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: The optical behavior of a four-level system in a ring cavity driven by two coherent laser fields is studied. One laser field is treated as the incident field and the other the control field, respectively. It is found that there is optical transparency when the difference between the two frequency detunings of the incident and control fields from the corresponding atomic transition frequencies is zero. Optical bistability can be produced and controlled by increasing the magnitude of the frequency difference. The bistable hysteresis becomes larger when the frequency difference is increased. Further increase of the latter can lead to onset of multistability.

Abstract: Nanostructured carbon materials show a tremendous variety in atomic structure, morphology, properties, and applications. As all properties are ultimately determined by the structure of the material, a thorough understanding of the growth mechanisms that give rise to the particular structure is critical. On many occasions, it has been shown that plasma enhanced growth can be strongly beneficial. This review will describe the authors current understanding of plasma enhanced growth of carbon nanotubes, the prototypical example of nanostructured carbon materials, as obtained from experiments, simulations, and modeling. Specific emphasis is put on where experiments and computational approaches correspond, and where they differ. Also, the current status on simulating PECVD growth of some other carbon nanomaterials is reviewed, including amorphous carbon, graphene, and metallofullerenes. Finally, computational challenges with respect to the simulation of PECVD growth are identified.

Abstract: A theoretical framework is described to understand the results of plasmon holography experiments leading to insight in the meaning of the experimental results and pointing out directions for future experiments. The framework is based on the formalism of mutual intensity to describe how coherence is transferred through an optical system. For the inelastic interaction with the object, an expression for the volume. plasmon excitations in a free electron gas is used as a model for the behaviour of aluminium. The formalism leads to a clear graphical intuitive tool for under-standing the experiments. It becomes evident that the measured coherence is solely related to the angular distribution of the plasmon scattering in the case of bulk plasmons. After describing the framework, the special case of coherence outside a spherical particle is treated and the seemingly controversial idea of a plasmon with a limited coherence length obtained front experiments is clarified. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: Ultrathin bismuth titanate films (Bi2Ti2O7, 5-25 nm) are deposited onto SiO2/Si substrates by aqueous chemical solution deposition and their evolution during annealing is studied. The films crystallize into a preferentially oriented, pure pyrochlore phase between 500 and 700 degrees C, depending on the film thickness and the total thermal budget. Crystallization causes a strong increase of surface roughness compared to amorphous films. An increase of the interfacial layer thickness is observed after anneal at 600 degrees C, together with intermixing of bismuth with the substrate as shown by TEM-EDX. The band gap was determined to be similar to 3 eV from photoconductivity measurements and high dielectric constants between 30 and 130 were determined from capacitance voltage measurements, depending on the processing conditions. (C) 2012 Elsevier Ltd. All rights reserved.

Abstract: Electrical transport properties of a two-dimensional electron gas axe studied in the presence of a perpendicular magnetic field B modulated weakly and periodically along one direction, B = (B + B0 cos Kx)z, with B0 much less than B, K = 2pi/a, and a being the period of the modulation. B0 is taken constant or proportional to B. The Landau levels broaden into bands and their width, proportional to the modulation strength B0, oscillates with B and gives rise to oscillations in the magnetoresistance at low B. These oscillations reflect the commensurability between the cyclotron diameter at the Fermi level and the period a and consequently hey are distinctly different from the Shubnikov-de Ha.as ones, at higher B, in period and temperature dependence. The bandwidth at the Fermi energy can be one order of magnitude larger, at low B, than that of the electric case for equal modulation strengths. The resulting magnetoresistance oscillations have a much higher amplitude than those of the electric case with which they are out of phase. Explicit asymptotic expressions are derived for the temperature dependence of the transport coefficients. The case when both electric and magnetic modulations are present is also considered. The position of the resulting oscillations depends on the ratio delta between the two modulation strengths. When the modulations are out of phase there is no shift in the position of the oscillations when delta varies and for a particular value of delta the oscillations are suppressed.

