102 research outputs found
Simulation of convergent-beam low-energy electron diffraction on Si(001) reconstructions
No full textResearch results based upon this code and data are published at http://doi.org/10.1016/j.apsusc.2019.05.274.
The image simulation of convergent beam low energy electron diffraction (CBLEED) patterns are used to determine the sensitivity of CBLEED to atomic-scale displacements of several reconstructed variants of the Si(001) surface. The CAVATN code is used to determine the dynamical LEED intensities as a function of the incident electron energy (Ei), angle (theta, phi) and at each of the miller indices (h,k), up to the third order. The CBLEED code then maps these intensities into reciprocal space, allowing the visualisation of CBLEED patterns to be made as a function of incident electron energy (Ei). The data files for the CBLEED simulations are stored in a .txt format, with an accompanying .png image displaying the result of the simulation. This data is then analysed to determine the sensitivity of CBLEED patterns to small atomic displacements.
CAVATN code: Relevant documentation, electron beam files and the crystal structure files are all included. The CAVATN dynamical LEED package, developed from the CAVLEED code, is also included, where the code employs the muffin-tin potential approximation and involves a set of phase shifts for each atom type (which are treated as spherically symmetric scatterers in a crystal) that can be evaluated using phase shift calculation packages or tables. In the simulations performed here, complex phase shifts were used to simulate temperature dependent scattering effects at T = 293K. The inner potential is treated as energy independent and is split into real Uor = 5 eV and imaginary Uoi = 10 eV parts to respectively treat refraction (via the vacuum and muffin-tin zero difference) and absorption (due to in- elastic processes). Multiple scattering between atoms within a layer is calculated using the chain method and the multiple scattering between layers is included by the renormalized forward scattering perturbation method to evaluate the wave amplitudes of diffracted beams at the surface, and hence the intensities of the LEED pattern.
CBLEED code: The dynamical CBLEED package is included as ‘cbleed_analysis_script.py’, where the CBLEED patterns are simulated by uniformly partitioning the convergent cone into square areas as shown in Figure 1. An incident electron beam is located at the centre of these squares and defined directionally by and . Each of the incident electron beams of the sampled convergent cone was then used as input to the dynamical LEED program CAVATN, so that the corresponding multiply scattered intensities could be determined and mapped into reciprocal space. All the output data files from the CBLEED code is available for the following structures in the ‘output’ folder; Si(001)-1x1-ideal, Si(001)-2x1-symmetric, Si(001)-2x1-buckled, Si(001)-2x1-dH (for dimer height displacements) and Si(001)-2x1-dL (for dimer length displacements). The data for the sensitivity to atomic-scale displacements is included in the ‘sensitivity_output’ folder, which determines both the partial and whole pattern sensitivities
Fabrication and characterization of metallic, two-dimensional dopant δ-layers in silicon
No full textWith the recent advances of deterministic atomic-scale patterning of phosphorous and arsenic on silicon, proposed architectures for silicon-based quantum computation are close to being realized. For future scalable devices, the role of atomically abrupt `delta' layer interfaces will be critical to device operation, so a further understanding is required in the two-dimensional (2D) physics involved. This thesis discusses a broad range of characterization methods that are employed to measure the properties of buried, 2D dopant δ-layers in silicon, whilst also developing a new method of resistless extreme ultra-violet (EUV) lithography on hydrogen passivated silicon. The first results chapter discusses the optimal method for quantifying secondary ion mass spectrometry (SIMS) depth profile measurements and the progress made towards standardizing scanning tunnelling microscopy (STM) based hydrogen desorption lithography at UCL. We then demonstrate that photoemission electron microscopy (PEEM) can be used to laterally image atomically-thin phosphorous and arsenic δ-layer patterns buried in silicon, with a minimum feature size of 25 nm. The second results chapter establishes the use of synchrotron radiation in the EUV range to desorb hydrogen on the Si(001)-(2x1):H surface. Using x-ray photoelectron spectroscopy (XPS) and STM data, we develop a method to quantify the surface dangling bond density, where the data reveals that the desorption mechanism is associated with valence band excitations mediated via secondary electrons. The third results chapter shows the first soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurements of phosphorous and arsenic δ-layers in silicon. We demonstrate that by measuring the kz extension of the out-of-plane valleys, this offers by far the most sensitive probe of electronic two-dimensionality of silicon δ-layers yet achieved. We found that arsenic δ-layers exhibit considerably more electronic two-dimensionality than their phosphorus counterparts and also measure the absolute charge densities, relative occupancies and donor sub-band minima of the δ-layers, which yield an excellent corroboration with theoretical predictions
