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A novel mechanism for understanding the strong enhancement of photoluminescence quantum yield in large-area monolayer MoS2 grown by CVD

Understanding the effect of the intrinsic properties of monolayer MoS2 (1L-MoS2) on the photoluminescence quantum yield (QY) is indispensable before seeking surface treatments for its improvement. Here, the effects of localized heat and heat-dissipation on the QY of 1L-MoS2 grown by chemical vapour deposition (CVD) are reported. The experimentally measured QY increases with an increase in the flake area of 1L-MoS2. The QY of the large-area flake is increased by more than one order as compared to the small-area flake, and this is attributed to a significant reduction in the local temperature rise with laser irradiation. The localized heating effects are substantially reduced in large-area flakes because of efficient heat dissipation. The reduced localized heating effects with flake area are established by power-dependent Raman and PL studies, which are further corroborated by Stokes and anti-Stokes Raman analysis. In the case of small-area (∼33 μm2) 1L-MoS2, there is an additional rise in the local temperature by ∼94 K as compared to the large-area case (∼5778 μm2) for the same laser power. A QY of ∼1.8% is achieved for the large-area 1L-MoS2, which is one order higher than that of the mechanically exfoliated 1L-MoS2 and two orders higher than that of CVD grown 1L-MoS2. The role of carrier density and defect passivation in the PL intensity enhancement was ruled out with the observation of the simultaneous enhancement of integrated PL intensities associated with the trions and excitons. In addition, the PL intensity of the large-area MoS2 monolayers was found to be influenced by the grain size. The role of grain size is understood by invoking the heat-dissipation mechanism at the grain boundaries. The reported results establish the importance of efficient heat-dissipation in realizing the potential of 1L-MoS2 and other 2D monolayer materials.