Dynamic Friction Unraveled by Observing an Unexpected Intermediate State in Controlled Molecular Manipulation.

The pervasive phenomenon of friction has been studied at the nanoscale via a controlled manipulation of single atoms and molecules with a metallic tip, which enabled a precise determination of the static friction force necessary to initiate motion. However, little is known about the atomic dynamics during manipulation. Here, we reveal the complete manipulation process of a CO molecule on a Cu(110) surface at low temperatures using a combination of noncontact atomic force microscopy and density functional theory simulations. We found that an intermediate state, inaccessible for the far-tip position, is enabled in the reaction pathway for the close-tip position, which is crucial to understanding the manipulation process, including dynamic friction. Our results show how friction forces can be controlled and optimized, facilitating new fundamental insights for tribology.

Radio frequency filter for an enhanced resolution of inelastic electron tunneling spectroscopy in a combined scanning tunneling- and atomic force microscope.

The combination of inelastic electron tunneling spectroscopy (IETS), also used for IET spectrum based on scanning tunneling microscopy with atomic force microscopy (AFM) enables us to measure the vibrational energies of a single molecule along with the force exerted by the tip of a microscope, which deepens our understanding on the interaction between the tip and the molecule on a surface. The resolution of IETS is a crucial factor in determining the vibrational energies of a molecule. However, radio frequency (RF) noise from the environment significantly deteriorates the resolution. We introduce an RF noise filtering technique, which enables high resolution IETS while maintaining uncompromised AFM performance, demonstrated by vibrational measurements of a CO molecule on a copper surface.

Vibrations of a molecule in an external force field.

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.

Force field analysis suggests a lowering of diffusion barriers in atomic manipulation due to presence of STM tip.

We study the physics of atomic manipulation of CO on a Cu(111) surface by combined scanning tunneling microscopy and atomic force microscopy at liquid helium temperatures. In atomic manipulation, an adsorbed atom or molecule is arranged on the surface using the interaction of the adsorbate with substrate and tip. While previous experiments are consistent with a linear superposition model of tip and substrate forces, we find that the force threshold depends on the force field of the tip. Here, we use carbon monoxide front atom identification (COFI) to characterize the tip's force field. Tips that show COFI profiles with an attractive center can manipulate CO in any direction while tips with a repulsive center can only manipulate in certain directions. The force thresholds are independent of bias voltage in a range from 1 to 10 mV and independent of temperature in a range of 4.5 to 7.5 K.

Logic operations of chemically assembled single-electron transistor.

Double-gate single-electron transistors (SETs) were fabricated by chemical assembling using electroless gold-plated nanogap electrodes and chemisorbed chemically synthesized gold nanoparticles. The fabricated SET showed periodic and stable Coulomb oscillations under application of voltages of both gates. The sole SET also exhibited all two-input logic operations-XOR, XNOR, NAND, OR, NOR, and AND-with an on/off ratio of 10(2). This demonstrates the potential of chemical assembling to give highly stable SETs exhibiting all logic operations.

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.

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.

Inelastic tunneling spectroscopy of alkanethiol molecules: high-resolution spectroscopy and theoretical simulations.

We investigate inelastic electron tunneling spectroscopy (IETS) for alkanethiol self-assembled monolayers (SAM) with a scanning tunneling microscope and compare it to first-principles calculations. Using a combination of partial deuteration of the molecule and high-resolution measurements, we identify and differentiate between methyl (CH3) and methylene (CH2) groups and their symmetric and asymmetric C-H stretch modes. The calculations agree quantitatively with the measured IETS in producing the weight of the symmetric and asymmetric C-H stretch modes while the methylene stretch mode is largely underestimated. We further show that inelastic intermolecular scattering is important in the SAM by plotting the theoretical current densities.

Inelastic electron tunneling spectroscopy of an alkanethiol self-assembled monolayer using scanning tunneling microscopy.

We report inelastic electron tunneling spectroscopy (IETS) of a C8 alkanethiol self-assembled monolayer using a scanning tunneling microscope (STM). High-resolution STM IETS spectra show clear features of the C-H bending and C-C stretching modes in addition to the C-H stretching mode, which enables a precise comparison with previously reported vibrational spectroscopy, especially electron energy loss spectroscopy data. Intensity variation of vibrational peaks with tip position is discussed with the STM IETS detection mechanism.