1,720,979 research outputs found
Thin Polymer Films: Simple Optical Determination of Molecular Diffusion Coefficients
No full textThe possibility to assess diffusion coefficients of small molecules in packaging polymer films directly on the shelf, or even along the fabrication line, without the use laboratory equipment commonly employed for gravimetric methods would represent a paradigm changer in the evaluation of barrier properties and byproduct formation in goods packaging and device encapsulation. In this work, we demonstrate a simple, effective and versatile method for the determination of the molecular diffusion coefficients that exploits simple UV-Vis spectroscopy and is suitable for any polymer film. This simple method also allows the direct identification of the intercalating molecule without the need for chemical targeting or of complex laboratory equipment. For this purpose, we report on the assessment of diffusion coefficients of both polar and non-polar molecules including water, ammonia, methanol, ethanol, toluene, and even hexafluorobenzene into polyvinyl chloride wrap commercialized for food packagin
(INVITED)Planar microcavities: Materials and processing for light control
No full textMicrocavities are a class of optical structures providing a versatile approach to engineering light matter interactions. In light of recent developments in materials processing technologies, in particular for organic and hybrid ones, and of the need for high efficiency optical systems, there has been extensive innovation and improvement in their design and realization leading to a multitude of structures and materials. Among these, closed multi-material microcavities or microresonators based on the effect of dielectric contrast have been attractive for their low losses, applicability in a wide spectral range, and customizability. High-dielectric contrast microcavities based on distributed Bragg reflectors have been adapted early on for their highly controlled fabrication and strong light confinement and proved to be essential in current technologies including lasers and light emitting diodes. In this review, we map their evolution from planar one-dimensional inorganic structures to more sophisticated designs incorporating various categories of organic and hybrid materials. Additionally, we provide an overview of state-of-the-art developments and limitations of this class of structures
New Polymer and Composite Structures for Photonic Applications
No full textThe focus of this thesis is the development of materials and architectures for all-polymer functional structures for photonic applications. The first part concerns the improvement and optimization of colorimetric and fluorescent sensing structures for the detection of various analytes in the vapor phase. Optical-readout sensors are portable and can provide an easy interpretation that needs no specialized training and can be visible to the naked eye. This makes them promising for applications in environmental control, health monitoring and food safety. The objective of the work was to investigate analyte diffusion processes into multilayered structures of polymer submicrometric films, and then optimizing the structure design and expanding the materials used in the field. First, sensors based on vapor diffusion in multilayered polymer dielectric mirrors with structural coloring were developed. Given their clear color change, this typology of sensors has been shown to be promising in the literature. However, as their response is limited by the diffusion speed of molecular species, they can suffer from slow detection of vapor-phase analytes. Next, I examine the use of fluorescent polymer films sensitive to microviscosity changes caused by exposure to volatile organic compounds and observing the changes in fluorescence during said exposure. The effect on the overall diffusion of capping layers deposited on top of the fluorescent polymer was investigated to quantify the effect of the barrier polymer on the selectivity of the sensor. Finally, I employed the solution processing protocols developed for novel low refractive index polymer suspensions that were initially utilized for the sensors to engineer structures for fluorescence control.
When two highly reflecting structures encapsulate a luminescent material in a submicrometric space, this changes the photoluminescence properties in structures called optical microcavities. While the highly reflecting structures can be metallic mirrors, these have limited reflectance intensity, high absorbance losses, as well as a lack of tunability. Instead, the use of dielectric mirrors enables very high reflectance at desired wavelengths. In addition, the use of compliant polymer materials allows the future use of these structures to construct more efficient flexible devices. I was able to develop highly reflecting microcavities for emitters in the visible range as well as in the near infrared. Besides achieving high amplification of fluorescence intensity, I was also able to report for the first time a change in the radiative rate of the fluorescence for polymer structures. As these effects were so far only observed in planar structures of inorganic nature or more complex polymer three-dimensional systems, this presents a breakthrough in the field.
