1,720,968 research outputs found
Frequency Stabilization and Optically Tunable Lasing in Colloidal Quantum Dot Superparticles
Get PDFSelf-assembled superparticles composed of colloidal quantum dots establish microsphere cavities that support optically pumped lasing from whispering gallery modes. Here, we report on the time- and excitation fluence-dependent lasing properties of CdSe/CdS quantum dot superparticles. Spectra collected under constant photoexcitation reveal that the lasing modes are not temporally stable but instead blue-shift by more than 30 meV over 15 min. To counter this effect, we establish a high-fluence light-soaking protocol that reduces this blue-shift by more than an order of magnitude to 1.7 ± 0.5 meV, with champion superparticles displaying mode blue-shifts of <0.5 meV. Increasing the pump fluence allows for optically controlled, reversible, color-tunable red-to-green lasing. Combining these two paradigms suggests that quantum dot superparticles could serve in applications as low-cost, robust, solution-processable, tunable microlasers
Nanocrystal Superparticles with Whispering-Gallery Modes Tunable through Chemical and Optical Triggers
No full textWhispering-gallery microresonators have the potential to become the building blocks for optical circuits. However, encoding information in an optical signal requires on-demand tuning of optical resonances. Tuning is achieved by modifying the cavity length or the refractive index of the microresonator. Due to their solid, non-deformable structure, conventional microresonators based
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on bulk materials are inherently difficult to tune. In this work, we fabricate irreversibly tunable optical microresonators by using semiconductor nanocrystals. These nanocrystals are first assembled into colloidal spherical superparticles featuring whispering-gallery modes. Exposing the superparticles to shorter ligands changes the nanocrystal surface chemistry, decreasing the cavity length of the microresonator by 20% and increasing the refractive index by 8.2%. Illuminating the superparticles with ultraviolet light initiates nanocrystal photo-oxidation, providing an orthogonal channel to decrease the refractive index of the microresonator in a continuous fashion. Through these approaches, we demonstrate optical microresonators tunable by several times their free spectral range
In Situ EXAFS-Based Nanothermometry of Heterodimer Nanocrystals under Induction Heating
No full textThe ability to heat nanocrystalline materials through magnetic
induction has been used in the fields of catalysis, biomedical sciences, and polymer
degradation. However, the working temperature to which the inductively coupled
material rises is still poorly understood. Herein, we use extended X-ray absorption
fine structure spectroscopy in conjunction with thermal imaging to improve the
understanding of heating in inductively coupled systems. After extraction of the
Debye−Waller factor from the spectroscopy, we obtain the temperature of
inductively heated nanocrystals from the correlated Debye model. We combine
carbon-supported iron oxide nanocrystals as induction heating agents with platinum
nanocrystals as thermal probes. By testing these nanocrystal species as both
unattached nanocrystals and heterodimers, we report that nanostructured systems
show a significant temperature difference of up to 73.60 °C when compared to their local support environment. This result has
implications for inductively heated catalysis, magnetic hyperthermia for targeted cell death, and polymer synthesis
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
Sub-5 nm Anisotropic Pattern Transfer via Colloidal Lithography of a Self-Assembled GdF3 Nanocrystal Monolayer
No full textPatterning materials with nanoscale features opens many research
opportunities ranging from fundamental science to technological applications.
However, current nanofabrication methods are ill-suited for sub-5 nm patterning
and pattern transfer. We demonstrate the use of colloidal lithography to transfer
an anisotropic pattern of discrete features into substrates with a critical dimension
below 5 nm. The assembly of monodisperse, anisotropic nanocrystals (NCs) with
a rhombic-plate morphology spaced by dendrimer ligands results in a well-ordered
monolayer that serves as a 2D anisotropic hard mask pattern. This pattern is
transferred into the underlying substrate using dry etching followed by removal of
the NC mask. We exemplify this approach by fabricating an array of pillars with a
rhombic cross-section and edge-to-edge spacing of 4.4 ± 1.1 nm. The fabrication
approach enables broader access to patterning materials at the deep nanoscale by
implementing innovative processes into well-established fabrication methods
while minimizing process complexity
Microwave Heating of Nanocrystals for Rapid,Low-Aggregation Intermetallic Phase Transformations
No full textThe use of intermetallic Pt-Co nanocrystals (NCs) for electrocatalytic oxygen reduction reaction is quickly gaining interest thanks to the higher electrochemical stability of the intermetallic L10 phase compared to a random alloy A1 phase. However, the thermal treatment that enables the intermetallic phase transformation also causes significant NC aggregation, resulting in a significant loss of electrochemically active surface area. Herein, we report the use of microwave radiation to induce the intermetallic phase transformation in Cu-doped Pt-Co NCs. We demonstrate that microwave radiation reduces NC aggregation while allowing for a complete phase transformation in only 30 seconds. These microwave-treated NCs demonstrate higher mass activity for oxygen reduction reaction while maintaining electrochemical stability similar to the thermally-annealed samples
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
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