Tiziana Di Luccio

Particle size dependence of resonant-tunneling effect induced by CdS nanoparticles in a poly(N-vinylcarbazole) polymer matrix

CdS nanoparticles of different sizes were synthesised in poly(N-vinylcarbazole) and studied in device structures glass/indium tin oxide (ITO)/PVK:CdS/Al. Electrical bistability and negative differential resistance (NDR) effects were observed in the current-voltage characteristics. In addition, the devices showed a considerable enhancement of the current magnitude. A dependence of the current conduction on the nanoparticle size and size distribution in the polymer was studied through electrical impedance measurements. The study revealed the importance of the charge effects of the nanoparticles resulting in a bistable behavior. A resonant tunneling current model was proposed to explain the NDR and its relation with the nanoparticle size and size distribution.

Tube Expansion Deformation Enables In Situ Synchrotron X-ray Scattering Measurements during Extensional Flow-Induced Crystallization of Poly l-Lactide Near the Glass Transition

Coronary Heart Disease (CHD) is one of the leading causes of death worldwide, claiming over seven million lives each year. Permanent metal stents, the current standard of care for CHD, inhibit arterial vasomotion and induce serious complications such as late stent thrombosis. Bioresorbable vascular scaffolds (BVSs) made from poly l-lactide (PLLA) overcome these complications by supporting the occluded artery for 3–6 months and then being completely resorbed in 2–3 years, leaving behind a healthy artery. The BVS that recently received clinical approval is, however, relatively thick (~150 µm, approximately twice as thick as metal stents ~80 µm). Thinner scaffolds would facilitate implantation and enable treatment of smaller arteries. The key to a thinner scaffold is careful control of the PLLA microstructure during processing to confer greater strength in a thinner profile. However, the rapid time scales of processing (~1 s) defy prediction due to a lack of structural information. Here, we present a custom-designed instrument that connects the strain-field imposed on PLLA during processing to in situ development of microstructure observed using synchrotron X-ray scattering. The connection between deformation, structure and strength enables processing–structure–property relationships to guide the design of thinner yet stronger BVSs.

Structural Characterization of Hybrid Organic–Inorganic Nanocomposites: X-ray

The properties of hybrid composites result from a complex cooperation between the organic and inorganic species constituting the nanocomposites. A continuous, increasing demand of the detailed knowledge of such properties at the nanoscale has contributed to the development of the characterization techniques. The chapter provides an overview on two specific characterization tools widely used in nanocomposites research: X-ray scattering (XRS) and Solid-State Nuclear Magnetic Resonance spectroscopy (SSNMR). We briefly describe some fundamentals of both the methods and present several relevant results from the literature in this field. Specific emphasis is devoted to nanoparticle polymer nanocomposites and polymer-layered silicate nanocomposites.

Structure of Nanocomposite films of CdS nanoparticles in a polymer matrix

A thiolate precursor was dispersed in a polymer solution and a precursor/polymer film was obtained by casting. Thermal annealing of the precursor/polymer film leads to the formation of a nanocomposite of nanometer-sized CdS dispersed in the polymer (thermolytic process). Different polymers were used as matrix material; in particular we employed a cyclo-olefin copolymer for its good optical properties and extremely low water absorption. After annealing with a temperature between 230 and 250°C in vacuum (pressure of about 6×10 −3 mbar) the CdS nanoparticles are found to be crystalline with a diameter of about 2nm in size. The nanoparticle size can be increased up to 15nm by annealing at higher temperatures (300°C). The details of the structural properties of the nanocomposite films have been investigated by small and wide angle x-ray scattering (SAXS and WAXS, respectively). A simple dependence of the nanoparticle dimensions on the annealing temperature was found. Furthermore, SAXS measurements indicate that the separation between the nanoparticles compares with their diameter.

Crimping-induced structural gradients explain the lasting strength of poly l-lactide bioresorbable vascular scaffolds during hydrolysis

