Sum frequency generation spectroscopy of fluorinated organic material-based interfaces: a tutorial review.

Molecular interactions at interfaces have a significant effect on the wetting properties of surfaces on a macroscale. Sum frequency generation (SFG) spectroscopy, one of a few techniques capable of probing such interactions, generates a surface vibrational spectrum sensitive to molecular structures and has been used to determine the orientation of molecules at interfaces. The purpose of this review is to assess SFG spectroscopy's ability to determine the molecular orientations of interfaces composed of fluorinated organic molecules. We will explore three different types of fluorinated organic material-based interfaces, naming liquid-air, solid-air, and solid-liquid interfaces, to see how SFG spectroscopy can be used to gain valuable and unique information regarding the molecular orientation of each interface. We hope this review will help to broaden the understanding of how to employ SFG spectroscopy to obtain more complex structural information for various fluorinated organic material-based interfaces in the future.

Evidence for Ion Association at the Gas-Liquid Interface of the Mixture of 1-Butyl-3-methylimidazolium Hexafluorophosphate and Benzonitrile: A Sum Frequency Generation Spectroscopy and Surface Tension Study.

The gas-liquid interface for the mixtures of [BMIM][PF6] and benzonitrile is studied by sum frequency generation (SFG) spectroscopy and surface tension measurements as an important solute to reduce the viscosity of ionic liquids. Solvation of ionic compounds in bulk solvent is not necessarily the same as that at the surface due to the lower dielectric medium at the air/liquid interface. The results from the temperature-dependent SFG spectroscopy and surface tension study suggest that the ionic liquid in a benzonitrile solvent is associated as ion pairs at the surface rather than as dissociated─solvated─ions in the bulk solution. The influence of ionic liquids on the surface structure of benzonitrile is investigated from 0 to 1.0 mole fraction of benzonitrile. The CH stretching mode of vibration of benzonitrile in the SFG spectrum begins from a 0.2 mole fraction (x) of benzonitrile, and the intensity of the peak constantly increases on increasing the concentration of benzonitrile. However, the addition of benzonitrile does not result in extra peaks or shifting of the peak frequency to the spectra of [BMIM][PF6]. The surface tension measurements further support the presence of benzonitrile at the gas-liquid interface. The surface tension data of the mixture smoothly decrease as the benzonitrile concentration increases. The apparent tilt angle of the terminal methyl group of the cation of [BMIM][PF6] is calculated from SFG polarization spectra and shows an apparent lowering with the addition of benzonitrile. The effect of temperature on the surface structure of the binary mixture is also reported at four different temperatures between -15 and 40 °C for both the SFG spectroscopy and surface tension study. Benzonitrile shows different behavior in the mixture at higher temperatures than pure benzonitrile, as observed in the SFG spectra. In contrast, it does not show any CN peak in the mixture below 0.9 mole fraction. The temperature dependence of the interfacial tension is used to evaluate thermodynamic functions such as surface entropy and surface enthalpy. Both were found to be lowered with the increasing concentration of benzonitrile. Both spectroscopic and thermodynamic analyses indicate that the ionic liquid is highly associated as ion pairs and the benzonitrile is more ordered at the surface at concentration <0.4×.

Chemical Imaging of Lipid Segregation: Determining Different Length Scales of Heterogeneity with Compressive-Sensing Sum Frequency Generation Microscopy and Brewster Angle Microscopy.

Membranes of various phospholipids may separate into different domains at micrometer length scales at the air-water interface. A significant challenge is to visualize the molecular organization and obtain chemical information on this surface. Langmuir-Blodgett monolayers of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) on fused silica were investigated with compressive sensing sum frequency generation (SFG) microscopy. Before and after mixing, SFG spectra for DSPC and DLPC revealed significant differences, indicating a structural change. Brewster angle microscopy images show phase separations that directly correlate the morphology of phospholipid mixtures with SFG images. Exploiting vibrational contrast in SFG images, exchange between the two phases was discovered, and quantitative thermodynamic analysis of lipid compositions in liquid-condensed and liquid-expanded (LE) phases was provided. Local SFG spectra reveal significant differences from one another, indicating the heterogeneity resulting from domain areas with distinct molecular orientation and conformation. Further heterogeneity across the domain boundary was presented on a finer scale, revealing an effect on DLPC due to the condensed phase DSPC, and the terminal methyl of DLPC perturbs the ordering of DSPC in the LE phase. This work demonstrates the heterogeneity of a two-dimensional binary lipid system mainly due to the aliphatic chain length and transition temperature difference.

