Infrared spectroscopic study of the CO‐mediated decrease of the percolation threshold during the growth of ultrathin metal films on MgO(001)

Abstract

Metals are known to grow three-dimensionally on clean ionic crystal surfaces with low surface energies. As a consequence, the naturally grown films consist of islands and are non-conductive up to the percolation threshold, which is at ∼1 nm for iron films grown on MgO(001) at room temperature and is much larger for noble metals and higher temperatures, respectively. In this paper we show that CO exposure at ∼10 −8 mbar during deposition modifies the growth on MgO(001) for iron and copper. The development of infrared transmission spectra versus metal film thickness clearly is changed by CO exposure. The spectral differences correspond to a significant percolation threshold decrease induced by CO. Infrared conductivity parameters calculated from transmittance spectra indicate that the surface relaxation of the free charge carriers is decreased due to CO exposure during growth but the plasma frequency is unchanged.

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

“…As known from IR transmission spectroscopy, the percolation threshold is lowered when CO is exposed during Fe and Cu film growth on MgO(001) [33]. CO exposure (4 × 10 -6 Pa) during Ag film growth on MgO(001) at room temperature did neither change IR transmission spectra nor AFM images.…”

IR spectroscopy of adsorbates on ultrathin metal films

“…Since the SEIRA intensities are at least comparable to the IRRAS intensities, the enhancement factor must be at least 30 -or even bigger because the area with adsorbates that contribute to the signal in SEIRA is certainly smaller than that in IRRAS, which corresponds to Im ε a⊥ < Im (-1/ε a⊥ ). The SEIRA-line intensities would be bigger for percolation of crystalline islands at lower average thickness, which can be obtained for example with MgO substrates [16,49]. Unfortunately, MgO is transparent only above 1300 cm -1 .…”

Present and Future of Surface-Enhanced Raman Scattering

“…Thus, metal particle layers with particle diameters of less than ∼100 nm only absorb IR light. There are many studies on the development of the IR absorption spectrum with particle geometry and density, ,, and many related SEIRA studies have been published. ,, The SEIRA signal intensity in such studies benefits from near-field hotspots and also from the multitude of adsorption sites for the molecules. The hotspots are, for example, located between particles in a close neighborhood, and thus, for a metal island layer as an ensemble of tiny plasmonic particles, the highest SEIRA signals are observed near the percolation threshold. ,, …”