Leonid V. Poperenko

In situ ellipsometric investigation of stainless steel corrosion behavior in buffered solutions with amino acids

Abstract The corrosion of metals is associated both with a release of ions and changes in optical surface properties. In this study, these two effects were correlated by a potentiodynamic corrosion test and in situ probing of the surface by ellipsometry. The studies were carried out with stainless steel (SS) AISI 304 and 316 in phosphate buffered saline (PBS) and in Dulbecco’s modified minimal essential medium (DMEM) at pH 7.4. In both media, 304 steel is more susceptible to corrosion than 316 grade. The 316 steel shows a higher corrosion potential and higher corrosion current density in PBS than in DMEM, for 304 steel this behavior is vice versa. Ellipsometry demonstrated a higher sensitivity than potentiodynamics to surface modification in the cathodic area. In DMEM the removal of a surface layer at negative potential and a further repassivation with increasing potential was characteristic. In PBS a surface layer started to grow immediately. X-ray photoelectron spectra of this layer formed in PBS are consistent with iron phosphate. Its formation is inhibited in DMEM; the presence of amino acids is discussed as the reason.

Optical and magneto-optical properties and magnetorefractive effect in metal-insulator CoFe-Al2O3 granular films

Optical and magneto-optical properties of metal-insulator (CoFe)x(Al2O3)1−x granular films have been investigated. The results have been compared with the experimental data of the magnetorefractive effect in the IR region. The optical and magneto-optical spectra of the films depend strongly on the volume fraction of ferromagnetic particles. It was found that the Kerr effect is substantially increased in the spectral region of the plasma frequency. It was revealed that appearance peculiarities observed in the magnetoreflection spectra at 7.5–9.5μm are associated with the excitation of longitudinal phonon modes in the Al2O3 dielectric matrix. It has been shown that both intraband and interband electron transitions contribute to the magnetoresistivity as well as to the IR magnetoreflection. The optical and magneto-optical properties of the (CoFe)x(Al2O3)1−x granular films can be interpreted in the frame of the effective-medium approximation. The magnetorefractive effect can be explained in terms of the modif...

Magnetic ordering effects in the Raman spectra of Sn1−xCoxO2

The room temperature ferromagnetism is observed in chemically synthesized powderlike samples of Sn1−xCoxO2 compound (x=0.07 and 0.1). The Raman spectra of the Sn1−xCoxO2 films were analyzed in terms of their structural and magnetic properties. In the Raman spectra of the Sn1−xCoxO2 films additional features were revealed which are associated with the excitation of phonon modes in the cobalt-oxide-defect complex. The intensities of the Raman bands showed a strong dependence on the doping concentration and on applied magnetic field. The softening of the ∼630cm−1 symmetric A1g mode and the field dependence of the intensity Raman scattering observed for Co-doped SnO2 films was ascribed to spin-phonon interaction. The absorption spectra show that there are changes of absorption edge toward low energy due to the influence of incorporated Co atoms on the electronic states of SnO2. These results can be described by the bound magnetic polarons theory.

Characterization of the surfaces of CdTe(111) single crystals after laser processing

Optical properties and morphology of the opposite polar faces of CdTe(111) single crystals subjected to irradiation with nanosecond pulses of the second harmonic of a YAG:Nd laser in a wide range of energy densities have been studied by multiple-angle-of-incidence single-wavelength ellipsometry and atomic force microscopy. The ellipsometric data have been interpreted by the semi-infinite medium model and it is assumed that there are no films on the crystal surface. Also the one-layer model is applied and the crystal relief is considered as a top layer. The refraction n and absorption k indexes of the surface layer are calculated using the fundamental ellipsometric equation and method of thickness curves. Laser irradiation results in an increase in the refractive index of the CdTe surface layer due to laser-induced transformation of the surface structure and it is associated with enriching of the CdTe surface with tellurium. The surface morphology of the crystals is modified at higher laser pulse energy densities because of the beginning of melting of prominent irregularities and thin surface layer. The effects of irradiation with nanosecond laser pulses on the polar surfaces of CdTe(111) crystals are analyzed.

Theoretical understanding of the SPR sensor response on the protein adsorption

This research deals with consideration of the SPR sensor response in the framework of the scattering matrix approach with the modeling of biomolecular layer using Green’s function formalism and effective medium theories. It was found out that modeling of the SPR sensor response using abovementioned approaches in the densely packed monolayer approximation gave not enough agreement with the experiment. The reason was that real molecular layers are rarely densely packed. They usually can be characterized by molecular surface concentration or monolayer filling factor. Approximation of these parameters allows obtaining their values, which correspond to real biomolecular layers, and gives better agreement with the experiment.

