João A. Ferreira

Macrocrystalline silicon thin films prepared by RF reactive magnetron sputter deposition

Abstract Hydrogenated microcrystalline silicon (μc-Si:H) thin films with Cu as a dopant material (about 2 wt.%) were deposited by RF planar magnetron sputtering in an argon/hydrogen plasma. The composition and microstructure of the films were analysed by SEM, ERD/RBS, X-ray diffraction and Raman spectroscopy. These techniques revealed a columnar film structure, each column consisting of several small (nano) crystals with a lateral dimension up to 100 A. The crystals are oriented, generally with the (111) plane parallel to the sample surface. The hydrogen content of the thin films is about 27–33 at.%. Low deposition rates and low sputter gas pressures favour crystallisation and grain growth. The behaviour can be understood in terms of the diffusion or relaxation length Λ of the deposited Si-atoms.

The effect of the nature of retained solvent on the fluorescence of Nile Red incorporated in sol–gel-derived matrices

Abstract The environmentally sensitive dye Nile Red was incorporated into glass monolith matrices produced using the sol–gel process from a tetraethylorthosilicate precursor. The effect of different solvents included in the matrix forming reaction and retained within the pore structure of the finished bulk matrix, on both the steady state and time-resolved fluorescence was monitored. The solvents were chosen both in terms of dielectric constant (ranging from 2.4 to 47), viscosity and the fact that some have an important role in alleviating stresses during the drying stage of the sol–gel process. It was found that the dye was solvated within the matrix pore structure and demonstrated that the peak fluorescence emission could be tuned by addition of solvents with different dielectric constants. The lower limit of this wavelength range was mainly determined by interaction with the host, while the upper limit related to emission from an added solvent and water mixture. An improvement of up to two times in the fluorescence yield of Nile Red, relative to the sample with no added solvent, was observed in matrices with retained solvent.

Probing Si and Ti based sol-gel matrices by fluorescence techniques

The photophysical behavior of several probes incorporated in sol-gel–derived matrices (both monoliths and thin films) has been studied using steady-state and time-resolved fluorescence, along with fluorescence anisotropy to study the matrix structure and to elucidate probe-matrix interactions. The probes studied include laser and solvatochromic dyes along with porphyrins and phthalocyanines. It was found that spectral shifts, time-resolved decays, and quantum yields depend on the type of matrix and its preparation conditions combined with the drying time and the nature of retained solvent, which can be added to act as an anticracking agent. The differences between the results in the TiO2 matrix, where electron transfer is most probably present, and SiO2 are shown.

Absorption coefficients and fluorescence quantum yields of porphyrin films determined by optical and photoacoustic spectroscopies

Abstract The optical properties (absorption coefficients, refractive indices) and the fluorescence quantum yields of solid films of tetraphenylporphins of hydrogen, magnesium and zinc were determined, using optical absorption and photoacoustic spectroscopy. Absorption spectra show clear differences relative to spectra in fluid solution. Fluorescence quantum yields reveal the influence of central atom substitution and are in all cases much lower than the yields in solution, supporting evidence of appreciable aggregation of the porphyrins in the solid phase. The optical and luminescent parameters are independent of sample thickness in the range between 0.2 and 1.0μm.

Photoluminescence of erbium doped microcrystalline silicon thin films produced by reactive magnetron sputtering

Microcrystalline silicon thin films doped with erbium were produced by RF sputtering and their structural, chemical and optical properties were studied by X-ray diffractometry at grazing incidence, Rutherford back scattering (RBS) and optical transmission spectroscopy. The samples exhibit a sharp photoluminescence (PL) spectrum from the Er centres with the strongest peak positioned at 1.536 μm with a full width at half maximum of about 8 nm. When the temperature varies between 5 and 300 K, the photoluminescence decreases only 5-fold, in contrast to the behaviour reported for monocrystalline silicon.

A genetic algorithm for the TOPdTW at operating rooms

This paper presents a genetic algorithm for the Team Orienteering Problem with double Time Windows (TOPdTW). The aim is to study TOPdTW to model a real problem that arises within the operating rooms in a hospital. The Genetic Algorithm uses a peculiar way to construct solutions that only generates valid solutions, which improves the global performance. This constructive algorithm reads the chromosome and decides which operation is scheduled next in the route. The algorithm was tested using some public instances of the TOPTW and instances generated for TOPdTW. The computational results are presented.

THE STRUCTURE AND PHOTOLUMINESCENCE OF ERBIUM DOPED NANOCRYSTALLINE SILICON THIN FILMS PRODUCED BY REACTIVE MAGNETRON SPUTTERING

We have produced and studied undoped and erbium-doped nanocrystalline silicon thin films in order to evaluate the erbium influence on the film microstructure and how this correlates with the photoluminescence properties. Films were grown by reactive RF sputtering. For the doped films metallic erbium was added to the c-Si target. The structural parameters and the chemical composition of the different samples were investigated by X-ray in the grazing incidence geometry, Raman spectroscopy, ellipsometry and Rutherford Back Scattering. The effect of the nc-Si/SiOx matrix ,i.e., nc-Si volume fraction and the presence of SiO and/or SiO2 phases, on the erbium photoluminescence efficiency is discussed.

Optical characterization of photoactive multilayers

Abstract The optical characterization (refractive index, absorption coefficient and thickness) of organic photoactive layers for photovoltaic cells is reported. Absorption efficiencies of solar-energy conversion devices based on multilayered arrangements of organic and semiconducting oxide films have been determined, since they are a determinant factor in the overall conversion yield. The films were prepared by vacuum sublimation and magnetron sputtering techniques.