Infrared-visible Sum Frequency Generation Research Articles

Activation Energy of Ion Desorption at Ionic Liquid/Pt Electrode Interfaces: A Sum-Frequency Generation Vibrational Spectroscopic Study.

Electrolyte/electrode interfaces of room-temperature ionic liquids (RTILs) exhibit hysteretic responses to different applied potentials owing to the differences in the ion adsorption/desorption processes; the ion desorption requires excess potential, which reflects the activation energy of ion desorption. Thus far, the contributions of the ion adsorption energy and the activation barrier for ion desorption toward the ion-dependent excess potential have not been quantified. Herein, we report on our infrared-visible sum-frequency generation vibrational spectroscopy study of the hysteretic responses of the anion adsorption/desorption at Pt electrode interfaces using neat, binary, and diluted RTILs composed of 1-butyl-3-methylimidazolium cations ([C4mim]+) and bis(trifluoromethanesulfonyl)amide ([TFSA]-) and trifluoromethanesulfonate ([OTf]-) anions. Experimental results are compared to the theoretical calculations for the electric double layer model. The hysteretic response of the RTIL/Pt interface derives predominantly from the activation energy of anion desorption, which causes the negative excess potential required for anion desorption. A comparison of the anion adsorption/desorption behaviors of neat RTILs with those of binary and diluted RTILs reveals that the large activation energy of anion desorption at the neat RTIL/Pt interface originates largely from the activation barrier for restructuring ionic layering in the diffuse layer.

Sum frequency generation spectroscopy of the attachment disc of a spider

The pyriform silk of the attachment disc of a spider was studied using infrared-visible vibrational sum frequency generation (SFG) spectroscopy. The spider can attach dragline and radial lines to many kinds of substrates in nature (concrete, alloy, metal, glass, plant branches, leaves, etc.) with the attachment disc. The adhesion can bear the spider's own weight, and resist the wind on its orb web. From our SFG spectroscopy study, the NH group of arginine side chain and/or NH2 group of arginine and glutamine side chain in the amino acid sequence of the attachment silk proteins are suggested to be oriented in the disc. It was inferred from the observed doublet SFG peaks at around 3300 cm−1 that the oriented peptide contains two kinds of structures.

Doubly resonant SFG and DFG spectroscopies: An analytic model for data analysis including distorted and rotated vibronic levels. I. Theory.

We present an analytic description of doubly resonant infrared-visible sum (SFG) and difference frequency generation (DFG) spectroscopies. Within the Born-Oppenheimer and Condon approximations for harmonic oscillators, we extend the usual theory, limited to linear electron-vibration coupling, and introduce the quadratic coupling phenomena (mode distortion and mode mixing) in the excited state. The excitation spectra of vibrations in SFG and DFG experiments are calculated in integral form for arbitrary mode distortions and small amplitude mode mixing between pairs of modes. Mode distortion modifies all orders of vibronic coupling including the fundamental process, whereas mode mixing appears as a perturbation added to the distorted mode case. For small quadratic coupling amplitudes, the results may be recast in simple analytic forms after the introduction of the overlap spectral function and developed in sums and products of Lorentzian functions.

Enhancement of the electrochemical stability of tetraglyme-Li[TFSA] electrolyte systems by adding [Bimps] zwitterion: An in-situ IV-SFG study

