Probing On-Surface Chemistry at the Nanoscale Using Tip-Enhanced Raman Spectroscopy
Zhen‐Feng Cai, Naresh Kumar, Renato Zenobi
Abstract
Open AccessCCS ChemistryMINI REVIEWS6 Oct 2022Probing On-Surface Chemistry at the Nanoscale Using Tip-Enhanced Raman Spectroscopy Zhen-Feng Cai, Naresh Kumar and Renato Zenobi Zhen-Feng Cai Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich CH-8093 , Naresh Kumar Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich CH-8093 and Renato Zenobi *Corresponding author: E-mail Address: [email protected] Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich CH-8093 https://doi.org/10.31635/ccschem.022.202202287 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Chemistry on solid surfaces is central to many research areas of practical interest, such as synthesis, catalysis, electrochemistry, photochemistry, and materials science. A comprehensive understanding of the nanoscale on-surface chemistry involved in these areas is important for establishing composition–structure–performance relationships. With the rapid development of tip-enhanced Raman spectroscopy (TERS), it has become possible to investigate physical and chemical processes on suitable surfaces at the nanoscale level and in real space. In this review, after a brief introduction of the background of on-surface chemistry and TERS, we systematically discuss the progress in the application of TERS in this field. Our focus is the applications of TERS to nanoscale coordination processes, decomposition reactions, polymerization processes, electrochemical reactions, catalytic chemistry, and functionalization chemistry on solid surfaces. We conclude by discussing the future challenges and development of TERS techniques and related applications in on-surface chemistry. Download figure Download PowerPoint Introduction Nanoscience and nanotechnology have been widely applied in different fields during the development of materials and devices and have great potential to overcome problems and challenges in the chemical and biological industries.1–3 Surfaces and interfaces play a crucial role in determining the quality and performance of nanostructure-based devices.4–6 Various chemical and physical processes occur in this region, including molecular diffusion, friction, adsorption and desorption, electron transfer, chemical reactions, surface reconstruction, and phase transitions.7,8 Each of these processes is highly dynamic in time and space; almost always, several of the processes occur simultaneously, leading to complex compositional and structural changes on the nanoscale. Therefore, it is crucial to have a comprehensive understanding of the nanoscale on-surface chemistry involved. The characterization of on-surface systems and related chemical processes requires structural information, chemical sensitivity, and high (nanoscale) spatial resolution. For traditional surface analytical tools, ensemble averaging obscures the vast complexity and heterogeneity of all accessible molecular microstates.9,10 However, important information concerning site-specific behavior, distributions in molecular dynamics, interactions, and reactions cannot be obtained on the ensemble level. Advanced scanning probe microscopy can characterize the surface topography with atomic resolution, but it is not able to reveal the chemical nature of the adsorbed species.11–15 It is essential to provide information for both the surface and its chemical makeup with molecular specificity and nanometer spatial resolution so that a complete understanding of the surface/interfacial processes and related composition–structure–performance relationships can be established. Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for obtaining deeper insight into the nanoscale mechanisms of physicochemical processes on surfaces and chemistry that are obscured by ensemble averaging. Tip-Enhanced Raman Spectroscopy TERS, first demonstrated in the year 2000, opened new ways to investigate surface phenomena with nanometer-scale spatial resolution, thanks to the strongly localized nature of the enhanced electromagnetic (EM) field at the tip.16–18 As illustrated in Figure 1a–c, a sharp metallic tip (typically a Au or Ag tip) irradiated with laser light is used to create a single hot spot between the tip and the sample. When the tip is brought into close proximity of a sample, this highly localized and enhanced field can selectively probe the local chemical and electronic structure by observing the Raman scattered light, with a spatial resolution of a few nanometers. If a metallic material with a high refractive index is used as a substrate, even stronger field intensity and enhancement can be generated due to electromagnetic coupling between the tip and the substrate. This so-called "gap-mode" TERS configuration further improves sensitivity and spatial resolution. When the tip scans over the sample, the spatially resolved Raman and topographic information can be recorded simultaneously. Figure 1 | Schematic diagram of a TERS setup and applications in different areas of on-surface chemistry. (a) TERS measurements on solid surfaces. (b) Illustration of the enhanced electromagnetic