Internal atomic-scale structure and photothermal dynamics of heterostructured nanomaterials

In this thesis we adopt a multimodal materials characterization approach to unravel the internal structure and the photoexcited electronic and geometric structural dynamics of Cd𝑥Zn1−𝑥Te/CdSe core/shell quantum dots (QDs). These QDs belong to a broader class of heterostructured II-VI semiconducting that are known for their applications in optoelectronics and biomedical imaging. The foundation of this thesis is the determination of the internal structure of II-VI core/shell quantum dots (CSQDs) using a combination of several characterization modes, with X-ray absorption spectroscopy (XAS) as a crucial element-specific technique. Through a combination of optical spectroscopy, electron microscopy, elemental analysis, and the global analysis of the extended X-ray absorption fine structure (EXAFS) spectrum, we show that the intended ZnTe/CdSe CSQDs, that are synthesized using a common one-pot synthesis procedure, are in actuality nanoparticles with an alloyed core and a patchy CdSe shell. Electronic structure calculations show that the CSQDs essentially behave as one-component QDs with a direct band gap. Cation exchange and the unintended reaction of molecular precursors prevent the formation of a sharp type-II ZnTe/CdSe interface with small lattice mismatch. Instead, the large interfacial strain between Cd𝑥Zn1−𝑥Te (𝑥 = 0.8) and pure-phase CdSe leads to the growth of islands on the QD surface. Our results corroborate the challenges associated with the synthesis of Zn/Cd chalcogenide type-II heterostructures due to facile ion exchange. This study is an example of how the assessment of heterogeneous nanomaterials on the basis of spectroscopy or size analysis alone is not always sufficient. While our XAS data were obtained at a large-scale synchrotron X-ray facility with specialized infrastructure and limited access, the advent of tunable high-brightness table-top X-ray sources will enable characterization studies on heterostructured photovoltaic and photocatalytic nanomaterials with much higher throughput and more experimental flexibility. We use density functional theory in combination with state-of-the art theoretical XAS codes to demonstrate the sensitivity of the X-ray absorption near-edge structure (XANES) to the local structure beyond the first coordination shell. In this way, we are able to corroborate the structural characterization of the alloyed Cd𝑥Zn1−𝑥Te (𝑥 = 0.8) core as determined by EXAFS analysis. This work underscores the power of XAS, in both experiment and simulation, for understanding the internal structure of heterogeneous nanoparticles. Ultrafast XAS is a powerful tool to unravel the electronic and geometric structures of photoexcited materials with femtosecond (fs)-nanosecond (ns) resolution. Using systematic DFT-based XAS simulations, we show that the time-resolved XANES spectra of nanoparticles at early time delays after photoexcitation (90 picoseconds, ps) are dominated by thermal effects, such as a 0.2% lattice expansion and disorder, while spectra at later times (2.5 ns) have clear signatures of excited carriers. In combination with heat diffusion simulations we derive the heat interface conductance 7-15 MW/m2/K for the colloidal Cd𝑥Zn1−𝑥Te/CdSe nanoparticle sample. Application of ultrafast XAS and the data analysis methods to other nanomaterials is an exciting perspective; in particular, in view of the recent development of intense free electron laser sources delivering v100 fs X-ray pulses. These state-of-the-art facilities open new opportunities for exploring photoinduced electronic properties of semiconductor nanomaterials on the ultrafast time scale.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemistry

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2021

Author dc:creator

• Gentle, Cecilia Marie

Contributors dc:contributor

• Murphy, Catherine J.
• van der Veen, Renske
• Shim, Moonsub
• Vura-Weis, Josh

• Quantum dots
• semiconductors
• X-ray absorption spectroscopy
• photothermal dynamics
• ultrafast spectroscopy

Statement dc:rights

• Copyright 2020 Cecilia Marie Gentle

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/109323

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/109323