Phase-Sensitive Vibrational Sum and Difference Frequency-Generation Spectroscopy Enabling Nanometer-Depth Profiling at Interfaces

Abstract

The unique physical and chemical properties of interfaces are governed by a finite depth that describes the transition from the topmost atomic layer to the properties of the bulk material. Thus, understanding the physical nature of interfaces requires detailed insight into the different structures, chemical compositions, and physical processes that form this interfacial region. Such insight has traditionally been difficult to obtain from experiments, as it requires a combination of structural and chemical sensitivity with spatial depth resolution on the nanometer scale. In this contribution, we present a vibrational spectroscopic approach that can overcome these limitations. By combining phase-sensitive sum and difference frequency-generation (SFG and DFG, respectively) spectroscopy and by selectively determining different nonlinear interaction pathways, we can extract precise depth information and correlate these to specific vibrationally resonant modes of interfacial species. We detail the mathematical framework behind this approach and demonstrate the performance of this technique in two sets of experiments on selected model samples. An analysis of the results shows an almost perfect match between experiment and theory, confirming the practicability of the proposed concept under realistic experimental conditions. Furthermore, in measurements with self-assembled monolayers of different chain lengths, we analyze the spatial accuracy of the technique and find that the precision can even reach the sub-nanometer regime. We also discuss the implications and the information content of such depth-sensitive measurements and show that the concept is very general and goes beyond the analysis of the depth profiles. The presented SFG/DFG technique offers new perspectives for spectroscopic investigations of interfaces in various material systems by providing access to fundamental observables that have so far been inaccessible by experiments. Here, we set the theoretical and experimental basis for such future investigations.

Polarization-Dependent Heterodyne-Detected Sum-Frequency Generation Spectroscopy as a Tool to Explore Surface Molecular Orientation and Ångström-Scale Depth Profiling

“…When wider probed region is required, using a novel technique to probe the nanometer scale depth profiling through the SFG and difference frequency generation spectra is available. 76 …”

“…Finally, we note that the probed region for the depth profile where the interfacial dielectric constant varies is | z | < ∼ 2 Å at the aqueous solution interface, while the SFG active region is at least | z | < 5 Å. − As such, the probed region for the depth profile is thinner than the SFG active region. When wider probed region is required, using a novel technique to probe the nanometer scale depth profiling through the SFG and difference frequency generation spectra is available …”

Determination of the Thickness of Interfacial Water by Time-Resolved Sum-Frequency Generation Vibrational Spectroscopy

“…In addition, this method measures the average thickness of the interfacial water and not the water at a specific depth. Considering that quantitative depth information can be obtained by polarization-dependent heterodyne-detected SFG spectroscopy (PD-HD-SFG), , and phase-sensitive sum and difference frequency-generation spectroscopy (PS-SFG-DFG), the current time-resolved SFG technique combined with PD-HD-SFG or PS-SFG-DFG will endow SFG with both depth and temporal resolution, in addition to its interfacial molecule specificity.…”

“…17−19,21,22,24−28 Recently, SFG-VS has also been exploited to access quantitative depth information by investigating the interfacial dielectric profile using polarization-dependent heterodyne-detected SFG spectroscopy 32,33 and selectively determining different nonlinear interaction pathways with the combination of phase-sensitive sum and difference frequency-generation spectroscopy. 34 Here, we propose an approach to the interfacial water thickness by measuring the relaxation lifetime of the 3200 cm −1 peak using ssp polarizations. In an IR pump-SFG probe experiment, a femtosecond IR pulse is used to excite the vibration of the OH stretch mode from the ground state (ν 0 ) to its first vibrational state (ν 1 ).…”

Self-Assembled Monolayers for Interfacial Engineering in Solution-Processed Thin-Film Electronic Devices: Design, Fabrication, and Applications

“…Getautis et al found that no vibrational bands could be identified in the VSFG spectrum of a 10% V1036 124 /90% C4 54 mixed SAMs, indicating a monolayer prepared from a mixed solution results in a more disordered structure compared to a monolayer from a pure V1036 solution, since the measured VSFG signal was proportional to the molecular ordering of the probed molecules . Moreover, VSFG has also been applied for studying some important aspects, including SAM growth mechanism exploration, conformational studies, − and functional head design . However, the high price, few commercial resources, complex data processing, and high sample preparation requirements make VSFG not a mainstream SAM-characterization tool at present.…”