Optical Spectroscopy of Surfaces, Interfaces, and Thin Films: A Status

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Review

Optical Spectroscopy of Surfaces, Interfaces, and Thin Films: A Status Report Kristen E Watts, Thomas J Blackburn, and Jeanne E Pemberton Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00735 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019

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Analytical Chemistry

Optical Spectroscopy of Surfaces, Interfaces, and Thin Films: A Status Report Kristen E. Watts, Thomas J. Blackburn, and Jeanne E. Pemberton* Department of Chemistry and Biochemistry University of Arizona 1306 East University Boulevard Tucson, AZ 85721

* Author to whom correspondence should be addressed: [email protected]

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Interfaces (i.e. inter-phases) can form between solids and liquids, solids and gases, liquids and gases, liquids and liquids, and liquids and gases. Surfaces and interfaces are ubiquitous in the chemical and biochemical sciences and play central roles in many critical processes and technologies in fields as diverse as biomedicine, heterogeneous catalysis, nanotechnology, energy conversion and storage, corrosion, lubrication, and chemical and biochemical sensing. This review covers work that uses optical spectroscopies in the ultraviolet-visible, infrared, and terahertz regions of the electromagnetic spectrum, or methods that probe events occurring at these frequencies, to probe surfaces, interfaces, and thin films reported between 2013 and 2018.

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Introduction Interfaces (i.e. inter-phases) can form between solids and liquids, solids and gases, liquids and gases, liquids and liquids, and liquids and gases. A single side of such an interface is considered a surface; processes occurring at or involving that surface are of primary interest in surface science. Alternately, processes in which events involving both sides of the interface are important define interfacial science more broadly. Surfaces and interfaces are ubiquitous in the chemical and biochemical sciences and play central roles in many critical processes and technologies in fields as diverse as biomedicine, heterogeneous catalysis, nanotechnology, energy conversion and storage, corrosion, lubrication, and chemical and biochemical sensing. In order to exert maximal control of and to extract maximal efficiency from such processes and technologies, understanding the details of the relevant surface and interfacial chemical and biochemical events at the atomic and molecular levels is essential. Toward that end, researchers can avail themselves of a vast array of surface and interface analysis tools. These are generally based on the interactions of electrons, electromagnetic radiation, or ions with surfaces and at interfaces to extract this understanding. Critical review of a scientific area of such breadth is certainly not readily feasible in the scope of a single manuscript, even for consideration of the research in a limited period of time. Thus, to provide a review that is more focused and hopefully more useful, we have restricted our consideration in this review to methods that involve the optical spectroscopies in the ultraviolet-visible (UV-vis), infrared (IR) and terahertz (THz) regions of the electromagnetic spectrum, or methods that probe events occurring at these frequencies, at interfaces and in molecular layers or thin films on surfaces. The time period generally covered by this review extends from January 2013 through October 2018. Obviously, given the breadth of even this more

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restricted scientific landscape, space limitations preclude a comprehensive review of this area. Thus, the authors endeavored to choose work for inclusion that was broadly representative of the diversity of optical methods being used for the investigation of surfaces, interfaces, and thin films and the new insight into chemical and biochemical details that these techniques are enabling. Omissions, inadvertent and otherwise, are the sole responsibility of the authors, with no negative connotations about the importance of any omitted work intended. We note that this is an area that has not been critically reviewed in Analytical Chemistry for many years. Indeed, although reviews of various forms of science and interface surface science and thin film analysis were regularly represented in the Analytical Chemistry reviews starting in the 1950’s, and many other aspects of surface and interfacial chemistry were reviewed in the context of specific applications areas such as catalysis and analytical sensors, the rapid growth and use of new surface and interface optical spectroscopic methodologies, along with the significant developments in more traditional methods, argue for a review of the current state of the field. Although there are very few truly new techniques represented in this review, significant innovations in traditional methods or their application, as well as further development of techniques first introduced within the past several decades, are allowing researchers to find increasingly creative ways to use optical methods to answer important questions in chemical and biochemical measurement science. Specific developments that deserve mention include innovations that allow microscopy and imaging at a lateral resolution better than the diffraction limit, especially in the area of vibrational spectroscopy, improved time resolution, and the increasing innovative use of plasmonic phenomena for chemical analysis. This review highlights selected works in the above areas chosen for their perceived significance either in terms of advancements in surface and interface analysis or the use of surface

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and interfacial phenomena for chemical analysis. The advances described here are organized into eight broad categories of impact: plasmonics, imaging and microscopy, biomedicine and biological systems, energy materials, electrochemical technologies, heterogeneous catalysis, buried interfaces, and cultural heritage and art conservation. Of course, the same collection of recent works could be discussed in a myriad of different ways; the approach to organization of this review was chosen simply for convenience. We note that certain sub-topics covered here have some overlap with more focused reviews that have been published in Analytical Chemistry over the past several years. In these cases, we have sought to update the status of these areas as they pertain more broadly to surface and interface analysis but not to fully replace the in-depth content of these previous reviews.

Plasmonics Continued advances in the understanding and control of surface plasmons has resulted in significant advances in the spectroscopy of surfaces and interfaces, leading to ultrasensitive detection strategies and diversifying new sensor possibilities. Although surface enhanced Raman spectroscopy (SERS) is the best-known method utilizing surface plasmon enhancement, surface plasmons are used as the basis for a variety of other enhanced spectroscopies as well, and new substrates that support surface plasmons continue to be identified. Surface plasmons have also been successfully harnessed to drive photocatalytic reaction chemistry at surfaces. Here, we discuss advances in substrate development for plasmonic enhancement, including plasmonics at two dimensional (2D) materials, and review recent advances in non-SERS plasmon enhanced spectroscopies. Representative applications of these plasmon-enhanced tools are highlighted.

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Developments in Plasmonic Substrates. The shape, size, and composition of nanomaterials have a significant impact on their use in plasmonic enhancement spectroscopies. The ability to carefully control not only the type of structure but the polydispersity of the particles can dramatically impact the reproducibility of a plasmon enhanced spectroscopic experiment. González-Rubio et al. used femtosecond laser pulse reshaping of Au nanorods for improved monodispersity. These researchers found that an optimized balance between energy deposited in the rods and the heat transferred to the surrounding medium improves the monodispersity of the resulting particles.1 Energy deposition into the nanorods can be controlled by means of the irradiation fluence, and energy transfer to the medium can be controlled by maintaining surfactant surface coverage, used for the seed-based nanoparticle growth, at the critical micelle concentration. Using such carefully controlled synthesis and modification methodologies, these researchers were able to reduce the spectral bandwidth of the Au nanorods as well as dramatically improve aspect ratio monodispersity. Fabrication of nanorods with this level of control can improve sensitivity, selectivity and reproducibility when used in sensing. Nanoparticle texturing has also been studied for plasmonic effects, as it is well understood that the maximum field enhancement occurs at points or vertexes on the surface of these nanostructures. Ma and coworkers describe a synthesis for Au “cabbage-like” microparticles (CLMPs) that are able to support an enhancement factor (EF) of 108. A benefit in addition to this large EF is that CLMPs can be used to monitor reactions at the single particle level because these CLMPs are significantly larger than wavelength of light and exhibit good stability in a wide range of solvents.2-3 The recent development of shell-isolated nanoparticles (SHINs) for enhanced Raman spectroscopy (SHINERS) has provided a highly versatile plasmonic substrate for enhanced

