Ion beam shaping of nanostructures
Recently it has been found at the Utrecht University (Netherlands) that spherical gold nanoclusters embedded in silica can be shaped into long rods and even wires by low-dose ion irradiation with energetic ( tens of MeV) heavy ions (see Fig. 1). This result has been reproduced with the Tandem accelerator of the FZD by irradiation of gold particles with 38 MeV 127I7+. More important, at the FZD it has been found that the same ion irradiation conditions deform Ge nanoclusters embedded in SiO2 too, but quite different from Au nanoparticles (see Fig. 2).
|Fig. 1: Ion beam shaping of gold spheres of 15 nm diameter embedded in SiO2. The XTEM image was taken by A. Vredenberg (Utrecht University) after irradiation at 300 K with 2.5x1014 Ag ions per cm2 of 54 MeV kinetic energy. In order to demonstrate the extreme shaping, size and location of the original Au nanospheres are plotted schematically. A fluence of 7.5x1014 Ag ions per cm2 leads to rods or better wires of up to 200 nm length, where the long wires grow on the expense of dissolved shorter ones (not shown here).|
Ge nanospheres become flattened in beam direction by ion irradiation.
So far, there has been no consistent theoretical model of ion beam shaping by swift heavy ions, neither for Au nor for Ge nanospheres embedded in SiO2. Thus, a cellular-automaton-based kinetic Monte-Carlo program, which were developed at the FZD more than 10 years ago and which were applied successfully to many fundamental and applied problems, has been employed to study ion beam shaping of Au with a few basic assumptions:
- Mass transport in the SiO2 layer and in the embedded Au nanoparticles is allowed exclusively within each track of a swift heavy ion which lasts some 10ps only.
- For the ion energies considered here, in SiO2 a cylindrical track of several nanometer diameter becomes molten for the track duration.
- The spatio-temporal temperature profile of tracks which were calculated by Toulemonde for SiO2 is adapted for Au nanoparticle containing SiO2 (critical assumption)
- In a track Au can dissolve in SiO2, and during cooling it can become supersaturated and precipitate according to its temperature-dependent (experimental) solubility.
- The migration energy of Au in SiO2 and along the Au-SiO2 interface were fitted to the experiment and were assumed to be equal.
No further assumptions were made. The Monte-Carlo method describes inherently thermodynamic properties like temperature and concentration dependent diffusivity, temperature dependent surface energy etc.
It was a surprise that the computer experiment with these few and simple assumptions reproduces the real experiment even in details Fig. 3): The sphere prolongs into a rod, the cloud of tiny Au precipitates surrounding the nanoparticles can be seen in the XTEM image as well as the simulation, and very long wires grow in an Ostwald ripening like manner on the expense of smaller ones. Only the experimental fluence is 10 times smaller due to the kinetics in the liquid instead of solid state diffusion used in the simulation. A detailed analysis of the computer experiment reveals that the shaping is due to thermocapillary and ripening comes from the extreme anisotropic diffusion (and therefore Au dissolution) along the tracks.