Raman Detection of Bond Breaking and Making of a Chemisorbed Up-Standing Single Molecule at Single-Bond Level

Resonant Tip-Enhanced Raman Spectroscopy of a Single-Molecule Kondo System

Tip-enhanced Raman spectroscopy (TERS) under ultrahigh vacuum and cryogenic conditions enables exploration of the relations between the adsorption geometry, electronic state, and vibrational fingerprints of individual molecules. TERS capability of reflecting spin states in open-shell molecular configurations is yet unexplored. Here, we use the tip of a scanning probe microscope to lift a perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecule from a metal surface to bring it into an open-shell spin one-half anionic state. We reveal a correlation between the appearance of a Kondo resonance in differential conductance spectroscopy and concurrent characteristic changes captured by the TERS measurements. Through a detailed investigation of various adsorbed and tip-contacted PTCDA scenarios, we infer that the Raman scattering on suspended PTCDA is resonant with a higher excited state. Theoretical simulation of the vibrational spectra enables a precise assignment of the individual TERS peaks to high-symmetry Ag modes, including the fingerprints of the observed spin state. These findings highlight the potential of TERS in capturing complex interactions between charge, spin, and photophysical properties in nanoscale molecular systems and suggest a pathway for designing single-molecule spin-optical devices.

Impact of Single-Melamine Tautomerization on the Excitation of Molecular Vibrations in Inelastic Electron Tunneling Spectroscopy

Vibrational quanta of melamine and its tautomer are analyzed at the single-molecule level on Cu(100) with inelastic electron tunneling spectroscopy. The on-surface tautomerization gives rise to markedly different low-energy vibrational spectra of the isomers, as evidenced by a shift in mode energies and a variation in inelastic cross sections. Spatially resolved spectroscopy reveals the maximum signal strength on an orbital nodal plane, excluding resonant inelastic tunneling as the mechanism underlying the quantum excitations. Decreasing the probe-molecule separation down to the formation of a chemical bond between the melamine amino group and the Cu apex atom of the tip leads to a quenched vibrational spectrum with different excitation energies. Density functional and electron transport calculations reproduce the experimental findings and show that the shift in the quantum energies applies to internal molecular bending modes. The simulations moreover suggest that the bond formation represents an efficient manner of tautomerizing the molecule.

Polypyrrole-Modified Nanocellulose Exhibits Superior Performance for Hg(II) Adsorption

Cellulose, a kind of polymer containing abundant functional groups, has widespread use in the adsorptive removal of environmental pollutants. An efficient and environmental friendly polypyrrole (PPy) coating approach is employed to modify the agricultural by-product straw derived cellulose nanocrystal (CNC) into excellent property adsorbents for removing the heavy metal ion of Hg(II). The FT-IR and SEM-EDS results demonstrated that PPy is formed on the surface of CNC. Consequently, the adsorption measurements proved that the obtained PPy-modified CNC (CNC@PPy) possesses a remarkably enhanced Hg(II) adsorption capacity of 1095 mg g^-1, owing to a plentiful functional group of doped Cl element on the surface of CNC@PPy by forming Hg₂Cl₂ precipitate. The results of the study suggest that the Freundlich model is more effective than the Langmuir model at describing the isotherms, while the pseudo-second order kinetic model is better suited to correlating with the experimental data compared to the pseudo-first order model. Further, the CNC@PPy exhibits an outstanding reusability, capable of maintaining 82.3% of its original Hg(II) adsorption capacity after five successive adsorption cycles. The findings of this work reveal a method to convert the agricultural by-product into high performance environmental remediation materials.

