Abstract: Nano-sized gold has become an important material in various fields of science and technology, where control over the size and crystallography is desired to tailor the functionality. Gold crystallizes in the face-centered cubic (fcc) phase, and its hexagonal closed packed (hcp) structure is a very unusual and rare phase. Stable Au hcp phase has been reported to form in nanoparticles at the tips of some Ge nanowires. It has also recently been synthesized in the form of thin graphene-supported sheets which are unstable under electron beam irradiation. Here, we show that stable hcp Au 3D nanostructures with well-defined crystallographic orientation and size can be systematically created in a process of thermally induced self-assembly of thin Au layer on Ge(001) monocrystal. The Au hcp crystallite is present in each Au nanostructure and has been characterized by different electron microscopy techniques. We report that a careful heat treatment above the eutectic melting temperature and a controlled cooling is required to form the hcp phase of Au on a Ge single crystal. This new method gives scientific prospects to obtain stable Au hcp phase for future applications in a rather simple manner as well as redefine the phase diagram of Gold with Germanium.

Abstract: The fast growing world population demands food to survive, and nitrogen-based fertilizers are essential to ensure sufficient food production. Today, fertilizers are mainly produced from non-sustainable fossil fuels<italic>via</italic>the Haber–Bosch process, leading to serious environmental problems. We propose here a novel rotating gliding arc plasma, operating in air, for direct NO_xproduction, which can yield high nitrogen content organic fertilizers without pollution associated with ammonia emission. We explored the efficiency of NO_xproduction in a wide range of feed gas ratios, and for two arc modes: rotating and steady. When the arc is in steady mode, record-value NO_xconcentrations up to 5.5% are achieved which are 1.7 times higher than the maximum concentration obtained by the rotating arc mode, and with an energy consumption of 2.5 MJ mol⁻¹(or<italic>ca.</italic>50 kW h kN⁻¹);<italic>i.e.</italic>the lowest value so far achieved by atmospheric pressure plasma reactors. Computer modelling, using a combination of five different complementary approaches, provides a comprehensive picture of NO_xformation in both arc modes; in particular, the higher NO_xproduction in the steady arc mode is due to the combined thermal and vibrationally-promoted Zeldovich mechanisms.

Abstract: The recent advantages of the fabrication of artificial nanochannels enabled new research on the molecular transport, permeance, and selectivity of various gases and molecules. However, the physisorption/chemisorption of the unwanted molecules (usually hydrocarbons) inside nanochannels results in the alteration of the functionality of the nanochannels. We investigated contamination due to hydrocarbon molecules, nanochannels made of graphene, hexagonal boron nitride, BC2N, and molybdenum disulfide using molecular dynamics simulations. We found that for a certain size of nanochannel (i.e., h = 0.7 nm), as a result of the anomalous hydrophilic nature of nanochannels made of graphene, the hydrocarbons are fully adsorbed in the nanochannel, giving rise to full uptake. An increasing temperature plays an important role in unclogging, while pressure does not have a significant role. The results of our pioneering work contribute to a better understanding and highlight the important factors in alleviating the contamination and unclogging of nanochannels, which are in good agreement with the results of recent experiments.

Abstract: Here, we study the effect of dynamic scattering on the projected geometric phase and strain maps reconstructed using dark-field electron holography (DFEH) for non-uniformly strained crystals. The investigated structure consists of a {SiGe/Si} superlattice grown on a (001)-Si substrate. The three dimensional strain held within the thin TEM lamella is modelled by the finite element method. The observed projected strain is simulated in two ways by multiplying the strain at each depth in the crystal by a weighting function determined from a recently developed analytical two-beam dynamical theory, and by simply taking the average value. We demonstrate that the experimental results need to be understood in terms of the dynamical theory and good agreement is found between the experimental and simulated results. Discrepancies do remain for certain cases and are likely to be from an imprecision in the actual two-beam diffraction conditions, notably the deviation parameter, and points to limitations in the 2-beam approximation. Finally, a route towards a 3D reconstruction of strain fields is proposed. (C) 2014 Elsevier B.V. All rights reserved.

Abstract: The compositional influence of Cr and Y on the microstructure of Mg―Cr―O, and Mg―Y―O films synthesized by reactive magnetron sputtering has been investigated by transmission electron microscopy, X-ray diffraction and molecular dynamics simulations. A decrease in crystallinity is observed in these films as the M (Cr or Y) content is increased. It is found that M forms a solid solution with MgO for metal ratios up to ~ 70% and ~ 50% for Cr and Y respectively. Above ~ 70% Cr metal ratio the Mg―Cr―O films are found to be completely amorphous. The Mg―Y―O films are composed of Mg(Y)O and Y2O3 nano crystallites, up to ~ 50% Y metal ratio. Above this ratio, only Y2O3 nano crystallites are found. The preferential < 111> MgO grain alignment is strongly affected by the increase in M content. For M metal ratios up to ~ 50%, there is a selective promotion of the < 100> MgO grain alignments and a decline in the < 111> grain alignments.

