Scanning Tunneling Microscope Light Emission Research Articles

Scanning tunneling microscope light emission spectra of polycrystalline [formula omitted] and [formula omitted

We have observed scanning tunneling microscope light emission (STM-LE) spectra of Ge 2Sb 2Te 5 and Sb 2Te 3. Although these chalcogenide alloys exhibit band gaps less than 0.5 eV, the STM-LE was observed with a narrow spectral width at a photon energy of 1.5 eV for both materials. By analyzing its bias voltage, polarity, and temperature dependencies combined with recently reported theoretical electronic structures, we concluded that the STM-LE is excited by electronic transitions taking place in the local electronic structure having a direct gap-like shape with a band gap of 1.5 eV, commonly found in the electronic structures of both materials.

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-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.

Vibration of H atomic chains on Ni(1 1 0) measured by scanning tunneling microscope (STM) light emission spectroscopy

We have measured the scanning tunneling microscope (STM) light emission spectrum of hydrogen atoms (H) adsorbed on a Ni(1 1 0) surface. 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 (D). Hence it is concluded that the periodic structures are caused by the vibrational motion of H.

Single molecule spectrum of rhodamine 6G on highly oriented pyrolytic graphite

We have measured the scanning tunneling microscope (STM) light emission spectrum of a single molecule of rhodamine 6G (R6G) adsorbed on highly oriented pyrolytic graphite (HOPG). Since the HOPG substrate radiates no STM light, we have succeeded in observing the spectrum radiated by R6G alone. The spectrum agrees well with the photoluminescence spectrum of R6G on HOPG with the exception of two structures that may arise from a triplet state whose transition is forbidden in photoluminescence. Based on this agreement, we have determined the STM light emission mechanism of adsorbed R6G.

STM light emission spectroscopy of individual quantum wells: measurement of transport parameters in real space

By spectroscopically analyzing the light emitted from the tip-sample gap of the scanning tunneling microscope (STM), we have investigated the carrier transport as well as the luminescence properties of AlGaAs/GaAs quantum wells (QW's). The emission intensity form a target well was measured as a function of the tip position on a cleaved (110) surface of the QW structures. The thermalization length and the diffusion length of the injected electrons were determined in real space.

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.

Electromagnetic enhancement effect in scanning tunneling microscope light emission from GaAs

The electromagnetic enhancement effect in scanning tunneling microscope (STM) light emission from GaAs has been investigated by the finite difference time domain method. We have calculated the intensity of light emitted by the recombination of minority carriers injected from the tip and majority carriers in the sample. The results depend not only on the material and the shape of the tip but also on light polarization. When the tip is tungsten whose dielectric function has a positive real part at 1.5 eV, the p-polarized light intensity depends strongly on the size of the tip and the location of recombination. However, the s-polarized light emission depends only weakly on these parameters. If the tip is a perfect metal, the p-polarized light intensity becomes a few times stronger than that for the W tip of the same shape. On the other hand, the s-polarized light intensity becomes weaker than that for the W tip due to the electromagnetic screening effect of a perfect metal. We conclude that the combination of the W tip and s-polarized light detection is suitable for precise measurement of the tip-position dependence of STM light emission properties. A tip with a negative value of the real part of dielectric function is preferable to the W tip, if one does not analyze the tip-position dependence of STM light emission, because STM light emission is enhanced relative to the case of the W tip.

Identification of O atoms on a Cu(110) surface by scanning tunneling microscope light emission spectra

We have identified the adsorption site of O atoms on the Cu(110)-O surface by studying the light emission spectra from individual atomic sites induced by scanning tunneling microscope (STM). Although the STM image contrast reverses depending on the tip condition, the emission spectrum from the oxygen site shows the same characteristic peaks, regardless of the image contrast. These characteristic peaks are identified with the electronic density of states associated with the O 2p orbitals.

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.

Optical observation of single-electron charging effect in metallic particles

We have investigated the single-electron charging effect of small Au particles by scanning tunneling microscope (STM) light emission spectroscopy and scanning tunneling spectroscopy (STS) at 4.7 K. A double tunnel junction was formed by an Al film substrate, an oxide layer, a small Au particle, a vacuum gap, and the STM tip. The high-energy cutoff of the emitted light became less than the quantum-cutoff value ${\mathrm{eV}}_{0}$ when the Coulomb staircase appeared in the STS spectrum. Here e is the elementary charge and ${V}_{0}$ is the bias voltage applied across the double tunnel junction. The observed decrement of the high-energy cutoff from ${\mathrm{eV}}_{0}$ agrees with the value calculated with the circuit parameters of the double tunnel junction determined from the STS spectrum. This result demonstrates that the STM light emission spectroscopy provides an alternate means of investigating the single-electron charging effect complementary to the ordinary electrical measurements.

Scanning Tunneling Microscope Light Emission Mechanism of Pb

We have investigated the scanning tunneling microscope (STM) light emission from Pb(100) at 4.7 K and 18 K, respectively below and above the superconductive transition temperature of Pb (7.18 K). The results were analyzed with the dielectric theory of STM light emission. The spectra and the intensity of STM light emission from the normal state Pb (T=18 K) were reproduced by the theory. The intensity for the superconducting state (T=4.7 K) was ∼2.5 times greater than that for the normal state of Pb. This enhancement of the STM light emission intensity cannot be explained by the usual dielectric theory of STM light emission.

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.

Tip shape dependence of the light emission efficiency for the scanning tunneling microscope

We have investigated the relation between the scanning-tunneling-microscope (STM) tip shape and the efficiency of STM light emission. The light intensity radiated by a current source placed in the tip-sample gap of the STM was calculated for various shapes of the tip using the finite differential time domain method. We found that the highest emission efficiency is obtained for the pyramidal tip with the flat top of 4 nm width and the apex angle of 90 °C.

Identification of electronic transitions localized at a single oxygen atom adsorbed on Cu(110)-(2×1) surface

We have identified electronic transitions tightly localized at a single oxygen atom adsorbed on the Cu(110)-(2×1) surface, by means of scanning tunneling microscope (STM) light emission spectroscopy. The emission spectrum measured with the STM tip located directly over the oxygen atom shows two peaks corresponding to states located ∼200 and ∼300 meV above the Fermi level of Cu. These peaks are identified with unoccupied states localized at a single oxygen atom.

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.

Time-resolved STM light emission from an evaporated Au film

STM (scanning tunneling microscope) light emission from an evaporated Au film irradiated by pico second laser pulses has been investigated for various bias voltages of the STM. The observed emission consists of two components. The first component that has the same duration as the incident laser pulse is excited by laser induced tunneling electrons. The second component that has a longer duration than that of the laser pulse is emitted from surface polariton plasmons excited non-linearly by the laser pulse.

STM light emission from Si(1 1 1)-(7 × 7) surface using a silver tip

Scanning tunneling microscope-light emission (STM-LE) from the Si(1 1 1)-(7×7) surface has been measured using silver tips. For silver tips photon emission was enhanced by more than 100 times as compared with that for tungsten or platinum–iridium alloy tips. A broad spectrum with a single peak at ∼2.25 eV was observed. The spectrum obtained can be reproduced by a theory based on the macroscopic dielectric response of the tip-sample system, indicating that the observed emission arises from the localized plasmons on the silver tip excited by tunneling electrons. Spatial variations in the emission intensity at the atomic scale was observed even under low bias voltage (2 V) and low tunneling current (1 nA) conditions.

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.