Cooperative Adsorption of Nonionic Triton X-100 and Dodecyldimethylamine Oxide Surfactant Mixtures at the Hydrophilic Silica-Water Interface Studied by Total Internal Reflection Raman Spectroscopy and Multivariate Curve Resolution.

The adsorption of dimethyldodecylamine oxide (DDAO) and Triton X-100 (TX) as single components and mixed systems at the silica-water interface has been studied using total internal reflection (TIR) Raman spectroscopy combined with multivariate curve resolution (MCR). In this study, the mixtures of DDAO and TX indicate minimal synergism in the bulk solution; however, the cooperative adsorption behavior on the silica surface is shown with various mixtures of DDAO (up to 1.3 mM) and TX (up to 1.1 mM). Adding the DDAO (up to 0.3 mM) to TX solution, the surface excess of TX shows 30% enhancement, from 1.2 to 1.8 µmol m-2. Adding the DDAO also shifts the TX adsorption isotherms, resulting in the Gibbs free energy change of -2.87 ± 0.73 kJ mol-1. This free energy change is interpreted as the decrease in surface energy when the silica surface charged sites are screened by the DDAO adsorbed layer. Alternatively, when a DDAO solution contains a small amount of TX molecules, i.e., < 30 µM, its adsorption on the silica surface quickly equilibrates. In addition, the formation of a more ordered liquid-crystalline adsorbed layer, as in the case of single-component DDAO adsorption, is not observed.

In situ quantitative study of the phase transition in surfactant adsorption layers at the silica-water interface using total internal reflection Raman spectroscopy.

Dimethyldodecylamine N-oxide (DDAO), a unique type of surfactant, shows high surface activity with two distinct energy states at the buried hydrophilic silica/aqueous solution interface studied by total internal reflection (TIR) Raman spectroscopy combined with ratiometric and kinetic analysis. Different from other types of surfactant, i.e., ionic and nonionic, the adsorption of DDAO demonstrates a specific critical surface aggregation concentration (csac) at 0.15 mM gives a complete surface coverage of 6.6 ± 0.3 µmol m-2, much lower than the bulk critical micellization concentration (cmc) at the same conditions (csac ≈ 0.072 cmc). A phase transition of adsorbed layers from liquid crystalline as the intermediate state to the disordered liquid phase is spectroscopically and energetically analyzed. The adsorption of DDAO on silica surfaces is described quantitatively in a potential energy curve.

Direct imaging of electric field behavior in 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene organic field-effect transistors by sum-frequency generation imaging microscopy.

Sum-frequency generation imaging microscopy combined with compressive-sensing (CS-SFG) is a powerful micro-spectroscopic technique for probing interfaces and surfaces with a spatial resolution where contrast is based on the chemical functional groups. We reported the use of the CS-SFG technique to probe the electric field due to charge accumulation and the internal electric field in operating organic field-effect transistors (OFETs) with the aluminum oxide and octadecylphosphonic acid (ODPA) self-assembled monolayer as the gate dielectric layer and 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT) as the semiconductor layer. In addition, the electric field behavior was discussed by a difference in the electric field induced SFG intensity between the open-circuit and the voltage application conditions. The SFG peak of CH stretching mode derived from methyl groups of ODPA and phenyl groups of DPh-BTBT could be observed at each interface of ODPA/DPh-BTBT or DPh-BTBT/Au, respectively. Moreover, the electric field induced SFG coming from ODPA/DPh-BTBT shows the presence of intense electric field due to charge injection and accumulation near the drain and source electrode edges under the operation of OFETs. Our studies show that the electric field-induced SFG imaging technique is useful for probing the local electric field distribution or charge accumulation behavior in OFETs under operating conditions.

Spectroscopic imaging of surfaces-Sum frequency generation microscopy (SFGM) combined with compressive sensing (CS) technique.

Surface chemistry is notoriously difficult to study, in part, due to the decreased number of molecules that contribute to the properties compared to the bulk phase but often has significant effects on the chemical activity of the material. This is especially true in topics such as corrosion, catalysis, wetting, and many others in nature and industry. Sum frequency generation (SFG) spectroscopy was developed for interface studies due to its high molecular selectivity and surface sensitivity, which is quite useful to study the effects of structural inhomogeneity in microscopy. Compressive sensing (CS) combined with SFG spectroscopy minimizes the imaging time while still producing quality images. Selected systems are presented here to demonstrate the capability of CS-SFG microscopy. CS-SFG microscopy successfully distinguished the static monolayer molecular mixtures, the orientations and adsorption of adsorbed molecules by the dip-coating technique, and the localized CO behaviors on polycrystalline Pt electrodes. Further discussion includes dynamic imaging as a future direction in CS-SFG microscopy. As materials and surfaces become more complex, imaging with chemical contrast becomes indispensable to understanding their performance and CS-SFG microscopy seems highly beneficial in this respect.

