470 research outputs found
Non-radiative resonance energy transfer in bi-polymer nanoparticles of fluorescent conjugated polymers
Cataloged from PDF version of article.his work demonstrates the comparative studies of non-radiative resonance energy transfer in bi-polymer nanoparticles based on fluorescent conjugated polymers. For this purpose, poly[(9,9-dihexylfluorene) (PF) as a donor (D) and poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as an acceptor (A) have been utilized, from which four different bi-polymer nanoparticle systems are designed and synthesized. Both, steady-state fluorescence spectra and time-resolved fluorescence measurements indicate varying energy transfer efficiencies from the host polymer PF to the acceptor polymer MEH-PPV depending on the D-A distances and structural properties of the nanoparticles. The first approach involves the preparation of PF and MEH-PPV nanoparticles separately and mixing them at a certain ratio. In the second approach, first PF and MEH-PPV solutions are mixed prior to nanoparticle formation and then nanoparticles are prepared from the mixture. Third and fourth approaches involve the sequential nanoparticle preparation. In the former, nanoparticles are prepared to have PF as a core and MEH-PPV as a shell. The latter is the reverse of the third in which the core is MEH-PPV and the shell is PF. The highest energy transfer efficiency recorded to be 35% is obtained from the last system, in which a PF layer is sequentially formed on MEH-PPV NPs. © 2010 Optical Society of America
Quantum efficiency enhancement in film by making nanoparticles of polyfluorene
Cataloged from PDF version of article.We report on conjugated polymer nanoparticles of polyfluorene that were formed to exhibit higher fluorescence quantum efficiency in film (68%) and reduce undesired emission peak wavelength shifts in film (by 20 nm), compared to the solid-state polymer thin film made directly out of the same polymer solution without forming nanoparticles. Using the facile reprecipitation method, solutions of poly[9,9-dihexyl-9H-fluorene] in THF were added at different volume ratios to obtain different size distributions of nanoparticle dispersions in water. This allowed us to control the sizedependent optical emission of our polyfluorene nanoparticles. Such organic nanoparticles hold great promise for use as efficient emitters in optoelectronic device applications. (C) 2008 Optical Society of America
Observation of Selective Plasmon-Exciton Coupling in Nonradiative Energy Transfer: Donor-Selective versus Acceptor-Selective
Cataloged from PDF version of article.We report selectively plasmon-mediated nonradiative energy transfer between quantum dot (QD) emitters interacting with each other via Forster-type resonance energy transfer (FRET) under controlled plasmon coupling either to only the donor QDs (i.e., donor-selective) or to only the acceptor QDs (i.e., acceptor-selective). Using layer-by-layer assembled colloidal QD nanocrystal solids with metal nanoparticles integrated at carefully designed spacing, we demonstrate the ability to enable/disable the coupled plasmon-exciton (plexciton) formation distinctly at the donor (exciton departing) site or at the acceptor (exciton feeding) site of our choice, while not hindering the donor exciton-acceptor exciton interaction but refraining from simultaneous coupling to both sites of the donor and the acceptor in the FRET process.. In the case of donor-selective plexciton, we observed a substantial shortening in the donor QD lifetime from 1.33 to 0.29 ns as a result of plasmon-coupling to the donors and the FRET-assisted exciton transfer from the donors to the acceptors, both of which shorten the donor lifetime. This consequently enhanced the acceptor emission by a factor of 1.93. On the other hand, in the complimentary case of acceptor-selective plexciton, we observed a 2.70-fold emission enhancement in the acceptor QDs, larger than the acceptor emission enhancement of the donor-selective plexciton, as a result of the combined effects of the acceptor plasmon coupling and the FRET-assisted exciton feeding. Here we present the comparative results of theoretical modeling of the donor- and acceptor-selective plexcitons of nonradiative energy transfer developed here for the first time, which are in excellent agreement with the systematic experimental characterization. Such an ability to modify and control energy transfer through mastering plexcitons is of fundamental importance, opening up new applications for quantum dot embedded plexciton devices along with the development of new techniques in FRET-based fluorescence microscopy
Rapid and Precise Determination of Zero-Field Splittings by Terahertz Time-Domain Electron Paramagnetic Resonance Spectroscopy
Zero-field splitting (ZFS) parameters are fundamentally tied to the
geometries of metal ion complexes. Despite their critical importance for
understanding the magnetism and spectroscopy of metal complexes, they are not
routinely available through general laboratory-based techniques, and are often
inferred from magnetism data. Here we demonstrate a simple tabletop
experimental approach that enables direct and reliable determination of ZFS
parameters in the terahertz (THz) regime. We report time-domain measurements of
electron paramagnetic resonance (EPR) signals associated with THz-frequency
ZFSs in molecular complexes containing high-spin transition-metal ions. We
measure the temporal profiles of the free-induction decays of spin resonances
in the complexes at zero and nonzero external magnetic fields, and we derive
the EPR spectra via numerical Fourier transformation of the time-domain
signals. In most cases, absolute values of the ZFS parameters are extracted
from the measured zero-field EPR frequencies, and the signs can be determined
by zero-field measurements at two different temperatures. Field-dependent EPR
measurements further allow refined determination of the ZFS parameters and
access to the g-factor. The results show good agreement with those obtained by
other methods. The simplicity of the method portends wide applicability in
chemistry, biology and material science.Comment: 36 pages, 30 figures, 1 tabl
Anisotropic Emission from Multilayered Plasmon Resonator Nanocomposites of Isotropic Semiconductor Quantum Dots
Cataloged from PDF version of article.We propose and demonstrate a nanocomposite localized surface plasmon resonator
embedded into an artificial three-dimensional construction. Colloidal semiconductor quantum dots
are assembled between layers of metal nanoparticles to create a highly strong plasmon-exciton
