890 research outputs found
Modal test of the Viking orbiter
A modal test of the Orbiter Development Test Modal (ODTM) has been conducted to verify, or update, the mathematical model used for load analysis. The approach used to assure the quality and validity of the experimental data is defined, the modal test is described, and test results are presented and compared with analysis results. Good correlation between the analyses and the test data assures an acceptable model for incorporation into the mathematical model of the launch system
Halogen vacancy migration at surfaces of CsPbBr<sub>3</sub>perovskites:Insights from density functional theory
Migration of halogen vacancies is one of the primary sources of phase segregation and material degradation in lead-halide perovskites. Here we use first principles density functional theory to compare migration energy barriers and paths of bromine vacancies in the bulk and at a (001) surface of cubic CsPbBr3. Our calculations indicate that surfaces might facilitate bromine vacancy migration in these perovskites, due to their soft structure that allows for bond lengths variations larger than in the bulk. We calculate the migration energy for axial-to-axial bromine vacancy migration at the surface to be only half of the value in the bulk. Furthermore, we study the effect of modifying the surface with four different alkali halide monolayers, finding an increase of the migration barrier to almost the bulk value for the NaCl-passivated system. Migration energies are found to be correlated to the lattice mismatch between the CsPbBr3 surface and the alkali halide monolayer. Our calculations suggest that surfaces might play a significant role in mediating vacancy migration in halide perovskites, a result with relevance for perovskite nanocrystals with large surface-to-volume ratios. Moreover, we propose viable ways for suppressing this undesirable process through passivation with alkali halide salts.</p
Halide perovskites from first principles: from fundamental optoelectronic properties to the impact of structural and chemical heterogeneity
Organic-inorganic metal-halide perovskite semiconductors have outstanding and widely tunable optoelectronic properties suited for a broad variety of applications. First-principles numerical modelling techniques are playing a key role in unravelling structure-property relationships of this structurally and chemically diverse family of materials, and for predicting new materials and properties. Herein we review first-principles calculations of the photophysics of halide perovskites with a focus on the band structures, optical absorption spectra and excitons, and the effects of electron- and exciton-phonon coupling and temperature on these properties. We focus on first-principles approaches based on density functional theory and Green’s function-based many-body perturbation theory and provide an overview of these approaches. While a large proportion of first-principles studies have been focusing on the prototypical ABX3 single perovskites based on Pb and Sn, recent years have witnessed significant efforts to further functionalize halide perovskites, broadening this family of materials to include double perovskites, quasi-low-dimensional structures, and other organic-inorganic materials, interfaces and heterostructures. While this enormous chemical space of perovskite and perovskite-like materials has only begun to be tapped experimentally, recent advances in theoretical and computational methods, as well as in computing infrastructure, have led to the possibility of understanding the photophysics of ever more complex systems. We illustrate this progress in our review by summarizing representative studies of first-principles calculations of halide perovskites with various degrees of complexity
Chemical Mapping of Excitons in Halide Double Perovskites
Halide double perovskites are an emerging class of semiconductors with
tremendous chemical and electronic diversity. While their bandstructure
features can be understood from frontier-orbital models, chemical intuition for
optical excitations remains incomplete. Here, we use \textit{ab initio}
many-body perturbation theory within the and the Bethe-Salpeter Equation
approach to calculate excited-state properties of a representative range of
CsBBCl double perovskites. Our calculations reveal that double
perovskites with different combinations of B and B cations display a broad
variety of electronic bandstructures and dielectric properties, and form
excitons with binding energies ranging over several orders of magnitude. We
correlate these properties with the orbital-induced anisotropy of
charge-carrier effective masses and the long-range behavior of the dielectric
function, by comparing with the canonical conditions of the Wannier-Mott model.
