Influence of morphology on adsorbate-induced changes in thin-film dynamic conductivity

Abstract

For ultrathin copper films of various morphology we studied adsorbate-induced changes in broadband infrared transmission at normal incidence of light. Smooth Cu films on Si(111) and mesoscopically rough Cu films on KBr(001) were exposed to CO and to C 2 H 4 . We observed significant broadband changes for each of these gases and for both surfaces. Applying a Drude-type model we calculated the optical spectra in accord with the experiment. We find that the effects related to a change in the electronic relaxation rate are weakly influenced by the mesoscopic roughness of the film, while the effects related to charge transfer are strongly enhanced due to such roughness. This paper shows that the real adsorbate-induced changes can be determined for both the homogeneous films and the inhomogeneous films beyond percolation. The increased surface area owing to mesoscopic roughness is merely one contribution to larger adsorbate-induced effects of inhomogeneous films. The other more interesting contribution is due to depolarization in rough metal films that is responsible for strong enhancement of charge transfer effects.

Free-electron response in reflectance anisotropy spectra

“…It remains possible that the behavior in the high energy region could arise from percolation effects. 36,39 The morphology of the wires is better known for this system than for Sn/ InAs, but there are insufficient data points to explore this possibility.…”

“…Additionally, the limited spectral range currently available may result in an error in the extrapolated dc value in cases where metallic nanostructures have significant percolation effects around the experimental cutoff of 0.5 eV. 39 The last problem is the limitation of the Drude free electron model itself, which leads to a dielectric function involving a merely phenomenological scattering parameter. However, it is possible to extend the model by allowing for screening ͑ ϱ ͒ and by using the effective mass of the electrons m * instead of the free electron mass, an approach used to describe the Drude absorption by free carriers in semiconductors.…”

IR spectroscopy of adsorbates on ultrathin metal films

“…Metal films with thicknesses of a few nanometers (nanofilms) are optically transparent and, therefore, their dynamic conductivity and the adsorbate induced changes can be studied easily via transmittance measurements [1,2]. Due to surface enhanced IR absorption (SEIRA) it is also possible to study the vibrations of adsorbates on sidewalls of metal-islands in the transmittance experiments with remarkable sensitivity [3,4].…”

Growth of silver on MgO(001) and infrared optical properties

“…For thin films, at least an additional relaxation rate s τ ω due to surface scattering and defects in the film has to be added to the bulk rate b ( ) [17,18]. Since quantum size effects can modify the plasma frequency in an ultrathin film [19] and incomplete volume filling due to island-like growth can cause other deviations from the bulk value pb ( ) ω ω [26], a frequency independent scaling factor β (ratio of film-plasma frequency to bulk one) is taken into account [17]. For clarity, the bulk metal corresponds to…”

“…An estimate relation is given in Ref. [26] on the basis of a comparison of the Drude dielectric function with the two-dimensional Bruggeman effective dielectric function for an island-like metal film. As shown in that paper, max ω shifts to lower frequencies with decreasing F, and above max ω it is no more possible to describe the effective medium dielectric function with the Drude model, since the spectral slope is qualitatively different there.…”