1,721,361 research outputs found
La mémoire utile, fond et forme au menu des années 2000
No full textÀ l’occasion du 50e anniversaire de la Division de la gestion de documents et des archives de l’Université de Montréal, Claude Minotto, son ancien directeur nous livre ses réflexions. Il nous présente d’abord un bref plaidoyer en faveur de la diversité, car à ses yeux, une fois les conditions de préservation assurées, l’important est l’utilité de ce qui est conservé. Il utilise l’expression qui sert d’entête au site Internet de la DGDA comme préambule à la présentation de divers défis qu’ont amené les changements technologiques de la première décennie des années 2000 à la discipline archivistique. Monsieur Minotto partage ensuite quelques anecdotes issues de ses années en tant que directeur de la DGDA afin d’illustrer l’importance des relations de cette dernière avec les autres entités de l’Université de Montréal. Il termine en soulignant différentes réalisations de l’équipe de la DGDA au cours de sa direction et confirme, en conclusion, la nécessité de préserver « la mémoire utile » et ce, peu importe le support et les évolutions technologiques.On the occasion of the 50th anniversary of the Division of Records Management and Archives of the University of Montreal, former director Claude Minotto shares his reflections. First he presents a plea on behalf of diversity, because in his eyes, once the conditions for preservation are in place, the important thing is to use what has been conserved. He uses the expression that is on the home page of the Division’s website as a preamble to the presentation on various challenges that technological change brought about in the discipline of archival science during the first decade of the new century. Mr. Minotto then shares several anecdotes from his years as director, with the goal of illustrating the importance of relations between the Division and the other units of the University of Montreal. He ends by highlighting various accomplishments of the Division’s team and affirms, in conclusion, the importance of preserving the useful memory regardless of material medium or technological evolution
Una prospettiva regionale? Le campagne del Veneto tardo-medievale nella medievistica italiana degli ultimi decenni.
No full textThese pages synthetically analyse the timeframe of research on medieval agrarian history in Italy, and more specifically in Veneto, in the last two decades. The main goal of this contribution is to understand if it is possible to conceive a regional perspective for the analysis of the Italian agrarian scenes. The opening section describes the framework within which the researches on Italian medieval contryside were carried out, whereas the second part goes more deeply into the historiographical background emerged in the last decades in connection with the study of the historical agrarian context of Veneto. The focus is also on the development of studies from the purely historical-agrarian field to the so-called environmental and ecological approach of countryside history
Characterization of CdSe-CdxZn1-xS core-shell QDs as active materials for compact micro-cavity lasers
Get PDFInnovation within the field of nanophotonics is fostering the progress in diverse technological fields, spanning light emitting, communication technologies, renewable energies, medical diagnostic and therapy. Among the different classes of nanomaterials that are contributing to such evolution, semiconductor nanocrystals, a.k.a Quantum Dots (QDs), are the most versatile ones. QDs are inorganic semiconductor nanostructures, whose outstanding light emitting performances make them promising competitors to more “conventional” bulk solid-state materials in many commercial applications. The interest on developing QD-based devices spread on a large scale with the development of colloidal synthesis methods. The colloidal approach expedites their processability and integration in light emitting devices with dimensions ranging from the macro- to the nano-scale. In particular, colloidal QDs are suitable active media for the fabrication of compact and flexible solid-state laser sources.
Optical properties of QDs are ruled by the Quantum Confinement (QC) regime. The latter occurs when the size of the material is reduced to levels comparable with the exciton Bohr radius. QC is a size-effect and consequently leads to size-dependent absorption and emission properties. Thanks to QC, QDs exhibit well-defined electronic levels, which enable molecular-like optically allowed absorption transitions. At the same, high absorption cross-sections and stabilities typical of bulk semiconductor materials are preserved.
In this thesis work an emerging class of colloidal QDs, namely CdSe-CdXZn1-XS core-shell QDs, is investigated. The attention is mainly focused on the optical gain, which represents one of the most inspected and promising applications for QDs. By investigating the Amplified Spontaneous Emission (ASE) of different series of CdSe-CdXZn1-XS heterostructures, this work demonstrates that key properties such as the ASE activation threshold and photo-stability can be optimized by a careful design of the core-shell heterostructure. Guidelines for the synthesis of such best performing optical gain QDs are drawn by means of optical spectroscopy, which provides insights into the correlation between the excitation and relaxation dynamics with the shell thickness, composition and, ultimately, the structure.
