Satoshi Katano

Reversible Control of Hydrogenation of a Single Molecule

Low-temperature scanning tunneling microscopy was used to selectively break the N-H bond of a methylaminocarbyne (CNHCH3) molecule on a Pt(111) surface at 4.7 kelvin, leaving the C-H bonds intact, to form an adsorbed methylisocyanide molecule (CNCH3). The methylisocyanide product was identified through comparison of its vibrational spectrum with that of directly adsorbed methylisocyanide as measured with inelastic electron tunneling spectroscopy. The CNHCH3 could be regenerated in situ by exposure to hydrogen at room temperature. The combination of tip-induced dehydrogenation with thermodynamically driven hydrogenation allows a completely reversible chemical cycle to be established at the single-molecule level in this system. By tailoring the pulse conditions, irreversible dissociation entailing cleavage of both the C-H and N-H bonds can also be demonstrated.

Geometrical characterization of pyrimidine base molecules adsorbed on Cu(1 1 0) surfaces: XPS and NEXAFS studies

The structures of pyrimidine base molecules of thymine and cytosine adsorbed onto Cu(110) surfaces (0 > 1.0 ML) have been discussed based on the results of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The NEXAFS analysis for azimuthal orientation of molecules on surfaces indicates that the adsorbed molecules are in an upright configuration with their n plane parallel to the row structure ([110] direction), well below saturation coverage (θ ∼ 0.2 ML). Based on the chemical shifts observed in XP spectra (N1s, O1s region), the molecules have been determined to interact with surfaces through their nitrogen atom (lone-pair) next to the carbonyl group (C=O).

Single-molecule reactions and spectroscopy via vibrational excitation

Inelastic tunnelling spectra of single C4 hydrocarbon molecules adsorbed on the Pd(110) surface are presented. Experimental evidence is given that the symmetry of the molecular orbital into which the tunnelling electron first enters determines which vibrational modes are excited. The action spectrum for cis–2–butene exhibits most of the vibrational modes that are expected to be excited except for ? (C=C), which may be because the molecule is p bonded to the substrate, thus making the lifetime of the excited state short.

THE EFFECT OF CO-ADSORPTION OF ON-TOP CO ON THE SUM-FREQUENCY GENERATION SIGNAL OF BRIDGE CO ON THE NI(111) SURFACE

Abstract Anomalous coverage dependence was observed on the sum-frequency generation signal of bridge CO on the Ni(111) surface when co-adsorbed by on-top CO. The bridge CO peak was distinct and normal from 0.04 to ∼0.40 monolayer (ML) but weakened after the appearance of the on-top CO peak at 0.47 ML and disappeared at 0.60 ML, although both peaks were observed in the infrared reflection absorption spectra up to 1.0 ML. The spectra of mixtures of C 16 O and C 18 O eliminate the possibility that the weakening of the bridge CO band is caused by its coupling with the on-top band.

Self-Assembly and Scanning Tunneling Microscopy Tip-Induced Motion of Ferrocene Adamantane Trithiolate Adsorbed on Au(111)

We have studied the self-assembled monolayers (SAMs) of adamantane-based trithiolate, which consists of a ferrocene derivative at the head (ferrocene adamantane trithiolate; ferrocene-ATT), on Au(111) using low temperature scanning tunneling microscopy (STM). It was found that the adsorption behavior of ferrocene-ATT is similar to that of bromine adamantane trithiolate (BATT) adsorbed on Au(111). This indicates that adsorption of adamantane-based trithiol is controlled by three legs (CH2S) connected to bridgehead positions of the adamantane cage. Molecules, which form an ordered structure, are stable under low-bias-voltage scanning, i.e., a sample bias voltage lower than 1 V. STM tip-induced diffusion, however, was observed both for small clustered molecules and for molecules bound around the edge of an ordered molecular island. Furthermore, applying a high bias voltage (5 V) resulted in the destruction of SAMs structures.

