Resonance Energy Transfer from ML-WS₂ to a Near Infrared Organic Dye

Van der Waals hybrid structures consisting of 2D transition metal dichalcogenides (TMDC) and organic molecules hold the promise to benefit from synergy effects that can improve optoelectronic properties and provide new functions. In this contribution we discuss incoherent resonance energy transfer (RET) from monolayer (ML) WS₂ to a highly photostabile organic dye molecule that converts the TMDC luminescence to the near infrared spectral range and enhances the light output. The dye features a narrow-band absorption spectrum with negligible overlap with the ML-WS₂ absorption so that dye and TMDC can be individually optically addressed. Time correlated single photon counting reveals that about 70% of the excitons created initially in ML-WS₂ are transferred to the dye molecules. The transferred excitons produce photoluminescence (PL) of the dye molecule which is proven by PL excitation spectroscopy. Photoelectron spectroscopy shows furthermore that the hybrid interface is of type I so that quenching of the luminescence due to interfacial exciton dissociation is avoided. As a result, the total PL yield in hybrid sample is about ten times larger than that of pristine ML-WS₂ since the RET process competes favorable with non-radiative recombination in the TMDC. As a consequence, a large fraction of the excitons, which would undergo non-radiative recombination in ML-WS₂ are converted into radiative excitons of the dye molecule. Key hereby is the high PL quantum yield of the molecules. Due to the close proximity of donor and acceptor both Förster and Dexter transfer can contribute to RET so that not only bright but also dark states of the TMDC can be involved in the RET process. Due to the short interaction length (< 5 nm) and the negligible spectral overlap of the TMDC and dye absorption, the RET process can be used to read out locally the optical properties of TMDCs with an unrivaled spatial resolution. In respect to practical applications, our findings pave the way for electrically driven nanoscale light source based for example on a TMDC split gate field-effect transistor geometry with tunable light output.