Abstract: The gravitational energy associated with variations in the radius of the Sun is huge: with a relative change of 1/2000 (the estimated difference between maximum and minimum solar activity) and taking only the convection zone to expand, corresponds to about 2 10(34)J, which is 1/10 of the total rotational energy of the Sun. Non-radial oscillations with a tiny nonlinearity can still yield energies comparable to or larger than the total magnetic energy of a full sunspot cycle or three or four orders more than the luminosity of the Sun, but that is not sufficient to account for the changes of the temperature on Earth. The expansion of the upper layers of the convection zone, however, may directly or indirectly affect the climate and the biological aspects on the Earth. Moreover this suits a qualitative explanation why the Sun expands during a minimum of the magnetic activity.

Abstract: Recording the electron energy loss spectroscopy data cube with a series of energy filtered images is a dose inefficient process because the energy slit blocks most of the electrons. When recording the data cube by scanning an electron probe over the sample, perfect dose efficiency is attained; but due to the low current in nanoprobes, this often is slower, with a smaller field of view. In W. Van den Broek et al. [Ultramicroscopy, 106 (2006) 269], we proposed a new method to record the data cube, which is more dose efficient than an energy filtered series. It produces a set of projections of the data cube and then tomographically reconstructs it. In this article, we demonstrate these projections in practice, we present a simple geometrical model that allows for quantification of the projection angles and we present the first successful experimental reconstruction, all on a standard post-column instrument.

Abstract: The present study analyses a specific laser ablation cell, the High Efficiency Aerosol Dispersion (HEAD) cell (see J. Pisonero et al., J. Anal. At. Spectrom., 2006, 21, 922931), by means of computational fluid dynamics (CFD) simulations. However, this cell consists of different modular parts, therefore, the results are probably of interest for the further development of other ablation cells. In the HEAD cell, the ablation spot is positioned below an orifice in the ceiling of the sample chamber. The particle transport through this orifice has been analysed for a ceiling height of 0.8 mm. The critical velocity for the onset of particle losses was found to be independent on the ejection angle at the crater spot. The deceleration of the particles can be described as the stopping in an effectively steady gas. Particle losses were negligible in this modular part of the cell at the evaluated laser ablation conditions. The transport efficiency through the Venturi chamber was investigated for different sample gas flow rates. In this case, it was found that small particles were predominantly lost at low flow rates, the large particles at higher flow rates. Making use of the simulation results, it was possible to design a modification of the HEAD cell that results in extremely short calculated washout times. The simulations yielded a signal of less than 10 ms, which was produced by more than 99% of the introduced mass.

Abstract: We present a study of Cd2+-for-Pb2+ exchange in PbSe nanocrystals (NCs) with cube, star, and rod shapes. Prolonged temperature-activated cation exchange results in PbSe/CdSe heterostructured nanocrystals (HNCs) that preserve their specific overall shape, whereas the PbSe core is strongly faceted with dominance of {111} facets. Hence, cation exchange proceeds while the Se anion lattice is preserved, and well-defined {111}/{111} PbSe/CdSe interfaces develop. Interestingly, by quenching the reaction at different stages of the cation exchange new structures have been isolated, such as coreshell nanorods, CdSe rods that contain one or two separated PbSe dots and fully zinc blende CdSe nanorods. The crystallographically anisotropic cation exchange has been characterized by a combined HRTEM/HAADF-STEM study of heterointerface evolution over reaction time and temperature. Strikingly, Pb and Cd are only intermixed at the PbSe/CdSe interface. We propose a plausible model for the cation exchange based on a layer-by-layer replacement of Pb2+ by Cd2+ enabled by a vacancy-assisted cation migration mechanism.

Abstract: The properties of free-standing silicon and germanium nanowires oriented along the [110] direction are studied using different first principles methods. We show the corrections due to quasi-particles to the band structures obtained using the local-density approximation. The formation energies of B and P doped nanowires are calculated, both in the absence and presence of dangling bond defects and we link these to experimental results. Furthermore, we report on the phonon properties of pure Si and Ge nanowires, as well as Ge/Si core-shell nanowires, and discuss the differences between them.

Abstract: We report single-crystal X-ray diffuse scattering measurements on C-60.C8H8 fullerene-cubane showing that the C-60 molecules are orientationally disordered at 300 and 150K and get ordered at low temperatures. Monte Carlo simulations provide further insight in the orientational behavior of both C-60 and C8H8 molecules; low-temperature molecular orientations are predicted.