Low Efficiency of Laser Heating of Gold Particles at the Plasmon Resonance: An X-ray Calorimetry Study

Abstract

Laser excitation of nanoparticles provides an appealing tool for particle engineering as well as nanoscale-localized photothermal material modification. In the case of plasmonic nanoparticles like gold, mostly on-resonant excitation is used as it appears to be the most efficient channel for laser heating. Yet, as will be shown in this paper, in the case of an excitation with picosecond pulse duration a drastic reduction of the energy uptake efficiency of gold nanoparticles is observed at on-resonant excitation with 532 nm compared to interband excitation at 400 nm. This observation is found irrespective of varied gold nanoparticle size or different synthesis methods. Based on the nanocalorimetry of laser-excited suspended gold nanoparticles with time-resolved X-ray scattering the transient particle temperature, heat-induced relaxation and particle expansion and nonreversible particle transformations were disentangled. At first glance, the lower heating efficiency appears related to ultrafast plasmon bleaching and its recovery in the picosecond range. Yet, following a caloric heat balance a significant residual dissipation channel remains in the case of on-resonant excitation, which is absent at interband excitation. Consequently, photothermal applications may suffer from lower heat generation at the expense of irreversible channels.

Physical Regimes and Mechanisms of Picosecond Laser Fragmentation of Gold Nanoparticles in Water from X-ray Probing and Atomistic Simulations

“…At this delay time, the electron−phonon coupling is completed, while the heat dissipation into the water environment is still negligible. 68 The results of the measurements of the lattice expansion performed for both CTAB and PLAL NPs are shown in Figure 1e. To connect the lattice expansion Δa/a 0 to the incident laser fluence F, we solve the following system of equations describing the increase in temperature T and lattice parameter a in response to the laser energy deposition to a NP with radius R:…”

“…The cooling of the NPs due to the heat transfer to the surrounding water, on the other hand, takes place on a longer time scale of hundreds of picoseconds. − Therefore, to probe the energy density deposited by the laser pulse through the corresponding lattice expansion, the delay between the laser pulse and the subsequent X-ray probe pulse is set to 60 ps. At this delay time, the electron–phonon coupling is completed, while the heat dissipation into the water environment is still negligible …”

“…Similar discrepancies between the theoretical Mie spectra and the NP absorption evaluated in pump−probe experiments have been reported in earlier studies. 68 Given this uncertainty, the establishment of quantitative links between experiment and theory is hindered, yet using the reference values of F 0 and ε 0 makes it possible to circumvent the lack of exact knowledge of the absorption cross sections and to align the fluence/energy scales used in the experiments and simulations. The WAXS measurements for CTAB NPs suggest that the scaling of the absorbed laser energy ε with fluence F remains roughly linear, at least up to the melting threshold (Figure 1e).…”

“…The wavelength of the pump pulse, 400 nm, is chosen to match the interband excitation of the NPs, so that efficiency losses due to intrapulse bleaching at the plasmon resonance are minimized. 68,85,86 The experimental setup is schematically illustrated in Figure 1c and described in detail in earlier publications. 36,68 A few key points are briefly outlined below.…”

“…The pump and the probe beams are focused on a free circular jet of a colloidal solution of mono- and polycrystalline gold NPs gained from chemical synthesis (labeled CTAB after the main surfactant) and PLAL, respectively. The wavelength of the pump pulse, 400 nm, is chosen to match the interband excitation of the NPs, so that efficiency losses due to intrapulse bleaching at the plasmon resonance are minimized. ,, The experimental setup is schematically illustrated in Figure c and described in detail in earlier publications. , A few key points are briefly outlined below.…”