Conductance states of molecular junctions for encoding binary information: a computational approach

Electronic devices, for logical and memory applications, are constructed based on bistable electronic units that can store binary information. Molecular electronics proposes the use of single molecules?with two distinctive states of conductance?as bistable units that can be used to create more complex electronic devices. The conductance of a molecule is strongly influenced by the contacts used to address it. The purpose of this work is to determine the electrical characteristics of several candidate molecular junctions, which are composed of a molecule and contacts. Specifically, we are interested in determining whether binary information, ?0? or ?1,? can be encoded in the low- and high-conductance states of the molecular junctions. First, we calculate quantum-mechanically the electronic structure of the molecular junction. Second, the continuous electronic states of the contacts, originated from their infinite nature, are obtained by solving the Schr?dinger equation with periodic boundary conditions. Last, the electron transport through the molecular junctions is calculated based on a chemical interpretation of the Landauer formalism for coherent transport, which involves the information obtained from the molecule and the contacts. Metal-molecule-metal and metal-molecule-semiconductor junctions are considered. The molecule used is an olygo(phenylene ethynylene) composed of three benzene rings and a nitro group in the middle ring; this molecule is referred hereafter as the nitroOPE molecule. Gold, silicon, and metallic carbon nanotubes are used as contacts to the molecule. Results from the calculations show that the molecular junctions have distinctive states of conductance for different conformational and charge states. High conductance is found in the conformation in which all the benzene rings of the nitroOPE are coplanar. If the middle benzene ring is made perpendicular to the others, low conductance is found. Also, the negatively charged junctions (anion, dianion) show low conductance. Whenever a semiconducting contact is used, a flat region of zero current is found at low bias voltages. The results indicate that the use of Si contacts is possible; however, because of the flat region, the operating point of the devices needs to be moved to higher voltages.