Can Single-Atom Change Affect Electron Transport Properties of Molecular Nanostructures such as C60 Fullerene?

At the nanoscale, even a single atom change in the structure can noticeably alter the properties, and therefore, the application space of materials. We examine this critical behavior of nanomaterials using fullerene as a model structure by a first-principles density functional theory method coupled with nonequilibriumGreen's function formalism. Two different configurations, namely, (i) endohedral (B@C60 and N@C60), in which the doping atom is encapsulated inside the fullerene cage, and (ii) substitutional (BC59 and NC59), in which the doping atom replaces a C atom on the fullerene cage, are considered. The calculated results reveal that the conductivity for the doped fullerene is higher than that of the pristine fullerene. In the low-bias regime, the current (I) voltage (V) characteristic of the endohedral as well as the substitutional configurations are very similar. However, as the external bias increases beyond 1.0 V, the substitutional BC59 fullerene exhibits a considerably higher magnitude of current than all other species considered, thus suggesting that it can be an effective semiconductor in p-type devices. SECTION Nanoparticles and Nanostructures M olecular nanoscale electronic devices have attracted a great deal of attention in recent years. Experimental and theoretical studies aimed at understanding the underlying physics of molecular and nanoelectronic devices have begun to shed light on electron transport mechanisms through molecules and engineered nanomaterials and help identify appropriate molecular and nanoscale architectures for effective functional elements in electronic devices. Among others, carbon fullerene is one of the most stable and well-known nanoscale molecular structures. Furthermore, its structure and electron transport properties have been the subject of extensive studies for its potential applications as spin valves and electro-mechanical amplifiers to name just a few. Carbon fullerenes are composed of a sheet of linked hexagonal rings separated by pentagonal (sometimes heptagonal) rings that help curve the structure into a spherical empty cage. By far the most common one is the Buckminster fullerene, C60, discovered accidently at Rice University in the late 1980s. It has a high symmetry of icosahedra, Ih, in which all the C atoms are equivalent with sp hybridization. C60, however, does not exhibit “superaromaticity”, i.e., the electrons in the hexagonal rings do not delocalize over themolecule. Therefore C60 often acts as a semiconductor quantum dot with an energy gap of about 1.5 eV. SinceC60 is a true nanoscale singlemolecule, its electronic structure and energy levels can be considerably affected by modifying a single atom in its structure. Thus, doping of fullerenes canbegenerally expected to change their electronic conductivity due to modification in the density of states near the Fermi surface. Indeed, a recent experimental study has shown that the NC59 molecule acts as a molecular rectifier in a double barrier tunnel junction via the single electron tunneling effect. Following the semiconductor analogy, onemay find that B andN atoms substituting C atoms in the fullerene cage can act as an “acceptor” and a “donor”, respectively, thus modifying Received Date: March 18, 2010 Accepted Date: April 27, 2010