Phase and Intercalation Exfoliations of Transition Metal Dichalcogenides to Produce 2D Framework

“…[17,98,99] Among all the matchless properties, their tunable bandgap is really complimentary that conveyed by strong PL and high excitonic binding energy, which makes them an exciting choice for a variety of optoelectronics including solar cells, LEDs, photodetectors, phototransistors, and photoelectric modulators. [19,38,43,100] Exemplify as, distinctive properties of MoS 2 comprising direct bandgap (1.8 eV, see Figure 3a), high charge mobility (≈700 cm 2 V −1 s −1 ), high current on/off ratio (≈10 7 -10 8 ), high optical absorption in visible range (≈10 7 m −1 ), and a giant rising of PL phenomenon from direct bandgap in case of a monolayer; consequently it is thoroughly researched for electronics and optoelectronics. [17][18][19]106] Electrons in crystals show honeycomb configuration structure own a couple of in-equivalent valleys in k-space electronic structure by an extra degree of freedom.…”