Mihai Anastasescu

Band gap bowing of binary alloys: Experimental results compared to theoretical tight-binding supercell calculations for CdxZn1-xSe

Compound semiconductor alloys of the type ABC find widespread applications as their electronic bulk band gap varies continuously with x, and therefore a tayloring of the energy gap is possible by variation of the concentration. We model the electronic properties of such semiconductor alloys by a multiband tight-binding model on a finite ensemble of supercells and determine the band gap of the alloy. This treatment allows for an intrinsic reproduction of band bowing effects as a function of the concentration x and is exact in the alloy-induced disorder. In the present paper, we concentrate on bulk CdZnSe as a well-defined model system and give a careful analysis on the proper choice of the basis set and supercell size, as well as on the necessary number of realizations. The results are compared to experimental results obtained from ellipsometric measurements of CdZnSe layers prepared by molecular beam epitaxy (MBE) and photoluminescence (PL) measurements on catalytically grown CdZnSe nanowires reported in the literature.

High-reflectivity II-VI-based distributed Bragg reflectors for the blue-violet spectral range

We report on the realization of a high quality distributed Bragg reflector for the blue-violet spectral range, with both high and low refractive index layers lattice matched to the GaAs substrate. Our structure is grown by molecular beam epitaxy (MBE). The high refractive index layer is made of ZnMgSSe, while the low index material consists of a short period superlattice containing MgS and ZnCdSe. The refractive index step of Δn = 0.43 results in a stop band width of 40 nm and the normalized reflectivity exceeds 99% for 21 Bragg pairs.

Enhancement of the electrochemical behaviour and biological performance of Ti–25Ta–5Zr alloy by thermo-mechanical processing

A new Ti-25Ta-5Zr alloy based only on non-toxic and non-allergic elements was elaborated in as-cast and thermo-mechanical processed, recrystallized states (XRD and SEM) in order to be used as candidate material for implant applications. Its long-term interactions with Ringer-Brown and Ringer solutions of different pH values and its cytocompatibility were determined. The thermo-mechanically processed alloy has nobler electrochemical behaviour than as-cast alloy due to finer microstructure obtained after the applied treatment. Corrosion and ion release rates presented the lowest values for the treated alloy. Nyquist and Bode plots displayed higher impedance values and phase angles for the processed alloy, denoting a more protective passive film. SEM micrographs revealed depositions from solutions that contain calcium, phosphorous and oxygen ions (EDX analysis), namely calcium phosphate. An electric equivalent circuit with two time constants was modelled. Cell culture experiments with MC3T3-E1 pre-osteoblasts demonstrated that thermo-mechanically processed Ti-25Ta-5Zr alloy supports a better cell adhesion and spreading, and enhanced cell proliferation. Altogether, these data indicate that thermo-mechanical treatment endows the alloy with improved anticorrosion and biological performances.

Nanostructured SnO2–ZnO composite gas sensors for selective detection of carbon monoxide

A series of SnO2–ZnO composite nanostructured (thin) films with different amounts of SnO2 (from 0 to 50 wt %) was prepared and deposited on a miniaturized porous alumina transducer using the sol–gel and dip coating method. The transducer, developed by our research group, contains Au interdigital electrodes on one side and a Pt heater on the other side. The sensing films were characterized using SEM and AFM techniques. Highly toxic and flammable gases (CO, CO2, CH4, and C3H8) were tested under lab conditions (carrier gas was dry air) using a special gas sensing cell developed by our research group. The gas concentrations varied between 5 and 2000 ppm and the optimum working temperatures were in the range of 210–300 °C. It was found that the sensing performance was influenced by the amount of oxide components present in the composite material. Improved sensing performance was achieved for the ZnO (98 wt %)–SnO2 (2 wt %) composite as compared to the sensors containing only the pristine oxides. The sensor response, cross-response and recovery characteristics of the analyzed materials are reported. The high sensitivity (R S = 1.21) to low amounts of CO (5 ppm) was reported for the sensor containing a composite sensitive film with ZnO (98 wt %)–SnO2 (2 wt %). This sensor response to CO was five times higher as compared to its response to CO2, CH4, and C3H8, thus the sensor is considered to be selective for CO under these test conditions.

Physicochemical Characterization and In Vitro Cytotoxic Effect of 3-Hydroxyflavone in a Silver Nanoparticles Complex.

The aim of this work was to characterize the physico-chemical properties of 3-hydroxyflavone (3-HF) in a silver nanoparticles complex (SNPs) using UV–vis and Fluorescence spectroscopy, Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) analysis. One also evaluated its effect on the cell viability and morphology of L929 mouse fibroblast cells in vitro. The contribution of the carrier protein, Bovine Serum Albumin (BSA) to 3-HF properties has also been investigated. 3-HF in BSA/SNPs systems presented no cytotoxic effect in L929 mouse fibroblast cells at any of the tested concentrations. The results are discussed with relevance to the oxidative stress process.

