Cavity-interface engineering of spatially programmable exciton-trion populations in monolayer WS₂

Spatially tuned and reconfigurable optical emission in hetero-interfaces of two-dimensional materials remains attractive for opto-electronic and photonic applications. Here, we demonstrate a method to investigate multiple hetero-interfaces in a WS₂ monolayer fabricated in a single sample, spatially engineering the exciton and trion populations. We fabricate WS₂/HfO₂/metal/SiO₂/Si interfaces with metals such as Au, Ni, Mo, and Pt, patterned in a WS₂ monolayer, where the HfO₂/SiO₂/Si stack creates parallel-plate reflectors and traps light, forming a Fabry-Pérot-type cavity. This yields a higher trion-to-exciton ratio of 1.34 in WS₂/HfO₂/SiO₂/Si also exhibiting the highest absorption, emission and free carrier density (∼3.97 × 10¹² cm^-2). The WS₂/HfO₂/metal/SiO₂/Si interfaces exhibit optical reflectance and partially trap the light resulting in different population ratios. In contrast, interfaces without the HfO₂ buffer exhibit charge transfer, yielding the lowest trion spectral weight. We demonstrate a unified approach for understanding cavity-interface coupled optical emissions for the fabrication of spatially programmable optoelectronic devices.