Basic parameters such as QD dimensions, size dispersion and photoluminescence quantum yield (QY) can be easily extracted from steady-state absorption and emission spectra. Steady-state absorption and phtoluminescence studies on CdSe-CdXZn1-XS QDs were employed as preliminary tools to prove that different shell materials induce distinct exciton confinement, size dispersion and QY.
In a second step, Surface Enhanced Raman Scattering (SERS) technique has been employed, for the first time, as a local probe for the study of the core-shell interfaces. SERS permits the analysis of the nanocrystals with the same structural features and lattice dynamics present when the QDs are employed as emitters in photonic devices. Results of this study revealed that the composition of the CdXZn1-XS shell entails a significant structural difference at the core-shell interface. This structural difference modifies the electronic structure within the QDs, since it directly tailors the QC of the electrons and holes.
The effect of the core-shell interface on optical properties has been unambiguously detected with the use of transient optical spectroscopy. In this thesis work, transient absorption (TA) and transient PL (tPL) techniques were employed to probe the exciton generation and recombination dynamics. The evolution of the exciton population was compared with kinetic models. Differently from steady-state techniques, transient techniques are sensitive to the nature and time-scales of the different radiative and non-radiative relaxation paths, whose control is crucial for guiding the heterostructure engineering process. The kinetic rates obtained revealed a clear dependence on the core-shell interface and the correlation with SERS results is discussed.
The correlation between structure and dynamics was detected from the nanosecond (tPL analysis) down to the sub-nanosecond time scales (TA analysis). A secondary mission of this thesis was also to find a global interpretation of the dynamics of all signals present in TA spectra of the different CdSe-CdXZn1-XS QD series. Pump fluence, shell thickness and composition are the coordinates along which the global analysis has been developed. This step is of pivotal importance in order to identify the mechanisms involved in the optical gain process, whose temporal evolution for QDs systems spans from the picosecond to the few nanosecond time-scale.
From the discussion of the results obtained from the different characterization techniques, it emerges that the most efficient way to boost the optical properties of CdSe QDs is the realization of a “graded” CdXZn1-XS shell, with Zn concentration (and confinement potential) gradually increasing along the radial direction. In a single entity, this solution should provide suitable confinement of the charge-carriers from the defective outer surface, prevent defect formation at the core-shell interface due the mismatch between the different materials and, eventually, limit the dot dimensions. Low QD dimensions increase the packing density and limits the scattering losses when QDs are included in a thin film and/or in a solid-state matrix. Such aspects have to be taken into serious consideration in order to increase the performances of a QD-based optical amplifier.
Finally, the validity of the hypothesis formulated is experimentally verified by characterizing the bi-exciton radiative recombination, which represents the photo-physical origin of ASE and thus defines the optical gain performances of differently engineered nano-heterostructures. As predicted, best optical gain performances have been achieved from ASE experiments by using CdSe QDs covered with a graded CdS-Cd0.5Zn0.5S-ZnS shell. Therefore, the results obtained from the spectroscopic characterization provide a guideline for the engineering of new synthetic approaches, addressed to the preparation of highly stable core-shell QDs with minimal optical gain activation threshold. Moreover, the rationalization of the dynamics involved in exciton and multi-exciton generation and recombination in core-shell QDs expedites their application in all types of light emitting devices
Lavorare l'acqua e la terra : l'incolto produttivo nella gronda lagunare di Venezia alla fine del Medioevo
Get PDFLa ricerca parte dall’esigenza di considerare le acque e l’incolto non solo come risorse naturali ed economiche (incolto produttivo), e conseguentemente come elemento importante per gli equilibri socio-economici delle società rurali, ma anche come risorse, occasioni, problemi di carattere politico nel senso lato del termine: si tratti di politica “inter-statuale” o di politica “intra-statale”, vale a dire di rapporti tra centro e periferia. Spesso infatti gli spazi incolti fungono anche da sorta di membrana-confine tra uno stato e un altro o tra diverse comunità, e lo studio dell’incolto si inserisce all’interno di più ampie discussioni storiografiche sul tema delle frontiere percepite in tutte le loro sfumature (politiche, economiche, giuridiche e giurisdizionali, naturali). Di conseguenza, analizzare la diffusione e lo sfruttamento, o la riduzione, di tali risorse significa anche esaminare le dinamiche giurisprudenziali atte a regolarne l’utilizzo.
