Abstract
Semiconducting monolayers of transition-metal dichalcogenides are outstanding platforms to study both electronic and phononic interactions as well as intra- and intervalley excitons and trions. These excitonic complexes are optically either active (bright) or inactive (dark) due to selection rules from spin or momentum conservation. Exploring ways of brightening dark excitons and trions has strongly been pursued in semiconductor physics. Here, we report on a mechanism in which a dark intervalley exciton upconverts light into a bright intravalley exciton in hBN-encapsulated WSe2 monolayers. Excitation spectra of upconverted photoluminescence reveals resonances at energies 34.5 and 46.0 meV below the neutral exciton in the nominal WSe2 transparency range. The required energy gains are theoretically explained by cooling of resident electrons or by exciton scattering with Λ- or K-valley phonons. Accordingly, an elevated temperature and a moderate concentration of resident electrons are necessary for observing the upconversion resonances. The interaction process observed between the inter- and intravalley excitons elucidates the importance of dark excitons for the optics of two-dimensional materials.
Highlights
Layered van-der-Waals heterostructures based on transitionmetal dichalcogenide (TMDC) monolayers attract attention due to prominent interaction effects which are determined by strong exciton−phonon and exciton−electron coupling
The two-dimensional confinement of charge carriers in TMDC monolayers leads to a reduced dielectric screening and to pronounced many-body effects mediated by the Coulomb interaction.[1−6] The optical properties of TMDC monolayers are governed by strongly bound excitons[7,8] and by various higher-order excitonic complexes whose binding energies considerably exceed those observed in conventional lowdimensional semiconductor structures.[9,10]
An additional hBN layer placed between the TMDC monolayer and SiO2/Si substrate acts as a buffer layer and affects the doping level in the monolayer.[5,27,28]
Summary
Layered van-der-Waals heterostructures based on transitionmetal dichalcogenide (TMDC) monolayers attract attention due to prominent interaction effects which are determined by strong exciton−phonon and exciton−electron coupling. The two-dimensional confinement of charge carriers in TMDC monolayers leads to a reduced dielectric screening and to pronounced many-body effects mediated by the Coulomb interaction.[1−6] The optical properties of TMDC monolayers are governed by strongly bound excitons[7,8] and by various higher-order excitonic complexes whose binding energies considerably exceed those observed in conventional lowdimensional semiconductor structures.[9,10] owing to the strong spin−orbit coupling in TMDC monolayers and the resulting valley contrasting spin-splitting of the energy gap at the K−/K+-points, the excitonic complexes possess both spin and valley degrees of freedom.[11] a large number of excitonic features positioned energetically below the bright intravalley exciton has been observed in the low-temperature emission spectra of tungsten-based materials. The involved transitions have been identified as bright “singlet” and “triplet” trions,[13,14] neutral and charged bright biexcitons,[15,16] spin-forbidden dark excitons[17,18] (denoted by D in Figure 1(a)), dark (gray) trions[19,20] and momentum-indirect dark excitons activated by scattering with defects or phonons,[1,21] denoted by I
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