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spectroscopies. First described in 2010 by the Tian group, the motivations for development of these SHINs or “smart dust” were isolation of the core metal to prevent direct interaction with the analyte (e.g. chemical reactions with the nanoparticle or analyte adsorption onto the nanoparticle) and the ability to create tip enhanced Raman spectroscopy (TERS)-like localization of the SERS response without the need for a traditional TERS geometry.4 Their report described a method for creating silica-coated gold nanoparticles to satisfy both desired attributes, providing an electric field enhancements of 85x with a 4 nm silica shell, and even larger with a 2 nm shell (124x). In addition, they found that they could obtain TERS-quality information on biomaterials that were previously inaccessible due to low Raman scattering cross-sections. In 2015, this group expanded the range of SHINs to include silver core nanoparticles, finding a synthetic method to overcome the silver oxide layer that is formed over time and creating a corrosion resistant nanoparticle with a long lifetime. The silica barrier layer around the silver nanoparticle also prevents signal distortions due to direct interaction of the analyte with the metal surface.5 Taking the SHINs concept one step further, Huang et al. showed that by applying silica, aluminum oxide, or titanium oxide coatings on metallic atomic force microscopy (AFM) tips using atomic layer deposition, not only is corrosion and analyte adhesion reduced, but simultaneous measurement of tip enhanced fluorescence spectroscopy (TEFS) and TERS is feasible with enhancements of ~103.6 This simultaneous measurement is possible due to separation of the analyte from the metal surface which prevents fluorescent quenching via nonradiative energy transfer. Moreover, by carefully controlling shell thickness, the ratio of TERS and TEFS intensities can be controlled. SHINs have also been used in plasmon enhanced luminescence experiments. Plasmon interaction with emitters has been shown to affect critical luminescence parameters such as

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absorption efficiency, radiation rate, and emission direction as well as emission quenching by the metal nanoparticles as noted above. Shell-isolated nanoparticles (SHINs) provide an avenue for careful control of system parameters to enable optimization of the distance of the emitter from the metal nanoparticle for optimum enhancement of the luminescent signal. Two new geometries for maximizing emission signal from emitters were demonstrated by Meng as shown in Figure 1.7 Coupling mode (Figure 1b), in which emitters are placed between a SiO2-coated Ag film and the SHINs with a silica shell thickness of 6 nm, was determined to be optimal compared with the non-coupling mode (Figure 1a) in which no Ag film beneath the SiO2 substrate Figure 1. Schematic diagram of experimental modes based on the Ru dye-functionalized quartz and SiO2/Ag film substrates. a) Noncoupling mode: the molecules anchored on the quartz substrate without coupling area. b) Coupling mode: the molecules located on the plasmonic gap. c) Three-dimensional FDTD simulation of Ag@SiO2 on a quartz substrate and d) 2 nm SiO2/Ag film excited by 488 nm laser. Reprinted from Meng, M.; Zhang, F.-L.; Yi, J.; Lin, L.-H.; Zhang, C.-L.; Bodappa, N.; Li, C. Y.; Zhang, S.-J.; Aroca, R. F.; Tian, Z.-Q.; Li, J.-F. Anal. Chem., 2018, 90, 10837-10842 (ref. 7.) Copyright 2018 American Chemical Society.

is used. The coupling mode gives rise to signal enhancement by a factor of 330 along with an increase in emission decay rate by a factor of 124. They also showed that the emitter phosphorescence was so enhanced that it could still be detected in an oxygen

environment despite O2 being a known quencher. They propose that this methodology could be used as an effective approach for trace oxygen detection in surface analysis. For time dependent studies and in situ monitoring of chemical reactions, several ultrafast surface enhanced techniques have been described. A recent review by Keller et al.8 describes these in more detail. One such technique, surface enhanced-femtosecond stimulated Raman spectroscopy (SE-FSRS), utilizes noble metal substrates. However, the required pump and probe laser powers necessary can cause significant sample damage unless specific substrates are used.

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More specifically, successful implementation of SE-FSRS had been shown to only be possible with commercially-available Au nanoparticle oligomers functionalized with trans-1,2-bis(4pyridyl)-ethylene and coated with a thick silica layer; no other substrates proved robust enough to make SE-FSRS possible with a broader range of adsorbates. This limitation was overcome using an approach developed by Negru et al. who reported SE-FSRS from Au nanoparticles oligomerized with a variety of adsorbates stabilized using a thin layer of polyvinylpyrrolidone on the outside of the Au oligomer aggregates.9 This protocol for Au oligomer synthesis should enable SE-FSRS to be more broadly applied for time-resolved experiments. Although noble metal nanoparticles are a standard substrate for plasmon-enhanced spectroscopies, transition metal nanostructures have been of recent interest due to their utility in ultraviolet (UV) SERS and their resistance to oxide formation in atmosphere, making them reusable. Rhodium-based nanostructures are promising as they effectively satisfy both of the above conditions for suitable UV and deep UV (DUV) SERS substrates. Das and Soni report a slightly modified synthesis of oxide free nanostructures and demonstrated their suitability for label-free, femtomolar detection of biomolecules as well as the in-situ monitoring of plasmon-induced degradation of such biomolecules.10 These substrates, when irradiated in the UV and DUV, also support signal enhancements of 105 over SERS excited at visible wavelengths due to resonance Raman effects. Two-dimensional materials including graphene, metal chalcogenides, boron nitride, and most recently, black phosphorous have been shown to support surface plasmon enhancement of electric fields. A recent review by Zhang et al. covers the discovery of graphene enhanced Raman spectroscopy (GERS), the opportunity it provides to more carefully probe the chemical enhancement mechanism in SERS, and the avenues that GERS opens for exploration of the

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plasmonic properties of other 2D materials.11 GERS can also be supplemented by conventional SERS with the introduction of metal nanoparticles for a slightly different type of spectroscopy known as graphene-supported SERS and graphene-veiled SERS.11 Finally, this review described GERS “tape” as a flexible, universal substrate for sensing and quantitative analysis for applications of interest like food science.