field localized at the tip apex. (c) Schematic diagram of the applications of TERS in different areas of on-surface chemistry. Download figure Download PowerPoint TERS combines the high spatial resolution of scanning probe microscopy with the high chemical sensitivity of surface-enhanced Raman spectroscopy (SERS).19 The diffraction limit does not play a role in TERS, because the Raman signals are dominated by the molecules in the nanocavity where the EM field is highly enhanced and confined. Compared to confocal Raman spectroscopy, TERS follows surface selection rules and will not reveal all Raman-active modes. A TERS spectrum mainly shows the Raman-active modes that have polarizability tensor components normal to the plane of the sample. Compared to SERS and shell-isolated nanoparticle-enhanced Raman spectroscopy,9,19,20 TERS provides nanometer spatial resolution that these methods cannot achieve. TERS enables a direct, noninvasive, and label-free spectroscopic investigation and can be applied in various environments, such as ambient air, ultrahigh vacuum (UHV), and electrochemical cells, as well as in solution.7,19,21 The spatial resolution of TERS can even reach the subnanometer regime under UHV conditions and at cryogenic temperatures, where drift is low. All of these features make TERS a promising technique for chemical analysis of on-surface systems and offer nanoscale insights into the composition–structure–performance relationships. The initial applications of TERS dealt with the detection of molecular films deposited on solid surfaces16 and, later on, with chemical imaging of carbon nanomaterials,22–26 nanostructured semiconductors,27,28 aromatic molecules,29–31 and biomolecules.32–35 In recent years, time-, potential-, and light-dependent TERS experiments, even in dynamic physical and chemical systems, were reported.36–41 In this review, we will mainly focus on the progress in visualizing on-surface chemistry by TERS, which is one of the techniques of choice to study these systems. Challenges and future developments of TERS techniques and related applications in the field will be discussed in the outlook. TERS Applications in On-Surface Chemistry On-surface coordination chemistry Bottom-up fabrication of functional metal-organic structures on surfaces provides coordination systems suitable for developing devices with controllable properties through different combinations and spatial arrangements of building blocks.42–44 The investigation of the fundamental details related to the molecular specificity, configuration, and orientation in these systems is of great importance to the understanding of structure–reactivity relationships. The spatial resolution of TERS is key to obtaining such information on the surface because coordination complexes present as minority species would simply be hidden in ensemble measurements. For example, Domke and Pettinger45 investigated the nanoscale chemical and topographic information of self-assembled adlayers of cobalt tetraphenylporphyrin (CoTPP) on Au(111) using TERS. By comparing the scanning tunneling microscopy (STM) images of different regions, they observed areas with a structurally ordered CoTPP adlayer and others with a disordered phase. Based on the chemical analysis of the TER spectra of these two phases, they found several new bands that appeared in the TER spectrum recorded from the disordered phase (Figure 2a), which were assigned to the on-surface coordination of axial CO and/or NO ligands on the Co center based on previous reports. Although measurements with environmental control were missing to provide further support, this study showed the potential of applying TERS to characterize the on-surface coordination chemistry of porphyrins. In later research in a UHV environment with the introduction of CO gas, the coordination effect of CO on the molecular configuration of adsorbed CoTPP was further studied by Lee et al.46 using TERS and density functional theory (DFT) calculations (Figure 2b). They found that the vibrational fingerprints of CoTPP molecules can be activated by the on-surface coordination of CO molecules to the central Co atom due to the molecular configuration changes. Figure 2 | Molecular specificity, configuration, and orientation in on-surface coordination systems revealed by TERS. (a) TER spectra of a well-ordered CoTPP monolayer and a coordinated CoTPP adlayer on a Au(111) surface. Reproduced with permission from ref 45. (b) Structural models showing bare CoTPP and its dicarbonyl with axial and bridging binding. Reproduced with permission from ref 46. (c) TER spectra with Lorentzian peak fitting showing a new 18O–18O stretching band at 1151 cm−1. Reproduced with permission from ref 47. (d) Scheme illustrating STM-TERS investigation of an on-surface coordination system. (e) Typical TER spectra of coordination complexes with different tilt angles on a Au(111) surface. (f and g) TERS peak intensity maps (840 cm−1) show that the orientation of the coordination species is more homogeneous in samples prepared by immersion compared to drop-casting. Reproduced with permission from ref 48. Download figure Download PowerPoint Except