636 research outputs found
Tight binding formulation of the dielectric response in semiconductor nanocrystals
We report on a theoretical derivation of the electronic dielectric response
of semiconductor nanocrystals using a tight-binding framework. Extending to the
nanoscale the Hanke and Sham approach [Phys. Rev. B 12, 4501 (1975)] developed
for bulk semiconductors, we show how local field effects can be included in the
study of confined systems. A great advantage of this scheme is that of being
formulated in terms of localized orbitals and thus it requires very few
computational resources and times. Applications to the optical and screening
properties of semiconductor nanocrystals are presented here and discussed.
Results concerning the absorption cross section, the static polarizability and
the screening function of InAs (direct gap) and Si (indirect gap) nanocrystals
compare well to both first principles results and experimental data. We also
show that the present scheme allows us to easily go beyond the continuum
dielectric model, based on the Clausius-Mossotti equation, which is frequently
used to include the nanocrystal surface polarization. Our calculations indicate
that the continuum dielectric model, used in conjunction with a size dependent
dielectric constant, underestimates the nanocrystal polarizability, leading to
exceedingly strong surface polarization fields.Comment: 9 pages, 5 figures; corrected typos, added reference
Role of local fields in the optical properties of silicon nanocrystals using the tight binding approach
The role of local fields in the optical response of silicon nanocrystals is
analyzed using a tight binding approach. Our calculations show that, at
variance with bulk silicon, local field effects dramatically modify the silicon
nanocrystal optical response. An explanation is given in terms of surface
electronic polarization and confirmed by the fair agreement between the tight
binding results and that of a classical dielectric model. From such a
comparison, it emerges that the classical model works not only for large but
also for very small nanocrystals. Moreover, the dependence on size of the
optical response is discussed, in particular treating the limit of large size
nanocrystals.Comment: 4 pages, 4 figure
Comorbid depressive disorders in ADHD. the role of ADHD severity, subtypes and familial psychiatric disorders
ObjectiveaaTo evaluate the presence of Major Depressive Disorder (MDD) and Dysthymic Disorder (DD) in a sample of Italian children with Attention Deficit Hyperactivity Disorder (ADHD) and to explore specific features of comorbid depressive disorders in ADHD. MethodsaaThree hundred and sixty-six consecutive, drug-naïve Caucasian Italian outpatients with ADHD were recruited and comorbid disorders were evaluated using DSM-IV-TR criteria. To evaluate ADHD severity, parents of all children filled out the ADHD Rating Scale. Thirty-seven children with comorbid MDD or DD were compared with 118 children with comorbid conduct disorder and 122 without comorbidity for age, sex, IQ level, family psychiatric history, and ADHD subtypes and severity. Resultsaa42 of the ADHD children displayed comorbid depressive disorders: 16 exhibited MDD, 21 DD, and 5 both MDD and DD. The frequency of hyperactive-impulsive subtypes was significantly lower in ADHD children with depressive disorders, than in those without any comorbidity. ADHD children with depressive disorders showed a higher number of familial psychiatric disorders and higher score in the Inattentive scale of the ADHD Rating Scale, than children without any comorbidity. No differences were found for age, sex and IQ level between the three groups. Conclusions: Consistent with previous studies in other countries, depressive disorders affect a significant proportion of ADHD children in Italy. Patient assessment and subsequent treatment should take into consideration the possible presence of this comorbidity, which could specifically increase the severity of ADHD attention problems
Engineering Silicon Nanocrystals: Theoretical study of the effect of Codoping with Boron and Phosphorus
We show that the optical and electronic properties of nanocrystalline silicon
can be efficiently tuned using impurity doping. In particular, we give
evidence, by means of ab-initio calculations, that by properly controlling the
doping with either one or two atomic species, a significant modification of
both the absorption and the emission of light can be achieved. We have
considered impurities, either boron or phosphorous (doping) or both (codoping),
located at different substitutional sites of silicon nanocrystals with size
ranging from 1.1 nm to 1.8 nm in diameter. We have found that the codoped
nanocrystals have the lowest impurity formation energies when the two
impurities occupy nearest neighbor sites near the surface. In addition, such
systems present band-edge states localized on the impurities giving rise to a
red-shift of the absorption thresholds with respect to that of undoped
nanocrystals. Our detailed theoretical analysis shows that the creation of an
electron-hole pair due to light absorption determines a geometry distortion
that in turn results in a Stokes shift between adsorption and emission spectra.
