Multiple light scattering and near-field effects in a fractal tree-like ensamble of dielectric nanoparticles

We numerically study light scattering and absorption in self-similar aggregates of dielectric nanoparticles, as generated by simulated ballistic deposition upon a surface starting from a single seed particle. The resulting structure exhibits a complex tree-like shape, intended to mimic the morphologic properties of building blocks of real nanostructured thin films produced by means of fine controlled physical deposition processes employed in nanotechnology. The relationship of scattering and absorption cross sections to morphology is investigated within a computational scheme which thoroughly takes into account both multiple scattering and near-field effects. Numerical results are compared with a pre-existing single scattering limited analytical treatment of light scattering in fractal aggregates of small dielectric particles.Comment: 10 pages, 9 figures. Accepted for publication in Physical Review

Engineering of Microbial Fuel Cells technology: Materials, Modelling and Architecture

A Microbial fuel cell (MFC) is a bio-electrochemical reactor, able to convert chemical energy, contained in organic substrate, in electrical energy, thanks to the metabolic activity of microorganisms. Firstly, a fluid-dynamic modelling of different Microbial Fuel Cell configurations to study trajectories and concentration profile of the liquid containing the organic substrate during operation of the device was developed. The study of the device was joined with the study and the synthesis of carbon based aerogels to be used as new electrode materials, both for the anode and the cathode. The aim of the modelling was to understand what happen, from a fluid-dynamic point of view, inside the cell during operation. It was based on the application of equations from fluid-dynamics in order to study both the particle trajectories (using Navier-Stokes equations) and diffusion of substrate inside the reactor (using Fick’s laws). Three different MFC architecture were investigated, starting from a circular shape. To increase the area of the reactor interested by flux exchange with respect to the one in the circular configuration, a new a squared MFC, with a non-alignment of the inlet and the outlet was proposed. Starting from results obtained during the simulation for the squared reactor, to accommodate the flux distribution, a further improvement in architecture was introduced: a drop-shape MFC, with a percentage of fluid area exchanged, higher than 96%. Another possibility to improve MFC performances, is the optimization of materials used as electrodes. To be an efficient electrode, a material must satisfy some important condition: biocompatibility, good electrically conductivity, resistance to electrolytic solutions and high surface area together with high porosity to allow the formation of the biofilm. Carbon based aerogels can satisfy all these properties. Organic aerogels were synthetized following a green approach, starting from marine polysaccharides, like agar and starch and then transformed in carbon based, thanks to a thermal process. The synthesis procedure is the sol-gel technique, followed by a drying process that can extract the liquid part of the gel, leaving the solid structure, without collapse the material. Synthetized materials were analyzed both structurally and morphologically in order to understand if porosity, surface area and chemical composition were appropriate. To enhance some of these properties, a post synthesis treatment was performed: the surface of the aerogel was treated with a KOH solution in order to enlarge pores and increase the porosity of the overall material. The optimized aerogel was tested, as anode, into the square shape MFC and compared with commercial carbon material having the same function. Due to their high surface area, high porosity and good interaction with microorganisms, aerogels presented better performances of commercial materials if used as anode in MFC. Considering, instead, the addition of amino acids as nitrogen source to the previous material, it allowed the used of polysaccharide aerogel, as cathode electrode able to catalyze the oxygen reduction reaction (ORR). They were tested in MFC, compared with the most used catalyst material in literature, that is platinum. Another alternative to platinum in the catalysis of the ORR, is represented by the metal oxide aerogels. In this work, aerogels based on MnxOy were tested. The synthesis of this material is similar to the previous one, with the difference of the addition of the manganese oxide directly between initial precursors. Through the thermal process, the organic part of the material is burned, leaving an oxide structure that is active from a catalytic point of view. After the morphological, structural and chemical analysis of the sample, the catalytic activity of the material was tested, as in the previous case, using the Rotating Ring Disk Electrode (RRDE) technique, in order to investigate its catalytic properties

Anisotropic Effects of Oxygen Vacancies on Electrochromic Properties and Conductivity of γ\gamma-Monoclinic WO3_3

