Adsorbate-induced changes in the broadband infrared transmission of ultrathin metal films

Physica Status Solidi (b)

2005

Metal films with thickness in the nanometer range are optically transparent. With IR transmittance spectroscopy the in-plane film conductivity with its correlation to the film-growth process can be studied without electrical contacts and, on metal-island films, adsorbate vibrations can be observed because of surface enhanced IR absorption (SEIRA). Their analysis enables insight into the adsorbate-metal bonding and therefore gives information on the available adsorption sites and the crystalline facets correlated to. As in IR reflection absorption spectra dipole -dipole interaction of molecules on different sites modifies the vibration lines according to the degree of disorder ("atomic roughness"). Depending on that roughness IR spectra of adsorbate vibrations may be further modified because of their interaction with electronic excitations of the film. So, the limited facet size on cold-condensed metal films leads to additional IR activity: Raman lines of certain centrosymmetric adsorbate molecules (C 2 H 4 ) are observed.

Section: Introductionmentioning

confidence: 98%

IR spectroscopy of adsorbates on ultrathin metal films

Physica Status Solidi (b)

2005

“…Infrared (IR) spectroscopy is an excellent alternative for studying the morphology development of ultrathin metal films [17][18][19][20][21] since in the IR range the low-frequency tails of metal-particle absorption as well as the dynamic conductivity of coalesced films give strong signals with typical spectral slope. In contrast to studies in the visible range, the analysis of IR spectra is straightforward since interband transitions start at much higher energies for most of the typical metals.…”

Section: Introductionmentioning

confidence: 99%

“…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].…”

Section: Introductionmentioning

confidence: 99%

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

Meng

Physica Status Solidi (b)

2007

Self Cite

Silver-film growth on MgO(001) was systematically studied with infrared spectroscopy in ultra-high vacuum for different substrate temperatures from 60 K to 500 K at a similar deposition rate. In all cases the spectral development clearly indicated the formation of islands. For 300 K the coalesced islands further developed to a film that reached bulk conductivity already at about 9 nm. The conductivity of the films beyond percolation was obtained from spectral fits to the Drude model of the dielectric function. Additionally to transmittance studies, infrared reflection spectra at oblique incidence of p-polarized light were measured during Ag film growth on MgO(001) at 300 K. Interestingly, a minimum reflectance (relative to the substrate reflectance) was observed at a thickness corresponding to the percolation threshold, which is a new experimental result. The island-like film morphology was directly proved with atomic force microscopy. Pictures taken immediately after film growth proved that particle size and coalescence strongly depend not only on average film thickness but also on the substrate temperature during deposition, which is related to temperature dependent diffusion.

“…It is important to consider the dependence of the Drude parameters ω τb and ω pb on ω in the mid IR. For transition metals where that effect is much stronger some details were described in reference [23]. Here we used the two Drude parameters ω τb (ω) and ω pb (ω) as they follow from bulk dielectric data for bulk silver at room temperature [24] using equation (1) in the wave-number range 500 cm -1 < ϖ < 6000 cm -1 .…”

Section: Resultsmentioning

confidence: 99%

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

Meng

Fahsold

2005

phys. stat. sol. (c)

Self Cite

The growth of silver on UHV cleaved MgO(001) at various temperatures is in situ studied with IR spectroscopy followed by ex situ AFM. In the whole substrate-temperature range for silver deposition used in this study (from about 70 K to 500 K) island formation is observed. Continuous films at higher thicknesses may have metallic properties close to typical silver-bulk data. Differences in the IR optical properties are clearly related to the film structure. The results from an IR spectral analysis of transmittance data with a Drude-type model are corroborated by IR spectral features of CO adsorbed on the films, and by AFM pictures.