Growth and kinetic Monte Carlo simulation of InAs quantum wires on vicinal substrates

AbstractSelf-assembled quantum structures have been successfully grown for some time now but control over their uniformity has proven difficult due to the stochastic nature of surface diffusion. We have investigated the effect of vicinal InP(001) substrates on the uniformity of InAs quantum wires grown on InGaAlAs lattice-matched to InP using molecular beam epitaxy. Dense quantum wires were grown on both nominally flat and vicinal substrates off-cut by 0.9° toward the [110] direction for comparison. The off-cut angle was chosen to provide terraces which match the orientation and spacing of wires grown on nominally flat substrates. A modest but statistically significant improvement in the size distribution of the wires was observed on vicinal substrates through the analysis of ultrahigh resolution scanning electron micrographs. The interface between the wires and the off-cut substrate was studied using cross-sectional high resolution scanning transmission electron microscopy. In addition, a kinetic Monte Carlo model of epitaxial growth including full strain calculations was developed to further investigate the nucleation process. Using an anisotropic bond model to account for the surface energy of different crystallographic facets, our simulations produced wires similar to those observed experimentally while demonstrating the importance of anisotropic bonding compared to anisotropic diffusion. Growth on vicinal substrates is also simulated here and indicates that off-cut substrates should indeed improve the size distribution of quantum wires under proper growth conditions

Atomic-scale identification of novel planar defect phases in heteroepitaxial YBa2_2Cu3_3O7−δ_{7-\delta} thin films

We have discovered two novel types of planar defects that appear in heteroepitaxial YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO123) thin films, grown by pulsed-laser deposition (PLD) either with or without a La$_{2/3}$Ca$_{1/3}$MnO$_3$ (LCMO) overlayer, using the combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging and electron energy loss spectroscopy (EELS) mapping for unambiguous identification. These planar lattice defects are based on the intergrowth of either a BaO plane between two CuO chains or multiple Y-O layers between two CuO$_2$ planes, resulting in non-stoichiometric layer sequences that could directly impact the high-$T_c$ superconductivity

Epitaxial thin films of multiferroic Bi2FeCrO6 with B-site cationic order

Epitaxial thin films of Bi2FeCrO6 have been synthesized by pulsed laser deposition on SrRuO3 on (100)- and (111)-oriented SrTiO3 substrates. Detailed X-ray diffraction and cross-section transmission electron microscopy analysis revealed a double perovskite crystal structure of the Bi2FeCrO6 epitaxial films very similar to that of BiFeO3 along with a particularly noteworthy Fe3+/Cr3+ cation ordering along the [111] direction. The films contain no detectable magnetic iron oxide impurities and have the correct cationic average stoichiometry throughout their thickness. They however exhibit a slight modulation in the Fe and Cr compositions forming complementary stripe patterns, suggesting minor local excess or depletion of Fe and Cr. The epitaxial BFCO films exhibit good ferroelectric and piezoelectric properties, in addition to magnetic properties at room temperature, as well as an unexpected crystallographic orientation dependence of their room temperature magnetic properties. Our results qualitatively confirm the predictions made using the ab-initio calculations: the double-perovskite structure of Bi2FeCrO6 films exhibit a Fe3+/Cr3+ cation ordering and good multiferroic properties, along with the unpredicted existence of magnetic ordering at room temperature.Comment: Accepted for publication in Journal of Materials Researc

Ionomer content optimization in nickel-iron-based anodes with and without ceria for anion exchange membrane water electrolysis

