Oliver Gessner

Pathways toward a sustainable economy will include the harvesting of sunlight to provide storable energy in the form of charges or chemical fuels. Hybrid solar light harvesting systems, consisting of combinations of molecules, metal nanoparticles, and transition metal oxides are among the most promising technical concepts to achieve these goals. We will present new insights into fundamental photoinduced dynamics in archetypical heterostructures based on femtosecond and picosecond time-resolved X-ray photoemission spectroscopy (TRXPS), as well as first steps toward taking time-domain X-ray spectroscopy of interfacial dynamics beyond the limitations of pump-probe measurements.

Photoinduced transient charge redistribution in the model hybrid system of nanoporous zinc oxide sensitized with ruthenium bipyridyl chromophores is probed independently from the viewpoints of the molecular electron donor and the semiconductor acceptor, revealing a multi-step charge injection process and nanoscale confinement of the injected electrons at the semiconductor surface. This strong confinement is the result of a pronounced downward band-bending inside the semiconductor toward the interface, which is further enhanced by the electron injection itself.

The self-trapping-like mechanism forces injected electrons to remain localized in the defect-rich surface region, which sheds light on the known challenges of using ZnO as electrode material in solar light harvesting applications.1

Heterogeneous light harvesting systems consisting of metal nanoparticles interfaced with wide bandgap transition metal semiconductors are among the most intensely studied platforms for solar fuel generation. Utilizing picosecond TRXPS at the Advanced Light Source synchrotron and femtosecond TRXPS measurements at the FLASH Free Electron Laser, we study photoinduced charge transfer dynamics in gold nanoparticle sensitized TiO2.2 The measurements provide interfacial site-specific real-time information of the charge buildup during the first ~ps and electron-hole recombination dynamics up to nanosecond timescales. A detailed picture of the photoinduced charge buildup beyond a single electron injection rate will be presented. We will discuss an empirical model capturing the multi-scale interfacial charge dynamics and complementary efforts to gain a deeper understanding of the underlying physics through highlevel ab initio calculations.

The presentation will also provide an update on the quest for strategies to monitor spontaneous, rather than externally triggered, dynamics in complex systems, which underly most of chemistry. A proof-of-principle experiment is presented that reinterprets key mathematical concepts underlying correlation-based dynamic scattering techniques, such as X-ray Photon Correlation Spectroscopy (XPCS), to extend time-resolved XPS beyond the conventional pump-probe approach. Time-Correlation X-ray Photoemission Spectroscopy (TCXPS) exploits temporal correlations between photoelectron detection events to reconstruct dynamic trends. Direct comparisons with the results of conventional pump-probe experiments will be discussed.3

1. S. Neppl, J. Mahl, et al., J. Phys. Chem. Lett. 12, 11951 (2021), doi: 10.1021/acs.jpclett.1c02648.
2. M. Borgwardt, J. Mahl, et al., J. Phys. Chem. Lett. 11, 5476 (2020), doi: 10.1021/acs.jpclett.0c00825.
3. F. Brausse, M. Borgwardt, et al., Struct. Dyn. 8, 044301 (2021), doi: 10.1063/4.0000099.