One of the central themes underlying the long-standing and continuing interest in nanomaterials is the recognition that the physical properties of nanoscale materials can be qualitatively different from the properties of bulk materials. Conceptually these differences can arise because some of the features that are negligible in bulk dominate the characteristics of nanoscale materials. One very well-known example of this type of effect is the surfacedominated behavior of nanoparticles as compared to bulk systems. The fraction of atoms at the surface of a nanoparticle scales as a/L, where a is the atomic spacing, typically on the order of 0.5 nm, and L is the size of the nanoparticle. For macroscopic values of L, the fraction of surface atoms is negligible. However, as L approaches the interatomic spacing, the fraction of surface atoms can become very large. The characteristic properties of these surface atoms has
been associated with the large chemical reactivity of gold nanoparticles,1-4 with the development of magnetism in semiconducting oxide nanoparticles,5-8 and the very high catalytic activity of nanoparticles,9-13 features that are absent in bulk materials.