Hydrogen production from anion exchange membrane water electrolysis (AEMWE) is an efficient cost-effective solution to renewable energy storage. Contrary to proton exchange membrane (PEM) electrolysis, AEMWE requires further optimization of its cell design, particularly for the kinetically unfavourable oxygen evolution anode half-cell reaction (OER). In this work we optimize the commercial Fumatech fumion ionomer content in AEMWE anodes using nickel (Ni) nanoparticles (NP) synthesized by chemical reduction. The optimal ionomer content is then applied to Ni-iron (Fe)-based NPs with and without ceria (CeO2), all prepared using the same method. Scanning Electron Microscopy (SEM) of the resulting electrode surfaces, Particle-size Distribution (PSD) of the catalyst inks, and in-situ testing of the monometallic Ni NPs show that the best and most active catalytic layer is obtained using 15 wt% ionomer. AEMWE performance and short-term durability are evaluated in different concentrations of potassium hydroxide (KOH), where the Ni90Fe10 is the best performing Ni-based electrode showing 1.72 V at 0.8 A cm−2 in 1 M KOH after IR-correction, and a degradation rate of 3.3 mV h−1. The addition of ceria to the Ni-based catalysts shows more consistent mass transfer over time likely due to more efficient water transport and bubble release.acceptedVersio

Carving Plasmon Modes in Silver Sierpiński Fractals

The surface plasmon resonance (SPR) modes of the first three generations of a Sierpiński fractal triangle are investigated using electron energy loss spectroscopy (EELS) complemented with finite difference time domain simulations. The Sierpiński fractal geometry is created in a subtractive manner, by carving triangular apertures into the triangular prism of the previous fractal generation. The ability of the fractal antenna to efficiently utilize space in coupling to long wavelength excitations is confirmed on the single nanostructure level via redshifting of the primary dipole mode as the fractal generation is increased. Through application of the Babinet principle, it is demonstrated that this spectral shift is caused by coupling of two orthogonal dipolar modes of a single triangle with two orthogonal dipole modes of the triangular aperture occupying the centre of the first generation fractal. It is also shown that the spectral position and strength of the dipole mode can be tuned by altering the size of the central 1 aperture, and thus the capacitance of the equivalent circuit, and the width of the conductive channels joining different fractal building blocks, thereby altering the circuit inductance. Importantly, placing the aperture on an anti-node of the SPR mode causes a shift in energy of this mode without changing the charge configuration; placing the aperture on a node of the SPR mode causes no shift in energy, but changes the field configuration, as revealed through EELS measurements. These fractal-specific properties provide new strategies to design, predict, and effectively exploit highly tunable SPR modes using simple building blocks

Tunable Syngas Production from CO2 and H2O in an Aqueous Photoelectrochemical Cell

Syngas, the mixture of CO and H2, is a key feedstock to produce methanol and liquid fuels in industry, yet limited success has been made to develop clean syngas production using renewable solar energy. We demonstrated that syngas with a benchmark turnover number of 1330 and a desirable CO/H2 ratio of 1:2 could be attained from photoelectrochemical CO2 and H2O reduction in an aqueous medium by exploiting the synergistic co‐catalytic effect between Cu and ZnO. The CO/H2 ratio in the syngas products was tuned in a large range between 2:1 and 1:4 with a total unity Faradaic efficiency. Moreover, a high Faradaic efficiency of 70 % for CO was acheived at underpotential of 180 mV, which is the lowest potential ever reported in an aqueous photoelectrochemical cell. It was found that the combination of Cu and ZnO offered complementary chemical properties that lead to special reaction channels not seen in Cu, or ZnO alone.Mixture is better: Syngas (CO+H2) with tunable composition is synthesized from the reduction of CO2 and H2O in an aqueous photoelectrochemcal cell. A turnover number of 1330 and a high Faradaic efficiency of 70 % for CO at underpotential of 180 mV are acheived. The excellent perfomance is attributed to the coupling effects of strong light harvesting of p‐n Si, efficient electron extraction of GaN nanowires, and fast surface reaction kinetics of Cu–ZnO co‐catalysts.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134441/1/anie201606424_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134441/2/anie201606424.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134441/3/anie201606424-sup-0001-misc_information.pd

