The Art of Catching and Probing Single Molecules

Abstract

This thesis presents several studies performed in the field of “molecular electronics” using scanning tunneling microscopy (STM) and spectroscopy. We have investigated electronic (transport) properties of a single octanethiol, a single CuPc molecule and molecular assemblies of s–cyclodextrin (s-CD) molecules immobilized on Pt/Ge(001), Au/Ge(001), and Au(111) substrates, respectively. A single octanethiol molecule is trapped between an STM tip and Pt/Ge(001) substrate, and the transport properties of this junction is investigated. The junction acts as a molecular switch which can be opened and closed by controlling the gap between the electrodes and voltage applied across them. The threshold electric field for the attachment/detachment was found to be 4-6 x 109 Vm-1. We have also studied CuPc molecules adsorbed in a “molecular bridge” configuration on Au-induced nanowires on a Ge(001) substrate, where the core, Cu2+ ion, of the CuPc molecule is decoupled from its environment. The charging of the core takes place at sample biases larger than 3.5 V as revealed from the STM images. Further, our STM measurements on a monolayer of s–CD molecules self-assembled on the Au(111) surface indicate a very rich dynamical behavior of single s–CD molecules. This dynamics is induced by electrons that tunnel inelastically and they are not observed in macroscopic molecular junctions. This indicates that a great care must be taken when interpreting transport measurements data obtained using macroscopic molecular junctions. Above studies corroborates that molecular electronics is very interesting from the fundamental and application point of view as we witness phenomena such as switching, charging and dynamics for various molecular junctions.