The potential-dependent interfacial structures of Pt/tetraglyme-lithium bis(trifluoromethylsulfonyl) amide ((Pt)/G4-Li[TFSA]) systems were assessed with and without 3-(1-butyl-1H-imidazol-3-ium-3-yl)propane-1-sulfonate ([Bimps]) zwitterions, to understand the impact of [Bimps] on the enhancement of the electrochemical stability of G4-Li[TFSA] electrolytes at Pt electrode surfaces. The aforementioned interfacial structures were investigated using in-situ infrared-visible sum frequency generation (IV-SFG) spectroscopy. The results of the linear sweep voltammetry (LSV) revealed that the oxidation limit of G4-Li[TFSA] at the Pt electrode shifted to a higher potential when [Bimps] was introduced, thus indicating that the electrochemical window is extended in the G4-Li[TFSA]-[Bimps] system. The SFG spectra of the G4-Li[TFSA] system confirmed that free G4 molecules and Li+-G4 complex cations are dominantly adsorbed on the Pt surface at 1 V (vs. Ag/Ag+). In contrast, the addition of [Bimps] to the G4-Li[TFSA] system resulted in almost full coverage of [Bimps] at the Pt surface by forming Li+-[Bimps] complex cations, even at 1 V (vs. Ag/Ag+), and no free G4 molecules were observed. The ab initio calculation indicated that the highest occupied molecular orbital (HOMO) energy level of free G4 was higher than that of the Li+-[Bimps] complex cation. This suggests that the Li+-[Bimps] complex was more stable for oxidation than the free G4, which accounts for the enhanced electrochemical stability of G4-Li[TFSA] system with [Bimps] additives.

Detection of surface structural changes during adsorption events using two-trace two-dimensional (2T2D) correlation spectroscopy

We demonstrate a method by which a pair of spectra acquired using surface-sensitive second-order nonlinear vibrational spectroscopy, visible-infrared sum-frequency generation, can be analyzed in order to readily determine whether changes that occur as a result of varying environmental conditions results in a change in the population of surface molecules, or a structural rearrangement of the surface. This is accomplished via two-trace two-dimensional correlation analysis, a recently developed technique that uses spectral phase information to provide a level of comparison beyond what can be achieved by simple spectral subtraction or division. While surface reorientation and/or a change in the degree of ordering are often difficult to detect from spectra obtained in a single beam polarization, without detailed analysis of the lineshape, we show that such changes provide strong asynchronous cross peaks in the frequency correlation map.

Surface Phonon Polariton Resonance Imaging Using Long-Wave Infrared-Visible Sum-Frequency Generation Microscopy

We experimentally demonstrate long-wave infrared-visible sum-frequency generation microscopy for imaging polaritonic resonances of infrared (IR) nanophotonic structures. This nonlinear-optical appr...

Doubly resonant sum frequency spectroscopy of mixed photochromic isomers on surfaces reveals conformation-specific vibronic effects.

Doubly resonant infrared-visible sum-frequency generation (DR-IVSFG) spectroscopy, encompassing coupled vibrational and electronic transitions, provides a powerful method to gain a deep understanding of nuclear motion in photoresponsive surface adsorbates and interfaces. Here, we use DR-IVSFG to elucidate the role of vibronic coupling in a surface-confined donor-acceptor substituted azobenzene. Our study reveals some unique features of DR-IVSFG that have not been previously reported. In particular, vibronic coupling resulted in prominent SFG signal enhancement of selective stretching modes that reveal electronic properties of coexisting photochromic isomers. Our analysis explores two concepts: (1) In partially isomerized azobenzene at the surface, coupling of the fundamental vibrations to the S0 → S1 transition is more prominent for the cis isomer due to symmetry breaking, whereas coupling to the S0 → S2 transition was dominant in the trans isomer. (2) A strong coupling between the fundamental vibrations and the valence π-electron density, promoted by the initial absorption of an infrared photon, may result in suppression of the intensity of the hot band vibronic transition. This may translate into a suppressed sum-frequency generation signal at sum frequency wavelengths resonant with the S0 → S2 transition of the trans isomer. The weaker coupling of the fundamental vibrations to the non-bonding electron density localized on the azo group can therefore produce detectable sum-frequency generation at the resonance wavelength of the weaker S0 → S1 transition in the cis form. These results are explained in the framework of a linear coupling model, involving both Franck-Condon and Herzberg-Teller coupling terms. Our theoretical analysis reveals the important role played by molecular conformation, orientation, and vibronic interference in DR-SFG spectroscopy.