Given this past comprehensive reiew, we cover here only

developments in 2D materials that have been published since this review. Of the reports on plasmonic enhancement by other 2D materials, SERS has been successfully demonstrated at semiconductors like MoS2 and boron nitride.12 Enhancement at these substrates arises from the resonant charge transfer state that forms between an analyte and the semiconductor at the energy of the excitation laser. Muehlethaler et al. reported a greatly improved EF for 4-mercaptopyridine on MoS2 of over 105 when using 467 nm (2.65 eV) excitation, which matches the energy of charge transfer from MoS2 to the analyte.13 Fabrication protocols for nanoarrays or nanoparticle-based substrates for surface enhanced spectroscopy experiments have traditionally been labor and cost-intensive. Commonly employed methods such as electron beam lithography are frequently prohibitively expensive, not readily scalable, low throughput, and require skilled operators. In addition, arrays produced by such approaches frequently possess a low density of plasmonic hot spots, leading to a reduction in overall enhanced signal. Nonetheless, such substrates are well suited for chemical analysis applications as analytes are readily drawn to active areas of the substrate for detection. Recent efforts have focused on new manufacturing approaches that are more scalable and that result in nanoarrays with better reproducibility of feature and hotspot monodispersity. A good discussion of these developments is found in a 2015 review by Dahlin on nanohole and nanopore arrays.14

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Here, we will focus on other nanoarray substrates that have been developed within the past two years since this review. Kanipe et al. described fabrication of highly reproducible substrates with a significant hotspot density based on traditional grating patterns using laser interference lithography, reactive ion etching, plasma-enhanced chemical vapor deposition, and e-beam deposition.15 Although this fabrication protocol is still complex, these nanorod arrays provided an EF for adsorbed thionine of 105. For more biocompatible nanoarrays, Lin et al. describe a method using hexagonally packed vertical Si nanorods loaded with adsorbed Au nanoparticles.16 The hexagonal packing creates a larger area not only for greater Au nanoparticle loading but also for large biomaterials, such as amyloid-β (Aβ) fibrils, to adsorb. With ordered packing, the Au nanoparticle “hotspots” contained on and coupled to the three dimensional (3D) semiconducting Si nanorod substrate, the SERS sensitivity of such substrates is enhanced over that of a random nanorod array while providing better spectral signal reproducibility. Finally, by adjusting the gap in the hexagonal nanorod array packing to 300 nm, these researchers also reported SERS spectra of a single, unlabeled Aβ fibril. Yap’s group reported a less complex fabrication method for Au nanocone arrays that provides flexible, transparent, high throughput, cost-effective, and sensitive SERS substrates of interest in multiple sensing applications such as food safety and military applications.17 Their approach uses nanoimprint lithography on polycarbonate sheets using a Ni mold with precisely defined periodicity, feature diameter, and feature-to-feature spacing. Of significance is that they demonstrated that this approach lends itself to scalability through a continuous roll-to-roll process employing polycarbonate sheets at a production rate of 3-5 m/min.

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In addition to finding more cost-effective ways to produce nanoarrays, efforts have also been undertaken to find new ways to pattern substrates beyond nanoholes, nanoantennas, nanorods, etc. Although some work had been done on creating enhancing substrates by replicating the nanostructures of biotemplates,18-21 Chou and coworkers took this approach one step further by developing a patterning method that takes advantage of the natural hydrophobic surface of a rose petal. The rose petal surface concentrates both the enhancing nanoparticles and analytes of interest in order to obtain sub-femtomolar limits of detection in a more eco-friendly and costeffective mode of preparing hydrophobic substrates. Their results demonstrated that the intact lower epidermis of a white rose petal is most effective in terms of signal enhancement. Additionally, they were able to take advantage of the “petal effect” by drying nanoparticles and analyte with the rose petal turned upside-down. The “petal effect” is described by a surface exhibiting both a high static contact angle and high contact angle hysteresis; therefore, a liquid droplet will not roll off, even if the surface is turned upside down in a natural application of the “Wenzel state” (homogeneous wetting of the rough, hydrophobic surface). An additional description of large EFs that can be obtained by highly patterned, superhydrophobic surfaces in SERS has been given by Yilmaz et al.22 Interestingly, the nanotextured films that these authors describe are not made from traditional metallic materials, but rather, are films of a known n-type small molecule organic semiconductor (OSC), α,ωdiperfluorohexylquaterthiophene (DFH-4T). Organic SERS substrates, particularly those of known semiconducting character, are of considerable interest because of the structural versatility they provide relative to their metallic counterparts in addition to more cost-effective and efficient production. Nanotextured films of DFH-4T supported SERS EFs of >103 without any metallic components. This EF increased to >1010 when the nanotextured film was coated with a thin layer

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of Au and was shown to yield sub-zeptomole levels of analyte detection. However, while the extra enhancement from the Au-plated films of these small molecule semiconductors is due to the added plasmonic nature of enhancement, the non-metallized films do not show any characteristic plasmonic transitions that rationalize the significant EF observed. Through quantum mechanical computations, the authors determined that the observed EF is due to an efficient resonant charge transfer from the DFH-4T film to the analyte (methylene blue) in addition to the highly nanotextured film they created through controlled vapor deposition of the DFH-4T. An approach for enhanced signal improvement has been developed that goes beyond nanomaterial patterning. Guo et al. describe a Fabry-Perot cavity substrate that augments typical enhancements in SERS or TERS experiments by approximately an order of magnitude by taking advantage of optical interference.23 This approach can be applied to virtually any SERS or TERS investigation without limitations on substrate or analyte and is particularly well-suited to experiments utilizing side illumination. In this approach, the sample is placed on top of a semitransparent mirror for coupling the cavity mode to the incident and scattered light with a transparent dielectric cavity spacer, in this case SiO2, with a precise thickness (optimized with experiment as described within the paper). The bottom layer is a fully reflective mirror with a thickness larger than the optical skin depth. Similarly, work on the development of improved substrates for surface enhanced IR absorbance spectroscopy (SEIRA) has also been reported in the review period. Pfitzner and coworkers reported an accessible process for nanoarray substrates for SEIRA.24 Using commercially available carbon-coated metal transmission electron microscopy grids as shadow masks, they developed disk-shaped Au nanoarrays by vapor deposition through regular holes in the carbon film with 2 µm dia and 3 µm pitch and tested these arrays in SEIRA measurements with

