Direct determination of monolayer MoS2 and WSe2 exciton binding energies on insulating and metallic substrates

Understanding the excitonic nature of exited states in two dimensional 2D transition metal dichalcogenides TMDCs is of key importance to make use of their optical and charge transport properties in optoelectronic applications. We contribute to this by the direct experimental determination of the exciton binding energy Eb,exc of monolayer MoS2 and WSe2 on two fundamentally different substrates, i.e., the insulator sapphire and the metal gold. By combining angle resolved direct and inverse photoelectron spectroscopy we measure the transport single particle band gap Et , and by reflectance measurements the optical excitonic band gap Eexc . The difference of these two energies is Eb,exc. The values of Et and Eb,exc are 2.11 eV and 240 meV for MoS2 on sapphire, and 1.89 eV and 240 meV for WSe2 on sapphire. On Au Eb,exc is decreased to 90 meV and 140 meV for MoS2 and WSe2, respectively. The significant Eb,exc reduction is primarily due to a reduction of Et resulting from enhanced screening by the metal, while Eexc is barely decreased for the metal support. Energy level diagrams determined at the K point of the 2D TMDCs Brillouin zone show that MoS2 has more p type character on Au as compared to sapphire, while WSe2 appears close to intrinsic on both. These results demonstrate that the impact of the dielectric environment of 2D TMDCs is more pronounced for individual charge carriers than for a correlated electron hole pair, i.e., the exciton. A proper dielectric surrounding design for such 2D semiconductors can therefore be used to facilitate superior optoelectronic device function