Abstract: The surface termination of MXenes greatly determines the electrochemical properties and ion kinetics on their surfaces. So far, hydroxyl-, oxygen-, and fluorine-terminated MXenes have been widely studied for energy storage applications. Recently, sulfur-functionalized MXene structures, which possess low diffusion barriers, have been proposed as candidate materials to enhance battery performance. We performed first-principles calculations on the structural, stability, electrochemical, and ion dynamic properties of Li-adsorbed sulfur-functionalized groups 3B, 4B, 5B, and 6B transition-metal (M)-based MXenes (i.e., M2CS2 with M = Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W). We performed phonon calculations, which indicated that all of the above M2CS2 MXenes, except for Sc, are dynamically stable at T = 0 K. The ground-state structure of each M2CS2 monolayer depends on the type of M atom. For instance, while sulfur prefers to sit at the FCC site on Ti2CS2, it occupies the HCP site of Cr-based MXene. We determined the Li adsorption configurations at different concentrations using the cluster expansion method. The highest maximum open-circuit voltages were computed for the group 4B element (i.e., Ti, Zr, and Hf)-based M2CS2, which are larger than 2.1 V, while their average voltages are approximately 1 V. The maximum voltage for the group 6B element (i.e., Cr, Mo, W)-based M2CS2 is less than 1 V, and the average voltage is less than 0.71 V. We found that S functionalization is helpful for capacity improvements over the O-terminated MXenes. In this respect, the computed storage gravimetric capacity may reach up to 417.4 mAh/g for Ti2CS2 and 404.5 mAh/g for V2CS2. Ta-, Cr-, Mo-, and W-based M2CS2 MXenes show very low capacities, which are less than 100 mAh/g. The Li surface diffusion energy barriers for all of the considered MXenes are less than 0.22 eV, which is favorable for high charging and discharging rates. Finally, ab initio molecular dynamic simulations performed at 400 K and bond-length analysis with respect to Li concentration verify that selected promising systems are robust against thermally induced perturbations that may induce structural transformations or distortions and undesirable Li release.

Abstract: Macro X-ray fluorescence scanning (MA-XRF) is gradually becoming an established technique for the non-invasive analytical investigation of painted surfaces. This paper discusses some of the benefits and limitations of employing MA-XRF for the study of manuscript illuminations. Art historical research on this type of artefacts that is based on scientific measurements is often limited by the fact that usually no sampling can take place. Hence there is a need for non-invasive analytical tools that make it possible to conduct systematic investigations. As a representative example of this type of objects, a 15th century Italian manuscript fragment from the collection of the Fitzwilliam Museum in Cambridge (UK) is investigated. The aims of the study were to gain insight into the materials and techniques employed by Renaissance illuminators and to help answer specific questions regarding the fragment's authorship and geographic origin. The complementarity and advantages of MA-XRF mapping versus site-specific analyses are discussed. For this purpose, MA-XRF data are evaluated and compared with the results of other analytical techniques. The interpretation of the elemental maps is discussed along with the challenges faced during the analysis. (C) 2015 Elsevier B.V. All rights reserved.

Abstract: V-shaped gold nanoantennas are the functional components of plasmonic metasurfaces, which are capable of manipulating light in unprecedented ways. Designing a metasurface requires the custom arrangement of individual antennas with controlled shape and orientation. Here, we show how highly crystalline gold nanorods in solution can be bend, one-by one, into a V-shaped geometry and printed to the surface of a solid support through a combination of plasmonic heating and optical force. Significantly, we demonstrate that both the bending angle and the orientation of each rod-antenna can be adjusted independent from each other by tuning the laser intensity and polarization. This approach is applicable for the patterning of V-shaped plasmonic antennas on almost any substrate, which holds great potential for the fabrication of ultrathin optical components and devices.

Abstract: Nanoparticle modified electrodes constitute an attractive way to tailor-make efficient carbon dioxide (CO2) reduction catalysts. However, the restructuring and sintering processes of nanoparticles under electrochemical reaction conditions not only impedes the widespread application of nanoparticle catalysts, but also misleads the interpretation of the selectivity of the nanocatalysts. Here, we colloidally synthesized metallic copper (Cu) and silver (Ag) nanoparticles with a narrow size distribution (<10%) and utilized them in electrochemical CO2 reduction reactions. Monometallic Cu and Ag nanoparticle electrodes showed severe nanoparticle sintering already at low overpotential of -0.8 V vs. RHE, as evidenced by ex situ SEM investigations, and potential-dependent variations in product selectivity that resemble bulk Cu (14% for ethylene at -1.3 V vs. RHE) and Ag (69% for carbon monoxide at -1.0 V vs. RHE). However, by co-deposition of Cu and Ag nanoparticles, a nanoparticle stabilization effect was observed between Cu and Ag, and the sintering process was greatly suppressed at CO2 reducing potentials (-0.8 V vs. RHE). Furthermore, by varying the Cu/Ag nanoparticle ratio, the CO2 reduction reaction (CO2RR) selectivity towards methane (maximum of 20.6% for dense Cu-2.5-Ag-1 electrodes) and C-2 products (maximum of 15.7% for dense Cu-1-Ag-1 electrodes) can be tuned, which is attributed to a synergistic effect between neighbouring Ag and Cu nanoparticles. We attribute the stabilization of the nanoparticles to the positive enthalpies of Cu-Ag solid solutions, which prevents the dissolution-redeposition induced particle growth under CO2RR conditions. The observed nanoparticle stabilization effect enables the design and fabrication of active CO2 reduction nanocatalysts with high durability.

