How defects influence the photoluminescence of TMDCs
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TRACT Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers, a class of ultrathin materials with a direct bandgap and high exciton binding energies, provide an ideal platform to study the photoluminescence (PL) of light-emitting devices. Atomically thin TMDCs usually contain various defects, which enrich the lattice structure and give rise to many intriguing properties. As the influences of defects can be either detrimental or beneficial, a comprehensive understanding of the internal mechanisms underlying defect behaviour is required for PL tailoring. Herein, recent advances in the defect influences on PL emission are summarized and discussed. Fundamental mechanisms are the focus of this review, such as radiative/nonradiative recombination kinetics and band structure modification. Both challenges and opportunities are present in the field of defect manipulation, and the exploration of mechanisms is expected to facilitate the applications of 2D TMDCs in the future.
KEYWORDS two-dimensional material, transition metal dichalcogenides (TMDCs), photoluminescence (PL), defect, defect engineering, quantum yield (QY)
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Introduction
Transition metal dichalcogenides (TMDCs) are a class of layered materials with the structural formula of MX2, where M represents the transition metal (Mo, W, etc.) and X is the chalcogen (S, Se, etc.). In recent years, these materials have attracted significant interest because of their emergent properties when scaled down to a monolayer. Specifically, monolayer TMDCs exhibit a direct bandgap ranging from visible to infrared wavelengths [1, 2], which affords great potential for applications in light-emitting and photonic devices [3–5]. Due to the reduced screening effect, electrons and holes are tightly bound in monolayer TMDCs, making the excitonic effect especially stronger [6, 7]. Excitons and exciton complexes (e.g., trions and biexcitons) dominate the optical properties, such as photoluminescence (PL). Moreover, various intrinsic lattice defects, including vacancies, substitutional impurities, adatoms, antisite defects, and grain boundaries, widely exist in TMDCs developed using a variety of fabrication methods. Previous works have revealed that defects have a great impact on the PL emission of twodimensional (2D) TMDCs from many viewpoints, such as the carrier concentration [8, 9], exciton dynamics [10–12] and the bandgap structure [13, 14]. This impact usually facilitates the dominance of nonradiative rather than radiative recombination pathways, resulting in a much lower quantum yield (QY) than expected [10, 12]. Thus far, numerous efforts have been made to manipulate the impacts of defects, and remarkable achievements have been rendered. Many gas molecules [9, 15–17] and chemicals [18–20] can effectively functionalize the surface of 2D TMDCs, modulate defect-induced doping, or heal the lattice structure, resulting in a large PL enhancement of up to a thousand times. While defects are not always detrimental,
they also provide an effective way to create functional prop
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