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Session A7 - Clusters I: A Look at Molecular Surfaces.
MIXED session, Monday morning, March 18
Room 261, America's Center

[A7.01] A Study of the Dopant Driven Transition of an Insulating Film of Molecular Icosahedra into a Semimetallic Phase

D. N. McIlroy, C. Waldfried, P. A. Dowben (Dept. of Physics and Astronomy, University of Nebraska-Lincoln), D. Heskett (Dept. of Physics and Astronomy, University of Rhode Island)

Using photoemission, inverse-photoemission, and electron energy loss spectroscopy we have observed the transition of a molecular film of orthocarborane (C_2B_10H_12) from an insulating phase to a semimetallic phase with Na doping. The film is semiconducting at low Na concentrations (x<2.5) and is metallic-like at higher Na concentrations. Two new occupied states are observed in the gap upon doping. Simultaneously, a new unoccupied density of states is also observed in the gap. In contrast to alkali doping of C60, the semimetallic phase of this system is achieved through the crossover of unoccupied states from above the Fermi level to below the Fermi level. The closing of the gap from approximately 11 eV to approximately 1.75 eV observed with photoemission and inverse-photoemission is supported by electron loss spectroscopy. The behavior of this system is very similarly to the theoretically predicted behavior of an infinite-dimensional Hubbard system as a function of decreasing correlation U.

Part A of program listing