Tip-induced Plasmon Research Articles

Tip influence on plasmon excitations in single gold particles in an STM

Photon-emission spectra have been measured for individual titania-supported gold particles excited by the electron current from an STM tip. Depending on the interaction strength between tip and cluster, the emission behavior changes from excitations with Mie-plasmon character at 2.3 eV to modes of tip-induced plasmons at 1.8 eV. The strong tip influence on plasmon excitations in Au particles results from the almost constant dielectric function of gold between 1.8 and 2.3 eV, making the plasmon resonance sensitive to small changes in the electromagnetic environment. The lifetime of collective modes, derived from homogeneous linewidths, is governed by dielectric losses in gold, in particular, by gold interband transitions.

Molecular spectroscopy of dye aggregates by scanning-tunneling-microscope-induced light emission

A bromide-modified silver (111) surface, covered by Jelly-type aggregates of a cyanine dye, is investigated by scanning tunneling microscopy (STM) in air. We show that, under appropriate conditions, light is emitted out of the junction by inelastic electron tunneling. Study of the relation between the light intensity and the tunnel bias clearly reveals a molecular influence on the light emission. Furthermore, through a spectral analysis, we show that sharp resonances appear for dye-covered surfaces by coupling of tip-induced plasmons with molecular excitations. This demonstrates that through STM-induced luminescence local spectroscopic information can be obtained on molecules.

Photon emission spectroscopy of NiAl(110) in the scanning tunneling microscope

Spectroscopic measurements have been carried out of the light emitted from the NiAl(110)/W tunnel junction of a scanning tunneling microscope. The data reveal two prominent emission lines in the visible and near-infrared region. Corresponding model calculations assign the observed light emission to the radiating decay of the tip-induced plasmon excited in the tip-sample cavity. In agreement with the theory, a low- and a high-energy mode of the plasmon can be distinguished in the experimental data. Since the excitation probability of the two modes is determined by the size of the tunnel cavity, it can be influenced by the radius of the tunnel tip. A blunted tip favors the observation conditions of the higher mode.

Light emission from a scanning tunneling microscope: Fully retarded calculation

The light emission rate from a scanning tunneling microscope (STM) scanning a noble metal surface is calculated taking retardation effects into account. As in our previous, non-retarded theory [Johansson, Monreal, and Apell, Phys. Rev. B 42, 9210 (1990)], the STM tip is modeled by a sphere, and the dielectric properties of tip and sample are described by experimentally measured dielectric functions. The calculations are based on exact diffraction theory through the vector equivalent of the Kirchoff integral. The present results are qualitatively similar to those of the non-retarded calculations. The light emission spectra have pronounced resonance peaks due to the formation of a tip-induced plasmon mode localized to the cavity between the tip and the sample. At a quantitative level, the effects of retardation are rather small as long as the sample material is Au or Cu, and the tip consists of W or Ir. However, for Ag samples, in which the resistive losses are smaller, the inclusion of retardation effects in the calculation leads to larger changes: the resonance energy decreases by 0.2-0.3 eV, and the resonance broadens. These changes improve the agreement with experiment. For a Ag sample and an Ir tip, the quantum efficiency is $\approx$ 10$^{-4}$ emitted photons in the visible frequency range per tunneling electron. A study of the energy dissipation into the tip and sample shows that in total about 1 % of the electrons undergo inelastic processes while tunneling.

On the difference between light emission versus voltage characteristics in air and in ultrahigh vacuum

The difference between the STM light emission versus voltage characteristics L(V) for the tunneling gap between two metals obtained in air and in ultrahigh vacuum (UHV) is explained. It arises because the distance between the tip and the sample in air has a stronger dependence on the voltage applied than in UHV. In air the different tip-induced plasmon modes can be observed not only in the spectrum of the emitted light as is the case in UHV, but in the L(V) characteristics as well.

Photon emission in scanning tunneling microscopy: Interpretation of photon maps of metallic systems.

We analyze maps of the integral photon intensity emitted from the tunneling gap of a scanning tunneling microscope obtained simultaneously with topography from a variety of metal films and single-crystal surfaces in ultrahigh vacuum. The effects of adsorbates and structures created with the scanning tunneling microscope on their local photon emission properties are investigated to explore the potential of the technique for chemical mapping. It is proposed that contrasts in photon maps on a scale of some tens of nanometers are attributable to local variations in the field strength of tip-induced plasmon modes which are determined by the surface geometry of the junction and its dielectric properties. On a (sub)nanometer scale, a second contrast mechanism is observed to occur, consistent with geometry-induced variations in the matrix element for inelastic tunneling. A comparison of electron spectroscopic data with bias-dependent photon maps indicates that contrasts on a subnanometer scale are further mediated by local modifications of the density of final states positioned one quantum of energy (h\ensuremath{\nu}) below the bottom of the elastic tunneling channel with respect to the Fermi level. These three mechanisms provide a framework for the interpretation of photon maps obtained on metallic systems.

PHOTON EMISSION FROM TRANSITION METAL SURFACES IN SCANNING TUNNELING MICROSCOPY

Light emission from noble metal surfaces excited by a scanning tunneling microscope has been interpreted as arising from in elastic tunneling excitation of tip-induced plasmon modes. We have extended this work to study the adsorption of oxygen on Ti and have observed the formation of structures with subnanometer lateral dimensions which give rise to clear contrasts in STM topographs and photon intensity maps. The experimental results strongly indicate that these contrasts are due to oxygen-induced variations of the local density of states.

Enhanced photon emission from the STM: a general property of metal surfaces

Light emission observed from noble-metal surfaces in an STM has been interpreted as arising from inelastic tunneling excitation of radiative tip-induced plasmon modes. We discuss recent extensions of this work to clean and oxygen-covered transition metal surfaces. Photon intensities observed in the tunnel mode permit mapping and spectroscopic investigation of sub-nanometer areas of the surfaces of Cu, Fe, and Ti. Our results corroborate the theory developed for noble metals. Contrasts in photon maps on an atomic scale caused by adsorbates are discussed.

The role of proximity plasmon modes on noble metal surfaces in scanning tunneling microscopy

Light emission from Cu(111) and Au(110) surfaces in the scanning tunneling microscope has been characterized. Fluorescence spectra are interpreted as arising from inelastic tunneling excitation of radiative tip-induced plasmon modes. Maps of integral phonon intensity measured simultaneously with topography are used to visualize the localized nature of these modes.

Inelastic tunneling excitation of tip-induced plasmon modes on noble-metal surfaces.

Light emission characteristics from the tunnel gap of a scanning tunneling microscope are used to elucidate the interaction of tunneling electrons with tip-induced plasmon modes on Ag, Au, and Cu surfaces. Enhanced redshifted spectra are observed in the tunneling regime. Model calculations of optical spectra in this range agree well with the experimental data. Isochromat spectra are used to demonstrate that the principal excitation process occurs via inelastic tunneling.