Search

Abstract: We report direct observations of the nucleation and early stages of growth of GaN nanowires. The nanowires were formed by exposing Au+Ga droplets to ammonia. The formation process was observed in situ, and controlled in real time using an environmental transmission electron microscope. Observations show that nuclei forms only at thin region of the Au+Ga, following only the VLS mechanism. The droplet–nanowire interface in the initial stages is multi-faceted, and become planar during the growth. Chemical and structural analysis shows that the resulting nuclei are GaN with the wurtzite structure. [Copyright &y& Elsevier]

Growth of wurtzite ScxAl1−xN (x < 0.23) by plasma-assisted molecular-beam epitaxy on c-plane GaN at high temperatures significantly alters the extracted lattice constants of the material due to defects likely associated with remnant phases. In contrast, ScAlN grown below a composition-dependent threshold temperature exhibits uniform alloy distribution, reduced defect density, and atomic-step surface morphology. The c-plane lattice constant of this low-temperature ScAlN varies with composition as expected from previous theoretical calculations and can be used to reliably estimate alloy composition. Moreover, lattice-matched Sc0.18Al0.82N/GaN multi-quantum wells grown under these conditions display strong and narrow near-infrared intersubband absorption lines that confirm advantageous optical and electronic properties. [ABSTRACT FROM AUTHOR]

Anomalous growth rate reduction and associated composition divergence with increasing aluminum flux in m-plane AlGaN grown by plasma-assisted molecular beam epitaxy at low temperature (565 °C) are observed and investigated. We find that the AlGaN growth rate under conventional gallium-rich conditions decreases rapidly with increasing aluminum flux, contrary to expectations. Moreover, the aluminum fraction of these layers increases super-linearly with aluminum flux, indicating substantial nitrogen and gallium loss from the crystal surface. Indium surfactant assisted epitaxy (ISAE) is found to mitigate this effect significantly. ISAE AlGaN layers do not exhibit a significant decrease in the growth rate with increasing aluminum flux, and their aluminum compositions increase linearly with aluminum flux. Transmission electron microscopy (TEM) images reveal the presence of high-aluminum composition defects within the conventionally grown AlGaN layers, which are significantly reduced in ISAE AlGaN layers. Spatial correlation of these defects with local areas of reduced growth rate observed in an (In)Al0.30Ga0.70N/In0.16Ga0.84N multiple quantum well (MQW) structure suggests that these phenomena have a causal relationship. We attribute the growth rate reduction to the loss of nitrogen and gallium due to site-blocking effects of aluminum adatoms. TEM imaging indicates that high-quality, virtually defect-free (In)Al0.24Ga0.76N/In0.16Ga0.84N MQWs can be grown at 565 °C with negligible indium incorporation into the barriers. [ABSTRACT FROM AUTHOR]

The use of an indium surfactant considerably alters the composition and morphology of low-temperature non-polar m-plane (10 1 ¯ 0) AlxGa1−xN (x ∼ 0.2) and of silicon-doped AlGaN/InGaN multiple quantum wells grown by plasma-assisted molecular beam epitaxy. This paper compares heterostructures grown with indium surfactant with those grown under conventional stoichiometric and gallium-rich conditions at the relatively low temperature necessary for growth of In0.16Ga0.84N quantum wells (565 °C). Stoichiometric growth results in rough, inhomogeneous AlGaN layers that are unsuitable for optical devices. Gallium-rich growth produces a smoother AlGaN layer, reduced inhomogeneities, and sharper interfaces as compared to stoichiometric growth. However, due to the low temperature, gallium-rich growth leads to the formation of an unintentional GaN layer on top of each AlGaN barrier, reducing the energies of confined electronic states in the quantum wells. An indium surfactant enables two-dimensional AlGaN growth at low temperature, producing atomically flat surface morphology and sharp heterostructure interfaces. Indium surfactant assisted epitaxy also eliminates the high aluminum alloy inhomogeneities observed with conventional stoichiometric and gallium-rich growth. Even though partial indium incorporation into the AlGaN layer is found at the studied temperatures, the high-quality, uniform non-polar In0.055Al0.19Ga0.755N/In0.16Ga0.84N quantum wells grown with indium surfactant display bright and narrow photoluminescence that is essential for device applications. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) spectroscopy of nonpolar m-plane InGaN thin films with indium composition up to 21% and nearly strain-balanced In0.09Ga0.91N/Al0.19Ga0.81N superlattices grown by plasma-assisted molecular beam epitaxy was performed as a function of temperature. The experimental transition energies are consistently lower than the calculation based on structural parameters extracted from x-ray diffraction measurements. This indicates the presence of indium composition fluctuations in InGaN and hence local bandgap reduction that produces charge localization centers. The spectral width of the low-temperature PL of our m-plane InGaN/AlGaN superlattices is narrower than previously reported for m-plane InGaN/GaN quantum wells grown by MOCVD. The PL integrated intensity drops rapidly, though, as the temperature is increased to 300 K, indicating strong non-radiative recombination at room temperature. Time-resolved PL at low temperatures was performed to characterize the relaxation time scales in an undoped and a doped superlattice. [ABSTRACT FROM AUTHOR]

