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

Understanding the excitonic nature of excited states in two-dimensional (2D) transition-metaldichalcogenides (TMDCs) is of key importance to make use of their optical and charge transportproperties in optoelectronic applications. We contribute to this by the direct experimentaldetermination of the exciton binding energy (Eb,exc) of monolayer MoS2 and WSe2 on twofundamentally different substrates, i.e. the insulator sapphire and the metal gold. By combiningangle-resolved direct and inverse photoelectron spectroscopy we measure the electronic band gap(Eg), and by reflectance measurements the optical excitonic band gap (Eexc). The difference of thesetwo energies is Eb,exc. The values of Eg and Eb,exc are 2.11 eV and 240 meV for MoS2 on sapphire, and1.89 eV and 240 meV for WSe2 on sapphire. On Au Eb,exc is decreased to 90 meV and 140 meV forMoS2 and WSe2, respectively. The significant Eb,exc reduction is primarily due to a reduction of Egresulting 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 MoS2has more p-type character on Au as compared to sapphire, while WSe2 appears close to intrinsic onboth. These results demonstrate that the impact of the dielectric environment of 2D TMDCs is morepronounced for individual charge carriers than for a correlated electron–hole pair, i.e. the exciton. Aproper dielectric surrounding design for such 2D semiconductors can therefore be used to facilitatesuperior optoelectronic device function.PAPER2018 Original content fromthis work may be usedunder the terms of theCreative CommonsAttribution 3.0 licence. Any further distributionof this work mustmaintain attributionto the author(s) and thetitle of the work, journalcitation and DOI.RECEIVED