Hierarchical Plasmon Resonances in Fractal Structures

An equilateral triangular prism is used as the fundamental building block to con- struct additive Sierpinski fractals, enabling new surface plasmon resonances (SPR) in the first three generations of Sierpinski triangles, as well as topological intermediaries between generations. The modes are characterized using electron energy loss spectroscopy accompanied by eigenmode calculations and optical finite-difference time domain simulations. The complex fractal geometries present a predictable hierarchy of new resonances, each arising from the previous generational building blocks used to construct the fractal. Intermediate structures break the polarization degeneracy of the equilateral fractals while maintaining a rich multiband spectral response. Engineering defects in the narrow conductive channels of the fractal allows further manipulation of the SPR response, emphasizing higher order SPR modes over the lowest energy peak. The knowledge gained is used to develop guidelines for engineering the response of more complex fractal-based structures, including the spectral response and hotspot distribution

Liquid Cell Transmission Electron Microscopy Sheds Light on The Mechanism of Palladium Electrodeposition

Electrodeposition is widely used to fabricate tunable nanostructured materials in applications ranging from biosensing to energy conversion. A model based on 3D island growth is widely accepted in the explanation of the initial stages of nucleation and growth in electrodeposition. However, there are regions in the electrodeposition parameter space where this model becomes inapplicable. We use liquid cell transmission electron microscopy along with post situ scanning electron microscopy to investigate electrodeposition in this parameter space, focusing on the effect of the supporting electrolyte, and to shed light on the nucleation and growth of palladium. Using a collection of electron microscopy images and current time transients recorded during electrodeposition, we discover that electrochemical aggregative growth, rather than 3D island growth, best describes the electrodeposition process. We then use this model to explain the change in the morphology of palladium electrodeposits from spherical to open clusters with nonspherical morphology when HCl is added to the electrolyte solution. The enhanced understanding of the early stages of palladium nucleation and growth and the role of electrolyte in this process provides a systematic route toward the electrochemical fabrication of nanostructured materials

Modeling and experimental characterization of stepped and v-shaped {311} defects in silicon

Producción CientíficaWe propose an atomistic model to describe extended {311} defects in silicon. It is based on the combination of interstitial and bond defect chains. The model is able to accurately reproduce not only planar {311} defects but also defect structures that show steps, bends, or both. We use molecular dynamics techniques to show that these interstitial and bond defect chains spontaneously transform into extended {311} defects. Simulations are validated by comparing with precise experimental measurements on actual {311} defects. The excellent agreement between the simulated and experimentally derived structures, regarding individual atomic positions and shape of the distinct structural {311} defect units, provides strong evidence for the robustness of the proposed model.Ministerio de Ciencia e Innovación (Proyect TEC2011-27701

Pnictogens Allotropy and Phase Transformation during van der Waals Growth

Pnictogens have multiple allotropic forms resulting from their ns2 np3 valence electronic configuration, making them the only elemental materials to crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout the group. Light group VA elements are found in the layered orthorhombic A17 phase such as black phosphorus, and can transition to the layered rhombohedral A7 phase at high pressure. On the other hand, bulk heavier elements are only stable in the A7 phase. Herein, we demonstrate that these two phases not only co-exist during the vdW growth of antimony on weakly interacting surfaces, but also undertake a spontaneous transformation from the A17 phase to the thermodynamically stable A7 phase. This metastability of the A17 phase is revealed by real-time studies unraveling its thickness-driven transition to the A7 phase and the concomitant evolution of its electronic properties. At a critical thickness of ~4 nm, A17 antimony undergoes a diffusionless shuffle transition from AB to AA stacked alpha-antimonene followed by a gradual relaxation to the A7 bulk-like phase. Furthermore, the electronic structure of this intermediate phase is found to be determined by surface self-passivation and the associated competition between A7- and A17-like bonding in the bulk. These results highlight the critical role of the atomic structure and interfacial interactions in shaping the stability and electronic characteristics of vdW layered materials, thus enabling a new degree of freedom to engineer their properties using scalable processes