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self-assembled monolayers (SAMs) of 4-mercaptobenzoic acid (4-MBA) and genetically engineered variants of the transmembrane protein sensory rhodopsin II from Natronomonas pharaonis (NpSRII). These disk shaped nanoarrays supported surface enhancements of >104 and enabled monitoring of chemical phenomena such as the in situ pH titration of the 4-MBA monolayer (ML) and the light-induced isomerization of tethered NpSRII by SEIRA difference spectroscopy. Au nanostars, which have been used successfully in SERS due to their excellent electric field enhancement at each “point” of the star, have also proven useful for SEIRA. Bibikova et al. described not only the first use of Au nanostars in SEIRA, but also the first use of these nanofeatures for simultaneous SERS and SEIRA measurements without chemical modification. Using crystal violet as the probe, they demonstrated EFs of 103 for SERS and 5 for SEIRA.25 The majority of nanoarrays for SEIRA are made with Ag, Au, or other noble metal nanoantennas, but as with SERS substrate, researchers continue to seek new substrates for SEIRA. Cerjan et al. describe a method using aluminum antennas based on the similarity of the optical properties of Al and Au in the IR region.26 They used a “cross junction” of four orthogonal “arms” with a nano-gap junction in the middle to provide maximum signal enhancement in the mid-IR for multiple functional groups. Although the enhancement they observed was not as great as for the corresponding Au junction format, the Al-based SEIRA system, in addition to being more costefficient, also has the benefit of providing a natural oxide layer that can be used to functionalize the array. Additionally, by tuning one pair of the antenna arms to be resonant with the dominant ν(Al−O) mode of the surface oxide, the inherent SEIRA signal from the oxide layer can be used as an internal standard for quantitation. They used this approach to quantify stearic acid layers formed on nanoantenna surfaces.

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Surface enhancement in the IR utilizing plasmons at 2D graphene substrates has also been realized. Graphene plasmons have numerous desirable properties for sensing, including their high electric field confinement and plasmon resonance tunability by controlling graphene structure or electrostatic “doping”. This allows its use for enhanced spectroscopy in the THz and IR regions. Hu et al. have recently reported a sensor based on graphene nanoribbons on a thin CaF2 film. This device was shown to be capable of sub-ML detection of adsorbates throughout the entire IR fingerprint region due to the limited plasmon-phonon coupling between the graphene and the substrate that usually limits the sensitivity in this spectral region.27 They also demonstrate that this device is capable of simultaneous in-plane and out-of-plane vibrational mode detection which will support surface molecular orientation analysis based on these enhanced IR spectra. Alonso-González et al. have demonstrated graphene plasmon-metal phonon coupling, noted as a downside in the previously discussed paper, as a probe in the THz regime. This coupling results in a wavelength decrease of the graphene plasmon to 1/66 that of the incident photon, placing the plasmon in the THz region.28 These researchers also developed a system for mapping THz plasmons with microscopy in a set up similar to scattering scanning near-field optical microscopy (s-SNOM, see below). In this case, the metal tip is illuminated with the THz beam of a gas laser. When this tip is in close proximity to the sample, the near field of the nanofocused THz electric field induces a current in the graphene sheet, similar to IR photocurrent nanoscopy. Recording the current as a function of tip position yields nanoscale spatially-resolved THz photocurrent images of the plasmons within the graphene sheet. These THz graphene plasmons could be significant for use in multiple technologically relevant applications. Advances in Non-SERS Plasmon Enhanced Spectroscopies. Although SERS has been the focus of most of the attention in the plasmonic spectroscopy literature, other plasmonically-enhanced

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spectroscopies are also of increasing interest to researchers. SEIRA is the natural complement to SERS methods and has been a modestly active area of research over the past decade. Neubrech and coworkers provided an extensive review of this area in 2017 that details developments in the fundamental theory behind SEIRA and optimization of the technique;29 thus, these aspects of SEIRA will not be treated in depth here. These researchers conclude their review with a discussion of promising future applications such as use of plasmonic near-fields, hyperspectral infrared chemical imaging, sensing/detection of proteins and real-time monitoring of lipid membranes.29 A recent paper by Mackin et al. demonstrated the utilization of SEIRA principles in 2DIR (surface enhanced 2DIR spectroscopy, SE-2DIR) to study thin layers of materials down to 1 nm.30 A key advantage of 2DIR is the ability to probe cross peaks, or changes that occur in the IR spectrum not in the region actively being pumped, that can be linked to molecular structures and their dynamics. However, in SE-2DIR, cross-peak analysis can be problematic, as the cross peaks are typically several orders of magnitude weaker than the diagonal peaks. Moreover, the typical plasmon resonance of the nanoantenna has a full width at half maximum of only ~200 cm-1, making measurement of cross peaks challenging. demonstrated

These the

ability

researchers to

employ

multiple modes of 2DIR for films as thin as Figure 2. SE-2DIR magnitude ν(NN)/ν(C=O) cross-peak spectrum of 1 nm thick azNHS/PS on a Au array measured at a waiting time of 3.8 ps. The attached panels show the total linear absorption of the sample on the nanoarray under parallel (blue) and perpendicular (green, ×3000) polarizations. Note that the broad plasmon absorption (blue) is significantly detuned from both molecular transitions. Reprinted from Mackin, R. T.; Cohn, B.; Gandman, A.; Leger, J. D.; Chuntonov, L.; Rubtsov, I. V. J. Phys. Chem. C, 2018, 122, 11015-11023 (ref. 30.) Copyright 2018 American Chemical Society.

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1

nm

of

4-azidobutyrate-N-

hydroxysuccinimide ester (azNHS) diluted in polystyrene (PS) utilizing plasmonic enhancement. EFs for the (NN)/(C=O)


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cross peaks, known to be relatively weak, were as high as ~104 for the thinnest films. Despite the small amplitude, with plasmonic enhancement, cross peaks can be readily measured for such thin films as shown in Figure 2. Successful implementation of plasmonic enhancement in 2DIR spectroscopy should enable future studies of structural dynamics in minute amounts of materials on the nanoscale on standard SEIRA nanoarrays. New developments in surface enhanced spectroscopies in the THz regime have also been reported. Traditionally, surface enhanced THz spectroscopies have been performed by embedding the analyte of interest into an array of nanoslits resonating at THz frequencies. This allows very sensitive detection of materials ranging from small molecules to biofilms, but requires a priori knowledge of the absorption properties of the investigated analyte and does not provide information about the analyte spectral fingerprint (absorption peak positions and bandwidths). Toma and coworkers were among the first to report the use of nanoantennas with broad resonances in the THz region, similar to those used in SEIRA experiments, for surface enhanced THz spectroscopy.31 These workers demonstrated the potential of this nanoantenna enhanced THz spectroscopy (NETS) approach using CdSe quantum dots as test-bed nano-objects, since they exhibit a clear phonon resonance in the THz frequency region near 6 m to which the enhanced THz field was coupled. This new approach should enable general THz measurements on analytes within nano-volumes with the possibility to characterize individual nano-objects. Weber et al. expanded on this idea of using traditional SEIRA substrates, pairing it with the known increase in plasmonic near-field intensity, and thereby enhancement increase, with the size of the resonantly excited nanostructure.32 This work demonstrated the resonant antennaenhanced spectroscopy of C60, C70 and the amino acid threonine over the spectral range of the molecular fingerprint region from 4.5 to 45 THz (6.7−67 μm wavelength, 150−1500 cm−1) using