Chemical Imaging of Surfaces with Sum Frequency Generation Vibrational Spectroscopy.

Surface chemistry is a key area of study in the chemical sciences, and many system properties are dominated by the chemistry at the interface between two bulk media. While the interface may have a large influence on the system behavior, there are relatively few molecules at the interface compared to the bulk; thus, probing their unique properties has become a specialized field in physical chemistry. In addition to the heterogeneous phase chemistry, surfaces also present spatial heterogeneity (Chemistry in Two Dimensions). This 2D chemistry affects the properties as much as the heterogeneous phases. If we consider the Cartesian z-axis as defining the dimension across the interface between the two bulk phases, then the x-y plane is the 2D region of the surface. We might even consider that the majority of surface chemistry has been concerned with this z-dimension, i.e., surface structure, partition excess, thermodynamics, etc. relative to the bulk, where the 2D distribution was only considered on average. This treatment is understandable since few techniques provide the spatial and chemical resolution needed to deduce the effects of 2D heterogeneity on the surface properties. It is desirable to use an all-optical technique for interface studies because the optical methods provide the chemical specificity through spectroscopy. Also, the use of second-order spectroscopy is typically surface-sensitive without background subtractions or enhancement mechanisms that could limit the range of systems to be investigated.In this Account, the development and selected results of sum frequency generation microscopy and its contributions to the surface chemistry are presented. Sum frequency generation (SFG) provides a unique probe for studying surface chemistry in ambient conditions with surface specificity. SFG provides image contrast based on multiple-chemically important-mechanisms such as chemical functional groups, molecular orientation, surface concentration, molecular conformation, local electric fields, among others. To understand the spatial distribution of heterogeneous chemistry, multiple microscopy methods have been developed which utilize the SFG process to yield spatial information with chemical sensitivity. These spectroscopic-microscopies come with unique advantages as well as challenges. Multiple solutions have been developed in this field to overcome the challenges and improve the advantages. In this Account, some of the leading SFG surface microscopies for surface studies are introduced. Initially, direct imaging of the SFG signal onto a CCD camera provided spatially and spectrally resolved imaging of monolayers on surfaces. However, to speed up the imaging process, the technique of compressive sensing was applied to SFG imaging. Most recently the use of machine learning methods and target factor analysis have improved the quality and acquisition speed of SFG images.

Distortion Correction for an Imaging Ellipsometer.

A simple imaging add-on utilizing the combination of a relay lens and an optical grating to a nulling ellipsometer for the purpose of imaging and correcting angular distortion is designed, built, and tested. Image contrast is achieved on a standardized silicon wafer, graphene transferred on silicon wafer, a self-assembled monolayer of 1-octadecanethiol on gold, and a 1:1 mixture of dilauroylphosphatidylcholine / distearoylphosphatidylcholine (DLPC/DSPC) on copper. The configuration used in this paper corrects the angular distortion and depth of focus with a spatial resolution of 4.38 µm in the laser's incident direction and 7.81 µm in the direction that is perpendicular to the incident plane together with 3 Å of normal resolution while still maintaining a large field of view of at least 720 µm × 550 µm in focus without executing any line scanning, post-image reconstruction, or specially customized objective method. Better resolution is possibly attained with higher NA optics.

From Micelles to Vesicle and Membrane Structures of Double-Strand Ionic Liquids in Water: Molecular Dynamics Simulation.

The structures of novel double-strand imidazolium iodide ionic liquids (ILs)-based lipid in the water phase have been studied in this research. We report the effect of alkyl chain length of 1,3-dimethyl-4,5-dialkyl-imidazolium iodide([2(Me)2(C n)im]I, n = 7, 11, 15) ILs on their structures in the aqueous solution by molecular dynamics simulation. The structure details of IL clusters lead to the various aggregation forms by increasing the alkyl chain length of ILs. The ILs with n = 7 and 11 can help develop micelle structures of different sizes, and the IL with n = 15, the benign IL, feasible to develop lipidlike vesicle. To obtain more details about bilayer properties, [2(Me)2(C15)im]I IL is investigated by different IL/water ratios in this study exclusively. The [2(Me)2(C15)im]I IL bilayer thickness and deuterium order parameters are compared with lipid membrane, and they reveal a small difference. The energies, radial distribution functions, spatial distribution function, cluster size, number density, and membrane properties all prove that the stable IL vesicle is formed in the dilute solution but the membrane is formed in the concentrated aqueous solution of [2(Me)2(C15)im]I.

