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.