VISCOELASTIC AND ELECTRICAL-PROPERTIES OF SELF-ASSEMBLED MONOLAYERS ON AU(111) FILMS

The interaction between Pt-13% Rh tips (of approximately 1000 angstrom radius) and (111) oriented gold films covered with alkanethiolate monolayers has been studied with an atomic force microscope (AFM) of the interferometer type. The thiol molecules, with structural formula H-S-(CH2)n-CH3 had 12 and 22 carbon atom chains (n = 11 and 21). The cantilever of the AFM was forced to oscillate near its resonance frequency while the amplitude and phase of the oscillation at the free end supporting the tip were measured. These two parameters changed rapidly when the tip approached within a few nanometers of the surface. This allows us to investigate the viscoelastic properties of the surface films. We found that the thiolate layers respond to the applied load in a reversible way. Upon contact with the layer, the tip oscillation amplitude decreased to 10-20 % of its original value and remained at this level until the load reached (7 to 15) x 10(-6) N. At loads below 1 x 10(-6) N, the tip-sample interaction is mainly elastic and no visible marks can be seen in the ac-AFM (attractive mode) images. Above that value, the gold substrate was found to yield plastically. This plastic threshold was identical to that found in bare gold. With an applied bias, tunnel current was found upon initial contact of the tip with the alkyl chain. This current is very sensitive to defects of the layer and to the compressive force applied by the tip. A theoretical calculation was performed to explain the effects of the adsorbed organic molecules on tunnel currents.