Temperature‐dependent continuous‐excitation and time‐resolved photoluminescence are studied to probe carrier localization and recombination in nearly strain‐balanced m‐plane In0.09Ga0.91N/Al0.19Ga0.81N multi‐quantum wells grown by plasma‐assisted molecular‐beam epitaxy. An average localization depth of 21 meV is estimated for the undoped sample. This depth is much smaller than the reported values in polar structures and m‐plane InGaN quantum wells. As part of this study, temperature and magnetic field dependence of time‐resolved photoluminescence is performed. At 2 K, an initial fast decay time of ≈0.3 ns is measured for both undoped and doped structures. The undoped sample also exhibits a slow decay component with a time scale of 2.2 ns. The existence of two relaxation paths in the undoped structure can be attributed to different localization centers. The fast relaxation decays are relatively insensitive to external magnetic fields, while the slower relaxation time constant decreases significantly with increasing magnetic fields. The fast decay time scale in the undoped sample is likely due to indium fluctuations in the quantum well. The slow decay time may be related to carrier localization in the barriers. The addition of doping leads to a single fast decay time likely due to stronger exciton localization in the InGaN quantum wells. [ABSTRACT FROM AUTHOR]

Ein edelmetallfreier Elektrokatalysator wurde für die Sauerstoffreduktionsreaktion entwickelt. Kohlenstoff ‐ fixiertes Cobalt ‐ Molybdän ‐ Oxynitrid wurde einfach durch Imprägnierung mit anschließender thermischer Ammonolyse hergestellt. Die elektrokatalytische Aktivität des Oxynitrids kam in 0.1 M KOH ‐ Lösung fast der von Platin gleich. Katalysatorstruktur und ‐ valenz wurden mithilfe komplementärer lokaler und Volumensonden charakterisiert. [ABSTRACT FROM AUTHOR]

A non‐noble metal electrocatalyst has been developed for the oxygen reduction reaction. Carbon‐supported cobalt molybdenum oxynitride was simply prepared by impregnation followed by thermal ammonolysis. The oxynitride electrocatalytic activity nearly matched that of platinum in 0.1 M KOH solution. The catalyst structure and valence have been characterized using complementary local and bulk probes. [ABSTRACT FROM AUTHOR]