for CO, coordinated O2- and O-species can also be simultaneously distinguished on the surface using TERS. Nguyen et al.47 used UHV-TERS to chemically analyze the interaction between O2 and cobalt phthalocyanines (CoPc) on Ag(111). Adsorption features with different contrasts in STM images were visualized and assigned to O2/CoPc/Ag(111) and O/CoPc/Ag(111) species. As displayed in Figure 2c, distinct TERS bands were also found for these species; and using supporting DFT simulations, the authors assigned the 1151 cm−1 band to 18O2 bound to CoPc as an axial ligand (18O–18O stretching vibration). The authors did not assign the weak 1216 cm−1 band because it overlaps with other features in the spectrum of 16O2-dosed CoPc/Ag(111), although it likely belongs to the 16O–16O stretching vibration. Expanding upon these results, our group demonstrated a TERS-based spectroscopic approach to investigate the formation and molecular orientation of a CoTPP-4-mercaptopyridine (4PySH) coordination system immobilized on Au surfaces with a spatial resolution of ca. 2 nm (Figure 2d).48 Figure 2e shows three typical TER spectra of the CoTPP-4PySH complex extracted from a high-resolution TERS map on an atomically flat Au(111) surface. The peak intensity of the bands highlighted by red stripes varies with the tilt angle, which suggests that the corresponding vibrational modes of these bands are enhanced to different degrees. In combination with DFT simulations, the adsorption configuration and molecular orientation of the coordination complexes are revealed in the TERS images. For instance, the tilt angles of the coordination complexes with a Raman fingerprint in Figure 2e correspond to ca. 90°, 60°, and 45°. Based on control experiments, we found that the orientation of the coordination species is more homogeneous in samples prepared by immersion with a well-ordered 4PySH adlayer (Figure 2f,g), which further supports our TERS results. We anticipate that in situ molecular-scale TERS imaging of on-surface coordination systems, like molecular electrocatalysts, metal–organic frameworks, catalytically active proteins, and supramolecular enzyme mimics, will enable further elucidation of catalytic mechanisms at the molecular scale. On-surface decomposition chemistry On-surface molecular decomposition is an important and intricate process in the chemical, biological, and material sciences.49–54 However, the spatially resolved investigation of such degradation processes on solid surfaces is a long-standing bottleneck in achieving mechanistic understanding. To achieve this goal, high sensitivity (submonolayer), molecular selectivity, and spatial resolution are required, which can be obtained through TERS. Xu et al.55 resolved the on-surface C–H bond breaking within a single pentacene molecule using UHV-TERS. The on-surface decomposition of pentacene was achieved by sequentially applying voltage pulses over a molecule, and chemical bond information of the resulting species was characterized by TERS. Figure 3a displays the TER spectra recorded from species α, β, and γ when the tip was placed around the middle and over the ends of pentacene. The authors were able to distinguish the on-surface bond-breaking phenomenon by the disappearance of the C–H stretching mode. These results demonstrate the chemical and structural sensitivity of TERS for probing bond breaking and making. Furthermore, 2D TERS mapping can provide further information about spatial distribution of the on-surface decomposition sites. Our group applied 2D TERS mapping to visualize the molecular decomposition process of a 4-PySH self-assembled monolayer (SAM) on Au(111) under ambient conditions.56 We showed that the decomposed and nondecomposed sites in a "partially degraded" sample can be spatially distinguished in the same image. Figure 3b shows a waterfall plot of the spectra measured in the TERS image, which confirms an inhomogeneous distribution of 4-PyS molecules over the Au(111) surface as highlighted by the arrows. This is a useful capability for evaluating the degradation degree of a SAM system. Figure 3 | Visualizing on-surface decomposition reactions using TERS (a) Determining C–H bond breaking within a single molecule by Raman spectra and maps. Reproduced with permission from ref 55. (b) Revealing the on-surface decomposition sites of a SAM on a Au(111) surface via a waterfall plot of the spectra measured in a TERS image. Reproduced with permission from ref 56. (c) On-surface dissociation of DMAB driven by plasmonic "scissors." Reproduced with permission from ref 58. (d) Laser powder-dependent TER spectra showing a plasmon-induced degradation process in a peptide sample. Reproduced with permission from ref 33. Download figure Download PowerPoint Even more interesting is that TERS can also be used as a tool to drive and control the on-surface decomposition reactions using hot electrons generated between the tip and the sample substrate.57 Sun et al.58 used TERS to achieve and visualize the on-surface decomposition of dimercaptoazobisbenzene (DMAB). They found that DMAB can be selectively snipped to p-aminothiophenol by hot electrons generated between the tip and the