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Tracking various chemical reactions, including electrochemical and photochemical reactions at the single-molecule level, is expected to yield a great deal of knowledge from both fundamental and applied aspects. In this study, we report on a methodology to track the electronic-state changes of redox reactions at the single-molecule level by using electrochemical scanning tunneling microscopy (EC-STM). EC-STM is powerful for single-molecule analysis of redox reactions, but previous studies have shown difficulties separating the structural and electronic contributions due to orientational changes during the redox reaction. Here, we visualize the electronic-state changes of a single ferrocene associated with redox reactions using EC-STM by synthesizing and fabricating a monolayer of structurally rigid tripodal molecules based on triptycene, which act as ideal anchors to preserve a constant distance between the electrode and the ferrocene moieties. This methodology paves the way for versatile single-molecule measurements of important phenomena at the solid–liquid interface, such as photochemistry and heterogeneous catalysis.

Microscopic studies on electrolyte solution/electrode interfaces provide the most fundamental information not only for understanding the electric double layer formed at the interfaces but also for ...

The formation and surface structures of self-assembled monolayers (SAMs) of octyl selenocyanate (OSC, CH3(CH2)7-SeCN) on Au(111) prepared by solution and ambient-pressure vapor deposition were inve...

Tip-enhanced Raman spectroscopy (TERS), which uses plasmons formed between the metallic tip and the sample upon laser irradiation, has emerged as a promising analytical method that is applicable in...

Self-assembled monolayers (SAMs) of aromatic thiols on metal surfaces are of interest as covering functional layers. Unlike alkanethiols, multicomponent SAMs of aromatic thiols have not been compre...

Unexpected phenomena displayed by low-boron-doped diamond (BDD) electrodes are disclosed in the present work. Generally, the presence of sp2 nondiamond carbon impurities in BDD electrodes causes un...

Focusing on the electric double layer formed at aqueous solution/graphite electrode interfaces, we investigated the relationship between the mobility of interfacial water and its hydrogen bonding networks by using molecular dynamics simulations. We focused on the mobility of the first hydration layer constructed nearest to the electrode. The mobility was determined by calculating the diffusion coefficient which showed an opposite trend to that of the applied potential polarity. The mobility decreased upon positive potentials while showing an increase upon negative potentials, which is rationalized by the strength of the interfacial hydrogen bonding networks.

We report the rapid improvement in the carrier mobility of the electric double layer field-effect transistor based on the ionic liquid (IL)/pentacene single crystal interface. Generally, the surface oxidation of the pentacene single crystal is unavoidable, and the considerable degradation restricts the performance of the field-effect transistor. However, the formation of the IL/pentacene single crystal interface resolves this problem by increasing the carrier mobility by approximately twice the initial value within a few hours. Furthermore, frequency-modulation atomic force microscopy revealed that the aforementioned rapid improvement is attributed to the appearance of a clean and flat surface of the pentacene single crystal via the defect-induced spontaneous dissolution of pentacene molecules into the IL.

Fluorescence and Raman scattering spectroscopies have been used in various research fields such as chemistry, electrochemistry, and biochemistry because they can easily obtain detailed information about molecules at interfaces with visible light. In particular, multimodal fluorescence and Raman scattering spectroscopy have recently attracted significant attention, which enables us to distinguish chemical species and their electronic states that are important for expressing various functions. However, a special strategy is required to perform simultaneous measurements because the cross sections of fluorescence and Raman scattering differ by as much as ∼1014. In this study, we propose a method for the simultaneous measurement of dye molecules on a metal surface using a monatomic layer of iodine as the dielectric layer. The method is based on adequately quenching the photoexcited state of the molecules near the metal surface to weaken the fluorescence intensity and using the resonance effect to increase the Raman signal. We have validated this concept by experiments with insulating layers of different thicknesses and dye molecules of different chemical structures. The proposed multimodal strategy paves the way for various applications such as catalytic chemistry and electrochemistry, where the adsorption structure and electronic states of molecular species near the metal surface determine functionalities.

