The design of active and robust bimetallic nanoparticle catalysts requires the control of the nanoscale alloying and phase-segregation structures and the correlation between the nanoscale phase structures and the catalytic properties. Here we describe new findings of a detailed investigation of such nanoscale phase structures and their structure-catalytic activity correlation for gold-platinum nanoparticles prepared with controllable sizes and compositions. The nanoscale alloying and phase-segregation were probed as a function of composition, size, and thermal treatment conditions using X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, electrochemical characterization, and density functional theory modeling. The results have provided the experimental evidence in support of the theoretically simulated dependence of alloying and phase segregation on particle size and temperature. More importantly, new insights have been gained into the control of the nanoscale phase properties of this bimetallic system among alloyed, partially alloyed, or partially phase segregated structures. In contrast to the largely alloyed character for the catalysts treated at 300-400°C, the higher-temperature treated catalysts (e.g., 800°C) are shown to consist of a Pt-rich alloy core and a Au shell or a phase-segregated Au domains enriched on the surface. This conclusion is further supported by the electrochemical and electrocatalytic data revealing that the catalytic activity is highly dependent on the nanoscale evolution of alloying and phase segregation. The thermal control of the nanoscale alloying, phase-segregation, and core-shell evolution of the nanoscale bimetallic catalysts provided the first example for establishing the correlation between the nanoscale phase structures and the electrocatalytic activity for oxygen reduction reaction correlation, which has profound implications to the design and nanoengineering of a wide variety of bimetallic or multimetallic nanostructures for advanced catalysts.

Understanding of the atomic-scale structure is essential for exploiting the unique catalytic properties of any nanoalloy catalyst. This report describes novel findings of an investigation of the nanoscale alloying of gold−copper (AuCu) nanoparticles and its impact on the surface catalytic functions. Two pathways have been explored for the formation of AuCu nanoparticles of different compositions, including wet chemical synthesis from mixed Au-and Cu-precursor molecules, and nanoscale alloying via an evolution of mixed Au-and Cuprecursor nanoparticles near the nanoscale melting temperatures. For the evolution of mixed precursor nanoparticles, synchrotron X-ray-based in situ real-time XRD was used to monitor the structural changes, revealing nanoscale alloying and reshaping toward an fcc-type nanoalloy (particle or cube) via a partial melting−resolidification mechanism. The nanoalloys supported on carbon or silica were characterized by in situ high-energy XRD/atomic pair disributoin function (PDF) analyses, revealing an intriguing lattice "expanding−shrinking" phenomenon depending on whether the catalyst is thermochemically processed under an oxidative or reductive atmosphere. This type of controllable structural changes is found to play an important role in determining the catalytic activity of the catalysts for carbon monoxide oxidation reaction. The tunable catalytic activities of the nanoalloys under thermochemically oxidative and reductive atmospheres are also discussed in terms of the bifunctional sites and the surface oxygenated metal species for carbon monoxide and oxygen activation. ■ INTRODUCTIONUnderstanding of the atomic-scale structure is essential for exploiting the unique catalytic properties of nanoalloy based catalysts. This understanding can be achieved by obtaining knowledge of the alloy formation process and the evolution of surface sites under reactive conditions. While the study of nanoscale alloy formation has been largely focused on the bottom-up approach, that is, the synthesis from the mixed metal precursors, recently, the focus has shifted to revealing the structural evolution of preformed metal or alloy nanoparticles. 1−4 One of the fundamental differences of metal particles confined to nanoscale dimensions from their bulk counterparts is the molecule-like chemical reactivity and the solid-like melting behavior. 5,6 This type of "molecule-solid duality" concept is applicable to multicomponent nanoscale systems (e.g., binary, ternary, etc.) in terms of alloying characteristics. Moreover, since metallic nanoparticles melt at lower temperatures than their bulk counterparts, the surface melting could take place at even lower temperatures, leading to an increased propensity of the nanoparticle surface atoms to detach, diffuse, and reattach. The knowledge of the dependency of surface melting on the size and strain 7 has been so far largely limited to unary metal particles. For small clusters of Au, Pt, Ag, and Cu with noncrystallographic 5-fold symmetry and irregular interatomic ordering, it i...

