Scanning Tunneling Microscope Light Emission Spectra Research Articles

STM Light Emission Spectroscopy of Self-Assembled Monolayer of Alkanethiol on Au Film

In this paper, scanning tunneling microscope (STM) light emission (STM-LE) from alkanethiol self-assembled monolayer (SAM)-covered Au film has been probed in the Kretschmann configuration. The films were deposited on the smooth plane of a hemispherical glass prism. STM-LEs from the tip–sample gap into the vacuum (tip-side emission) and into the prism (prism-side emission) were investigated. Our experimental results showed that the tip-side emission was scarcely found, and the prism-side emissions were successfully detected due to the enhancement of Kretschmann configuration. It was also found from the experimental study that the peak intensity of STM-LE spectra become smaller, accompanying the redshift of the peak position with the rise in thickness of the alkanethiol SAM film. The main focus of this paper is to explore the behavior of electron tunneling into the SAM-covered Au film. In order to explain these phenomena, we have designed different models regarding the tip–sample gap structure. Among these models, the antenna factor successfully explains the cutoff energy shift of STM-LE spectra. According to this model, the cutoff energy shows redshift when the electronic transition occurs at the Au–S interface layer and the amount of the shift depends on the strength of the transitions.

Pump–probe scanning-tunneling-microscope light-emission spectroscopy of Sb2Te3

We have investigated the temporal behavior of Sb2Te3 after irradiation with picosecond (ps) pump-laser pulses using scanning-tunneling-microscope (STM) light emission synchronized with ps-probe-laser pulses delayed by times t after individual pump pulses. We determined the gap energy ΔE at the F point in the band diagram of Sb2Te3 from the STM light-emission spectra as a function of the delay time t. We found that ΔE increased monotonically with t from the original value of 1.62 eV for a delay time of 0 to 15.3 ps and it decreased toward that original value (1.62 eV) for t in the time span between 15.3 and 28 ps; we saw no change in ΔE for t≥28 ps. By comparing this t-dependence of ΔE with the dielectric functions of the crystalline and amorphous phases of Sb2Te3, we have concluded that the phase transition from the crystalline phase toward the amorphous phase is induced by the pump pulses. The phonon energy of the A2u mode is clearly seen in the STM light-emission spectra when ΔE is 1.62 eV, but not when ΔE is increased above 1.62 eV by pump-pulse irradiation. This feature appears to be consistent with the Raman signals, which are much stronger for the crystalline phase than for the amorphous phase.

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).

Classical electricity analysis of the coupling mechanisms between admolecule vibrations and localized surface plasmons in STM for vibration detectability

The presence of a dynamic dipole moment in the gap between the tip of a scanning tunneling microscope (STM) and a substrate, both of which are made of metal, produces a large dynamic dipole moment via the creation of localized surface plasmons (LSPLs). With regard to the vibration-induced structures that have been experimentally observed in STM light emission spectra, we have incorporated the effect of the phonon vibrations of an admolecule below the STM tip into the local response theory, and we have evaluated the enhancement of the dynamic dipole involving phonon vibrations. Our analysis shows how effectively this vibration becomes coupled with the LSPLs. This was shown using three mechanisms that considered the vibrations of a dipole-active molecule and the vibrations of a charged molecule emitting and receiving tunneling electrons. In each of the mechanisms, phonon vibrations with angular frequency ωp shifted each LSPL resonance by ℏωp or by a multiple of ℏωp. The phonon effect was negligibly small when the position of the dipole-active molecule vibrated with ωp, but it was largest and most detectable when the point charge corresponding to the admolecule at the surface of the tip vibrated with ωp. It was found that a series of LSPL resonances with or without phonon-energy shifts can be characterized by a few dominant orders of multipole excitations, and these orders become higher as the resonance energy increases.

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.

Finite-Difference Time-Domain Analysis of Scanning Tunneling Microscope Light Emission Spectra

The finite-difference time-domain (FDTD) method has been applied to the spectral analysis of scanning tunneling microscope (STM) light emission. We found that FDTD analysis provides predictions that are consistent with those of the dielectric theory of STM light emission if the radius of curvature of the tip front is less than 50 nm. In the radius range above 50 nm, discrepancies between these theories were considerbale in predictions of the efficiency of STM light emission but moderate in those of its spectral shapes. STM light emission spectra of rods and disks embedded in substrates, to which the dielectric theory of STM light emission is not applicable, were calculated by the FDTD method, and we deduced that the spectral features specific to the rod material become distinct when the lateral size of the rod is comparable to or larger than that of localized surface plasmons determined by the dielectric theory of STM light emission; a thickness of 3–5 nm is additionally required for the disk.

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.