Characterization of functional layers of CdTe crystals subjected different surface processing

The surface functional layers of commercial detector-grade CdTe crystals after commonly used surface treatments as polishing chemical etching in bromine-containing solutions and laser annealing with nanosecond pulses of the second harmonic of YAG:Nd laser are characterized. Effects of etching and irradiation with energy density near the melting threshold on the CdTe surface properties are studied using ellipsometry and photoluminescence and also crystals after storage in air were used. The intrinsic emission band at 1.56 eV and broad defect band peaked at 1.45 eV are found in the PL spectra, excited by a semiconductor laser with wavelength of 405 nm at 80 K. The emission in 1.45 eV was associated with carrier recombination at the defects in the surface region, so called the surface state band.

Nonlinear characteristics of laser-induced incandescence of rough carbon surfaces

In this work the possibility of laser overheating of light-absorbing surfaces of bulk carbon samples to incandescent temperatures with the use of a moderate-power Q-switched YAG-Nd3+ laser (wavelength 1064 nm, pulse duration 20 ns, power density 3-10 MW/cm2) was studied. We observed laser-induced incandescence (LII) of carbon surfaces and investigated its properties. When the surface was irradiated by a sequence of laser pulses, unusual changes of LII intensity were discovered in the experiments. Also significant nonlinearity in the dependence of LII intensity on the laser pulse power density was observed. The average temperature of irradiated surface was estimated by approximating the experimental LII spectrum by Planks function and by computer simulations of laser heating of the carbon surface. For typical experimental conditions, the value of 2400 K was obtained. Both of the estimates of temperature are in a good agreement. The model, which is proposed to explain the observed effects, is based on the equation of heat conduction. Well-known thermal and optical properties of carbon are taken into account. The observed effects can be explained by essential nonuniformity of heating of rough surfaces and dominant evaporation of carbon from the tops of surface asperities.

Detection of the Nanostructures in ZnO Thin Films by Using Optical Methods

The ZnO films deposited by reactive magnetron sputtering are investigated. SEM provides information on the composition of the films and enables us to observe the nanostructures. AFM provides information on the roughness and the texture of the surfaces. Fourier transform, photoluminescence and Raman scattering are used for confirmation of appropriate results. The spectral dependences of ellipsometric parameters Δ and Ψ are determined by ellipsometry and the effective values of optical constants n and k are calculated. As it can be indicated, the nanostructures in ZnO film are formed when reducing base pressure down to 2.5•10-7 mbar and elevating the temperature of the substrate up to 5500C. The amorphous structure is formed in most of other films (crystalline structure is formed at rather special conditions). Films containing considerable portions of impurities (generally carbon impurities) are formed at reduced base pressure. The values of optical constants of some of the films fit the appropriate Dumont and Jellison tabulations satisfactorily.

Study of the systems thin metal film/thick lithium niobate substrate implanted by Ar+ ions and its application for production of pyroelectric photodetectors

The optical properties (the reflectance and the absorption) of the systems thin metal film - thick lithium niobate substrate (with film thicknesses of 15, 20, 30 and 40 nm) implanted by Ar+ ions with energies of 50, 100 and 150 keV and doses of 1015 - 1016 cm-2 are investigated in the wide spectral range (λ = 0.2 - 15 μm). It is shown that ion implantation influence on the absorption and the reflectance of such systems. Intensive atoms intermixing on the interface “film - substrate” change the optical properties of such systems. Different materials result to different effects. The changes of surface structure of Ni and Mo film on lithium niobate substrate slowly influence on their optical properties. But Pd film on lithium niobate substrate result to independence of reflectance from wavelength in the wide spectral range with magnification of an adhesion of the film to the substrate. AFM investigations of such systems show changes of the surface structure of the samples after the implantation. The effective use of such materials for pyroelectric photodetectors is proposed. The characteristics of pyroelectric photodetectors on the base of the systems “thin metal (Ni, Mo, Pd) film - thick lithium niobate substrate” implanted by Ar+ ions were investigated and compared with similar photodetectors. Such photodetectors have high sensitivity, response, radiation stability, detectivity, large dynamic range. Their functional characteristics not worse that characteristics of similar photodetectors. Owing to high radiation stability (large adhesion of metal film to substrate) such metal film based pyroelectric photodetectors can effectively use for registration of power laser radiation. The production process of metal film based pyroelectric photodetectors implanted by Ar+ ions is also proposed.

Metal-film-based pyroelectric detectors and their optical characteristics

The pyroelectric detectors on the basis of systems: thin nickel film--thick lithium niobate substrate, thin molybdenum film--thick lithium niobate substrate and thin titanium film--thick lithium niobate substrate implanted by argon ions as sensitive elements were designed. The optical constants of such systems with film thicknesses of d=15, 20, 30, and 40 nm in the wide spectral range were measured. It is shown that the reflectance of the system thin nickel film--lithium niobate increases and the reflectance of the systems thin molybdenum film--thick lithium niobate substrate and thin titanium film--thick lithium niobate substate decreases owing to the implantation. The influence of ion implantation on the surface structure of such systems is studied by electron microscopy. The optical characteristics and the functional parameters of pyroelectric detectors such as spectral sensitivity, radiation stabilty are researched. The number of advantages of such pyroelectric detectors among other detectors of similar type is found.