Tunneling Microscope Light Emission Spectroscopy Research Articles

Determining the phonon energy of highly oriented pyrolytic graphite by scanning tunneling microscope light emission spectroscopy

We have investigated the scanning tunneling microscope (STM) light emission spectra of isolated single Ag nanoparticles lying on highly oriented pyrolytic graphite (HOPG). The STM light emission spectra exhibited two types of spectral structures (step-like and periodic). Comparisons of the observed structures and theoretical predictions indicate that the phonon energy of the ZO mode of HOPG [M. Mohr et al., Phys. Rev. B 76, 035439 (2007)] can be determined from the energy difference between the cutoff of STM light emission and the step in the former structure, and from the period of the latter structure. Since the role of the Ag nanoparticles does not depend on the substrate materials, this method will enable the phonon energies of various materials to be measured by STM light emission spectroscopy. The spatial resolution is comparable to the lateral size of the individual Ag nanoparticles (that is, a few nm).

Capacitance Behavior of Alkanethiol Self-Assembled Monolayer Studied by Scanning Tunneling Microscope Light Emission Spectroscopy

We have studied the capacitance behavior of alkanethiol self-assembled monolayer (SAM)-covered Au film by scanning tunneling microscope light emission (STM-LE) spectroscopy in the Kretschmann geometry. Although, the STM-LE from tip-sample gap into the vacuum (tip-side emission) is fundamentally weak and very difficult to detect, we have succeeded in detecting the STM-LE radiated into the prism (prism-side emission) by virtue of the enhancement of prism-coupled geometry. Our experimental results shows that the cutoff energy of STM-LE spectra have been redshifted with increase in thickness (chain length) of the SAM film. In order to explain the cutoff energy shift, we have designed a two-layer tunnel junction model by considering the capacitance response of the SAM film depending on the molecular chain length. It has been found from the model analysis, that the capacitance of the SAM changes with changing of the molecular thickness. Hence, it is concluded that the shift of the cutoff energy has originated from the change of the capacitance of the SAM film depending on the molecular chain length.

Measurement of phonon energy of Sb2Te3 by scanning tunneling microscope light-emission spectroscopy

We have measured the energy of the A2u phonon mode of Sb2Te3 by scanning tunneling microscope light-emission (STM-LE) spectroscopy combined with ps-pulsed laser irradiation of the STM tip-sample gap. It is known that the STM-LE from Sb2Te3 is excited by electronic transitions at a photon energy of around 1.6eV, i.e, a single peak with a narrow spectral width appears at the photon energy in the STM-LE spectrum. For STM-LE occurring with the STM tip-sample gap irradiated by ps laser pulses, several supplementary peaks appear in the STM-LE spectra (in addition to the single original peak). The difference in energy between adjacent peaks gives the phonon energy of the A2u mode of Sb2Te3.

Nanoscale properties of single atoms and molecules investigated by scanning tunneling microscope light emission spectroscopy

Nanoscale properties of single atoms and molecules investigated by scanning tunneling microscope light emission spectroscopy

Vibrational Excitation of a Single Benzene Molecule Adsorbed on Cu(110) Studied by Scanning Tunneling Microscope Light Emission Spectroscopy

We have investigated the vibrational excitation of a single benzene molecule adsorbed on Cu(110) using scanning tunneling microscope light emission (STM-LE) spectroscopy. The STM-LE spectrum of the benzene molecule has a broad light emission peak and is similar to that of the Cu substrate, indicating that the STM-LE is radiated via the excitation of localized surface plasmons (LSP) confined in the tip−sample gap. However, careful analysis revealed that the STM-LE spectrum of the benzene molecule exhibits a steplike structure appearing at an energy near the quantum cutoff. Isotope-labeled experiments revealed that the fine structure is associated with the vibrational excitation of the benzene molecule, that is, the CH out-of-plane bending mode. We concluded that the vibrational excitation of benzene observed in the present study is not caused by the tunneling electrons through the inelastic tunneling process but by the Raman process due to the strong electric field generated just under the STM tip.

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.

Local Density of States of Partially Oxidized Ag(110) Surfaces Observed Using Scanning Tunneling Microscope Light-Emission Spectroscopy

We have observed the scanning tunneling microscope (STM) light-emission spectra of partially oxidized Ag(110) surfaces with atomic spatial resolution. The STM image visualized Ag–O chains along the [001] direction as light rows, separated by a distance n ×a from adjacent light rows. Here, n is an integer between 2 and 7, and a is the surface lattice constant of Ag in the [110] direction. The STM light-emission spectra were measured for tip locations over the light rows and the midpoints of the darkly imaged areas sandwiched by two adjacent light rows. While the spectral shapes of the STM light-emissions were identical, their amplitudes showed atomic-site dependence, i.e., the STM light-emission intensities showed atomic-site dependence without any spectral differences. The STM light-emission intensities at the midpoints of the dark areas increased with n. The STM light-emission intensities of the light rows were independent of the widths of the adjacent dark areas. The experimental results were analyzed by the dielectric theory of STM light-emission, and it was found that the observed atomic-site dependence is caused by electronic states localized along the light rows. These states are identified as the py and pz surface bands induced by adsorbed oxygen.

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.

Scanning Tunneling Microscope Light Emission Spectroscopy with Good Signal-to-noise Ratio

Scanning Tunneling Microscope Light Emission Spectroscopy with Good Signal-to-noise Ratio

Vibration of H atomic chains on Ni(110) measured by scanning tunneling microscope light emission spectroscopy.

We have measured the visible light spectrum emitted by hydrogen atoms adsorbed on an Ni(110) surface, excited by the tunneling current from the scanning tunneling microscope. The spectrum contains periodic fine structures, whose period corresponds to the vertical vibrational energy of the adsorbed H atom. This energy showed the expected isotope shift when H was replaced by deuterium, and further it depended on the H-atom chain length.

Superconducting niobium tip for scanning tunneling microscope light emission spectroscopy

Niobium tips for use in scanning tunneling microscope (STM) light emission measurements have been fabricated by an electrochemical etching method. The typical radius of curvature of the tip is less than 100 nm. The Nb tip has been demonstrated to be capable of producing STM images of highly oriented pyrolytic graphite and Au(110)-(2×1) surfaces with atomic spatial resolution. It has been confirmed by STM light emission spectroscopy that the front of the Nb tip becomes superconducting for temperatures below the superconducting transition temperature of Nb.

Scanning tunneling microscope light emission spectroscopy with picosecond time resolution

Scanning tunneling microscope (STM) light emission spectroscopy with picosecond time resolution has been developed and applied to the measurement of time-resolved spectra of STM light emission from an evaporated Au film. The spectra consist of two components in the time-energy domain. The first component is excited by the picosecond tunneling current pulses that are generated by picosecond laser irradiation at the STM tip-sample gap. The second component arises from the plane-wave surface plasmon polaritons that are excited at the Au surface by the laser and made radiative by the presence of the STM tip that breaks the translational symmetry of the surface.