Invited Paper SA-TuA3
Real-time X-ray Photoelectron Spectroscopy Studies of Electronic Dynamics at Molecule-Semiconductor Interfaces
Tuesday, November 11, 2014, 3:00 pm, Room 312
Interfacial charge transfer processes in molecular and nanoscale systems play an increasingly important role in emerging concepts for renewable energy technologies. Rational design decisions, however, rely on our capability to monitor the pathways of charge carriers on an atomic scale and with a temporal resolution that is commensurate with the timescales of interfacial electron motion. We introduce a new approach to characterize the location of a migrating electron at a molecule-semiconductor interface with sub-nanometer spatial sensitivity and sub-picosecond temporal resolution. Employing the unique capabilities of the Linac Coherent Light Source (LCLS) X-ray Free Electron Laser we use femtosecond time-resolved X-ray photoelectron spectroscopy (tr-XPS) to monitor the nature of an intermediate state that precedes free charge carrier generation in dye-sensitized ZnO nanocrystals after photoexcitation with visible light. Using the element specificity of inner-shell photoemission lines and, in particular, their sensitivity to transient local valence electronic structures, tr-XPS employs the Ru center of the dye molecule as a local reporter atom to provide a unique perspective on ultrafast interfacial charge flow. The underlying physics are explored in a concerted effort with constrained density functional theory (CDFT) calculations of the interfacial electronic structure. The results are discussed with respect to a significantly reduced rate of free charge carrier generation in N3/ZnO systems compared to other material combinations such as N3/TiO2.
Femtosecond time-resolved experiments at the LCLS are complemented by a new picosecond time-resolved XPS effort at the Advanced Light Source (ALS). Using a novel time-stamping technique in combination with a high-power picosecond laser system, tr-XPS experiments on dye-sensitized semiconductor substrates can be performed with up to MHz repetition rates in all operating modes of the ALS (multi-bunch and two-bunch). The measurements simultaneously monitor chemical shifts of particular photolines and transient surface photovoltages of the semiconductor substrate. Results will be discussed with respect to a possible correlation between intramolecular electron dynamics and transient interfacial charge carrier concentrations in the semiconductor. Routes and first steps toward the implementation of in operando X-ray studies of interfacial photoelectrochemical processes will be outlined.