Fe-Doped Ni-Based Catalysts Surpass Ir-Baselines for Oxygen Evolution Due to Optimal Charge-Transfer Characteristics
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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Engineering topics
Norman, Andrew G.
Publications and source records attributed to Norman, Andrew G..
Mn Additive Improves Zr Grain Boundary Diffusion for Sintering of a Y-Doped BaZrO 3 Proton Conductor
Not Available
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Thin-film TaAs: Developing a platform for Weyl semimetal devices
MX monopnictide compounds (M = Nb,Ta, X = As,P) are prototypical 3D Weyl semimetals (WSMs) that have been shown in bulk single crystal form to have potential for a wide variety of novel devices due to topologically protected band structures and high mobilities. However, very little is known about thin-film synthesis, which is essential to enable device applications. We synthesize TaAs(001) epilayers by molecular beam epitaxy on GaAs(001) and provide an experimental phase diagram illustrating conditions for single-phase, single-crystal-like growth. We investigate the relationship between nanoscale defects and electronic structure using angle-resolved photoemission spectroscopy, kelvin probe force microscopy, and transmission electron microscopy. Our results provide a roadmap and platform for developing 3D WSMs for device applications.
36 MATERIALS SCIENCE↗
Synthesis and Calculations of Wurtzite Al 1– x Gd x N Heterostructural Alloys
Not Available
36 MATERIALS SCIENCE↗
Epitaxial Dirac Semimetal Vertical Heterostructures for Advanced Device Architectures
Exploiting the extraordinary transport and optical properties of 3D topological semimetals for device applications requires epitaxial integration with semiconductors to carefully control carrier transport, yet no studies have established heteroepitaxy on top of any topological semimetals to date. Here, a novel approach toward fabricating heterostructures is demonstrated by epitaxially incorporating the Dirac semimetal Cd 3 As 2 between Zn x Cd 1-x Te and CdTe layers via molecular beam epitaxy on GaAs (001) substrates. The approach utilizes the higher energy (001) surface of Cd 3 As 2 to stabilize 2D epitaxy of zinc blende semiconductors. To demonstrate the impact heterostructure formation offers to device performance, an all-epitaxial, barrier-type vertical photodetector is fabricated that accesses a different carrier separation mechanism than previously reported non-epitaxial junctions and consequently exhibits significantly reduced dark currents. Finally, the results highlight the important role that epitaxial integration can play in accessing advanced architectures for topological semimetal-based devices.
36 MATERIALS SCIENCE↗
High Mobility Cd 3 As 2 (112) on GaAs(001) Substrates Grown via Molecular Beam Epitaxy
Not Available
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Accelerating Hydrogen Absorption and Desorption Rates in Palladium Nanocubes with an Ultrathin Surface Modification
Exploiting the high surface-area-to-volume ratio of nanomaterials to store energy in the form of electrochemical alloys is an exceptionally promising route for achieving high-rate energy storage and delivery. Nanoscale palladium hydride is an excellent model system for understanding how nanoscale-specific properties affect the absorption and desorption of energy carrying equivalents. Hydrogen absorption and desorption in shape-controlled Pd nanostructures does not occur uniformly across the entire nanoparticle surface. Instead, hydrogen absorption and desorption proceed selectively through high-activity sites at the corners and edges. Such a mechanism hinders the hydrogen absorption rates and greatly reduces the benefit of nanoscaling the dimensions of the palladium. To solve this, we modify the surface of palladium with an ultrathin platinum shell. This modification nearly removes the barrier for hydrogen absorption (89 kJ/mol without a Pt shell and 1.8 kJ/mol with a Pt shell) and enables diffusion through the entire Pd/Pt surface.
25 ENERGY STORAGE↗
Surface Conversion of Single-Crystal Bi 2 Se 3 to β-In 2 Se 3
In this work, we demonstrate that the surface layers of single-crystal layered-2D Bi 2 Se 3 can be converted to layered-2D rhombohedral ß-In 2 Se 3 by annealing under a trimethylindium (TMIn) flux. Samples were prepared in a metalorganic chemical vapor deposition (MOCVD) chamber, then transferred under vacuum to a surface analysis chamber for analysis with low-energy electron diffraction (LEED) and Auger electron spectroscopy. Additional ex situ characterization included x-ray diffraction, transmission-electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS) elemental mapping, and Raman spectroscopy. The resulting single-crystal ß-In 2 Se 3 adopts the rhombohedral crystal structure (space group R-3m) and orientation of the underlying Bi 2 Se 3 , and the excess Bi atoms generated by this process create an underlying region of Bi-rich Bi x Se y . Due to the difference in bandgap between Bi 2 Se 3 and In 2 Se 3 , this conversion reaction presents a pathway to lateral heterojunctions if only selected regions are converted by masking the surface to spatially define the TMIn exposure. The conversion may also have implications for heteroepitaxy, because the in-plane lattice constants of Bi 2 Se 3 and In 2 Se 3 (0001) surfaces match those of InP and GaAs (111), respectively, and the natural cleavage planes of a layered-2D crystal facilitate substrate removal and reuse.
