Mauro Casalboni

Self-assembled nanoparticles of functional copolymers for photonic applications

A modified emulsion copolymerization of phenylacetylene (PA) with hydrophilic monomers having different functions, i.e., acrylic acid (AA) and N,N-dimethylpropargylamine (DMPA) respectively, yields functionalized polymeric P(PA-co-AA) and P(PA-co-DMPA) nanoparticles. The systematic investigation on the experimental parameters affecting size, surface charge and polydispersity of the copolymers (initiator concentration, reaction time, cosolvent and PA/comonomer ratios) allows to modulate the nanoparticle physico-chemical properties. Spherical shaped particles with diameters in the range 80-500 nm, low polydispersity (PI values in the range 1.11-1.30) and different surface charge densities, between 0.44 and 2.87 microC/cm(2), have been consistently obtained and characterized by means of Dynamic Light Scattering (DLS), laser Doppler electrophoretic and Scanning Electron Microscopy (SEM) studies. XPS measurements have provided information on the nanoparticles chemical surface structure and suggest that AA and DMPA units are preferentially distributed on the surface of the spheres. The nanospheres self-assemble giving large domains (9.5 x 14.5 microm). Photonic analysis of the self-assembled copolymeric nanoparticles has been performed by means of Spectroscopic Ellipsometry (SE) and Bragg reflection spectroscopy, both of them demonstrating a three-dimensional photonic crystal property of these systems.

Optical characterization of alkyl-thiophenic monomers functionalized with second-order nonlinear chromophores

A series of thiophenic monomers functionalized with different chromophoric groups have been optically characterized in view of preparation of polymeric films for second-order nonlinear optics and electrooptics. In order to investigate the optical behaviour inside a solid host matrix, the molecules have been physically dispersed in poly(methylmethacrylate) (PMMA) films and poled through high-temperature corona-poling for characterization through in situ second harmonic generation and ex situ angle-resolved spectroscopy. The values of dipole moment μ and optical hyperpolarizability β have been estimated inside the host matrix and related to the molecular structure. For one of the chromophores the long-term reorientation dynamics has been monitored in order to investigate the stability and the interaction with the polymeric environment.

High-energy angle resolved reflection spectroscopy on three-dimensional photonic crystals of self-organized polymeric nanospheres

We report on the optical characterization of three-dimensional opal-like photonic crystals made by self-organized nanospheres of poly[styrene-(co-2-hydroxyethyl methacrylate)] having a face centred cubic (fcc) structure oriented along the [111] direction. A detailed optical characterization of the samples is presented using angle resolved reflection spectroscopy in specular geometry. The investigated energies are between a/lambda=0.5 and a/lambda=1.5 (where a is the lattice parameter and lambda is the light wavelength), a region in which both first and second-order Bragg diffraction are expected. Some interesting features as branching of the Bragg peak dispersion and high energy reflection peaks are revealed. We compare the experimental data with theoretical calculations using both Bragg diffraction and band structure approach. A comparison with recent results reported in the literature is also presented.

Hybrid organic–inorganic materials containing poled zwitterionic push–pull chromophores

Dihydroxy functionalised zwitterionic push-pull chromophores have been introduced in 3-glycidoxypropyltrimethoxysilane, tetraethylorthosilicate and N-[(3-trimethoxysilyl)propyl]ethylenediamine derived hybrid materials.Hybrid films have been deposited as thick layers via spin-coating.The amine groups introduced with the organically modified alkoxide bearing amine functionalities have been found to have an effective scavenger effect of the dye photobleaching.The addition, during the synthesis of the precursor sol, of N-hydroxyl carbazole has allowed to reach up to 20% of chromophore concentration avoiding the formation of aggregates within the matrix.The NLO properties of the material, after poling, have shown a good temporal stability, with retention of ca.70% of the initial signal value, after several months, providing a d 33 value of � 50� 70 pm V � 1 at the wavelength of 1.064 mm.

From nanospheres to microribbons: Self-assembled Eosin Y doped PMMA nanoparticles as photonic crystals

A modified emulsion synthesis of poly(methylmethacrylate) (PMMA) with the Eosin Y (EY), commercial chromophore, yields dye doped polymeric nanoparticles (PMMA@EY). A systematic investigation on the experimental parameters (monomer and initiator concentration, reaction time and MMA/EY molar ratio) has been explored to modulate physico-chemical properties of the dye doped polymeric colloids. Spherical shaped particles, doped with EY (0.5-3.0 wt%; loading efficiency η = 11-15%), with controlled diameters in the range 240-510 nm, low dispersity and ζ-potential values in the range between -42 mV and -59 mV, have been synthesized and characterized by means of UV-Vis spectrometry, Dynamic Light Scattering (DLS), laser Doppler electrophoresis and Scanning Electron Microscopy (SEM). Microribbons based on PMMA@EY nanoparticles have been fabricated by room temperature self-assembly of aqueous colloidal suspension on highly wettable glass substrates. Surface chemical treatment assisted the formation of long (up to few centimeters) regular ribbons with rectangular section. Lateral size and height of the structures have been controlled by changing the suspension concentration and/or the deposition volume: the higher suspension concentration produces larger and thicker ribbons and the higher deposited volume produces thicker ribbons (up to 23 μm with 198 μL of a 3 wt% suspension). Moreover, a transition from a film-like to a ribbon-like growth has been observed with increasing nanoparticles concentration. Short range ordering and photonic crystal features have been maintained in the fluorescent ribbon microarchitecture, resulting in a self-assembled material with excellent potential for the development of mirror-less and random lasers.

