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.
A novel way to grow MoS2 on a large scale with uniformity and in desired patterns is developed. We use Au film as a catalyst on which Mo(CO)6 vapor decomposes to form a Mo‐Au surface alloy that is an ...ideal Mo reservoir for the growth of atomic layers of MoS2. Upon exposure to H2S, this surface alloy transforms into a few layers of MoS2, which can be isolated and transferred on an arbitrary substrate. By simply patterning Au catalyst film by conventional lithographic techniques, MoS2 atomic layers in desired patterns can be fabricated.
When a gold surface reacts with vaporized Mo(CO)6 at 300 °C, a surface alloy forms, which in turn becomes an ideal large‐scale atom‐thick Mo reservoir. When this alloy further reacts with H2S, atomic layers of MoS2 are specifically formed on Au, which can be isolated by means of etching.
We report that high-quality single-layer graphene (SLG) has been successfully synthesized directly on various dielectric substrates including amorphous SiO2/Si by a Cu-vapor-assisted chemical vapor ...deposition (CVD) process. The Cu vapors produced by the sublimation of Cu foil that is suspended above target substrates without physical contact catalyze the pyrolysis of methane gas and assist nucleation of graphene on the substrates. Raman spectra and mapping images reveal that the graphene formed on a SiO2/Si substrate is almost defect-free and homogeneous single layer. The overall quality of graphene grown by Cu-vapor-assisted CVD is comparable to that of the graphene grown by regular metal-catalyzed CVD on a Cu foil. While Cu vapor induces the nucleation and growth of SLG on an amorphous substrate, the resulting SLG is confirmed to be Cu-free by synchrotron X-ray photoelectron spectroscopy. The SLG grown by Cu-vapor-assisted CVD is fabricated into field effect transistor devices without transfer steps that are generally required when SLG is grown by regular CVD process on metal catalyst substrates. This method has overcome two important hurdles previously present when the catalyst-free CVD process is used for the growth of SLG on fused quartz and hexagonal boron nitride substrates, that is, high degree of structural defects and limited size of resulting graphene, respectively.
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 ...various environments. However, the practical use of TERS combined with scanning tunneling microscopy (STM) in an ambient environment has not reached its full potential thus far; this is because the atomic-scale geometry near the tip apex plays a crucial role in stable STM imaging, whereas plasmon enhancement for TERS is achieved on a submicron-scale tip structure, rendering it difficult to select an appropriate tip in advance. Herein, we propose a specific and simple method for tip conditioning that utilizes local heating through plasmon excitation in a self-consistent manner. This method is established on the basis of the characterizations of the tip shape and plasmonic properties. Accordingly, we demonstrate stable STM imaging compatible with reliable TERS measurements using the preconditioned tip.
Underpotential deposition (UPD) in electrochemistry can be used to form heterometal deposits on substrates at a monolayer level, altering the surface characteristics. In this work, we demonstrated ...the application of UPD metals in catalyzing photoreactions of adsorbed molecules under the excitation of surface plasmons. For a monolayer of 4-nitrothiophenol (4-NTP) on a roughened Au substrate, the reduction and dimerization to 4,4′-dimercaptoazobenzene (DMAB) were monitored under conditions of Ag UPD using electrochemical surface-enhanced Raman spectroscopy (EC-SERS). Formation of DMAB was enhanced on the Ag surface deposited via UPD between the Au substrate and the 4-NTP layer. The structures of the 4-NTP layer and the plasmonic surface remained intact during Ag UPD. Therefore, we suggest that the surface of UPD Ag played a catalytic role in reduction of the nitro group. Overall, surface modification via UPD offers a potential strategy for controllably tuning the surface and interface chemistry.
We present a fundamental study of solid–electrolyte interphase (SEI) layers toward a better understanding of interfacial electrochemistry. In particular, water-in-salt electrolytes yield SEIs with a ...simple composition that describes the electrolyte–electrode interface explicitly. The 21 m lithium bis(trifluoromethanesulfonyl)imide formed a porous SEI film on a highly oriented pyrolytic graphite (HOPG) electrode at −2 V (vs Ag/AgCl). The significant hydrogen evolution reaction (HER) made holes in a thin SEI film and defect sites in the HOPG. In addition, the SEI comprised fragmented TFSI without including any Li compounds. We suggested that fragments of TFSI– were precipitated out by the addition of the hydrogen atoms, which were yielded through the Volmer step and detached from the HOPG surface before HER. Subsequently, a nonporous and LiOH-rich film was formed by −4 V. The OH– and Li+ ions were enriched during the continuous HER, and their chemical reaction produced a thick film and nanoneedles. However, there was no evidence of Li+ intercalation into graphitic layers of the HOPG, presumably caused by sluggish Li+-ion transport in the Li-deficient SEI layer. This study shows variable interfacial reactions over a wide range of applied potential and the HER impact on SEI films associated with the performance of aqueous Li-ion batteries.
Singlet oxygen (1O2) is one of the most critical species leading to parasitic side reactions and poor reversibility in non‐aqueous Li−O2 batteries. 1O2 is generated via the disproportionation of the ...superoxide radical (O2.−) in O2/Li2O2 electrochemistry. The mechanistic and computational studies on 1O2 formation revealed the significant roles of the associated cations, solvation ability of aprotic solvents, H+ source, and catalyst/electrode materials. Along with efforts to alleviate 1O2 production, trapping and eliminating 1O2 have been attempted using molecular agents. Anthracene derivatives trap 1O2 and form endoperoxides, which can be quantitatively detected using in situ fluorescence analysis. Physical quenchers that convert 1O2 to 3O2 are desirable for cycling of Li−O2 cells because quencher molecules are reusable. We highlight the recent reports on the formation and elimination of 1O2, and challenges and perspectives of suppressing the 1O2 effect on the performance of Li−O2 cells.
The limiting species: Singlet oxygen (1O2) is one of the critical species leading to side reactions and poor cyclability of non‐aqueous Li−O2 battery. This minireview covers recent achievements on the major factors determining 1O2 formation and development of 1O2 trapping/quenching agents for improving the reversibility of Li−O2 electrochemistry.
We studied solid electrolyte interphase (SEI) on metallic sodium (Na) electrodes. Among sodium hexafluorophosphate (NaPF6), sodium triflate (NaOTf), and sodium bis(trifluoromethanesulfonyl)imide ...(NaTFSI) electrolytes, sodium fluoride (NaF)‐rich and compact SEI was only formed by chemical reduction of NaPF6. Excellent rigidity and insolubility of NaF‐rich SEI layer enhanced electrochemical cycling performances for both Na/Na symmetric cells and sodium–oxygen (Na–O2) cells. By contrast, the Na electrodes using NaOTf and NaTFSI formed porous and carbonaceous SEI layers rather than NaF. Soluble carbonaceous species were detached from the Na electrode, which led to the undesired decomposition of electrolyte solution. It resulted in the substantial formation of the dead Na and dendritic Na and caused cycling failure of Na–O2 cells within 10 cycles, demonstrated by NaOTf.
Studies of solid electrolyte interface of Na metals and their effects for Na/Na nad Na–O2 cell performances.
