Probing Interlayer Excitons in a Vertical van der Waals p-n Junction using Scanning Probe Microscopy Technique

Two dimensional (2D) semiconductors feature exceptional optoelectronic properties controlled by strong confinement in one dimension. In this contribution, we studied interlayer excitons in a vertical p-n junction made of bilayer n-type MoS2 and few layers p-type GaSe using current sensing atomic force microscopy (CSAFM). The p-n interface is prepared by mechanical exfoliation onto highly ordered pyrolytic graphite (HOPG). Thus the heterostructure creates an ideal layered system with HOPG serving as the bottom contact for the electrical characterization. Home-built Au tips are used as the top contact in CSAFM mode. During the basic diode characterization, the p-n interface shows strong rectification behavior with a rectification ratio of 104 at ±1 V. The I-V characteristics reveal pronounced photovoltaic effects with a fill factor of 0.55 by excitation below the band gap. This phenomenon can be explained by means of the dissociation of interlayer excitons at the interface. The possibility of the interlayer exciton formation is indicated by density functional theory (DFT) calculations on this heterostructure: the valence band of GaSe and the conduction band of MoS2 contribute to an excitonic state at an energy of about 1.5 eV. The proof of such excitonic transition is provided by photoluminescence measurement at the p-n interface. Finally, photocurrent mapping at the interface under 785 nm excitation provides evidence of efficient extraction of such excitons. Our results demonstrate two dimensional device for future optoelectronics and light harvesting assisted by interlayer excitons in van der Waals heterostructure.