371 research outputs found
Optical Transitions in Single-Wall Boron Nitride Nanotubes
Optical transitions in single-wall boron nitride nanotubes are investigated by means of optical absorption spectroscopy. Three absorption lines are observed. Two of them (at 4.45 and 5.5 eV) result from the quantification involved by the rolling up of the hexagonal boron nitride (h-BN) sheet. The nature of these lines is discussed, and two interpretations are proposed. A comparison with single-wall carbon nanotubes leads one to interpret these lines as transitions between pairs of van Hove singularities in the one-dimensional density of states of boron nitride single-wall nanotubes. But the confinement energy due to the rolling up of the h-BN sheet cannot explain a gap width of the boron nitride nanotubes below the h-BN gap. The low energy line is then attributed to the existence of a Frenkel exciton with a binding energy in the 1 eV range
Proyecto de consultorĂa integral en una empresa dedicada a la organizaciĂłn de eventos en el municipio de Zapopan
ITESO, A.C
Ventricular fusion during auricular entrainment of idiopathic fascicular tachycardias. Implications on ventricular tachycardia circuit
Tuning complex shapes in Pt(0) nanoparticles : from cubic dendrites to five-fold stars
A platinum star performance: Quasi-single-crystalline Pt nanoparticles with peculiar morphologiesâcubic dendrites, planar tripods, and fivefold starsâwere synthesized in high yield. Shape selectivity was achieved by finely tuning the growth kinetics under a dihydrogen atmosphere
Graphene oxide: key to efficient charge extraction and suppression of polaronic transport in hybrids with poly (3-hexylthiophene) nanoparticles
Nanoparticles (NPs) of conjugated polymers in intimate contact with sheets of graphene oxide (GO) constitute a promising class of water-dispersible nanohybrid materials of increased interest for the design of sustainable and improved optoelectronic thin-film devices, revealing properties exclusively pre-established upon their liquid-phase synthesis. In this context, we report for the first time the preparation of a P3HTNPsâGO nanohybrid employing a miniemulsion synthesis approach, whereby GO sheets dispersed in the aqueous phase serve as a surfactant. We show that this process uniquely favors a quinoid-like conformation of the P3HT chains of the resulting NPs well located onto individual GO sheets. The accompanied change in the electronic behavior of these P3HTNPs, consistently confirmed by the photoluminescence and Raman response of the hybrid in the liquid and solid states, respectively, as well as by the properties of the surface potential of isolated individual P3HTNPsâGO nano-objects, facilitates unprecedented charge transfer interactions between the two constituents. While the electrochemical performance of nanohybrid films is featured by fast charge transfer processes, compared to those taking place in pure P3HTNPs films, the loss of electrochromic effects in P3HTNPsâGO films additionally indicates the unusual suppression of polaronic charge transport processes typically encountered in P3HT. Thus, the established interface interactions in the P3HTNPsâGO hybrid enable a direct and highly efficient charge extraction channel via GO sheets. These findings are of relevance for the sustainable design of novel high-performance optoelectronic device structures based on water-dispersible conjugated polymer nanoparticles
Light-induced sulfur transport inside single-walled carbon nanotubes
This article belongs to the Section 2D and Carbon Nanomaterials.Filling of single-walled carbon nanotubes (SWCNTs) and extraction of the encapsulated species from their cavities are perspective treatments for tuning the functional properties of SWCNT-based materials. Here, we have investigated sulfur-modified SWCNTs synthesized by the ampoule method. The morphology and chemical states of carbon and sulfur were analyzed by transmission electron microscopy, Raman scattering, thermogravimetric analysis, X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopies. Successful encapsulation of sulfur inside SWCNTs cavities was demonstrated. The peculiarities of interactions of SWCNTs with encapsulated and external sulfur species were analyzed in details. In particular, the donorâacceptor interaction between encapsulated sulfur and host SWCNT is experimentally demonstrated. The sulfur-filled SWCNTs were continuously irradiated in situ with polychromatic photon beam of high intensity. Comparison of X-ray spectra of the samples before and after the treatment revealed sulfur transport from the interior to the surface of SWCNTs bundles, in particular extraction of sulfur from the SWCNT cavity. These results show that the moderate heating of filled nanotubes could be used to de-encapsulate the guest species tuning the local composition, and hence, the functional properties of SWCNT-based materials.This work was supported by the Russian Science Foundation (Project 18-72-00017), the bilateral Program âRussian-Germany Laboratory at BESSY IIâ in the part of XPS and C K-edge NEXAFS measurements, and shared research center SSTRC on the basis of the Novosibirsk VEPP-4 - VEPP-2000 complex at BINP SB RAS, using equipment supported by project RFMEFI62119X0022 in the part of S K-edge NEXAFS measurements. R.A. acknowledges the support from the Spanish Ministerio de Economia y Competitividad (MAT2016-79776-P, AEI/FEDER, EU), from the European Unionâs Horizon 2020 programme under the project âESTEEM3â (823717) and from the Government of Aragon and the European Social Fund under the project âConstruyendo Europa desde Aragonâ 2014â2020 (grant number E13_17R, FEDER, EU).Peer reviewe
HD 51844: An Am delta Scuti in a binary showing periastron brightening
Pulsating stars in binary systems are ideal laboratories to test stellar
evolution and pulsation theory, since a direct, model-independent determination
of component masses is possible. The high-precision CoRoT photometry allows a
detailed view of the frequency content of pulsating stars, enabling detection
of patterns in their distribution. The object HD 51844 is such a case showing
periastron brightening instead of eclipses. We present a comprehensive study of
the HD 51844 system, where we derive physical parameters of both components,
the pulsation content and frequency patterns. Additionally, we obtain the
orbital elements, including masses, and the chemical composition of the stars.
