Film Thickness and Surface Plasmon Tune the Contribution of the SFG Signals from Buried Interface and Air Surface

Sum frequency generation vibrational spectroscopy (SFG-VS) is a powerful technique for determining molecular structures at both buried interface and air surface. Distinguishing the contribution of SFG signals from buried interface and air surface is crucial to the applications in devices such as microelectronics and bio-tips. Here we demonstrate that the SFG spectra from buried interface and air surface can be differentiated by controlling the film thickness and employment of surface-plasmon enhancement. Using substrate-supported PMMA films as a model, we have visualized the variations in the contribution of SFG signals from buried interface and air surface. By monitoring carbonyl and C-H stretching groups, we found that SFG signals are dominated by the moieties (-CH2, -CH3, -OCH3 and C=O) segregated at the PMMA/air surface for the thin films while they are mainly contributed by the groups (-OCH3 and C=O) at the substrate/PMMA buried interface for the thick films. At the buried interface, the tilt angle of C=O decreases from 65° to 43° as the film preparation concentration increases; in contrast, the angles at the air surface fall in the range between 38° and 21°. Surface plasmon generated by gold nanorod can largely enhance SFG signals, particularly the signals from the buried interface.