Organically tailorable monolayers of ferrocene-terminated alkanethiol self-assembled monolayers (Fc SAM, Fig. 1a) on Au surfaces are attractive model systems to elucidate the molecular-level physicochemical properties of solid-liquid electrochemical (EC) interfaces [1]. In particular, Fc SAM has been shown to be an attractive spectroscopic molecular probe for photoelectron spectroscopy because the Fe heteroatom is at a fixed position above the electrode surface. Thus, this allows photoelectron spectroscopy to ascertain the electronic and structural properties at a fixed position above the electrode surface [2-3]. The ability to obtain molecular-level structure-function-property relationships with Fc SAM has relevance to bio(chemical) sensors, micro-actuators, and molecular electronics. Here, we use X-ray and ultraviolet photoelectron spectroscopy combined with an electrochemical cell (EC-XPS/UPS) to electrochemically control the Fc SAM and spectroscopically probe the corresponding electronic and structural changes [2-3]. To perform our experiments, electrochemical (EC) measurements are performed in a separate and dedicated EC chamber in a solution environment under Ar atmosphere, followed by evacuation and finally transfer to the XPS/UPS analysis chamber [2-3]. The EC chamber contains a retractable PTFE (polytetrafluorethylene) cell and utilizes a hanging meniscus setup. I will discuss the merits and challenges of EC-XPS/UPS methodology in the context of other approaches. Using EC-XPS/UPS, we are able to discriminate anion dependencies (Fig. 1b-d) with respect to the Fe oxidation state, monolayer thickness, and changes to the valence structure regarding the highest occupied molecular orbital (HOMO. Lastly, we are able to spectroscopically probe the changes relating to the interfacial potential distribution across the electrode-monolayer-electrolyte interface or so-called “potential drop” in the form of systematic potential-induced binding energy shifts in the XPS/UPS spectra. Further details will be provided at the presentation. Fig. 1 (a) Ferrocene-terminated alkanethiol SAM on Au. Schematic of electrode-monolayer-electrolyte interface and interfacial potential distribution. (b) Cyclic voltammograms at 50 mV s-1 in 0.1 M NaTFSI or NaPF6 (c-d) Fe 2p3/2 spectra of Fc SAM, pristine and after anodic/cathodic polarization at 0.625 and 0 V, respectively. References [1] K. Uosaki, Electrochemistry 12, 1105-1113 (1999). [2] R. A. Wong, Y. Yokota, M. Wakisaka, J. Inukai, Y. Kim, J. Am. Chem. Soc. 140, 13672-13679 (2018). [3] R. A. Wong, Y. Yokota, M. Wakisaka, J. Inukai, Y. Kim, Nat. Commun. 11, 1-9 (2020). Figure 1

Lithium-ion (Li-ion) batteries have become ubiquitous to modern-day electronics and electric vehicles. A detailed understanding regarding the reversible and irreversible nature of Li-ion electrode processes is a prerequisite to rationally delineate the factors affecting device performance and clarify degradation mechanisms. This includes, for instance, the role of electrode cracking and solid solid-electrolyte-interphase (SEI) formation which can contribute toward irreversible capacity loss. In terms of experimental approaches, aside from the high surface area electrodes used in practical systems, one powerful approach is to incorporate the study of well-defined electrodes and surface science techniques.1 Herein, we employ highly oriented pyrolytic graphite (HOPG) as a well-defined model anode in Li-ion batteries and reveal the chemical and morphological changes at low cathodic potentials prior to the potentials of bulk SEI formation.2 To achieve this, we employ the use of X-ray photoelectron spectroscopy (XPS) and ultrahigh vacuum scanning tunneling microscopy (UHV-STM). We utilize a so-called UHV-EC setup to provide a snapshot-like analysis of the electrode surface without the involvement of electrode washing or exposure to air. Upon cathodic polarization at ca. 1.75V (Figure 1), STM imaging shows the onset and occurrence of graphite exfoliation which can originate from the intercalation of solvated Li+ ions. Meanwhile, XPS shows a minimal amount of residual electrolyte along with the observation of LiF indicating the decomposition of the Li salt (LiPF6). The future extensions of our methodology and additional details will be provided at the presentation. Figure 1. (a) Cathodic linear sweep voltammogram in 1.1 M LiPF6 EC/DMC. Following polarization at ca. 1.75 V (b) STM imaging, and (c) XPS F 1s and Li 1s spectra. References [1] T. Minato, T. Abe, Prog. Surf. Sci. 92, 240-280 (2017). [2] R. A. Wong, Y. Yokota, E. Kazuma, M. Oniki, Y. Kim, J. Phys. Chem. C 125, 21093-21100 (2021). Figure 1