substrate under laser illumination. A schematic diagram of DMAB decomposing to form p-aminothiophenol by UHV-TERS is shown in Figure 3c. Their interpretation was that dissociation of the N=N bond in DMAB is driven by the reaction energy provided by the hot electrons. For biological samples, TERS can also be applied to trigger and visualize the dissociation process. Our group used TERS to both drive the reactions and to monitor their products.33 Peptide backbone bonds are found to dissociate in the hot spot, which is reflected in the disappearance of the amide I band in the TER spectra (Figure 3d). We revealed that the observed fragmentation pathway is probably caused by dissociative capture of plasmon-induced hot electrons. The ability to control and visualize decomposition reactions of chemical and biological samples on the nanoscale using TERS has promising applications in the mechanistic investigation of dynamic processes and/or conversions on solid surfaces. On-surface polymerization chemistry On-surface polymerization represents a novel bottom-up approach for preparing inaccessible polymer architectures with unique structures and properties.59–63 Insights into the mechanisms of polymer formation are thus of great importance not only for theoretical understanding but also for applications of polymeric materials. It has been proven that TERS is one of the most powerful techniques for monolayer analysis,64–71 which combines ultrahigh sensitivity with nanometer spatial resolution and Raman spectroscopic fingerprint information in a label-free fashion. These attractive features have enabled, for example, quantifying the reaction conversion in monolayer polymerization leading to 2D polymers.66 In this regard, TERS provides an interesting alternative for corroborating anticipated 2D polymer structures by their unique vibrational modes when neither high-resolution atomic force microscopy (AFM) nor STM can be successfully applied. Our group has worked extensively on TERS imaging of 2D polymer networks synthesized both at the air–water interface and on Au(111) surfaces, providing spectroscopic insights into the chemical bonds,70 substrate effects,65 homogeneity,69 step-growth mechanism,71 and so on. Furthermore, the high resolution and sensitivity of TERS enabled visualization of molecular orientation68 and identification and quantification of nanodefects67 within a 2D imine-linked polymer monolayer with a spatial resolution of <10 nm. Furthermore, TERS even allows us to monitor on-surface polymerization processes in real time and space. As shown in our recent work, the polymerization of an anthracene-based monomer (Figure 4a) on Au(111) can be flexibly triggered and monitored simultaneously by TERS.64 The monomer SAM was prepared and transferred using a Langmuir–Blodgett trough. Figure 4b shows the DFT-optimized molecular packing model of monomer 1 on Au(111). The pink square represents a single TERS imaging pixel, which contains ≈6 monomer molecules that are hexagonally arranged around a pore. The polymerization reaction sites can be distinguished by the appearance of new dimer bands (e.g., the mode at 965 cm−1) and the absence of spectroscopic features from unreacted anthracene blades. The images in Figure 4c–e display the consecutive TERS intensity maps of the characteristic 965 cm−1 band for the polymer at the same location after sequential illumination. To better reflect the chemical reality at the molecular scale, a hexagonal network model based on Figure 4b was built and is displayed in Figure 4f–h. The reacted regions in each image are labeled with a particular color (yellow for the first imaging, pink for the second imaging, and green for the third imaging), while the unreacted areas are indicated using blue. Interestingly, by overlaying the reacted regions in Figure 4f–h, it was found that the newly polymerized areas connect preferentially to existing polymerized locations. Such observations indicate that the newly grown polymer sites were probably triggered by an existing polymerized area, which points toward a self-accelerating growth mechanism for on-surface polymerization. We anticipate that the capability of TERS for on-surface dynamic polymerization analysis will play an increasingly important role in the on-surface dynamics. Figure 4 | Nanoscale chemical imaging of an on-surface dynamic polymerization process. (a) Chemical structures of two monomers and scheme of a photochemical reaction between two anthracenes. (b) Schematic illustration of the proposed molecular packing on Au(111). (c–e) Consecutive TERS imaging showing the plasmon-induced 2D polymerization process within a SAM on Au(111). (f–h) Proposed molecular model showing the corresponding growth kinetics of the 2D polymer. Reproduced with permission from ref 64. Download figure Download PowerPoint On-surface electrochemistry Electrochemistry is gaining widespread interest due to its broad application in various fields like energy conversion,72,73 sensors,74,75 corrosion,76 synthesis,77 catalysis,78 and so on. It is desirable to have a comprehensive understanding of the various complex processes (e.g., adsorption, desorption, diffusion, reactions, electron