Defense Date

2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Dr. Katharine Tibbetts

Second Advisor

Dr. Dusan Bratko

Third Advisor

Dr. Soma Dhakal

Fourth Advisor

Dr. Scott Gronert

Fifth Advisor

Dr. Massimo Bertino

Abstract

Metal-silicon nanostructures are a growing area of research due to their applications in multiple fields such as biosensing and catalysis. In addition, silicon can provide strong support effects to metal nanoparticles while being more cost effective than traditionally used supports, like titania. Traditional wet-chemical methods are capable of synthesizing metal-silicon nanostructures with a variety of composition and nanoparticle shapes, but they often require high temperatures, toxic solvents, strong reducing agents, or need capping agents added to stabilize the nanoparticles. Laser processing is an emerging technique capable of synthesizing metal-silicon composite surfaces that offers a faster, simpler, and greener synthesis route to these structures.

Reactive laser ablation in liquid (RLAL) is a single-step process that can be considered both a “top-down” and “bottom-up” approach. It combines pulsed laser ablation in liquid (PLAL) and laser reduction in liquid (LRL) by ablating a solid target in a metal salt solution. RLAL has been studied previously for synthesizing silver-silicon nanostructured surfaces for use in SERS. However, little is known about the chemical composition of these laser-processed surfaces and the reaction mechanisms leading to their formation are poorly understood. In this work, we synthesized and characterized various silicon-metal nanostructures through femtosecond-RLAL (fs-RLAL). Furthermore, we discuss the relationship between the pH of the precursor solution, processing silicon simultaneously or sequentially, the concentration of the precursor solution, and sample translation rate on the resulting metal-silicon nanostructured surfaces.

First, silicon wafers were immersed in pH-controlled solutions of KAuCl4 and Cu(NO3)2, then processed with ultrashort laser pulses. For both copper and gold, two syntheses were compared: (1) simultaneous deposition, wherein a silicon wafer was laser-processed in aqueous metal salt solution, and (2) sequential deposition, wherein the silicon wafer was laser-processed in water and then exposed to aqueous metal salt solution. Gold deposition on the silicon wafers was found to xv depend upon the pH of the precursor solution: near-neutral solutions (pH ~6.3) resulted in much higher gold deposition than acidic or basic solutions. X-ray photoelectron spectroscopy and depth profiling showed the existence of both gold (Au0) and gold-silicide (AuxSi) phases on the surfaces of simultaneous and sequential samples. For copper, only simultaneous deposition resulted in high Cu loading and cubic Cu NPs deposited on the surface. Solution pH near ~6.8 maximized Cu deposition. When Cu(NO3)2 concentration was varied, it affected the Cu NP shape, but not Cu loading. When sample translation rate was varied, the Cu NP size and Cu loading was affected. Silver and various alloy combinations were used in the fs-RLAL synthesis to determine if silver or alloys could be deposited onto the silicon nanostructures. Silver deposition was greatly enhanced by slowing the sample translation rate and utilizing NH3 as the base instead of KOH resulted in smaller Ag NPs. This synthesis method was unable to efficiently synthesize alloy structures on silicon, but initial data suggests that the addition of a second metal into the synthesis invokes a galvanic replacement type effect, enhancing deposition of the metal with the higher reduction potential. We propose mechanisms that explain the observed gold penetration depth and its deposition dependence on solution pH, the morphology of cubic Cu NPs deposited on silicon and their dependence on various parameters, the deposition of silver, as well as the sacrificial nature of using additional metals in RLAL. The mechanistic understanding gained in this work may have use for synthesizing a variety of metal-silicon composite surfaces through laser processing to prepare functional materials such as catalysts and surface-enhanced Raman spectroscopy substrates.

Rights

© Eric J. Broadhead

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

5-3-2021

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