Quasi-coherent thermal emitter based on refractory plasmonic materials

The thermal emission of refractory plasmonic metamaterial - a titanium nitride 1D grating - is studied at high operating temperature (540 {\deg}C). By choosing a refractory material, we fabricate thermal gratings with high brightness that are emitting mid-infrared radiation centered around 3 $\mu$m. 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 32{\lambda} (angular divergence of 1.8{\deg}) 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.Comment: 10 pages, 5 figure

Surface morphology evolution of m-plane (1(1)over-bar00) GaN during molecular beam epitaxy growth: Impact of Ga/N ratio, miscut direction, and growth temperature

We present a systematic study of morphology evolution of [1 (1) over bar 00] m-plane GaN grown by plasma-assisted molecular beam epitaxy on free-standing m-plane substrates with small miscut angles towards the -c [000 (1) over bar] and +c [0001] directions under various gallium to nitrogen (Ga/N) ratios at substrate temperatures T = 720 degrees C and T = 740 degrees 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 [000 (1) over bar] miscut substrates. (C) 2013 AIP Publishing LLC

Saturation of intersubband transitions in p-doped GaAs/AlGaAs quantum wells

Optical saturation experiments have been performed on hh1-hh2 intersubband transitions in two samples of p-doped GaAs/AlGaAs quantum wells. The transitions had energies of 183 and 160 meV and the measured population relaxation times were 2±1.5 and 0.3±0.1 ps, respectively. Modeling of the quantum wells with a 6×6 k·p method shows that intersubband scattering by LO phonons can account for these relaxation times. The valence bandstructure is typically more complicated than the conduction bandstructure in a quantum well but these measurements show that LO phonons are the dominant intersubband scattering mechanism in both cases

Nanoalloying and phase transformations during thermal treatment of physical mixtures of Pd and Cu nanoparticles

Nanoscale alloying and phase transformations in physical mixtures of Pd and Cu ultrafine nanoparticles are investigated in real time with in situ synchrotron-based x-ray diffraction complemented by ex situ high-resolution transmission electron microscopy. The combination of metal-support interaction and reactive/non-reactive environment was found to determine the thermal evolution and ultimate structure of this binary system. At 300 degrees C, the nanoparticles supported on silica and carbon black intermix to form a chemically ordered CsCl-type (B2) alloy phase. The B2 phase transforms into a disordered fcc alloy at higher temperature (\u3e 450 degrees C). The alloy nanoparticles supported on silica and carbon black are homogeneous in volume, but evidence was found of Pd surface enrichment. In sharp contrast, when supported on alumina, the two metals segregated at 300 degrees C to produce almost pure fcc Cu and Pd phases. Upon further annealing of the mixture on alumina above 600 degrees C, the two metals interdiffused, forming two distinct disordered alloys of compositions 30% and 90% Pd. The annealing atmosphere also plays a major role in the structural evolution of these bimetallic nanoparticles. The nanoparticles annealed in forming gas are larger than the nanoparticles annealing in helium due to reduction of the surface oxides that promotes coalescence and sintering

Coherent vertical electron transport and interface roughness effects in AlGaN/GaN intersubband devices

We investigate electron transport in epitaxially-grown nitride-based resonant tunneling diodes (RTDs) and superlattice sequential tunneling devices. A density-matrix model is developed, and shown to reproduce the experimentally measured features of the current–voltage curves, with its dephasing terms calculated from semi-classical scattering rates. Lifetime broadening effects are shown to have a significant influence in the experimental data. Additionally, it is shown that the interface roughness geometry has a large effect on current magnitude, peak-to-valley ratios and misalignment features; in some cases eliminating negative differential resistance entirely in RTDs. Sequential tunneling device characteristics are dominated by a parasitic current that is most likely to be caused by dislocations, however excellent agreement between the simulated and experimentally measured tunneling current magnitude and alignment bias is demonstrated. This analysis of the effects of scattering lifetimes, contact doping and growth quality on electron transport highlights critical optimization parameters for the development of III-nitride unipolar electronic and optoelectronic devices

Gold-Copper Nanoparticles: Nanostructural Evolution and Bifunctional Catalytic Sites

National Science Foundation [CHE 0848701, CMMI 1100736]; DOE-BES [DE-SC0006877]; NYSERDA-NYBEST [18514]; DOEUnderstanding 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 Cu-precursor 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

Gold-copper nanoparticles: Nanostructural evolution and bifunctional catalytic sites

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 Cu-precursor 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. 漏 2012 American Chemical Society