Polarization and phase characteristics of nonresonant sum-frequency generation response from a silicon (111) surface.

Silicon (111) [Si(111)] surfaces both with and without a thin film of polystyrene are investigated with sum-frequency generation (SFG) spectroscopy with s-polarized visible and p-polarized infrared inputs. On uncoated Si, the nonresonant polarization changes from s to p, with the maximum signal in each polarization component repeating every 60° of sample rotation. With polystyrene on Si(111), the resonant features go in and out of phase with the nonresonant signal every 120°. The resonant response is only s polarized, as expected; however, the nonresonant response again switches between s and p polarizations. Implications for proper collection and interpretation of SFG spectra from crystalline substrates are discussed.

Plasma treatment of polystyrene thin films affects more than the surface.

Plasma treatment of polymer materials introduces chemical functionalities and modifies the material to make the native hydrophobic surface more hydrophilic. It is generally assumed that this process only affects the surface of the material. We used vibrationally resonant sum-frequency generation spectroscopy to observe changes in the orientation of phenyl groups in polystyrene (PS) thin films on various substrates before and after plasma treatment. VR-SFG selectively probes regions of broken symmetry, such as surfaces, but can also detect the emergence of anisotropy. On dielectric substrates, such as fused silica, the spectroscopic peak corresponding to the symmetric stretching (ν2) mode of the phenyl rings was undetectable after plasma treatment, showing that surface phenyl rings were altered. This peak also diminished on conducting substrates, but the intensity of another peak corresponding to the same mode in a bulklike environment increased significantly, suggesting that plasma treatment induces partial ordering of the bulk polymer. This ordering is seen on conducting substrates even when the polymer is not directly exposed to the plasma. Annealing reverses these effects on the polystyrene bulk; however, the surface phenyl rings do not return to the orientation observed for untreated films. These results call into question the assumption that the effects of plasma treatment are limited to the free surface and opens up other possibilities for material modification with low-temperature plasmas.