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Materiali Pleccitonici Colloidali: design e dinamiche
No full textL’interazione fra luce e materia, studiata ormai quasi da un secolo, può generare differenti fenomeni a seconda della forza dell’accoppiamento di queste due entità. La conoscenza dei meccanismi che la regolano e delle dinamiche degli stati energetici coinvolti ne potrebbe permettere l’applicazione in ambito fotovoltaico, in fotonica, nell’intero panorama emergente delle tecnologie quantistiche e, più in generale, laddove sia importante controllare il flusso dell’energia sia temporalmente che spazialmente, anche a livello nanometrico.
Una classe di sistemi in cui è possibile generare stati elettronici ibridi luce-materia è quella dei nanomateriali pleccitonici colloidali, ottenuti attraverso l’assemblaggio di nanoparticelle plasmoniche e fluorofori organici in forma aggregata o meno. Questi materiali sono economici, facilmente caratterizzabili e la loro sintesi scalabile semplicemente. Tuttavia, il loro studio ad oggi risulta limitato sia nell’ottimizzazione del loro design che nella razionalizzazione delle dinamiche in differenti regimi di accoppiamento.
Con questa tesi di dottorato voglio portare il mio contributo in entrambi gli ambiti. Ho analizzato il design di questi materiali sia attraverso un lavoro di catalogazione e interpretazione della letteratura esistente, sia attraverso la preparazione di due famiglie di nuovi materiali pleccitonici. Lo studio della dinamica si concentra proprio su tali famiglie di materiali, sondate con tecniche di spettroscopia elettronica ultraveloce all’avanguardia.
Nel complesso, attraverso la correlazione fra le conoscenze ottenute in questi due ambiti, mai d’altro canto totalmente scindibili, ho dimostrato la versatilità di questi materiali. L’interazione fra luce e materia è, infatti, regolata profondamente dal design dei nanosistemi, che ne influenza fortemente il regime di accoppiamento. Inoltre, sempre attraverso il design di questi materiali, è stato possibile modulare i meccanismi di rilassamento, in particolare rallentandone i tempi caratteristici fino a due ordini di grandezza, e, soprattutto, ingegnerizzando la loro natura quantistica.The light-matter coupling has been studied during the previous and the present centuries because of the plethora of different phenomena achievable depending on the coupling strength. The knowledge of the mechanisms ruling this coupling, and the dynamics of the involved states could find applications in photonics, solar cells, in the whole emerging panorama of the quantum technologies and, overall, wherever it becomes relevant to control the energy flux either temporally and spatially, even at the nanometric scale.
Among the systems exploited to generate hybrid light-matter states, the class of Colloidal Plexcitonic Materials (CPMs) is defined as the assembly of coupled colloidal plasmonic nanoparticles and organic dyes. CPMs are cheap, easy to characterize, and their synthesis is easy to scale up. However, to date, only a limited number of studies have been devoted to the investigation of the optimization of their properties and their dynamics in different coupling regimes.
With this Thesis, I want to bring my contribution to both these aspects. For this reason, I analysed the design of these materials either by cataloguing and interpreting the existing literature, and by preparing two new families of CPMs. The study of the dynamics was thus focussed on these families, sifted with the state-of-the-art ultrafast electronic spectroscopy techniques.
Overall, the knowledge gained studying the design properties and the dynamics of these systems revealed that, first, these two aspects are strongly correlated, and second, that these materials are highly versatile. Indeed, their design has been found to play a central role in the light-matter strength of interaction and coupling. Moreover, again with the fine tuning of these materials, it was possible to modulate their relaxation mechanisms: their lifetimes could be increased up to two orders of magnitude and, mostly, their quantum nature can be engineered
Plexcitonic Nanohybrids Based on Gold Nanourchins: The Role of the Capping Layer
Full text linkPlexcitonic nanohybrids are plasmonic-excitonic novel materials whose peculiar properties are attracting considerable attention in photonics, solar cells, and sensing. These materials can be synthesized and characterized easily by assembling organic or inorganic dyes on plasmonic nanoparticles as support. However, the main factors controlling the assembly process and the occurrence of the plexcitonic coupling are still unclear. To fill this gap, in this work, we studied the plexciton coupling of 12 different dyes with a series of gold nanourchins with various coatings and sizes. Among 24 combinations tested, we observed the formation of plexcitonic hybrids only in five cases. Most of them had cyanine J-aggregates as excitonic counterparts. Stronger plexcitonic couplings were obtained when nanourchins were coated with an exchangeable citrate capping layer rather than a strong ionic thiols capping layer. We propose that the presence of a strongly bound capping layer, as in this latter ca..
Identification of Design Principles for the Preparation of Colloidal Plexcitonic Materials
Full text link: Colloidal plexcitonic materials (CPMs) are a class of nanosystems where molecular dyes are strongly coupled with colloidal plasmonic nanoparticles, acting as nanocavities that enhance the light field. As a result of this strong coupling, new hybrid states are formed, called plexcitons, belonging to the broader family of polaritons. With respect to other families of polaritonic materials, CPMs are cheap and easy to prepare through wet chemistry methodologies. Still, clear structure-to-properties relationships are not available, and precise rules to drive the materials' design to obtain the desired optical properties are still missing. To fill this gap, in this article, we prepared a dataset with all CPMs reported in the literature, rationalizing their design by focusing on their three main relevant components (the plasmonic nanoparticles, the molecular dyes, and the capping layers) and identifying the most used and efficient combinations. With the help of statistical analysis, we also found valuable correlations between structure, coupling regime, and optical properties. The results of this analysis are expected to be relevant for the rational design of new CPMs with controllable and predictable photophysical properties to be exploited in a vast range of technological fields
Molecularly Detailed View of Strong Coupling in Supramolecular Plexcitonic Nanohybrids
No full textPlexcitons constitute a peculiar example of light-matter hybrids (polaritons) originating from the (strong) coupling of plasmonic modes and molecular excitations. Here we propose a fully quantum approach to model plexcitonic systems and test it against existing experiments on peculiar hybrids formed by Au nanoparticles and a well-known porphyrin derivative, involving the Q branch of the organic dye absorption spectrum. Our model extends simpler descriptions of polaritonic systems to account for the multilevel structure of the dyes, spatially varying interactions with a given plasmon mode, and the simultaneous occurrence of plasmon-molecule and intermolecular interactions. By keeping a molecularly detailed view, we were able to gain insights into the local structure and individual contributions to the resulting plexcitons. Our model can be applied to rationalize and predict energy funneling toward specific molecular sites within a plexcitonic assembly, which is highly valuable for designing and controlling chemical transformations in the new polaritonic landscapes
Ultrafast Dynamics of Multiple Plexcitons in Colloidal Nanomaterials: The Mediating Action of Plasmon Resonances and Dark States
Full text link[Image: see text] Plexcitons, that is, mixed plasmon-exciton states, are currently gaining broad interest to control the flux of energy at the nanoscale. Several promising properties of plexcitonic materials have already been revealed, but the debate about their ultrafast dynamic properties is still vibrant. Here, pump–probe spectroscopy is used to characterize the ultrafast dynamics of colloidal nanohybrids prepared by coupling gold nanoparticles and porphyrin dyes, where one or two sets of plexcitonic resonances can be selectively activated. We found that these dynamics are strongly affected by the presence of a reservoir of states including plasmon resonances and dark states. The time constants regulating the plexciton relaxations are significantly longer than the typical values found in the literature and can be modulated over more than 1 order of magnitude, opening possible interesting perspectives to modify rates of chemically relevant molecular processes
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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