Adsorption of heptane-toluene binary mixtures on a hydrophobic polymer surface.

Polymer coatings offer a means to modulate the adsorption of molecules onto solid surfaces by offering a surface functionality, charge, roughness, and hydrophobicity that is different from the underlying substrate. One application is to provide anti-fouling functions for metal surfaces. Understanding solvent-surface interactions is an essential component to gaining mechanistic insight into the adsorption process. In this work, we study the adsorption of toluene-heptane binary mixtures onto a perflurorinated polymer surface. We use a combination of IR absorption and Raman scattering spectroscopy to study the mixture in the bulk phase, and surface-specific visible-infrared sum-frequency generation to probe the surface layers. Through the use of homo- and heterospectral two-dimensional correlation spectroscopy, we conclude that the adsorption of the two solvents is reversible and that the surface structure is generally independent of the surface composition, with a small change in toluene orientation as the toluene content increases. We also find that the hydrophobic fluropolymer has very little preference for either solvent, similar to previous studies on hydrophilic surfaces.

In Situ Nanoscale Characterization of Water Penetration through Plasma Polymerized Coatings.

The search continues for means of making quick determinations of the efficacy of a coating for protecting a metal surface against corrosion. One means of reducing the time scale needed to differentiate the performance of different coatings is to draw from nanoscale measurements inferences about macroscopic behavior. Here we connect observations of the penetration of water into plasma polymerized (PP) protective coatings and the character of the interface between the coating and an oxide-coated aluminum substrate or model oxide-coated silicon substrate to the macroscopically observable corrosion for those systems. A plasma polymerized film from hexamethyldisiloxane (HMDSO) monomer is taken as illustrative of a hydrophobic coating, while a PP film from maleic anhydride (MA) is used as a characteristically hydrophilic coating. The neutron reflectivity (NR) of films on silicon oxide coated substrates shows that water moves more readily through the hydrophilic PP-MA film. Off-specular X-ray scattering indicates the PP-MA film on aluminum is less conformal with the substrate than is the PP-HMDSO film. Measurements with infrared-visible sum frequency generation spectroscopy (SFG), which probes the chemical nature of the interface, make clear that the chemical interactions between coating and aluminum oxide are disrupted by interfacial water. With this water penetration and interface disruption, macroscopic corrosion can occur much more rapidly. An Al panel coated with PP-MA corrodes after 1 day in salt spray, while a similarly thin (∼30 nm) PP-HMDSO coating protects an Al panel for a period on the order of one month.

Insight on Structure of Water and Ice Next to Graphene Using Surface-Sensitive Spectroscopy.

The water/graphene interface has received considerable attention in the past decade due to its relevance in various potential applications including energy storage, sensing, desalination, and catalysis. Most of our knowledge about the interfacial water structure next to graphene stems from simulations, which use experimentally measured water contact angles (WCAs) on graphene (or graphite) to estimate the water-graphene interaction strength. However, the existence of a wide spectrum of reported WCAs on supported graphene and graphitic surfaces makes it difficult to interpret the water-graphene interactions. Here, we have used surface-sensitive infrared-visible sum frequency generation (SFG) spectroscopy to probe the interfacial water structure next to graphene supported on a sapphire substrate. In addition, the ice nucleation properties of graphene have been explored by performing in situ freezing experiments as graphitic surfaces are considered good ice nucleators. For graphene supported on sapphire, we observed a strong SFG peak associated with highly coordinated, ordered water next to graphene. Similar ordering was not detected next to bare sapphire, implying that the observed ordering of water molecules in the former case is a consequence of the presence of graphene. Our analysis indicates that graphene behaves like a hydrophobic (or negatively charged) surface, leading to enhanced ordering of water molecules. Although liquid water orders next to graphene, the ice formed is proton disordered. This research sheds light on water-graphene interactions relevant in optimizing the performance of graphene in various applications.

