Abstract
The luminescence yield of transition metal dichalcogenide monolayers frequently suffers from the formation of long-lived dark states, which include excitons with intervalley charge carriers, spin-forbidden transitions, and a large center-of-mass momentum located outside the light cone of dispersion relations. Efficient relaxation from bright exciton states to dark states suppresses the quantum yield of photon emission. In addition, the radiative recombination of excitons is heavily influenced by Auger-type exciton-exciton scattering, which yields another nonradiative relaxation channel at room temperature. Here, we show that Auger-type scattering is promoted not only between (bright) excitons but also between excitons and long-lived dark states. We studied the luminescence dynamics of monolayer WS2 capped with hexagonal BN over broad time ranges of picoseconds to milliseconds using carefully designed pump-and-probe techniques. We observed that luminescence quenching associated with Auger-type scattering occurs on 1-100 microsecond time scales, which thus correspond to the lifetimes of the relevant dark states. The broad distribution of the measured lifetimes implies the impact of various types of long-lived states on the exciton annihilation process.
Highlights
The high-yield light emission associated with exciton recombination makes transition metal dichalcogenide (TMD) monolayers unique platforms on which to demonstrate various exciting physics phenomena [1,2,3,4,5,6,7,8,9]
The luminescence yield of transition metal dichalcogenide monolayers frequently suffers from the formation of long-lived dark states, which include excitons with intervalley charge carriers, spin-forbidden transitions, and a large center-of-mass momentum located outside the light cone of dispersion relations
Mechanisms behind the strong light emission include (1) strong interband transitions since both conduction and valence electron states consist of strongly localized metal d orbitals and the absorption coefficients of TMD commonly exceed 10% per monolayer depending on wavelength [10,11] and (2) large exciton binding energies of the order of 0.5 eV [12,13,14,15,16,17,18], which ensures the stable formation of excitons even at room temperature
Summary
The high-yield light emission associated with exciton recombination makes transition metal dichalcogenide (TMD) monolayers unique platforms on which to demonstrate various exciting physics phenomena [1,2,3,4,5,6,7,8,9]. States, which include intervalley excitons, spin-forbidden triplet excitons, and excitons with nonzero center-of-mass momentum; and Auger-type scattering that leads to nonradiative exciton annihilation [28,29,30,31]. Of these phenomena, Auger scattering, i.e., nonradiative energy transfer between excitons, is regarded as a dominant reason for the room-temperature quantum yield being limited, as the effect appears even for moderate densities lower than 1010 cm−2 [32,33]. V, we discuss the identification of the measured long-lived states
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