Bond-Order Discrimination by Atomic Force Microscopy

Science 14 September 2012: 
Vol. 337 no. 6100 pp. 1326-1329 
DOI: 10.1126/science.1225621

1. Leo Gross¹,*, 
2. Fabian Mohn¹, 
3. Nikolaj Moll¹, 
4. Bruno Schuler¹, 
5. Alejandro Criado², 
6. Enrique Guitián²,
7. Diego Peña², 
8. André Gourdon³, 
9. Gerhard Meyer¹

We show that the different bond orders of individual carbon-carbon bonds in polycyclic aromatic hydrocarbons and fullerenes can be distinguished by noncontact atomic force microscopy (AFM) with a carbon monoxide (CO)–functionalized tip. We found two different contrast mechanisms, which were corroborated by density functional theory calculations: The greater electron density in bonds of higher bond order led to a stronger Pauli repulsion, which enhanced the brightness of these bonds in high-resolution AFM images. The apparent bond length in the AFM images decreased with increasing bond order because of tilting of the CO molecule at the tip apex