Norio Okabayashi

Site Selective Inelastic Electron Tunneling Spectroscopy Probed by Isotope Labeling

We study inelastic scattering in alkanethiol self-assembled monolayers using isotope labeling and unambiguously determine which molecular vibrations are active in the inelastic electron tunneling spectroscopy. The selective deuteration of the molecule also allows us to show that the different parts of the molecule contribute approximately equally to inelastic signal. Our first principles calculations confirm the experimental results and provide insights on electron transport through molecules.

Influence of atomic tip structure on the intensity of inelastic tunneling spectroscopy data analyzed by combined scanning tunneling spectroscopy, force microscopy, and density functional theory

Achieving a high intensity in inelastic scanning tunneling spectroscopy (IETS) is important for precise measurements. The intensity of the IETS signal can vary by up to a factor of 3 for various tips without an apparent reason accessible by scanning tunneling microscopy (STM) alone. Here, we show that combining STM and IETS with atomic force microscopy enables carbon monoxide front-atom identification, revealing that high IETS intensities for CO/Cu(111) are obtained for single-atom tips, while the intensity drops sharply for multiatom tips. Adsorption of the CO molecule on a Cu adatom [CO/Cu/Cu(111)] such that the molecule is elevated over the substrate strongly diminishes the tip dependence of IETS intensity, showing that an elevated position channels most of the tunneling current through the CO molecule even for multiatom tips, while a large fraction of the tunneling current bypasses the CO molecule in the case of CO/Cu(111).

Identification of a deuterated alkanethiol inserted in a hydrogenated alkanethiol self-assembled monolayer by mapping of an inelastic tunneling signal

We show an experimental technique for visualizing distributions of vibrational modes of molecules through mapping of an inelastic tunneling signal with a scanning tunneling microscope. A topographic information and d(2)I/dV(2) signal processed by a lock-in amplifier were simultaneously imaged, where the feedback loop for the tunneling gap was engaged and a modulation voltage was superimposed to the gap voltage. The current signal used for the tunneling gap control was tuned by the filtering in order to minimize the response of the feedback loop caused by the modulation voltage. The effectiveness of this technique was demonstrated for a self-assembled monolayer composed of a mixture of normal and deuterated hexanethiol molecules, where both molecules have the same molecular length and the former was embedded in the matrix of the latter. Two types of molecules were successfully discriminated by chemical properties.

Inelastic electron tunneling spectroscopy with a dilution refrigerator based scanning tunneling microscope

This paper presents the design and operations of a compact cryogenic scanning tunneling microscope system combined with a dilution refrigerator. We obtained a minimum temperature of 260 mK at the sample position. Taking advantage of low-temperature measurement for the spectroscopy, inelastic electron tunneling spectroscopy (IETS) at 4.4 K was demonstrated for the octanethiol molecules in a self-assembled monolayer. The spectrum showed many vibrational features as in the case of a high-resolution electron energy loss spectroscopy. We discuss the resolution of the IET signals focusing on its changes with the modulation voltage and the sample temperature. IETS at 260 mK is also presented with similar quality as in the case of 4.4 K, indicating that the vibration-dumping methods for the operation of the dilution refrigerator described in this paper are adequate.

Analysis of STM images with pure and CO-functionalized tips : A first-principles and experimental study

We describe a first-principles method to calculate scanning tunneling microscopy (STM) images, and compare the results to well-characterized experiments combining STM with atomic force microscopy (AFM). The theory is based on density functional theory with a localized basis set, where the wave functions in the vacuum gap are computed by propagating the localized-basis wave functions into the gap using a real-space grid. Constant-height STM images are computed using Bardeens approximation method, including averaging over the reciprocal space. We consider copper adatoms and single CO molecules adsorbed on Cu(111), scanned with a single-atom copper tip with and without CO functionalization. The calculated images agree with state-of-the-art experiments, where the atomic structure of the tip apex is determined by AFM. The comparison further allows for detailed interpretation of the STM images.

Vibrations of a molecule in an external force field

Significance The vibration of a molecule adsorbed on a surface contains essential information on the molecule–surface bond, which is important to understand the surface reactions that occur, e.g., in catalytic reactions. Accessing the vibrational energies of a single molecule is possible by combining scanning tunneling microscopy with inelastic electron spectroscopy. However, the tip of a microscope exerts a force on a nearby molecule, and possibly even induces slight structural changes. To study this problem, we have further incorporated atomic force microscopy. The relationship between the exerted forces and vibrational energies is well reproduced by a classical mechanical model. This combined technique opens the possibility to study the atomic-scale interaction of a molecule on a surface with unprecedented precision. The oscillation frequencies of a molecule on a surface are determined by the mass distribution in the molecule and the restoring forces that occur when the molecule bends. The restoring force originates from the atomic-scale interaction within the molecule and with the surface, which plays an essential role in the dynamics and reactivity of the molecule. In 1998, a combination of scanning tunneling microscopy with inelastic tunneling spectroscopy revealed the vibrational frequencies of single molecules adsorbed on a surface. However, the probe tip itself exerts forces on the molecule, changing its oscillation frequencies. Here, we combine atomic force microscopy with inelastic tunneling spectroscopy and measure the influence of the forces exerted by the tip on the lateral vibrational modes of a carbon monoxide molecule on a copper surface. Comparing the experimental data to a mechanical model of the vibrating molecule shows that the bonds within the molecule and with the surface are weakened by the proximity of the tip. This combination of techniques can be applied to analyze complex molecular vibrations and the mechanics of forming and loosening chemical bonds, as well as to study the mechanics of bond breaking in chemical reactions and atomic manipulation.