Electronic configurations

Bulk gold is a renowned conductor of electricity, with a conductivity value that is beaten only by copper and silver. This property arises directly from its electronic configuration. However, even this familiar ‘fact’ is overturned at the nanoscale. Gold structures at the bottom end of the nanoscale may, depending on shape and substrate, actually be semiconductor with a significant value of band gap.

The transition occurs somewhere between 1 and 3 nm, corresponding to a hemispherical cluster containing between 15 and 150 atoms. The special electronic configuration of small nanoparticles results, from the fact that their physical dimensions are smaller than the characteristic dimension of the electron wave function of the bulk material. Such tiny particles, termed as quantum dots or artificial atoms if they are disc-shaped, have been proposed as the basis of a new generation of nanoscale electronic devices. Recently the electronic properties of gold nanoparticles have been examined on the basis of density functional theory (DFT). They have shown that for Au8 , the energy difference between HOMO and LUMO is highest and the frontier wave function has considerable mixing of s and d character. The catalytic activities of such exceedingly small clusters have been found to be acutely size dependent, peaking in one example at a cluster diameter of close to 3 nm, and falling sharply within 0.5 nm either side . One interpretation of this is that the best catalytic activity is actually derived from a particular value of the band gap, and too great a gap, or none at all, is less favorable. It should be noted that gold is not unique in this respect, and that other noble metals, such as Pd and Pt, exhibit similar properties.