1,720,963 research outputs found
Triplet-triplet energietransfer en ladingstransport in amorfe organische lagen gedopeerd met halfgeleidernanodeeltjes
No full textIn this thesis we chose the combination of blue-emitting colloidal CdSe/ZnS quantum dots (QDs) as a triplet emitter and poly(9-vinylcarbazole) (PVK) as matrix material for the emitting layer. PVK and related compounds are considered as reliable emitting materials as well as suitable charge carrier transport matrices when blue emission is involved. Furthermore the optical and charge transport properties of this polymer have been studied extensively.Due to the requirements for Förster and Dexter energy transfer efficient energy transfer from respectively the singlet and triplet excited state of the matrix to the QDs requires a short average distance between the donor and acceptor. To achieve this condition with a minimum loading of QDs requires, besides a small size of the QDs, a homogeneous distribution (no clustering) of the QDs.The physicochemical behavior and emission efficiency of the QDs dispersed in the polymer matrix is typically determined by their organic capping layer. Hence the organic ligand must on one hand strongly bind to the surface of the QDs to ensure a strong capping and avoid an extensive aggregation of the QDs in the doped polymer film. On the other hand it must have sufficient affinity for the host polymer to avoid a phase separation and achieve in this way a homogeneous distribution of the QDs. We therefore developed novel ligands; fatty acid ligands terminated by a carbazole group with series of alkyl-chain lengths (C6, C11, and C16) as well as a procedure to either put those ligands on naked QDs or to replace the initial ligands on the QDs by those novel ligands. The grafting density of the carbazole ligands was estimated by solution NMR experiment (chapter 2 and 3). By adding a known amount of CH2Br2 to a solution of the QDs in CDCl3 the number of ligands per QD was estimated by comparing the ratio of the area under the CH2Br2 resonance and the area under one of the carbazole resonances. In this way a density of 76 and 44 ligands/QD corresponding to 2.25 and 1.3 ligands/nm2 was obtained for C6 and C11 (chapter 2) when the samples were prepared by ligand exchange starting from commercial long chain alkylamine (LCA) capped QDs (EVIDOT). When the samples were prepared by post-synthetic (direct) encapsulation of QDs prepared at UGent a density of 44 and 62 ligands/QD corresponding to 1.2 and 1.7 ligands/nm2 was obtained for C11 and C16 (chapter 3). This suggests, based on comparison with a literature values, that only the 30 to 50 % of QD surface is covered by carbazole ligands.In chapter 2 and chapter 3, we showed that by encapsulating the QDs with the carbazole ligands the miscibility between the QDs and PVK was improved significantly compared to the situation where commonly used ligands such as LCA and OA were used. The improvement of the miscibility results in less clustering of the QDs in the PVK film and thus allows more efficient singlet and triplet energy transfers from the polymer to the QDs. At the same dopant (QD) loading a more efficient energy transfer from the carbazole moieties to the QDs was observed in the PVK film doped with the C16 capped QDs compared to the film doped with C11 capped QDs which in turn showed more efficient energy transfer than the film doped with C6 capped QDs. More efficient energy transfer in the doped PVK films with C16 capped QDs corresponds to less and/or smaller clusters of the QDs which was confirmed by AFM and confocal fluorescence images. The estimation for the singlet and triplet energy transfer efficiencies based on time-resolved fluorescence experiment and stationary PL spectra indicated that the increase of the overall energy transfer efficiency mainly corresponds to the increase of the triplet energy transfer efficiency, which requires a short distance between the carbazole moieties and the QD cores as it occurs by an electron exchange interaction. Decreased clustering leads at the same load of QDs to a higher concentration of individual QDs and hence to a shorter average distance between the carbazole moieties and the QDs, resulting in a more efficient triplet energy transfer. In addition to the increase of the triplet transfer efficiency, the less extensive clustering of the QDs also produces a smoother film when the PVK film was doped with C16 capped QDs. As known for organic electronic devices, a smoother organic film is another key factor to ensure a better interface with the electrodes or other organic layers and hence a more efficient charge injection in the polymer layer by other organic layers and/or a metallic electrode. Although we observed that the ligand properties are critically important to enhance the miscibility and energy transfer between the carbazole moieties and QDs, at the highest QD concentrations used a saturation of the energy transfer still occurred in the PVK matrix. At high concentration (30 wt%) even the C16 capped QDs started to cluster in the PVK matrix, which limits the efficiency of triplet energy transfer. Therefore we explored alternatives for PVK. As alternative to PVK we chose poly(9-(2,3-epoxypropyl) carbazole) (PEPK) (chapter 4) which has a much smaller molecular weight and shows less extensive excimer formation, the latter indicating less extensive ground state interactions between the carbazole moieties. Furthermore this compound was reported to have better film forming properties compared to PVK. At a concentration of 30 wt% of QDs capped with C16 ligands more efficient singlet and triplet energy transfer to the QDs was observed in the films of PEPK compared to doped PVK films. E.g. at a loading of 30 wt% QDs the quantum