Mechanism of Improved Au Nanoparticle Size Distributions Using Simultaneous Spatial and Temporal Focusing for Femtosecond Laser Irradiation of Aqueous KAuCl₄

Abstract

The production of gold nanoparticles (AuNPs) by irradiation of aqueous [AuCl4]− with femtosecond laser pulses is investigated using simultaneous spatial and temporal focusing (SSTF) and compared to the results of conventional geometric focusing (GF). The effects of capping agent, laser power, reaction conditions in the cuvette, and laser chirp are studied, and we find that SSTF produces smaller particles with fewer irregular structures and fewer outlying large particles than GF in all cases except for one, in which the particle size distributions are the same. The difference is primarily ascribed to the intrinsic plasma properties of the two geometries: SSTF produces a plasma that is more homogeneous and spatially symmetric than that of GF, promoting efficient intrinsic mixing of the solution.

Elucidating Strong Field Photochemical Reduction Mechanisms of Aqueous [AuCl4]−: Kinetics of Multiphoton Photolysis and Radical-Mediated Reduction

“…The latter effects have been shown to adversely affect the size distributions of AuNPs synthesized using fs laser irradiation. 31 A schematic illustration of the apparatus is shown in Figure 1. The laser pulses were spectrally dispersed using a grating pair (1200 l/mm, G 1 and G 2 in Figure 1), then focused with an f = 50 mm aspheric lens (L) into a quartz fluorimeter cuvette containing 3.0 mL of aqueous [AuCl 4 ] − solution (S).…”

Resolving the source of blue luminescence from alkyl-capped silicon nanoparticles synthesized by laser pulse ablation

“…Luminescent alkyl-capped Si NPs were synthesized by single-step laser ablation of a Si (111) wafer target using simultaneously spatially and temporally focused (SSTF) 28,29 femtosecond pulses generated using a homebuilt apparatus (details in ESI †). In brief, 35 fs, 100 µJ laser pulses centred at 790 nm generated by a titanium-sapphire-based chirped-pulse amplifier laser were first shaped using SSTF and then focused onto the surface of a piranha cleaned Si (111) wafer immersed in neat 1-octene solvent through a quartz cuvette.…”

Au Nanoparticle Synthesis Via Femtosecond Laser-Induced Photochemical Reduction of [AuCl4]−

“…At a given laser wavelength, the peak-intensity needed to reach critical electron density for OB is highly dependent on pulse duration, due to the interplay between the photoionization and cascade ionization rates [33,34]. To illustrate this effect over a wide range of pulse durations used in recent AuCl 4 ½ À photochemical reduction studies [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], Eq. (2) coupled to the appropriate formulas for each rate [33,34] was solved using the Runge-Kutta integrator ode45 incorporated into MATLAB, as in our previous work [16].…”

“…The high pulse-energies of up to 5 mJ and tight-focusing conditions often used in AuCl 4 ½ À reduction experiments [14][15][16][17][18][19][20][21] produce peak intensities that significantly exceed the OB threshold. For instance, irradiation with 1 mJ pulses under the conditions described above results in a peak electron-density that surpasses the OB threshold by at least factor of 50 and even exceeds the maximum electron-density of 4 Â 10 22 cm À3 achievable in liquid water [42] for shorter pulses (dotted line, Figure 1(b)).…”

“…The second is by irradiating a metal-salt solution to produce reducing agents via solvent-molecule photolysis [14][15][16]. Controlling nucleation and growth of the nanoparticles during metal-salt reduction by changing laser parameters (focusing conditions, pulse duration, pulse energy, irradiation time), and chemical parameters (metal-ion concentration, solvent composition, presence of capping agents), determines the size, shape, and stability of the colloidal products [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. Laser-assisted AuNP fabrication requires a simple setup, which facilitates experimentation [12].…”