Microscope Light Emission Research Articles

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.

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.

Atomic-site-dependent light emission fromAu(110)−(2×1)surface induced by scanning tunneling microscope

We have investigated the scanning-tunneling microscope (STM) light-emission mechanism of the $\mathrm{Au}(110)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ surface. We found that the light stimulated by the STM is emitted through three different channels. The first channel is the emission through excitation of localized surface plasmons (LSP). The other two channels are through the recombination of d-band holes and s-p electrons in Au. When the sample bias voltage is positive (i.e., electrons are injected into the sample), d-band holes are created by impact ionization. The intensity due to this process is greater when the tip is located between the Au atomic rows than over the row. This process is the origin of the atomic-site-dependent spectra that we reported in a previous paper [Y. Uehara, T. Fujita, and S. Ushioda, Phys. Rev. Lett. 83, 2445 (1999)]. When the bias voltage is negative, d-band holes are created by tunneling of d electrons from Au to the tip. The light-emission intensity due to the recombination of these d holes with sp electrons is about twice as strong as that emitted through the excitation of LSP.

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.

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.

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.

Relation between the Radius of Tip Curvature and the Light Emission Efficiency from Scanning Tunneling Microscope

The relation between the efficiency of scanning tunneling microscope light emission (STM-LE) and the radius of tip curvature has been studied using silver tips. The emission efficiency increases with decreasing radius of tip curvature ρ for ρ>80 nm. This result agrees qualitatively with the prediction of a theory that includes the effect of electromagnetic retardation.

Scanning tunneling microscope (STM) light emission spectroscopy of surface nanostructures

We present an overview of the experimental method and physical principles of scanning tunneling microscope (STM) light emission spectroscopy (STM-LES). By this new spectroscopic technique one can obtain the optical emission spectra of specific and individual surface nanostructures that are imaged by the STM. This method has been used to investigate the electronic transitions in surface nanostructures such as the protrusions of porous Si, the quantum wells of a semiconductor, and the rows and valleys of the reconstructed Au(110) surface.

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.