Imaging and controlling coherent phonon wave packets in single graphene nanoribbons

The motion of atoms is at the heart of any chemical or structural transformation in molecules and materials. Upon activation of this motion by an external source, several (usually many) vibrational modes can be coherently coupled, thus facilitating the chemical or structural phase transformation. These coherent dynamics occur on the ultrafast timescale, as revealed, e.g., by nonlocal ultrafast vibrational spectroscopic measurements in bulk molecular ensembles and solids. Tracking and controlling vibrational coherences locally at the atomic and molecular scales is, however, much more challenging and in fact has remained elusive so far. Here, we demonstrate that the vibrational coherences induced by broadband laser pulses on a single graphene nanoribbon (GNR) can be probed by femtosecond coherent anti-Stokes Raman spectroscopy (CARS) when performed in a scanning tunnelling microscope (STM). In addition to determining dephasing (~440 fs) and population decay times (~1.8 ps) of the generated phonon wave packets, we are able to track and control the corresponding quantum coherences, which we show to evolve on time scales as short as ~70 fs. We demonstrate that a two-dimensional frequency correlation spectrum unequivocally reveals the quantum couplings between different phonon modes in the GNR.

Toward a New Era of SERS and TERS at the Nanometer Scale: From Fundamentals to Innovative Applications

Surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) have opened a variety of exciting research fields. However, although a vast number of applications have been proposed since the two techniques were first reported, none has been applied to real practical use. This calls for an update in the recent fundamental and application studies of SERS and TERS. Thus, the goals and scope of this review are to report new directions and perspectives of SERS and TERS, mainly from the viewpoint of combining their mechanism and application studies. Regarding the recent progress in SERS and TERS, this review discusses four main topics: (1) nanometer to subnanometer plasmonic hotspots for SERS; (2) Ångström resolved TERS; (3) chemical mechanisms, i.e., charge-transfer mechanism of SERS and semiconductor-enhanced Raman scattering; and (4) the creation of a strong bridge between the mechanism studies and applications.

Precise tracking of tip-induced structural variation at the single-chemical-bond limit

Sub-nanometer-resolved TERS provides a systematic way for investigating tip-molecule interaction and molecular motions, enabling a promising approach to examine on-surface reaction mechanisms and catalysis at the microscopic level.

Ultrashort Pulse Excited Tip-Enhanced Raman Spectroscopy in Molecules

Vibrational fingerprints of molecules and low-dimension materials can be traced with subnanometer resolution by performing Tip-enhanced Raman spectroscopy (TERS) in a scanning tunneling microscope (STM). Strong atomic-scale localization of light in the plasmonic nanocavity of the STM enables high spatial resolution in STM-TERS; however, the temporal resolution is so far limited. Here, we demonstrate stable TERS measurements from subphthalocyanine (SubPc) molecules excited by ∼500 fs long laser pulses in a low-temperature (LT) ultrahigh-vacuum (UHV) STM. The intensity of the TERS signal excited with ultrashort pulses scales linearly with the increasing flux of the laser pulses and exponentially with the decreasing gap-size of the plasmonic nanocavity. Furthermore, we compare the characteristic features of TERS excited with ultrashort pulses and with a continuous-wave (CW) laser. Our work lays the foundation for future experiments of time-resolved femtosecond TERS for the investigation of molecular dynamics with utmost spatial, temporal, and energy resolutions simultaneously.

Visualizing On-Surface Decomposition Chemistry at the Nanoscale Using Tip-Enhanced Raman Spectroscopy

Chemical imaging of molecular decomposition processes at solid-liquid interfaces is a long-standing problem in achieving mechanistic understanding. Conventional analytical tools fail to meet this challenge due to the lack of required chemical sensitivity and specificity at the nanometer scale. In this work, we demonstrate that high-resolution hyperspectral tip-enhanced Raman spectroscopy (TERS) imaging can be a powerful analytical tool for studying on-surface decomposition chemistry at the nanoscale. Specifically, we present a TERS-based hyperspectral approach to visualize the on-surface decomposition process of a pyridine-4-thiol self-assembled monolayer on atomically flat Au(111) surfaces under ambient conditions. Reactive intermediates involved in the degradation process are spectroscopically detected with 5 nm spatial resolution. With supporting density functional theory simulations, a key species could be assigned to the disulfide reaction intermediate. This work opens a new application area for studying on-surface decomposition chemistry and related dynamics quantitatively at solid-liquid interfaces with nanometer spatial resolution.

