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Atomic-scale control of photochemistry facilitates extreme miniaturisation of optoelectronic devices. Localised surface plasmons, which provide strong confinement and enhancement of electromagnetic fields at the nanoscale, secure a route to achieve sub-nanoscale reaction control. Such local plasmon-induced photochemistry has been realised only in metallic structures so far. Here we demonstrate controlled plasmon-induced single-molecule switching of peryleneanhydride on a silicon surface. Using a plasmon-resonant tip in low-temperature scanning tunnelling microscopy, we can selectively induce the dissociation of the O-Si bonds between the molecule and surface, resulting in reversible switching between two configurations within the nanojunction. The switching rate can be controlled by changing the tip height with 0.1-Å precision. Furthermore, the plasmon-induced reactivity can be modified by chemical substitution within the molecule, suggesting the importance of atomic-level design for plasmon-driven optoelectronic devices. Thus, metal-single-molecule-semiconductor junctions may serve as a prominent controllable platform beyond conventional nano-optoelectronics.
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Tip-enhanced Raman spectroscopy (TERS), combined with low-temperature scanning tunnelling microscopy (STM), has emerged as a highly sensitive method for chemical characterization, offering even sub-molecular resolution. However, its exceptional sensitivity is generally limited to molecules adsorbed onto plasmonic surfaces. Here we demonstrate single-molecule TERS for fullerene (C60) adsorbed on the Si(111)-(7 × 7) reconstructed surface. Distinct adsorption geometries of C60 are manifested in the TERS spectra. In addition, we reveal that formation of a molecular-point-contact (MPC) drastically enhances Raman scattering and leads to the emergence of additional vibrational peaks, including overtones and combinations. In the MPC regime, the anti-Stokes peaks are observed, revealing that vibrationally excited states are populated through optical excitation of the MPC junction, whereas showing no significant vibrational heating by current flow via inelastic electron-vibration scattering. Our results will open up the possibility of applying TERS for semiconducting surfaces and studying microscopic mechanisms of vibrational heating in metal-molecule-semiconductor nanojunctions.
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Tip-enhanced vibrational spectroscopy has advanced to routinely attain nanoscale spatial resolution, with tip-enhanced Raman spectroscopy even achieving atomic-scale and submolecular sensitivity. Tip-enhanced infrared spectroscopy techniques, such as nano-FTIR and AFM-IR spectroscopy, have also enabled the nanoscale chemical analysis of molecular monolayers, inorganic nanoparticles, and protein complexes. However, fundamental limits of infrared nanospectroscopy in terms of spatial resolution and sensitivity have remained elusive, calling for a quantitative understanding of the near-field interactions in infrared nanocavities. Here, we demonstrate the application of nano-FTIR spectroscopy to probe the amide-I vibration of a single protein consisting of â¼500 amino acid residues. Detection with higher tip tapping demodulation harmonics up to the seventh order leads to pronounced enhancement in the peak amplitude of the vibrational resonance, originating from sub-tip-radius geometrical effects beyond dipole approximations. This quantitative characterization of single-nanometer near-field interactions opens the path toward employing infrared vibrational spectroscopy at the subnanoscale and single-molecule levels.
Assuntos
Rádio (Anatomia) , Vibração , Microscopia de Força Atômica , Nanotecnologia/métodos , Espectroscopia de Infravermelho com Transformada de FourierRESUMO
We demonstrate on-surface deprotection of methylenedioxy groups which yielded graphene nanoribbons (GNRs) with edges functionalized by hydroxy groups. While anthracene trimer precursors functionalized with hydroxy groups did not yield GNRs, it was found that hydroxy groups are first protected as methylenedioxy groups and then deprotected during the cyclo-dehydrogenation process to form GNRs with hydroxy groups. The X-ray photoemission spectroscopy and non-contact atomic force microscopy studies revealed that â¼20% of the methylenedioxy turned into hydroxy groups, while the others were hydrogen-terminated. The first-principles density functional theory (DFT) study on the cyclo-dehydrogenation process was performed to investigate the deprotection mechanism, which indicates that hydrogen atoms emerging during the cyclo-dehydrogenation process trigger the deprotection of methylenedioxy groups. The scanning tunneling spectroscopy study and DFT revealed a significant charge transfer from hydroxy to the Au substrate, causing an interface dipole and the HOMO being closer to the Fermi level when compared with hydrogen-terminated GNR/Au(111). This result demonstrates on-surface deprotection and indicates a possible new route to obtain GNRs with desired edge functionalization, which can be a critical component for high-performance devices.
