Dependence of “Reststrahlen” bands in far-infrared reflectivity on configuration of GaAs/AlAs multiple quantum well heterostructures

Far infrared reflectivity measurements are performed on a series of GaAs/AlAs multiple quantum well (MQW) heterostructures with systematically varied thicknesses of the constituent layers. In addition to the artificial anisotropy we observe two distinct bulk-like Reststrahlen regions. The widths of the GaAs-like and the AlAs-like Reststrahlen bands strongly depend on the relative thicknesses of the constituent layers of the MQW heterostructures, in excellent agreement with the predictions of the effective-medium theory.Prof. Aldo Cingolani passed away just before the publication of this article. We would like to dedicate this paper to his memory     

Au nanoparticles prepared by physical method on Si and sapphire substrates for biosensor applications

Gold nanoparticles heavily functionalized with oligonucleotides have been used in a variety of DNA detection methods. The optical properties of three-dimensional aggregates of Au nanoparticles in solution or deposited onto suitable surfaces have been analyzed to detect hybridization processes of specific DNA sequences as possible alternatives to fluorescent labeling methods. This paper reports on the preparation of gold nanoparticles directly deposited onto the surface of silicon (Si) and sapphire (Al2O3) substrates by a physical methodology, consisting in the thermal evaporation of a thin Au film and its successive annealing. The method guarantees the preparation of monodispersed single-crystal Au nanoparticles with a strong surface plasmon resonance (SPR) peak centered at about 540 nm. We show that the changes of SPR excitation before and after DNA functionalization and subsequent hybridization of Au nanoparticles immobilized onto Si and Al2O3 substrates can be exploited to fabricate specific biosensors devices in solid phase.     

Synthesis of hybrid organic–inorganic nanocomposite materials based on CdS nanocrystals for energy conversion applications

Efficient solar energy conversion is strongly related to the development of new materials with enhanced functional properties. In this context, a wide variety of inorganic, organic, or hybrid nanostructured materials have been investigated. In particular, in hybrid organic–inorganic nanocomposites are combined the convenient properties of organic polymers, such as easy manipulation and mechanical flexibility, and the unique size-dependent properties of nanocrystals (NCs). However, applications of hybrid nanocomposites in photovoltaic devices require a homogeneous and highly dense dispersion of NCs in polymer in order to guarantee not only an efficient charge separation, but also an efficient transport of the carriers to the electrodes without recombination. In previous works, we demonstrated that cadmium thiolate complexes are suitable precursors for the in situ synthesis of nanocrystalline CdS. Here, we show that the soluble [Cd(SBz)₂]₂·(1-methyl imidazole) complex can be efficiently annealed in a conjugated polymer obtaining a nanocomposite with a regular and compact network of NCs. The proposed synthetic strategies require annealing temperatures well below 200 °C and short time for the thermal treatment, i.e., less than 30 min. We also show that the same complex can be used to synthesize CdS NCs in mesoporous TiO₂. The adsorption of cadmium thiolate molecule in TiO₂ matrix can be obtained by using chemical bath deposition technique and subsequent thermal annealing. The use of NCs, quantum dots, as sensitizers of TiO₂ matrices represents a very promising alternative to common dye-sensitized solar cells and an interesting solution for heterogeneous photocatalysis.     

Nanostructure size evolution during Au-catalysed growth by carbo-thermal evaporation of well-aligned ZnO nanowires on (100)Si

We report the structural and morphological properties of well-aligned ZnO nanowires grown at 750 °C on Au-deposited and annealed (100)Si substrates using carbo-thermal evaporation. As-grown nanowires are made of wurtzite ZnO, have cylindrical shape and carry droplet-like nanoparticles (NPs) at their tips, as expected for vapour–liquid–solid (VLS) growth. Grazing incidence X-ray diffraction measurements demonstrate that the NPs are made of pure fcc Au. No secondary Au/Zn alloy phases were detected. Bragg diffraction patterns confirmed that the nanowires were grown with their crystal c-axes parallel to the [100] direction of Si (i.e. normal to the substrate surface), while Au NPs are mostly (111)-oriented. The diameter distribution of ZnO nanowires mimics that of the Au NPs at their tips. A quantitative study of the nanostructure size distribution after sequential annealing and growth steps evidences the occurrence of three nanoscale processes: (i) Ostwald ripening and/or coalescence of Au NPs before nanowire nucleation, (ii) Au-catalysed VLS nucleation and axial growth of ZnO nanowires and (iii) radial growth of nanowires by a vapour–solid process. These processes originate the NP and nanowire size evolution during the experiments. The present findings are interpreted in terms of Zn vapour pressure changes during carbo-thermal evaporation. PACS 61.46.+w; 68.65.-k; 81.16.Dn     

Synthesis and characterization of PMMA/silylated MMTs

Commercially available Sodium clay (Dellite HPS) and organo-clay (Dellite 72T) are modified via a silylation reaction. These silylated clays are characterized by IR, XRD, thermogravimetric analyses, and their equilibrium contact angles are measured. They are used to prepare nanocomposites at different loading percentage (1, 3, 5% wt) by in situ intercalative polymerization of Methyl methacrylate and morphology and thermal properties of nanocomposites are examined. SEM images of nanocomposites fractured surface show the absence of clays aggregates, confirming a good dispersion and distribution of montmorillonites in the polymer matrix. The effects of modified clays on the thermal properties of nanocomposites are analyzed by differential scanning calorimetry and thermogravimetric analyses showing an increase of glass and decomposition temperatures of all nanocomposites respect to homopolymer ones. The best results are obtained in the presence of silylated montmorillonites, clearly the organosilane improves the compatibility between polymer matrix and clay and as effect the properties of nanocomposites.     

Formation of metallic nanophases in silica by ion beam mixing. Part II: cluster formation

2 matrix by ion-beam mixing of SiO₂/Ag multilayers is studied via Rutherford backscattering spectrometry, optical absorption, and transmission electron microscopy experiments. In a first step, irradiation with MeV heavy ions transforms the continuous Ag layers into a string of micrometer-sized Ag inclusions. This mechanism can be attributed to lateral segregation of metallic atoms induced by irradiation. In a second step, the Ag inclusions are broken up by incoming ions and Ag nanoclusters are formed by agglomeration of mobile Ag atoms. The latter mechanism is likely due to a combination of ballistic mixing and radiation-induced segregation or radiation-enhanced diffusion processes. The size of the metallic nanoclusters formed depends also on the irradiation temperature. Received: 27 October 1997/Accepted: 3 February 1998