Formation and Characterization of Nanoparticles Via Laser Ablation in Solution

Open Access
Golightly, Justin Samuel
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
March 21, 2007
Committee Members:
  • Albert Welford Castleman Jr., Committee Chair
  • Thomas E Mallouk, Committee Member
  • Nicholas Winograd, Committee Member
  • Henry Foley, Committee Member
  • Laser Ablation
  • Nanoparticles
  • irradiation
  • metal nanoparticles
  • synthesis
The work presented in this thesis encompassed laser ablation of various transition metals within a liquid environment. The ablation process within liquid produces nanoparticles of an assortment of size and composition. The transition metals studied represent some of the transition metals studied previously within the Castleman research group, as the purpose of this research was to use prior studies and discoveries as a framework to achieve a materials-oriented synthesis by means of similar experimental methods. In creating nanoparticles via this solution-phase approach, discoveries were made regarding the behavior of metals in the ablation process. It was found that this technique could produce a variety of nanoparticles by variation of solvent and laser parameters. Much of the work is devoted towards creating new nanomaterials by controlling factors such as laser fluence and pulse duration, as well as choosing solvents with specific properties. By focusing on characterization of these nanoparticles at each condition, a better understanding of how the variables affect nanoparticle formation was attained. Through an improved understanding of the ablation process, control over the properties of the resultant nanoparticles can be obtained, and thusly nanoparticles can be tailored with specific properties. Creation of nanoparticles via laser ablation in solution is a relatively young technique for nanoparticle synthesis, and the work presented should prove useful in guiding further exploration in ablation processes in liquids for nanomaterial production. When a laser is focused onto a target under a liquid environment, the target material and its surrounding liquid are vaporized. The concoction of vapor is ejected normal to the surface as a bubble. The bubble has a temperature reaching the boiling point of the metal, and has a gradient to the boiling point of the solvent. The bubble expands until it reaches a critical volume, and then subsequently collapses. It is within this bubble that nanoparticle formation occurs. As the bubble expands, the vapor cools and nanoparticle growth transpires. During the bubble collapse, pressures reaching GigaPascals have been reported, and a secondary nanoparticle formation occurs as a result of these high pressures. Chapter 1 delves a little more into the nanoparticle formation mechanisms, as well as an introduction to the analytical techniques used for characterization. Transmission electron microscopy (TEM) techniques were used as a primary method of analysis. A combination of TEM images, selected area electron diffraction (SAED) patterns, and energy dispersive x-ray analysis (EDS/EDAX) provided a great deal of information about the as-formed nanoparticles. The images provided size and shape information, as well as electron density information, which correlates to metal content. The diffraction patterns supplied information about the crystallinity of the nanoparticles, helpful in determining their composition, as well as providing clues about the formation process. The EDS analysis provided elemental content to compliment the SAED patterns and TEM images. Additional analysis was provided by Raman spectroscopy, utilizing a micro-Raman tool, which aided characterization by yielding bonding information. The UV-Vis spectrum showed the absorption behavior by the nanoparticles in their nascent solution. Electrospray ionization (ESI) was a useful tool for analyzing the colloidal solutions in search of stable nanoparticles too small to be analyzed by electron microscopy methods. Ablation of titanium took place in isopropanol, ethanol, water, and n-hexane, under various fluences, with a 532 nm Nd:YAG operating at 10 Hz. It was found that a myriad of nanoparticles could be made with vastly different compositions that were both solvent and fluence dependent. Nanoparticles were made that incorporated carbon and oxygen from the solvent, showing how solvent choice is an important factor in nanoparticle creation. Chapter 3 discusses the results of the titanium work in great detail and demonstrates carbide production with ablation in isopropyl alcohol. Ablation in n-hexane also showed diffraction patterns correlating with carbides, and water showed oxygen incorporation. These results showed the ability to utilize the solvent in tailoring nanoparticles to achieve desired properties. Zirconium and nickel were ablated with the Nd:YAG at 532 nm. These studies utilized a stainless steel chamber designed and built to improve control over the experimental variables. The nickel study showcased the new chamber’s ability for reproducibility in a size dependence study based upon laser fluence. The results of ablation with the Nd:YAG were compared to femtosecond ablation experiments performed with a titanium:sapphire femtosecond laser system. The Ti:sapphire femtosecond laser operated at 10 Hz, produced femtosecond pulses centered at ~795 nm. The pulse duration was varied from 100 fs to 390 fs, the nanoparticles created from each condition were characterized, and the results are presented in chapters 5 and 6. Aluminum nanoparticles were made using both nanosecond and femtosecond laser ablation techniques. Aluminum nanoparticles have a great deal of potential for use as fuel additives as well as in paints and coatings. The nanosecond ablation process rendered large nanoparticles (over 200 nm) and the results are briefly shown in Appendix A. The femtosecond system produced a much smaller distribution of nanoparticles, with nanoparticles remaining in suspension for over a month’s time, as evidenced by their unique UV-Vis absorbance. These nanoparticles were produced in isopropyl alcohol, and were stabilized by the solvent, as TEM analysis showed nanoparticles with very little oxygen incorporation. The solvent is bound to the nanoparticles as a result of the formation process and as a result forms a protective coating, which prevents further oxidation over time. The remarkable stability of these aluminum nanoparticles is a testament of employing the high energy scheme of the laser ablation process in a manner to tailor the production of novel nanomaterials.