The goal of this research program is to gain a fundamental understanding of the elementary processes and molecular-level mechanisms of the matrix assisted laser ablation synthesis of nanoparticles, and to apply the mechanistic understanding to design and optimize this promising technique for an efficient, controllable, inexpensive, and environmentally friendly generation of nanoparticles. Experimental investigations governing the growth, physical limits, and capabilities of the technique will be guided by the microscopic insights into the mechanisms of nanoparticle nucleation and growth obtained in computer modeling performed with coarse-grained molecular dynamics method. A comprehensive investigation of the atomic/cluster mobility during the process of the explosive disintegration of the target material and the expansion of the multi-phase ablation plume consisting of liquid droplets, matrix vapor, and products of chemical decomposition will provide useful information for several emerging nanoscale materials processing techniques. This research program is closely integrated with the current educational activities of the PIs in the areas of laser-materials interactions and computational materials science, thus providing immediate benefits for training of graduate students while stimulating related research activities for undergraduates with additional exposure through the REU program. The project will continue a strong existing interaction between the PIs, aiding in the continued development of a core activity on laser-material interactions at the Materials Science and Engineering department. The research results of this project will be integrated into graduate courses on Materials Characterization, Thin Films, and Modeling in Materials Science developed and currently taught by the PIs.
James M. Fitz-Gerald and Leonid V. Zhigilei, University of Virginia The results obtained within this research program provide a fundamental understanding of the elementary processes and molecular-level mechanisms of the matrix assisted laser ablation synthesis of nanoparticles. The mechanistic understanding of the ablation process enabled us to design and optimize this promising technique for an efficient, controllable, inexpensive, and environmentally friendly generation of nanoparticles. Experimental investigations of the physical limits and capabilities of the technique were guided by the microscopic insights into the mechanisms of nanoparticle nucleation and growth obtained in computer modeling performed with coarse-grained molecular dynamics method. A comprehensive investigation of the atomic/cluster mobility during the process of the explosive disintegration of the target material and the expansion of the multi-phase ablation plume consisting of liquid droplets, matrix vapor, and products of chemical decomposition provided useful information for several emerging nanoscale materials processing techniques. Findings: The matrix assisted deposition of metal-based acetates has shown the ability to synthesize nanoparticles of several classes of inorganic materials. Nanoporous films of metallic and oxide systems have been grown successfully on room temperature substrates from non-volatile precursors under low vacuum conditions. Results from atomic scale characterization of stoichiometric crystalline oxides and ordered phase nanoparticles demonstrate the ability to synthesize complex particles with relevant applications in areas of chemical sensing and next generation magnetic materials. Based on observations, it is hypothesized that acetates photothermally decompose within the frozen matrix, releasing metal ions. A strong dependence on precursor decomposition temperature was observed for nanoparticle formation and attributed in part to the limitation placed on the achievable temperature of the water matrix by the onset of the explosive boiling at ~550 K. Subsequent irradiation following decomposition leads to the formation of nanoparticles within the target along with enhanced decomposition of neighboring acetate systems with higher decomposition temperatures. Results also suggest that nanoparticle ejection occurs upon achieving a critical number density. Overall, the combined attributes of the matrix assisted process provide a strong platform for the controlled deposition of complex nanoparticle and nanoporous films onto a variety of substrate configurations. Experiments were conducted in the following areas: 1) Growth of Nanoporous Films of Ag, Pd, Au, Sn, and Cu 2) Growth of Multicomponent Nanoporous Films of Y-Ba-Cu-O 3) Growth of Fe-Pd Nanoparticles 4) MAPLE Ablation Studies 5) Melting Target Experiments 6) Nanoparticle Formation A novel multi-scale computational model capable of describing the processes responsible for the nucleation, growth, and transport of metal nanoparticles in matrix-assisted laser ablation synthesis technique has been developed. A combination of atomistic, mesoscopic, and continuum models is used to address different aspects of the ablation process at different length-scales and with different levels of detail. In particular, continuum-level model is designed and used for a computationally-efficient estimation of the ablation depth and the extent of the photolytic and thermal decomposition of the metallo-organic precursors in the irradiated target. The conclusions from the continuum-level simulations are used in the design of a coarse-grained molecular dynamics model capable of reproducing the same physical regime of laser ablation as in experiment, albeit at a reduced length- and time-scales. The simulations performed with the multi-scale model reveal the dependence of different steps involved in nucleation, growth, and ejection of nanoparticles on the laser irradiation conditions and characteristics of the target material. Computational study was also extended to the analysis of the ability of MAPLE to transfer and deposit larger structural units, such as individual carbon nanotubes (CNTs) and nanotube bundles, and to generate nanocomposite thin films with complex structure. In simulations performed for MAPLE targets loaded with CNTs, the ejection of individual nanotubes, CNT bundles, and tangles with sizes comparable or even exceeding the laser penetration depth is observed. No significant splitting and thinning of CNT bundles in the ejection process is observed in the simulations, suggesting that fragile structural elements or molecular agglomerates with complex secondary structures may be transferred and deposited to the substrate with the MAPLE technique.
