Pulsed-laser induced gold microparticle fragmentation by thermal strain

TL;DR

This study tackles the problem of understanding how gold microparticles fragment under ultrafast laser irradiation in liquid. It combines time-resolved x-ray scattering with two-temperature model simulations to map the spatiotemporal heating and structural evolution within single microparticles. The key finding is that fragmentation arises from thermoelastic stress due to sharp front-back temperature gradients and stress confinement, with a fragmentation threshold below $750 \ \mathrm{J/m^2}$; at higher fluence, photothermal effects drive nanocluster formation and more extensive fragmentation, evidenced by a ~10× increase in surface area and fragments around 80 nm, along with transient bubble formation. This work clarifies fragmentation pathways and provides actionable insights for optimizing laser-induced fragmentation to produce targeted nanoscale products in a controlled manner.

Abstract

Laser fragmentation of suspended microparticles is an upcoming alternative to laser ablation in liquid (LAL) that allows to streamline the the delivery process and optimize the irradiation conditions for best efficiency. Yet, the structural basis of this process is not well understood to date. Herein we employed ultrafast x-ray scattering upon picosecond laser excitation of a gold microparticle suspension in order to understand the thermal kinetics as well as structure evolution after fragmentation. The experiments are complemented by simulations according to the two-temperature model to verify the spatiotemporal temperature distribution. It is found that above a fluence threshold of 750 J/m$^2$ the microparticles are fragmented within a nanosecond into several large pieces where the driving force is the strain due to a strongly inhomogenous heat distribution on the one hand and stress confinement due to the ultrafast heating compared to stress propagation on the other hand. The additional limited formation of small clusters is attributed to photothermal decomposition on the front side of the microparticles at the fluence of 2700 J/m$^2$.

Figures (10)

• Figure 1: a) Scanning electron microscopy image of a powder of quasi-spherical gold microparticles. b) X-ray scattering distribution of an aqueous colloid containing 400 mg/l gold microparticles with full scattering including the water phase in blue, the extracted scattering from the gold particles in black, mainly showing the (111) and (200) gold powder peaks at 2.67 and 3.1 Å$^{-1}$ , respectively. Upon laser excitation the powder peaks shift transiently and eventually are reduced in intensity as marked by the difference scattering at 45 ps delay between laser and x-ray pulses in red, respectively.
• Figure 2: Numerical simulation (cross section through the center) of the heat transfer within a single gold microparticle upon irradiation from the top at a simulated fluence of 85 J/m$^2$. The temperature is color coded, reaching 1170 K rise within 100 ps in a top layer facing the laser footprint (a), with full melting assumed, if the temperature exceeds a rise of 1488 K (see text) and in a superheated/partially molten state, if the temperature exceeds a rise of 1040 K to the melting point of bulk gold (b). The panels c) - e) show the temperature flow through the particles and later cooling at a delay of 1 ns, 10 ns and 100 ns, respectively.
• Figure 3: Time-resolved lattice expansion at several incident laser fluences of 320, 1600 and 2700 J/m$^2$ together with simulations of temperature rise $\Delta$T at corresponding fluences of 85 and 425 J/m$^2$ taking into account only parts of the volume showing a temperature increase of below 1040 K. At the simulated curve at 425 J/m$^2$ the volume fraction below a rise of 1488 K is too small for reliable averaging between 800 ps and 200 ns, thus marked by a dashed line at the maximum observable crystal temperature.
• Figure 4: Numerical simulation of the melting state of a single gold microparticle as function of delay at a laser fluence of 425 (a - c) and 717 J/m$^2$ (d-f) with the color coding from fig. \ref{['map85']} marking fully molten regions in blue and superheated regions in light blue.
• Figure 5: a) Powder profiles of the (111) reflection of gold as function of delay at 2700 J/m$^2$. The lines are fits with a sum of Gaussian peaks. The vertical bars mark the position of the cold (111) peak (black) and the position of maximum expansion at the melting point. b) False color map of the extracted temperature-increase probability distribution as function of delay. The color scale maps the spectral weight of each temperature bin, see text for details.