Dipole-Moment Reversal in a Polar Organic Monolayer Probed by Sum and Difference Frequency Spectroscopy

We investigate the adsorption of pyridine on Cu(110) in ultra-high vacuum with a combination of work function measurements and femtosecond infrared-visible sum and difference frequency generation (SFG/DFG). A monolayer of pyridine substantially reduces the work function by 2.9 eV due to the large pyridine dipole. We perform density functional theory (DFT) calculations that provide us with a dipole moment change upon adsorption in very good agreement with the experimental results. The pyridine dipole strongly enhances the sum frequency response of the surface electrons, but surprisingly reduces the surface difference frequency signal. We propose a model based on the static electric field-induced nonlinear optical response generated by the collective electric field of the adsorbate layer. The pyridine dipole switches direction from the ground to the excited electronic state, as charge moves from nitrogen to the ring. SFG can then be enhanced by the electric field of adsorbed pyridine in its ground electronic state, while the 2.33 eV incident photon in DFG excites electrons into the pyridine LUMO, which reverses the electric field in the adsorbate layer and reduces the nonlinear optical response. The model is verified by 2.33 eV pump – SFG probe spectroscopy, where the pump pulse is found to reduce the surface electron response on a subpicosecond timescale. This demonstrates the potential to manipulate the work function in organic electronic devices by photon-induced dipole moment reversal.

Polar Dynamics at a Functional Group Level: Infrared‐Visible Sum‐Frequency Generation Study on Polar Columnar Liquid Crystals

Organic ferroelectrics (OFEs) are an attractive topic in material science because of their usefulness in modern technologies. There are a variety of OFEs that have been developed through various approaches using various materials. To further design and understand OFEs, it is essentially important to evaluate the dynamical behaviors of individual molecules during polarization switching. Although recent developments in theoretical calculations offer powerful tools to predict how molecules assemble into complex structures at the molecular level, it is still too difficult to determine the dynamics in response to external stimuli at the functional group level. Here, this study adopts infrared‐visible sum‐frequency generation (IV‐SFG) to probe the electric‐field‐induced dynamics at the functional group level and reports on the switching mechanism of the bulk ferroelectricity and paraelectricity in an organic system for the first time. This paper demonstrates an IV‐SFG study on a recently‐discovered polar columnar liquid crystal system of tapered‐paraffinic‐wedge molecules, whose favorable wet‐processability may provide a suitable design for “paintable” organic ferroelectrics. Dynamic motions of dipoles, such as of phthalonitriles and amides, are successfully probed. In ferroelectric compounds, the phthalonitriles are responsible both for the ferroelectric and paraelectric switching, whereas the amides only for the ferroelectric switching.

Experimental and Theoretical Investigation of the Restructuring Process Induced by CO at Near Ambient Pressure: Pt Nanoclusters on Graphene/Ir(111).

The adsorption of CO on Pt nanoclusters grown in a regular array on a template provided by the graphene/Ir(111) Moiré was investigated by means of infrared-visible sum frequency generation vibronic spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy from ultrahigh vacuum to near-ambient pressure, and ab initio simulations. Both terminally and bridge bonded CO species populate nonequivalent sites of the clusters, spanning from first to second-layer terraces to borders and edges, depending on the particle size and morphology and on the adsorption conditions. By combining experimental information and the results of the simulations, we observe a significant restructuring of the clusters. Additionally, above room temperature and at 0.1 mbar, Pt clusters catalyze the spillover of CO to the underlying graphene/Ir(111) interface.

