Emission of nanoparticles during ultrashort laser irradiation of silicon targets

Abstract

We have experimentally observed that nanoparticles emission is a peculiar feature of matter removal during ultrashort (fs) laser pulse irradiation of silicon followed by vacuum expansion. Time-resolved optical emission spectroscopy has been used to characterize the plume expansion dynamics, while atomic force microscopy analysis of silicon nanoparticles deposited after 100 fs laser ablation has shown an average particle size of ≈ 7 nm, with a pretty narrow size distribution. This is in agreement with the predictions of recent theoretical models on the mechanisms underlying ablation of solids in this ultrashort temporal regime. Finally, a phenomenological model of nanoparticles cooling during their vacuum expansion has allowed us to single out radiative cooling as the most effective cooling mechanism in the late stage of plume expansion.

Droplet evolution in expanding flow of warm dense matter

“…For Si nanoparticles, the same estimate yields T rad ≃ 2190K. This is consistent with the measurements reported in [12] where the cooling of Si nanoparticles formed by laser ablation is found to be well explained by radiation at expansion times 5 − 150µs and for temperatures below 2000K, although the evaporative cooling rates that we obtain within our model are significantly larger than the estimates they report. As an example, for Si at 2000K, our crude model predicts a radiative cooling rate of ≃28K/µs, while the evaporative coling rate in vacuum is still ≃6.5K/µs.…”

“…Recently, there has been significant progress in the observation of those two-phase flows, from the early ablation and plume expansion stages in the the ps and ns timescales [8][9][10][11] to the late µs timescale evolution including "post-mortem" analysis of the clusters [12,13].…”

Growth and magnetic characterization of Co nanoparticles obtained by femtosecond pulsed laser deposition

“…We also performed a similar statistical on AFM images of selected samples, yielding similar results except for the small diameter range, where the convolution effect with the AFM tip radius ͑ϳ10 nm͒ affects the results. Although previous studies reported a characteristic particle diameter between 5 and 10 nm, 13,24,25 our experimental data do not show a peak in the particle size distribution, but rather a continuous increase in the number of particles as their size is reduced until the resolution of the available images prohibits determining such statistics. Another factor playing a role could be the different optical, thermal, and rheological properties of the materials studied in this work compared with Co, being therefore likely to produce slightly different particle size distributions.…”

“…On the other hand, several groups have recently started to study the deposition and physical properties of nanoparticles obtained by fs-PLD from a variety of targets, such as Ge, 22 Permalloy, 23 Si, 24,25 Ti, 26 Al, 27 Ni, 28,29 or Fe. 30 The common feature is that different materials seem to behave roughly the same when irradiated with intensities around the threshold of plasma formation, typically 10 12 -10 13 W / cm 2 , leading to nanoparticle formation, with radii of tens of nm.…”

Mechanisms of Nanoparticle Formation by Laser Ablation

“…This approach is appropriate for interpretation of the experimental findings obtained in laser ablation with hundred of nanoseconds and longer laser pulses, and in the presence of a background gas (Ohkubo et al, 2003;Boldarev et al, 2001). In the PLA with shorter laser pulses, however, clusters can be ejected directly from the target as a result of the target disintegration by laser-induced explosion-like process (Bulgakov, 2004;Amoruso et al, 2004;Zhigilei, 2003). In this case, the common thermal desorption and condensation model is insufficient and only a detailed molecular-level simulation can provide a complete description of the nanoparticle formation process.…”