1,720,979 research outputs found
Silver nanoflowers for single-particle SERS with 10 pM sensitivity
No full textSurface-enhanced Raman scattering (SERS) has received considerable attention as a noninvasive optical sensing technique with ultrahigh sensitivity. While numerous types of metallic particles have been actively investigated as SERS substrates, the development of new SERS agents with high sensitivity and their reliable characterization are still required. Here we report the preparation and characterization of flower-shaped silver (Ag) nanoparticles that exhibit high-sensitivity single-particle SERS performance. Ag nanoflowers (NFs) with bud sizes in the range 220-620 nm were synthesized by the wet synthesis method. The densely packed nanoscale petals with thicknesses in the range 9-22 nm exhibit a large number of hot spots that significantly enhance their plasmonic activity. A single Ag NF particle (530-620 nm) can detect as little as 10(-11) M 4-mercaptobenzoic acid, and thus provides a sensitivity three orders of SERS magnitude greater than that of a spherical Ag nanoparticle. The analytical enhancement factors for single Ag NF particles were found to be as high as 8.0 x 10(9), providing unprecedented high SERS detectivity at the single particle level. Here we present an unambiguous and systematic assessment of the SERS performances of the Ag NFs and demonstrate that they provide highly sensitive sensing platforms by single SERS particle. © 2017 IOP Publishing Ltd Printed in the UK1341sciescopu
Observation of Charge Transfer in Heterostructures Composed of MoSe2 Quantum Dots and a Monolayer of MoS2 or WSe2
No full textMonolayer transition metal dichalcogenides (TMDs) are atomically thin semiconductor films that are ideal platforms for the study and engineering of quantum heterostructures for optoelectronic applications. We present a simple method for the fabrication of TMD heterostructures containing MoSe2 quantum dots (QDS) and a MoS2 or WSe2, monolayer. The strong modification of photoluminescence and Raman spectra that includes the quenching of MoSe2 QDs and the varied spectral weights of trions for the MoS2 and WSe2 monolayers were observed, suggesting the charge transfer Occurring in these TMD heterostructures. Such optically active heterostructures, which can be conveniently fabricated by dispersing TMD QDs onto TMD monolayers, are likely to have various nanophotonic applications because of their versatile and controllable properties. © 2017 American Chemical Society91
Highly elastic conductive sponges by joule heat-driven selective polymer reinforcement at reduced graphene oxide junctions
No full text© 2019 Elsevier LtdPolymer reinforcement of reduced graphene oxide (rGO) sponges is widely employed to enhance mechanical strength and elasticity. However, the surplus polymer decreases electrical conductivity by passivating the conductive surface of rGO flakes. Here we firstly report the selective polydimethylsiloxane (PDMS) reinforcement at the flake junction of rGO sponge by the Joule heating process utilizing the high electrical contact resistance. The preferential Joule heating of the junction is theoretically simulated by finite element modeling and experimentally confirmed by micro-thermal infrared imaging. The local temperature increase results in the further reduction of rGO and preferential PDMS curing at the flake junction only. The PDMS/rGO mass ratio was carefully optimized at 3.96. The electrical conductivity (0.087 S m−1 at 0% strain) is more than an order of magnitude higher than that (0.00251 S m−1) of the conventional oven-heated sponge with a similar PDMS/rGO mass ratio. The mechanical strength is equivalent (210.3 kPa at 70% strain), in spite of the preferential polymer coating at the rGO flake junction only, with excellent elasticity. The Joule heating method is an excellent curing strategy to selectively reinforce flake junctions for conductive elastic rGO-polymer sponges11sciescopu
Atomic Observation of Filling Vacancies in Monolayer Transition Metal Sulfides by Chemically Sourced Sulfur Atoms
No full textChemical
treatment using bis(trifluoromethane) sulfonimide (TFSI)
was shown to be particularly effective for increasing the photoluminescence
(PL) of monolayer (1L) MoS2, suggesting a convenient method
for overcoming the intrinsically low quantum yield of this material.
