Electron-hole liquid in a van der Waals heterostructure photocell at room temperature

In semiconductors, photo-excited charge carriers exist as a gas of electrons and holes, bound electron-hole pairs (excitons), biexcitons and trions(1-4). At sufficiently high densities, the non-equilibrium system of electrons (e(-)) and holes (h(+)) may merge into an electronic liquid droplet(5-10). Here, we report on the electron-hole liquid in ultrathin MoTe2 photocells revealed through multi-parameter dynamic photoresponse microscopy (MPDPM). By combining rich visualization with comprehensive analysis of very large data sets acquired through MPDPM, we find that ultrafast laser excitation at a graphene-MoTe2-graphene interface leads to the abrupt formation of ring-like spatial patterns in the photocurrent response as a function of increasing optical power at T = 297 K. The sudden onset to these patterns, together with extreme sublinear power dependence and picosecond-scale photocurrent dynamics, provide strong evidence for the formation of a two-dimensional electron-hole liquid droplet. The electron-hole liquid, which features a macroscopic population of correlated electrons and holes, may offer a path to room-temperature optoelectronic devices that harness collective electronic phenomena.