| AVS 54th International Symposium | |
| Surface Science | Friday Sessions |
| Session SS1-FrM |
| Session: | Surface Dynamics |
| Presenter: | M.A. Freedman, The University of Chicago |
| Authors: | M.A. Freedman, The University of Chicago S.J. Sibener, The University of Chicago |
| Correspondent: | Click to Email |
We have studied the vibrational dynamics of self-assembled monolayers (SAMs) and polymer thin films, two complex systems of great technological interest, using the highly surface-sensitive and non-perturbative technique of low-energy helium atom scattering. While this technique has traditionally been used for single crystals and simple adsorbates on crystals, recent work in our group and elsewhere has expanded these studies to ordered organic monolayers. We have further broadened the range of studies performed with helium atom scattering to determine how much information can be gleaned from soft, disordered systems. We have shown that this technique provides a novel means for studying surface vibrational dynamics at these complex interfaces. We explored the interaction between the alkanethiol chains of the SAMs and the Au(111) substrate by investigating the energy of single phonon modes as a function of chain length. The fact that the observed phonon mode is dispersionless and exhibits no dependence on chain length indicates that the interaction between the adsorbate and substrate in the sagittal plane is dominated by single-chain harmonic-oscillator-type interactions. The energy of the single-phonon mode of decanethiol is larger than other chain lengths because its commensurate structure to the reconstructed Au(111) surface leads to stronger binding. We investigated the polymer thin film interface to determine the effect of nanoconfinement and polymer species on the vibrational dynamics. We find that helium atom scattering is a sensitive probe of the effect of nanoconfinement on the surface dynamics of poly(methyl methacrylate) (PMMA). Specifically, a reduction in the mean-square displacement derived from the elastic scattering and the reduced propensity for annihilation events in inelastic spectra indicate that thinner films are stiffer, perhaps due to substrate interactions. The inelastic spectra of PMMA, polystyrene, and polybutadiene have similar rates of intensity decay and absolute values of full width half maximum with increasing surface temperature. Deviations from a semiclassical scattering model and the linear increase in full width half maximum with increasing surface temperature indicate changes in the vibrational dynamics. We have shown that helium atom scattering is a sensitive probe of complex systems and may indeed give insight on whether the onset of glassy dynamics in the bulk affects dynamics at the topmost interface.