The energy required to remove an electron from a piece of bulk metal is called the work function (WF). By making the piece of metal smaller and smaller eventually the limit will be reached where you only have one atom left. The energy required to remove an electron from a single atom is the ionization potential (IP). What if you have a small piece of metal, a so called cluster, which is larger than one atom but substantially smaller than what we would normally call a bulk metal. What would be their value of IP or WF?

Traditionally, the work function of metals are measured on a clean surface prepared in ultrahigh vacuum. Minuscule amounts of contaminants on the surface may cause significant errors on the measurement. Moreover, the exact crystallographic orientation of the surface has to be precisely determined. Due to these reasons current published values of the metal work functions are only accurate to within a few percent.

On the other hand, the energy required to remove an electron from a single atom, called ionization potential, has been measured with enormous precision for nearly all elements on the periodic chart. The measurements are done with photoelectron spectroscopy on gaseous atoms of the element of interest. Between these two regimes of the length scale from a single atom to a piece of bulk metal are the nano clusters.

A nano cluster is simply an agglomeration of certain number of atoms to form a particle in the size range of nanometers. As the cluster is made bigger and bigger, eventually the limit will be reached where the cluster is so large that it behaves the same as bulk metal. Similarly, as the cluster is made smaller and smaller we will end up with just one atom. Because the production of clusters is often much easier than the preparation of a clean surface in ultrahigh vacuum, photoelectron spectroscopy on large cluster beams is a convenient alternative method to measure the work function of bulk metals.

By studying the energy required to remove an electron from clusters of different sizes, it's possible to bridge the entire gap from the atom to the bulk.  For example, by analyzing this size dependence in terms of the image charge method, one can explain the very curious empirical observation that the ratio between the ionization potential and the work function for all metallic elements is approximately equal to 2. The study of nano clusters is therefore interesting not only because it will teach us about novel phenomena in the nanometer scale, but it also have the potential to enhance our current understanding of atomic and bulk properties.

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Greg Pawin wrote on 2004-08-04 04:50:42 Hey you didn't tell me about this! This is pretty cool. I would have never put those two ideas together--work function and ionization potential.

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