Temperature dependence of exciton recombination in semiconducting single-wall carbon nanotubes

We study the excitonic recombination dynamics in an ensemble of (9,4) semiconducting single-wall carbon nanotubes by high sensitivity time-resolved photo-luminescence experiments. Measurements from cryogenic to room temperature allow us to identify two main contributions to the recombination dynamics. The initial fast decay is temperature independent and is attributed to the presence of small residual bundles that create external non-radiative relaxation channels. The slow component shows a strong temperature dependence and is dominated by non-radiative processes down to 40 K. We propose a quantitative phenomenological modeling of the variations of the integrated photoluminescence intensity over the whole temperature range. We show that the luminescence properties of carbon nanotubes at room temperature are not affected by the dark/bright excitonic state coupling

Reducible valgus flat-foot: Assessment of posterior subtalar joint surface displacement by posterior arthroscopy during sinus tarsi expansion screwing

AbstractIntroductionSubtalar arthroereisis corrects childhood and adult reducible valgus flat-foot in certain indications. Inserting an expansion screw in the sinus tarsi simultaneously corrects the calcaneal valgus of the talocalcaneal divergence and first-ray pronation if these are reducible. The displacement induced in the posterior subtalar joint (decoaptation, translation, rotation) is, however, poorly known. The present study involved arthroscopic assessment of posterior subtalar joint surface displacement during insertion of a talocalcaneal arthroereisis screw, with the hypothesis that displacement varies in three dimensions according to screw size.Material and methodEight specimens were used for the study. All ankles were supple, taken from adult subjects. A 4.5-mm arthroscope was used and measurements were taken with a graduated palpator in the posterior subtalar joint. Three sinus tarsi expansion screws of incremental diameter were assessed. Before and after insertion measurements were made of posterolateral and posteromedial talar exposure on the calcaneus, anteroposterior and lateromedial translation, and talocalcaneal joint-line opening.ResultsMedial rotation, varization and anterior translation of the calcaneus were comparable in all cases. Mean lateral opening of the posterior subtalar joint was 0.88mm with 8-mm screws and 1.25mm with 16-mm screws. Significant differences between 8 and 16mm screws were found for lateral subtalar joint opening (P=0.028) and for lateromedial translation (P=0.004).ConclusionSinus tarsi expansion screwing corrects hindfoot valgus and talocalcaneal divergence by inducing medial translation of the calcaneus under the talus and talar medial rotation and varization, proportional to screw size (medial translation and lateral opening of the subtalar joint).Level of evidenceIII

Exciton Spin Dynamics in Semiconductor Quantum Wells

In this paper we will review Exciton Spin Dynamics in Semiconductor Quantum Wells. The spin properties of excitons in nanostructures are determined by their fine structure. We will mainly focus in this review on GaAs and InGaAs quantum wells which are model systems.Comment: 55 pages, 27 figure

Excitonic recombination dynamics in shallow quantum wells

We report a comprehensive study of carrier-recombination dynamics in shallow AlxGa1-xAs/GaAs quantum wells. At low crystal temperature (2 K), the excitonic radiative recombination time is shown to be strongly enhanced in shallow quantum wells with x>0.01, consistently with a model that takes into account the thermal equilibrium between the three-dimensional exciton gas of the barrier and the two-dimensional exciton gas, which are closer in energy as x decreases. Furthermore, we demonstrate the existence of a thermally activated escape mechanism due to the low effective barrier height in these structures. The nonradiative recombination is shown to dominate the carrier dynamics for temperatures as low as 10 K for x≈0.01. Our experimental observations are analyzed using three different variational exciton calculations. In particular, we study the crossover from the two-dimensional to the three-dimensional behavior of the exciton, which occurs for x as low as 0.01 and affects mainly the oscillator strength, whereas the transition energies in shallow quantum wells can be calculated, to a large extent, using the same approximations as for conventional quantum wells. The peculiar behavior of the oscillator strength at the crossover to the weak confinement regime is obtained by expansion in a large basis

The big problem of small particles : a comparison of methods for determination of particle size in nanocrystalline anatase powders

We compare different methods for particle size determination in nanocrystalline anatase (TiO2) powders: transmission electron microscopy (TEM), nitrogen adsorption measurements, mercury porosimetry, and X-ray diffraction(XRD). The main source of errors in TEM is the sampling of the powder population, whereas in XRD the deconvolution of peak broadening due to instrument, microstrains, and crystalline domain size is delicate and can lead to unreliable results. Different approaches including Scherrer and Williamson-Hall equations are discussed. The presence of mesopores due to agglomerate formation is clearly revealed in adsorption measurements and porosimetry