132 research outputs found
Superradiance of J-aggregates: correspondence between infinite disordered chain and regular chain of finite length
The radiation decay of excitons in a slightly disordered molecule chain is considered. The degree of disordering dictates the excitonic state length that determines the enhancement of the decay rate Г (superradiance). Г was calculated versus disordering degree and temperature and compared with the case of a regular chain where the length of excitonic state is determined by the chain length, N. For every degree of disordering, the value of N was found that provides practically the same decay rate as a function of temperature
Kinetics of cluster growth in expanding rare-gas jet
The growth of clusters in an overcooled atomic jet is calculated with the simultaneous allowance for two processes: the condensation of supersaturated gas and the cluster-cluster aggregation. These processes are consistently described within a quasi-equilibrium approach for large enough clusters, while their nucleation stage is allowed for phenomenologically by the use of a fitting parameter. The theory quantitatively describes the experimental dependence of the mean cluster size on the initial gas temperature within a broad range of sizes. The mean size of clusters, their distribution over sizes and the contribution of clusters to the total mass of the jet are calculated versus the distance from the gas ejection point (nozzle entry). It is shown that these characteristics are mainly determined by the initial gas entropy which is equivalent to the empirical law of corresponding jets. The size distribution function has a halfwidth exceeding the mean cluster size and is close (on a relative scale) to an universal curve. The broad size distribution of clusters should be taken into account when analyzing spectroscopic data on the size dependence of electronic processes in a cluster
Scaling analysis of electron transport through metal-semiconducting carbon nanotube interfaces: Evolution from the molecular limit to the bulk limit
We present a scaling analysis of electronic and transport properties of
metal-semiconducting carbon nanotube interfaces as a function of the nanotube
length within the coherent transport regime, which takes fully into account
atomic-scale electronic structure and three-dimensional electrostatics of the
metal-nanotube interface using a real-space Green's function based
self-consistent tight-binding theory. As the first example, we examine devices
formed by attaching finite-size single-wall carbon nanotubes (SWNT) to both
high- and low- work function metallic electrodes through the dangling bonds at
the end. We analyze the nature of Schottky barrier formation at the
metal-nanotube interface by examining the electrostatics, the band lineup and
the conductance of the metal-SWNT molecule-metal junction as a function of the
SWNT molecule length and metal-SWNT coupling strength. We show that the
confined cylindrical geometry and the atomistic nature of electronic processes
across the metal-SWNT interface leads to a different physical picture of band
alignment from that of the planar metal-semiconductor interface. We analyze the
temperature and length dependence of the conductance of the SWNT junctions,
which shows a transition from tunneling- to thermal activation-dominated
transport with increasing nanotube length. The temperature dependence of the
conductance is much weaker than that of the planar metal-semiconductor
interface due to the finite number of conduction channels within the SWNT
junctions. We find that the current-voltage characteristics of the metal-SWNT
molecule-metal junctions are sensitive to models of the potential response to
the applied source/drain bias voltages.Comment: Minor revision to appear in Phys. Rev. B. Color figures available in
the online PRB version or upon request to: [email protected]
Rate-equation calculations of the current flow through two-site molecular device and DNA-based junction
Here we present the calculations of incoherent current flowing through the
two-site molecular device as well as the DNA-based junction within the
rate-equation approach. Few interesting phenomena are discussed in detail.
Structural asymmetry of two-site molecule results in rectification effect,
which can be neutralized by asymmetric voltage drop at the molecule-metal
contacts due to coupling asymmetry. The results received for poly(dG)-poly(dC)
DNA molecule reveal the coupling- and temperature-independent saturation effect
of the current at high voltages, where for short chains we establish the
inverse square distance dependence. Besides, we document the shift of the
conductance peak in the direction to higher voltages due to the temperature
decrease.Comment: 12 pages, 6 figure
Siberian Snake and Depolarizing Resonance Studies at the Cooler Ring
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
Green function techniques in the treatment of quantum transport at the molecular scale
The theoretical investigation of charge (and spin) transport at nanometer
length scales requires the use of advanced and powerful techniques able to deal
with the dynamical properties of the relevant physical systems, to explicitly
include out-of-equilibrium situations typical for electrical/heat transport as
well as to take into account interaction effects in a systematic way.
