  Single-layer transition metal dichalcogenides are at the center of an ever
increasing research effort both in terms of fundamental physics and
applications. Exciton-phonon coupling plays a key role in determining the
(opto)electronic properties of these materials. However, the exciton-phonon
coupling strength has not been measured at room temperature. Here, we develop
two-dimensional micro-spectroscopy to determine exciton-phonon coupling of
single-layer MoSe2. We detect beating signals as a function of waiting time T,
induced by the coupling between the A exciton and the A'1 optical phonon.
Analysis of two-dimensional beating maps combined with simulations provides the
exciton-phonon coupling. The Huang-Rhys factor of ~1 is larger than in most
other inorganic semiconductor nanostructures. Our technique offers a unique
tool to measure exciton-phonon coupling also in other heterogeneous
semiconducting systems with a spatial resolution ~260 nm, and will provide
design-relevant parameters for the development of optoelectronic devices.