Redox reactions on electrode surfaces have been widely applied to fundamental technologies such as energy conversion and storage, metal refining, and electronics. Recently, it has become clear that local structures at the single-atom and single-molecule level affect the efficiency and selectivity of redox reactions [1]. EC-STM, a combination of scanning tunneling microscopy (STM) and electrochemical (EC) measurements, allows direct observation of electrochemical interfaces with high spatial resolution [2], and we aimed to observe the redox reactions of single molecules using this technique. As a target molecule, ferrocene (Fc) was coupled to a tripodal base molecule [3], which was known to form an ordered self-assembled monolayer (SAM) on a gold electrode. By preparing mixed SAM with the molecules with and without Fc moieties, we could stably deposit Fc species on the Au (111) electrode in an isolated and dispersed state, which enables us to observe clear changes in the EC-STM images upon the redox reaction of the single Fc derivative. A mixture of 8,13-trimercaptotriptycene (Trip) and its Fc derivative (Fc-Trip) was deposited on an Au(111) electrode (the molecules were provided by the Fukushima Lab., Chemical Biology Laboratory, Tokyo Institute of Technology, and the Suzuki Lab., Graduate School of Science, Hokkaido University). Cyclic voltammetry (CV) was performed with a potentiostat (HZ-7000, Hokuto Denko) at a sweep rate of 100 mV/s in 0.1 M HClO4 solution. STM measurements were performed using an MS-10 STM (Bruker), controlled by a NanoScope V (Bruker). A Pt/Ir wire coated with Apiezon wax was used as a probe in the home-made electrochemical cell. The sample potential (Esample) was varied from 0.2 V (vs Ag/AgCl) to 0.4 V while the potential difference between the tip and the sample was kept at 0.3 V. The CV results are shown in Fig. 1 (a), where the peak originating from the redox reactions of Fc-Trip is observed (dotted line is only for Trip), indicating that the redox potential of Fc-Trip is ~0.3 V. Since the full width at half maximum of the peak is about 0.1 V, Fc-Trip do not interact with each other and are considered to be isolated and dispersed on the Au(111) electrode [4]. Corresponding EC-STM images are shown in Fig. 1 (c)-(e). While the bright spots derived from Fc-Trip are observed at Esample = 0.2 V (the tip potential Etip is 0.5 V), as indicated by white circles, the bright spots almost disappeared at Esample = 0.4 V (Etip = 0.7 V). We confirmed that the change of the height is reversible with respect to the applied potentials, as shown in Fig. 1(c) and 1(e). The height of the bright spot in the EC-STM image reflects the electron transfer rate through the redox molecule between the tip and the sample. When the sample potential is set to 0.2 V, Fc-Trip is formally in the reduced state, but the electrons are readily transferred to the tip regulated at 0.5 V. On the other hand, when the sample potential is set to settle the oxidized state (0.4 V), the molecule cannot pass electrons to the high potential tip. Therefore, in the case of Fig. 1(d), the current between the tip and the sample is not increased with the presence of the Fc moiety, leading to the apparent disappearance of spots. We noticed that some bright spots did not disappear at Esample = 0.4 V (an arrow in Fig. 1(d)), which suggests that there is heterogeneity in the redox reactivity. In the future, we plan to investigate the heterogeneity by combining spectroscopic techniques with the EC-STM measurement. [1] J. H. K. Pfisterer et al., Nature , 74, 549 (2017) [2] Y. Yokota et al., J. Phys. Chem. C, 111, 7561 (2007) [3] F. Ishiwari et al., J. Am. Chem. Soc., 141, 5995 (2019) [4] C. E. D. Chidsey et al., J. Am. Chem. Soc., 112, 4301 (1990) Figure 1