Sum Frequency Generation Imaging Microscopy of Self-Assembled Monolayers on Metal Surfaces: Factor Analysis of Mixed Monolayers.

Sum frequency generation (SFG) images of microcontact patterned self-assembled alkanethiol monolayers on metal surfaces were analyzed by factor analysis (FA) to determine the spatial distribution of the patterned monolayers over the images. Additionally, each significant abstract factor produced by FA was assessed to determine the information contained within it. These results indicate that FA of the SFG spectra is a promising method to determine the composition and identities of mixed alkanethiol systems that show different vibrational spectra and image contrast. Factor analysis has successfully been applied to SFG images obtained with low signals, which reduces the time required for full spectral SFG imaging.

Sum frequency generation spectroscopy of tetraalkylphosphonium ionic liquids at the air-liquid interface.

Sum frequency generation (SFG) spectroscopy is a nonlinear vibrational spectroscopic technique used in the study of interfaces, due to its unique ability to distinguish surface molecules that have preferential ordering compared to the isotropic bulk. Here, a series of alkyltrioctylphosphonium chloride ionic liquids, systematically varied by cation structure, were characterized at the air-liquid interface by SFG. The effect on surface structure resulting from molecular variation (i.e., addition of cyano- and methoxy-functional groups) of the cation alkyl chain was investigated. SFG spectra in the C-H stretching region (2750-3100 cm-1) for [P8 8 8 n ][Cl], where n = 4, 5, 8, 10, 12, or 14, showed characteristic changes as the alkyl chain length was increased. Spectral profiles for n = 4, 5, 8, or 10 appeared similar; however, when the fourth alkyl chain was sufficiently long (as in the case of n = 12 or n = 14), abrupt changes occurred in the spectra. Molecular dynamics (MD) simulation of a slab of each ionic liquid (with n = 8, 10, or 12) confirmed gauche defects, with enhancement for the long alkyl chain and an abrupt increase of gauche occurrence from n = 8 to n = 10. A comparison of the tilt angle distribution from the simulation and the SFG analysis show a broad distribution of angles. Using experimental SFG spectra in conjunction with MD simulations, a comprehensive molecular picture at the surface of this unique class of liquids is presented.

Chemical Imaging of Self-Assembled Monolayers on Copper Using Compressive Hyperspectral Sum Frequency Generation Microscopy.

Sum frequency generation microscopy is a label-free optical imaging technique with intrinsic molecular vibrational contrast for surface studies. Recent developments of compressive sensing broad-band hyperspectral SFG microscopy have demonstrated the potential application for imaging monolayer at metal surfaces with micrometer spatial resolution. Here is presented the capability of chemical imaging of spatially patterned monolayers of 1-octadecanethiol (ODT) and 16-methoxy-1-hexadecanethiol (MeOHT) molecules assembled on a copper surface. The spatial distributions of the monolayer with vibrational-spectral contrast are well-demonstrated at different frequency regions through reconstruction of the hypercube using a 3-dimensional total variation regularization algorithm (3DTV). Spatial-chemical distributions of each component are also reconstructed directly from the compressive measurements by endmember unmixing (CEU) scheme. Compared to 3DTV algorithm, the reconstruction from CEU shows spatial distribution of each component on the surfaces, and demonstrates the ability to characterize the domains of mixed-molecules on surfaces.

Distortion Correction for a Brewster Angle Microscope Using an Optical Grating.

A distortion-corrected Brewster angle microscope (DC-BAM) is designed, constructed, and tested based on the combination of an optical grating and a relay lens. Avoiding the drawbacks of most conventional BAM instruments, this configuration corrects the image propagation direction and consequently provides an image in focus over the entire field of view without any beam scanning or imaging reconstruction. This new BAM can be applied to both liquid and solid subphases with good spatial resolution in static and dynamic studies.

Adsorption of Dimethyldodecylamine Oxide and Its Mixtures with Triton X-100 at the Hydrophilic Silica/Water Interface Studied Using Total Internal Reflection Raman Spectroscopy.

Adsorption of dimethyldodecylamine oxide (DDAO) and its mixtures with Triton X-100 (TX-100) at the hydrophilic silica/water interface has been studied using total internal reflection (TIR) Raman spectroscopy and target factor analysis (TFA). The use of a linear vibrational spectroscopic technique helps obtain information on molecular behavior, adsorbed amount, and conformational order of surfactant molecules at the interface. The results obtained from polarized Raman measurements of pure DDAO show insignificant changes in the orientation and conformational order of surface molecules as a function of DDAO bulk concentrations. The adsorption isotherm of pure DDAO shows a change in the structure of the adsorbed layer at concentrations close to the critical micelle concentration (cmc). TFA reveals that, for a low concentration of DDAO (0.30 mM in this study), adsorption of both DDAO and TX-100 in the mixed surfactants was enhanced at low TX-100 concentrations. The synergistic effect is dominant at low concentrations of TX-100, with enhanced adsorption of both surfactants. Although competitive adsorption is effective at high concentrations of TX-100, the presence of a small amount of DDAO at the interface still enhances TX-100 adsorption. When DDAO concentrations are increased to 1.00 mM, TX-100 replaces DDAO molecules on the surface when TX-100 concentration is increased.