L’analisi è stata condotta entro i confini geografici coincidenti con le terre della gronda lagunare meridionale di Venezia nel periodo storico che va dalla fine del Medioevo agli inizia dell’età moderna. Tra le parti in causa compaiono monasteri, comunità rurali ed esponenti dell’aristocrazia veneziana, variamente coinvolti – l’intreccio è sempre molto stretto – in questioni confinarie e in strategie di accaparramento delle risorse. In particolare, il peso giocato dagli enti ecclesiastici veneziani – fruitori e detentori di boschi, acque e valli – nelle controversie provocate da usi differenti dell’ambiente incolto perilagunare è importante, in un momento nel quale la rilevanza economica di questo ambiente appare crescente. Va segnalato infine che buona parte delle fonti proviene dagli archivi monastici, e ciò pone delicati problemi di metodo che vengono discussi nell’elaborato.The present research moves from the necessity to consider from two different perspectives waters and untamed lands in the southern area of the so-called gronda lagunare of Venice (the contact area between the lagoon and mainland) from the end of Middle Age to the beginning of Modern Age. As for the first perspective is concerned, waters and untamed lands have been studied as natural and economical resources (productive untamed lands), and as an important element for socio-economical balances in rural societies. As for the second one, waters and untamed lands have been examined from a “political” point of view considering their presence in “inter-state” policies as well as in “intra-state” policies. Untamed lands frequently function as a particular form of boundary between one State and another or between different communities. Therefore, the study of untamed lands is usually part of wider historiographical debates on boundaries and borders, conceived in all their different meanings (political, economical, legal and juridical, natural). As a consequence, the analysis of the diffusion, exploitation and reduction of these particular elements involves also the analysis of the legal debates and of the dynamics which were necessary to control their use. In the geo-historical context considered in the study, acted monasteries, rural communities and Venetian aristocratic exponents, being involved at various degrees in disputes on boundaries as well as in resources hoarding strategies. Different uses of untamed lands in the area provoked controversies, in which Venetian monasteries role was very important. Holding and using woods, waters and valleys, monasteries had a growing economical importance in this period. Finally, it must be stressed that big part of the analysed sources comes from monastic archives, and this causes important methodological problems discussed in the thesis
A fluid monastic body: questions of re-bordering in Northern Italy (12th-16th centuries)
No full textIn this study, we aim to analyse some of the dynamics adopted by monastic communities in defining their environmental and jurisdictional space through boundaries. The wide geographical area under consideration and the long chronological span are appropriate dimensions on which to investigate phenomena that are not limited to the local level. The comparative analysis of sources concerning boundaries, produced by diverse and distant monastic communities, attests how the medieval monastic landscape was defined according to specific norms and customs that operated within the relationship between monks and their environmental resources, creating physical limits of inclusion and exclusion
Raccontare, disegnare, incidere l'acqua e la terra. Storie di geografia popolare nella laguna meridionale di Venezia nel Medioevo.
No full textIn the final centuries of the Middle Ages, the voice and memory of people create a sort of "popular" geography which tells of deeply rooted customs and practices in the aqueous "contado" south of the Venetian lagoon. The ability to evoke a territory strongly characterized by water and emerged land, is clearly revealed in witnesses' testimony given during the long proceedings aiming to define the boundaries of property and territories with uncertain borders. Strategies of public and private actors who owned these lands and waters were influenced by political and economical interests which affected also the overall design of such boundaries. They define spaces which are contended because of their highly important political and economic value. The territory told through the testimonies of those who lived and worked in this area takes shape also in the Venetian art which, according to different periods, follows in a more or less realistic way the delicate balance between man and nature
Bridging Energetics and Dynamics of Exciton Trapping in Core-Shell Quantum Dots
Get PDFThe widespread application of quantum dots greatly profits from their broad absorption band. However, the variable nature of excitations within these bands is expected to result in undesired excitation energy dependence of steady state emission properties. We demonstrate the different role played by hot and cold carrier trapping in determining fluorescence quantum yields. Our analysis relates the energetic parameters with the available knowledge on the dynamics of charge trapping. It turns out that detrapping processes play a pivotal role in determining steady state emission properties. We studied excitation dependent photoluminescence quantum yields (PLQY) in different CdSe/CdxZn1–xS (x = 0, 0.5, and 1) quantum dots to identify best performing heterostructures in terms of shell thickness and composition. Our rationalization of the observed behavior is focused on the modulation of trapping and detrapping rates. The combination of experimental results and PLQY kinetics modeling reveals the need to consider hot-carrier trapping, supporting recent dynamics observations. This work provides a deeper insight into the trapping process in quantum dots, relating its energetics and dynamics
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
- …
