DOI
https://doi.org/10.25772/QPKB-DD28
Author ORCID Identifier
https://orcid.org/0009-0000-8861-4442
Defense Date
2025
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Chemistry
First Advisor
Dr. Katharine Tibbetts
Second Advisor
Dr. Matthew Hartman
Third Advisor
Dr. M. Samy El-Shall
Fourth Advisor
Dr. Joseph Reiner
Abstract
Metal nanoparticle (NP) synthesis is a continually evolving area of research, owing to the widespread application of NPs in fields such as catalysis, drug delivery, and biosensing. Compared to bulk metal material, NPs contain a larger proportion of surface atoms and exhibit unique optical properties, thus making them ideal for applications where maximizing surface interactions is key. Moreover, their compositions can be altered in several different ways, as evidenced by metal alloy, core/shell, supported, and defective NPs. While conventional synthesis methods are capable of producing NPs in large quantities, they often require conditions of high temperature and pressure, strong reducing agents, or surfactants that hinder NP activity. Therefore, sustainable NP synthesis methods are in high demand. To this end, laser synthesis methods have emerged as a green alternative, comprising pulsed laser ablation in liquid (PLAL, a top down method), laser reduction in liquid (LRL, a bottom up method), or a combination of the two called reactive laser ablation in liquid (RLAL).
Several classes of nanomaterials are uniquely accessible through these laser synthesis methods. For example, using RLAL, silicon laser induced periodic surface structures (LIPSS) containing plasmon active metals can be synthesized that exhibit unique optical properties. While their application in surface enhanced Raman spectroscopy (SERS) has allowed for the detection of target analytes at the single molecule level, achieving high metal content on the silicon surface can only be achieved through multi-step and time consuming processes. Likewise, these methods give only limited control over the obtained metal NP densities and sizes. Therefore, in an effort to enhance the efficiency and flexibility in which plasmonic metals can be deposited, we first use a chemically-motivated method to promote the deposition of gold and copper nanostructures onto silicon LIPSS in a single step using galvanic replacement (GR). We show that the deposited metal content, surface morphology, and metal crystallite size can be tuned based on the difference in electrochemical potentials of the deposited and sacrificial metal. Compared to single-metallic Au and Cu reference samples where no GR was possible, GR more than doubled the metal content on the LIPSS and reduced metal crystallite sizes by up to 20\%.
We then turn to PLAL, which is commonly performed in organic solvents to prevent the oxidation of metal NPs. As a result of solvent decomposition during ablation, reactive radical species form that participate in the formation of metal carbide NPs or even NPs surrounded by thick carbon shells. In addition, stable liquid byproducts are produced which may also be responsible for the carbon coatings observed in NPs synthesized by PLAL. However, many of these products have yet to be identified, and the chemical reaction pathways responsible for their formation are poorly understood. Because it is unclear whether these reaction pathways are target-dependent, using a Cu and Si target, we examined how the target material impacts the distribution of reaction products obtained from ablation in acetone. We found that with a Cu target, the resulting Cu NPs contained amorphous carbon on their surfaces, whereas with a Si target, Si NPs contained fewer carbon species, but more fluorescent carbon dot byproducts were produced in solution. However, the presence of a Cu or Si target reduced the yield of molecular byproducts formed from the recombination of acetone radicals. Additionally, ablated copper species were found to catalyze the formation of pinacol as a byproduct. After providing a mechanistic understanding of the laser-induced solvent decomposition reactions that occur in acetone as well as in the presence of two targets, we turn to another phenomenon, namely, the formation of crystal defects in PLAL synthesized NPs. Planar defects such as grain boundaries are inherent to these structures and have been identified as the active sites for several catalytic reactions. However, no strategies exist that can either control or maximize their formation during ablation, and imaging of these defects must be conducted at high magnifications, which limits the scope of analysis to a single particle at a time. Using a novel TEM imaging technique called precession-illumination-hollow cone dark field (PI-HCDF) imaging , we provide an efficient method that rapidly identifies grain boundaries in a wide array of NPs. In an effort to determine the synthesis conditions that maximize grain boundary formation, a Cu target was ablated in three different solvents (acetone, methanol, ethanol). Here, it was inconclusive whether variation of the solvent led to a significant difference in Cu NP defect density.
Rights
© Nicholas G. Simpson
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
5-6-2025