Abstract: Quantum thermal transport in armchair and zigzag graphene nanoribbons is investigated in the presence of single atomic vacancies and subject to different boundary conditions. We start with a full comparison of the phonon polarizations and energy dispersions as given by a fifth-nearest-neighbor force-constant model (5NNFCM) and by elasticity theory of continuum membranes (ETCM). For free-edge ribbons, we discuss the behavior of an additional acoustic edge-localized flexural mode, known as fourth acoustic branch (4ZA), which has a small gap when it is obtained by the 5NNFCM. Then, we show that ribbons with supported edges have a sample-size dependent energy gap in the phonon spectrum which is particularly large for in-plane modes. Irrespective to the calculation method and the boundary condition, the dependence of the energy gap for the low-energy optical phonon modes against the ribbon width W is found to be proportional to 1/W for in-plane, and 1/W-2 for out-of-plane phonon modes. Using the 5NNFCM, the ballistic thermal conductance and its contributions from every single phonon mode are then obtained by the nonequilibrium Green's function technique. We found that, while edge and central localized single atomic vacancies do not affect the low-energy transmission function of in-plane phonon modes, they reduce considerably the contributions of the flexural modes. On the other hand, in-plane modes contributions are strongly dependent on the boundary conditions and at low temperatures can be highly reduced in supported-edge samples. These findings could open a route to engineer graphene based devices where it is possible to discriminate the relative contribution of polarized phonons and to tune the thermal transport on the nanoscale.

Abstract: By considering a cylindrically symmetric generalization of a plane wave, the first-order Born approximation of screened Coulomb scattering unfolds two new dimensions in the scattering problem: transverse momentum and orbital angular momentum of the incoming beam. In this paper, the elastic Coulomb scattering amplitude is calculated analytically for incoming Bessel beams. This reveals novel features occurring for wide-angle scattering and quantitative insights for small-angle vortex scattering. The result successfully generalizes the well-known Rutherford formula, incorporating transverse and orbital angular momentum into the formalism.

Abstract: Site selective doping of aligned carbon nanostructures represents a promising approach for their implementation in actual devices. In the present work we report on alkali metals decoration on low density vertically aligned carbon nanotubes, disclosing the possibility of engineering site selective depositions of potassium atoms on the carbon systems. Photoemission measurements were combined with microscopy demonstrating the effective spatial control of alkali deposition. The changes of electronic structures of locally doped carbon regions were studied by exploiting the ability of the scanning photoemission microscopy technique. From the analysis of experimental data supported by theoretical calculations, we show the tuning of the charge transfer from potassium to carbon atoms belonging to neighboring nanotubes or along the same tube structure.

Abstract: The lowest energy electronic states of a donor located near a semiconductor-insulator-metal interface are investigated within the effective mass approach. The effect of the finite thickness of the insulator between the semiconductor and the metallic gate on the energy levels is studied. The lowest energy states are obtained through a variational approach, which takes into account the influence of all image charges that arise due to the presence of the metallic and the dielectric interfaces. We compare our results with a numerical exact calculation using the finite element technique.

Abstract: We investigate the magnetic interface states of graphene quantum dots that contain p-n junctions. Within a tight-binding approach, we consider rectangular quantum dots in the presence of a perpendicular magnetic field containing p-n as well as p-n-p and n-p-n junctions. The results show the interplay between the edge states associated with the zigzag terminations of the sample and the snake states that arise at the p-n junction due to the overlap between electron and hole states at the potential interface. Remarkable localized states are found at the crossing of the p-n junction with the zigzag edge having a dumb-bell-shaped electron distribution. The results are presented as a function of the junction parameters and the applied magnetic flux. DOI: 10.1103/PhysRevB.87.035426

Abstract: The main drawbacks of silicon as the most promising anode material for lithium-ion batteries (theoretical capacity=3572 mAh g−1) are lithiation-induced volume changes and the continuous formation of a solidelectrolyte interphase (SEI) upon cycling. A recent strategy is to focus on the influence of coatings and composite materials. To this end, the evolution of the SEI, as well as an applied carbon coating, on nanosilicon electrodes during the first electrochemical cycles is monitored. Two specific techniques are combined: Transmission Electron Microscopy (TEM) is used to study the surface evolution of the nanoparticles on a very local scale, whereas electrochemical impedance spectroscopy (EIS) provides information on the electrode level. A TEMEELS fingerprint signal of carbonate structures from the SEI is discovered, which can be used to differentiate between the SEI and a graphitic carbon matrix. Furthermore, the shielding effect of the carbon coating and the thickness evolution of the SEI are described.