A fundamental question has been jointly addressed recently by TP B6 and TP G2 together with the groups of G. Schmid (Essen) and P. Oelhafen (Basel):

Is alloy formation in metallic nanoparticles size dependent?

This question is related to the fact that by reducing the size of a metallic nanoparticle its electronic properties eventually will be dramatically changed from a metallic to an insulating molecular state. Thus, it would have been plausible that these changed electronic properties are reflected as a changed alloy formation behavior.

To test this possibility, a series of ensembles of Au-nanoparticles of different size were prepared on top of Si substrates. Subsequently, 4 monolayers of Indium were evaporated on top of the Au nanoparticles at room temperature (R.T.) and the resulting alloy formation into AuIn 2 , which is expected to occur in this system at R.T., could be monitored in situ by a corresponding chemical shift in the Au-4f-XPS spectra.

The following figure shows the corresponding result:

Comparison to the pure Au reference (lowest spectrum) and the related vertical line indicating the spectral 4f-position for pure Au, clearly demonstrates a chemical shift pointing to an AuIn 2 formation for all sizes of Au nanoparticles (the diameter of the Au nanoparticles is attached ti each spectrum) except for those with a diameter of 1.4 nm corresponding to the magic number clusters of Au-55. Additionally, the letters M and I in the figure indicate the size dependent electronic state of the particles: M for metallic and I for insulating. Obviously, the dramatic difference in electronic properties between “M”- and “I”-particles is nor influencing the alloy formation, which is observed in both cases. It is only the geometrically extreme stability of the Au-55 clusters, which impedes the alloy formation.

More details on this experiments can be found in:

H.-G. Boyen, A. Ethirajan, G. Kästle, F. Weigl, P. Ziemann, G. Schmid, M.G. Garnier, M. Büttner, P. Oelhafen: Alloy formation of supported Gold nanoparticles at their transition from clusters to solids: Does size matter? , Phys. Rev. Lett. 94 , 016804 (2005).