Germanene Reformation from Oxidized Germanene on Ag(111)/Ge(111) by Vacuum Annealing.

For group 14 mono-elemental 2D materials, such as silicene, germanene, and stanene, oxidation is a severe problem that alters or degrades their physical properties. This study shows that the oxidized germanene on Ag(111)/Ge(111) can be reformed to germanene by simple heating ≈500 °C in a vacuum. The key reaction in reforming germanene is the desorption of GeO and GeO during heating ≈350 °C.

Visualization of Multiple-Resonance-Induced Frontier Molecular Orbitals in a Single Multiple-Resonance Thermally Activated Delayed Fluorescence Molecule.

The spatial distribution and electronic properties of the frontier molecular orbitals (FMOs) in a thermally activated delayed fluorescence (TADF) molecule contribute significantly to the TADF properties, and thus, a detailed understanding and sophisticated control of the FMOs are fundamental to the design of TADF molecules. However, for multiple-resonance (MR)-TADF molecules that achieve spatial separation of FMOs by the MR effect, the distinctive distribution of these molecular orbitals poses significant challenges for conventional computational analysis and ensemble averaging methods to elucidate the FMOs' separation and the precise mechanism of luminescence. Therefore, the visualization and analysis of electronic states with the specific energy level of a single MR-TADF molecule will provide a deeper understanding of the TADF mechanism.

Nanoscale thermal imaging of hot electrons by cryogenic terahertz scanning noise microscopy.

Nanoscale thermal imaging and temperature detection are of fundamental importance in diverse scientific and technological realms. Most nanoscale thermometry techniques focus on probing the temperature of lattice or phonons and are insensitive to nonequilibrium electrons, commonly referred to as "hot electrons." While terahertz scanning noise microscopy (SNoiM) has been demonstrated to be powerful in the thermal imaging of hot electrons, prior studies have been limited to room temperature.

Revealing the Influence of Molecular Chemisorption Direction on the Reaction Selectivity of Dehalogenative Coupling on Au(111): Polymerization versus Cyclization.

The control of reaction selectivity is of great interest in chemistry and depends crucially on the revelation of key influencing factors. Based on well-defined molecule-substrate model systems, various influencing factors have been elucidated, focusing primarily on the molecular precursors and the underlying substrates themselves, while interfacial properties have recently been shown to be essential as well. However, the influence of molecular chemisorption direction on reaction selectivity, as a subtle interplay between molecules and underlying substrates, remains elusive.

Anti-Kasha emissions of single molecules in a plasmonic nanocavity.

Kasha's rule generally holds true for solid-state molecular systems, where the rates of internal conversion and vibrational relaxation are sufficiently higher than the luminescence rate. In contrast, in systems where plasmons and matter interact strongly, the luminescence rate is significantly enhanced, leading to the emergence of luminescence that does not obey Kasha's rule. In this work, we investigate the anti-Kasha emissions of single molecules, free-base and magnesium naphthalocyanine (HNc and MgNc), in a plasmonic nanocavity formed between the tip of a scanning tunneling microscope (STM) and metal substrate.

Excited States of Metal-Adsorbed Dimethyl Disulfide: A TDDFT Study with Cluster Model.

The optical near field refers to a localized light field near a surface that can induce photochemical phenomena such as dipole-forbidden transitions. Recently, the photodissociation of the S-S bond of dimethyl disulfide (DMDS) was investigated using a scanning tunneling microscope with far- and near-field light. This reaction is thought to be initiated by the lowest-energy highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition of the DMDS molecule under far-field light.

Atomic-Scale Photon Mapping Revealing Spin-Current Relaxation.

A nanoscopic understanding of spin-current dynamics is crucial for controlling the spin transport in materials. However, gaining access to spin-current dynamics at an atomic scale is challenging. Therefore, we developed spin-polarized scanning tunneling luminescence spectroscopy (SP STLS) to visualize the spin relaxation strength depending on spin injection positions.

Steering the Reaction Pathways of Terminal Alkynes by Introducing Oxygen Species: From C-C Coupling to C-H Activation.

Selective regulation of chemical reactions is crucial in chemistry. Oxygen, as a key reagent in ubiquitous oxidative chemistry, exhibits great potential in regulating molecular assemblies, and more importantly, chemical reactions in molecular systems supported by metal surfaces. However, the unique catalytic performance and reaction mechanisms of oxygen species remain elusive, which are essential for understanding reaction selection and regulation.

Orbital-resolved visualization of single-molecule photocurrent channels.

Given its central role in utilizing light energy, photoinduced electron transfer (PET) from an excited molecule has been widely studied. However, even though microscopic photocurrent measurement methods have made it possible to correlate the efficiency of the process with local features, spatial resolution has been insufficient to resolve it at the molecular level. Recent work has, however, shown that single molecules can be efficiently excited and probed when combining a scanning tunnelling microscope (STM) with localized plasmon fields driven by a tunable laser.

Dissociation of Single O Molecules on Ag(110) by Electrons, Holes, and Localized Surface Plasmons.

A detailed understanding of the dissociation of O molecules on metal surfaces induced by various excitation sources, electrons/holes, light, and localized surface plasmons, is crucial not only for controlling the reactivity of oxidation reactions but also for developing various oxidation catalysts. The necessity of mechanistic studies at the single-molecule level is increasingly important for understanding interfacial interactions between O molecules and metal surfaces and to improve the reaction efficiency. We review single-molecule studies of O dissociation on Ag(110) induced by various excitation sources using a scanning tunneling microscope (STM).

Dissociation Mechanism of a Single O Molecule Chemisorbed on Ag(110).

