Elin Cartwright: Ultrafast Charge Dynamics at Interfaces Studied with Hard X-ray Core-Hole Clock Spectroscopy

Abstract

Charge transfer dynamics govern the functionality and performance of numerous electronic applications. This thesis explores the fundamental aspects of ultrafast charge transfer at interfaces of semiconductor and adsorbate systems using X-ray-based spectroscopy. The studied material systems provide an array of parameters which influence electron delocalization, including composition, structure, and quantum confinement. To understand the effects of these parameters on charge dynamics, we employ Core-Hole Clock Spectroscopy in the framework of Resonant Auger Spectroscopy, supplemented by Hard X-ray Photoelectron Spectroscopy to study electronic structures. Probing these systems with X-rays allows selective excitation of core-levels in an element-specific and chemically specific way, providing detailed atomic-level insight into charge dynamics on subfemtosecond timescales.

Three categories of materials were investigated. The first involves xenon monolayers adsorbed on gold, silver, and copper. Comparisons across these substrates show that even minor variations in metal work function and xenon-metal hybridization significantly impact both core-level binding energies and electron transfer rates. Secondly, studies of donor-acceptor polymer bulk heterojunctions under varying donor-acceptor ratios show how polymer chain ordering and shifts in core-level binding energies influence charge dynamics. Donor-acceptor bilayers, using multiple donor-acceptor combinations, were also examined to clarify charge transfer at well-defined interfaces, further deepening our understanding of dynamics in organic semiconductor systems vital for applications including organic electronics and solar cells. Additionally, the effects of prolonged X-ray irradiation on both electronic structure and charge dynamics were assessed to distinguish intrinsic material responses from beam-induced changes in spectroscopic data. Finally, our results on lead sulfide quantum dots of varying sizes illustrate how quantum confinement restricts available electronic states and alters charge dynamics at the nanoscale. Core-hole clock measurements with chemical specificity reveal nearly identical electron transfer timescales from both lead and sulfur, indicating that both elements play comparable roles in charge transport.

These findings advance our fundamental understanding of how interfacial properties govern electronic structure and ultrafast charge transfer in various semiconducting and molecular systems. This knowledge, in turn, can inform the design of material interfaces in technologies such as solar cells, flexible electronics, and quantum-dot-based sensors.