Seed - Aromatic Isocyanide Self-assembled Monolayers on Gold Investigated by Ab-initioCalculations.
Giulia Galli, Chemistry, University of California, Davis.
Galli (in collaboration with Scott, Swanson) has studied the structural, electronic, and vibrational properties of the interface between a gold surface and aromatic isocyanide self-assembled monolayers (SAMs), using density-functional theory. The calculations predict a herringbone arrangement at high coverage, instead of the conventional structure with Ö3xÖ3 periodicity. The most favorable geometry is, however, found at very low coverage, where the interaction between molecules is negligible. The binding energies obtained here are smaller than for benzenethiols and independent of the number of aromatic rings.
In addition, the calculations show that the energy barriers between differently tilted geometries are small compared to room temperature. (Figure 5). These results are consistent with the disordered patterns recently observed in room temperature scanning tunneling microscope measurements (measurements Liu), and point at the presence of SAM structure comprising molecules with different heights on top of the metal surfaces. We have also computed the C-N stretching frequencies for aromatic isocyanides adsorbed on different sites, and observed a blue shift for molecules adsorbed straight on the atop site. This blue shift either vanishes or is reversed for molecules with tilted geometries. These findings are consistent with the existence of two peaks found in experimental vibrational spectra, one centered at the free C-N vibrational frequency and the other at higher frequencies. It would be interesting to investigate whether the relative intensities of these two peaks vary as a function of temperature, thus indicating a tendency to ordering when the sample is cooled down. Likewise, it would be interesting to study whether STM images show a tendency to ordering as T is decreased.
Figure 5. Several possible configurations of PDI on Au.
Although our results on disordered patterns of PDI on Au indicate possible difficulties in using isocyanide SAM for molecular devices, we note that the use of longer and possibly functionalized molecules may induce order of the SAM also at room temperature. Work is in progress to study suitable functionalizations.
The calculations also give insight into the energy level alignment of the molecular orbitals with respect to the gold Fermi level, and the charge redistribution at the interface, which provide essential guidance to understand and predict transport properties of these SAMs. The results show that PDI SAMs enhance hole over electron injection.