In this introduction I will give a wide but deep overview of the optics of multilayered polymer films, their diffusion peculiarities, and use for sensing. Furthermore, I will address the topic of solid-state organic fluorophores and controlling their photoluminescence through engineering the dielectric environment. This will be followed by a chapter-by-chapter exploration of the results obtained during the doctoral training as adapted from already published or drafted work. Finally, the outlook and possible future implications and developments of this research will be examined
All-polymer Planar Photonic Crystals as an Innovative Tool for the Analysis of Air
No full textThe possibility to evaluate the molecular diffusivity in polymer thin films used for packaging and device encapsulation directly in-situ would represent a paradigm changer in the assesment of barrier properties and of air quality. Indeed, employing the packaging itself as a smart sensor could lead to waste reduction and mitigate food poisoning effects. In this work, we demonstrate a new technique that exploits simple UV-Vis reflectance spectroscopy to identify the kinetic of diffusion of small molecules in the vapor phase through polymer thin films and polymer multilayered structures. The new method allows then to assess the presence of the analyte in air and its diffusion coefficient in agreement with gravimetric data reported in literature
Mild Sol-Gel Conditions and High Dielectric Contrast: A Facile Processing toward Large-Scale Hybrid Photonic Crystals for Sensing and Photocatalysis
No full textSolution processing of highly performing photonic crystals has been a towering ambition for making them technologically relevant in applications requiring mass and large-area production. It would indeed represent a paradigm changer for the fabrication of sensors and for light management nanostructures meant for photonics and advanced photocatalytic systems. On the other hand, solution-processed structures often suffer from low dielectric contrast and poor optical quality or require complex deposition procedures due to the intrinsic properties of components treatable from solution. This work reports on a low-temperature sol–gel route between the alkoxides of Si and Ti and poly(acrylic acid), leading to stable polymer–inorganic hybrid materials with tunable refractive index and, in the case of titania hybrid, photoactive properties. Alternating thin films of the two hybrids allows planar photonic crystals with high optical quality and dielectric contrast as large as 0.64. Moreover, low-temperature treatments also allow coupling the titania hybrids with several temperature-sensitive materials including dielectric and semiconducting polymers to fabricate photonic structures. These findings open new perspectives in several fields; preliminary results demonstrate that the hybrid structures are suitable for sensing and the enhancement of the catalytic activity of photoactive media and light emission control
Strategies for Dielectric Contrast Enhancement in 1D Planar Polymeric Photonic Crystals
No full textHistorically, photonic crystals have been made of inorganic high refractive index materials coupled to air voids to maximize the dielectric contrast and in turn the light confinement. However, these systems are complex, costly, and time-demanding, and the fabrication processes are difficult to scale. Polymer structures promise to tackle this issue thanks to their easy solution and melt processing. Unfortunately, their low dielectric contrast limits their performance. In this work, we propose a concise but exhaustive review of the common polymers employed in the fabrication of planar 1D photonic crystals and new approaches to the enhancement of their dielectric contrast. Transfer matrix method modeling will be employed to quantify the effect of this parameter in standardized structures and to propose a new polymer structure for applications dealing with light management
Controlled Molecular Diffusion in Fluorescent Polymer Films for Label‐Free Detection of Volatile Organic Compounds
Full text linkAggregation-induced emission has eliminated the problem of fluorescence quenching in the solid state, making molecules with this property excellent candidates for vapor sensing due to their portability and ease of interpretation. Here, films of polystyrene / 2-[4-vinyl(1,10-biphenyl)-40-yl]-cyanovinyljulolidine copolymers are reported that exhibit aggregation-induced emission behavior for the detection of toluene, m-xylene, dichloromethane, and chloroform. After exposure to the analytes, the emission of the copolymers shows significant changes in intensity and spectral shape corresponding to the reduced microviscosity of the molecular environment. However, these changes are similar for different analytes, resulting in low chemical selectivity. Therefore, label-free selectivity is achieved by controlling the molecular diffusion of the four vapor analytes within the films using the Flory–Huggins solution theory with capping layers of cellulose acetate (CA) and poly(vinyl alcohol) (PVA) polymers
All-Polymer Microcavities for the Fluorescence Radiative Rate Modification of a Diketopyrrolopyrrole Derivative
Full text link[Image: see text] Controlling the radiative rate of emitters with macromolecular photonic structures promises flexible devices with enhanced performances that are easy to scale up. For instance, radiative rate enhancement empowers low-threshold lasers, while rate suppression affects recombination in photovoltaic and photochemical processes. However, claims of the Purcell effect with polymer structures are controversial, as the low dielectric contrast typical of suitable polymers is commonly not enough to provide the necessary confinement. Here we show all-polymer planar microcavities with photonic band gaps tuned to the photoluminescence of a diketopyrrolopyrrole derivative, which allows a change in the fluorescence lifetime. Radiative and nonradiative rates were disentangled systematically by measuring the external quantum efficiencies and comparing the planar microcavities with a series of references designed to exclude any extrinsic effects. For the first time, this analysis shows unambiguously the dye radiative emission rate variations obtained with macromolecular dielectric mirrors. When different waveguides, chemical environments, and effective refractive index effects in the structure were accounted for, the change in the radiative lifetime was assigned to the Purcell effect. This was possible through the exploitation of photonic structures made of polyvinylcarbazole as a high-index material and the perfluorinated Aquivion as a low-index one, which produced the largest dielectric contrast ever obtained in planar polymer cavities. This characteristic induces the high confinement of the radiation electric field within the cavity layer, causing a record intensity enhancement and steering the radiative rate. Current limits and requirements to achieve the full control of radiative rates with polymer planar microcavities are also addressed
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