387 research outputs found
Photoemission study and band alignment of GaN passivation layers on GaInP heterointerface
III-V semiconductor-based photoelectrochemical (PEC) devices show the highest
solar-to-electricity or solar-to-fuel conversion efficiencies. GaInP is a
relevant top photoabsorber layer or a charge-selective contact in PEC for
integrated and direct solar fuel production, due to its tunable lattice
constant, electronic band structure, and favorable optical properties. To
enhance the stability of its surface against chemical corrosion which leads to
decomposition, we deposit a GaN protection and passivation layer. The n-doped
GaInP(100) epitaxial layers were grown by metalorganic chemical vapor
deposition on top of GaAs(100) substrate. Subsequently, thin 1-20 nm GaN films
were grown on top of the oxidized GaInP surfaces by atomic layer deposition. We
studied the band alignment of these multi-junction heterostructures by X-ray
and ultraviolet photoelectron spectroscopy. Due to the limited emission depth
of photoelectrons, we determined the band alignment by a series of separate
measurements in which we either modified the GaInP(100) surface termination or
the film thickness of the grown GaN on GaInP(100) buffer layers. On
n-GaInP(100) surfaces prepared with the well-known phosphorus-rich (2x2)/c(4x2)
reconstruction we found up-ward surface band bending (BB) of 0.34 eV, and Fermi
level pinning due to the present surface states. Upon oxidation, the surface
states are partially passivated resulting in a reduction of BB to 0.12 eV and a
valence band offset (VBO) between GaInP and oxide bands of 2.0 eV. Between the
GaInP(100) buffer layer and the GaN passivation layer, we identified a VBO of
1.8 eV. The corresponding conduction band offset of -0.2 eV is found to be
rather small. Therefore, we evaluate the application of the GaN passivation
layer as a promising technological step not only to reduce surface states but
also to increase the stability of the surfaces of photoelectrochemical devices
Harmonic Generation from Relativistic Plasma Surfaces in Ultra-Steep Plasma Density Gradients
Harmonic generation in the limit of ultra-steep density gradients is studied
experimentally. Observations demonstrate that while the efficient generation of
high order harmonics from relativistic surfaces requires steep plasma density
scale-lengths () the absolute efficiency of the harmonics
declines for the steepest plasma density scale-length , thus
demonstrating that near-steplike density gradients can be achieved for
interactions using high-contrast high-intensity laser pulses. Absolute photon
yields are obtained using a calibrated detection system. The efficiency of
harmonics reflected from the laser driven plasma surface via the Relativistic
Oscillating Mirror (ROM) was estimated to be in the range of 10^{-4} - 10^{-6}
of the laser pulse energy for photon energies ranging from 20-40 eV, with the
best results being obtained for an intermediate density scale-length
Synergistic Catalytic Effects of Alloys of Noble Metal Nanoparticles Supported on Two Different Supports:Crystalline Zeolite Sn-Beta and Carbon Nanotubes for Glycerol Conversion to Methyl Lactate
Two multifunctional catalytic systems comprising Sn-based/doped crystalline zeolite Beta were synthesized, and they were employed as heterogeneous catalysts in the selective conversion of glycerol to methyl lactate. The first catalytic system, named Au-Pd-Sn-deAl-7.2-Beta-DP, was created through the post-synthesis dealumination of the parent zeolite Beta (Si/Al = 10) using 7.2 M HNO3. Subsequently, it was grafted with 27 mmol of SnCl4, resulting in Sn-deAl-7.2-Beta. Following this, Au and Pd nanoparticles were supported on this catalyst using the depositionâprecipitation (DP) method. The second catalytic system was a physical mixture of Au and Pd nanoparticles supported on functionalized carbon nanotubes (Au-Pd-F-CNTs) and Sn-containing zeolite Beta (Sn-deAl-7.2-Beta). Both catalytic systems were employed in glycerol partial oxidation to methyl lactate under the following conditions: 140 °C for 4.5 h under an air pressure of 30 bar. The Au-Pd-Sn-deAl-7.2-Beta-DP catalytic system demonstrated 34% conversion of glycerol with a 76% selectivity for methyl lactate. In contrast, the physical mixture of Au-Pd-F-CNTs and Sn-deAl-7.2-Beta exhibited higher activity, achieving 58% glycerol conversion and a nearly identical selectivity for methyl lactate (77%). The catalytic results and catalyst structure were further analyzed using various characterization techniques, such as X-ray diffraction (XRD), N2 physisorption, scanning electron microscopy (SEM), X-ray fluorescence (XRF), transmission electron microscopy (TEM), UV-vis spectroscopy, and pyridine Fourier transform infrared (FTIR). These analyses emphasized the significance of adjusting the quantity of active sites, particle size, and active sites proximity under the chosen reaction conditions.</p
