G. Fahsold

Resonances of individual metal nanowires in the infrared

With infrared spectroscopic microscopy using synchrotron light, the authors studied resonant light scattering from single metal nanowires with diameters in the 100nm range and with lengths of a few microns. The Au and Cu nanowires were electrochemically grown in polycarbonate etched ion-track membranes and transferred on infrared-transparent substrates. Significant antennalike plasmon resonances were observed in good agreement with exact light-scattering calculations. The resonances depend not only on length and diameter but also on the dielectric surrounding of the nanowire. The observed maximum extinction at resonance corresponds to an electromagnetic far-field enhancement by a factor of about 5.

The correlation between film thickness and adsorbate line shape in surface enhanced infrared absorption

Various experimental results on surface enhanced infrared absorption reveal asymmetric line shapes. Whereas the order of magnitude of the enhancement can be understood from electromagnetic field enhancement the unusual line shape remains without satisfactory explanation. An interaction with electron-hole pairs would lead to an asymmetric line but this should be restricted to the first monolayer. However, asymmetry is also observed for vibrations at larger distances from the metal-film surface. Here we show strongly asymmetric lines and their enhancement as a consequence of the interaction of adsorbate vibrations with surface plasmons of metal islands. Both the effects and also the baseline change can be estimated by a proper application of well established effective-media models.

ASYMMETRIC LINE SHAPES AND SURFACE ENHANCED INFRARED ABSORPTION OF CO ADSORBED ON THIN IRON FILMS ON MGO(001)

We have measured infrared transmission spectra of CO adsorbed on in situ grown iron films on MgO(001) under ultrahigh vacuum conditions. Even at normal incidence we observed strong absorption lines in the C–O stretch region with number, intensities, positions, and shapes dependent on CO coverage and Fe-film thickness and morphology. This absorption must be due to vibrational dipoles oblique to the substrate surface, e.g., due to molecules at island walls. The distinct absorption lines can be assigned to distinct adsorption sites on crystalline facets of epitaxial Fe islands on MgO(001). For each of the films the strongest CO line shows an asymmetric shape. Additionally, the observed absorption is enhanced by at least two orders of magnitude with respect to adiabatic intensities of, e.g., CO on NaCl. Line shapes and intensity let us suggest nonadiabatic coupling of the adsorbate vibration to electronic transitions.

Anomalous infrared transmission of adsorbates on ultrathin metal films: Fano effect near the percolation threshold

IR absorption lines of CO on ultrathin epitaxial iron films are both enhanced and asymmetric. In this letter we show new experimental results which demonstrate a correlation of the asymmetry of the CO-stretching line to electronic properties of the underlying metal film. The new finding indicates the important role of metal film morphology for nonadiabatic effects. Such effects are strongest slightly above the percolation threshold as our results show.

Quantum size effects manifest in infrared spectra of single bismuth nanowires

Infrared transmission spectroscopy measurements on single bismuth nanowires of various diameters d are presented. The spectra show a strong absorption whose onset is blueshifted proportionally to 1∕d2. We ascribe the absorption to interband transitions. The blueshift results from quantum size effects since they lead to the d-dependent splitting of the energy bands and to a respective shift of energy gaps.

Enhanced Infrared Absorption of SERS‐Active Lines of Ethylene on Cu

Vibrational modes of adsorbates on rough metal films can show surface enhanced Raman scattering (SERS) as well as surface enhanced infrared (IR) absorption (SEIRA). Both effects are a consequence of field enhancement and of a so-called chemical effect. Our experiments show that chemical contributions to SERS and to SEIRA from ethylene on Cu may have the same origin. We investigated ethylene adsorbed on ultrathin Cu films at about 100 K and below with IR transmission spectroscopy. At exposures up to 2 L the IR spectra show lines only at the positions of the very intense SERS E-bands. At higher exposures lines at the frequencies of the infrared active vibrations of ethylene molecules develop. All lines are stronger than one expects from molecular IR activities. Co-adsorption experiments performed with ethylene and CO show that E-lines in SEIRA do not occur after pre-coverage of Cu with CO.

An IR-transmission spectroscopical study of the influence of substrate surface defects on the morphology and the electronic structure of ultrathin Fe grown on MgO(001)

Abstract IR broad-band properties are sensitive to electronic transport and charge localization. This allows to investigate the electronic structure of ultrathin metal films by means of IR-transmission spectroscopy. As this electronic structure is determined by the interfaces of the film, the dynamic conductivity also reflects its morphology. We performed in-situ transmission spectroscopy in the middle infrared (MIR) during the evaporation of Fe on MgO(001) at room temperature in ultrahigh vacuum. We varied the quality of the substrate surface by using MgO(001) cleaved in air and cleaved in ultrahigh vacuum. The difference in the thicknesses for the onset of continuous transport properties of the growing films is well detected by their different IR-spectroscopical behavior. For continuous films, we calculate the thickness dependent scattering of electrons and the thickness dependent effective IR-optical oscillator strength from their IR-optical data. For MgO cleaved in air compared with cleavage in ultrahigh vacuum, the continuous thin film model becomes relevant at a smaller thickness, i.e. the substrate is covered faster in this case.

He-scattering studies of the BaF2(111) surface

Abstract BaF 2 (111) surfaces were prepared by cleavage in UHV and investigated with He atom scattering. Diffraction measurements were used for a characterization of the crystalline surface structure, of the thermal stability, and of the adsorption behavior of water. It follows from the experimental results that cleavage produces step heights with odd numbers of FBaF layers, probably up to three. Phonon dispersions of the Rayleigh mode and of two other surface modes are derived from time-of-flight measurements of inelastically scattered He atoms and explained in the framework of a nearest-neighbour central force model that involves the bulk parameters. An effective surface Debye temperature of about 200 K follows from the investigation of specular intensity as a function of temperature. Heating up to 720 K causes etch pits. At 130 K the formation of a (1 × 1) water adsorbate structure is proved. The findings are discussed regarding recent results from CaF 2 (111).

In-situ investigation of Fe ultrathin film growth by infrared transmission spectroscopy

IR-transmission spectroscopy is sensitive to charge transport and charge localization. Applied to low-dimensional metallic systems, this offers the possibility to investigate morphology on a nanometer scale during thin film growth by IR-optical methods. We performed in-situ IR-transmission spectroscopy during deposition of Fe on UHV-cleaved MgO(001) at 313 K and at 121 K. At thicknesses corresponding to a few monolayers of Fe, we find extremely weak IR-absorption that is due to island morphology as known from our previous helium-atom-scattering results for the same system. For complete substrate coverages, the comparison of IR-transmittance with calculations for homogeneous films indicates significant attenuation of IR-absorption due to additional scattering of charge carriers at the irregular surface as a result of preceding island growth.

Surface-enhanced infrared absorption?

In ultra-high vacuum (UHV) we have measured infrared transmission spectra of CO on ultra-thin films of iron grown at about 315 K on UHV-cleaved MgO(001). Even at normal incidence of light we observe several asymmetric CO stretching lines with positions, intensities and shapes dependent on film morphology. The CO stretching lines observed are enhanced by at least two orders of magnitude with respect to adiabatic values. Enhancement and asymmetry are correlated to the curvature of transmission spectra of the bare iron films which is a measure of the dynamic conductivity, i.e., of the degree of continuity of the films formed by the growth of three-dimensional epitaxial islands.