Nanoscale coupling of photons to vibrational excitation of Ag nanoparticle 2D array studied by scanning tunneling microscope light emission spectroscopy

Scanning tunneling microscope light emission (STM-LE) spectroscopy has been utilized to elucidate the luminescence phenomena of Ag nanoparticles capped with myristate (myristate-capped AgNP) and 2-methyl-1-propanethiolate (C(4)S-capped AgNP) on the dodecanethiol-precovered Au substrate. The STM imaging revealed that myristate-capped AgNPs form an ordered hexagonal array whereas C(4)S-capped AgNPs show imperfect ordering, indicating that a shorter alkyl chain of C(4)S-capped AgNP is not sufficient to form rigid interdigitation. It should be noted that such a nanoparticle ordering affects the luminescence properties of the Ag nanoparticle. We found that the STM-LE is only detected from the Ag nanoparticles forming the two-dimensional superlattice. This indicates that the STM-LE of the Ag nanoparticle is radiated via the collective excitation of the local surface plasmon resonance (LSPR) spread over the Ag nanoparticles. Note that the STM-LE spectra of the Ag nanoparticles exhibit spike-like peaks superimposed on the broad light emission peak. Using Raman spectroscopy, we concluded that the spike-like structure appearing in the STM-LE spectra is associated with the vibrational excitation of the molecule embedded between Ag nanoparticles.

Screening of SFG signals from bridged CO on Ni(111) by the coexistence of linear CO

A spectroscopic study of adsorbed CO on Ni(111) was carried out using infrared-visible sum-frequency generation (SFG) and infrared reflection absorption spectroscopy (IRAS). An anomalous coverage-dependence of the SFG signal intensities of bridged and linear CO was found. While the SFG signal of bridged CO at 1920 cm -1 gained intensity with increasing CO coverage in the region between 0 and 0.4 ML ( 1 monolayer, ML: saturation coverage at 130 K), it weakened and disappeared when linear CO (2074 cm -1 ) was present on the surface above 0.47 ML. The IRA peak of bridged CO, however, did not disappear at the increased coverage suggesting that the weakening of the SFG signal of bridged CO is not caused by the decrease in the coverage but by the suppression of the SFG process of bridged CO by the coexistence of linear CO. The spectra of isotope mixtures of C 16 O and C 18 O clearly indicated that the weakening of SFG signals is not caused by the dipole-dipole coupling between the vibrational modes of bridged and linear CO. The anomalous coverage-dependence of the SFG signal was ascribed to the change of the Raman tensor by the coadsorption of linear CO.

Tailoring electronic states of a single molecule using adamantane-based molecular tripods

Adsorption structures and electronic states of molecular tripods, having a Br atom (BATT) and a ferrocene derivative (Ferrocene-ATT) at the head part of the adamantane-based trithiolate, adsorbed on Au(111) have been investigated using low temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). We found that BATT and Ferrocene-ATT form self-assembled monolayers (SAMs), and their orderings are identical to one another, which suggests that the adsorption structure of adamantane-based molecular tripods is independent of the type of functional substituent attached to the head part. The electronic states originated from the ferrocene group were confirmed in the STS spectrum of Ferrocene-ATT whereas those are absent in the BATT spectrum. We note that the ferrocene part has few interactions with the Au substrate owing not only to the upright geometry of Ferrocene-ATT but also to the insulative properties of the adamantane base. The STS mapping revealed the spatial distribution of the electronic state of Ferrocene-ATT.

Prism-Coupled Scanning Tunneling Microscope Light Emission Spectroscopy of Au Film Covered with Self-Assembled Alkanethiol Monolayer

We have investigated scanning tunneling microscope (STM) light emission from bare and alkanethiol self-assembled monolayer (SAM)-covered Au films in the Kretschmann geometry. The films were deposited on the flat bottom of a hemispherical prism, and the STM light emissions from the tip–sample gap into the vacuum (tip-side emission) and into the prism (prism-side emission) were measured. The prism-side emission was much stronger than the tip-side emission for the bare Au film. Theoretical analysis revealed that this enhancement of emission intensity is caused by the fact that surface plasmon polaritons (SPPs) localized at the Au surface become radiative on the prism side. This geometry was applied to the investigations of STM light emission from the Au film covered with an alkanethiol SAM. The prism-side emission was successfully detected by virtue of the enhancement of STM light emission.

Nanoscale structural variation observed on the vicinal SrTiO3(001) surface

The vicinal (001) surface of a Nb-doped SrTiO3 single crystal has been investigated by scanning tunneling microscopy and low energy electron diffraction. The stepped surface prepared by annealing in ultrahigh vacuum at 250 °C exhibits a complex atomic structure composed of four types of reconstructions, which shows short-range variation within nanoscale regions. SrO layers show a c(6×2) structure being stable up to 1000 °C, while √13×√13-R33.7°, c(√13×√13)-R33.7°, and c(√2×√18)-R45° structures are formed on TiO2 layers, which disappear at 450–750 °C followed by the formation of 2×2 and √5×√5-R26.6° structures. These results indicate instability of the reconstructions on the TiO2 terminated surface due to the variation in Sr adatom density caused by multikinetic processes, in contrast to the thermodynamically stable SrO terminated surface.