Applications of Shaped Femtosecond near-IR Laser Irradiation in the Generation of Metal Nanoparticles
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Applications of Shaped Femtosecond near-IR Laser Irradiation in the Generation of Metal Nanoparticles Behzad Tangeysh, Katharine Moore Tibbetts, Johanan H. Odhner, Bradford B. Wayland* and Robert J. Levis* Department of Chemistry and Center for Advanced Photonics Research, College of Science and Technology, Temple University, Philadelphia, PA 19122, U.S.A. ABSTRACT Femtosecond near IR laser irradiation is explored as a general methodology to produce metal nanoparticles from metal precursor solutions. Initial studies of the formation and transformations of gold nanoparticles in aqueous solution are used as model processes to evaluate the effects of laser parameters, reaction medium and surfactants in controlling metal nanoparticle formation. The addition of polymer surfactants such as poly(ethylene glycol) (PEG) was found to significantly accelerate Au(III) reduction as compared to surfactant-free systems. Photoreduction for aqueous solutions of Au(III) in the presence of PEG results in relatively small narrowly dispersed spherical gold nanoparticles compared to relatively large well-formed crystalline nanoparticles that are observed in the absence of surfactants. Varying the concentration of PEG is an effective approach to tune the diameter and size distribution from 3.9±0.7 nm to 11±2.4 nm for Au nanoparticles produced by laser processing. INTRODUCTION Laser irradiation methods such as ablation and photoreduction of precursor solutions have been used effectively to produce transition metal particles and alloys [1-3]. Plasmonic nanoparticles such as gold and silver are among the most extensively studied nanomaterials and thus serve as a benchmark to evaluate general methodologies for formation and stabilization of metal nanoparticles. Femtosecond (fs) laser irradiation of aqueous [AuCl4]- in the presence of a variety of surfactants in solution has been shown to generate and stabilize Au NPs [4-6]. Reduction of aqueous [AuCl4]- during fs laser irradiation has been attributed to the reactive reducing radicals such as (H.) and solvated free electrons (eaq-) produced from dissociation and ionization of the water that occurs in the region of the laser focus [7,8]. In this work the application of simultaneous spatio-temporal focusing (SSTF) of fs near IR laser irradiation in controlled formation of gold nanoparticles from precursor solution is presented. Systematic variations of reaction parameters is shown to provide effective strategies to achieve size control by using shaped fs laser pulses. EXPERIMENTAL Aqueous [AuCl4]- and [AuCl4]-: PEG solutions were irradiated with pulses from a titaniumsapphire-based chirped-pulse amplifier delivering 35 femtosecond (fs) pulses with bandwidth centered at 790 nm and a 1 kHz repetition rate. Prior to focusing with an f=50-mm lens into a quartz cuvette containing 3.0 mL of liquid sample, the fs pulses were spectrally dispersed using a grating pair. The lens focuses the laser pulse simultaneously in space and time, and the temporal profile of the pulse is restored when all of the spec
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