Abstract: Photoemission spectroscopy techniques – wherein one photoemits an electron from a material using a high-energy photon to study its properties – have provided unparalleled insight into materials and condensed matter systems over the past several decades. Among these, there are two particularly powerful and complementary techniques: angle-resolved photoemission spectroscopy (ARPES), which resolves the momentum of the photoemitted electron in the material; and photoemission electron microscopy (PEEM), which resolves its spatial coordinate. Recently, the merger of these techniques into multi-dimensional platforms of photoemission spectroscopy, along with access to the temporal dimension by further incorporating ultrafast spectroscopy techniques, have enabled powerful visuals of the evolving electronic structure of photoexcited systems in real and momentum space.  

I will discuss the development of this multi-dimensional photoemission spectroscopy platform in my lab over the better part of a decade. Then, I will discuss some recent work in my lab in visualizing photoexcited carriers in space, time and energy. Applying these techniques to state-of-the-art perovskite photovoltaic films, we will image the performance limiting nanoscale defect clusters in these next-gen solar materials, and understand their role in charge trapping and degradation.  

Finally, we will turn our attention to imaging momentum space in photoexcited 2D semiconductors and heterostructures. Thereby, we will directly image the distribution of an electron around a hole in an exciton – a hydrogen-like state that forms when a semiconductor absorbs light; visualize dark excitonic states that have largely remained hidden to optical experiments, and observe the structure of a moiré trapped interlayer exciton. 

Bio: Keshav Dani is currently an Associate Professor at the Okinawa Institute of Science and Technology (OIST), Graduate University in Okinawa, Japan. He joined OIST in Nov. 2011 as a tenure-track Assistant Professor after completing a Director’s Postdoctoral Fellowship at the Center for Integrated Nanotechnologies at Los Alamos National Laboratory. Keshav graduated from UC Berkeley with a PhD in Physics and obtained a BS from Caltech in Mathematics.