Strong Single- and Two-Photon Luminescence Enhancement by Nonradiative Energy Transfer across Layered Heterostructure

Abstract

The strong light-matter interaction in monolayer transition metal dichalcogenides (TMDs) is promising for nanoscale optoelectronics with their direct band gap nature and the ultrafast radiative decay of the strongly bound excitons these materials host. However, the impeded amount of light absorption imposed by the ultra-thin nature of the monolayers impairs their viability in photonic applications. Using a layered heterostructure of a monolayer TMD stacked on top of strongly absorbing, non-luminescent, multi-layer SnSe2, we show that both single-photon and two-photon luminescence from the TMD monolayer can be enhanced by a factor of 14 and 7.5, respectively. This is enabled through inter-layer dipole-dipole coupling induced non-radiative Förster resonance energy transfer (FRET) from SnSe2 underneath which acts as a scavenger of the light unabsorbed by the monolayer TMD. The design strategy exploits the near-resonance between the direct energy gap of SnSe2 and the excitonic gap of monolayer TMD, the smallest possible separation between donor and acceptor facilitated by van der Waals heterojunction, and the inplane orientation of dipoles in these layered materials. The FRET driven uniform single-and twophoton luminescence enhancement over the entire junction area is advantageous over the local enhancement in quantum dot or plasmonic structure integrated 2D layers, and is promising for improving quantum efficiency in imaging, optoelectronic, and photonic applications. KEYWORDS: MoS2, WS2, SnSe2, van der Waals heterostructure, photoluminescence enhancement, two-photon luminescence, Förster Resonance Energy Transfer (FRET), charge transfer.transfer across WS2/MoSe2 hetero-bilayer stack through FRET across higher order exciton transitions. Nonetheless, the donor's absorption is still constrained by its physical thickness at the monolayer limit despite the high efficiency of FRET at the closest possible physical separation.Here, we demonstrate enhanced PL of monolayer MoS2 (and WS2) across its vdW heterojunction with multi-layer SnSe2 via FRET with single and two-photon excitation. Counteracting the charge transfer across highly staggered conduction bands of MoS2 and SnSe2, MoS2 single-photon luminescence (1P-PL) shows ~14-fold enhancement at room temperature with resonant excitation and ~5-fold enhancement with non-resonant excitation, while two-photon luminescence (2P-PL) of MoS2 shows up to ~7.5-fold enhancement with non-resonant excitation. Even with the insertion of few-layer hBN between MoS2 and SnSe2, the 1P-PL enhancement persists up to 5 times with resonant excitation. We demonstrate modulation of the degree of the PL enhancement by systematic parameter variation, including donor material, acceptor material, their thickness, physical separation between donor and acceptor, sample temperature, and excitation wavelength which corroborate FRET aided PL enhancement. We emphasize the intrinsic advantage of realizing FRET with SnSe2 as a donor and elucidate the impact of multiple parameters on the luminescence enhance...

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