Abstract: The physical processes in an Ar/O2 magnetron discharge used for the reactive sputter deposition of TiOx thin films were simulated with a 2d3v particle-in-cell/Monte Carlo collisions (PIC/MCC) model. The plasma species taken into account are electrons, Ar+ ions, fast Arf atoms, metastable Arm* atoms, Ti+ ions, Ti atoms, O+ ions, O2+ ions, O− ions and O atoms. This model accounts for plasmatarget interactions, such as secondary electron emission and target sputtering, and the effects of target poisoning. Furthermore, the deposition process is described by an analytical surface model. The influence of the O2/Ar gas ratio on the plasma potential and on the species densities and fluxes is investigated. Among others, it is shown that a higher O2 pressure causes the region of positive plasma potential and the O− density to be more spread, and the latter to decrease. On the other hand, the deposition rates of Ti and O are not much affected by the O2/Ar proportion. Indeed, the predicted stoichiometry of the deposited TiOx film approaches x=2 for nearly all the investigated O2/Ar proportions.

Abstract: Organic photovoltaic (OPV) devices are on the verge of commercialization being long-term stability a key challenge. Morphology evolution during lifetime has been suggested to be one of the main pathways accounting for performance degradation. There is however a lack of certainty on how specifically the morphology evolution relates to individual electrical parameters on operating devices. In this work a case study is created based on a thermodynamically unstable organic active layer which is monitored over a period of one year under non-accelerated degradation conditions. The morphology evolution is revealed by compositional analysis of ultrathin cross-sections using nanoscale imaging in scanning transmission electron microscopy (STEM) coupled with electron energy-loss spectroscopy (EELS). Additionally, devices are electrically monitored in real-time using the non-destructive electrical techniques capacitance-voltage (C-V) and Impedance Spectroscopy (IS). By comparison of imaging and electrical techniques the relationship between nanoscale morphology and individual electrical parameters of device operation can be conclusively discerned. It is ultimately observed how the change in the cathode contact properties occurring after the migration of fullerene molecules explains the improvement in the overall device performance. (C) 2014 Elsevier B.V. All rights reserved.

Abstract: Strain engineering is an effective way to tune the intrinsic properties of a material. Here, we show by using first-principles calculations that both uniaxial and biaxial tensile strain applied to monolayer TiS3 are able to significantly modify its intrinsic mobility. From the elastic modulus and the phonon dispersion relation we determine the tensile strain range where structure dynamical stability of the monolayer is guaranteed. Within this region, we find more than one order of enhancement of the acoustic phonon limited mobility at 300 K (100 K), i.e. from 1.71 x 10(4) (5.13 x 10(4)) cm(2) V-1 s(-1) to 5.53 x 10(6) (1.66 x 10(6)) cm(2) V-1 s(-1). The degree of anisotropy in both mobility and effective mass can be tuned by using tensile strain. Furthermore, we can either increase or decrease the band gap of TiS3 monolayer by applying strain along different crystal directions. This property allows us to use TiS3 not only in electronic but also in optical applications.

Abstract: A generalization of the real-space Greens-function approach is presented for ab initio calculations of relativistic electron energy loss spectra (EELS) which are particularly important in anisotropic materials. The approach incorporates relativistic effects in terms of the transition tensor within the dipole-selection rule. In particular, the method accounts for relativistic corrections to the magic angle in orientation resolved EELS experiments. The approach is validated by a study of the graphite C  K edge, for which we present an accurate magic angle measurement consistent with the predicted value.

Abstract: To date, several experimental groups reported measurements of the thickness dependence of T-c of atomically uniform single-crystalline Pb nanofilms. The reported amplitude of the T-c oscillations varies significantly from one experiment to another. Here we propose that the reason for this unresolved issue is an interplay of the quantum-size variations in the single-electron density of states with thickness-dependent oscillations in the phonon-mediated electron-electron coupling. Such oscillations in the coupling depend on the substrate material, the quality of the interface, the protection cover, and other details of the fabrication process, changing from one experiment to another. This explains why the available data do not exhibit one-voice consistency about the amplitude of the T-c oscillations. Our analyses are based on a numerical solution of the Bogoliubov-de Gennes equations for a superconducting slab.