Cross-material exciton-phonon coupling in van der Waals heterostructures

Exciton-phonon interaction plays a major role in the optical response of van der Waals (vdW) heterostructures, which, if appropriately controlled, is a valuable tool in engineering optoelectronic devices based on 2D semiconductors. Here, we report a novel exciton-phonon interaction at the interface between different vdW atomic layers, as well as with the substrates. By interfacing a monolayer WSe$_{\mathrm{2}}$ with atomically-thin hexagonal boron nitride (hBN), we observe the activation of Raman silent hBN A$_{\mathrm{2u}}$ mode stemming from the coupling between WSe$_{\mathrm{2}}$ exciton and hBN phonons. A comparison between hBN-overlaid and hBN-sandwiched WSe$_{\mathrm{2}}$ samples shows that the coupling between hBN A$_{\mathrm{2u}}$ phonon and WSe$_{\mathrm{2}}$ exciton is suppressed in the latter, while that between hBN A$_{\mathrm{2u}}$, WSe$_{\mathrm{2}}$ A'$_{\mathrm{1}}$ phonons and WSe$_{\mathrm{2}}$ exciton remains strong. This demonstrates that the interfacial exciton-phonon coupling can be manipulated by symmetry reconstruction. Moreover, we observe enhanced Raman signals originating from surface mode in SiO$_{\mathrm{2}}$, as well as E$_{\mathrm{g}}$ mode in sapphire substrates. This highlights the ubiquity of interfacial exciton-phonon coupling in vdW heterostructures. Due to their nanoscaled geometries, they are highly susceptible to vibrational surroundings, especially from supporting substrates, an aspect commonly overlooked.