yield for singlet energy transfer, as determined from the analysis of the fluorescence decays, amounted to 41±2% in the PEPK matrix while it was only 12±0.6% in the PVK matrix. This can be related to the smaller number of excimer sites present in PEPK. While those sites do not block direct energy transfer from an excited carbazole to the QDs they will block exciton hopping between the carbazole moieties. Furthermore also a better miscibility with the QDs can play a role at this high loading of 30 wt%. This better miscibility can also explain why the triplet energy transfer was more efficient in PEPK where it amounted to 72% at a loading of 30 wt% QDs while under those conditions in PVK only a value of 37% was found. Excimer sites are not expected to hinder the hopping of triplet states through the polymer matrix as they lack, contrary to the singlet state, the stabilization by exciton interaction. In addition to the doped polymer films, we also observed the energy transfer from the bound carbazole ligands to the QDs in a solution of the QDs in chlorobenzene (chapter 2 and 3). By increasing the length of the carbazole ligand from C6 to C16, i.e. increasing the distance between the carbazole moiety and the QD, less efficient energy transfer was observed. This is in agreement with the distance dependence predicted in the Förster and Dexter theory, but contradicts the results we obtained in PVK films (chapter 2 and 3). The estimation of the efficiency for triplet transfer in a solution of C6 capped QDs from Evidot (chapter 2) and C11 and C16 capped QDs from UGent (chapter 3) yielded values much in excess of 100 %, which are physically unacceptable (significantly over 100 %). We attributed these deviations to accumulation of experimental errors related mainly an overestimation of the value of the relative intensity of the QD luminescence in the fluorescence spectra (due to the wavelength dependence of the refractive index of the solution) and to the determination of the fluorescence quantum yield of the QDs. Due to the procedure used to determine the efficiency of triplet transfer, underestimation of the fluorescence quantum yield of the QDs will increase the observed efficiency for triplet energy transfer drastically. In this respect one should also mention the batch to batch variation of the fluorescence quantum yield of the QDs and the observation that this parameter was observed to change during the optical measurements and the storage time of the samples after preparation. In chapter 5 the hole and electron transports in pristine PVK films and PVK films doped with the QDs are discussed. The carrier mobilities were determined using the time-of-flight (TOF) method. In pristine PVK films the field dependence of the mobility shows at 298 K a positive Pool-Frenkel slope for the hole mobility, while a small negative slope is observed for the electron mobility. Moreover the hole mobility is over two orders of magnitude larger than the electron mobility in range of applied fields (~ 4.0 - 6.5×105 V/cm) that was investigated.The experiments were extended to the hole transport in PVK films doped with C11 capped QDs. Upon increasing the QD concentration the hole mobility decreased by nearly 2 orders of magnitude at a doping concentration of 3.87×1018 dots/cm3 (12 wt%) at T = 298 K. For pristine PVK as well as for PVK doped with the QDs up to a concentration of 3.87×1018 dots/cm3 the field and temperature dependence of the hole mobility could be analyzed formally in the framework of the Bässler disorder model. This analysis shows the increaseof the energetic (σ) and spatial (Σ) disorder upon increasing the QD concentration in a range from zero to 3.87×1018 dots/cm3. This indicates that the presence of QDs enlarges formally the spread of energy levels and intersite distances of the holes. In contrast to the zero-field mobility obtained in the temperature range from 298 K to 338 K (μ0), which is reduced upon increasing QD concentration, the logarithm of the zero field mobility extrapolated at infinite temperature (μ00) is observed to increase almost linearly with the doping concentration. Considering the modest trapping in spite of the high concentration of QDs compared to systems where a deep trap was introduced, one has to conclude that the traps generated by the QDs are rather shallow. Although a study of the dependence of the ZnS shells thickness on the optical properties of the QDs did not suggest a major delocalization of the electron or hole over the ZnS shell or trapping of the exciton by surface states on the ZnS shell or at the interface between the ZnS shell and the CdSe core, this should not exclude that surface states on the ZnS shell close or slightly above the HOMO of PVK can accommodate holes and act as traps and at higher concentration also as alternative transport sites. A further increase of the QD concentration up to 7.1×1018 dots/cm3 (22 wt%) leads however again to an increase of the hole mobility. Furthermore at this QD doping level of 7.1×1018 dots/cm3, the data can no longer be analyzed formally in the framework of the Gaussian disorder model. Considering that this concentration of 7.1×1018 dots/cm3 corresponds to a situation where 30.4 volume% of the film is occupied by the C11 capped QDs, the limitation of the model is attributed to hopping of holes over percolating QD clusters. This high volume fraction is a consequence of the layers of ZnS shell (0.6 nm) and C11 ligands (0.46 nm) of the QDs. The formation of those percolating clusters will be enhanced by the limited miscibility between the C11 capped QDs and PVK at this relatively high concentration.status: Publishe
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
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
- …
