Optoelectronic properties of clusters and small supported particles

Optoelectronic properties of clusters and small supported particles 

It is a well-known fact that, as the size of a metal particle is decreased, the overlap of the bands of valence electrons, with which we are mainly concerned, diminishes, and finally they are replaced by discrete energy levels characteristic of the isolated atom. This results in the loss of electrical conductivity and in the Mie plasmon resonance, an effect that has been noted with the AU55 and smaller clusters, on the basis of which they were described above as being molecular . The extent of band overlap is temperature-sensitive because of thermal excitation, i.e. bands tend to convert to levels as temperature falls thus metallic properties may be seen at high temperature and insulator properties at low temperature. As an approximate guide we may take the relation 

It has been a major point of interest in the study of small metal particles to determine the precise point (if indeed there is a precise point) at which metallic character is lost. The broader context of this problem as it relates to other metals such as mercury and sodium has been discussed in a series of important papers by Peter Edwards and his associates.135,136 The difficulty seems to be that there is no agreed criterion by which membership of the metallic state can be judged, and various physical techniques give somewhat different answers because they sense slightly different aspects of electron behaviour. The question has been earnestly addressed in the case of gold, partly because of the familiar sensitivity of catalytic activity to particle size as noted above (Section 2.1) the way in which electrons are used to form metallic bonds determines the character of the free valences at the surface, and hence the kind of chemisorption bond that is formed with the reactants. 

There have been quite a number of studies of small gold particles by XPS,7,54,80,92,95,137-139 and it is almost always found that the binding energy of the 4f7/2 core level increases by 0.8-1 eV as the size falls below 

Diameter (nm) Height (nm) Layers No. of atoms Band gap (eV) Type 

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