198 research outputs found
Size-dependent fine-structure splitting in self-organized InAs/GaAs quantum dots
A systematic variation of the exciton fine-structure splitting with quantum
dot size in single InAs/GaAs quantum dots grown by metal-organic chemical vapor
deposition is observed. The splitting increases from -80 to as much as 520
eV with quantum dot size. A change of sign is reported for small quantum
dots. Model calculations within the framework of eight-band k.p theory and the
configuration interaction method were performed. Different sources for the
fine-structure splitting are discussed, and piezoelectricity is pinpointed as
the only effect reproducing the observed trend.Comment: 5 pages, 5 figure
Control of fine-structure splitting and excitonic binding energies in selected individual InAs/GaAs quantum dots
A systematic study of the impact of annealing on the electronic properties of
single InAs/GaAs quantum dots (QDs) is presented. Single QD cathodoluminescence
spectra are recorded to trace the evolution of one and the same QD over several
steps of annealing. A substantial reduction of the excitonic fine-structure
splitting upon annealing is observed. In addition, the binding energies of
different excitonic complexes change dramatically. The results are compared to
model calculations within eight-band k.p theory and the configuration
interaction method, suggesting a change of electron and hole wave function
shape and relative position.Comment: 4 pages, 4 figure
Charge-induced conformational changes of dendrimers
We study the effect of chargeable monomers on the conformation of dendrimers
of low generation by computer simulations, employing bare Coulomb interactions.
The presence of the latter leads to an increase in size of the dendrimer due to
a combined effect of electrostatic repulsion and the presence of counterions
within the dendrimer, and also enhances a shell-like structure for the monomers
of different generations. In the resulting structures the bond-length between
monomers, especially near the center, will increase to facilitate a more
effective usage of space in the outer-regions of the dendrimer.Comment: 7 pages, 12 figure
Modification with alkyl chains and the influence on thermal and mechanical properties of aromatic hyperbranched polyesters
Soft Interaction Between Dissolved Dendrimers: Theory and Experiment
Using small-angle neutron scattering and liquid integral equation theory, we
relate the structure factor of flexible dendrimers of 4th generation to their
average shape. The shape is measured as a radial density profile of monomers
belonging to a single dendrimer. From that, we derive an effective interaction
of Gaussian form between pairs of dendrimers and compute the structure factor
using the hypernetted chain approximation. Excellent agreement with the
corresponding experimental results is obtained, without the use of adjustable
parameters. The present analysis thus strongly supports the previous finding
that flexible dendrimers of low generation present fluctuating structures akin
to star polymers.Comment: 20 pages, 4 figures, submitted to Macromolecules on July 24, 200
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Comparison of nanotubes produced by fixed bed and aerosol-CVD methods and their electrical percolation behaviour in melt mixed polyamide 6.6 composites
The electrical percolation behaviour of five different kinds of carbon nanotubes (CNTs) synthesised by two CVD techniques was investigated on melt mixed composites based on an insulating polyamide 6.6 matrix. The electrical percolation behaviour was found to be strongly dependent on the properties of CNTs which varied with the synthesis conditions. The lowest electrical percolation threshold (0.04 wt.%) was determined for as grown multi-walled carbon nanotubes without any purification or chemical treatment. Such carbon nanotubes were synthesised by the aerosol method using acetonitrile as ferrocene containing solvent and show relatively low oxygen content near the surface, high aspect ratio, and good dispersability. Similar properties could be found for nanotubes produced by the aerosol method using cyclohexane, whereas CNTs produced by the fixed bed method using different iron contents in the catalyst material showed much higher electrical percolation thresholds between 0.35 and 1.02 wt.%. © 2009 Elsevier Ltd. All rights reserved
Electronic features of cotton fabric e-textiles prepared with aqueous carbon nanofiber inks
Cotton woven fabrics functionalized with aqueous inks made with carbon nanofibers (CNFs) and anionic surfactant are prepared via dip-coating followed by heat treatment, and their electronic properties are discussed. The e-textiles prepared with the inks made with the highest amount of CNFs (6.4 mg mL−1 ) show electrical conductivities (σ) of ∼35 S m−1 and a negative Seebeck (S) of −6 μV K−1 at 30 °C, which means that their majority carriers are electrons. The σ(T) of the e-textiles from 30 to 100 °C shows a negative temperature effect, interpreted as a thermally activated hopping mechanism across a random network of potential wells by means of the 3D variable range hopping (VRH) model. Likewise, their S(T) from 30 to 100 °C shows a negative temperature effect, conveniently depicted by the same model proposed for describing the negative Seebeck of doped multiwall carbon nanotube mats. From this model, it is deduced that the cause of the negative Seebeck in the e-textiles may arise from the contribution of the impurities found in the as-received CNFs, which cause sharply varying and localized states at approximately 0.085 eV above their Fermi energy level (EF). Moreover, the possibility of a slight n-doping from the cellulose fibers of the fabrics and the residuals of the anionic surfactant onto the most external CNF graphitic shells present in the e-textiles is also discussed with the help of the σ(T) and S(T) analysis.This research was funded by the project UID/CTM/00264/2021 of 2C2T under the COMPETE and FCT/MCTES
(PIDDAC) cofinanced by FEDER through the PT2020 program. E.M. acknowledges financial support from ANID
Anillo ACT/192023 and Fondecyt No 1190361
Thermoelectric properties of polypropylene carbon nanofiber melt-mixed composites: exploring the role of polymer on their Seebeck coefficient
The effect of polypropylene (PP) on the Seebeck coefficient (S) of carbon nanofibers (CNFs) in melt-extruded PP composites filled with up to 5 wt. % of CNFs was analyzed in this study. The as-received CNFs present an electrical conductivity of ~320 S m−1 and an interesting phenomenon of showing negative S-values of −5.5 μVK−1, with 10−2 µW/mK2 as the power factor (PF). In contrast, the PP/CNF composites with 5 wt. % of CNFs showed lower conductivities of ~50 S m−1, less negative S-values of −3.8 μVK−1, and a PF of 7 × 10−4 µW/mK2. In particular, the change in the Seebeck coefficient of the PP/CNF composites is explained by a slight electron donation from the outer layers of the CNFs to the PP molecules, which could reduce the S-values of the as-received CNFs. Our study indicates that even insulating polymers such as PP may have a quantifiable effect on the intrinsic Seebeck coefficient of carbon-based nanostructures, and this fact should also be taken into consideration to tailor conductive polymer composites with the desired thermoelectric (TE) properties.The authors affiliated with 2C2T acknowledge
support from FCT-Foundation for Science and Technology within the
scope of project UID/CTM/00264/2020. In addition, support through project IF/
A. J. Paleo et al.
00894/2015 and within the scope of the project CICECO-Aveiro
Institute of Materials, UIDB/50011/2020 and UIDP/50011/2020 and
access to the Navigator platform (LCA-UC) through the Advanced
Computing Project CPCA/A2/2524/2020, financed by national funds
through the Portuguese Foundation for Science and Technology I.P./
MCTES, is gratefully acknowledged
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