Crystals2014, 4(4), 450-465; doi:10.3390/cryst4040450 - published 20 November 2014 Show/Hide Abstract
Abstract: Separating functionalized single-wall carbon nanotubes (SWCNTs) from functionalized amorphous carbon is challenging, due to their polydispersity and similar physicochemical properties. We describe a single-step, dialytic separation method that takes advantage of the ability of heavily functionalized SWCNTs to bundle in a polar environment while maintaining their solubility. Experiments on functionalized SWCNTs were compared with functionalized, C60 fullerenes (buckyballs) to probe the general applicability of the method and further characterize the bundling process. This approach may simultaneously be used to purify a functionalization reaction mixture of unreacted small molecules and of residual solvents, such as dimethylformamide.
Crystals2014, 4(4), 439-449; doi:10.3390/cryst4040439 - published 19 November 2014 Show/Hide Abstract
Abstract: The crystal structures of three salts of anionic 5-aminotetrazole are described. The tetramethylammonium salt (P ) forms hydrogen-bonded ribbons of anions which accept weak C–H···N contacts from the cations. The cystamine salt (C2/c) shows wave-shaped ribbons of anions linked by hydrogen bonds to screw-shaped dications. The tetramethylguanidine salt (P21/c) exhibits layers of anions hydrogen-bonded to the cations.
Crystals2014, 4(4), 427-438; doi:10.3390/cryst4040427 - published 14 October 2014 Show/Hide Abstract
Abstract: We survey our recent results on the observation and studies of the effects accompanying the dynamical Bragg diffraction in one-dimensional photonic crystals (PhC). Contrary to the kinematic Bragg diffraction, the dynamical one considers a continuous interaction between the waves travelling within a spatially-periodic structure and is the most pronounced in the so called Laue geometry, leading to a number of exciting phenomena. In the described experiments, we study the PhC based on porous silicon or porous quartz, made by the electrochemical etching of crystalline silicon with the consequent thermal annealing. Importantly, these PhC are approximately hundreds of microns thick and contain a few hundreds of periods, so that the experiments in the Laue diffraction scheme are available. We discuss the effect of the temporal splitting of femtosecond laser pulses and show that the effect is quite sensitive to the polarization and the phase of a femtosecond laser pulse. We also show the experimental realization of the Pendular effect in porous quartz PhC and demonstrate the experimental conditions for the total spatial switching of the output radiation between the transmitted and diffracted directions. All described effects are of high interest for the control over the light propagation based on PhC structures.
Crystals2014, 4(3), 417-426; doi:10.3390/cryst4030417 - published 17 September 2014 Show/Hide Abstract
Abstract: We revisit the standard Nicolson–Ross–Weir method of effective permittivity and permeability restoration of photonic structures for the case of subwavelength metal-dielectric multilayers. We show that the direct application of the standard method yields a false zero-epsilon point and an associated spurious permeability resonance. We show how this artifact can be worked around by the use of the cycle shift operator to the periodic multilayer in question.
Crystals2014, 4(3), 404-416; doi:10.3390/cryst4030404 - published 5 September 2014 Show/Hide Abstract
Abstract: Crystal structures of five hexaethylguanidinium salts (PF6−1, FeCl4−2, CuCl42−·H2O 3, CoBr42−/Br−·H2O 4, bistriflimide 5) were determined. Short interionic contacts were identified. Cyclic voltammetry of 2 revealed an electrochemical window from +1.0 to −1.5 V with a single Fe(III)/Fe(II) redox event at −0.27 V on a gold electrode versus Ag/AgCl.
Crystals2014, 4(3), 390-403; doi:10.3390/cryst4030390 - published 3 September 2014 Show/Hide Abstract
Abstract: The geometrical shape of ions in crystals and the concept of ionic radii are re-considered. The re-investigation is motivated by the fact that a spherical modelling is justified for p valence shell ions on cubic lattice sites only. For the majority of point groups, however, the ionic radius must be assumed to be an anisotropic quantity. An appropriate modelling of p valence ions then has to be performed by ellipsoids. The approach is tested for pyrite-structured dichalcogenides MX2, with chalcogen ions X = O, S, Se and Te. The latter are found to exhibit the shape of ellipsoids being compressed along the <111> symmetry axes, with two radii r|| and r⊥ describing their spatial extension. Based on this ansatz, accurate interatomic M–X distances can be derived and a consistent geometrical model emerges for pyrite-structured compounds. Remarkably, the volumes of chalcogen ions are found to vary only little in different MX2 compounds, suggesting the ionic volume rather than the ionic radius to behave as a crystal-chemical constant.