Altering electrochemical interfaces by using electrolyte effects or so-called “electrolyte engineering” provides a versatile means to modulate the electrochemical response. However, the long-standing challenge is going “beyond cyclic voltammetry” where electrolyte effects are interrogated from the standpoint of the interfacial properties of the electrode/electrolyte interface. Here, we employ ferrocene-terminated self-assembled monolayers as a molecular probe and investigate how the anion-dictated electrochemical responses are translated in terms of the electronic and structural properties of the electrode/monolayer/electrolyte interface. We utilise a photoelectron-based spectroelectrochemical approach that is capable of capturing “snapshots” into (1) anion dependencies of the ferrocene/ferrocenium (Fc/Fc+) redox process including ion-pairing with counter anions (Fc+–anion) caused by differences in Fc+–anion interactions and steric constraints, and (2) interfacial energetics concerning the electrostatic potential across the electrode/monolayer/electrolyte interface. Our work can be extended to provide electrolyte-related structure-property relationships in redox-active polymers and functionalised electrodes for pseudocapacitive energy storage., Conveying electrochemistry in terms of the electrode/electrolyte interfacial properties remains challenging. Here, the authors employ a surface-bound molecular probe and photoelectron spectroscopy to peer into the anion-dictated and potential-induced interfacial electronic and structural properties.

A molecular-scale understanding of electrolyte/electrode interfaces has long been a challenging issue in electrochemistry. Spectroscopic tools with high spatial resolution are required for advancing beyond conventional electrochemical measurements such as cyclic voltammetry (CV). In this study, we developed tip-enhanced Raman spectroscopy (TERS), which is based on an electrochemical scanning tunneling microscope (EC-STM), and demonstrated electrochemical TERS (EC-TERS) measurements of benzenethiol self-assembled monolayers (SAMs) on Au(111). A specially designed cell enables us to carry out reproducible CV, EC-STM, and EC-TERS measurements, which indicates consistent results among these techniques for the oxidative desorption of the SAMs. We also present direct evidence that the measured EC-TERS signals originate from molecules adsorbed on Au(111) and not from those on the STM tip.

We explore the redox-dependent electronic and structural changes of ferrocene-terminated self-assembled monolayers (Fc SAMs) immersed in aqueous solution. By exploiting X-ray and ultraviolet photoelectron spectroscopy combined with an electrochemical cell (EC-XPS/UPS), we can electrochemically control the Fc SAMs and spectroscopically probe the induced changes with the ferrocene/ferrocenium (Fc/Fc+) redox center (Fe oxidation state), formation of 1:1 Fc+–ClO4– ion pairs, molecular orientation, and monolayer thickness. We further find the insignificant involvement of interfacial water in the Fc SAMs irrespective of redox state. Electrolyte dependencies could be identified with 0.1 M NaClO4 and HClO4 when probing partially oxidized Fc/Fc+ SAMs. Corroborating the occurrence of electrochemically induced oxidation, EC-UPS shows that oxidation to Fc+ is accompanied by a shift of the highest occupied molecular orbital toward higher binding energy. The oxidation to Fc+ is also met with an increase in work functio...

Gold (Au) tips have wide application in local spectroscopies not only because of their high chemical stability but also their strong localized surface plasmon resonance (LSPR) in the visible and near-infrared regions. The energy and intensity of LSPR strongly depend on the tip shape. However, the conventional fabrication method of Au tips using electrochemical etching with two electrodes has problems regarding both the controllability and reproducibility of the tip shape. Here, we demonstrate a novel three-electrode electrochemical etching method to fabricate the Au tips by precisely tuning the applied electrochemical potential. The sharpness of the tip is well controlled by the applied potential, with high reproducibility.

