Abstract
ABSTRACTWe use electrochemical methods to control the adsorption of molecules onto an electrode for imagingin-situby scanning tunneling microscopy. Measurements of the barrier for electron tunneling show that the mechanism of electron transfer differs from vacuum tunneling. Barriers depend upon the direction of electron tunneling, indicating the presence of permanently aligned dipoles in the tunnel gap. We attribute a sharp dip in the barrier near zero field to induced polarization. We propose a ‘tunneling’ process consisting of two parts: One is delocalization of quantum-coherent states in parts of the molecular adlayer that hybridize strongly (interaction ≥kT) with Bloch states in the metal. This gives rise to a quantum-point-contact conductance,Gc≤ 2e2/hat a height zo. The other part comes from the exponential decay of the tails of localized states, G =Gcexp{−2K(z − z0)}. Because measured decay lengths, (2K‘)−1, are small (≈ 1 Å), STM contrast is dominated by the contour along which G[z0(x,y)] = Gc. Measured changes in z0are used to calculate images which are in reasonable agreement with observations. We illustrate this with images of synthetic DNA oligomers.
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