Introduction to Nanotechnology

4.2. METAL NANOCLUSTERS 83
Figure 4.10. Density functional calculation of excited state energy levels of Be, B8, and BI2
nanoparticles. Photon-induced transitions between the lowest level and the upper levels
determine the color of the particles. (F. J. Owens, unpublished.)
One method of studying the electronic structure of nanoparticles is UV photo-
electron spectroscopy, which is described in more detail in Chapter 3. An incident
UV photon removes electrons from the outer valence levels of the atom. The
electrons are counted and their energy measured. The data obtained from
the measurement are the number of electrons (counts) versus energy. Because the
clusters have discrete energy levels, the data will be a series of peaks with
separations corresponding to the separations of the energy levels of the cluster.
Figure 4.1 1 shows the UV photoelectron spectrum of the outer levels of copper
clusters having 20 and 40 atoms. It is clear that the electronic structure in the valence
region varies with the size of the cluster. The energy of the lowest peak is a measure
of the electron affinity of the cluster. The electron a&i@ is defined as the increase in
electronic energy of the cluster when an electron is added to it. Figure 4.12 is a plot
of the measured electron affinities versus the size of Cu clusters, again showing
peaks at certain cluster sizes.
4.2.5. Reactivity
Since the electronic structure of nanoparticles depends on the size of the particle, the
ability of the cluster to react with other species should depend on cluster size. This
has important implications for the design of catalytic agents.
There is experimental evidence for the effect of size on the reactivity of
nanoparticles. Their reactivity with various gases can be studied by the apparatus
sketched in Fig. 4.2, in which gases such as oxygen are introduced into the region of
the cluster beam. A laser beam aimed at a metal disk dislodges metallic particles that
are carried along to a mass spectrometer by a flow of helium gas. Down stream