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Helicene is a functional material with chirality caused by its characteristic helical geometry. The inversion of its helicity by external stimuli is a challenging task in the advanced control of the molecular chirality. This study fabricated a novel helical molecule, specifically a pentahelicene-analogue twisted aromatic hydrocarbon fused with a graphene nanoribbon, via on-surface synthesis using multiple precursors. Noncontact atomic force microscopy imaging with high spatial resolution confirmed the helicity of the reaction products. The helicity was geometrically converted by pushing a CO-terminated tip into the twisted framework, which is the first demonstration of helicity switching at the single-molecule scale.
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The reaction of nitric oxide (NO) on Cu(100) is studied by scanning tunneling microscopy, electron energy loss spectroscopy and density functional theory calculations. The NO molecules adsorb mainly as monomers at 64 K, and react and dissociate to yield oxygen atoms on the surface at â¼70 K. The temperature required for the dissociation is significantly low for Cu(100), compared to those for Cu(111) and Cu(110). The minimum energy pathway of the reaction is via (NO)2 formation, which converts into a flat-lying ONNO and then dissociates into N2O and O with a considerably low activation energy. We propose that the formation of (NO)2 and flat-lying ONNO is the key to the exceptionally high reactivity of NO on Cu(100).
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Formic acid (HCOOH) can be catalytically decomposed into H2 and CO2 and is a promising hydrogen storage material. As H2 production catalysts, Cu surfaces allow selective HCOOH decarboxylation; however, the on-surface HCOOH decomposition reaction pathway remains controversial. In this study, the temperature dependence of the HCOOH/Cu(111) adsorption structures is elucidated by scanning tunneling microscopy and non-contact atomic force microscopy, establishing the adsorbate chemical species using density functional theory. 2D HCOOH islands at 80 K, linear chains of HCOOH and monodentate formate at 150 K, chain-like assemblies of monodentate and bidentate formate at 200 K, and bidentate formate clusters at 300 K are observed. At each temperature, the adsorbates experience attractive interactions among themselves. Such aggregation stabilizes them against desorption and decomposition. Thus, accurate evaluation of intermolecular interactions is essential to understand catalytic reactivity.
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Formiatos , Hidrogênio , Adsorção , CatáliseRESUMO
Performing bottom-up synthesis by using molecules adsorbed on a surface is an effective method to yield functional polycyclic aromatic hydrocarbons (PAHs) and nanocarbon materials. The intramolecular cyclodehydrogenation of hydrocarbons is a critical process in this synthesis; however, thus far, its elementary steps have not been elucidated thoroughly. In this study, we utilize the metal tip of a low-temperature noncontact atomic force microscope as a manipulable metal surface to locally activate dehydrogenation for PAH-forming cyclodehydrogenation. This method leads to the dissociation of a H atom of an intermediate to yield the cyclodehydrogenated product in a target-selective and reproducible manner. We demonstrate the metal-tip-catalyzed dehydrogenation for both benzenoid and nonbenzonoid PAHs, suggesting its universal applicability as a catalyst for nanographene synthesis.
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On-surface synthesis is a powerful method for fabricating atomically precise graphene nanoribbons (GNRs), but the products always include defective structures. In this study, scanning tunnelling microscopy and atomic force microscopy were used to determine the length distribution of armchair-edge GNRs with a width of seven carbon atoms (7-AGNRs) synthesised on Au(111) and to characterise defective structures. The product quality was improved by increasing the precursor deposition amount because of a preference for intermolecular polymerisation over intramolecular cyclodehydrogenation at a high coverage. However, the annealing rate had a complex effect on the quality, with a low rate elongating 7-AGNRs but degenerating the length uniformity. These insights advance the understanding of the critical parameters for obtaining high-quality products in high yield by on-surface synthesis.
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Localized electronic spin state in molecules has a relatively long spin lifetime and has thus attracted much attention. In this study, we characterize the magnetoresistance of a system comprising Pt and Fe(II)-phthalocyanine (FePc) molecules. The magnetoresistance measurement with the weak antilocalization analysis reveals that a magnetic moment in FePc acts as magnetic impurities for conduction electrons in Pt. Moreover, we find that the magnetoresistance involves a component that possesses the same symmetry as spin-Hall magnetoresistance. These results reveal the spin-angular momentum transfer from metallic Pt to a magnetic moment in FePc molecules, which can be used as a spin torque in a molecular system.
