Ultrafast charge injection dynamics in quantum dot sensitized metal oxides

Dykstra, Conner Patrick

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https://hdl.handle.net/2142/121929 Copy

Description

Title

Ultrafast charge injection dynamics in quantum dot sensitized metal oxides

Author(s)

Dykstra, Conner Patrick

Issue Date

2023-09-21

Director of Research (if dissertation) or Advisor (if thesis)

van der Veen, Renske

Doctoral Committee Chair(s)

Vurar-Weis, Josh

Committee Member(s)

• Murphy, Catherine
• Shim, Moonsub

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Quantum Dots
• CdSe
• ZnO
• charge transfer
• quantum dot heterostructures

Abstract

This thesis unravels the complex interactions between CdSe quantum dots (QDs) and ZnO nanorods during and after charge transfer using multi-modal time-resolved optical and X-ray techniques. CdSe - ZnO is a foundational system in photovoltaics, however the inability to study charge injection from the ZnO point of view has left open questions about the interaction between carriers post-charge injection. Direct photoexcitation of ZnO was studied with a novel implementation of picosecond resolved X-ray absorption spectroscopy (XAS) by exploiting the oriented nature of the nanorods. ZnO has a wurtzite crystal structure, which is anisotropic and allows for the application of X-ray linear dichroism (XLD) to extract orbital dependent dynamics. The thermal impact of laser excitation was subtracted with a careful calibration in the extended X-ray absorption fine structure (EXAFS) region of the spectrum. The resulting nonthermal X-ray transient absorption (XTA) spectrum and transient XLD reveals a reduction in the absorption cross-section as a result of nonlocal core-hole screening. This effect had not previously been reported in XTA spectra. Upon excitation with low laser fluence, which limits the thermal impact from photoexcitation, the first instance of phase-space filling (PSF) in the hard X-ray regime was measured. This signal offers the possibility for the measurement of carrier- and material-specific dynamics since the holes and electrons have spectrally separated signals. Next, we sought to estimate the expected signal in an XTA experiment on the heterostructured CdSe - ZnO system, in which charges are transferred from the QD moiety to the ZnO. PSF in the X-ray regime was correlated with a corresponding optical PSF signal at the ZnO band edge. Ultraviolet (UV) probe optical transient absorption (OTA) with direct excitation of ZnO was used to estimate the optical PSF amplitude in ZnO that matches the necessary carrier concentration to measure PSF in the X-ray regime. CdSe - ZnO was then measured with optical excitation of the CdSe to test the feasibility of an optical pump - X-ray probe charge injection experiment. It was found that the efficiency of charge injection and resulting carrier concentration is insufficient under ideal circumstances to measure X-ray PSF within a reasonable averaging timeframe at synchrotron facilities. These experiments demonstrate the power of an alternative approach using purely optical techniques. UV probe OTA is a powerful technique that directly interrogates the dynamics within wide band gap metal oxides. The direct band gap of ZnO is located at 3.3 eV, with an excitonic transition located at 365 nm. This transition gives strong signals far to the blue of the CdSe band gap. Applying dual UV/visible probe OTA directly correlates the kinetics of CdSe and ZnO. The implementation of a holistic kinetic model shows that charge injection is split into two time regimes one of which derives from heterogeneous ZnO states, likely due to facet-specific band bending. This work also addresses a longstanding issue within sensitized QD systems, which is how electrons and QD-bound holes interact after charge transfer. The holistic kinetic model reveals that there is a very short-lived intermediate state (<1 ps) that decreases with smaller QD sizes. The identity of this state is likely a localized surface state that arises from band bending in ZnO caused by adsorbed organic species, which contrasts with previous interpretations of a Coulombically bound interfacial exciton. We have demonstrated the power, and limitations, of a novel time-resolved X-ray technique and its applicability to metal oxide materials and interfacial charge transfer in heterostructured materials. Two novel effects, hard X-ray PSF and nonlocal core-hole screening, were uncovered. While powerful, We showed that the achievable signal-to-noise ratio (SNR) at synchrotron based optical pump - X-ray probe experiments are not sufficient for charge injection studies in QD-sensitized nanomaterials. These results were followed up with optical measurements to show that charge injection in ZnO nanorods is strongly dependent on the surface chemistry of ZnO, in addition to the well-known dependence on CdSe chemistry. The assignment of the intermediate state after charge injection to ZnO surface states offers a clear view on how ZnO based devices can be optimized. The UV-probe OTA approach demonstrated in this work offers opportunities for investigating the parameter space for QDHs, such as the dependence of charge injection on ZnO morphology, pump energy, and core-shell structures. Large scale X-ray facilities that offer high time resolution coupled with large X-ray fluxes can also be powerful avenues.

Graduation Semester

2023-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Conner Patrick Dykstra

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