The dissociation of O molecules chemisorbed on silver surfaces is an essential reaction in industry, and the dissociation mechanism has long attracted attention. The detailed dissociation mechanism was studied at the single-molecule level on Ag(110) by using a scanning tunneling microscope (STM). The dissociation reaction was found to be predominantly triggered by inelastically tunneled holes from the STM tip due to the significantly distributed density of states below the Fermi level of the substrate.

Monatomic Iodine Dielectric Layer for Multimodal Optical Spectroscopy of Dye Molecules on Metal Surfaces.

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 ∼10.

Localized Graphitization on Diamond Surface as a Manifestation of Dopants.

Doped diamond electrodes have attracted significant attention for decades owing to their excellent physical and electrochemical properties. However, direct experimental observation of dopant effects on the diamond surface has not been available until now. Here, low-temperature scanning tunneling microscopy is utilized to investigate the atomic-scale morphology and electronic structures of (100)- and (111)-oriented boron-doped diamond (BDD) electrodes.

Visualization of Frontier Molecular Orbital Separation of a Single Thermally Activated Delayed Fluorescence Emitter by STM.

Because the spatial distribution of frontier molecular orbitals (FMOs) regulates the thermally activated delayed fluorescence (TADF) property, researchers synthesize TADF emitters by designing their FMO distribution. However, it remains challenging to clarify how the FMO distribution is altered at molecular interfaces. Thus, visualizing the FMOs at molecular interfaces helps us to understand the working behavior of TADF emitters.

Boron position-dependent surface reconstruction and electronic states of boron-doped diamond(111) surfaces: an study.

Boron-doped diamond (BDD) has attracted much attention in semi-/superconductor physics and electrochemistry, where the surface structures and electronic states play crucial roles. Herein, we systematically examine the structural and electronic properties of the unterminated and H-terminated diamond(111) surfaces by using density functional theory calculations, and the effect of the boron position on them. The surface energy increases compared to that of the undoped case when the boron is located at a deeper position in the diamond bulk, which indicates that boron near the surface can facilitate the surface stability of the BDD in addition to the H-termination.

Single-molecule laser nanospectroscopy with micro-electron volt energy resolution.

Ways to characterize and control excited states at the single-molecule and atomic levels are needed to exploit excitation-triggered energy-conversion processes. Here, we present a single-molecule spectroscopic method with micro-electron volt energy and submolecular-spatial resolution using laser driving of nanocavity plasmons to induce molecular luminescence in scanning tunneling microscopy. This tunable and monochromatic nanoprobe allows state-selective characterization of the energy levels and linewidths of individual electronic and vibrational quantum states of a single molecule.

Chemical Identification and Bond Control of π-Skeletons in a Coupling Reaction.

Highly unsaturated π-rich carbon skeletons afford versatile tuning of structural and optoelectronic properties of low-dimensional carbon nanostructures. However, methods allowing more precise chemical identification and controllable integration of target sp-/sp-carbon skeletons during synthesis are required. Here, using the coupling of terminal alkynes as a model system, we demonstrate a methodology to visualize and identify the generated π-skeletons at the single-chemical-bond level on the surface, thus enabling further precise bond control.

Probing consequences of anion-dictated electrochemistry on the electrode/monolayer/electrolyte interfacial properties.

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.

Tunable Optical Transition in 2H-MoS via Direct Electrochemical Engineering of Vacancy Defects and Surface S-C Bonds.

Although surface engineering has been regarded to be a great approach to modulate the optical and electrical properties of nanomaterials, the spontaneous covalent functionalization on semiconducting 2H-MoS is a notoriously difficult process, while several reactions have been performed on metallic 1T-MoS. This limitation in functionalization is attributed to the difficulty of electron transfer from 2H-TMD to the reacting molecules due to its semiconducting property and neutral charge state. Unfortunately, this is an all too important prerequisite step toward creating chemically reactive radical species for surface functionalization reactions.

Controlling the Resonance Raman Effect in Tip-Enhanced Raman Spectroscopy Using a Thin Insulating Film.

Both surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) are widely used for the investigation of nanoscale materials. One of the most critical aspects of both SERS and TERS is the control of both the plasmon and molecular resonance precisely. Here, we demonstrate single-molecule TERS under molecular resonance conditions using a scanning tunneling microscope.

Single-Molecule Study of a Plasmon-Induced Reaction for a Strongly Chemisorbed Molecule.

Chemical reactions induced by plasmons achieve effective solar-to-chemical energy conversion. However, the mechanism of these reactions, which generate a strong electric field, hot carriers, and heat through the excitation and decay processes, is still controversial. In addition, it is not fully understood which factor governs the mechanism.

Centimeter-Scale and Highly Crystalline Two-Dimensional Alcohol: Evidence for Graphenol (COH).

We report a chemical route to synthesize centimeter-scale stoichiometric "graphenol (COH)", a 2D crystalline alcohol, via vapor phase hydroxylation of epitaxial graphene on Cu(111). Atomic resolution scanning tunneling microscopy revealed this highly-ordered configuration of graphenol and low energy electron diffraction studies on a large-area single crystal graphene film demonstrated the feasibility of the same superstructure being achieved at the centimeter length scale. Periodic density functional theory (DFT) calculations about the formation of C6(OH) and its electronic structure are also reported.

Single-molecule resonance Raman effect in a plasmonic nanocavity.

Tip-enhanced Raman spectroscopy (TERS) is a versatile tool for chemical analysis at the nanoscale. In earlier TERS experiments, Raman modes with components parallel to the tip were studied based on the strong electric field enhancement along the tip. Perpendicular modes were usually neglected.