Vibration-induced structures in scanning tunneling microscope light emission spectra of Ni(1 1 0)-streaky (1 × 2)-H

We have measured the scanning tunneling microscope (STM) light emission spectra of Ni(1 1 0)-streaky (1 × 2) surfaces. When the tip was fixed over atomic hydrogen adsorbed on the surfaces, two types of vibration-induced structure were observed in the STM light emission spectra. One is the periodic fine structures that were already reported in our previous paper [Y. Uehara, S. Ushioda, Phys. Rev. Lett. 92 (2004) 066102] and the other newly found in the present experiments is a stepwise structure that is located at the vibrational energy of hydrogen below the cutoff energy of the STM light emission. They are ascribed to different excitation mechanisms of the vibration in the STM light emission process; the periodic fine structures appear when the vibrating motion is directly excited by the electrons injected from the tip. Conversely, the stepwise structure is observed when it is excited by the electromagnetic fields confined in the tip-sample gap, i.e., by localized surface plasmons.

Electron transport in the barriers of AlGaAs/GaAs quantum well structures observed by scanning-tunneling-microscope light-emission spectroscopy

Using scanning-tunneling-microscope light-emission (STM-LE) spectroscopy, we have investigated the transport properties of minority carriers in p-Al0.3Ga0.7As/p-GaAs quantum well (QW) structures. The optical measurements were performed on a cleaved (110) surface at room temperature. The STM-LE spectra were measured by injecting hot electrons from the STM tip positioned at different distances from the QWs. The emission intensity from individual wells as a function of the tip-well distance was found to decay with two distinct decay constants.

Identification of atoms on the Ni(110)–(2×1) O surface by scanning tunneling microscope (STM) light emission spectra

We have investigated the Ni(110)–(2×1) O surface by scanning tunneling microscope (STM) light emission spectroscopy. A mosaic of light and dark domains was seen in the STM image of the surface. The STM light emission spectra of both domains were measured to compare with that of the clean Ni(110)–(1×1) surface. We found that the dark domains are covered by the O atoms and the light domains correspond to the exposed Ni surface.

Diffusion process of electrons injected from STM tip into AlGaAs/GaAs quantum wells

We have investigated the electron diffusion process in Al 0.3Ga 0.7As/GaAs quantum well (QW) structures by means of scanning tunneling microscope light emission (STM-LE) spectroscopy. The optical measurements were performed on a cleaved (1 1 0) surface at room temperature. The STM-LE spectra were measured by injecting hot electrons from the tip positioned at different distances from the QWs. The emission intensity from individual wells as a function of the tip-well distance was found to decay with two distinct decay constants. From comparison with Monte Carlo simulations for hot electron relaxation, we found that the intervalley scattering from the Γ valley to the L and X valleys has the most significant effect on the diffusion process of the injected electrons.

Spatially resolved STM light emission spectra of cleaved (1 1 0) AlGaAs/GaAs heterostructures

We have measured the spectra of light emitted from individual single GaAs quantum wells of cleaved (1 1 0) AlGaAs/GaAs heterostructures using the STM (scanning tunneling microscope) tip as a local electron injection source. The cross-sectional STM images of the quantum wells were obtained, and the light emission spectra were measured by locating the STM tip over individual quantum wells of interest. Single emission peaks were observed to shift to the high-energy side with decreasing well width. The peak positions agree with the calculated transition energies for the corresponding well width. The thermalization length of the injected electron was estimated by observing the change in the emission intensity as the tip is moved at different distances from a given well.

Comparison of Light-Emission Efficiencies from Si–Metal-Oxide-Semiconductor Junctions and from Si in Scanning Tunneling Microscopy

We have investigated the mechanism of scanning tunneling microscope (STM) visible light emission from n-type Si(100). The current fluctuation theory that explained the light emission characteristics of the Si–metal-oxide-semiconductor (Si–MOS) tunnel junction is applied to the STM light emission from n-type Si(100). This theory reproduces the observed STM light emission spectra, and predicts that the light emission efficiency from the STM geometry is ∼1.5×104 times greater than that from the Si–MOS junctions. This prediction is consistent with our experimental result. Experimentally, the light emission intensity from the Si surface under the STM is comparable to that from the Si–MOS junction, although the tunneling current in the STM is 10-3 of the current in the Si–MOS junction. That is, the light emission efficiency from the STM is at least 103 times greater than that from the Si–MOS.

Scanning Tunneling Microscope Light Emission Spectra ofAu(110)−(2×1)with Atomic Spatial Resolution

We have measured the STM (scanning tunneling microscope) light emission spectra of $\mathrm{Au}(110)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ at 80 K with atomic spatial resolution. The individual Au atoms along the $(2\ifmmode\times\else\texttimes\fi{}1)$ rows could be resolved in the STM image. Distinctly different emission spectra are observed depending on whether the tip is located over the atomic row or over the valley between the rows. The spectrum obtained over the row atoms can be ascribed to the emission by localized surface plasmons. The spectrum found over the valley contains emission from excitations atomically localized in the valley.