Chiara Toccafondi

Optical properties of Yeast Cytochrome c monolayer on gold: an in situ spectroscopic ellipsometry investigation.

The adsorption of Yeast Cytochrome c (YCC) on well defined, flat gold substrates has been studied by Spectroscopic Ellipsometry (SE) in the 245-1000 nm wavelength range. The investigation has been performed in aqueous ambient at room temperature, focusing on monolayer-thick films. In situ δΨ and δΔ difference spectra have shown reproducibly well-defined features related to molecular optical absorptions typical of the so-called heme group. The data have been reproduced quantitatively by a simple isotropic optical model, accounting for the molecular absorption spectrum and film-substrate interface effects. The simulations allowed a reliable estimate of the film thickness and the determination of the position and the shape of the so-called Soret absorption peak that, within the experimental uncertainty, is the same found for molecules in liquid. These findings suggest that YCC preserves its native structure upon adsorption. The same optical model was able to reproduce also ex situ results on rinsed and dried samples, dominated by the spectral features associated to the polypeptide chain that tend to overwhelm the heme absorption features.

Photochromic and photomechanical responses of an amorphous diarylethene-based polymer: a spectroscopic ellipsometry investigation of ultrathin films

This work deals with very thin (14–40 nm) films of a polyester containing diarylethene units in the main chain spin cast on a silicon wafer. By irradiation with UV light the colourless form turns blue due to the appearance of a strong absorption band centred at about 600 nm. The coloured state is thermally stable and the backward conversion can be triggered with visible light. Comparison of broadband (245–1700 nm) spectroscopic ellipsometry data with simulations based on an isotropic, Kramers–Kronig consistent, multiple-resonance model allowed us to determine the complex index of refraction n of the film in its blue and colourless forms. The refractive index of the blue phase neatly exceeds that of the transparent form for wavelengths in the NIR. In particular, out of resonance, at λ ∼ 1700 nm, ΔRe(n) ∼ 0.05. Parallel to the ΔRe(n) increase, the analysis of ellipsometry data suggests a decrease of the film thickness (about 1.5%) during the transition from the open (transparent) to the closed (coloured) form.

Optical properties of nanogranular and highly porous TiO2 thin films

We present a systematic investigation of the optical properties of cluster-assembled TiO2 films (20–150 nm nominal thickness) deposited onto Si wafers and processed with two types of ex-situ oxidation treatments. Spectroscopic ellipsometry (0.7–5 eV energy range), supported by x-ray photoelectron spectroscopy and atomic force microscopy measurements, allowed us to obtain information on the depth-dependent morphology of the films in a non-destructive mode. The characteristic grainy and porous structure of the films was modelled by means of a flexible effective-medium approach, allowing us to obtain a reliable estimate of the amount and distribution of voids into the films and their accessibility to foreign liquids. The absorption-edge broadening and significant optical absorptions within the gap were attributed to the intrinsically nanogranular morphology and to the interrelated defective structure/stoichiometry of the particles, also in relation to the occurrence of poorly accessible interstices or pores in the inner parts of the film.

Photochromic Polyurethanes Showing a Strong Change of Transparency and Refractive Index

Photochromic polymers have been studied as rewritable systems for optical elements with tunable transparency in the visible and refractive index in the NIR. Six diarylethene monomers have been synthesized to give thin films of photochromic polyurethanes. The absorption properties of the monomers in solution and of the corresponding polymeric films have been evaluated showing that a transparency contrast in the visible spectrum of the order of 103 can be obtained by a suitable choice of the chemical structure and illumination wavelength. The change in the refractive index in the NIR have been determined by ellipsometry showing changes larger than 10−2. A trend of this variation with the absorption properties has been also highlighted. Fresnel lenses working on the basis of both a change of the transparency and the refractive index (amplitude and phase) have been demonstrated.