Singlet and triplet trions in WS₂ monolayer encapsulated in hexagonal boron nitride

Abstract

Embedding a WS monolayer in flakes of hexagonal boron nitride allowed us to resolve and study the photoluminescence response due to both singlet and triplet states of negatively charged excitons (trions) in this atomically thin semiconductor. The energy separation between the singlet and triplet states has been found to be relatively small reflecting rather weak effects of the electron-electron exchange interaction for the trion triplet in a WS monolayer, which involves two electrons with the same spin but from different valleys. Polarization-resolved experiments demonstrate that the helicity of the excitation light is better preserved in the emission spectrum of the triplet trion than in that of the singlet trion. Finally, the singlet (intravalley) trions are found to be observable even at ambient conditions whereas the emission due to the triplet (intervalley) trions is only efficient at low temperatures.

Emission Excitation Spectroscopy in WS2 Monolayer Encapsulated in Hexagonal BN

“…The collection time of each spectrum was only 1 s. Figure 1 shows the intensity of the optical response as a function of excitation energy measured on monolayer WS 2 encapsulated in h-BN in the form of the color-coded map. The presented spectral window covers the energy range that corresponds to the emission of light due to recombination of the neutral exciton (X 0 ) in the vicinity of so-called A exciton [14]. It can be seen that the line shape of the detected signal significantly depends on the excitation energy.…”

Dynamics of Free and Localized Excitons in Two‐Dimensional Transition Metal Dichalcogenides

“…Since the discovery of semiconducting monolayer TMDCs, a variety of experimental and theoretical studies have been carried out seeking to understand the exciton populations decay . Actually, the exciton kinetics is strongly sensitive to the experimental conditions such as temperature, excitation power, doping density, the dielectric environment (substrate effect: gold, SiO 2 , or h‐BN), quality (chemically treated, or encapsulated with h‐BN), and sample treatment method . Moreover, it is depending on the sample type (Mo/W) .…”

Filtering the photoluminescence spectra of atomically thin semiconductors with graphene

“…In the case of Molybdenum based-TMDs, X 0 is the lowest lying excitonic state, resulting in rather intense emission at low temperature, whereas, a spin-dark state lies lower than X 0 in Tungsten-based TMDs. As a result, X 0 and X emission dominate the PL spectrum of Mo-based TMDs [9], whereas the emission spectra of W-based TMDs display a complex series of lines stemming from X 0 , bi-excitons (XX 0 )[12-15], charged excitonic states (including X [10,16] and charged biexcitons (XX ) [12][13][14][15]), spin-dark excitons [17][18][19], defectinduced emission and exciton-phonon sidebands [20].Considerable progress has been made to determin-istically observe intrinsic TMD emission features. In particular, encapsulation of TMDs in hexagonal boron nitride (BN) films results in narrower neutral exciton linewidth [21,22], approaching the radiative limit [6,23,24], without however, getting rid of the other emission features mentioned above.…”