On the observation of electron–hole liquid luminescence under low excitation in Al2O3‐passivated c‐Si wafers

We report on low‐temperature photoluminescence (PL) from aluminum oxide (Al₂O₃)‐passivated c‐Si wafers, which surprisingly exhibits clear signature of the formation of the so‐called electron–hole liquid (EHL), despite the use of excitation powers for which the condensed phase is not usually observed in bulk Si. The elevated incident photon densities achieved with our micro‐PL setup together with the relatively long exciton lifetimes associated with a good quality, indirect band‐gap semiconductor such as our float‐zone c‐Si, are considered the key aspects promoting photogenerated carrier densities above threshold. Interestingly, we observe a good correlation between the intensity of the EHL feature in PL spectra and the passivation performance of the Al₂O₃ layer annealed at different temperatures. The change in the extension of the sub‐surface space‐charge region that results from the balance between the induced fixed charge in the Al₂O₃ and the defect states at the alumina/Si interface is at the origin of the observed correlation. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Hardy-type estimates for Dirac operators

We prove some Hardy type inequalities related to the Dirac operator by elementary methods, for a large class of potentials, which even includes measure valued potentials. Optimality is achieved by the Coulomb potential. When potentials are smooth enough, our estimates provide some spectral information on the operator.     

Diffusion and Velocity Correlations of the Phase Transitions in a System of Macroscopic Rolling Spheres

We study an air-fluidized granular monolayer composed of plastic spheres which roll on a metallic grid. The air current is adjusted so that the spheres never lose contact with the grid and so that the dynamics may be regarded as pseudo two dimensional (or two dimensional, if the effects of the sphere rolling are not taken into account). We find two surprising continuous transitions, both of them displaying two coexisting phases. Moreover, in all the cases, we found the coexisting phases display a strong energy non-equipartition. In the first transition, at a weak fluidization, a glass phase coexists with a disordered fluid-like phase. In the second transition, a hexagonal crystal coexists with the fluid phase. We analyze, for these two-phase systems, the specific diffusive properties of each phase, as well as the velocity correlations. Surprisingly, we find a glass phase at a very low packing fraction and for a wide range of granular temperatures. Both phases are also characterized by strong anticorrelated velocities upon a collision. Thus, the dynamics observed for this quasi two-dimensional system unveil phase transitions with peculiar properties, very different from the predicted behavior in well-know theories for their equilibrium counterparts.     

New palladium (II) complexes containing phosphine‐nitrogen ligands and their use as catalysts in aminocarbonylation reaction

Aminocarbonylation of aryl halides, homogeneously catalysed by palladium, is an efficient method that can be employed for obtaining amides for pharmaceutical and synthetic applications. In this work, palladium (II) complexes containing P^N ligands were studied as catalysts in the aminocarbonylation of iodobenzene in the presence of diethylamine. Two types of systems were used: a palladium (II) complex formed in situ; and one prepared prior to the catalytic reaction. In general, the palladium complexes studied achieved high conversions in an average reaction time of less than 2 hr, which is less than that for the standard system (Pd (II)/PPh₃) used. The pre‐synthesized complexes were faster than their in situ counterparts, as the latter require an induction time to form the Pd/P^N species. The structure and electronic properties of the ligand P^N can influence both the activity and the selectivity of the reaction, stabilizing the acyl‐palladium intermediates formed in a better manner.     

Phase behavior and viscoelastic properties of entangled block copolymer gels

Triblock copolymers in midblock‐selective solvents can form physical gels. However, at low triblock contents (near the percolation threshold), the bridging of chains between micelles can lead to macrophase separation. Adding a styrene–isoprene diblock to a styrene–isoprene–styrene triblock copolymer in squalane can eliminate macrophase separation, yielding a wide range of stable, single‐phase gels with a disordered arrangement of micelles. The plateau modulus of these triblock gels scales with the 2.2 power of polymer content, indicating the importance of entanglements in dictating the modulus. Comparing gels made from the midblock‐saturated derivative of the same polymer [styrene‐(ethylene‐alt‐propylene)‐styrene] in squalane reveals that the modulus differences in the gels are a direct consequence of the difference in the entanglement molecular weight of the midblock homopolymer in bulk. Finally, the broad relaxation spectrum of these triblocks is well‐described by a recent theory for the dynamics of entangled star polymers, with the breadth of the relaxation spectrum dictated by the number of entanglements per midblock in the gel. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2183–2197, 2001     

Solubility of NaCl in water by molecular simulation revisited

In this paper, the solubility of NaCl in water is evaluated by using computer simulations for three different force fields. The condition of chemical equilibrium (i.e., equal chemical potential of the salt in the solid and in the solution) is obtained at room temperature and pressure to determine the solubility of the salt. We used the same methodology that was described in our previous work [E. Sanz and C. Vega, J. Chem. Phys. 126, 014507 (2007)] although several modifications were introduced to improve the accuracy of the calculations. It is found that the predictions of the solubility are quite sensitive to the details of the force field used. Certain force fields underestimate the experimental solubility of NaCl in water by a factor of four, whereas the predictions of other force fields are within 20% of the experimental value. Direct coexistence molecular dynamic simulations were also performed to determine the solubility of the salt. Reasonable agreement was found between the solubility obtained from free energy calculations and that obtained from direct coexistence simulations. This work shows that the evaluation of the solubility of salts in water can now be performed in computer simulations. The solubility depends on the ion-ion, ion-water, and water-water interactions. For this reason, the prediction of the solubility can be quite useful in future work to develop force fields for ions in water.     

Density functional calculations on Jahn-Teller effect of tetrachloromethane cation

Density functional (DF) calculations of the tetrachloromethane cation and its most important competitive process, the formation of CCl⁺₃, were carried out to explain the possible stability of CCl⁺₄. From results obtained with B-LYP and B-P86 methods, it is possible to produce a slight Jahn-Teller (JT) effect for a C_s planar structure of the cation type CCl₂(SINGLE BOND)Cl(SINGLE BOND)Cl⁺ compatible with the experimental data obtained by electron-spin resonance spectroscopy. A complex of C_3v structure CCl⁺₃(SINGLE BOND)Cl which is similar to the previous one found in CF⁺₄ appears when symmetry-broken wave functions are used in HF-LYP and HF-P86 methods. Depending of the DF method employed, either one of the minima [C_s (planar) and C_3v] is the most stable and competes with the dissociation of the molecular ion to give CCl⁺₃. The JT stabilization energy is smaller when the JT active coordinates are considered. © 1997 John Wiley & Sons, Inc.