Nanothin Bi_xO_ySe_z-Enabled Modulation of Photoluminescent Properties of WSe₂

The bright photoluminescent (PL) properties of monolayer transition metal dichalcogenide (TMD) systems like WSe₂ and WS₂ make them appealing for applications such as next-generation optoelectronics and quantum information sciences. The PL originates from tightly bound exitons that are sensitive to changes in the surrounding dielectric environment. Here we show that by growing few-layer Bi₂Se₃ on WSe₂, the PL signal is quenched. Interestingly, the PL signal can be restored in discrete, sub-micrometer spots by exposing the Bi₂Se₃–WSe₂ heterostructure to a high-power laser (393 μW) in an oxygen-rich environment. The PL intensity can further be modulated through control of the laser power and oxygen partial pressure to achieve a continuum of PL values, demonstrating spatially selective tunability of the PL signal.<br/>Raman spectroscopy and optical imaging indicate that the Bi₂Se₃ undergoes a structural phase change during the first laser-oxygen exposure. The laser-oxygen exposed spots were investigated using scanning transmission electron microscopy (STEM) and energy dispersive x-ray spectroscopy (EDS), which shows a 47% oxygen increase and 87% selenium decrease at those locations. Selected area electron diffraction (SAED) of the pristine Bi₂Se₃ and laser-exposed spots show a change in the crystal lattice constant. The SAED pattern of regions exposed to the laser-oxygen environment match well with the rhombohedral R3m Bi₂O₃ phase calculated from first-principles density functional theory (DFT). Bi₂O₃ is a good candidate for solid oxide fuel cells (SOFC) and oxygen separation membranes. Bi₂O₃ exists with a number of different polymorphs, and the properties vary dramatically with the different crystal phases. For instance, the monoclinic α-Bi₂O₃ is stable at room temperature, but it has poor oxygen ion conduction, while the rhombohedral phase offers more competitive oxygen ion transport. However, this phase is difficult to stabilize, resulting in few experimental studies without heavy dopant concentrations. To the best of our knowledge, we have synthesized for the first time a few-layer, nanothin phase of R3m Bi₂O₃ at room temperature and without high dopant concentrations. STEM-EDS measures a small amount of Se remaining, and thus we refer to this phase as Bi_xO_ySe_z.<br/>The Bi_xO_ySe_z phase allows for oxygen transport into the interlayer region of the Bi_xO_ySe_z–WSe₂ heterostructure, thus decoupling the two layers and restoring the WSe₂ PL signal. The PL intensity modulation is a result of intercalating and deintercalating oxygen between the two materials. The PL intensity cycles with partial pressure of oxygen during laser exposure, suggesting that the PL changes are dependent on an oxygen interaction that doesn’t structurally alter the heterostructure, which is consistent with oxygen intercalating and deintercalating between the layers. Using Fick’s 2^nd Law of Diffusion, we measure oxygen diffusion through the Bi_xO_ySe_z to be 2.61e-18 m²/s under laser exposure at room temperature, several orders of magnitude above other room-temperature oxygen transporters. Other Bi₂O₃ phases have demonstrated faster oxygen diffusion but require temperatures of 500 °C or greater.<br/>We have demonstrated the fabrication of nanothin Bi_xO_ySe_z, closely resembling the rare rhombohedral phase of Bi₂O₃. In addition to the ability to modulate the PL signal of WSe₂, nanothin Bi_xO_ySe_z is a promising, unexplored room-temperature oxygen transporter that could advance low-temperature SOFC technology and syngas production.