Nanoscale Electrostatic Control of Interlayer Excitons in MoSe2-WSe2 Semiconductor Heterostructures

Abstract

Interlayer excitons in 2D semiconductors have been the subject of intensive study due to their long lifetimes and spin-valley physics, with long standing goals including single IX trapping and diffusion control for valleytronic applications. In this work, we use nano-patterned graphene gates to create a quasi-zero and one-dimensional electrostatic IX traps. This approach uses the IX’s permanent dipole moment in a sharply spatially varying electric field to create a nanoscale custom potential energy landscape for IXs, with features as small as 17 nm. In zero dimensional structures, etched holes in the graphene gate create a quantum dot-like potential for IXs. The trapped IXs show the predicted electric field dependent energy, saturation at low excitation power, and increased lifetime, all signatures of strong spatial confinement. Additionally, photoluminescence from these trapped IXs exhibits a unique power dependent blue-shift, where we observe narrow linewidth IX emission (<0.9 meV) and discrete jumps in the emission energy with increasing power. These jumps can be attributed to quantized increases of the number occupancy of IXs within the electrostatically defined trap. These traps are advantageous over other trapping methods involving strain or moiré traps, due to their deterministic placement in the lithographically defined process, 100 meV energy tunability by applied gate voltage, and scalability to create large arrays of single quantum emitters with controllable placement and spacing. Next, we explore slanted one-dimensional traps for excitons, defined by a triangular etch in the graphene gate to create a water slide-like potential, where IXs created at the top of the water slide will flow unidirectionally to the bottom. By performing spatially and temporally resolved photoluminescence measurements, smoothly varying IX energy along the structure and high speed exciton flow are measured. Furthermore, IX current can be controlled by saturating exciton population within the channel using a second laser pulse, demonstrating the basic principle of an optically gated excitonic transistor. This work paves the way towards low loss excitonic circuits, the study of bosonic transport in one-dimensional channels, and custom potential energy landscapes for IXs in van der Waals heterostructures.