Electronic and mechanical characterization of self-assembled alkanethiol monolayers by scanning tunneling microscopy combined with interaction-force- gradient sensing

We have used scanning tunneling microscopy to study self-assembled monolayers of mercaptohexadecanol in ultrahigh vacuum. In addition to tunneling, the interaction force gradient acting between tip and sample was measured. Analysis of the force-gradient data shows that the tip is in mechanical contact with the surface of the monolayer which, in turn, is elastically compressed. The lateral dimensions of the mechanical contact are substantially (approximately five times) larger than the width of the tunneling-current filament. The results suggest that the compression of the monolayer constitutes an integral part of tunneling through the molecules. This view is further supported by spectroscopic measurements showing that the density of states at the Fermi level as seen by tunneling depends on the initial tunneling voltage used to define the gap width. © 1993 The American Physical Society.