Time series analysis using standard tools was mployed to extract the pulsation
frequencies. Photospheric abundances of 21 chemical elements were derived by
means of spectrum synthesis. We derived orbital elements both by fitting the
observed radial velocities and the light curves, and we did asteroseismic
modelling as well. We found that HD 51844 is a double lined spectroscopic
binary. The determined abundances are consistent with delta Delphini
classification. We determined the orbital period (33.498 +- 0.002 d), the
eccentricity (0.484 +- 0.020), the mass ratio (0.988 +- 0.02), and the masses
to 2.0 +- 0.2 M_sun for both components. Only one component showed pulsation.
Two p modes (f_22 and f_36) and one g mode (f_orb) may be tidally excited.
Among the 115 frequencies, we detected triplets due to the frequency
modulation, frequency differences connected to the orbital period, and
unexpected resonances (3:2, 3:5, and 3:4), which is a new discovery for a delta
Scuti star.Comment: 15 pages, 11 figures, accepted for publication in A&
BN domains included into carbon nanotubes: role of interface
We present a density functional theory study on the shape and arrangement of
small BN domains embedded into single-walled carbon nanotubes. We show a strong
tendency for the BN hexagons formation at the simultaneous inclusion of B and N
atoms within the walls of carbon nanotubes. The work emphasizes the importance
of a correct description of the BN-C frontier. We suggest that BN-C interface
will be formed preferentially with the participation of N-C bonds. Thus, we
propose a new way of stabilizing the small BN inclusions through the formation
of nitrogen terminated borders. The comparison between the obtained results and
the available experimental data on formation of BN plackets within the single
walled carbon nanotubes is presented. The mirror situation of inclusion of
carbon plackets within single walled BN nanotubes is considered within the
proposed formalism. Finally, we show that the inclusion of small BN plackets
inside the CNTs strongly affects the electronic character of the initial
systems, opening a band gap. The nitrogen excess in the BN plackets introduces
donor states in the band gap and it might thus result in a promising way for
n-doping single walled carbon nanotubes
Spectroscopic survey of Kepler stars. I. HERMES/Mercator observations of A- and F-type stars
The Kepler space mission provided near-continuous and high-precision photometry of about 207 000 stars, which can be used for asteroseismology. However, for successful seismic modeling it is equally important to have accurate stellar physical parameters. Therefore, supplementary ground-based data are needed. We report the results of the analysis of high-resolution spectroscopic data of A- and F-type stars from the Kepler field, which were obtained with the HERMES spectrograph on the Mercator telescope. We determined spectral types, atmospheric parameters and chemical abundances for a sample of 117 stars. Hydrogen Balmer, Feâi, and Feâii lines were used to derive effective temperatures, surface gravities, and microturbulent velocities. We determined chemical abundances and projected rotational velocities using a spectrum synthesis technique. The atmospheric parameters obtained were compared with those from the Kepler Input Catalogue (KIC), confirming that the KIC effective temperatures are underestimated for A stars. Effective temperatures calculated by spectral energy distribution fitting are in good agreement with those determined from the spectral line analysis. The analysed sample comprises stars with approximately solar chemical abundances, as well as chemically peculiar stars of the Am, Ap, and λ Boo types. The distribution of the projected rotational velocity, vsinâi, is typical for A and F stars and ranges from 8 to about 280 km sâ1, with a mean of 134 km sâ1
Spectroscopic survey of Kepler stars. I. HERMES/Mercator observations of A- and F-type stars
The Kepler space mission provided near-continuous and high-precision photometry of about 207000 stars, which can be used for asteroseismology. However, for successful seismic modeling it is equally important to have accurate stellar physical parameters. Therefore, supplementary ground-based data are needed. We report the results of the analysis of high-resolution spectroscopic data of A- and F-type stars from the Kepler field, which were obtained with the HERMES spectrograph on the Mercator telescope. We determined spectral types, atmospheric parameters and chemical abundances for a sample of 117 stars. Hydrogen Balmer, Feâi, and Feâii lines were used to derive effective temperatures, surface gravities, and microturbulent velocities. We determined chemical abundances and projected rotational velocities using a spectrum synthesis technique. The atmospheric parameters obtained were compared with those from the Kepler Input Catalogue (KIC), confirming that the KIC effective temperatures are underestimated for A stars. Effective temperatures calculated by spectral energy distribution fitting are in good agreement with those determined from the spectral line analysis. The analysed sample comprises stars with approximately solar chemical abundances, as well as chemically peculiar stars of the Am, Ap, and λBoo types. The distribution of the projected rotational velocity, vsinâi, is typical for A and F stars and ranges from 8 to about 280kmsâ1, with a mean of 134kmsâ
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