Abstract
In general, the quantum yields (QYs) of monolayer transition metal dichalcogenides (1L-TMDs) are low, typically less than 1% in their pristine state, significantly limiting their photonic applications. Many methods have been reported to increase the QYs of 1L-TMDs; however, the technical difficulties involved in the reliable estimation of these QYs have prevented the general assessment of these methods. Herein, we demonstrate the estimation of the QYs of 1L-TMDs using a poly methyl methacrylate (PMMA) thin film embedded with rhodamine 6G (R6G) as a reference specimen for measuring the QYs of 1L-TMDs. The PMMA/R6G composite films with thicknesses of 80 and 300 nm demonstrated spatially homogeneous emissions with the incorporation of well-dispersed R6G molecules, and may, therefore, be used as ideal reference specimens for the QY measurement of 1L-TMDs. Using our reference specimens, for which the QY ranged from 5.4% to 22.2% depending on the film thickness and R6G concentrations, we measured the QYs of the exfoliated or chemical vapor deposition (CVD)-grown 1L-WS2, -MoSe2, -MoS2, and -WSe2 TMDs. The convenient procedure proposed in this study for preparing the thin reference films and the simple protocol for the QY estimation of 1L-TMDs may enable accurate comparisons of the absolute QYs between the 1L-TMD samples, thereby enabling the development of a method to improve the QY of 1L-TMDs.
Highlights
Layered transition metal dichalcogenides (TMDs) with chemical formulas of MX2 have been extensively investigated for fundamental science and device applications [1,2]
A high density of structural defects in these systems has been proven to be the main cause of the low quantum yields (QYs) of 1L-TMDs [9,10,11,12,13]
The thicknesses and emission profiles of the reference films must be spatially uniform at micro-scale dimensions to permit comparison with the local PL of micro-sized 1L-TMDs
Summary
Layered transition metal dichalcogenides (TMDs) with chemical formulas of MX2 (wherein M represents a transition metal atom, such as Mo or W, and X represents a chalcogen atom, such as S, Se, or Te) have been extensively investigated for fundamental science and device applications [1,2]. The QY represents the efficiency of the conversion of absorbed photons into emitted photons [22,23], and it has been estimated by directly measuring the absorptions and emissions of the photons using an integrating sphere [24,25,26] Such a direct measurement of the absolute QY is not applicable for 1L-TMDs because of their small sizes and significantly low QYs. Alternatively, the QY of the reference sample is first measured using the integrating sphere. The thicknesses and emission profiles of the reference films must be spatially uniform at micro-scale dimensions to permit comparison with the local PL of micro-sized 1L-TMDs. R6G in low-concentration solutions has a QY close to 100% [1,31,32]; this dye tends to exhibit an extremely low QY in its solid form due to fluorescence quenching that originates from the aggregation of the solid particles [33]. The results were consistent between measurements using two different thicknesses of reference specimens
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