The ability to determine the atomic arrangement in nanoalloy catalysts and reveal the detailed structural features responsible for the catalytically active sites is essential for understanding the correlation between the atomic structure and catalytic properties, enabling the preparation of efficient nanoalloy catalysts by design. Herein we describe a study of CO oxidation over PdCu nanoalloy catalysts focusing on gaining insights into the correlation between the atomic structures and catalytic activity of nanoalloys. PdCu nanoalloys of different bimetallic compositions are synthesized as a model system and are activated by a controlled thermochemical treatment for assessing their catalytic activity. The results show that the catalytic synergy of Pd and Cu species evolves with both the bimetallic nanoalloy composition and temperature of the thermochemical treatment reaching a maximum at a Pd : Cu ratio close to 50 : 50. The nanoalloys are characterized structurally by ex situ and in situ synchrotron X-ray diffraction, including atomic pair distribution function analysis. The structural data show that, depending on the bimetallic composition and treatment temperature, PdCu nanoalloys adopt two different structure types. One features a chemically ordered, body centered cubic (B2) type alloy consisting of two interpenetrating simple cubic lattices, each occupied with Pd or Cu species alone, and the other structure type features a chemically disordered, face-centered cubic (fcc) type of alloy wherein Pd and Cu species are intermixed at random. The catalytic activity for CO oxidation is strongly influenced by the structural features. In particular, it is revealed that the prevalence of chemical disorder in nanoalloys with a Pd : Cu ratio close to 50 : 50 makes them superior catalysts for CO oxidation in comparison with the same nanoalloys of other bimetallic compositions. However, the catalytic synergy can be diminished if the Pd50Cu50 nanoalloys undergo phase segregation into distinct chemically-ordered (B2-type) and disordered (fcc-type) domains. This finding is significant since it provides a rational basis for streamlining the design and preparation of Pd-based nanoalloy catalysts in terms of atomic structure and phase state.

The thermal emission of refractory plasmonic metamaterial -a titanium nitride 1D grating -is studied at high operating temperature (540 °C). By choosing a refractory material, we fabricate thermal gratings with high brightness that are emitting mid-infrared radiation centered around 3 µm. We demonstrate experimentally that the thermal excitation of plasmon-polariton on the surface of the grating produces a well-collimated beam with a spatial coherence length of 32λ (angular divergence of 1.8°) which is quasi-monochromatic with a full width at half maximum of 70 nm. These experimental results show good agreement with a numerical model based on a two-dimensional full-wave analysis in frequency domain.

Low-aluminum composition AlGaN/GaN double-barrier resonant tunneling structures were grown by plasma-assisted molecular-beam-epitaxy on free-standing c-plane GaN substrates grown by hydride-vapor phase epitaxy. Clear, exactly reproducible, negative-differential resistance signatures were observed from 4 Â 4 lm 2 devices at 1.5 V and 1.7 V at 77 K. The relatively small value of the maximum peak-to-valley ratio (1.03) and the area dependence of the electrical characteristics suggest that charge transport is affected by leakage paths through dislocations. However, the reproducibility of the data indicates that electrical traps play no significant role in the charge transport in resonant tunneling diodes grown by molecular-beam-epitaxy under Ga-rich conditions on free-standing GaN substrates. V

