Guido Scavia

Blueshift of optical band gap in c-axis oriented and conducting Al-doped ZnO thin films

We have investigated the structural, optical, and electrical properties of alumina-doped zinc oxide (AZO) thin films, grown by pulsed laser deposition. The optical transmittance of the films is over 80% in the visible region, and the absorption edge shifts from about 380 nm of the undoped sample to 320 nm of the AZO film. The calculated optical band gap (Eg) of 2 wt % AZO films shows a widening up to 3.82 eV with respect to the undoped film (3.28 eV). Higher doping concentration (6 wt %) leads to films with larger Eg (4.1 eV), but also epitaxial properties are affected. A further widening of the gap occurs when the AZO films are deposited by lowering the substrate temperature (Ts) from 450 to 250 °C. These blueshifts are respectively attributed to the increase in carrier concentration, induced by Al-donor doping, and also a lower degree of crystalline order. AZO films with doping concentration of 2 wt % show resistivity values of about 10−3 Ω cm and the local I-V curves, measured by scanning tunneling spe...

A controlled approach to iron oxide nanoparticles functionalization for magnetic polymer brushes

In this article, we report a detailed study of surface modification of magnetite nanoparticles by means of three different grafting agents, functional for the preparation of magnetic polymer brushes. 3-Aminopropyltriethoxysilane (APTES), 3-chloropropyltriethoxysilane (CPTES), and 2-(4-chlorosulfonylphenyl)ethyltrichlorosilane (CTCS) were chosen as grafting models through which a wide range of polymer brushes can be obtained. By means of accurate thermogravimetric analysis a good control over the amount of immobilized molecules is achieved, and optimal operating conditions for each grafting agent are consequently determined. Graft densities ranging from approximately 4 to 7 molecules per nm(2) are obtained, depending on the conditions used. In addition, the surface-initiated atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) carried out with CTCS-coated nanoparticles is presented as an example of polymer brushes, leading to a well-defined and dense polymeric coating of around 0.6 PMMA chains per nm(2).

Unsoluble ordered polymeric pattern by breath figure approach

Herein, the ability of an alternated polyfluorene copolymer (PFOTHP) bearing tetrahydropyranyl groups on its lateral chains to turn into a solvent resistant material after a thermal treatment is exploited for the fabrication of versatile micropatterned thin films. Nanoporous films are formed by casting PFOTHP from a volatile solvent solution in the presence of humidity. The use of polydimethylsiloxane as a template of the hexagonal pattern enables the retention of structure during the thermal annealing which triggers the formation of unsoluble highly ordered honeycomb structured films. This prevents any restriction in the use of polymeric porous films together with organic solvents. Solutions of a second polymer can be drop on top of these films without modifying their porous surface, so that bicomponent arrays of two different photoluminescent polymers are achievable. The straightforwardness of this approach suggests interesting perspectives in the field of micro and nano-organized materials and offers valuable opportunities of application not only in optoelectronic but also in bioanalysis devices.

Structural Iridescent Tuned Colors from Self-Assembled Polymer Opal Surfaces

Structural colors are the object of a wide scientific interest, not only for the potential technical applications of their intriguing optical properties but also for the need of coloring agents to replace toxic and carcinogenic dyes. We present a simple methodology to obtain polymer opal surfaces of self-assembled core-shell nanoparticles with different degree of order for structural color applications. Polymer nanospheres prepared by surfactant-free emulsion radical copolymerization of an hydrophobic and an hydrophilic comonomer (styrene and methacrylic acid) spontaneously assemble into core-shell particles. Nanoparticles with identical composition and different diameters were prepared by modulating the degree of ionization of the weakly acidic comonomer. We report experimental results revealing how the synthesis parameters affect the properties of the core-shell particles and their influence on the optical properties of the final polymer opal surfaces, which depend on size, charge, and packing arrangement of the constituent nanoparticles.