• Aluminum thin films epitaxially grown on two-dimensional InAs quantum wells are characterized. • The normal orientation of aluminum film changes as the surface lattice parameter is varied. • The driving force of the aluminum orientation is analyzed. We present a detailed study of 7-nm thick Al layers epitaxially grown on In x Al 1−x As or Al 1−x Ga x Sb y As 1−y (0 0 1) semiconductors using high-resolution transmission electron microscopy (HRTEM) and synchrotron X-ray diffraction (XRD). These techniques provide high spatial and high reciprocal space resolution information about Al lattice orientations and interfacial structure, highlighting the existence of a strong correlation between the surface lattice parameter of the semiconductor and the normal orientation of the aluminum film. The normal orientation of Al layers is found to be [1 1 1] when the in-plane semiconductor surface lattice parameter, a S , is smaller than 5.98 Å, and is [1 1 0] when a S is larger than this value. In the transition between these lattice parameters where one orientation is preferred, both orientations may be observed. Concomitant with the change in normal orientation is an inversion in the strain state of the aluminum film. When the normal orientation is [1 1 1], the Al film is compressively strained; while for [1 1 0] normal orientations, the Al in-plane strain is tensile. An energy balance model, accounting for surface, interfacial, side-face, and strain energy, and the registry of (1 1 1 ¯) lattice planes across the semiconductor/aluminum interface provide explanations for the observed normal orientation and strain inversion. [ABSTRACT FROM AUTHOR]

The design of functional interfaces is central to both fundamental and applied research in materials science and energy technology. We introduce a new, broadly applicable technique for the precisely controlled high‐throughput preparation of well‐defined interfaces containing polyatomic species ranging from small ions to nanocrystals and large protein complexes. The mass‐dispersive deposition of ions onto surfaces is achieved using a rotating‐wall mass analyzer, a compact device which enables the separation of ions using low voltages and has a theoretically unlimited mass range. We demonstrate an efficient deposition of singly charged Au144(SC4H9)60 ions (33.7 kDa), which opens up exciting opportunities for the structural characterization of nanocrystals and their assemblies using transmission electron microscopy. Our approach also enables the high‐throughput deposition of mass‐selected ions from multicomponent mixtures, which is of interest to the controlled preparation of surface gradients and rapid screening of molecules in mixtures for a specific property. [ABSTRACT FROM AUTHOR]

The design of functional interfaces is central to both fundamental and applied research in materials science and energy technology. We introduce a new, broadly applicable technique for the precisely controlled high‐throughput preparation of well‐defined interfaces containing polyatomic species ranging from small ions to nanocrystals and large protein complexes. The mass‐dispersive deposition of ions onto surfaces is achieved using a rotating‐wall mass analyzer, a compact device which enables the separation of ions using low voltages and has a theoretically unlimited mass range. We demonstrate an efficient deposition of singly charged Au144(SC4H9)60 ions (33.7 kDa), which opens up exciting opportunities for the structural characterization of nanocrystals and their assemblies using transmission electron microscopy. Our approach also enables the high‐throughput deposition of mass‐selected ions from multicomponent mixtures, which is of interest to the controlled preparation of surface gradients and rapid screening of molecules in mixtures for a specific property. [ABSTRACT FROM AUTHOR]

Nonpolar AlGaN/GaN heterostructures have the potential to supplant polar heterostructures in infrared optoelectronic devices due to their theoretical advantages stemming from the absence of built‐in polarization fields along nonpolar directions of the nitride wurtzite lattice. However, development of nonpolar m‐plane infrared devices in a broad spectral range has been hampered, so far, by challenges to grow homogeneous high Al‐composition AlGaN on m‐plane GaN. AlxGa1‐xN layers with 0.6< x <0.8 are found to be kinetically unstable under metal‐rich growth conditions by plasma‐assisted molecular‐beam epitaxy. After reviewing recent progress in the field, this paper focuses on the effect of the structure of m‐plane AlxGa1−xN/GaN (x <0.6) superlattices on near‐infrared intersubband absorption. Even at these intermediate Al‐compositions, the effective growth rate of AlGaN is drastically reduced, and the AlGaN‐GaN interface roughness is unexpectedly high. Consequently, accurate determination of layer thicknesses and alloy composition necessitates structural characterization by a combination of scanning transmission electron microscopy and high‐resolution X‐ray diffraction. The energy and linewidth of near‐infrared intersubband transitions are also significantly affected by this unusual growth behavior. The experimental results for intersubband absorption of m‐ and c‐plane superlattices are compared to each other and with numerical calculations, and the main reasons for discrepancies are discussed. [ABSTRACT FROM AUTHOR]