IV-SFG studies on the effect of Li+ in extending the electrochemical window at the Pt |[C2mim][FSA] interface

The effect of Li+ addition at the interface of a 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide ([C2mim][FSA]) room-temperature ionic liquid (RTIL) and a Pt electrode is investigated by infrared-visible sum-frequency generation (IV-SFG) vibrational spectroscopy. Addition of Li+ to the Pt|[C2mim][FSA] system results in the extension of the electrochemical window (EW) by >1.0V at its negative edge. The potential dependence of the SF signal reveals that the [FSA]− anion of neat [C2mim][FSA] is desorbed at −1.5V while it remains in place even at −2.0V when Li+ is added. The SFG spectra indicate that the [FSA]− anion at the Pt|[C2mim][FSA] interface interacts with Li+ at the interface with the negatively-charged Pt electrode. This [FSA]− anion layer anchored through Li+ suppresses [C2mim]+ cation adsorption on the negatively-charged Pt electrode, resulting in a wider electrochemical window.

Effect of Surface Energy on Freezing Temperature of Water.

Previous studies have found that superhydrophobic surfaces are effective in delaying freezing of water droplets. However, the freezing process of water droplets on superhydrophobic surfaces depends on factors such as droplet size, surface area, roughness, and cooling rate. The role of surface energy, independent of any other parameters, in delaying freezing of water is not understood. Here, we have used infrared-visible sum frequency generation spectroscopy (SFG) to study the freezing of water next to solid substrates with water contact angles varying from 5° to 110°. We find that the freezing temperature of water decreases with increasing surface hydrophobicity only when the sample volume is small (∼10 μL). For a larger volume of water (∼300 μL), the freezing temperature is independent of surface energy. For water next to the surfaces with contact angle ≥54°, we observe a strong SFG peak associated with highly coordinated water. This research sheds new light on understanding the key factors in designing new anti-icing coatings.

Reverse Water-Gas Shift or Sabatier Methanation on Ni(110)? Stable Surface Species at Near-Ambient Pressure.

The interaction of CO, CO2, CO + H2, CO2 + H2, and CO + CO2 + H2 with the nickel (110) single crystal termination has been investigated at 10(-1) mbar in situ as a function of the surface temperature in the 300-525 K range by means of infrared-visible sum frequency generation (IR-vis SFG) vibrational spectroscopy and by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). Several stable surface species have been observed and identified. Besides atomic carbon and precursors for graphenic C phases, five nonequivalent CO species have been distinguished, evidencing the role of coadsorption effects with H and C atoms, of H-induced activation of CO, and of surface reconstruction. At low temperature, carbonate species produced by the interaction of CO2 with atomic oxygen, which stems from the dissociation of CO2 into CO + O, are found on the surface. A metastable activated CO2(-) species is also detected, being at the same time a precursor state toward dissociation into CO and O in the reverse water-gas shift mechanism and a reactive species that undergoes direct conversion in the Sabatier methanation process. Finally, the stability of ethylidyne is deduced on the basis of our spectroscopic observations.

Interfacial Water at Polyurethane-Sapphire Interface.

Infrared-visible sum frequency generation spectroscopy (SFG) was used to directly probe water between polyurethane (PU) and sapphire substrates after exposing samples to liquid water and water vapor. For liquid water, the observation of SFG peaks associated with H2O bands (3000-3400 cm(-1)) and D2O bands (2300-2600 cm(-1)) indicated water molecules diffused to the buried interface and existed in the form of a hydrogen-bonded water network. The water layer disrupted interactions between polyurethane and sapphire. When PU films were exposed to water vapor, the SFG peak intensities of PU hydrocarbon and sapphire hydroxyl groups changed significantly, which suggested water molecules had reached the interface. However, no hydrogen-bonded water bands were present; instead, the H2O peak at 3550 cm(-1) and D2O peaks (2600-2700 cm(-1)) were observed. We assigned these peaks to low-coordination water molecules or hydroxyl groups hydrogen bonded with carboxyl groups of PU at the interface. The water molecules did not form a uniform layer at the interface and as a consequence did not completely disrupt the PU/sapphire interactions. These results provide important implications for understanding interfacial adhesion, coatings, and corrosion.