transfer, etc.) that occur at the electrochemical interface, which requires both nanoscale chemical and spatial information in the local environment. Electrochemical TERS (EC-TERS) is a promising tool for nanoscale analysis of electrochemical interfaces and processes. Zeng et al.39 performed a potential-dependent EC-TERS investigation of a SAM of 4′-(pyridin-4-yl)biphenyl-4-yl)methanethiol molecules on Au(111). Based on the careful analysis of the EC-TERS spectra, they found that the substrate potential can induce a substantial change in the molecular configuration. As displayed in Figure 5a, doublet peaks at 1592 and 1605 cm−1 were distinguished at 0.3 V (vs Pt), which were assigned to the vibrational modes of the pyridine ring and benzene ring. When the substrate potential was shifted to 0.7 V (vs Pt), the 1592 cm−1 peak was red-shifted and overlapped with the 1605 cm−1 peak, which was interpreted as protonation of the pyridine ring. Such a phenomenon was not evident in the SERS control experiments, which shows that EC-TERS can be more sensitive in revealing structural changes at the electrochemical electrode. In the same period, Kurouski et al.79 also developed AFM-based EC-TERS to investigate the nanoscale redox behavior of Nile Blue (NB) molecules and compared the results with conventional cyclic voltammetry. They showed that when the substrate potential was swept from 0 to −0.6 V (vs Ag/AgCl), the overall intensity of the TER spectra decreased, in agreement with the change in the electronic state from NBOX to NBRED (Figure 5b). These results demonstrate the capability of EC-TERS in monitoring on-surface electrochemical redox reactions. Figure 5 | Electrochemical processes/reactions analyzed by EC-TERS. (a) Potential-dependent EC-TERS spectra showing the reversible protonation process of organic molecules. Reproduced with permission from ref 39. (b) Potential dependence of the integrated TERS intensity of the cm−1 band of Reproduced with permission from ref and TERS analysis of the phenomenon during an Reproduced with permission from ref (e) In situ 2D chemical imaging of catalytic using EC-TERS. Reproduced with permission from ref Download figure Download PowerPoint In situ EC-TERS is also of the of molecular during electrochemical reactions. et investigated the chemical change and degradation of during reactions using They found that a reversible change in the TER spectrum of can be observed within several when the substrate potential was between 0.7 V (vs reversible to V (vs reversible However, when the catalytic reactions were for more changes The vibrational bands at and cm−1 from and new peaks assigned to appeared at and cm−1 (Figure This provides for in which is for its during the Furthermore, the topography and of active sites can be simultaneously recorded under reaction conditions using EC-TERS. and showed an EC-TERS investigation of the electrochemical process of Au on a Au(111) electrode. When the substrate potential was at a potential of V (vs the band at cm−1 was not observed at the sites (Figure When the potential was to V (vs to trigger the the band appeared at the which suggests that the surface play a role in the formation of spatially resolved analysis of the EC-TERS enabled the authors to the in the formation of and species. They that on of the while on sharp The results obtained so that EC-TERS will be able to provide new insights into various electrochemical systems at On-surface catalytic chemistry the mechanism of catalytic reactions at the nanoscale is essential to the performance and conversion and To achieve both chemical and topographic information related to catalytic active and their spatial distribution are in the local environment. TERS allows one to such information, in even with a sensitivity to only a few molecules. coupling reactions have been used as model systems in SERS and TERS research for several In the TERS a of research have the conversion of to DMAB and to DMAB as catalytic However, the role of the on the of these model reactions has to leading to of results. our group has applied nanoscale TERS imaging in combination with ambient STM and DFT to investigate the role of in the coupling of to DMAB on both and Au surfaces using the setup shown in Figure samples were a sample by of on a Au(111) or Au substrate and an sample by a Au(111) or Au substrate in the TERS imaging with ca. 3 nm spatial resolution revealed a catalytic on the sample (Figure compared to the immersion sample (Figure results were obtained on the DMAB system. details of the self-assembled structures were obtained using high-resolution which revealed a disordered phase of the adlayer on Au in the sample (Figure and the ordered phase in the immersion sample (Figure an structure–reactivity of the mechanistic insights obtained from DFT based on the from STM imaging indicated that a combination of effect and the reaction in the ordered phase of the This study the structure–reactivity in coupling and highlighted the crucial role of the molecular in the of on-surface coupling reactions in Figure | of high-resolution STM and TERS imaging to reveal the in a model coupling (a) Scheme