The nonmetal to metal transition with alkali doping of films of molecular icosahedra

Using photoemission, inverse photoemission, and electron energy loss spectroscopy, we have probed the changes in the electronic structure of molecularly adsorbed closo-1,2-dicarbadodecaborane (closo-C2B10H12 or orthocarborane) films as a function of Na doping. The insulator gap of this film of molecular icosahedra closes with Na doping and both new occupied and unoccupied states are introduced. The Na-doped system resembles a Mott-Hubbard insulator, even at high doping levels, and demonstrates the defining role of correlation.