Outstanding Poling Stability of a New Cross-Linked Nonlinear Optical (NLO) Material from a Low Molecular Weight Chromophore

In this paper we report the synthesis and characterization of a trihydroxylated nonlinear optical (NLO) azochromophore and its functionalization with 2,4-tolylendiisocyanate (TDI) to give an amorphous mixture of isomers that was used as a starting compound for the preparation of cross-linked electro-optic (EO) thin films. An unedited type of thermal cross-linking reaction was used, exploiting the reactivity of isocyanate groups themselves in the presence of N,N-dimethylacetamide, without the addition of any hydroxylated comonomer as usual in the preparation of polyurethanes. Thin films were prepared by spin coating and corona poled during thermal cross-linking. A d(33) value of 33 pm/V was obtained by second-harmonic generation (SHG) measurements on poled films, and an excellent stability of SHG signal was shown upon aging at 130 °C and during dynamic thermal stability measurements.

Entrapping of Push-Pull Zwitterionic Chromophores in Hybrid Matrices for Photonic Applications

A new class of heterocycle-based push-pull chromophores showing enhanced nonlinear properties characterized by an aromatic and highly zwitterionic ground state and a quinoid/neutral excited state have been synthesized to be incorporated in sol-gel hybrid systems. This class of compounds shows very large first molecular hyperpolarizabilities (βμ as high as 27000 × 10−48 esu) and is a promising candidate for photonic applications where large second order non-linearities are required. In spite of their very large hyperpolarizability and chemical and thermal stability, these chromophores are decomposed in presence of light and oxygen (photobleaching) and are sensitive to acidic environments due to the carbanionic nature of the donor moiety. A hybrid matrix, based on N-[(3-trimethoxysilyl)propyl]ethylenediamine and 3-glycidoxypropyltrimethoxysilane, has been specifically designed to allow the incorporation of such zwitterionic compounds assuring at the same time a good temporal stability of the optical properties. Amine functionalisation has been found very effective in reducing the photobleaching by acting on these chromophores via the singlet oxygen. Second harmonic generation has been observed on poled films, and an order parameter, Φ, of 0.17 has been estimated. The nonlinear coefficient deff of the samples has been thus estimated at a value two times larger than for d11 of quartz that, from literature data, is about 0.335 pm/V.

Applications of synchrotron radiation in defect and color center research

Abstract VUV synchrotron radiation in the energy interval between 4 and 100 eV has been widely used in the study of the optical properties of large gap insulators either pure or doped with various impurities such as heavy metals and rare earths. Besides the absorption and emission properties, important results such as photon multiplication and Auger-free luminescence have been revealed in these investigations. The radiative damage produced in ionic crystals by synchrotron radiation is also discussed, with a brief description of the lattice defects produced and the atom or ion desorption from the irradiated crystals.

Effect of dielectric Bragg grating nanostructuring on dye sensitized solar cells

We report on a theoretical investigation on the influence of different wavelength scale periodic grating architectures on dye sensitized solar cell (DSC). A broadband absorption enhancement is expected in such solar cells thanks to diffraction effects produced by these structures. Their optimal size has been analyzed in terms of pitch grating, height and position along the solar cell layers. Numerical simulations indicate that nanostructuring the interface between the active and the electrolyte layer by integrating a dielectric grating produces an absorption enhancement of 23.4%. The presented results have been also evaluated in view of feasible realistic structures compatible with low cost soft lithographic techniques.

Hydrophilic silver nanoparticles with tunable optical properties: application for the detection of heavy metals in water

Due their excellent chemo-physical properties and ability to exhibit surface plasmon resonance, silver nanoparticles (AgNPs) have become a material of choice in various applications, such as nanosensors, electronic devices, nanobiotechnology and nanomedicine. In particular, from the environmental monitoring perspective, sensors based on silver nanoparticles are in great demand because of their antibacterial and inexpensive synthetic method. In the present study, we synthesized AgNPs in water phase using silver nitrate as precursor molecules, hydrophilic thiol (3-mercapto-1-propanesulfonic acid sodium salt, 3MPS) and sodium borohydride as capping and reducing agents, respectively. The AgNPs were characterized using techniques such as surface plasmon resonance (SPR) spectroscopy, dynamic light scattering (DLS), zeta potential (ζ-potential) measurements and scanning tunneling microscopy (STM). Further, to demonstrate the environmental application of our AgNPs, we also applied them for heavy metal sensing by detecting visible color modification due to SPR spectral changes. We found that these negatively charged AgNPs show good response to nickel (II) and presented good sensibility properties for the detection of low amount of ions in water in the working range of 1.0–0.1 ppm.