Interlayer Exciton-Phonon Bound State in Bi₂Se₃/Monolayer WS₂ van der Waals Heterostructures

The ability to assemble layers of two-dimensional (2D) materials to form permutations of van der Waals heterostructures provides significant opportunities in materials design and synthesis. Interlayer interactions can enable desired properties and functionality, and understanding such interactions is essential to that end. Here we report formation of interlayer exciton-phonon bound states in Bi₂Se₃/WS₂ heterostructures, where the Bi₂Se₃ A₁⁽³⁾ surface phonon, a mode particularly susceptible to electron-phonon coupling, is imprinted onto the excitonic emission of the WS₂. The exciton-phonon bound state (or exciton-phonon quasiparticle) presents itself as evenly separated peaks superposed on the WS₂ excitonic photoluminescence spectrum, whose periodic spacing corresponds to the A₁⁽³⁾ surface phonon energy. Low-temperature polarized Raman spectroscopy of Bi₂Se₃ reveals intense surface phonons and local symmetry breaking that allows the A₁⁽³⁾ surface phonon to manifest in otherwise forbidden scattering geometries. Our work advances knowledge of the complex interlayer van der Waals interactions and facilitates technologies that combine the distinctive transport and optical properties from separate materials into one device for possible spintronics, valleytronics, and quantum computing applications.