307 research outputs found
Accuracy of dielectric-dependent hybrid functionals in the prediction of optoelectronic properties of metal oxide semiconductors: a comprehensive comparison with many-body GW and experiments
Understanding the electronic structure of metal oxide semiconductors is crucial to their numerous technological applications, such as photoelectrochemical water splitting and solar cells. The needed experimental and theoretical knowledge goes beyond that of pristine bulk crystals, and must include the effects of surfaces and interfaces, as well as those due to the presence of intrinsic defects (e.g. oxygen vacancies), or dopants for band engineering. In this review, we present an account of the recent efforts in predicting and understanding the optoelectronic properties of oxides using ab initio theoretical methods. In particular, we discuss the performance of recently developed dielectric-dependent hybrid functionals, providing a comparison against the results of many-body GW calculations, including G 0 W 0 as well as more refined approaches, such as quasiparticle self-consistent GW. We summarize results in the recent literature for the band gap, the band level alignment at surfaces, and optical transition energies in defective oxides, including wide gap oxide semiconductors and transition metal oxides. Correlated transition metal oxides are also discussed. For each method, we describe successes and drawbacks, emphasizing the challenges faced by the development of improved theoretical approaches. The theoretical section is preceded by a critical overview of the main experimental techniques needed to characterize the optoelectronic properties of semiconductors, including absorption and reflection spectroscopy, photoemission, and scanning tunneling spectroscopy (STS)
Framework for selecting manufacturing simulation software in industry 4.0 environment
Even though the use of simulation software packages is widespread in industrial and manufacturing companies, the criteria and methods proposed in the scientific literature to evaluate them do not adequately help companies in identifying a package able to enhance the efficiency of their production system. Hence, the main objective of this paper is to develop a framework to guide companies in choosing the most suitable manufacturing simulation software package. The evaluation framework developed in this study is based on two different multi-criteria methods: analytic hierarchy process (AHP) integrated with benefits, opportunities, costs, risks (BOCR) analysis and the best-worst method (BWM). The framework was developed on the basis of the suggestions from the literature and from a panel of experts, both from academia and industry, trying to capture all the facets of the software selection problem. For testing purposes, the proposed approach was applied to a mid-sized enterprise located in the south of Italy, which was facing the problem of buying an effective simulation software for Participatory Design. From a practical perspective, the application showed that the framework is effective in identifying the most suitable simulation software package according to the needs of the company. From a theoretical point of view, the multi-criteria methods suggested in the framework have never been applied to the problem of selecting simulation software; their usage in this context could bring some advantages compared to other decision-making tools
Bulk Cr tips for scanning tunneling microscopy and spin-polarized scanning tunneling microscopy
A simple, reliable method for preparation of bulk Cr tips for Scanning
Tunneling Microscopy (STM) is proposed and its potentialities in performing
high-quality and high-resolution STM and Spin Polarized-STM (SP-STM) are
investigated. Cr tips show atomic resolution on ordered surfaces. Contrary to
what happens with conventional W tips, rest atoms of the Si(111)-7x7
reconstruction can be routinely observed, probably due to a different
electronic structure of the tip apex. SP-STM measurements of the Cr(001)
surface showing magnetic contrast are reported. Our results reveal that the
peculiar properties of these tips can be suited in a number of STM experimental
situations
Low-frequency modes in the Raman spectrum of sp-sp2 nanostructured carbon
A novel form of amorphous carbon with sp-sp2 hybridization has been recently
produced by supersonic cluster beam deposition showing the presence in the film
of both polyynic and cumulenic species [L. Ravagnan et al. Phys. Rev. Lett. 98,
216103 (2007)]. Here we present a in situ Raman characterization of the low
frequency vibrational region (400-800 cm-1) of sp-sp2 films at different
temperatures. We report the presence of two peaks at 450 cm-1 and 720 cm-1. The
lower frequency peak shows an evolution with the variation of the sp content
and it can be attributed, with the support of density functional theory (DFT)
simulations, to bending modes of sp linear structures. The peak at 720 cm-1
does not vary with the sp content and it can be attributed to a feature in the
vibrational density of states activated by the disorder of the sp2 phase.Comment: 15 pages, 5 figures, 1 tabl
Two-dimensional TiOx nanostructures on Au(111): a Scanning Tunneling Microscopy and Spectroscopy investigation