Abstract: An experimental determination of the mean vortex velocity in superconductors mostly relies on the measurement of flux-flow resistance with magnetic field, temperature, or driving current. In the present work we introduce a method combining conventional transport measurements and a frequency-tuned flashing pinning potential to obtain reliable estimates of the vortex velocity. The proposed device is characterized using the time-dependent Ginzburg-Landau formalism, where the velocimetry method exploits the resonances in mean vortex dissipation when temporal commensuration occurs between the vortex crossings and the flashing potential. We discuss the sensitivity of the proposed technique on applied current, temperature and heat diffusion, as well as the vortex core deformations during fast motion.

Abstract: Glass treated on its surface with silver compounds and an aluminosilicate, such as ochre or clay, at higher temperatures (between 550 and 650 °C) accepts a wide variety of a yellow colour. It is the aim of this study to investigate the parameters of the manufacturing process affecting the final colour of silver stained glass and to correlate them with the final colour and colour intensity. Therefore, defined mixtures of ochre and a silver compound (AgCl, AgNO3, Ag2SO4, Ag3PO4, Ag2O) were prepared and applied on soda-lime glass. The firing process was modified within the range from 563 to 630 °C and glass samples were analysed after treatment with energy dispersive X-ray fluorescence analysis (EDXRF), scanning electron microscopy (SEM/EDX), transmission electron microscopy (TEM), as well as ion beam analysis (IBA) with an external beam. Within the scope of IBA simultaneous measurements using particle-induced X-ray emission (PIXE), particle-induced gamma-ray emission (PIGE), and Rutherford backscattering spectrometry (RBS) were carried out in order to obtain the thickness of the Ag-rich surface layer and the depth distribution of Ag. By means of TEM the microstructure of the silver particles was visualised. XRF results show that the lowest amount of Ag could be detected on glass samples treated with silver stain mixtures containing AgCl and Ag2O. A low kiln temperature (e.g. 563 °C) results in a higher silver concentration at the surface and lower penetration depths. Furthermore, the results obtained with SEM/EDX at cross-sections of the glass samples could be confirmed by PIXE, PIGE, RBS, and TEM.

Abstract: Electron tomography has become a cornerstone technique for the visualization of nanoparticle morphology in three dimensions. However, to obtain in-depth information about a nanoparticle beyond surface faceting and morphology, different electron microscopy signals must be combined. The most notable examples of these combined signals include annular dark-field scanning transmission electron microscopy (ADF-STEM) with different collection angles and the combination of ADF-STEM with energy-dispersive X-ray or electron energy loss spectroscopies. Here, the experimental and computational development of various multimode tomography techniques in connection to the fundamental materials science challenges that multimode tomography has been instrumental to overcoming are summarized. Although the techniques can be applied to a wide variety of compositions, the study is restricted to metal and metal oxide nanoparticles for the sake of simplicity. Current challenges and future directions of multimode tomography are additionally discussed.

Abstract: Yamuna River sediments are more enriched in metals than those of the Ganges and average Indian river sediments. Variation of metals in suspended, bed and core sediment are due to the varying proportions of grain size and mineral content. Iron, Mn and Pb show a preference for the oxide fraction, whereas Cu and Zn are predominant in organic and carbonate fractions of sediments. Of the total elemental content, 80% Mn, 78% Fe, 69% Pb, 67% Cu and 55% Zn are available in chemically mobile phases of the sediments. The high partition coefficient of metals with respect to Mn suggests similar chemical mobility and preferences for solid phases. River sediments in the vicinity of Delhi show an increase in sorption of metals downstream, consequently metals are retained in sediments. The high correlation coefficient and significant regression relation among the metals indicate their similar behaviour during transport. At Allahabad, the contribution of the Yamuna to the Ganges is 3200 t Pb year−1, 12 100 t Zn year−1 and 8500 t Cu year−1 in particulate form.

