Voltage-induced chemical contrast in scanning tunneling microscopy using tips chemically modified with hydrogen bond donors

In-situ voltage treatment to induce chemical contrast in scanning tunneling microscopy images has previously been demonstrated only in one case, i.e., with tips modified with a hydrogen bond acceptor. In this work, this method has been applied to induce chemical contrast using tips modified with a self-assembled monolayer of 4-mercaptobenzoic acid, a hydrogen bond donor. Chemical contrast enhancement, which allows recognition of particular functional groups, is ascribed to the chemically selective interaction of the hydrogen bond donating carboxyl group of 4-mercaptobenzoic acid and the hydrogen bond accepting ether oxygens in sample monolayers formed on highly oriented pyrolytic graphite. Surfaces initially showing no chemical contrast enhancement were made to show such contrast enhancement by applying a + 2.0 V pulse to the STM tip for 15 s. The chemical contrast could also be removed from images using 15 s treatments with a voltage of the opposite sign, i.e., - 2.0 V. The effectiveness of inducing chemical contrast using this voltage treatment and the tip lifetime are compared for tips modified with 4-mercaptopyridine and 4-mercaptobenzoic acid.