Scanning Tunneling Microscopy (STM) on Physisorbed Chemical Groups of Individual Immobilized Molecules

Abstract

ABSTRACT: Most scanning tunneling microscopy (STM) studies of adsorbed molecules are performed either on molecules that are chemically bound to the surface or are part of a molecular monolayer. In the first case, the chemical reaction with the surface changes the surface local density of states. In the second one, the electrons tunnel through a physisorbed molecule. In the study described in this paper STM imaging is performed on isolated molecules immobilized on a clean semiconductor surface where atomic resolution is observed. This is an intermediate case.The adsorption behavior of diene molecules on Si(111)7 × 7 was investigated. The molecules adsorb by reaction of the double bonds on the surface. In the reaction, the C=C bond is opened to create two neighboring Si‐C bonds. Two such C‐C bonds are used to immobilize the chain to the surface. The adsorption chemistry is affected not only by the nature of the Si‐C bond but also by steric limitations.Performing STM imaging on such systems ensures that the position of the immobilized chemical group (in this case an aliphatic chain) could be clearly identified and that it was possible to image parts of the molecule that are not chemically bound to the surface. STM images of 1,13‐tetradecadiene revealed a white protrusion in the location of the aliphatic chain. The increase in transmission is explained by an additional channel for tunneling: through bond tunneling. Long‐range electron transfer through molecules is a widely known chemical and biochemical process. This observation opens the possibility to investigate this fundamental phenomenon in large details. The potential applications for molecular electronics are discussed.