The metallicity of thin films and overlayers

Thin-film materials, normally considered metals, can exhibit behavior that is very nonmetallic. There is an increasing body of experimental information that this phenomenon is quite general at submonolayer coverages. With increasing coverage and/or temperature there is a nonmetal to metal transition. This unusual behavior can occur even for films deposited on metal surfaces. Insight and identification of this electronic phase transition originates from several experimental probes of metallicity. Among the several techniques that have now been applied to this problem are angle-resolved photoemission, core level photoemission, resonant photoemission, inverse photoemission, high resolution electron energy loss, scanning tunneling microscopy, electron and atom beam scattering and optical techniques such as second harmonic generation and optical conductivity. These techniques are often consistent and suggestive of a many body transition that can, in some respects, be discussed in the context of the Mott–Hubbard transition. The results lead to the interesting speculation that many metal overlayers may be nonmetallic. A number of aspects of the nonmetal to metal transition in thin films and overlayers can be compared with measurements probing the metallicity of free clusters.