1,720,971 research outputs found
Metal Halides for Optoelectronic Applications
No full textThis Thesis focused on the improvement of the stability and the emission efficiency of different luminescent materials, in order to obtain promising compounds for optoelectronic devices. The main project was the synthesis of emissive hybrid metal halides as crystals, using different organic cations: methylammonium MA (CH6N+), butylammonium BA (C4H12N+), benzylammonium Bz (C7H10N+). In the first composition, expressed generally as MAxMnyBrz, two hybrid phases were synthesized varying the amount of NMF precursor, with yellow-green and red emissions, and also a temperature-dependent photoluminescence change. Using BA and Bz, the growing dimensions of the organic cations favored the formation of 2D layered crystal structures. In BA2(Pb,Mn)Br4, the precipitation of two different phases (pure-Pb and pure-Mn) was observed, with the typical blue emission of Pb2+ and the red one of Mn2+. Instead, in Bz2(Pb,Mn)Br4, all samples showed a red emission from Mn2+ (except for pure-Pb). Concerning other metal halide compounds, the synthesis of CsMnBr3 and Cs3MnBr5 nanocrystals (NCs) were optimized, with red and green emission. The doping of both phases with different lanthanides (Nd3+, Er3+, Tm3+ and Yb3+) was attempted, enabling an intense NIR emission only in CsMnBr3. Studying other perovskite compounds, CsPbX3 (with X = Br-, Cl-, I-) NCs were successfully synthesized using R-limonene as solvent. Then, the investigation of persistent luminescence (PeL) in Cs2NaInCl6:Mn2+ halide DPs demonstrated a dependence from the particles size: micrometric crystals presented a longer PeL than single crystals, whereas it disappeared in nanocrystals. Moreover, layered double perovskites (DPs) NCs with Cs4MnxCd1-xSb2Cl12 composition were synthesized. All the samples showed an absorption and a red emission stemming from Sb3+ and Mn2 (except for x = 0). Finally, the last project was the study of AECuSi4O10 (AE: Ca, Sr, Ba) silicates and their NIR photoemission, given by Cu2+
From Bulk to Nano: The Effect on the Persistent Luminescence
No full textPersistent luminescence materials are compounds able to glow in the dark after proper excitation. After the discovery in the 1990s of the long persistent luminescence in SrAl2O4:Eu2+,Dy3+ lasting 12 hours, the interest in this class of materials has grown quickly. Today, thousands of compounds are known to possess persistent luminescence, and some of them have found real industrial applicability. However, the mechanism driving the persistent luminescence remains somewhat unknown. The scenario is further complicated by moving from bulk materials to nanocrystals. Herein, we give an overview of the most important goals achieved in persistent luminescence materials and a critical perspective on this peculiar phenomenon at the nanoscale
Red-emissive nanocrystals of Cs4MnxCd1-xSb2Cl12 layered perovskites
No full textLayered double perovskites are currently being investigated as emerging halide-based materials for optoelectronic applications. Herein, we present the synthesis of Cs4MnxCd1-xSb2Cl12 (0 <= x <= 1) nanocrystals (NCs). X-ray powder diffraction evidences the retention of the same crystal structure for all the inspected compositions; transmission electron microscopy revealed monodisperse particles with a mean size between 10.7 nm and 12.7 nm. The absorption spectra are mostly determined by transitions related to Sb3+, whereas Mn2+ induced a red emission in the 625-650 nm range. The photoluminescence emission intensity and position vary with the Mn2+ content and reach the maximum for the composition with x = 0.12. Finally, we demonstrate that the photoluminescence quantum yield of the latter NCs was increased from 0.3% to 3.9% through a post-synthesis treatment with ammonium thiocyanate. The present work expands the knowledge of colloidal layered double perovskite nanocrystals, stimulating future investigations of this emerging class of materials
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
- …