We demonstrate THz intersubband absorption (15.6-26.1 meV) in m-plane AlGaN/GaN quantum wells. We find a trend of decreasing peak energy with increasing quantum well width, in agreement with theoretical expectations. However, a blue-shift of the transition energy of up to 14 meV was observed relative to the calculated values. This blue-shift is shown to decrease with decreasing charge density and is therefore attributed to many-body effects. Furthermore, a ∼40% reduction in the linewidth (from roughly 8 to 5 meV) was obtained by reducing the total sheet density and inserting undoped AlGaN layers that separate the wavefunctions from the ionized impurities in the barriers.PACS numbers: 78.67. De, 78.66.Fd The terahertz (THz) spectral region has attracted attention due to potential applications in medical diagnostics, security screening and quality control. GaAs/AlGaAs quantum cascade lasers (QCLs) have already demonstrated potential as THz sources in the 1.2-5 THz range. 1-5 However, the operating range of GaAs QCLs is limited by the longitudinal optical (LO) phonon emission at 36 meV (8.7 THz). Fortunately, GaN-based QCLs have the potential to operate in this range due to the larger LO-phonon energy (90 meV).To date, most studies of intersubband transitions in the III-nitrides have utilized the polar c-plane orientation. 6-10 Spontaneous emission from c-plane AlGaN/GaN QCLs in the THz region has been reported, although full laser operation has remained elusive. 11 The built-in polarization fields in c-plane heterostructures place a lower limit on the transition energy, and the inherent asymmetry in the conduction band profile reduces the dipole moment at the larger well widths required for operation in the THz region. These limitations have been partially mitigated by the implementation of more complex step-well designs. 9,12 However, the transition energies of these step-wells are highly sensitive to structural parameters. 13,14 Moreover, the additional layers significantly increase the complexity of design and growth of practical devices. The challenges of the built-in polarization fields can be circumvented by utilizing non-polar nitride heterostructures. Non-polar nitride structures may be achieved using either the cubic phase or the m-plane orientation of the wurzite phase. Near-infrared intersubband absorption in the cubic AlGaN/GaN has been observed, 15 and m-plane oriented We have already demonstrated molecular beam epitaxy (MBE) growth of high-quality m-plane AlGaN/GaN superlattices. 17,18 In this study, the samples consist of 26 quantum wells (QWs) grown on free-standing m-plane

We present a systematic study of morphology evolution of [11¯00] m-plane GaN grown by plasma-assisted molecular beam epitaxy on free-standing m-plane substrates with small miscut angles towards the –c [0001¯] and +c [0001] directions under various gallium to nitrogen (Ga/N) ratios at substrate temperatures T = 720 °C and T = 740 °C. The miscut direction, Ga/N ratio, and growth temperature are all shown to have a dramatic impact on morphology. The observed dependence on miscut direction supports the notion of strong anisotropy in the gallium adatom diffusion barrier and growth kinetics. We demonstrate that precise control of Ga/N ratio and substrate temperature yields atomically smooth morphology on substrates oriented towards +c [0001] as well as the more commonly studied –c [0001¯] miscut substrates.

We report a systematic and quantitative study of near-infrared intersubband absorption in strained AlGaN/GaN and lattice-matched AlInN/GaN superlattices grown by plasma-assisted molecular-beam epitaxy as a function of Si-doping profile with and without δ doping. For AlGaN/GaN, we obtained good theoretical agreement with experimental measurements of transition energy, integrated absorbance and linewidth by considering many-body effects, interface roughness, and calculations of the transition lifetime that include dephasing. For the AlInN/GaN system, experimental measurements of the integrated absorbance due to the superlattice transitions produced values more than one order of magnitude lower than AlGaN/GaN heterostructures at similar doping levels. Furthermore, observed transition energies were roughly 150 meV higher than expected. The weak absorption and high transition energies measured in these structures is attributed to columnar alloy inhomogeneity in the AlInN barriers observed in high-angle annular dark-field scanning transmission electron microscopy. We simulated the effect of these inhomogeneities using three-dimensional band-structure calculations. The inhomogeneities were modeled as AlInN nanorods with radially varying In composition embedded in the barrier material of the superlattice. We show that inclusion of the nanorods leads to the depletion of the quantum wells (QWs) due to localization of charge carriers in high-In-containing regions. The higher energy of the intersubband transitions was attributed to the relatively uniform regions of the QWs surrounded by high Al (95%) composition barriers. The calculated transition energy assuming Al 0.95 In 0.05 N barriers was in good agreement with experimental results.