The structure of silica supported Pt and Pt–In bimetallic catalysts with nominal In:Pt atomic ratios of 0.7 and 1.4 were determined by in situ synchrotron XAS and XRD. It was seen that the addition of In led to the formation of two different intermetallic alloy phases. At an In:Pt ratio of 0.7 the Pt 3 In phase with a Cu 3 Au structure was formed. When the ratio was increased to 1.4 a shell of PtIn 2 having a CaF 2 structure formed around a core of Pt 3 In. The catalysts were tested for ethane dehydrogenation at 600 °C to determine the effect of alloying on ethylene selectivity and turnover rate (TOR). The monometallic Pt catalysts was 73% selective for ethylene and had an initial TOR of 0.7 s −1 . Both alloy catalysts were ≈100% selective for dehydrogenation and had higher initial TOR, 2.8 s −1 and 1.6 s −1 for In:Pt ratio of 0.7 and 1.4, respectively. The increase in selectivity is attributed to the elimination of large Pt ensembles resulting from geometric changes to the catalyst surface upon alloying. Electronic changes due to the formation of Pt–In bonds are thought to be responsible for the increases in TOR in the alloy catalysts. [ABSTRACT FROM AUTHOR]

Magnetron-sputtering inert-gas condensation is an emerging technique offering single-step, chemical-free synthesis of nanoparticles with well-defined morphologies optimized for specific applications. In this study, the authors report a flexible approach to produce Fe nanocubes as building blocks for high-performance NO2 gas sensor devices, and hybrid FeAu nanocubes with magneto-plasmonic properties. Considering that nucleation happens within a short distance from the sputtering target, the authors utilize the high-permeability and resultant screening effect induced by magnetic Fe targets of various thicknesses to manipulate the magnetic field configuration and plasma confinement. The authors thus readily switch from bimodal to single-Gaussian size distributions of Fe nanocubes by modifying their primordial thermal environments, as explained by a combination of modeling methods. Simultaneously, the authors obtain a material yield increase of more than one order of magnitude compared to experiments using postgrowth mass filtration. The effectiveness of the method is demonstrated by the deposition of Fe nanocubes on microhotplate devices, leading to unprecedented NO2 detection performance for Fe-based chemoresistive gas sensors. The exceedingly low detection limit down to 3 ppb is attributed to a morphological change in operando from Fe/Fe-oxide core/shell to specific hollow-nanocube structures, as revealed by in situ environmental transmission electron microscopy. [ABSTRACT FROM AUTHOR]

Magneto-plasmonic hybrid nanoparticles (HNPs) are promising for a large number for dual magneto-optical bioapplications. Gas-phase techniques offer a good alternative to chemical routes for the generation of tailored HNPs. Here, we present a novel method to synthesize ternary HNPs composed of multiple dumbbell-like FeAg cores encapsulated by an amorphous Si shell. The method involves a simultaneous sputtering of Fe, Ag and Si targets under controlled conditions. We demonstrate that the morphology and the size of the HNPs can be modulated by tuning experimental parameters such as the energy and the cooling rate, or the collision and coalescence processes experienced by the HNPs during their formation. We find that by increasing the residence time of the HNPs in the aggregation zone, we increase both the size of the HNPs, and the thickness of the Si shell. HNPs exhibit ferromagnetic behavior and show an enhanced, red-shifted, light absorption band due to the strong near-field coupling between the Ag cores and the Si shell. A mechanism of formation of these HNPs is suggested, combining the physico-chemical properties of the materials (Fe, Ag, Si) with the experimental conditions. [ABSTRACT FROM AUTHOR]

Carbon supported cobalt molybdenum oxynitrides, CoxMo1-xOyNz/C (x = 0.25, 0.50, 0.75) are prepared from aqueous solutions of Co(NO3)2, (NH4)6Mo7O24, and carbon black followed by ammonolysis of the obtained powders at 773-973 K for 3 h. [ABSTRACT FROM AUTHOR]