Synergistic Catalytic Effects of Alloys of Noble Metal Nanoparticles Supported on Two Different Supports:Crystalline Zeolite Sn-Beta and Carbon Nanotubes for Glycerol Conversion to Methyl Lactate
Two multifunctional catalytic systems comprising Sn-based/doped crystalline zeolite Beta were synthesized, and they were employed as heterogeneous catalysts in the selective conversion of glycerol to methyl lactate. The first catalytic system, named Au-Pd-Sn-deAl-7.2-Beta-DP, was created through the post-synthesis dealumination of the parent zeolite Beta (Si/Al = 10) using 7.2 M HNO3. Subsequently, it was grafted with 27 mmol of SnCl4, resulting in Sn-deAl-7.2-Beta. Following this, Au and Pd nanoparticles were supported on this catalyst using the depositionâprecipitation (DP) method. The second catalytic system was a physical mixture of Au and Pd nanoparticles supported on functionalized carbon nanotubes (Au-Pd-F-CNTs) and Sn-containing zeolite Beta (Sn-deAl-7.2-Beta). Both catalytic systems were employed in glycerol partial oxidation to methyl lactate under the following conditions: 140 °C for 4.5 h under an air pressure of 30 bar. The Au-Pd-Sn-deAl-7.2-Beta-DP catalytic system demonstrated 34% conversion of glycerol with a 76% selectivity for methyl lactate. In contrast, the physical mixture of Au-Pd-F-CNTs and Sn-deAl-7.2-Beta exhibited higher activity, achieving 58% glycerol conversion and a nearly identical selectivity for methyl lactate (77%). The catalytic results and catalyst structure were further analyzed using various characterization techniques, such as X-ray diffraction (XRD), N2 physisorption, scanning electron microscopy (SEM), X-ray fluorescence (XRF), transmission electron microscopy (TEM), UV-vis spectroscopy, and pyridine Fourier transform infrared (FTIR). These analyses emphasized the significance of adjusting the quantity of active sites, particle size, and active sites proximity under the chosen reaction conditions.</p
Solid-State NMR Spectroscopy:Towards Structural Insights into Starch-Based Materials in the Food Industry
Solid-state NMR is a nondestructive and noninvasive technique used to study the chemical structure and dynamics of starch-based materials and to bridge the gap between structureâfunction relationships and industrial applications. The study of crystallinity, chemical modification, product blending, molecular packing, amyloseâamylopectin ratio, end chain motion, and solventâmatrix interactions is essential for tailoring starch product properties to various applications. This article aims to provide a comprehensive and critical review of research characterizing starch-based materials using solid-state NMR, and to briefly introduce the most advanced and promising NMR strategies and hardware designs used to overcome the sensitivity and resolution issues involved in structureâfunction relationships
A Survey on Continuous Time Computations
We provide an overview of theories of continuous time computation. These
theories allow us to understand both the hardness of questions related to
continuous time dynamical systems and the computational power of continuous
time analog models. We survey the existing models, summarizing results, and
point to relevant references in the literature
Dynamics of mechanical waves in periodic grapheme nanoribbon assemblies
We simulate the natural frequencies and the acoustic wave propagation characteristics of graphene nanoribbons (GNRs) of the type (8,0) and (0,8) using an equivalent atomistic-continuum FE model previously developed by some of the authors, where the C-C bonds thickness and average equilibrium lengths during the dynamic loading are identified through the minimisation of the system Hamiltonian. A molecular mechanics model based on the UFF potential is used to benchmark the hybrid FE models developed. The acoustic wave dispersion characteristics of the GNRs are simulated using a Floquet-based wave technique used to predict the pass-stop bands of periodic mechanical structures. We show that the thickness and equilibrium lengths do depend on the specific vibration and dispersion mode considered, and that they are in general different from the classical constant values used in open literature (0.34 nm for thickness and 0.142 nm for equilibrium length). We also show the dependence of the wave dispersion characteristics versus the aspect ratio and edge configurations of the nanoribbons, with widening band-gaps that depend on the chirality of the configurations. The thickness, average equilibrium length and edge type have to be taken into account when nanoribbons are used to design nano-oscillators and novel types of mass sensors based on periodic arrangements of nanostructures
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