Molecular-Scale Structure of a Nitrobenzene Monolayer on Si(001)

Nitrobenzene molecules can be attached to Si(001) by exposing the Si(001) (2 x 1) surface to nitrobenzene gas at room temperature. The resulting monolayer lacks long-range order in scanning tunneling microscopy images but shows signs of a local periodic arrangement of molecules. Self-consistent plane wave density functional theory calculations find that the energy gained per molecule in the adsorption of nitrobenzene is nearly constant as a function of nitrobenzene coverage and that it is energetically favorable to form coverages as high as one molecule per Si dimer. Ab initio molecular dynamics (AIMD) simulations show that the migration of oxygen transforms an initial configuration in which the nitrobenzene molecule bridges a Si dimer into lower energy configurations with oxygen separated from the nitrobenzene phenyl group. A previously unknown low-energy configuration, characteristic of SiO2 bonding, is identified by the AIMD calculations. The energy barriers for O migration into Si backbonds are calculated by using the climbing-image nudged elastic band method. The calculated dipole moments of nitrobenzene on Si(001) varied from 0.11 to 0.45 D, depending on the molecular configuration. Nitrogen is observed using X-ray photoelectron spectroscopy in a concentration consistent with the attachment of one nitrobenzene molecule per Si dimer.