Well-defined surfaces of Pt have been extensively studied for various catalytic processes. However, industrial catalysts are mostly composed of fine particles (e.g., nanocrystals), due to the desire for a high surface to volume ratio. Therefore, it is very important to explore and understand the catalytic processes both at nanoscale and on extended surfaces. In this report, a general synthetic method is described to prepare Pt nanocrystals with various morphologies. The synthesized Pt nanocrystals are further purified by exploiting the "self-cleaning" effect which results from the "colloidal recrystallization" of Pt supercrystals. The resulting high-purity nanocrystals enable the direct comparison of the reactivity of the {111} and {100} facets for important catalytic reactions. With these high-purity Pt nanocrystals, we have made several observations: Pt octahedra show higher poisoning tolerance in the electrooxidation of formic acid than Pt cubes; the oxidation of CO on Pt nanocrystals is structure insensitive when the partial pressure ratio pO2/pCO is close to or less than 0.5, while it is structure sensitive in the O2-rich environment; Pt octahedra have a lower activation energy than Pt cubes when catalyzing the electron transfer reaction between hexacyanoferrate (III) and thiosulfate ions. Through electrocatalysis, gas-phase-catalysis of CO oxidation, and a liquid-phase-catalysis of electron transfer reaction, we demonstrate that high quality Pt nanocrystals which have {111} and {100} facets selectively expose are ideal model materials to study catalysis at nanoscale. [ABSTRACT FROM AUTHOR]

Well-defined surface, such as surface of a single crystal, is being used to provide precise interpretation of catalytic processes, while the nanoparticulate model catalyst more closely represents the real catalysts that are used in industrial processes. Nanocrystal superlattice, which combines the chemical and physical properties of different materials in a single crystalline structure, is an ideal model catalyst, that bridge between conventional models and real catalysts. We identify the active sites for carbon monoxide (CO) oxidation on Au-FeOx catalysts by using Au-FeOx binary superlattices correlating the activity to the number density of catalytic contacts between Au and FeOx. Moreover, using nanocrystal superlattices, we propose a general strategy of keeping active metals spatially confined to enhance the stability of metal catalysts. With a great range of nanocrystal superlattice structures and compositions, we establish that nanocrystal superlattices are useful model materials through which to explore, understand, and improve catalytic processes bridging the gap between traditional single crystal and supported catalyst studies. [ABSTRACT FROM AUTHOR]

Large-area icosahedral-AB13-type Pt-Pd binary superlattices (BNSLs) are fabricated through self-assembly of 6 nm Pd nanocrystals (NCs) and 13 nm Pt octahedra at a liquid-air interface. The Pt-Pd BNSLs enable a high activity toward electrocatalysis of oxygen reduction reaction (ORR) by successfully exploiting the shape effects of Pt NCs and synergistic effects of Pt-Pd into a single crystalline nanostructure. The Pt-Pd BNSLs are promising catalysts for the oxygen electrode of fuel cells. [ABSTRACT FROM AUTHOR]

Interactions between ceria (Ce02) and supported metals greatly enhance rates for a number of important reactions. However, direct relationships between structure and function in these catalysts have been difficult to extract because the samples studied either were heterogeneous or were model systems dissimilar to working catalysts. We report rate measurements on samples in which the length of the ceria-metal interface was tailored by the use of monodisperse nickel, palladium, and platinum nanocrystals. We found that carbon monoxide oxidation in ceria-based catalysts is greatly enhanced at the ceria-metal interface sites for a range of group VIII metal catalysts, clarifying the pivotal role played by the support. [ABSTRACT FROM AUTHOR]

In article number 1605328 Mukhles Sowwan and co‐workers synthesize multi‐functional hybrid Fe nanocubes via a flexible gas phase method providing morphological control during formation. This approach enabled tailoring the magneto‐plasmonic properties of Fe nanocubes by Au doping and realizing Fe‐based gas sensors with remarkable performance for NO2 detection down to concentrations as low as 3 ppb. [ABSTRACT FROM AUTHOR]