  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
provide a path to new 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.