Tungsten trioxide (WO$_3$) is a paradigmatic electrochromic material, whose peculiar optical properties in the presence of oxygen vacancies or intercalated alkali atoms have been observed and investigated for a long time. In this paper we propose a rationalization of experiments based on first-principles calculations of optical and electrical properties of oxygen deficient (reduced) WO$_3$. Our approach is based on a parameter-free dielectric-dependent hybrid density functional methodology, used in combination with the charge transition levels formalism, for studying excitation mechanisms in the presence of defects. Our results indicate that oxygen vacancies lead to a different physics in $\gamma$-monoclinic WO$_3$, depending on the orientation of the W-O-W chain where the vacancy is created, thus evidencing strong anisotropic effects rooted in the peculiar structural properties of the original nondefective monoclinic cell. Different types of oxygen vacancies can hence be classified on the basis of the calculated ground state properties, electronic structure, and excitation/emission energies, giving a satisfactory explanation to a range of experimental observations made on oxygen deficient WO$_3$.Comment: Accepted for publication in J. Phys. Chem.

Electronic structure and phase stability of oxide semiconductors: Performance of dielectric-dependent hybrid functional DFT, benchmarked against GWGW band structure calculations and experiments

We investigate band gaps, equilibrium structures, and phase stabilities of several bulk polymorphs of wide-gap oxide semiconductors ZnO, TiO2,ZrO2, and WO3. We are particularly concerned with assessing the performance of hybrid functionals built with the fraction of Hartree-Fock exact exchange obtained from the computed electronic dielectric constant of the material. We provide comparison with more standard density-functional theory and GW methods. We finally analyze the chemical reduction of TiO2 into Ti2O3, involving a change in oxide stoichiometry. We show that the dielectric-dependent hybrid functional is generally good at reproducing both ground-state (lattice constants, phase stability sequences, and reaction energies) and excited-state (photoemission gaps) properties within a single, fully ab initio framework.Comment: Minor changes in the final published versio

Defect calculations in semiconductors through a dielectric-dependent hybrid DFT functional: the case of oxygen vacancies in metal oxides

We investigate the behavior of oxygen vacancies in three different metal-oxide semiconductors (rutile and anatase TiO2, monoclinic WO3, and tetragonal ZrO2) using a recently proposed hybrid density-functional method in which the fraction of exact exchange is material-dependent but obtained ab initio in a self-consistent scheme. In particular, we calculate charge-transition levels relative to the oxygen-vacancy defect and compare computed optical and thermal excitation/emission energies with the available experimental results, shedding light on the underlying excitation mechanisms and related materials properties. We find that this novel approach is able to reproduce not only ground-state properties and band structures of perfect bulk oxide materials, but also provides results consistent with the optical and electrical behavior observed in the corresponding substoichiometric defective systems.Comment: Accepted for publication in J. Chem. Phy

Communication: Hole localization in Al-doped quartz SiO2 within ab initio hybrid-functional DFT

We investigate the long-standing problem of the hole localization at the Al impurity in quartz SiO$_2$, using a relatively recent DFT hybrid-functional method in which the exchange fraction is obtained \emph{ab initio}, based on an analogy with the static many-body COHSEX approximation to the electron self-energy. As the amount of the admixed exact exchange in hybrid functionals has been shown to be determinant for properly capturing the hole localization, this problem constitutes a prototypical benchmark for the accuracy of the method, allowing one to assess to what extent self-interaction effects are avoided. We obtain good results in terms of description of the charge localization and structural distortion around the Al center, improving with respect to the more popular B3LYP hybrid-functional approach. We also discuss the accuracy of computed hyperfine parameters, by comparison with previous calculations based on other self-interaction-free methods, as well as experimental values. We discuss and rationalize the limitations of our approach in computing defect-related excitation energies in low-dielectric-constant insulators.Comment: Accepted for publication in J. Chem. Phys. (Communications

Towards age-independent acoustic modeling

International audienceIn automatic speech recognition applications, due to significant differences in voice characteristics, adults and children are usually treated as two population groups, for which different acoustic models are trained. In this paper, age-independent acoustic modeling is investigated in the context of large vocabulary speech recognition. Exploiting a small amount (9 hours) of children's speech and a more significant amount (57 hours) of adult speech, age-independent acoustic models are trained using several methods for speaker adaptive acoustic modeling. Recognition results achieved using these models are compared with those achieved using age-dependent acoustic models for children and adults, respectively. Recognition experiments are performed on four Italian speech corpora, two consisting of children's speech and two of adult speech, using 64k word and 11k word trigram language models. Methods for speaker adaptive acoustic modeling prove to be effective for training age-independent acoustic models ensuring recognition results at least as good as those achieved with age-dependent acoustic models for adults and children

Acoustic variability and automatic recognition of children’s speech

International audienc