Film thickness and surface plasmon tune the contribution of SFG signals from buried interface and air surface

Abstract

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 (poly(methyl methacrylate)) 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 from 38° to 21°. Surface plasmon generated by gold nanorods can largely enhance SFG signals, particularly the signals from the buried interface.

Evidence for a Local Field Effect in Surface Plasmon-Enhanced Sum Frequency Generation Vibrational Spectra

“…The spectra are dominated by two peaks at 1737 and 1724 cm −1 , which originate from carbonyl groups at the PMMA/air surface and CaF 2 /PMMA (or Au/PMMA) interface, respectively. 65 The intensity of these two peaks in the presence of gold nanoparticles is greater than the ones in the absence of gold nanoparticles for both ppp and ssp combinations. The local plasmon resonance peak of gold nanoparticles with a diameter of 40 nm is ∼545 nm (Figure 4a); because of its wide visible absorption bands, there is still a certain light absorption at the position of the visible (801 nm) and SFG (703 nm) beams that can excite the plasmon of gold nanoparticles.…”

“…Figure shows the ppp and ssp spectra of PMMA films in the absence and presence of gold nanoparticles. The spectra are dominated by two peaks at 1737 and 1724 cm –1 , which originate from carbonyl groups at the PMMA/air surface and CaF 2 /PMMA (or Au/PMMA) interface, respectively . The intensity of these two peaks in the presence of gold nanoparticles is greater than the ones in the absence of gold nanoparticles for both ppp and ssp combinations.…”

Molecular Insights into Adhesion at a Buried Silica-Filled Silicone/Polyethylene Terephthalate Interface

“…This SFG signal is then collected and plotted as a function of the input IR beam wavenumber, generating a vibrational spectrum of the molecules at the surface or interface. The silicone films used here were sufficiently thick to avoid penetration of the visible and IR beams to the other side of the silicone sample, so only the buried silicone/d4-PET interface contributes to the SFG aliphatic C–H stretching signal from silicone adhesives, and there is no need to consider the signal contribution from multiple interfaces . Under the experimental condition used, there was no sample damage by the input laser beams.…”

“…The silicone films used here were sufficiently thick to avoid penetration of the visible and IR beams to the other side of the silicone sample, so only the buried silicone/d4-PET interface contributes to the SFG aliphatic C−H stretching signal from silicone adhesives, and there is no need to consider the signal contribution from multiple interfaces. 72 Under the experimental condition used, there was no sample damage by the input laser beams. The SFG spectra were collected from the buried silicone/d4-PET interfaces using the SSP polarization combination of the input and output beams.…”

Polymer Adsorbed Layer Promotes Dipole Ordering in Gate Dielectrics for Organic Field-Effect Transistors

“…Thus, the value of |χ eff,ppp (2) /χ eff,ssp (2) | = |(A q,ppp /Γ q,ppp )/(A q,ssp /Γ q,ssp )| for the carbonyl group was obtained by spectral fitting using eq 1, and then the orientation of the carbonyl groups was qualitatively evaluated. According to previous studies, the larger the value of |χ eff,ppp (2) /χ eff,ssp (2) |, the smaller the tilt angle of the carbonyl groups, 64,65 which means that the orientation of carbonyl groups depending on the adsorption time exhibits a distribution. With the extension of the adsorption time, the carbonyl groups become more perpendicular relative to the surface plane.…”