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standard Fourier transform IR (FTIR) spectroscopy. They observed an increase of the SEIRA enhancement of up to 2 orders of magnitude, with increasing enhancements being observed at lower frequencies. The observed scaling behavior of the SEIRA enhancement was in excellent agreement with the integrated near-field intensities of the corresponding resonantly excited nanoantennas, obtained from both numerical finite difference time-domain (FDTD) simulation as well as classical antenna theory. Moreover, the SEIRA enhancement was shown to exhibit a proportionality to λ3. These results are promising and suggest that plasmonic enhancement can be used to overcome the small absorption cross sections of molecules normally encountered in THZ spectroscopy due to the mismatch between molecular size and the THz wavelength. The application of plasmonics to nonlinear surface spectroscopies has also been reported. Davidson et al. described fabrication of a Au serrated nano-gap deposited onto a dielectric material with an inversion symmetry, here polymethyl methacrylate (PMMA), coupled with second harmonic generation (SHG).33 Their approach relies on higher harmonic conversion efficiencies than those attainable in bulk materials due to nanoscale electric-field confinement (i.e. near-field). Upon irradiation with an ultrafast laser pulse polarized perpendicular to the nanogap, the electric fields in this device oscillate with a period of ~2-3 fs, inducing polarization within the PMMA film and rendering it susceptible to second-order transitions that can be observed with an orthogonal probe laser. These researchers propose that this plasmon-enhanced SHG spectroscopy enabled by their light modulation set-up could allow differentiation of second-harmonic light within a dielectric material generated by resonant plasmons from light scattered by the plasmon itself. Understanding Plasmonic Effects. In addition to creating enhanced electric fields at plasmonic surfaces, plasmonic materials can also convert light into other forms of energy for applications such as photocatalysis, photovoltaics, or photothermal therapies. One challenge has been

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understanding the distribution of plasmon energy into photoexcited versus thermal pathways in these plasmon-assisted processes.

As noted above, Keller has written a review describing

advances in ultrafast surface-enhanced Raman spectroscopies primarily used for understanding plasmon-driven chemistries such as plasmon photocatalysis and plasmonic-based photovoltaic devices.8 This review includes a thorough discussion of the main ultrafast surface enhanced methods of femtosecond stimulated Raman spectroscopy (FSRS), coherent anti-Stokes Raman spectroscopy (SE-CARS), and time-resolved coherent anti-Stokes Raman spectroscopy (TR-SECARS) and applications thereof prior to 2015. Here, we restrict our discussion to new advances in understanding thermal effects in plasmonic chemistry and the theory of plasmon spectroscopic interpretation, especially that involving the convolution of near- and far-field effects. Previously employed methodologies for studying temperatures induced by plasmonic excitation have only provided ensemble effects without focusing directly on the hotspot at which the plasmonic-driven catalysis occurs. Keller and Frontiera describe the first Raman-based thermometry probe on the ultrafast time scale in order to have more catalytically relevant time scales for plasmonic and heat dissipation measurements.34 Using the relative intensities of Stokes and anti-Stokes scattering, temperatures corresponding to specific modes can be calculated using the Boltzmann relationship, giving ultrafast Raman spectroscopy the hot-spot selectivity that other temperature probes have lacked. These researchers were able to show that energy deposited into species adsorbed onto plasmonic structures depletes very rapidly with temperature increases of only a few degrees Kelvin. Their conclusion that plasmon-driven chemistry is not thermally driven has important implications for plasmon-based photovoltaics and photocatalysis. To better understand the effects of plasmons in near-field versus far-field experiments, which until recently have been only poorly understood, Kurouski and coworkers investigated near-

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field and far-field relationships in 3D SERS substrates. They found that the near-field contribution was due primarily to dimer, trimer, and higher order nanoparticle clusters, because these clusters contribute to the local surface plasmon resonance in the near-field, but concluded that understanding of the far-field response would require additional technical and computational advancements.35 In a follow up study, Kurouski and coworkers examined two types of 2D filmover-nanosphere (FON) SERS substrates, which are characterized by a metal film over a layer of closely packed silica or polystyrene nanospheres supported on a substrate.36 The first FON substrate studied contained relatively uniform nanopillars formed by spinning the nanosphere substrate during metal vapor deposition (SP-FON). The second type of FON substrate resulted when the nanosphere substrate is held stationary during metal deposition to form more heterogeneous blob-like structures (ST-FON). The near and far-field effects were compared and contrasted for each type of FON for both gold and silver films using wavelength scan SERS between 725 and 825 nm. Despite the fact that the SERS spectra from each type of surface are similar, their near- and far-field effects are different. The SP-FONs showed no spectral offset between the maximum of the near-field response reported by the enhancement factor and the minimum of the far-field response reported by reflectance spectra; however, ST-FONs had about a 45-60 nm red-shift of the near-field EF maximum from the far-field reflectance minimum. Differences in the spectral alignment of these two types of FON substrates was attributed to the different degrees of polydispersity at the nanoscale in the feature sizes and shapes with SP-FONs being considerably more monodisperse. This work has important implications for the control of surface feature morphology necessary for complete control of near- and far-field behaviors of plasmonic materials.

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Related to the understanding of near-field versus far-field plasmonic effects is the poor interlaboratory reproducibility that has plagued TERS. Poliani and coworkers assert that one of the main reasons for this lies in the poor understanding of near-field light-matter interactions despite the now well-developed fundamental understanding of SERS. To advance understanding of near-field light-matter interactions, these researchers undertook TERS experiments using polarized excitation on three different materials, germanium-doped gallium nitride, graphene, and carbon nanotubes.37 They investigated the dependence of TERS on incoming light polarization, demonstrating independence of the TERS signal on incoming light polarization. They explained their experimental results using a quantum mechanical description that accounts for plasmon polaritons and the corresponding evanescent field at the tip. They postulate that the selection rules break down in TERS because of photon tunneling by perturbation of the evanescent field with concomitant annihilation of the plasmon polariton. An additional problem with SERS and other surface enhanced techniques for studying surface/adsorbate interactions or very small concentrations of analyte is that the Raman signal is frequently distorted. In the process of spectral distortion, the chemical information about the analyte is then lost. This is due to the localized surface plasmon resonance (LSPR) that provides electric field hotspots for the signal enhancement to occur, but at the same time, also induces a plasmonic spectral shaping effect (PSSE) that yields widely varying signal intensities and distorted spectral features. Lin et al. showed that the photoluminescence (PL) spectra of nanomaterials frequently used in these experiments also have a PSSE that can distort the relative strength of the peaks in the Raman spectrum.38 Such distortion can greatly complicate interpretation of the spectra to extract information such as molecular orientation. Recognizing that the PL spectrum is frequently considered to be the source of the SERS continuum background, these workers

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developed a rigorous background correction to retrieve the original chemical information of the analyte without the distortion induced by the PSSE. We note that although this work was published prior to the work of Poliani et al. noted above, it suggests that the violation of Raman selection rules in TERS might be due to PSSEs. To better understand dye interactions with metallic nanoparticles, Darby and coworkers developed a novel integrating sphere UV-vis absorbance spectrometer to make ultrasensitive absorbance measurements in strongly scattering media. They applied this instrument to investigate changes in dye absorbance properties upon adsorption to metallic nanoparticles.39 Through careful analysis of the differential cross-section and working under conditions of low dye surface coverage, these researchers showed that weak molecular resonances of an adsorbed dye can be disentangled from the optical resonance of the nanoparticles, the strongly-coupled plasmonmolecule resonances, and dye-dye resonance interactions.