On the sensitivity of convergent beam low energy electron diffraction patterns to small atomic displacements
No full textMultiple scattering simulations are developed and applied to assess the potential of convergent beam low-energy electron diffraction (CBLEED) to distinguish between various reconstructions of the Si(001) surface. This is found to be readily achievable through changes in pattern symmetry. A displacement R-factor approach is used to incorporate the angular content of CBLEED discs and identify optimal energy ranges for structure refinement. Defining a disc R-factor, optimal diffraction orders are identified which demonstrate an enhanced sensitivity to small atomic displacements. Using this approach, it was found that respective dimer height and length displacements as small as ±0.06 Å and ±0.20 Å could be detected
Comprehensive Review on Two-Step Thermochemical Water Splitting for Hydrogen Production in a Redox Cycle
No full textThe interest in and need for carbon-free fuels that do not rely on fossil fuels are constantly growing from both environmental and energetic perspectives. Green hydrogen production is at the core of the transition away from conventional fuels. Along with popularly investigated pathways for hydrogen production, thermochemical water splitting using redox materials is an interesting option for utilizing thermal energy, as this approach makes use of temperature looping over the material to produce hydrogen from water. Herein, two-step thermochemical water splitting processes are discussed and the key aspects are analyzed using the most relevant information present in the literature. Redox materials and their compositions, which have been proven to be efficient for this reaction, are reported. Attention is focused on non-volatile redox oxides, as the quenching step required for volatile redox materials is unnecessary. Reactors that could be used to conduct the reduction and oxidation reaction are discussed. The most promising materials are compared to each other using a multi-criteria analysis, providing a direction for future research. As evident, ferrite supported on yttrium-stabilized zirconia, ceria doped with zirconia or samarium and ferrite doped with nickel as the core and an yttrium (III) oxide shell are promising choices. Isothermal cycling and lowering of the reduction temperature are outlined as future directions towards increasing hydrogen yields and improving the cyclability.ChemE/Catalysis Engineerin
Computational Investigation of Microreactor Configurations for Hydrogen Production from Formic Acid Decomposition Using a Pd/C Catalyst
No full textThe need to replace fossil fuels with sustainable alternatives has been a critical issue in recent years. Hydrogen fuel is a promising alternative to fossil fuels because of its wide availability and high energy density. For the very first time, novel microreactor configurations for the formic acid decomposition have been studied using computational modeling methodologies. The decomposition of formic acid using a commercial 5 wt % Pd/C catalyst, under mild conditions, has been assessed in packed bed, coated wall, and membrane microreactors. Computational fluid dynamics (CFD) was utilized to develop the comprehensive heterogeneous microreactor models. The CFD modeling study begins with the development of a packed bed microreactor to validate the experimental work, subsequently followed by the theoretical development of novel microreactor configurations to perform further studies. Previous work using CFD modeling had predicted that the deactivation of the Pd/C catalyst was due to the production of the poisoning species CO during the reaction. The novel membrane microreactor facilitates the continuous removal of CO during the reaction, therefore prolonging the lifetime of the catalyst and enhancing the formic acid conversion by approximately 40% when compared to the other microreactor configurations. For all microreactors studied, the formic acid conversion increases as the temperature increases, and the liquid flow rate decreases. Further studies revealed that all microreactor configurations had negligible internal and external pore diffusion resistances. The detailed models developed in this work have provided an interesting insight into the intensification of the formic acid decomposition reaction over a Pd/C catalyst
Changes in General and Specific Psychopathology Factors Over a Psychosocial Intervention
No full textOBJECTIVE: Recent research suggests that comorbidity among child and adolescent psychiatric symptoms can be explained by a single general psychopathology ('p') factor, as well as more specific factors summarizing clusters of symptoms. We investigated within- and between-person changes in the general and specific psychopathology factors over a psychosocial intervention. METHOD: We ran a secondary analysis of the Systemic Therapy for At-Risk Teens study, a pragmatic randomized controlled trial that compared the effects of multisystemic therapy to management-as-usual for reducing antisocial behavior in 684 adolescents (82% male; 11-18 at baseline) over an 18-month period. The general p factor, as well as specific antisocial, attention, anxiety, and mood factors, were estimated from a symptom-level analysis of a set of narrow-band symptom scales measured repeatedly over the study. General and specific psychopathology factors were assessed for