Significance The molecular basis of the mysterious ability of the clinically approved bioresorbable vascular scaffold to maintain radial strength after 9 mo of hydrolysis—despite a ∼40% reduction in the poly l-lactide molecular weight Mn—is an unexpected gradient in degradation revealed by X-ray microdiffraction. Hydrolysis is slowest in the 100- × 30-µm region where stress is concentrated during arterial contractions. This favorable gradient in hydrolysis arises from gradients in morphology created during crimping onto the angioplasty balloon: Small regions that undergo elongational deformation (<3% of scaffold) resist hydrolysis after deployment. This remarkably small amount of material (too low to detect in Mn measurements) has a disproportionate impact on strength because it forms precisely where the scaffold is most vulnerable to failure. Biodegradable polymers open the way to treatment of heart disease using transient implants (bioresorbable vascular scaffolds, BVSs) that overcome the most serious complication associated with permanent metal stents—late stent thrombosis. Here, we address the long-standing paradox that the clinically approved BVS maintains its radial strength even after 9 mo of hydrolysis, which induces a ∼40% decrease in the poly l-lactide molecular weight (Mn). X-ray microdiffraction evidence of nonuniform hydrolysis in the scaffold reveals that regions subjected to tensile stress during crimping develop a microstructure that provides strength and resists hydrolysis. These beneficial morphological changes occur where they are needed most—where stress is localized when a radial load is placed on the scaffold. We hypothesize that the observed decrease in Mn reflects the majority of the material, which is undeformed during crimping. Thus, the global measures of degradation may be decoupled from the localized, degradation-resistant regions that confer the ability to support the artery for the first several months after implantation.

Effect of tungsten disulfide (WS_2) nanotubes on structural, morphological and mechanical properties of poly(L-lactide) (PLLA) films

Poly(L-lactide) (PLLA) is a semicrystalline, biocompatible and biodegradable polymer widely employed in many applications (food packaging, biomedical devices, drug delivery systems). This work deals with nanocomposites of PLLA and tungsten disulfide (WS_2) nanotubes (NTs) as a novel material to obtain thinner and stronger bioresorbable vascular scaffolds. We studied the influence of WS_2 NTs on the mechanical properties of PLLA-WS_2 films. Polarized optical microscopy reveals a high degree of orientation of the polymer molecules in stretched films that further increases with a post-stretching annealing treatment. At the same time, X-ray diffraction (XRD) and Raman spectroscopy confirm enhancement of the crystallinity induced by the WS_2 NTs.

In-situ GISAXS on Nanocomposite Films of CdS Nanoparticles and Polymers

We investigated the growth of CdS nanoparticles in polymer films by means of ex-situ and in-situ x-ray scattering experiments using synchrotron radiation. The CdS nanoparticles were synthesized by thermal decomposition of a Cd thiolate precursor dispersed in a cyclic olefin copolymer. The films were deposited by spin coating. Grazing incidence diffraction (GID) reveals the Bragg reflections of the CdS nanoparticles. In-situ diffraction and grazing incidence small angle scattering (GISAXS) experiments were recorded during the thermal treatment of the precursor/polymer films from room temperature up to 250°C. The diffraction curves show that the initial precursor structure is soon lost at 100°C. Correspondingly, the GISAXS data show a peak at a momentum transfer value q ∼ 0.2A −1 that shifts towards smaller values with the temperature. Under UV excitation the films show photoluminescence in the range 400 – 700 nm.

Morphological and Structural Characterization of TiN/ZrN Superlattices Deposited by Reactive R.F. Magnetron Sputtering

TiN/ZrN multilayers coatings have been deposited using a reactive R. F. magnetron sputtering process. Nitride layers with different sputtering conditions have been stacked in order to obtain superlattices having different preferred orientations. The deposition rate has been considered as independent parameter which changes the energy and the momentum transfer of the backscattered particles, influencing films structure evolution.

Evolution of Pt Nanoclusters Morphology on PEMFC Electrode due to Methanol Oxidation Reaction Studied by Electron Microscopy and Synchrotron Grazing Incidence X-Ray Diffraction

The proton exchange membrane fuel cells (PEMFC) have been developed mainly as a power source for vehicles, power generation and consumer electronics since they combine high energy conversion efficiency at relatively low temperatures without pollutants emission in the environment. An electrode for a PEMFC is a layered structure composed by a catalyst layer deposited on a porous carbon substrate. The substrate is usually covered by a diffusion layer that enhances the gas and water flow. Platinum nanoparticles supported by carbon microparticles are commonly employed as catalyst layer. In this work an extreme ultra-low loading of Pt catalyst (< 0.02 mg/cm2) has been deposited by magnetron sputtering on gas diffusion electrodes, with different carbon supports (Vulcan and SuperP), in order to enhance the activity of PEM fuel cells. The morphology (shape and grain size) and microstructure have been studied combining field emission scanning electron microscopy (FEG-SEM), grazing incidence synchrotron x-ray diffraction (GIXRD) and x-ray photoelectron spectroscopy (XPS). The results presented here concern the evolution of the cluster size and shape after the ageing, induced by cyclic voltammetry for methanol oxidation reaction.