Imaging & Microscopy The ultimate goal of recent efforts in imaging and microscopy is to maximize spatial resolution while attaining high information content on sample properties (chemical, mechanical, etc.) For information about molecular properties, vibrational spectroscopy methods have been of considerable importance. As noted above, TERS has played a prominent role in this area, although developments in IR imaging have also been substantial in the time period reviewed here. Certain developments in optics as well as increased use of advanced light sources such as quantum cascade lasers, synchrotron radiation, and free electron lasers have also been notable, contributing to progress in chemical mapping with simultaneous high-resolution imaging.

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A number of excellent reviews and perspectives have been published recently that describe the theory, implementation, and application of a wide variety of optical spectroscopy imaging tools. Verma produced a comprehensive review on TERS combined with scanning near-field optical microscopy (SNOM) for nanoscale chemical imaging, including considerations for instrument development, choice of tip metal, choice of imaging mode, as well as describing some novel applications.40 A recent review by Pozzi and coworkers discusses TERS theory and the implementation of ultrahigh vacuum (UHV) TERS. UHV TERS has the added benefit of an inert sampling environment, decreasing the rate of sample photobleaching, and greatly enhancing spatial resolution (frequently demonstrated below 5 nm).41 In another review, Zhang, Fang, and Zhu discuss the fundamentals of aperture SNOM paired with TERS for chemical imaging, with a specific focus on the implementation of imaging aperture TERS, innovations in aperture tip design, as well as novel applications for the technique.42 Centrone wrote a thorough review on the tipbased IR imaging techniques of s-SNOM and photothermal-induced resonance (PTIR).43 Muller, Pollard, and Raschke recently provided an interesting perspective on the implementation of IR imaging techniques and how these provide important imaging information, such as molecular orientation, and dynamic information with certain ultrafast iterations.44 Finally, a recent review by Dazzi and Prater provides an updated discussion of the history, development, theory, and instrumentation for atomic force microscopy (AFM)-based IR spectroscopy (AFM-IR) and its diverse applications.45 Reviews of other optical microscopy and imaging methods have also appeared. Li, Kuang, and Liu wrote a concise perspective describing novel innovations and prospects for fluorescence nanoscopy; notable is the ability of these imaging modes to overcome the diffraction limit that has plagued the spatial resolution of optical microscopy in the past.46

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Since the aforementioned reviews are extensive, we focus here on important recent innovations in instrumentation and modalities of these imaging techniques as they have been applied to advance the study of surfaces and interfaces. Specifically, we focus on novel techniques of vibrational spectroscopic imaging, THz imaging, and optical imaging (e.g. fluorescence and refractive index microscopies). Finally, we discuss prominent innovations in optics as a nod to future directions for chemical and morphological spectroscopic imaging techniques for furthering the understanding of surfaces and interfaces. Details of the use of some of these techniques is deferred to later sections describing specific applications areas as appropriate. Vibrational Spectroscopy Imaging. Approaches to overcome the diffraction limit of µm’s for nanoscale IR chemical imaging has become a highly active area of research in recent years. Typically, these approaches have employed tip-based technologies operating in the near field such as s-SNOM or near-field thermal emission (NFTE). This latter technique relies on thermal emission from the sample itself, thereby avoiding the need for a radiation source. Peragut et al. describe an instrument that allows them to combine both techniques, detecting near-field effects based on either thermal emission from the sample via scanning tunneling microscopy (STM, thermal radiation scanning tunneling microscopy) or externally-generated near-field using a synchrotron source illuminating the tip-sample area. They demonstrated the efficacy of their instrument in providing sub-wavelength broadband imaging of Au features on a SiC substrate, as well as excellent spatial and spectral resolution in chemical maps based on the SiC phonons.47 Sources have been an important consideration in recent implementations of IR s-SNOM with many instruments using synchrotron radiation sources. These instruments usually make use of an asymmetric Michelson interferometer that provides spectral data on both components of the complex refractive index of a sample but whose geometry necessitates that the AFM probe be

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located within the interferometer, rendering it vulnerable to minor fluctuations in alignment over the long sample arm (several cm). In an attempt to mitigate this drawback, Pollard et al. utilized a symmetric Michelson interferometer geometry that uses the undesired signal from the far-field electric field as an internal reference. This so-called self-referenced interferometry model allowed accurate determination of vibrational peak shape.48 They also demonstrated that this technique can be applied to synchrotron IR nanospectroscopy (SINS), allowing more rapid measurements with better reproducibility than the less stable asymmetric interferometer design. In addition to synchrotron sources, Kehr et al. described their implementation of infrared s-SNOM, which they call “s-SNIM” for scattering scanning near-field IR microscopy, at the free-electron laser in Dresden, Germany.49 Their source has a range from the mid-IR to the THz regime and has been demonstrated to yield high quality morphological images as well as chemical maps with the added capability of being able to image samples at temperatures as low as 4 K. Most IR s-SNOM nano-imaging studies have focused on the near- and mid-IR spectral regimes, limited in part by the availability of continuous wave sources as well as detector capabilities. Khatib et al. have recently extended the range of IR nanoscopy into the far infrared (FIR) region through use of the coherent, broadband radiation emitted from a synchrotron source in addition to development of a novel MHz bandwidth Ge:Cu photoconductor.50 Their instrumental scheme was tested at two beamlines at the Advanced Light Source, one with a modified AFM with IR detection and the other with a commercial IR nanoscope, and was successful in imaging FIR phenomena ranging from graphene plasmons to sapphire phonons with a spatial resolution down to 30 nm. These developments have successfully opened the door for new IR nano-imaging applications and further demonstrate the importance of synchrotron radiation in the advancement of vibrational nanoscopic imaging.

