Supporting Information Wiley-VCH 2007 69451 Weinheim, Germany
Crossover Site-Selectivity in the Adsorption of the Fullerene Derivative PCBM on Au(111) David Écija, a Roberto Otero, a Luis Sánchez, b José María Gallego, c Yang Wang, d Manuel Alcamí, d Fernando Martín, d Nazario Martín, b * Rodolfo Miranda a * a Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid (Spain), e-mail: rodolfo.miranda@uam.es b Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Ciudad Universitaria s/n. 28040 Madrid (Spain); Fax: (+) 34913944103; e-mail: nazmar@quim.ucm.es. c Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas. Cantoblanco, 28049 Madrid (Spain). d Departamento de Química, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid (Spain); e-mail: fernando.martin@uam.es. 1
Experimental Section PCBM was synthesized by following the previously reported procedure. [ 1 ] Preparation of thin PCBM films and STM experiments were carried out in a Ultra-High Vacuum (UHV) chamber with a base pressure of 5 10-10 Torr, and equipped with standard facilities for metal surface preparation, a lowtemperature effusion cell and an Aarhus-type variable-temperature, fast scanning STM purchased from SPECS. Atomically clean, crystalline Au(111) surfaces were prepared by standard sputter/anneal procedures (sputter at 1 kv for 15 min followed by annealing to 800 K for another 15 min), which resulted in large terraces (about 200 nm wide), separated by monoatomic steps and displaying the 22 3 herringbone reconstruction. PCBM was deposited from a resistively heated glass crucible at 400 K onto the clean Au(111) substrate held at room temperature. Once the sample has been prepared by the aforementioned protocol, it is transferred to the STM, which has been previously cooled to 150 K. Measurements are carried out at low temperatures in order to freeze molecular diffusion. Tunneling conditions are chosen so as to not disturb individual PCBM molecules (V 1 V bias and I 100-500 pa). Theoretical calculations were performed in the framework of the density functional theory (DFT) with the B3LYP fuctional for exchange and correlation. Due to the large size of PCBM, all calculations were done for molecules in the gas phase without considering the surface. Geometry optimizations for PCBM monomer and dimers were performed with 6-31G* basis set, which has been shown to provide accurate geometries and relative energies of fullerenes and their derivatives. [ 2] The geometries of PCBM tetramers were optimized with the smaller 3-21G basis set. All calculations have been carried out with the Gaussian98 program. [ 3] The herringbone reconstruction. The reconstruction of Au(111), usually referred to as herringbone or 22x 3, has its origin in an uniaxial contraction along the close-packed direction in which 23 atoms are distributed among 22 bulk sites, resulting in the formation of regions where the surface atoms occupy, alternatively, fcc and hcp sites separated by regions (called dislocation lines or domain walls), where the atoms are out of registry with the substrate. In addition, a more isotropic stress relief is obtained by the formation of stress domains in which the contraction alternates by 120º between two equivalent domains. These stress domains give rise to the periodic network of surface lattice dislocations that make up the herringbone pattern. The places where the dislocation lines meet are the elbows of the reconstruction. The appearance of the dislocation lines alternates from one to the other. In the y-type dislocation [1] J. C. Hummelen, B. W. Knight, F. LePeq, F. Wudl, J. Yao, C. L. Wilkins, J. Org. Chem., 1995, 60, 532-538 [2] S. Díaz Tendero, M. Alcamí, F. Martín, J Chem. Phys. 2003, 119, 5545. [3] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. M. Jr., R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, P. Salvador, J. J. Dannenberg, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian 98, Revision A.11, Gaussian, Inc., Pittsburgh PA, 2001. 2
line the elbows are all smooth and round, while in the x-type, which have an additional point dislocation at each bend, the elbows alternatively seem to bulge out the dislocation line (pinch-out elbow) or sink in (pinch-in elbow) [ 4 ]. While for many systems preferential nucleation takes place equally in all the elbows (both pinch-in and pinch-out ) of the x-type dislocation lines, PCBM shows a pronounced preference for nucleating only at the pinch-out elbows of the x- type dislocation lines. Figure S-1. STM images of the herringbone reconstruction. [4] a) J. V. Barth, H. Brune, G. Ertl, R. J. Behm, Phys. Rev. B 1990, 42, 9307-9318; b) D. D. Chambliss, R. J. Wilson, and S. Chiang, Phys. Rev. Lett. 1991, 66, 1721; c) W. G. Cullen, P. N. First, Surf. Sci. 1999, 420, 43. 3
Figure S-2. STM image of the Au(111) surface after depositing 0.2 ML of PCBM. The white lines in (b) indicate the close-pack directions of the surface. The surface has been drawn with different color scales in order to highlight (a) the exclusivity of the preferential nucleation at the fcc areas, and (b) the molecular arrangement within the zigzag structures. Note how the individual rows are parallel (±15º) to a close-packed direction of the Au surface Figure S-3. Detail of the nanoscale spiderweb showing intramolecular resolution 4
Figure S-4. Proposed model for the molecular arrangement in the low coverage phase. The C 60 cages double rows are located within the fcc areas, while the tails of the molecule avoid the dislocation lines. 5