Filtering the photoluminescence spectra of atomically thin semiconductors with graphene

Abstract

Atomically thin semiconductors made from transition metal dichalcogenides (TMDs) are model systems for investigations of strong light-matter interactions and applications in nanophotonics, opto-electronics and valley-tronics. However, the typical photoluminescence spectra of TMD monolayers display a large number of intrinsic and extrinsic features that are particularly challenging to decipher. On a practical level, monochromatic TMD-based emitters would be beneficial for low-dimensional devices but no solution has yet been found to meet this challenge. Here, using a counter-intuitive strategy that consists in interfacing TMD monolayers with graphene, a system known as an efficient luminescence quencher, we demonstrate bright, single and narrow-line photoluminescence arising solely from TMD neutral excitons. This observation stems from two effects: (i) complete neutralization of the TMD by the adjacent graphene leading to the absence of optical features from charged excitons (ii) selective non-radiative transfer of TMD excitons to graphene, that is sufficiently rapid to quench radiative recombination of long-lived excitonic species without significantly affecting bright excitons, which display much shorter, picosecond radiative lifetimes at low temperatures. Our approach is systematically applied to four tungsten and molybdenumbased TMDs and establishes TMD/graphene heterostructures as a unique set of opto-electronic building blocks. Graphene not only endows TMDs monolayers with superior optical performance and enhanced photostability but also provides an excellent electrical contact, suitable for TMDbased electroluminescent systems emitting visible and near-infrared photons at near THz rate with linewidths approaching the lifetime limit.TMD monolayers (thereafter simply denoted TMD), such as MoS 2 , MoSe 2 , WS 2 , WSe 2 are direct-bandgap semiconductors [1,2], featuring short Bohr radii, large exciton binding energy (near 500 meV [3]) and picosecond excitonic radiative lifetimes at low temperature [4][5][6], all arising from their strong 2D Coulomb interactions, reduced dielectric screening and large effective masses [3,7]. Since the first investigations of light emission from TMDs, it has been clear that their lowtemperature spectra was composed of at least two prominent features, stemming from bright neutral excitons (X 0 ) and charged excitons (trions, X ) [8-10] endowed with a binding energy of typically 20 to 40 meV relative to X 0 . Among the vast family of TMDs, one may distinguish between so-called dark and bright materials [11]. 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 (...

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