Discrimination of functional groups with scanning tunneling microscopy using chemically modified tips: Recognition of ether oxygens through hydrogen bond interactions

Gold tips modified with self-assembled monolayers were used for the scanning tunneling microscopy (STM) observation of monolayers of dihexadecyl ether and 1,11-bis(hexadecyloxy)undecane physisorbed onto highly ordered pyrolytic graphite. The ether oxygens gave dark lines in STM images measured with unmodified and thiophenol-modified tips. In contrast, they appeared as bright lines in images observed with tips modified with 4-mercaptobenzoic acid, allowing the selective recognition of ether oxygens. These changes in image contrasts for ether oxygens appear to result from the facilitation of electron tunneling by hydrogen bond interactions between the ether oxygens of the samples and the carboxyl group of 4-mercaptobenzoic acid on the tips.