Melamine self-assembly and dehydrogenation on Ag(111) studied by tip-enhanced Raman spectroscopy

The adsorption and self-assembly structures of melamine molecules on an Ag(111) surface are studied by low temperature scanning tunneling microscopy (STM) combined with tip-enhanced Raman spectroscopy (TERS). Two ordered self-assembly phases of melamine molecules on Ag(111) were studied by STM and TERS, combining with first-principles simulations. The α-phase consists of flat-lying melamine molecules, while the β-phase consists of mixed up-standing/tilted melamine molecules. Moreover, dehydrogenation of melamine can be controlled by annealing the sample as well as by a tip-enhanced photo-catalytic effect. Our work demonstrates TERS as a powerful tool not only for investigating the configuration and vibration properties of molecules on a metal surface with high spatial resolution but also for manipulating the chemical reactions with tip and photo-induced effects.

Progress of tip-enhanced Raman scattering for the last two decades and its challenges in very recent years

Tip-enhanced Raman scattering (TERS) has recently attracted remarkable attention as a novel nano-spectroscopy technique. TERS, which provides site-specific information, can be performed on any material surface regardless of morphology. Moreover, it can be applied in various environments, such as ambient air, ultrahigh vacuum (UHV), solutions, and electrochemical environments. This review reports on one hand progress of TERS for the last two decades, and on the other hand, its challenges in very recent years. Part of the progress of TERS starts with the prehistory and history of TERS, and then, the characteristics and advantages of TERS are described. Significant emphasis is put on the development of TERS instrumentation and equipment such as ultrahigh vacuum TERS, liquid TERS, electrochemical-TERS, and tip-preparations. Applications of TERS, particularly those with nanocarbons, biological materials, and surface and interface analysis, are mentioned in some detail. In the part on challenges, we focus on the very recent advances in TERS; progress in spatial resolution to the angstrom scale is the hottest topic. Recent TERS studies performed under UHV, for example chemical imaging at the angstrom scale and Raman detection of bond breaking and making of a chemisorbed up-standing single molecules at single-bond level, are reviewed. Of course, there is no clear border between the two parts. In the last part the perspective of TERS is discussed.

Plasmonic Photoelectrochemical Coupling Reactions of para-Aminobenzoic Acid on Nanostructured Gold Electrodes

Surface plasmon resonance (SPR) bridges photonics and photoelectrochemistry by providing an effective interaction between absorption and confinement of light to surface electrons of plasmonic metal nanostructures (PMNs). SPR enhances the Raman intensity enormously in surface-enhanced Raman spectroscopy (SERS) and leads to the plasmon-mediated chemical reaction on the surface of nanostructured metal electrodes. To observe variations in chemical reactivity and selectivity, we studied the SPR photoelectrochemical reactions of para-aminobenzoic acid (PABA) on nanostructured gold electrodes. The head-to-tail coupling product "4-[(4-imino-2,5-cyclohexadien-1-ylidene)amino]benzoic acid (ICBA)" and the head-to-head coupling product p,p'-azodibenzoate (ADBA) were obtained from PABA adsorbed on PMN-modified gold electrodes. In particular, under acidic and neutral conditions, ICBA was obtained as the main product, and ADBA was obtained as the minor product. At the same time, under basic conditions, ADBA was obtained as the major product, and ICBA was obtained as the minor product. We have also provided sufficient evidence for the oxidation of the tail-to-tail coupling reaction product that occurred in a nonaqueous medium rather than in an aqueous medium. The above finding was validated by the cyclic voltammetry, SERS, and theoretical calculation results of possible reaction intermediates, namely, 4-aminophenlylenediamine, 4-hydroxyphenlylenediamine, and benzidine. The theoretical adsorption model and experimental results indicated that PABA has been adsorbed as para-aminobenzoate on the gold cluster in a bidentate configuration. This work offers a new view toward the modulation of selective surface catalytic coupling reactions on PMN, which benefits the hot carrier transfer efficiency at photoelectrochemical interfaces.