R. Bernardini

Asymmetries in the optical properties of vacuum-deposited organic films illuminated at the substrate and non-substrate surfaces

Abstract Tetracene, pentacene and nickel phthalocyanine films were prepared by vacuum deposition onto glass substrates. Their reflection spectra on illumination of the substrate (S) and the non-substrate (NS) surfaces were measured as a function of the sample thickness d. The reflectivity R and band shape of the spectra change with d and differ when measured from the S surface and the NS surface. These changes and asymmetries are interpreted as due to four factors: (i) the difference in surface roughnness, (ii) the difference in the refraction index at the film-air interface (NS) and the film-glass interface (S), (iii) the presence of stresses in the films and (iv) the Rayleigh-type scattering of light enhanced by the contact medium. A detailed analysis indicates the first and second factors to be dominant. Previous data for asymmetries in photoconductivity and photovoltaic effects can be consistently explained in terms of roughness-induced differentiation in the energy and spatial distributions of the structural defects in the near-surface layers of such films.

Near edge x-ray absorption and x-ray photoelectron diffraction studies of the structural environment of Ge-Si systems

Near edge X-ray absorption spectroscopy (XAS), X-ray photoelectron diffraction (XPD) and Auger electron diffraction (AED) are powerful techniques for the qualitative study of the structural and electronic properties of several systems. The recent development of a multiple scattering approach to simulating experimental spectra opened a friendly way to the study of structural environments of solids and surfaces. This article reviews recent X-ray absorption experiments using synchrotron radiation which were performed at Ge L edges and core level electron diffraction measurements obtained using a traditional X-ray source from Ge core levels for ultrathin Ge films deposited on silicon substrates. Thermodynamics and surface reconstruction have been found to play a crucial role in the first stages of Ge growth on Si(001) and Si(111) surfaces. Both techniques show the occurrence of intermixing processes even for room-temperature-grown Ge/Si(001) samples and give a straightforward measurement of the overlayer tetragonal distortion. The effects of Sb as a surfactant on the Ge/Si(001) interface have also been investigated. In this case, evidence of layer-by-layer growth of the fully strained Ge overlayer with a reduced intermixing is obtained when one monolayer of Sb is predeposited on the surface.

Magnetic anisotropy in epitaxial Cu/Ni/Cu/Si(1 1 1) ultrathin films studied by magneto-optical Kerr effect and Brillouin spectroscopy

Epitaxial ultrathin Cu/Ni/Cu films with Ni thickness of 3 and 6 nm, have been grown by UHV evaporation on the Si(1 1 1)-7 x 7 surface. Magneto-optical Kerr-effect measurements have shown that the preferential direction of magnetization lies in the film plane for both films. Brillouin light scattering was then exploited to study the spin-wave dispersion as a function of both the applied magnetic field and the wave vector direction. This enabled us to determine, in addition to the other magnetic parameters, both the in-plane and the out-of-plane anisotropy constants. This is the first Brillouin scattering investigation of magnetic anisotropy in Ni films.

Thermoreflectance spectrum of tetracene single crystal

Abstract The b-polarized thermoreflectance spectra of the (001) face of tetracene single crystals have been measured, at room temperature and 100 K below the structural phase transitions at 182 and 144 K, for the first time. Evidence for the asymmetric broadening of the Frenkel exciton due to the weak exciton (phonon) coupling is found in thermoreflectance spectra. A large exciton red shift at the structural phase transition has been found by decreasing the temperature.

The character of metal-insulator phase transition in V2O3 from the plasmon behaviour

Abstract The metal-to-insulator transition (MIT) in V2O3 has been studied by thermoreflectance spectroscopy. The behaviour of the plasmon resonance as a function of crystal temperature has been measured. This experimental method is a sensitive probe of the effects of electron-electron correlation and/or of the band gap opening at the MIT phase transitions. A blue shift of the plasmon energy in cooling the sample throught the phase transition temperature has been found. This behaviour, with a shift in the opposite direction of that found in VO2, provides a direct experimental evidence that the electron-electron interaction plays a minor role in driving the V2O3 phase transition.