| AVS 62nd International Symposium & Exhibition | |
| Nanometer-scale Science and Technology | Wednesday Sessions |
| Session NS+EN+MG+SS+TF-WeA |
| Session: | Nanoscale Catalysis and Surface Chemistry |
| Presenter: | Ehsan Monazami, University of Virginia |
| Authors: | E. Monazami, University of Virginia J.B. McClimon, University of Pennsylvania P. Reinke, University of Virginia |
| Correspondent: | Click to Email |
The interaction of f C60 molecules with metal surfaces is a topic of considerable interest and discussed in the context of molecular electronics and organic solar cell applications. Our work focuses on the interaction of C60 with W, which is a carbide forming transition metal. It has generally been assumed that the C60 cage breaks up readily on a W surface, but our observations reveal a more complex, temperature induced reaction sequence leading to the formation of nanospheres with variable bandgap, which has been studied with in-situ STM and STS analysis.
The nanospheres are synthesized by initiating the reaction between C60 deposited on a W-thin film surface grown on MgO(001) at about 450 K. In contrast to previous reports, the molecules do not collapse but the spherical shape is retained up to 800 K and the electronic structure changes gradually from wide bandgap C60 to fully metallic nanospheres. The size distribution of the nanospheres is centered at 1.5 nm (600 K) and shifts for higher temperatures to about 2 nm with a concurrent decrease in height resulting in an ellipsoidal shape. These nanosheres present an exceptional resistance to sintering which is a unique feature for metallic nanoparticles. The densely packed C60 and isolated C60 molecules show the same transition in shape and electronic structure which confirms that the transformation is controlled by the reaction with the W substrate.
The transition in the electronic structure progresses gradually from the wide-bandgap molecule (2.3 eV) to a bandgap of ~1eV at 700 K, and a metallic surface at 800 K. We will illustrate this progression with a series of ST spectra and maps. The bandgap variation offers for the first time a pathway to the formation of nanoscale clusters (nanospheres) with variable bandgap while retaining the surface curvature in narrow range. The detailed structure of the nanospheres is, however, still subject to discussion. We currently favor, based on our experimental results, a substitution model where the W-atoms from the substrate react with the C60 cage which acts as a scaffold. The W atoms are incorporated substitutionally forming a carbide type bonding, whose presence is indicated by XPS analysis. The variation in bandgap is then driven by the degree of substitution and the fullerene molecule acts as a scaffold. We predict that other carbon scaffolds such as nanotubes and carbide forming transition metals can react in the same manner leading to a new class of nanostructures with unique adaptability of the electronic structure. The nanospheres are an excellent testbed for the physics and chemistry of highly curved surfaces.
Supported by NSF-DMR Ceramics DMR-100580.