Materials Science Research Lecture

Abstract: Condensation - the familiar process underlying the formation of clouds and the distillation of ethyl alcohol into whiskey - is the phase transition of gas into liquid. In semiconductors, at sufficiently high electron-hole (e-h) densities or low temperatures, the gas of non-equilibrium electrons and holes may undergo condensation. Negatively charged electrons and positively charged holes merge to become a neutral electronic liquid. In this talk, I report on the gas-to-liquid phase transition of electrons and holes in ultrathin van der Waals heterostructure 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-molybdenum ditelluride-graphene interface leads to the abrupt formation of ring-like spatial patterns in the photocurrent response as a function of increasing optical power. These patterns, together with extreme sublinear power dependence and picosecond-scale photocurrent dynamics, provide strong evidence for the formation of a two-dimensional e-h condensate. Remarkably, our imaging experiments mark the first observation (in over 50 years of study) of an e-h liquid at room temperature.

Bio: Having established himself as a rising leader in quantum optoelectronics research at Cornell University (PhD Physics 2012) and MIT (Postdoctoral Fellow Physics), Prof. Nathaniel Gabor began his academic career at the University of California Riverside in 2013. Nathaniel's research focuses on quantum optoelectronic measurements of nanoscale materials, and has lead to the direct observation of highly efficient electron-hole pair multiplication in carbon nanotubes and the unusual hot carrier transport regime in graphene. He has also established leadership in the design and understanding of advanced energy harvesting and storage technologies, having been recognized with numerous national and international awards. Based on several key discoveries regarding light harvesting in biology, Prof. Gabor was named a Scialog Advanced Energy Storage Fellow by the Research Corporation for Science Advancement, as well as a CIFAR Azrieli Global Scholar within the Canadian Institute for Advanced Research. In addition to his successful funding as a NSF Career Award and AFOSR Young Investigator Program (YIP) Award recipient, his recent work has been recognized as potentially explaining one of the longest standing questions in photosynthesis research: Why are plants green?

Refreshments served at 3:30pm in the Spalding Laboratory Lobby.