RII Track-4: Time-Resolved Mossbauer Spectroscopy, a New Tool for Investigating Ultrafast Dynamics in Solid-State Photocatalytic and Photovoltaic Materials

University Of Rhode Island, Kingston RI

Investigators

Abstract

Nontechnical Description Improving the function of solar harvesting devices made from cheap, durable materials is necessary to make solar technology an integral part of a clean, renewable energy strategy. The efficiency of a photovoltaic or photocatalytic device is generally determined by events that occur within the first few picoseconds to nanoseconds following absorption of light, and thus a thorough understanding of electronic and chemical dynamics on this ultrafast timescale is crucial for guiding the design of next-generation materials. Transient absorption spectroscopy has long been deployed to this end, wherein an ultrashort laser pump pulse synchronously launches processes in a sample and a probe pulse reports on the state of the system at a later time. But while this approach has proven enormously successful in providing molecular movies of reactions in solution, laser-induced heating inevitably gives rise to spurious signals in solid-state materials. This project aims to solve that dilemma by pioneering an entirely new ultrafast X-ray technique that probes atomic nuclei instead of electrons. Because this technique requires the exceptionally bright X-ray pulses provided by large-scale storage ring light sources, this work will be performed at the Advanced Photon Source at Argonne National Laboratory. A residency at this world-class facility will provide the University of Rhode Island research team the unique opportunity to demonstrate TRSRM for the first time and track the formation of short-lived chemical species in several solid-state solar energy conversion materials. Technical Description Recent work in ultrafast optical and X-ray transient absorption spectroscopy has highlighted the deficiencies of these traditional approaches for studying photochemical dynamics in the solid-state due to the presence of massive thermal artifacts that arise from laser-induced heating. This project aims to develop and demonstrate time-resolved synchrotron radiation M?ssbauer spectroscopy (TRSRM), a novel and difficult X-ray technique that will provide an unambiguous probe of excited state dynamics in a wide variety of solid-state photocatalytic and photovoltaic materials, including hematite, iron titanate, and organolead halide perovskites. M?ssbauer spectroscopy retains element specificity and oxidation/spin state sensitivity of X-ray techniques while boasting exceptionally narrow linewidths, straightforward assignments, and most importantly for ultrafast implementation of the technique a relative insensitivity to large thermal variations. TRSRM may also be performed under electrochemical working conditions, an absolute requirement for investigating the function of photoelectrochemical materials such as hematite. By providing clear mechanistic insight into how these materials function, this work will pave the way for critical assessments (and reassessments) of the chemical and physical properties that enhance or diminish performance in many photoactive materials and guide the efforts of materials engineers to improve solar energy conversion efficiency through rational design. Because TRSRM can only be performed at a time-resolved hard X-ray spectroscopy beamline, the research team will travel to the Advanced Photon Source to prepare and execute the initial experiments. Following successful demonstration of picosecond-resolved TRSRM, the team will seek to identify not only transient species but also transition states in solid-state materials through femtosecond-resolved TRSRM at hard X-ray free electron laser facilities. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.