Scanning surface harmonic microscopy of self-assembled monolayers on gold

Abstract

Adsorption of self-assembled monolayers on gold surfaces greatly enhances the local probe-induced third-harmonic (TH) generation at microwave frequency. At a given location the TH signal is, as a function of gap distance, proportional to the tunneling current. A constant TH signal image of self-assembled monolayers made from 11-mercaptoundecanol shows the same depression pattern with similar resolution as that obtained with tunneling microscopy. The tunneling current remains constant across the depressions at constant TH signal. However, the TH signal is enhanced on certain protrusions of the monocomponent layers and on net-like patterns on the layers made from mixtures of perfluorononanoyl-2-mercaptoethylamide and 1-mercaptododecane.