Surface topography and optical properties of Ge-Sb(As)-S-Te thin films by atomic-force microscopy and variable angle spectroscopic ellipsometry

Thin films were evaporated from powdered GexSb(As)40−xS50Te10 (x = 10, 20 and 27) glassy materials and their optical properties and surface morphology were studied by spectroscopic ellipsometry, performed in the UV-VIS-NIR range, and AFM imaging. For both kinds of quaternary systems, the optical constants (n and k) values decreased with increasing the Ge content in the films, their values being smaller for the GexAs40−xS50Te10 compositions. All films were transparent (k = 0) above 800 nm. The optical band gap energy increased with the Ge content; its value was larger for the GexAs40−xS50Te10 compositions. The AMF images revealed surfaces which are cracks-free and fully covered with uniformly distributed grains having particular structure depending on the film composition. For all the evaporated films, the average mean square (RMS) roughness did not exceed 5 nm, giving evidence for sufficiently high smoothness.

Optical and Structural Properties of SnO2-Based Sol-Gel Thin Films

SnO2 thin films have been deposited on glass from Sn (II)-2-ethylhexanoate precursor using the dip-coating sol-gel method. Based on spectroscopic ellipsometry (SE) measurements optical and structural properties of the SnO2 films have been obtained. It was found that the concentration of tin oxide sols used for deposition strongly influences the thickness and the optical properties of the film, such as optical constants (n and k) or optical conductivity (sigma)

Influence of the substrate type on the microstructural, optical and electrical properties of sol–gel ITO films

AbstractIndium tin oxide (ITO) is recognized as the best transparent and conductive material [transparent conducting oxide (TCO)] until now and its properties are dependent on the preparation method. In the present work ITO films with In:Sn atomic ratio 9:1 were prepared by a sol–gel route on different substrates (microscope glass slides, microscope glass covered with one layer of SiO2 and Si wafers) for TCO applications. The multilayer ITO films were obtained by successive deposition by the dip-coating method and the films were characterized from the structural, morphological, optical, and electrical points of view using X-ray diffraction, scanning electron microscopy, atomic force microscopy, spectroscopic ellipsometry and by Hall effect measurements, respectively. The results showed that the thickness, optical constants and carrier numbers depend strongly on the type of substrate, number of deposited layers and sol concentration. The optical properties of ITO films are closely related to their electrical properties. The enhancement of the conductivity was possible with the increase of crystallite size (which occurred after thermal treatment) and with the reduction of surface roughness.

Characterization and Corrosion Resistance of Anodic Electrodeposited Titanium Oxide/Phosphate Films on Ti-20Nb-10Zr-5Ta Bioalloy

Neste trabalho, a eletrodeposicao galvanostatica anodica de um filme de oxidacao contendo fosfatos em liga de Ti-20Nb-10Zr-5Ta de uma solucao de acido ortofosforico e apresentada. Sua composicao foi determinada por difratometria de raios X (XRD), espectroscopia no infravermelho por transformada de Fourier (FTIR) e micro-espectroscopia Raman, e a topografia por microscopia de forca atomica (AFM). A resistencia a corrosao da liga recoberta em fluido humano simulado (por metodo de polarizacao linear, monitoramento de potenciais de circuito aberto e gradientes do potencial de circuito aberto correspondentes) assim como a caracterizacao do recobrimento (por espectroscopia Raman e perfil de profundidade por espectroscopia fotoeletronica de raios X (XPS)) depositado por um periodo de imersao de 300 h em fluido humano simulado foram estudadas. O filme eletrodepositado era composto de dioxido de titânio amorfo e grupos fosfato. A resistencia a corrosao da liga Ti-20Nb-10Zr-5Ta recoberta em solucoes de Ringer neutra e alcalina foi maior que a da liga pura devido as propriedades protetoras do filme eletrodepositado. Os parâmetros de corrosao foram aprimorados com passar do tempo como resultado do espessamento do filme da superficie por deposicao da solucao fisiologica. O recobrimento depositado apresentou uma composicao variavel em profundidade, na camada mais profunda hidroxiapatita nanocristalina nucleada e na camada exterior fosfato de calcio amorfo.

Aggregation Behavior of Some Asymmetric Porphyrins versus Basic Biological Tests Response

Fractal analysis of free bases porphyrins was computed on atomic force microscopy (AFM) micrographs using two different methods: the correlation function method and the variable length scale method. The correlation function method provides fractal dimension only for short scale range; results indicate that only few images have fractal properties for short ranges; for the rest of them, no fractal dimension was found using the correlation function method. The variable length scale method occur information for long range scaling. All samples have fractal properties at higher scaling range. For three samples the correlation function method leads to the same fractal dimension as the variable length scale method and scaling ranges for both methods overlap. Results show the necessity to use both methods to describe the fractal properties of AB3  meso-porphyrins that may be used to predict their relative cell localization. In order to emphasize the influence of fractal and textural properties the results regarding their self-similarity and texture/morphology were further compared with their behavior in biological assessment, that is, functionality of some Jurkat cell lines.