Advanced organic/inorganic materials for applications including solar energy conversion and optoelectronics require the creation of interfaces with carefully controlled structural and electronic properties. Functional monolayers at these interfaces can have associated electronic states or dipole moments and thus provide a new degree of control over device properties. IRG 2 has already shown how functional interfaces incorporating electron acceptors and reconfigurable molecular dipoles can modify the optoelectronic properties of field-effect transistors (Adv. Mater. 20, 4180 (2008) and Adv. Mater. 19, 4353 (2007)). The precise formation of interfaces, remains challenging because the process is conducted in a solvent solution. We have addressed this problem by with a combined theoretical and experimental study of the interface between nitrobenzene, a prototypical molecule exhibiting a large electric dipole moment, and Si (001).
We find that nitrobenzene forms a monolayer on Si(001), but that this layer lacks long-range order in scanning tunneling microscopy (STM) images (Figure 1). Ab initio molecular dynamics simulations show that the migration of oxygen transforms an initial configuration in which the nitrobenzene molecule bridges a Si dimer into lower energy configurations (Figure 2). The disorder observed experimentally results from the incomplete conversion of the molecules to the lowest energy configuration. Calculated dipole moments of nitrobenzene on Si(001) varied from 0.11 to 0.45 Debye, depending on the molecular configuration, providing the basis for designing devices with well-defined interface electronic properties.
Left: (a) STM image of the Si(001) 2 x 1 surface prior to exposure to nitrobenzene. (b) STM image after exposure to 30 Langmuir of nitrobenzene. The inset arrow in (b) indicates the direction of the rows of nitrobenzene molecules, which are perpendicular to the substrate dimer rows.
Right: Potential energy surface of the structural evolution of nitrobenzene adsorbed on Si(001) at 1 ML coverage. Numbers in blue give the relative heights of the topmost H of the phenyl ring referenced to the height of that atom in the NO2 configuration. Numbers in black and red give the activation energy barrier in eV for the transformations between configurations shown in the insets.
G. Peng, S. Seo, R. E. Ruther, R. J. Hamers, M. Mavrikakis, and P. G. Evans, “Molecular-Scale Structure of a Nitrobenzene Monolayer on Si(001),” J. Phys. Chem. C, in press (2011). Available online through J. Phys. C. ASAP at http://pubs.acs.org/doi/abs/10.1021/jp1069434