We investigated the growth of titanium oxide two-dimensional (2D) nanostructures on Au(111), produced by Ti evaporation and post-deposition oxidation. Scanning tunneling microscopy and spectroscopy (STM and STS) and low-energy electron diffraction (LEED) measurements characterized the morphological, structural and electronic properties of the observed structures. Five distinct TiOx phases were identified: the honeycomb and pinwheel phases appear as monolayer films wetting the gold surface, while nanocrystallites of the triangular, row and needle phases grow mainly over the honeycomb or pinwheel layers. Density Functional Theory (DFT) investigation of the honeycomb structure supports a (2 x 2) structural model based on a Ti-O bilayer having Ti2O3 stoichiometry. The pinwheel phase was observed to evolve, for increasing coverage, from single triangular crystallites to a well-ordered film forming a (4*sqrt(7) x 4*sqrt(7))R19.1° superstructure, which can be interpreted within a moire-like model. Structural characteristics of the other three phases were disclosed from the analysis of high-resolution STM measurements. STS measurements revealed a partial metallization of honeycomb and pinwheel and a semiconducting character of row and triangular phases
AAV-mediated liver-specific MPV17 expression restores mtdna levels and prevents diet-induced liver failure
Mutations in human MPV17 cause a hepatocerebral form of mitochondrial DNA depletion syndrome (MDS) hallmarked by early-onset liver failure, leading to premature death. Liver transplantation and frequent feeding using slow-release carbohydrates are the only available therapies, although surviving patients eventually develop slowly progressive peripheral and central neuropathy. The physiological role of Mpv17, including its functional link to mitochondrial DNA (mtDNA) maintenance, is still unclear. We show here that Mpv17 is part of a high molecular weight complex of unknown composition, which is essential for mtDNA maintenance in critical tissues, i.e. liver, of a Mpv17 knockout mouse model. On a standard diet, Mpv17(-/-) mouse shows hardly any symptom of liver dysfunction, but a ketogenic diet (KD) leads these animals to liver cirrhosis and failure. However, when expression of human MPV17 is carried out by adeno-associated virus (AAV)-mediated gene replacement, the Mpv17 knockout mice are able to reconstitute the Mpv17-containing supramolecular complex, restore liver mtDNA copy number and oxidative phosphorylation (OXPHOS) proficiency, and prevent liver failure induced by the KD. These results open new therapeutic perspectives for the treatment of MPV17-related liver-specific MDS
Three-dimensional approach to scanning tunneling spectroscopy and application to Shockley states
The problem of the interpretation of scanning tunneling spectroscopy (STS) data is analytically solved using a three-dimensional (3D) transfer Hamiltonian approach. We present an analytical model capable of including both the electronic structure of the sample and the symmetry of the tip states (s, pz, dz2, ...) and we discuss the role of these 3D aspects in tunneling. We applied this model to the case of Shockley states. This system, allowing a full analytical treatment, led us to a detailed simulation and comprehension of the tunneling process. A procedure for the recovery of the sample local density of states from STS measurements is then proposed and applied to both the simulated and the experimental STS data of Shockley states. Comparing this approach with other methods proposed in the literature, the importance of considering the 3D aspects in treating and interpreting STS data is demonstrated
Hierarchically organized nanostructured TiO2 for photocatalysis applications
A template-free process for the synthesis of nanocrystalline TiO2
hierarchical microstructures by reactive Pulsed Laser Deposition (PLD) is here
presented. By a proper choice of deposition parameters a fine control over the
morphology of TiO2 microstructures is demonstrated, going from classical
compact/columnar films to a dense forest of distinct hierarchical assemblies of
ultrafine nanoparticles (<10 nm), up to a more disordered, aerogel-type
structure. Correspondingly, film density varies with respect to bulk TiO2
anatase, with a degree of porosity going from 48% to over 90%. These structures
are stable with respect to heat treatment at 400 centigrade degrees, which
results in crystalline ordering but not in morphological changes down to the
nanoscale. Both as deposited and annealed films exhibit very promising
photocatalytic properties, even superior to standard Degussa P25 powder, as
demonstrated by the degradation of stearic acid as a model molecule. The
observed kinetics are correlated to the peculiar morphology of the PLD grown
material. We show that the 3D multi-scale hierarchical morphology enhances
reaction kinetics and creates an ideal environment for mass transport and
photon absorption, maximizing the surface area-to-volume ratio while at the
same time providing readily accessible porosity through the large inter-tree
spaces that act as distributing channels. The reported strategy provides a
versatile technique to fabricate high aspect ratio 3D titania microstuctures
through a hierarchical assembly of ultrafine nanoparticles. Beyond
photocatalytic and catalytic applications, this kind of material could be of
interest for those applications where high surface-to-volume and efficient mass
transport are required at the same time.Comment: 10 pages, 7 figures, Nanotechnology (accepted
Stabilization of linear carbon structures in a solid Ag nanoparticle assembly
Linear sp carbon nanostructures are gathering interest for the physical
properties of one-dimensional (1D) systems. At present, the main obstacle to
the synthesis and study of these systems is their instability. Here we present
a simple method to obtain a solid system where linear sp chains (i.e. polyynes)
in a silver nanoparticle assembly display a long term stability at ambient
conditions. The presence and the behavior of linear carbon is investigated by
Surface Enhanced Raman Scattering (SERS) exploiting the plasmon resonance of
the silver nanoparticles assembly. This model system opens the possibility to
investigate an intriguing form of carbon nanostructures
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