Electroluminescence from multi-particle exciton complexes in transition metal dichalcogenide semiconductors

Abstract

Light emission from higher-order correlated excitonic states has been recently reported in hBN-encapsulated monolayer WSe 2 and WS 2 upon optical excitation. These exciton complexes are found to be bound states of excitons residing in opposite valleys in momentum space, a promising feature that could be employed in valleytronics or other novel optoelectronic devices. However, electrically-driven light emission from such exciton species is still lacking. Here we report electroluminescence from bright and dark excitons, negatively charged trions and neutral and negatively charged biexcitons, generated by a pulsed gate voltage, in hexagonal boron nitride encapsulated monolayer WSe 2 and WS 2 with graphene as electrode. By tailoring the pulse parameters we are able to tune the emission intensity of the different exciton species in both materials. We find the electroluminescence from charged biexcitons and dark excitons to be as narrow as 2.8 meV.

Resonant optical Stark effect in monolayer WS2

“…The photo-induced absorption feature is located approximately 48 ± 7 meV below the A-exciton, consistent with recent measurements of biexciton emission in monolayer WS 2 39 . Because this absorption band is also near the emission band reported for charged biexcitons in the tungsten-based TMDs 23–25 , we cannot exclude their participation in this photo-induced absorption. It should be noted that in those measurements the TMDs were encapsulated in hBN and graphene, both of which have been shown to reduce exciton binding energies through screening of Coulomb interactions 40 .…”

“…Though this picture treats the excited states as noninteracting particles, it has been applied to low-dimensional semiconductors where reduced electrostatic screening produces strong many-body interactions. For example, many-body Coulomb interactions give rise to observations of biexciton formation 22 , many-particle complexes 23–25 , efficient Auger recombination 26–28 , enhanced bandgap renormalization 29–31 , and the formation of electron-hole liquids 32 in monolayer TMDs. Such many-body effects require a theoretical description that goes beyond the dressed-atom picture in order to understand the OSE in the strongly interacting limit.…”

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.…”

Valley phenomena in the candidate phase change material WSe2(1-x)Te2x

“…52 The sign of the spin-orbit coupling is the same for all W-based TMDs 52 and so we expect no change in the optical activity of the lowest exciton state with Te substitution. The presence of broad features at lower energies compared to X 0 and X T originate from a combination of higher-order excitonic complexes 53 and localized exciton states from lattice defects, strain, and residual impurities introduced during fabrication. 10,13,54 We explore band gap tunability of 1H-WSe2(1-x)Te2x by extracting X 0 energy as a function of temperature, which is plotted in Fig.…”