Furthermore, we derive chemically intuitive rules for predicting the nature of
excitons in halide double perovskites using electronic structure information
obtained from computationally inexpensive DFT calculations
Mapping Charge-Transfer Excitations in Bacteriochlorophyll Dimers from First Principles
Photoinduced charge-transfer excitations are key to understand the primary
processes of natural photosynthesis and for designing photovoltaic and
photocatalytic devices. In this paper, we use Bacteriochlorophyll dimers
extracted from the light harvesting apparatus and reaction center of a
photosynthetic purple bacterium as model systems to study such excitations
using first-principles numerical simulation methods. We distinguish four
different regimes of intermolecular coupling, ranging from very weakly coupled
to strongly coupled, and identify the factors that determine the energy and
character of charge-transfer excitations in each case. We also construct an
artificial dimer to systematically study the effects of intermolecular distance
and orientation on charge-transfer excitations, as well as the impact of
molecular vibrations on these excitations. Our results provide design rules for
tailoring charge-transfer excitations in Bacteriochloropylls and related
photoactive molecules, and highlight the importance of including
charge-transfer excitations in accurate models of the excited-state structure
and dynamics of Bacteriochlorophyll aggregates
Coordination-driven magnetic-to-nonmagnetic transition in manganese doped silicon clusters
The interaction of a single manganese impurity with silicon is analyzed in a
combined experimental and theoretical study of the electronic, magnetic, and
structural properties of manganese-doped silicon clusters. The structural
transition from exohedral to endohedral doping coincides with a quenching of
high-spin states. For all geometric structures investigated, we find a similar
dependence of the magnetic moment on the manganese coordination number and
nearest neighbor distance. This observation can be generalized to manganese
point defects in bulk silicon, whose magnetic moments fall within the observed
magnetic-to-nonmagnetic transition, and which therefore react very sensitively
to changes in the local geometry. The results indicate that high spin states in
manganese-doped silicon could be stabilized by an appropriate lattice
expansion
Pain Management in Patients with Cancer: Focus on Opioid Analgesics
Cancer pain is generally treated with pharmacological measures, relying on using opioids alone or in combination with adjuvant analgesics. Weak opioids are used for mild-to-moderate pain as monotherapy or in a combination with nonopioids. For patients with moderate-to-severe pain, strong opioids are recommended as initial therapy rather than beginning treatment with weak opioids. Adjunctive therapy plays an important role in the treatment of cancer pain not fully responsive to opioids administered alone (ie, neuropathic, bone, and visceral colicky pain). Supportive drugs should be used wisely to prevent and treat opioids’ adverse effects. Understanding the pharmacokinetics, pharmacodynamics, interactions, and cautions with commonly used opioids can help determine appropriate opioid selection for individual cancer patients
Interferon beta 1b following natalizumab discontinuation: one year, randomized, prospective, pilot trial
Background: Natalizumab (NTZ) discontinuation leads to multiple sclerosis reactivation. The objective of this study is to compare disease activity in MS patients who continued on NTZ treatment to those who were switched to subcutaneous interferon 1b (IFNB) treatment. Methods: 1-year randomized, rater-blinded, parallel-group, pilot study (ClinicalTrial.gov ID: NCT01144052). Relapsing remitting MS patients on NTZ for ≥12 months who had been free of disease activity on this therapy (no relapses and disability progression for ≥6 months, no gadolinium-enhancing lesions on baseline MRI) were randomized to NTZ or IFNB. Primary endpoint was time to first on-study relapse. Additional clinical, MRI and safety parameters were assessed. Analysis was based on intention to treat. Results: 19 patients (NTZ n=10; IFNB n=9) with similar baseline characteristics were included. 78% of IFNB treated patients remained relapse free (NTZ group: 100%), and 25% remained free of new T2 lesions (NTZ group: 62.5%). While time to first on-study relapse was not significantly different between groups (p=0.125), many secondary clinical and radiological endpoints (number of relapses, proportion of relapse free patients, number of new T2 lesions) showed a trend, or were significant (new T2 lesions at month 6) in favoring NTZ. Conclusions: De-escalation therapy from NTZ to IFNB over 1 year was associated with some clinical and radiological disease recurrence. Overall no major safety concerns were observed
- …