Manipulating Trion and Biexciton Emissions in Monolayer WS₂ by Sandwiching with Ultrathin ZnO Layers for Excitonic Light Emission Applications

Two-dimensional (2D) layered materials provide an ideal platform for visualizing several quantum mechanical phenomena, such as quantum confinement, within a sheet of materials. In contrast to the case of quantum wells (QWs) realized through conventional semiconductor heterostructures, the QW made with 2D semiconductors offers a unique platform to explore many-body effects with its optical excitation and emission characteristics. Herein, we delve into the effect on the photoluminescence (PL) emission spectrum from chemical vapor deposition-grown monolayer (1L)-WS2 flakes upon being encapsulated by ultrathin ZnO films. We assemble a sandwich-type structure with 1L-WS2 using ZnO, a higher band gap semiconductor, and investigate the modulation of the PL emission from the WS2 flakes of subnanometer thickness resulting from the quantum confinement and doping effect. We have adopted excitation power- and temperature-dependent micro-PL spectral analysis to comprehend the contributions of neutral excitons, trions, biexcitons, and defects in the tunable PL from the sandwich structure. The PL of 1L-WS2 is partly influenced by strain and doping. Raman spectroscopy is utilized to understand the strain and doping effects induced by the ZnO layer on 1L-WS2. Unlike a typical quantum-well case, here, ZnO being a higher band gap semiconductor injects carriers onto the 1L-WS2, which, along with escalating the exciton density, causes the formation of multibody quasiparticles, such as trions and biexcitons. Cryogenic temperatures and high laser powers favor biexciton emission in monolayer WS2, restricting the QW-induced excitonic PL enhancement only to low excitation powers. This work offers insights into comprehension of the carrier dynamics in monolayer transition-metal dichalcogenides by encapsulation with ultrathin semiconductor layers and the modulation of its PL emission for applications in excitonic light emission.