Equilibrium Green function techniques and their extension to non-equilibrium
situations via the Keldysh formalism build one of the pillars of current
state-of-the-art approaches to quantum transport which have been implemented in
both model Hamiltonian formulations and first-principle methodologies. We offer
a tutorial overview of the applications of Green functions to deal with some
fundamental aspects of charge transport at the nanoscale, mainly focusing on
applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references,
submitted to Springer series "Lecture Notes in Physics
Ion conducting and paramagnetic d-PCL(530)/siloxane-based biohybrids doped with Mn 2+ ions
Amorphous α,ω-hidroxylpoly(ε-caprolactone) (PCL(530))/siloxane ormolytes doped
with manganese perchlorate (Mn(ClO4)2) (d-PCL(530)/siloxanenMn(ClO4)2) with n =
20, 50, and 100), thermally stable up to at least 200 ºC, were synthesized by the sol-gel
method. Ionic conductivity values up to 4.8×10−8 and 2.0×10−6 S cm−1 at about 25 and
100 ºC, respectively, where obtained for n = 20. FT-IR data demonstrated that the
hydrogen bonding interactions present in the non-doped d-PCL(530)/siloxane host
hybrid matrix were significantly influenced by the inclusion of Mn(ClO4)2 which
promoted the formation of more oxyethylene/urethane and urethane/urethane
aggregates. In addition, the Mn2+ ions bonded to all the “free” C=O groups of the
urethane cross-links and to some of the “free” ester groups of the amorphous PCL(530)
chains. In the electrolytes, the ClO4
− ions were found “free” and bonded to the Mn2+
ions along a bidentate configuration. The magnitude of the electron paramagnetic
resonance (EPR) hyperfine constant of the analyzed samples (A ≈ 90×10-4
cm−1
)
suggested that the bonding between Mn2+ ions and the surrounding ligands is
moderately ionic. The synthetized d-PCL(530)/siloxanenMn(ClO4)2 biohybrids have
potential application in paramagnetic, photoelectrochemical and electrochromic devices.This work was supported by Fundacao para a Ciencia e a Tecnologia (FCT) and Feder (contracts PTDC/CTM-BPC/112774/2009, PEst-OE/QUI/UI0616/2014 and PEst-C/QUI/UI0686/2013) and COST Action MP1202 "Rational design of hybrid organic-inorganic interfaces". R.F.P.P. acknowledges FCT for a grant (SFRH/BPD/87759/2012). M.M.S. acknowledges CNPq (PVE grant 406617/2013-9), for a mobility grant. The financial support of the Brazilian agencies Capes and CNPq are gratefully acknowledged. Research was partially financed by the CeRTEV, Center for Research, Technology and Education in Vitreous Materials, FAPESP 2013/07793-6.info:eu-repo/semantics/publishedVersio
Synthesis and characterization of polypyrrole decorated graphene/β-cyclodextrin composite for low level electrochemical detection of mercury (II) in water
Mercury (Hg(II)) is considered as one of the most toxic element that directly affects the human health and the environment. Therefore, in this study, we propose a sensitive and disposable electrochemical sensor for the detection of Hg(II) in various water samples using polypyrrole (PPy) decorated graphene/-cyclodextrin (GR-CD) composite modified screen-printed carbon electrode (SPCE). The GRCD/PPy composite was synthesized by chemical oxidation of PPy monomer in GR-CD solution using FeCl3. Differential pulse voltammetry (DPV) is used for the detection of Hg(II) and the DPV results reveal that GR-CD/PPy composite modified SPCE has high sensitivity towards Hg(II) than bare, GR, GR-CD and PPy modified SPCEs. The optimization studies such as effect of pH, accumulating time and effect of scanning potential towards the detection of Hg(II) were investigated. The GR-CD/PPy composite modified SPCE could detect the Hg(II) up to 51.56 M L−1 with the limit of detection (LOD) of 0.47 nM L−1. The obtained LOD was well below the guideline level of Hg(II) set by the World’s Health Organization (WHO) and U.S. Environmental Protection Agency (EPA). In addition, the fabricated GR-CD/PPy composite modified SPCE selectively detected the Hg(II) in the presence of potentially interfering metal cations
Assessing associations between the AURKAHMMR-TPX2-TUBG1 functional module and breast cancer risk in BRCA1/2 mutation carriers
While interplay between BRCA1 and AURKA-RHAMM-TPX2-TUBG1 regulates mammary epithelial polarization, common genetic variation in HMMR (gene product RHAMM) may be associated with risk of breast cancer in BRCA1 mutation carriers. Following on these observations, we further assessed the link between the AURKA-HMMR-TPX2-TUBG1 functional module and risk of breast cancer in BRCA1 or BRCA2 mutation carriers. Forty-one single nucleotide polymorphisms (SNPs) were genotyped in 15,252 BRCA1 and 8,211 BRCA2 mutation carriers and subsequently analyzed using a retrospective likelihood appr
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