Synthesis and characterisation of fluorenone–thiophene-based donor–acceptor oligomers: role of moiety sequence upon packing and electronic properties

A series of oligomers constituting alternating thiophene and fluorenone residues, designed to evaluate the effect upon both electronic properties and packing of the moiety sequence in the light of donor–acceptor concept, were synthesised and studied by cyclic voltammetry, optical spectroscopy, and XRD investigations into crystals, films, and powders. Moreover, ab initio calculations, based on Density Functional Theory, were carried out on selected molecules. A clear indication of close-packing induction by fluorenone in the oligomers, together with the factors determining the electrical and optical properties, are derived and presented. In the light of the performance of selected compounds in prototypes of p-type FET devices and bulk heterojunction solar cells, some general indications of chemical structure requirements for applications such as photovoltaic cells are described.

Bifunctional microstructured films and surfaces obtained by soft lithography from breath figure arrays

Breath figure films of polystyrene and polyalkylthiophene presenting regular patterns of 600 nm and 5 µm are used as templates to prepare negative polydimethylsiloxane stamps containing an entrapped quantity of the primary polymer. These breath figure replica are used to generate single or double layers of the entrapped conjugated polymer in the form of networks on any substrates. Besides this, polystyrene printed in this way can be used as a resist mask that allows the polymerization of aniline in a regular micrometric arrangement on a platinum electrode or to generate a regular acid/base patterning on a plain polyaniline film.

Fluoro-functionalization of vinylene units in a polyarylenevinylene for polymer solar cells

A low band-gap copolymer PDTBTFV alternating bis-thienyl-(bis-alkoxy)-benzothiadiazole blocks with difluorovinylene units and its non-fluorinated counterpart PDTBTV have been synthesized and characterized as donor materials in bulk heterojunction (BHJ) solar cells with PCBM as the acceptor. The solar cells with the fluorinated polymer show better photovoltaic performances than those recorded with the non-fluorinated material. Comparative spectroscopic and computational studies, together with morphological, electrical and optical characterization of thin films, have been carried out to shed light on the reasons for the improvement of performances as induced by the double bond fluorination. Our study introduces the fluorinated double bond as a new conjugated unit in donor polymers for BHJ solar cells.

Molecular cluster studies of the crystal growth process in mbe and mo-mbe: i. structure and thermodynamics of gas phase reactants [as2, as4, ga(ch3)3]

Abstract The structure and thermodynamics of some gas phase molecular reactants [As 2 , As 4 , Ga(CH 3 ) 3 ] used in MBE (molecular beam epitaxy) and MO-MBE (metal-organic molecular beam epitaxy) has been studied by Hartree-Fock-Roothaan molecular-orbitals linear-combination-of-atomic-orbitals self-consistent-field (HFR-MO-LCAO-SCF) ab initio methods. A first attempt to simulate the sticking of a gallium atom at the (100) surface of GaAs has also been made.

The effect of Co doping on the conductive properties of ferromagnetic ZnxCo1−xO films

The investigation of the electrical properties of Co-doped ZnO thin films provides two unexpected results: a decrease of the electrical conductivity and the contemporary occurrence of a reduction of conductivity and of an enhancement of ferromagnetic order. The former result is surprising since Zn atoms are replaced with iso-valent Co atoms. The latter finding questions previously suggested beneficial effects of n-type doping on the ZnO:Co magnetic behavior. While morphological and structural characterization permits us to exclude an influence of the morphology and of the presence of Co metal or Co-oxide phases on present experimental findings, with the aid of first-principles electronic structure calculations, we propose a qualitative picture which can explain in a coherent way both the changes in conductivity and ferromagnetic behavior ensuing from the Co doping and the above-mentioned, beneficial effects of n-type doping.

Light harvesting of CdSe/CdS quantum dots coated with β-cyclodextrin based host–guest species through resonant energy transfer from the guests

Films of nano-hybrids based on red emitting CdSe/CdS QDs functionalized with perthiolated β-cyclodextrin hosting a green emitting nitrobenzoxadiazole derivative show emission harvested by the host–guest organic system through resonant energy transfer from the organic host–guest species to the QD.