Emission properties of colloidal quantum dots on polyelectrolyte multilayers

We present steady state and time-resolved photoluminescence (PL) characteristics of differently charged CdTe quantum dots (QDs) adsorbed onto a polyelectrolyte (PE) multilayer. The PE multilayer is built up using a layer-by-layer assembly technique. We find that the diffusion of the QDs into the PE multilayer is an important factor in the case of 3-mercapto-1, 2-propanediol stabilized QDs (neutral surface charge), resulting in a ～31-fold enhancement in PL intensity accompanied by a blue shift in the PL spectra and an increase in decay lifetime from 3.74?ns to a maximum of 11.65?ns. These modified emission properties are attributed to the enhanced surface related emission resulting from the interaction of the QD's surface with the PE. We find that diffusion does not occur for thioglycolic acid (TGA) stabilized QDs (negative surface charge) or 2-mercaptoethylamine stabilized QDs (positive surface charge), indicating localization of the QDs on top of the PE multilayer. However, the PL lifetime of the TGA stabilized QDs decreases from 9.58 to 5.78?ns with increasing PE multilayer thickness. This provides evidence for increased intrinsic exciton recombination relative to surface related emission, which results in an overall reduction in the average lifetime. Our studies indicate the importance of the QD surface charge in determining the interaction with the PE multilayers and the subsequent modification of the QD emission properties.     

Theoretical limit of localized surface plasmon resonance sensitivity to local refractive index change and its comparison to conventional surface plasmon resonance sensor

In this paper, the theoretical sensitivity limit of the localized surface plasmon resonance (LSPR) to the surrounding dielectric environment is discussed. The presented theoretical analysis of the LSPR phenomenon is based on perturbation theory. Derived results can be further simplified assuming quasistatic limit. The developed theory shows that LSPR has a detection capability limit independent of the particle shape or arrangement. For a given structure, sensitivity is directly proportional to the resonance wavelength and depends on the fraction of the electromagnetic energy confined within the sensing volume. This fraction is always less than unity; therefore, one should not expect to find an optimized nanofeature geometry with a dramatic increase in sensitivity at a given wavelength. All theoretical results are supported by finite-difference time-domain calculations for gold nanoparticles of different geometries (rings, split rings, paired rings, and ring sandwiches). Numerical sensitivity calculations based on the shift of the extinction peak are in good agreement with values estimated by perturbation theory. Numerical analysis shows that, for thin (≤10 nm) analyte layers, sensitivity of the LSPR is comparable with a traditional surface plasmon resonance sensor and LSPR has the potential to be significantly less sensitive to temperature fluctuations.     

Cycling stability of RNi5 (R = La,La+Ce) hydrides during the operation of metal hydride hydrogen compressor

This work presents results of the experimental studies (XRD, SEM, PCT) of hydride forming intermetallides used in the first (LaNi₅) and the second (La_0.5Ce_0.5Ni₅) stages of industrial-scale metal hydride hydrogen compressor providing H₂ compression from 3.5 to 150 atm with the productivity about 10 Nm³/h. During the operation, both materials underwent 18,180 hydrogenation/dehydrogenation (h/d) cycles which included H₂ absorption at the pressure of 3.5 atm (LaNi₅) and 35–38 atm (La_0.5Ce_0.5Ni₅) at T = 15–20 °C followed by H₂ desorption at the pressure of 35–38 atm (LaNi₅) and 150 atm (La_0.5Ce_0.5Ni₅) at T = 150–160 °C. It was found that the observed ～30% drop of the productivity of the compressor by the end of its operation is associated with a degradation of the first stage hydride material (LaNi₅) under conditions specified above. The cycling resulted in the appearance of Ni and LaH_2+x phases in addition to the parent intermetallide. In turn, the cycled LaNi₅ exhibited more than 20% lower hydrogen storage capacity than the alloy at the beginning of the cycling; the cycling was also found to result in a noticeable sloping of initially flat plateau. Conversely, the degradation effects in La_0.5Ce_0.5Ni₅ were found to be much less pronounced, in spite of the higher operating H₂ pressures. The observed effect was associated with the decrease of thermodynamic driving force (TDF) of AB₅ disproportionation in H₂ when substituting La with Ce.     

