Excitonic Effects in Tungsten Disulfide Monolayers on Two-Layer Graphene

Abstract

Light emission in atomically thin heterostructures is known to depend on the type of materials and the number and stacking sequence of the constituent layers. Here we show that the thickness of a two-dimensional substrate can be crucial in modulating the light emission. We study the layer-dependent charge transfer in vertical heterostructures built from monolayer tungsten disulfide (WS2) on one- and two-layer epitaxial graphene, unravelling the effect that the interlayer electronic coupling has on the excitonic properties of such heterostructures. We bring evidence that the excitonic properties of WS2 can be effectively tuned by the number of supporting graphene layers. Integrating WS2 monolayers with two-layer graphene leads to a significant enhancement of the photoluminescence response, up to 1 order of magnitude higher compared to WS2 supported on one-layer graphene. Our findings highlight the importance of substrate engineering when constructing atomically thin-layered heterostructures.

Probing exciton species in atomically thin WS2–graphene heterostructures

“…The samples were characterised by SKPM and reveal various morphologies (star-, triangular-and trapezoidal-shaped) for WS 2 , with the surface potential increasing for both WS 2 and graphene with the number of layers, as illustrated in figure 1(a). In accordance with DFT calculations [17], the SKPM shows that the WS 2 layer is largely transparent to the underlying graphene in the surface potential measurements ( figure 1(a)). The optical properties of monolayer WS 2 deposited on 1LG (WS 2 /1LG) change when an additional graphene layer is introduced, going from a quenched PL signal for WS 2 /1LG to a less quenched one for WS 2 on 2LG (WS 2 /2LG) (figure 1(b)), as evidenced by a previous study [17].…”

“…In accordance with DFT calculations [17], the SKPM shows that the WS 2 layer is largely transparent to the underlying graphene in the surface potential measurements ( figure 1(a)). The optical properties of monolayer WS 2 deposited on 1LG (WS 2 /1LG) change when an additional graphene layer is introduced, going from a quenched PL signal for WS 2 /1LG to a less quenched one for WS 2 on 2LG (WS 2 /2LG) (figure 1(b)), as evidenced by a previous study [17]. This is due to the difference in electronic structure between 1LG and 2LG.…”

“…The two components are associated with the emission from neutral exciton states at 1.93 eV, with the peak FWHM of 30 meV, and from charged exciton states (trions) at 1.9 eV and peak FWHM of 50 meV. The formation of trions requires an electron-or hole-rich environment, which is provided for WS 2 /2LG by the interaction with graphene, and owing to the higher carrier density in 2LG compared to 1LG, as confirmed by previous transport experiments and calculations [17,23].…”

“…P-n junctions with high quantum efficiency (>60%) were demonstrated in GaTe/MoS 2 heterostructures [14] and interfacing TMDC with graphene achieved very efficient photocarrier extraction [15,16]. A recent study evidenced that the excitonic properties of monolayer WS 2 can be effectively tuned with the number of supporting graphene layers [17], leading to enhancement of the photoluminescence (PL) response for WS 2 supported on two-layer graphene (2LG) compared to WS 2 on one-layer graphene (1LG). The excitonic effects in TMDC heterostructures exhibit rich features, such as neutral and charged excitons (trions), biexcitons, localised excitons [18], which become more evident at low temperature, and with properties and behaviour that are far from understood.…”

Graphene‐TMDC‐Graphene Hybrid Plasmonic Metasurface for Enhanced Biosensing: A Theoretical Analysis

“…The interactions between metal, graphene and TMDC materials could be assumed as the physical mechanism for the enhanced sensing performance: (i) the metal layer provides abundant free electrons for the excitation of surface plasmon polaritons; (ii) the efficiency of exciton relaxation is enhanced attributed to the strong spin‐orbit interfacial and localized Coulomb interactions between TMDC materials and graphene layer, which leads to a higher light absorption rate; (iii) more free electrons are transferred to the sensing surface to enhance the resonant electric field due to interlayer coupling, leading to the perfect absorption of the incident light and related phase jump; (iv) more energy is transferred to afford the resonant oscillations of the free electrons on the sensing surface due to the efficient light absorption of graphene and TMDC materials; (v) more electrons can be transferred to the sensing surface to participate in the resonant oscillations. This is attributed to the work function difference between metal, graphene and TMDC (i.e., Au‐5.54 eV, MoS 2 ‐5.1 eV, graphene‐4.5 eV), since the continuity of the Fermi levels of the whole system needs to be maintained.…”