Abstract: A novel triple ion beam cutting technique was employed to prepare high-quality surfaces of diamond/Al composites for interfacial characterization, which has been unachievable so far. Near-perfect and artifact-free surfaces were obtained without mechanical pre-polishing. Hence, the as-prepared surfaces are readily available for further study and also, ready to be employed in a focus ion beam system for preferential selection of transmission electron microscopy samples. Dramatically different diamond/Al interface configurations – sub-micrometer Al2O3 particles and clean interfaces were unambiguously revealed.

Abstract: Compared with dust-free plasmas, the existence of dust particles in plasmas may greatly influence the plasma properties. such as the plasma density, electron temperature, sheath properties, electron energy distribution function (EEDF) as well as the heating mechanism. In this work, a 1D hybrid fluid/MC model has been developed to investigate the interaction between dust and plasma in a low-pressure silane discharge sustained in a radio frequency capacitively coupled plasma, in which we assume spherical dust particles with a given radius are generated by taking the sum of the production rate of Si2H4- and Si2H5- as the nucleation rate. From our simulation, the plasma may experience definite perturbation by dust particles with a certain radius (more than 50nm) with an increase in electron temperature first, which further induces a rapid rise in the positive and negative ion densities. Then, the densities begin to decline due to the gradual lack of sufficient seed electrons. In addition, as the dust radius increases, the high energy tails of the EEDFs will be enhanced for discharge maintenance, accompanied by a decline in the population of low-energy electrons in comparison with those of pristine plasma. Furthermore, an obvious bulk heating is observed apart from the a-mode and local field reversal heating. This may contribute to the enhanced bulk electric field (also called the drift field) as a result of electron depletion via the dust. In addition, large-sized dust particles that accumulate near the sheaths tend to form two stable density peaks with their positions largely influenced by the time-averaged sheath thickness. A detailed study of the effects of the external parameters, including pressure, voltage and frequency, on the spatial distribution of dust particles is also conducted.

Abstract: The nonreciprocal charge transport property which depends on the polarity of the applied current, such as the diode effect and the rectification effect, is of great importance for both theoretical research and engineering application. The nonreciprocal transport property in superconductors generally requires to break both the spatial inversion symmetry and the time-reversal symmetry, and therefore becomes one of the fundamental issues in superconductivity. Of particular interest, the superconducting diode effect, which exhibits one-way superconductivity, can potentially be applied to dissipationless diode devices, as a consequence has received extensive attention in recent years. In this Ph. D thesis, we simulate vortex dynamics with heat dissipation by numerically solving time-dependent Ginzburg-Landau equations and heat transfer equation. The nonreciprocal transport properties of the following three superconducting systems are studied. We study a superconducting film patterned with a conformal pinning array and find a giant rectification effect which is consistent with the experimental observation. In presence of the funneling effect due to the geometry of the conformal pinning array, Joule heating of the accumulating vortices creates hot spots and drives the sample to the normal state. Meanwhile, the density gradient of vortex does not match the gradient of pinning. The two mechanisms together lead to the giant rectification effect. We study the nonreciprocal charge transport property in a pinning-free superconducting nano-ring. We systematically calculate the response of the ratchet signal to various parameters in both D.C. and A.C. currents. By analyzing the vortex potential, we find that the nonreciprocal transport property is caused by the asymmetry potential barriers for vortex entry and exit. We study a superconductor/nanoscale-magnetic-dot hybrid structure. It takes advantage of the external current to control the nucleation of vortex-antivortex pairs, and can produce superconducting diode effect without applied magnetic fields. Our vortex dynamics simulation details the progress of the superconducting-normal phase transition due to motion of vortex pairs and heat dissipation. The nonreciprocal transport properties of the above three systems are all based on the broken symmetry of spatial inversion, which is caused by the anisotropic pinning array, the asymmetric geometry, and the nonuniform distribution of the magnetic field, respectively. The mechanisms we discuss in this thesis do not require special property of the materials and thus can be applied to any kinds of conventional superconductors. The present studies would provide solid theoretical basis for the future design and application of the dissipationless superconducting devices.

Abstract: We study the ratchet effect in a narrow pinning-free superconductive ring based on time-dependent Ginzburg-Landau (TDGL) equations. Voltage responses to external dc and ac currents at various magnetic fields are studied. Due to asymmetric barriers for flux penetration and flux exit in the ring-shaped superconductor, the critical current above which the flux-flow state is reached, as well as the critical current for the transition to the normal state, are different for the two directions of applied current. These effects cooperatively cause ratchet signal reversal at high magnetic fields, which has not been reported to date in a pinning-free system. The ratchet signal found here is larger than those induced by asymmetric pinning potentials. Our results also demonstrate the feasibility of using mesoscopic superconductors to employ a superconducting diode effect in versatile superconducting devices.