Nondestructive Characterization of Molecular Structures at Buried Copper/Epoxy Interfaces and Their Relationship to Locus of Failure Analysis

Delamination at heterogeneous metal/polymer interfaces during reliability testing of packaged devices continues to be a reliability issue in microelectronic packaging. Although interfacial adhesion properties are largely determined by molecular structures at buried interfaces, structure-property relationships at buried metal/polymer interfaces are not well understood due to a lack of nondestructive interface sensitive analytical techniques. In this paper, methodology was developed to nondestructively characterize the molecular structure at buried copper/epoxy interfaces in situ using infrared-visible sum-frequency generation (SFG) vibrational spectroscopy. The methodology was applied to elucidate the relationship between molecular structure at the buried copper/epoxy interface and delamination behavior by correlating molecular structure with the locus of failure analysis. SFG and locus of failure analysis suggest that molecular ordering and silane behavior at the copper interface both contribute to a gradient in network structure and mechanical properties near the copper interface which was correlated with a locus of failure at an interfacial weak zone. The combined destructive and nondestructive characterization methodology developed here is general and can be directly applied to selectively characterize relationships between molecular structure at buried metal/polymer interfaces and interfacial properties such as adhesion, delamination, and interfacial moisture transport during Joint Electron Device Engineering Council qualification testing.

Polarization entanglement of sum-frequency photons: A tool to probe the Markovian limit

The article addresses the possibility of entanglement-specific infrared-visible sum-frequency generation spectroscopy. In the case of an anisotropic interface, it is possible to employ SSS and PSS polarizations to detect responses not only specific to ${\ensuremath{\chi}}_{YYY}$ and ${\ensuremath{\chi}}_{XYY}$ nonlinearities, but also to higher-order ${\ensuremath{\chi}}_{(YYY)(XYY)}$ and ${\ensuremath{\chi}}_{(XYY)(YYY)}$ nonlinearities. Using quantum mechanical studies of the rhenium complex $[\mathrm{Re}{(\mathrm{OH})}_{3}{(\mathrm{CO})}_{3}]$ as a molecular model, we demonstrate that if such complexes would form an anisotropic orientational distribution at a surface, under the considered geometry and the polarization settings, we may prepare quantum correlated $\mathrm{C}=\mathrm{O}$ vibrational states to emit polarization-entangled photons. Accordingly, we explore the possibility of a polarization-measurement protocol to extract spectral signatures of the entangled states. The response would be informative on intramolecular interactions. As a result, we discuss the possible practical implications in probing dynamics at interfaces, and different opportunities in the preparation of entangled vibrational states of quantified fidelity.

Nondestructive in situ characterization of molecular structures at the surface and buried interface of silicon-supported low-k dielectric films.

As low-k dielectric/copper interconnects continue to scale down in size, the interfaces of low-k dielectric materials will increasingly determine the structure and properties of the materials. We report an in situ nondestructive characterization method to characterize the molecular structure at the surface and buried interface of silicon-supported low-k dielectric thin films using interface sensitive infrared-visible sum frequency generation vibrational spectroscopy (SFG). Film thickness-dependent reflected SFG signals were observed, which were explained by multiple reflections of the input and SFG beams within the low-k film. The effect of multiple reflections on the SFG signal was determined by incorporating thin-film interference into the local field factors at the low-k/air and Si/low-k interfaces. Simulated thickness-dependent SFG spectra were then used to deduce the relative contributions of the low-k/air and low-k/Si interfaces to the detected SFG signal. The nonlinear susceptibilities at each interface, which are directly related to the interfacial molecular structure, were then deduced from the isolated interfacial contributions to the detected SFG signal. The method developed here is general and demonstrates that SFG measurements can be integrated into other modern analytical and microfabrication methods that utilize silicon-based substrates. Therefore, the molecular structure at the surface and buried interface of thin polymer or organic films deposited on silicon substrates can be measured in the same experimental geometry used to measure many optical, electrical, and mechanical properties.