It has been observed that the properties of room temperature ionic liquids near solid substrates are different from those of bulk liquids, and these properties play an important role in the development of catalysts, lubricants, and electrochemical devices. In this paper, we report microscopic studies of ionic liquid/solid interfaces performed using molecular dynamics simulations. The structural and dynamic properties of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIM-TFSI) on mica and graphite interfaces were thoroughly investigated to elucidate the microscopic origins of the formation of layered structures at the interfaces. Our investigation included the observation of structural and orientational changes of ions as a function of distance from the surfaces, and contour mappings of ions parallel and perpendicular to the surfaces. By virtue of such detailed analyses, we found that, during the 5 ns simulation, the closest layer of BMIM-TFSI behaves as a two-dimensional ionic crystal on mica and as a liquid or liquid crystal on graphite.

Electric double-layer transistors based on ionic liquid/organic semiconductor interfaces have been extensively studied during the past decade because of their high carrier densities at low operation voltages. Microscopic structures and the dynamics of ionic liquids likely determine the device performance; however, knowledge of these is limited by a lack of appropriate experimental tools. In this study, we investigated ionic liquid/organic semiconductor interfaces using molecular dynamics to reveal the microscopic properties of ionic liquids. The organic semiconductors include pentacene, rubrene, fullerene, and 7,7,8,8-tetracyanoquinodimethane (TCNQ). While ionic liquids close to the substrate always form the specific layered structures, the surface properties of organic semiconductors drastically alter the ionic dynamics. Ionic liquids at the fullerene interface behave as a two-dimensional ionic crystal because of the energy gain derived from the favorable electrostatic interaction on the corrugated periodic substrate.

An understanding of the characteristics of ionic liquid/graphite interfaces is highly important for electrochemical devices such as batteries and capacitors. In this paper, we report microscopic studies of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIM-TFSI) on charged graphite electrodes using molecular dynamics simulations to reveal the two-dimensional arrangement of the ions and their dynamics at the interfaces. Analyses of surface distribution and mobility of ions revealed that the ion arrangement changes from a bilayer type to a checkerboard type with increasing applied potential. Whereas the bilayer type arrangement increases the ionic mobility parallel to the interfaces with the negative potential, the ions arranged in the checkerboard type tend to localize because of the increased lateral electrostatic interactions. Furthermore, we revealed that the inhomogeneity of ionic distribution at the positive potential propagates up to a few nanometers from the interface.

The progression of corrosion in polycrystalline boron-doped diamond (BDD) thin film electrode is explored as the electrode undergoes high-current density anodic treatments with organic compounds. Micro-Raman spectroscopy and spectral mapping indicate that anodic corrosion is initiated by the conversion of sp3 diamond to amorphous sp2 carbon at the surface, which are then removed after longer anodic treatment. Polarized Raman analysis reveals that corrosion-induced changes on the surface are specific to (100)-grain facets and (111)-grain edges. X-ray photoelectron spectroscopic measurements suggest that carbonyl groups consequently form on these specific sites and act as an intermediate toward the etching of the surface. This process exposes and subsequently removes the subsurface boron atoms, thus reducing the doping density. The observed crystal grain orientation dependence of the corrosion process provides new insights toward a better understanding of degradation in BDD electrodes.

Unexpected phenomena displayed by low-boron-doped diamond (BDD) electrodes are disclosed in the present work. Generally, the presence of sp

Phonons provide information on the physicochemical properties of a crystalline lattice from the material’s vibrational spectrum. Optical phonons, in particular, can be probed at both micrometre and nanometre scales using light-based techniques, such as, micro-Raman and tip-enhanced Raman spectroscopy (TERS), respectively. Selection rules, however, govern the accessibility of the phonons and, hence, the information that can be extracted about the sample. Herein, we simultaneously observe both allowed and forbidden optical phonon modes of defect-free areas in monolayer graphene to study nanometre scale strain variations and plasmonic activation of the Raman peaks, respectively, using our home-built TERS system in ambient. Through TERS imaging, strain variations and nanometre-sized domains down to 5 nm were visualised with a spatial resolution of 0.7 nm. Moreover, such subnanometric confinement was found to activate not only the D and D’ forbidden phonon modes but also their D + D’ combination mode. With our TERS in ambient system, the full phonon characterisation of defect-free graphene and other 2D nanomaterials is now possible, which will be useful for subnanometre strain analysis and exploring the inherent properties of defect-free materials.

