Electroless formation of silver nanoaggregates: an experimental and molecular dynamics approach

Abstract

The ability to manipulate matter to create non-conventional structures is one of the key issues of material science. The understanding of assembling mechanism at the nanoscale allows us to engineer new nanomaterials, with physical properties intimately depending on their structure.

This paper describes new strategies to obtain and characterise metal nanostructures via the combination of a top-down method, such as electron beam lithography, and a bottom-up technique, such as the chemical electroless deposition. We realised silver nanoparticle aggregates within well-defined patterned holes created by electron beam lithography on silicon substrates. The quality characteristics of the nanoaggregates were verified by using scanning electron microscopy and atomic force microscopy imaging. Moreover, we compared the experimental findings to molecular dynamics simulations of nanoparticles growth. We observed a very high dependence of the structure characteristics on the pattern nanowell aspect ratio. We found that high-quality metal nanostructures may be obtained in patterns with well aspect ratio close to one, corresponding to a maximum diameter of 50 nm, a limit above which the fabricated structures become less regular and discontinuous. When regular shapes and sizes are necessary, as in nanophotonics, these results suggest the pattern characteristics to obtain isolated, uniform and reproducible metal nanospheres.

Keywords:

• metal nanoparticles
• superclusters
• electroless deposition
• molecular dynamics simulations
• nanoscale systems

Acknowledgments

M. Monteferrante and L. Chiodo would like to thank G. D’Adamo for useful discussions, L. Ferraro and S. Meloni for providing the code to compute the local bond order parameters, and CASPUR-CINECA (project IscraB_SNaMT) and IIT for computational CPU facilities.

Notes

1. Equation (1) can be obtained from Equation (2) by simple algebra: after product development, the definition of the centre of mass is substituted in Equation (2); then, terms are rewritten, from products of summation to double summation of products; finally, some terms cancel out and the others can be collected to get Equation (1).

2. In the experimental case, we have (from AFM images) a set of points (x, y) on a regular grid and, for each point, a corresponding surface height z: these data can be directly used to compute the power spectrum using Fourier transform. In the case of simulations, since we have atomic positions, to proceed in the same way, at first the height of the external atoms z as function of (x, y) should be interpolated with a smooth surface, f = f(x, y), and then the data should be given on a regular grid in (x, y). The use of a different function defined in real space is in this case more direct.

3. The theorem of addition of spherical harmonics states that, given two unitary vectors , the sum , with being the Legendre polynomial of degree l, depends only on the relative angle .

4. Expanding the exponential in Equation (5) to the first order in q, for low q, only a constant term is retained. Indeed, translational invariance implies and, therefore, .

Additional information

Funding

This work was supported by the EU Commission, the European Social Fund and the Calabria Region under Grant POR Calabria FSE 2007–2013; Italian Minister of Health [grant number GR-2010-2320665]; IIT SEED project ‘SIMBEDD’; SFI [grant number 08-IN.1-I1869]; CASPUR-CINECA under Grant IscraB_SNaMT.