Enhanced Sum Frequency Generation for Monolayers on Au Relative to Silica: Local Field Factors and SPR Effect

Abstract

Using metals as signal magnified substrates, surface plasmon-enhanced sum frequency generation (SFG) vibrational spectroscopy is a promising technique to probe weak molecular-level signals at surfaces and interfaces. In this study, the vibrational signals of the n-alkane monolayer on the gold (Au) and silica substrates are investigated using the broadband femtosecond SFG. The enhancement factors are discovered to be up to ∼1076 and ∼31 for the methyl symmetric and asymmetric stretching (ss and as) modes of the monolayer, respectively. By systematically analyzing the second-order nonlinear susceptibility tensor components (χ ijk s), the Fresnel coefficients (F ijk s), and the surface plasmon resonance (SPR) effect, we find that the interplay between F ijk and χ ijk terms and the SPR effect dominate the SFG signal enhancement. Our study reveals that the relative contributions of different influencing factors (i.e., Fresnel coefficients and SPR) to the SFG signal enhancement provide an approach to interpreting enhanced SFG vibrational signals detected from probe molecules on distinct substrates and may finally guide the design of the experimental methodology to improve the detection sensitivity and signal-to-noise ratio.

The Surface of Colloidal Metal–Organic Framework Nanoparticles Revealed by Vibrational Sum Frequency Scattering Spectroscopy

“…Nonresonant backgrounds are often observed in the spectra of substrates with complex or metallic compositions. As with these data, they often appear in the C–H stretching region and arise from linker-based electronic transitions that can be reduced by detiming the visible pulse in the experiment. − This approach, however, resulted in poor resolution of the resonant CH modes, prompting us to report spectra from the coherent VSFSS measurement.…”

Probing Interfacial Aging of Model Adhesion Joints under a Hygrothermal Environment at a Molecular Level

“…In this research, we implemented SFG vibrational spectroscopy to explore the buried interfacial structures between the epoxy and solid substrates. The SFG theory and experimentation have been described in a great number of previous literature. − In brief, when two input beams, namely, a frequency-tunable infrared (IR) one and a frequency-fixed visible (VIS) one, overlap spatially and temporally at a surface or interface, an SFG beam will be radiated from the overlapped position. For the SFG system in this study, customized by EKSPLA, a 1064 nm and 50-Hz fundamental beam with a pulse width of ∼20 ps, originating from a picosecond Nd:YAG laser, was doubled and tripled to generate a 532 nm beam and 355 nm beam, respectively.…”