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PTIR-based AFM-IR was first developed by Dazzi et al. in 2005.51 Since its advent, it has been used in a range of non-scattering IR imaging modes on the nanoscale as well as supporting the acquisition of chemical and topographical information about samples. However, in its early phases, PTIR was limited in its sensitivity and was applicable only to films >15 nm in thickness. Moreover, the spatial resolution in PTIR was limited to the thermal diffusion length through the sample. Lu, Jin, and Belkin described an important modification to this technique by detecting mechanical forces exerted by photothermally expanding molecules on the AFM tip, resulting in sensitivity improvements of multiple orders of magnitude over both traditional PTIR and IR sSNOM techniques as well as providing spatial resolution limited by the size of the AFM tip.52 They demonstrated the ability to image sub-ML surface coverage by taking advantage of electromagnetic enhancement at a sharp Au tip over a coated Au substrate, in addition to the mechanical enhancement realized by operating the quantum cascade laser (QCL) at a pulse frequency equivalent to the cantilever bending frequency. Donaldson et al. described the use of synchrotron radiation to enable two types of photothermal near-field IR spectroscopy: cantilever resonant thermal expansion and scanning thermal microscopy.53 However, they note extremely long spectral acquisition times compared with the pulsed QCL source approach just described, making chemical image acquisition untenable. They noted that even though they were able to obtain decent spectra with an IR based scanning thermal microscopic measurement compared to previous measurements of the same type, the spectra resulting from the resonance enhanced photothermal expansion FTIR experiment were of far superior signal-to-noise ratio. One of the main drawbacks with AFM-IR is that the effects of sample mechanical properties on the resulting IR spectral data are frequently not considered, mostly due to an

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incomplete understanding of probe-sample mechanical interactions. Kenkel et al. addressed this through development of a mathematical deconvolution method based on description of the cantilever transfer function and correcting for the mechanical properties of the sample beneath the cantilever.54 They demonstrated the efficacy of their approach in correcting artifacts in the AFMIR imaging of polymers and epithelial cell lines which therein dramatically improved the accuracy and sensitivity of this technique. Jin et al. developed the ability to perform AFM-IR in water with a technique called resonantly enhanced infrared photoexpansion nanospectroscopy (REINS).55 In this technique, the sample is placed onto a prism for evanescent illumination from beneath the sample and a Au AFM tip is brought into contact with the sample droplet. The sample and tip are subsequently covered by a glass cover slip positioned above the AFM tip and area to be imaged. The IR beam is nanofocused onto the sample, and the photothermal expansion is analyzed to circumvent the water absorption observed in the IR analysis. Since the AFM tip is operating in contact mode, mechanical dampening of the tip is prevented. Liquid dampening does affect REINS but is compensated through control of the laser excitation pulse rate. By careful control of the sampling noted above, Jin et al. were able to expand IR imaging to aqueous environments while obtaining a spatial resolution of 25 nm. Although PTIR-based AFM-IR has been shown to be useful in many applications, it is incapable of measuring time-dependent thermal phenomena directly, as would be necessary to assess material thermal conductivity. Katzenmeyer and coworkers recently described implementation of a common thermal spectroscopy, photothermal microspectroscopy (PTMS), in an imaging mode called scanning thermal IR microscopy (STIRM) to meet this need.56 STIRM differs from PTIR in that it directly measures local heating of a sample instead of thermal

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expansion in addition to also providing topographic and scanning thermal microscopy (SThM) information. With modification of nanoscale tips traditionally used for scanning thermal microscopy and utilization of total internally reflected light to prevent IR absorption in the tip, these researchers were able to overcome the simultaneous spatial resolution and sensitivity limits previously associated with PTMS. Another recent innovation in tip-based IR imaging techniques is photo-induced force microscopy (PiFM). PiFM relies on the interaction between the dipole induced in the sample through s-SNOM techniques with the mirror dipole induced in the scanning tip. In contrast to PTIR, PiFM can be operated in non-contact or tapping modes, and thus, can be used for characterization of soft matter films. Nowak et al. describe the first implementation of PiFM in the IR region for sub-diffraction limit imaging in tandem with fingerprint region chemical information acquisition for differentiation of block co-polymer species within a mixed film.57 In addition to high spatial resolution, the experiment allows rapid sample image acquisition without sacrificing spatial resolution. An additional drawback of PTIR is that it cannot provide simultaneous chemical and mechanical information with topography. To address this shortcoming, Wang et al. describe another variant of AFM-IR nano-imaging called peak force IR microscopy (PFIR) that uses a synchronized frequency-tunable QCL laser that impinges on the AFM cantilever at half the tipsurface tapping frequency, allowing time-gated detection of the laser-induced mechanical responses in the sample.58 The mechanical properties of the sample are obtained by a position sensor on the laser noting the extent of difference in cantilever deflection with and without IR light, taking advantage of the same thermal expansion principles as in PTIR. They demonstrated this technique on a range of sample types, including polymer islands, block co-polymers, boron

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nitride nanotubes, and perovskites, showing ~10 nm spatial resolution while obtaining information regarding the mechanical properties of the materials, such as modulus and adhesion. Figure 3 shows an example of the excellent spatial resolution obtained for islands of poly(2-vinylpyridine) (P2VP) on a Au surface.

Figure 3. PFIR imaging and spectroscopy on P2VP. A) Topography of nanoscale P2VP polymer islands on a Au substrate. B) The PFIR image of the same area with an IR frequency of 1433 cm−1. Islands of P2VP show increased PFIR response. C) and D) The measured modulus and adhesion maps of the same area. The mechanical responses are simultaneously collected with the PFIR image. E) PFIR spectra based on the oscillation amplitude of contact resonance (blue curve) and baseline offset (red curve) from the marked locations (black circle) in B). The FTIR spectrum (dashed curve) is included as a reference. Reprinted from Wang, L.; Wang, H.; Wagner, M.; Yan, Y.; Jakob, D. S.; Xu, X. G., Sci. Adv. 2017, 3, e1700255 (ref. 58) licensed under Creative Commons license CC BY 4.0. Copyright 2017 American Association for the Advancement of Science.

Amenabar et al. reported hyperspectral IR nanoimaging by nano-FTIR spectroscopy using a tunable mid-IR laser continuum. In their approach, multiple bandwidth-limited nano-FTIR spectra are recorded and combined over the spectral range from 1,000 to 1,900 cm-1 at each of about 5,000 pixels of a 2D sample area after correction for drift. These researchers demonstrated their method by mapping polymer-polymer interfaces with nanoscale spatial resolution, and demonstrated that standard multivariate analysis of the hyperspectral images can be performed.59 New applications of these tip-based IR nanospectroscopy approaches are continuing to emerge. A recent application of infrared s-SNOM includes the field of corrosion science. Johnson and Böhmler describe one such application of s-SNOM to the study of corrosion processes on copper.60 They were able to show Cu2O and Cu(HCOO)2 corrosion products on a copper plate with 29



