Chemically synthesized gold nanoparticles embedded in a ${\mathrm{SiO}}_{\mathbf{2}}$ matrix: A model system to give insights into nucleation and growth under irradiation

The control over the size and the dispersion of the ion beam-synthesized nanoparticles is generally poor. This is partially due to the scarce knowledge of the behavior of the precipitate phase in the first stages of the nucleation and growth processes. To overcome this difficulty, we explore the formation and growth under MeV ion irradiation of satellites clusters around chemically synthesized Au particles embedded in a silica matrix. Our results show that the satellites evolve in an Ostwald ripening regime limited by the diffusion in an open system. In addition, the complete evolution of the gold supersaturation is obtained and the temporal window for the nucleation regime is determined. It allows us to give a guideline method to improve the control of the particles monodispersity during the ion beam synthesis. Moreover, an estimation of the threshold concentration for nucleation, an effective value for the surface tension of the Au nanoparticles and the gold diffusivity under irradiation are given.

Figure 1

[(a)–(c)] TEM micrographs of Au NCs irradiated at increasing fluences with $4\mathrm{MeV}$ Au ions at room temperature. (c) The circle represents an eyeball separation of the areas of growth of the two generations of satellites.

Figure 2

Size distributions of satellites as a function of the irradiation fluence.

Figure 3

Evolution of the satellite density with irradiation fluence.

Figure 4

Evolution of the satellites mean size $\left(R\right)$, square $\left(R^2\right)$, and cube $\left(R^3\right)$ as a function of the irradiation fluence.

Figure 5

Schematic representation of [(a)–(c)] implantation and [(d)–(g)] nanoimplantation. (g) In an open system the NC can be replaced by a fictive solute reservoir.

Figure 6

(a) Evolution of the nucleation fraction of the satellites with fluence, i.e., the fraction of satellites smaller than $1\mathrm{nm}$. (b) The evolution of the Au solute concentration with fluence can be estimated by considering two opposite terms: (i) The full black circles represent the continuous monomers addition into the matrix due to the irradiation-induced NC dissolution. (ii) The full gray circles represent the evolution of solute concentration into the matrix above the nucleation threshold. The difference between the two curves is related to the amount of solute which is absorbed by the growing nanoparticles. Finally, the dashed line describes the complete evolution of the Au solute concentration with fluence.

Figure 7

The full circles represent the experimental threshold values for gold nucleation in silica within the temperature range $500–800°\mathrm{C}$, as taken from Ref. 4. The extrapolation of these to lower temperatures is given by the dotted line. The open circles represent our estimates at room temperature.

Figure 8

Monte Carlo simulation showing the decrease of the surface tension under irradiation ${\sigma }^{MC}$ with respect to the equilibrium value $\sigma$, as a function of the ballistic force $u$, for different values of the mixing length $\lambda$, from Ref. 39.

Figure 9

(a) Bright-field image of a sample irradiated with a fluence of $1\times {10}^{16}{\mathrm{cm}}^{-2}$. The maximum satellite-clusters distance is indicated by the dotted circle (b). Increase of the maximum satellite-clusters distance (as calculated from the original cluster surface) as a function of the fluence of the irradiating ions.