14 SOLAR ENERGY↗
Insights into the Dynamic Interfacial and Bulk Composition of Copper-Modified, Hydrogen-Alloyed, Palladium Nanocubes under Electrocatalytic Conditions
Understanding and controlling the alloying properties of nanomaterials under electrochemical conditions are critically important for fields ranging from energy storage and catalysis to electrochromic window technology. Hydrogen-absorbing nanomaterials, like palladium, are especially interesting due to their ability to reversibly absorb and store hydrogen in their lattice at near-stoichiometric amounts. Palladium’s work function is also significantly deeper than that of most transition metals, which enables electrochemical underpotential deposition of conformal monolayer and submonolayer amounts of transition metals onto the palladium surface. The simultaneous existence of these two properties is unique and opens new and exciting avenues for electrochemical applications. However, the intersection of surface-modified, hydrogen-alloyed palladium nanomaterials is poorly understood, and specifically, how these structures evolve during electrochemical operating conditions remains an open question. Here, we synthesize {100}-terminated palladium nanocubes and deposit between 0.5 and 22 monolayers of copper at the palladium surface. We then electrochemically alloy these surface-modified structures with hydrogen. Using a combination of analytical electrochemistry, spectroscopy, and microscopy, we track the positional evolution of the Cu at the surface of Pd, its impact on palladium’s ability to absorb hydrogen, and copper’s effect on hydrogen evolution electrocatalysis. We show that Cu readily alloys into the palladium nanocube at potentials more negative than the Cu 2+/0 deposition, but a 0.5 monolayer thick copper layer remains at the Pd surface regardless of potential. Finally, we discuss the implications of these findings within the framework of CO 2 reduction catalysis for carbon–carbon bond-forming chemistry.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Mg x Zn 1−x O contact to CuGa 3 Se 5 absorber for photovoltaic and photoelectrochemical devices
Abstract CuGa 3 Se 5 is a promising candidate material with wide band gap for top cells in tandem photovoltaic and photoelectrochemical (PEC) devices. However, traditional CdS contact layers used with other chalcopyrite absorbers are not suitable for CuGa 3 Se 5 due to the higher position of its conduction band (CB) minimum. Mg x Zn 1− x O (MZO) is a transparent oxide with adjustable band gap and CB position as a function of magnesium composition, but its direct application is hindered by CuGa 3 Se 5 surface oxidation. Here, MZO is investigated as a contact (n-type ‘buffer’ or ‘window’) material to CuGa 3 Se 5 absorbers pretreated in Cd 2+ solution, and an onset potential close to 1 V vs reversible hydrogen electrode in 10 mM hexaammineruthenium (III) chloride electrolyte is demonstrated. The Cd 2+ surface treatment changes the chemical composition and electronic structure of the CuGa 3 Se 5 surface, as demonstrated by photoelectron spectroscopy measurements. The performance of CuGa 3 Se 5 absorber with Cd 2+ treated surface in the solid-state test structure depends on the Zn/Mg ratio in the MZO layer. The measured open circuit voltage of 925 mV is promising for tandem PEC water splitting with CuGa 3 Se 5 /MZO top cells.
36 MATERIALS SCIENCE↗
Tandem Heterogeneous Catalysis for Polyethylene Depolymerization via an Olefin-Intermediate Process
Not Available
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Application of templated vapor-liquid-solid growth to heteroepitaxy of InP on Si
Direct growth of III–V semiconductors on Si promises to combine the superior optoelectronic properties of III–Vs with the existing large-scale fabrication capabilities for Si. Vapor-liquid-solid-based growth techniques have previously been used to grow optoelectronic-quality III–Vs in polycrystalline films and various photolithography-defined features. We show that templated vapor-liquid-solid growth can produce epitaxial material when performed on crystalline substrates. In templated vapor-liquid-solid growth, the metal group-III precursor is evaporated along with a capping SiO 2 layer on the crystalline substrate, then melted and converted with exposure to a vapor-phase group-V precursor. We demonstrate homoepitaxial growth of InP on InP wafers using two forms of the SiO x capping layer to confine the liquid metal: evaporated SiO 2 and solgel SiO x , the latter of which is necessary for growth on Si. We then demonstrate heteroepitaxial growth of InP islands on Si substrates from both evaporated and electroplated In metals. The templated vapor-liquid-solid process provides better material utilization and growth rates than common vapor-phase techniques, with similar control and convenience, providing a path toward the large-scale fabrication of integrated optoelectronic components.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Six-junction III–V solar cells with 47.1% conversion efficiency under 143 Suns concentration
Single-junction flat-plate terrestrial solar cells are fundamentally limited to about 30% solar-to-electricity conversion efficiency, but multiple junctions and concentrated light make much higher efficiencies practically achievable. Until now, four-junction III–V concentrator solar cells have demonstrated the highest solar conversion efficiencies. Here, we demonstrate 47.1% solar conversion efficiency using a monolithic, series-connected, six-junction inverted metamorphic structure operated under the direct spectrum at 143 Suns concentration. Furthermore, when tuned to the global spectrum, a variation of this structure achieves a 1-Sun global efficiency of 39.2%. Nearly optimal bandgaps for six junctions were fabricated using alloys of III–V semiconductors. To develop these junctions, it was necessary to minimize threading dislocations in lattice-mismatched III–V alloys, prevent phase segregation in metastable quaternary III–V alloys and understand dopant diffusion in complex structures. Further reduction of the series resistance within this structure could realistically enable efficiencies over 50%.
14 SOLAR ENERGY↗