a spatial resolution of 20 nm. They were further able to demonstrate that the formation of Cu(HCOO)2 is inhomogenous, existing as very small patches on the surface. Kim et al. applied sSNOM to investigate the stacking domains in few-layer graphene (FLG), and they were able to distinguish two domains.61 The ability to distinguish between the domains has suggested s-SNOM would be applicable to other local properties such as strain, doping, and monitoring functionalization of a surface. This enables s-SNOM to be applicable to a wide variety of molecular structure analysis for a myriad of fields. PTIR has been successfully employed for studies in environmental chemistry, specifically for study of atmospheric aerosols, organic and inorganic, deposited onto a substrate. Or et al. recently described the use of PTIR AFM-IR to characterize the morphology, chemical composition, reactivity and mechanical properties of these aerosols and demonstrated the effects of humidity on their size, shape, and IR spectra.62 Kelchtermans et al. utilized PTIR to analyze a multilayer cling film of unknown structure comprised of polyethyelene and polyamide.63 Of particular note, these researchers observed a unique interface layer between the polyethylene and polyamide with distinct spectral and mechanical properties that was ultimately identified as a less-branched polyethylene layer grafted with maleic anhydride to ensure good covalent bonding with the amide groups on the polyamide. AFM-IR techniques are also frequently used in tandem with traditional IR methods to provide greater insight into interface chemistry, with more traditional IR methods signaling the average response of the system and AFM-IR illuminating more localized information. Morsch et al. utilized AFM-IR and FTIR to study water uptake in organic coatings, with FTIR providing initial evidence for increased atmospheric water uptake after presoaking the sample.64 AFM-IR provided definitive evidence for highly localized water uptake on the coating surface. Collectively,

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the information from these two methods enabled determination of a definitive water uptake mechanism in these films. Delen and coworkers applied AFM-IR and IR reflection-absorption spectroscopy (IRRAS) to analyze the growth mechanism of a surface-tethered metal-organic framework (MOF).65 IRRAS showed fast MOF growth only when complete drying between steps was allowed, while AFM-IR provided additional evidence that MOF growth was facilitated by the rapidity of the aforementioned drying process. The combination of techniques provides two different sets of information, as IRRAS represents an average response of the system while AFMIR is a highly localized technique. As an alternative to tip-based IR chemical and morphological imaging, the imaging ability of SEIRA has also been explored. Hasenkampf and coworkers demonstrated that the imaging capabilities of SEIRA could be greatly enhanced through use of a QCL coupled with a plasmonic nanoantenna array allowing imaging of a several mm2 region of the surface in as little as five minutes at a spatial resolution of ~9 µm with 2.5 cm-1 spectral resolution.66 Kühner et al. have also been interested in furthering the application of imaging SEIRA in order to map the location of materials across a surface of plasmonic nanoantennas.67 In their proof of concept experiment, these researchers imaged C60 and pentacene thin films on fabricated gold nanoantenna on a CaF2 substrate, and were able to spatially resolve 30-100 nm thick films on adjacent nanoantenna. No enhanced signal was obtained from regions of the substrate that did not contain these plasmonic hot spots. Although this approach does not provide the sub-diffraction limit spatial resolution of its AFM-IR imaging counterparts, SEIRA-based chemical imaging has potentially interesting implications for broadly accessible thin film sensing. Discrete frequency (DF) imaging as developed by Wrobel et al. utilizes the polarization of light to determine orientation in a manner similar to polarization-modulated IRRAS (PM-

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IRRAS).68 However, it does so without the need for a polarization-switching filter; instead, it relies on a polarized IR quantum cascade laser of discrete frequency. These researchers demonstrated the ability of this technique to provide orientation information under conditions of rapid spectral data collection with a speed enhancement of ~180x. The new technique is designed to image large areas in the tens of mm size range, thereby expanding the image size considerably compared to conventional techniques. The combination of image size and speed make DF imaging applicable to monitoring surface dynamics that occur rapidly and over large areas that currently cannot be monitored. Ostrander et al. reported the first application of 2DIR image collection using a wide-field microscope with diffraction-limited spatial resolution, demonstrated by imaging of metal carbonyl beads.69 As noted in the previous section, 2DIR can provide a wealth of information about a sample, ranging from molecular dynamics to energy transfer and vibrational coupling. By developing a method to incorporate microscopic imaging with 2DIR, the trove of chemical information contained in the 2D spectral signature can now be correlated to the sample within a spatial context. As noted above, TERS as a high-resolution chemical mapping probe is well known across many scientific communities. TERS combines the chemical information gained from Raman spectroscopy with the spatial resolution of SPM. A nanofabricated SPM tip made out of a plasmonic materials, such as Au or Ag, enhances the electric field up to a billion-fold by focusing the field to the tip apex. When carried out under UHV conditions, TERS can probe adsorbatesubstrate interactions allowing determination of the molecular structure of a thin film. In 2015, Jiang et al. showed the tremendous power of TERS coupled with UHV low-temperature STM by demonstrating the ability of this technique to resolve adjacent molecules with similar structures

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adsorbed on a surface. This work demonstrated the spectral distinction of adjacent porphyrin structures, one zinc chelated and the second as the free base, present on a surface with a spatial resolution of 0.6 nm, smaller than the size of either molecule.70 In similar work, Chiang and coworkers employed both TERS and TEFS to investigate the free-base meso-tetrakis-(3,5ditertiarybutylphenyl)-porphyrin. By selectively exciting Q-bands of this porphyrin, these researchers were able to elucidate information about vibronic excited states using TERS, whereas TEFS results indicated weak coupling of the molecular adlayer to the Ag(111) substrate.71 In a following study, these researchers used UHV TERS to distinguish conformational changes between neighboring porphyrin molecules with Å resolution.72 More recently, Chiang and coworkers used UHV TERS to study intermolecular interactions of N-N′-bis(2,6-diisopropylphenyl)-perylene-3,4:9,10-bis(dicarboximide) (PDI) MLs on Ag(111) and Ag(110). Their results revealed vibrational symmetry breaking in self-assembled PDI islands due to strong lateral intermolecular interactions between the diisopropylphenyl end groups and the center perylene rings of PDI. These interactions were confirmed by time dependent density functional theory (TDDFT).73 Similarly, Jiang et al. used TERS to study layers of N-N′bis(2,6-diisopropylphenyl)-1,7-(4′-t- butylphenoxy)perylene-3,4:9,10-bis(dicarboximide) (PPDI) on Ag(100). At room temperature, both condensed and diffusing domains of PPDI were identified and spatially resolved. In combination with TDDFT, the orientation of PPDI molecules at dynamic domain boundaries was determined with ~4 nm lateral resolution.74 The Zenobi group used TERS and density functional theory (DFT) to investigate adsorbed fluorinated hydrocarbon (carboxylic-fantrip) MLs formed by Langmuir-Blodgett deposition and sub-ML formed by spin coating.75 Through analysis of Raman peak shifts and line width changes that occur due to dispersion effects and surface selection rules, these researchers were able to

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distinguish between randomly oriented molecules and those in a locked orientation at a spatial resolution

Optical Spectroscopy of Surfaces, Interfaces, and Thin Films: A Status

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