We conducted the operando atomic force microscopy (AFM) of ionic liquid (IL)/rubrene single crystal interfaces under the operation of an electric double-layer (EDL)-gated field-effect transistor (FET). We developed a top-side-gated EDL-FET and performed the simultaneous measurement of device characteristics and frequency-modulation AFM in the IL droplet. The AFM images revealed microscopic and macroscopic changes in the rubrene single crystal surface upon carrier injection by applying a gate voltage.

An ionic liquid forms a characteristic solvation structure on a substrate. For example, when the surface of the substrate is negatively or positively charged, cation and anion layers are alternately aligned on the surface. Such a solvation structure is closely related to slow diffusion, high electric capacity, and chemical reactions at the interface. To analyze the periodicity of the solvation structure, atomic force microscopy is often used. The measured force curve is generally oscillatory and its characteristic oscillation length corresponds not to the ionic diameter, but to the ion-pair diameter. However, the force curve is not the solvation structure. Hence, it is necessary to know the relationship between the force curve and the solvation structure. To find physical essence in the relationship, we have used statistical mechanics of a simple ionic liquid. We found that the basic relationship can be expressed by a simple equation and the reason why the oscillation length corresponds to the ion-pair diameter. Moreover, it is also found that Derjaguin approximation is applicable to the ionic liquid system.

The maximum cause to make mechanical toughness of a weld metal reduce in process management is known to be a mixture of nitrogen including in the atmosphere by breaking the shield condition. Mixtur...

Ferrocene-terminated self-assembled monolayers (SAMs) have been widely studied in the past quarter century to reveal the electrochemical properties of chemically modified electrodes. It has been well-known that the formal potential of the system strongly depends on the local environment of the ferrocene moiety. Although electronic states of ferrocene-terminated SAMs should directly affect the electrochemical properties, knowledge concerning electronic structures with respect to different local environment is very limited. In this study, we performed density functional theory calculations of ferrocene-terminated SAMs with different coadsorbed species to reveal the relationship between the electronic structures and the local environment of ferrocene moieties. Depending on the local electrostatic potential, density of states derived from the highest-occupied molecular orbital (HOMO) and its vicinities (HOMO–1 and HOMO–2) of the ferrocene moiety were found to largely shift with respect to the electrode Fermi ...

Carbon-based materials are regarded as an environmentally benign alternative to the conventional metal electrode used in electrochemistry from the viewpoint of sustainable chemistry. Among various carbon electrode materials, boron-doped diamond (BDD) exhibits superior electrochemical properties. However, it is still uncertain how surface chemical species of BDD influence the electrochemical performance, because of the difficulty in characterizing the surface species. Here, we have developed in situ spectroscopic measurement systems on BDD electrodes, i.e., in situ attenuated total reflection infrared spectroscopy (ATR-IR) and electrochemical X-ray photoelectron spectroscopy (EC-XPS). ATR-IR studies at a controlled electrode potential confirmed selective surface hydroxylation. EC-XPS studies confirmed deprotonation of C–OH groups at the BDD/electrolyte interface. These findings should be important not only for better understanding of BDD’s fundamentals but also for a variety of applications.

Ferrocene-terminated self-assembled monolayers (SAMs) have been studied in the last quarter-century to determine the relationship between their electrochemical properties and microscopic structures as a prototypical chemically modified electrode. Although electronic structures are directly related to the electrochemical properties, knowledge of such a relation concerning ferrocene-terminated SAMs is extremely limited. In this study, we deposited a small amount of ionic liquid onto three types of ferrocene-terminated SAMs possessing different ionization energies and performed X-ray and ultraviolet photoelectron spectroscopies to elucidate the changes in the electronic structure resulting from the adsorption of the ionic liquid. For a low-ionization-energy system, spontaneous oxidation of the ferrocene moieties occurred, while their electrochemical activities were retained; for other systems, only minor changes were observed. This result indicates that the interaction between the ionic liquid and ferrocene-...