Compressive Broad-Band Hyperspectral Sum Frequency Generation Microscopy to Study Functionalized Surfaces.

A broad-band sum frequency generation microscope has been developed for the study of molecular monolayers on surfaces. Because sum frequency generation is a vibrational spectroscopy based on a second-order optical process, it is uniquely sensitive to detecting a molecule's vibrational fingerprints specifically at interfaces. In this microscope, a structured illumination beam generated by a spatial light modulator is used to irradiate the sample with a series of sparsifying pseudorandom patterns. The spectra associated with each pattern are then input into a reconstruction algorithm to compressively recover the full hyperspectral image cube. As a proof-of-principle, this system performed molecule-specific imaging of a microcontact-printed self-assembled monolayer of alkanethiolate on copper. This hyperspectral compressive imaging effectively recovered both spatial and spectral surface features with compression greater than 80%, meaning more than a 5-fold decrease in acquisition time compared to traditional methods.

Grain Structures and Boundaries on Microcrystalline Copper Covered with an Octadecanethiol Monolayer Revealed by Sum Frequency Generation Microscopy.

An octadecanethiol (ODT) self-assembled monolayer on microcrystalline copper was investigated by sum frequency generation (SFG) imaging microscopy. The crystal grain and grain boundaries of the copper surface were mapped in the SFG image based on the strong brightness contrast of the SFG signal across the boundary. Local SFG spectra reveal significant difference with each other as well as the average SFG spectra, indicating the heterogeneity of the copper surface resulting from copper grains with distinct crystallographic facets and orientations. It is demonstrated that the SFG signal of crystalline domain areas contains azimuthal anisotropy with respect to the plane of incidence. In addition, the statistical orientation analyses of amplitude ratio of CH3-sym/CH3-asym and corresponding contour maps imply that the orientation of ODT molecules is affected by the underlying copper.

Sum frequency generation spectroscopy study of an ionic liquid at a graphene-BaF2 (111) interface.

Sum frequency generation (SFG) vibrational spectroscopy and contact angle measurements of an ionic liquid, 1-butyl-3-methylimidazolium dicyanamide [BMIM][DCA], at solid-liquid interfaces are reported. Bare solid single crystal BaF2 (111) surface, a single and few layer graphene-coated BaF2 (111) surface are used as the solid substrates. The SFG results indicate that both [BMIM](+) and [DCA](-) can be detected specifically on the graphene-coated BaF2 (111) surface, without coating only [DCA](-) are observed. [DCA](-) anions are attracted to the positively charged BaF2 (111) surface and occupy the first layer at the solid-liquid interface. The graphene coating shields the charged crystal surface and allows both cations and anions to exist at the interface. Furthermore, increase in the contact angle of BaF2 surface after graphene layers deposition suggests that the graphene coating lowers the surface energy.

Efficient solar water-splitting using a nanocrystalline CoO photocatalyst.

The generation of hydrogen from water using sunlight could potentially form the basis of a clean and renewable source of energy. Various water-splitting methods have been investigated previously, but the use of photocatalysts to split water into stoichiometric amounts of H2 and O2 (overall water splitting) without the use of external bias or sacrificial reagents is of particular interest because of its simplicity and potential low cost of operation. However, despite progress in the past decade, semiconductor water-splitting photocatalysts (such as (Ga1-xZnx)(N1-xOx)) do not exhibit good activity beyond 440 nm (refs 1,2,9) and water-splitting devices that can harvest visible light typically have a low solar-to-hydrogen efficiency of around 0.1%. Here we show that cobalt(II) oxide (CoO) nanoparticles can carry out overall water splitting with a solar-to-hydrogen efficiency of around 5%. The photocatalysts were synthesized from non-active CoO micropowders using two distinct methods (femtosecond laser ablation and mechanical ball milling), and the CoO nanoparticles that result can decompose pure water under visible-light irradiation without any co-catalysts or sacrificial reagents. Using electrochemical impedance spectroscopy, we show that the high photocatalytic activity of the nanoparticles arises from a significant shift in the position of the band edge of the material.