Plasmonic-hot-electron mediated room-temperature generation of charged biexciton in monolayer WS2

Transition-metal dichalcogenide (TMDC) monolayers exhibit strong many-body Coulomb interaction, which induces rich excitonic states, including neutral exciton, trion (charged exciton), biexciton, and even charged biexciton. Despite the fact that the biexcitons in pristine two-dimensional (2D) TMDCs have great potential in forming correlated exciton pairs at distinct valleys, they lack efficient emission and optical tunability at room temperature, hindering their applications in photonic and optoelectronic devices. Here, we report room-temperature strong emission of the charged biexcitons from a gold nanosphere capped ${\mathrm{WS}}_{2}$ monolayer through localized surface plasmon (LSP) induced hot-electron injection, and demonstrate all-optical control of the emission intensity through on- or off-resonant LSP excitation. Under on-resonant LSP excitation at 2.08 eV, the photoluminescence (PL) intensity shows a superlinear scaling with the laser power density, signifying the existence of charged biexcitons; On the contrary, the off-resonant LSP excitation at 2.33 eV results in no detectable PL signal of the charged biexcitons. By calculating the energy difference between the charged biexciton and trion, we further determine the binding energy of the former to be 25 meV. Our findings offer an opportunity for understanding the many-body effect associated with the charged biexcitons of 2D TMDCs, and could facilitate their potential quantum and optoelectronic applications under ambient environment.