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We demonstrated that a nitric oxide (NO) molecule on Cu(110) acts as an "ON-OFF-ON toggle switch" that can be turned on and off by repulsive force and electron injection, respectively. On the surface, NO molecules exist in three configurations: flat along the [001] direction (ON), upright (OFF), and flat along [001[over ¯]] (ON). An NO-functionalized tip, which was characterized by scanning tunneling microscopy and inelastic electron tunneling spectroscopy, can convert an upright NO adsorbate into a flat-lying NO. Atomic force microscopy and a simulation of the interactions between the NO molecules reveal that a repulsive force not aligned with the N-O bond provides the torque that detrudes the NO toggle; i.e., the upright NO adsorbate is tilted away from the tip. Therefore, the NO adsorbate behaves as a nonvolatile sensor for the detection of locally applied repulsive torque.
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A core-expanded, pyrrole-fused azacoronene analogue containing two unusual N-doped heptagons was obtained from commercially available octafluoronaphthalene and 3,4-diethylpyrrole in two steps as a heteroatom-doped nonplanar nanographene. Full fusion with the formation of the tetraazadipleiadiene framework and the longitudinally twisted structure was unambiguously confirmed by single-crystal X-ray diffraction analysis. The edge-to-edge dihedral angle along the acene moiety was 63°. This electron-rich π-system showed four reversible oxidation peaks. Despite the nonplanar structure, the Hückel aromaticity owing to a peripheral π-conjugation in the dicationic state was concluded from the bond-length alternation and nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations.
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We observed individual sodium (Na) atoms and their complexes with water molecules on Cu(110) with scanning tunneling microscopy at 6 K. We induced the reaction of a Na adatom with one or two water molecules, which yielded two kinds of Na-water complexes. Density functional theory calculations were performed to study the structure of the complexes, which revealed that the water molecules are bonded to a Na atom along the [11[combining macron]0] direction via an oxygen atom with the hydrogen atoms pointing toward the Cu atoms of the surface. The 1 : 1 Na-water complex is stablized by 225 meV upon bond formation, and the ligand water moves back and forth around the Na atom. The complex can accommodate another water molecule to yield a 1 : 2 Na-water complex with an energy gain of 214 meV. The atomic-scale identification of the alkali-water complexes would give fundamental insights into the hydration process of alkali cations and their specific adsorption onto metal electrodes.
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Controlling the structural deformation of organic molecules can drive unique reactions that cannot be induced only by thermal, optical or electrochemical procedures. However, in conventional organic synthesis, including mechanochemical procedures, it is difficult to control skeletal rearrangement in polycyclic aromatic hydrocarbons (PAHs). Here, we demonstrate a reaction scheme for the skeletal rearrangement of PAHs on a metal surface using high-resolution noncontact atomic force microscopy. By a combination of organic synthesis and on-surface cyclodehydrogenation, we produce a well-designed PAH-diazuleno[1,2,3-cd:1',2',3'-fg]pyrene-adsorbed flatly onto Cu(001), in which two azuleno moieties are highly strained by their mutual proximity. This local strain drives the rearrangement of one of the azuleno moieties into a fulvaleno moiety, which has never been reported so far. Our proposed thermally driven, strain-induced synthesis on surfaces will pave the way for the production of a new class of nanocarbon materials that conventional synthetic techniques cannot attain.
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Local defects in water layers growing on metal surfaces have a key influence on the wetting process at the surfaces; however, such minor structures are undetectable by macroscopic methods. Here, we demonstrate ultrahigh-resolution imaging of single water layers on a copper(110) surface by using non-contact atomic force microscopy (AFM) with molecular functionalized tips at 4.8 K. AFM with a probe tip terminated by carbon monoxide predominantly images oxygen atoms, whereas the contribution of hydrogen atoms is modest. Oxygen skeletons in the AFM images reveal that the water networks containing local defects and edges are composed of pentagonal and hexagonal rings. The results reinforce the applicability of AFM to characterize atomic structures of weakly bonded molecular assemblies.
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Using low-temperature scanning tunneling microscopy (STM), the adsorption and reaction of hydrogen sulfide (H2S) and its fragments (SH and S) on Cu(110) are investigated at 5 K. H2S adsorbs molecularly on the surface on top of a Cu atom. With voltage pulses of STM, it is possible to induce sequential dehydrogenation of H2S to SH and S. We found two kinds of adsorption structures of SH. The short-bridge site is the most stable site for SH, while the long-bridge site is the second. Density functional theory calculations show that the S-H axis is inclined from the surface normal for both species. The reaction of H2S with OH and O was directly observed to yield SH and S, respectively, providing a molecular-level insight into catalyst poisoning.