However, the underlying atomic mechanism of the PL enhancement has
remained elusive. Here, we report the microscopic origin of the defect
healing observed in TFSI-treated 1L-MoS2 through a correlative
combination of optical characterization and atomic-scale scanning
transmission electron microscopy, which showed that most of the sulfur
vacancies were directly repaired by the extrinsic sulfur atoms produced
from the dissociation of TFSI, concurrently resulting in a significant
PL enhancement. Density functional theory calculations confirmed that
the reactive sulfur dioxide molecules that dissociated from TFSI can
be reduced to sulfur and oxygen gas at the vacancy site to form strongly
bound SMo. Our results reveal how defect-mediated nonradiative
recombination can be effectively eliminated by a simple chemical treatment
method, thereby advancing the practical applications of monolayer
semiconductors
Local Strain Induced Band Gap Modulation and Photoluminescence Enhancement of Multilayer Transition Metal Dichalcogenides
No full textThe photocarrier relaxation between direct and indirect band gaps along the high symmetry K-Gamma line in the Brillion zone reveals interesting electronic properties of the transition metal dichalcogenides (TMDs) multilayer films. In this study, we reported on the local strain engineering and tuning of an electronic band structure of TMDs multilayer films along the K-Gamma line by artificially creating one-dimensional wrinkle structures. Significant photoluminescence (PL) intensity enhancement in conjunction with continuously tuned optical energy gaps was recorded at the high strain regions. A direct optical band gap along K-K points and an indirect optical gap along Gamma-K points measured from the PL spectra of multilayer samples monotonically decreased as the strain increased, while the indirect band gap along Lambda-Gamma was unaffected owing to the same level of local strain in the range of 0%-2%. The experimental results of band gap tuning were in agreement with the density functional theory calculation results. Local strain modified the band structure in which K-conduction band valley (CBV) was aligned below the Lambda-CBV, and this explained the observed local PL enhancement that made the material indirect via the K-Gamma transition. The study also reported experimental evidence for the funneling of photogenerated excitons toward regions of a higher strain at the top of the wrinkle geometry. © 2017 American Chemical Society7711sciescopu
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
Enhanced Light Emission from Monolayer Semiconductors by Forming Heterostructures with ZnO Thin Films
No full textMonolayer transition-metal dichalcogenides (1L-TMDs) are atomically thin direct band gap semiconductors, from which the emission of light is determined by optical transitions of exciton complexes such as neutral excitons and trions. While the quantum yields of 1L-TMDs are quite low, the ability to control the populations of exciton complexes in 1L-TMDs through various doping processes is an interesting advantage, and provides ample possibilities for engineering the optical properties of these semiconductor monolayers. Here we demonstrate a simple method of controlling the populations of excitons and trions to enhance the light emission of 1L-TMDs by having them form heterostructures with ZnO thin films (TFs). 1Ls of MoS2 or MoSe2 showed up to 17-fold increases in photoluminescence (PL) when they were placed on ∼50 nm thick ZnO TFs. This enhancement of the PL was due to charge exchanges occurring through the 1L-TMD/ZnO interface. The PL enhancements and changes in the PL spectra of the 1L-TMDs were greater when the 1L-TMD/ZnO heterostructures were subjected to 355 nm wavelength laser excitation than when they were excited with a 514 nm wavelength laser, which we attributed to the onset of energy transfer by photoexcited excitons and/or the additional p-doping by photoexcited holes in ZnO. The p-doping phenomenon and the enhanced light emission of 1L-TMD/ZnO heterostructures were unambiguously visualized in spatially resolved PL and Raman spectral maps. Our approach using the 1L-TMD/ZnO TF heterostructure suggests that a rich variety of options for engineering the optical properties of 1L-TMDs may be made available by carrying out simple and intuitive manipulations of exciton complexes, and these endeavors may yield practical applications for 1L-TMDs in nanophotonic devices. © 2016 American Chemical Society110111sciescopu
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