Upconversion photoluminescence excitation reveals exciton–trion and exciton–biexciton coupling in hBN/WS$$_{2}$$/hBN van der Waals heterostructures

Abstract

Monolayers of transition-metal dichalcogenides with direct band gap located at the binary $$K_{-}/K_{+}$$ K - / K + points of the Brillouin zone are promising materials for applications in opto- and spin-electronics due to strongly enhanced Coulomb interactions and specific spin-valley properties. They furthermore represent a unique platform to study electron–electron and electron–phonon interactions in diverse exciton complexes. Here, we demonstrate processes in which the neutral biexciton and two negative trions, namely the spin-triplet and spin-singlet trions, upconvert light into a bright intravalley exciton in an hBN-encapsulated WS$$_{2}$$ 2 monolayer. We propose that the energy gains required in the polarized upconversion photoluminescence originate from different interactions including resonant optical phonons, a cooling of resident electrons and a non-local and an anisotropic electron–hole exchange, respectively. The temperature dependence (7–120 K) of the excitonic upconversion intensity obtained at excitation energies corresponding to the biexciton and trions provides insight into an increasing phonon population as well as a thermally enhanced electron scattering. Our study sheds new light on the understanding of excitonic spin and valley properties of van der Waals heterostructures and improves the understanding of photonic upconversion mechanisms in two-dimensional quantum materials.

Biexciton and Singlet Trion Upconvert Exciton Photoluminescence in a MoSe2 Monolayer Supported by Acoustic and Optical K-Valley Phonons

“…By comparison, the upconverted X PL is enhanced with rising temperature: I X,UPC is maximum at about 50 K, for resonantly exciting T S , and is reduced by further increasing the temperature. For exciting XX 0 , I X,UPC exhibits a maximum at about 60 K. This thermal behavior differs from that observed in WS 2 monolayers, 16 , 18 where the upconverted PL of the exciton becomes significantly intensified with rising temperature and even exceeds the regular PL intensity.…”

“…In this context, upconversion photoluminescence (PL) spectroscopy recently provided thorough insights into exciton–exciton and exciton–phonon interactions and was successfully employed to reveal the coupling between various excitonic complexes in the optically darkish hBN-encapsulated WSe 2 and WS 2 monolayers. In particular, upconversion of light from dark excitons, trions, and biexcitons to bright excitons was observed. − …”

“…In particular, upconversion of light from dark excitons, trions, and biexcitons to bright excitons was observed. 14 − 16 …”