Influence of Landau level mixing on the properties of elementary excitations in graphene in strong magnetic field

ABSTRACT: Massless Dirac electrons in graphene fill Landau levels with energies scaled as square roots of their numbers. Coulomb interaction between electrons leads to mixing of different Landau levels. The relative strength of this interaction depends only on dielectric susceptibility of surrounding medium and can be large in suspended graphene. We consider influence of Landau level mixing on the properties of magnetoexcitons and magnetoplasmons-elementary electron-hole excitations in graphene in quantizing magnetic field. We show that, at small enough background dielectric screening, the mixing leads to very essential change of magnetoexciton and magnetoplasmon dispersion laws in comparison with the lowest Landau level approximation.PACS: 73.22.Pr; 71.35.Ji; 73.43.Mp; 71.70.Gm.     

Pulsed neutron source measurements in the subcritical ADS experiment YALINA-Booster

A subcritical zero-power source-driven coupled core, the YALINA-Booster, has been constructed for experimental investigations of neutron kinetics of source-driven systems. In this study, the reactivity of two subcritical configurations has been determined by the area ratio method. The prompt neutron decay constants have been evaluated through slope fitting of the prompt neutron decay as well as through the pulsed Rossi-α method. It is shown that the slope fitting method and the pulsed Rossi-α method give stable results whereas the area ratio method results show spatial dependence. The reasons for the spatial spread are addressed.     

Calculation of Some Properties of the Cubic Oxides of II-VIII Group Metals

Experimental data on the bulk modulus of elasticity, thermal expansion coefficient, specific heat capacity, specific volume, Grueneisen constant, and atomization energy of the cubic monoxides of Mg, Ca, Sr, Cd, Ba, Eu, Th, U, Pu, Ti, Zr, V, Nb, Ta, Cr, Mn, Fe, Co, and Ni were analyzed. Using six correlation relationships values of unknown properties and adjusted values of known properties at 298 K were calculated.

     

Probing Cell‐Type‐Specific Intracellular Nanoscale Barriers Using Size‐Tuned Quantum Dots

The compartmentalization of size‐tuned luminescent semiconductor nanocrystal quantum dots (QDs) in four distinctive cell lines, which would be representative of the most likely environmental exposure routes to nanoparticles in humans, is studied. The cells are fixed and permeabilized prior to the addition of the QDs, thus eliminating any cell‐membrane‐associated effects due to active QD uptake mechanisms or to specificity of signaling routes in different cell types, but leaving intact the putative physical subcellular barriers. All quantitative assays are performed using a high content analysis (HCA) platform, thereby obtaining robust data on large cell populations. While smaller QDs 2.1 nm in diameter enter the nuclei and localize to the nucleoli in all cell types, the rate and dynamics of their passage vary depending on the cell origin. As the QD size is increased to 4.4 nm, penetration into the cell is reduced but each cell line displays its own cutoff size thresholds reflecting cell‐type‐determined cytoplasmic and nuclear pore penetration specificity. These results give rise to important considerations regarding the differential compartmentalization and susceptibility of organs, tissues, and cells to nanoparticles, and may be of prime importance for biomedical imaging and drug‐delivery research employing nanoparticle‐based probes and systems.     

Rietveld refinement of amorphous SiO2 prepared via sol–gel method

Using the Rietveld refinement, we analyze the structural evolution under thermal treatment of silica xerogel samples prepared by the sol–gel method with molar ratio (R) of water to TEOS of R = 5 and R = 11.66. We refine the structure of compounds using the Maud program and we found the unit cell parameters and atomic positions of the refined silica amorphous atoms for whole samples. The results show us that the amorphous structure is quartz-like or low-cristobalite-like, depending on the molar ratio values and the heat-treatment of the samples. For R = 5 different quartz structures are obtained, whereas for R = 11.66 we obtain close structures in which a transformation of quartz-like amorphous to low-cristobalite amorphous phase occurs at about 600 °C. These results give some scopes to explain the partial crystallization of the silica at relatively low temperatures obtained when metallic species are present.