reliability, validity, and within- and between-person change using a parallel process multilevel growth model. RESULTS: A revised bifactor model that included a general p factor and specific anxiety, mood, antisocial, and attention factors with cross-loadings fit the data best. While the factor structure was multidimensional, p accounted for most of the variance in total scores. The p, anxiety, and antisocial factors predicted within-person variation in external outcomes. p and antisocial factors showed within-person reductions, while anxiety showed within-person increases over time. Despite individual variation in baseline factor scores, adolescents showed similar rates of change. CONCLUSION: The bifactor model is useful for teasing apart general and specific therapeutic changes which are conflated in standard analyses of symptom scores. CLINICAL TRIAL REGISTRATION INFORMATION: START (Systemic Therapy for At Risk Teens): A National Randomised Controlled Trial to Evaluate Multisystemic Therapy in the UK Context. http://www.isrctn.com; ISRCTN77132214
Hadron structure
No full text© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. This is a review of recent developments in hadron structure within the framework of Lattice QCD. The main focus is on recent achievements in the evaluation of nucleon quantities, such as the axial charge, electromagnetic form factors, the Dirac and Pauli radii, the quark momentum fraction and the spin content of the nucleon, in view of simulations at pion masses very close to their physical value. A discussion of the systematic uncertainties and the computation of the disconnected contributions using dynamical simulations is also included. Results emerging the propetries of particles other than the nucleon are summarized, highlighting selected hyperon and meson form factors
Spatially Resolved Dielectric Loss at the Si/SiO2 Interface
No full textThe Si=SiO2 interface is populated by isolated trap states that modify its electronic properties. These
traps are of critical interest for the development of semiconductor-based quantum sensors and computers,
as well as nanoelectronic devices. Here, we study the electric susceptibility of the Si=SiO2 interface with
nm spatial resolution using frequency-modulated atomic force microscopy. The sample measured here is a
patterned dopant delta layer buried 2 nm beneath the silicon native oxide interface. We show that charge
organization timescales of the Si=SiO2 interface range from 1–150 ns, and increase significantly around
interfacial traps. We conclude that under time-varying gate biases, dielectric loss in metal-insulatorsemiconductor capacitor devices is in the frequency range of MHz to sub-MHz, and is highly spatially
heterogeneous over nm length scales
Post - Independence Cypriot Dramaturgy (1960 Onwards)
No full textHow did playwriting evolve in Cyprus since independence (1960 and afterwards)? There is a systematic absence of studies on the subject. The author of this article analyzes the work of some of the most important playwrights of the period (Rina Katselli, Michalis Pitsillidis, Panos Ioannidis, Michalis Pasiardis and Yiorgos Neophytou) and describes their main carcteristics. Even though the writers of the 1960s continue to write in the naturalistic mode, others courageously follow more contemporary trends.How did playwriting evolve in Cyprus since independence (1960 and afterwards)? There is a systematic absence of studies on the subject. The author of this article analyzes the work of some of the most important playwrights of the period (Rina Katselli, Michalis Pitsillidis, Panos Ioannidis, Michalis Pasiardis and Yiorgos Neophytou) and describes their main carcteristics. Even though the writers of the 1960s continue to write in the naturalistic mode, others courageously follow more contemporary trends.Comment évolue l’écriture théâtrale pendant la période de l’indépendance (depuis 1960)? Il n’existe pas d’études systématiques sur ce sujet. Dans cet article sont analysées des œuvres de quelques auteurs dramatiques les plus représentatifs (Rina Katselli, Michalis Pitsillidis, Panos Ioannidis, Michalis Pasiardis et Yiorgos Neophytou) et sont résumées les lignes directrices de leur œuvre. Bien que les auteurs de la décennie 1960 continuent de créer en suivant la tradition du théâtre éthographique (théâtre de mœurs), on assiste à des efforts plus audacieux, qui tendent à une écriture théâtrale plus contemporaine
Spatially resolved dielectric loss at the Si/SiO interface
No full textThe Si/SiO interface is populated by isolated trap states which modify
its electronic properties. These traps are of critical interest for the
development of semiconductor-based quantum sensors and computers, as well as
nanoelectronic devices. Here, we study the electric susceptibility of the
Si/SiO interface with nm spatial resolution using frequency-modulated
atomic force microscopy to measure a patterned dopant delta-layer buried 2 nm
beneath the silicon native oxide interface. We show that surface charge
organization timescales, which range from 1-150 ns, increase significantly
around interfacial states. We conclude that dielectric loss under time-varying
gate biases at MHz and sub-MHz frequencies in metal-insulator-semiconductor
capacitor device architectures is highly spatially heterogeneous over nm length
scales.
Supplemental GIFs can be found at
https://doi.org/10.6084/m9.figshare.2554668
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