Kelvin probe and scanning tunneling microscope characterization of Langmuir-Blodgett sapphyrin films

The work function of solid layers of increasing thicknesses of E2M8- sapphyrin, deposited on a gold substrate by the Langmuir-Blodgett method, has been measured by the Kelvin-probe technique. The results show that the contact-potential-difference values depend upon the layer thickness, reaching saturation after a certain amount of deposited sapphyrin. Scanning tunneling microscope images taken at the same coverages show that corresponding with this threshold, sapphyrin forms a true continuous layer on gold, completely covering the substrate. Evolution of the layer towards its completion is accompanied by a continuous variation of the work-function value, consistent with an increasing dipole term due to the interaction of sapphyrin with the metal substrate. (C) 1999 American Institute of Physics. [S0003-6951(99)04235-7].