Figure 4.10. Density functional calculation of excited state energy levels of Be, Be, 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 (F. J. Owens, unpublished.)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 die cluster. Figure 4.11 shows the UV photoelectron spectrum of die 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 die lowest peak is a measure of die election affinity of the cluster. The electron affinity 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 die 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

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