ABSTRACT
Molecular electronics is the research field that deals with the design and implementation of electronic devices that rely on a single or a few molecules. The idea to use single molecules as elementary electronic building blocks has been put forward in 1974 by Aviram and Ratner in a seminal paper [1]. It is evident that a proper understanding of the properties of an individual molecule is of utmost importance for molecular electronics. The transport properties of a single molecule are evidently the most relevant properties for basically all molecular electronics applications [2-9]. Measuring the resistance or conductance of a single molecule seems trivial: One connects both ends of the molecule to macroscopic electrical contacts and records a current-voltage (I-V) trace. However, there are several challenging hurdles that have to be overcome before a successful measurement can be executed. One cannot simply take two alligator clips and connect them to both ends of the molecule. The size of a molecule is of the order
of a nanometer, and therefore one has to apply clever tricks to capture a molecule between two macroscopic electrical contacts. In the next section, we will briefly discuss several methods that have been applied to “catch” a single molecule between two macroscopic electrical contacts. Once the molecule is properly contacted, the transport experiment is rather straightforward; however, the interpretation of the current-voltage traces is far from trivial. The molecular orbitals of the molecule can hybridize with the electronic states of the contacts leading in general to shifts and broadening of the molecular orbitals. Rather than measuring the conductance of a single molecule, one measures the conductance of the complete contact-molecule-contact junction. As pointed out by K. W. Hipps in 2001 in a Science article entitled “It’s all about the contacts” molecular electronics is mainly a “contact” problem [10]. Regarding the properties of single molecules, we will restrict ourselves in this contribution to transport properties only. It should be pointed out here that recently many studies in the field of molecular electronics have been performed that go beyond the electronic transport characterization of single molecules. These studies involve thermoelectric, optoelectronic, mechanical, and spintronic phenomena. We would like to emphasize that these studies fall outside the scope of this chapter.