interaction in the plasmonic cavity. In such a multilayered plasmonic resonator architecture of
isotropic CdTe quantum dots, we observed polarized light emission of 80% in the vertical
polarization with an enhancement factor of 4.4, resulting in a steady-state anisotropy value of
0.26 and reaching the highest quantum efficiency level of 30% ever reported for such CdTe quantum
dot solids. Our electromagnetic simulation results are in good agreement with the experimental
characterization data showing a significant emission enhancement in the vertical polarization, for
which their fluorescence decay lifetimes are substantially shortened by consecutive replication of our
unit cell architecture design. Such strongly plasmon-exciton coupling nanocomposites hold great
promise for future exploitation and development of quantum dot plasmonic biophotonics and
quantum dot plasmonic optoelectronics
White emitting polyfluorene functionalized with azide hybridized on near-UV light emitting diode for high color rendering index
Cataloged from PDF version of article.We develop and demonstrate high-quality white light generation that relies on the use of a single-type simple conjugated polymer of polyfluorene functionalized with azide groups (PFA) integrated on a near-UV LED platform. The high-quality white emission from the polyfluorene is achieved by using the azide functionalization to facilitate cross-linking intentionally when cast into solid-state form. Hybridized on n-UV InGaN/GaN LED at 378 nm, the PFA emitters collectively generate a very broad down-converting photoluminescence at longer wavelengths across the entirety of the visible spectrum, yielding high color rendering indices up to 91. (c) 2008 Optical Society of America
Phase diagram of neutron-rich nuclear matter and its impact on astrophysics
Dense matter as it can be found in core-collapse supernovae and neutron stars
is expected to exhibit different phase transitions which impact the matter
composition and equation of state, with important consequences on the dynamics
of core-collapse supernova explosion and on the structure of neutron stars. In
this paper we will address the specific phenomenology of two of such
transitions, namely the crust-core solid-liquid transition at sub-saturation
density, and the possible strange transition at super-saturation density in the
presence of hyperonic degrees of freedom. Concerning the neutron star
crust-core phase transition at zero and finite temperature, it will be shown
that, as a consequence of the presence of long-range Coulomb interactions, the
equivalence of statistical ensembles is violated and a clusterized phase is
expected which is not accessible in the grand-canonical ensemble. A specific
quasi-particle model will be introduced to illustrate this anomalous
thermodynamics and some quantitative results relevant for the supernova
dynamics will be shown. The opening of hyperonic degrees of freedom at higher
densities corresponding to the neutron stars core modifies the equation of
state. The general characteristics and order of phase transitions in this
regime will be analyzed in the framework of a self-consistent mean-field
approach.Comment: Invited Talk given at the 11th International Conference on
Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1,
2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference
Series (JPCS
Stability window and mass-radius relation for magnetized strange quark stars
The stability of magnetized strange quark matter (MSQM) is investigated
within the phenomenological MIT bag model, taking into account the variation of
the relevant input parameters, namely, the strange quark mass, baryon density,
magnetic field and bag parameter. We obtain that the energy per baryon
decreases as the magnetic field increases, and its minimum value at vanishing
pressure is lower than the value found for SQM. This implies that MSQM is more
stable than non-magnetized SQM. Furthermore, the stability window of MSQM is
found to be wider than the corresponding one of SQM. The mass-radius relation
for magnetized strange quark stars is also derived in this framework.Comment: 12 pages, 6 figures, 3 table
Neutron rich matter, neutron stars, and their crusts
Neutron rich matter is at the heart of many fundamental questions in Nuclear
Physics and Astrophysics. What are the high density phases of QCD? Where did
the chemical elements come from? What is the structure of many compact and
energetic objects in the heavens, and what determines their electromagnetic,
neutrino, and gravitational-wave radiations? Moreover, neutron rich matter is
being studied with an extraordinary variety of new tools such as Facility for
Rare Isotope Beams (FRIB) and the Laser Interferometer Gravitational Wave
Observatory (LIGO). We describe the Lead Radius Experiment (PREX) that is using
parity violation to measure the neutron radius in 208Pb. This has important
implications for neutron stars and their crusts. Using large scale molecular
dynamics, we model the formation of solids in both white dwarfs and neutron
stars. We find neutron star crust to be the strongest material known, some 10
billion times stronger than steel. It can support mountains on rotating neutron
stars large enough to generate detectable gravitational waves. Finally, we
describe a new equation of state for supernova and neutron star merger
simulations based on the Virial expansion at low densities, and large scale
relativistic mean field calculations.Comment: 10 pages, 2 figures, Plenary talk International Nuclear Physics
Conference 2010, Vancouver, C
Comparative study of electroabsorption in InGaN/GaN quantum zigzag heterostructures with polarization-induced electric fields
Cataloged from PDF version of article.We present a comparative study on InGaN/GaN quantum zigzag structures embedded in p-i-n diode architecture that exhibit blue-shifting electroabsorption in the blue when an electric field is externally applied to compensate for the polarization-induced electric field across the wells. With the polarization breaking their symmetry, the same InGaN/GaN quantum structures redshift their absorption edge when the external field is applied in the same direction as the well polarization. Both computationally and experimentally, we investigate the effects of polarization on electroabsorption by varying compositional content and structural parameters and demonstrate that electroabsorption grows stronger with weaker polarization in these multiple quantum well modulators. (c) 2008 American Institute of Physics
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