In order to give a deeper insight in this effect, in one case we have
calculated the absorption and emission spectra going beyond the single-particle
approach showing the important role played by many-body effects. The entire set
of results we have collected in this work give a strong indication that with
the doping it is possible to tune the optical properties of silicon
nanocrystals.Comment: 14 pages 19 figure
Quaternary structure change as a mechanism for the regulation of thymidine kinase 1-like enzymes
SummaryThe human cytosolic thymidine kinase (TK) and structurally related TKs in prokaryotes play a crucial role in the synthesis and regulation of the cellular thymidine triphosphate pool. We report the crystal structures of the TK homotetramer from Thermotoga maritima in four different states: its apo-form, a binary complex with thymidine, as well as the ternary structures with the two substrates (thymidine/AppNHp) and the reaction products (TMP/ADP). In combination with fluorescence spectroscopy and mutagenesis experiments, our results demonstrate that ATP binding is linked to a substantial reorganization of the enzyme quaternary structure, leading to a transition from a closed, inactive conformation to an open, catalytic state. We hypothesize that these structural changes are relevant to enzyme function in situ as part of the catalytic cycle and serve an important role in regulating enzyme activity by amplifying the effects of feedback inhibitor binding
Il COSIM per la riqualificazione e l’ampliamento delle pertinenze del Teatro alla Scala di Milano
Nell’ambito del progetto PRIN 2010-2011, la ricerca dipartimentale ha affrontato, tra gli altri, il tema della ‘federazione’ del Construction Site Information Modeling (CoSIM) con il BHIMM, al fine di indagare i vantaggi e i limiti di aggregazione dell’apparato informativo nello stadio di progettazione e di produzione del processo edilizio, con particolare attenzione, in questo caso, agli interventi sul patrimonio architettonico esistente. Tra i diversi casi di studio ai quali si è applicata la modellazione informativa ergotecnica, quello del rifacimento delle pertinenze del Teatro alla Scala Milano, ha permesso di indagare gli aspetti del CoSIM relativi alla virtualizzazione 3D del layout di cantiere e all’analisi 4D del processo in presenza di vincoli di contesto di forte impatto sulla sua programmazione operativa. Come risultato si è ottenuto un affinamento del template di contestualizzazione del CoSIM, l’ampliamento di una libreria di modelli di elementi cantieristici parametrizzati e la creazione di un data base di macchine e attrezzature da cantiere tale da consentire un approccio dinamico, per LOD successivi, alla modellazione ergotecnica nello stadio di progettazione e di esecuzione, così da favorire anche la gestione in corso d’opera di un intervento, nell’ottica di prevenire criticità operative poi difficilmente dominabili dal punto di vista dei costi e dei tempi di realizzazione
Silicon nanofluidic membrane for electrostatic control of drugs and analytes elution
Individualized long-term management of chronic pathologies remains an elusive goal despite recent progress in drug formulation and implantable devices. The lack of advanced systems for therapeutic administration that can be controlled and tailored based on patient needs precludes optimal management of pathologies, such as diabetes, hypertension, rheumatoid arthritis. Several triggered systems for drug delivery have been demonstrated. However, they mostly rely on continuous external stimuli, which hinder their application for long-term treatments. In this work, we investigated a silicon nanofluidic technology that incorporates a gate electrode and examined its ability to achieve reproducible control of drug release. Silicon carbide (SiC) was used to coat the membrane surface, including nanochannels, ensuring biocompatibility and chemical inertness for long-term stability for in vivo deployment. With the application of a small voltage (≤ 3 V DC) to the buried polysilicon electrode, we showed in vitro repeatable modulation of membrane permeability of two model analytes—methotrexate and quantum dots. Methotrexate is a first-line therapeutic approach for rheumatoid arthritis; quantum dots represent multi-functional nanoparticles with broad applicability from bio-labeling to targeted drug delivery. Importantly, SiC coating demonstrated optimal properties as a gate dielectric, which rendered our membrane relevant for multiple applications beyond drug delivery, such as lab on a chip and micro total analysis systems (µTAS)
Screening in semiconductor nanocrystals: \textit{Ab initio} results and Thomas-Fermi theory
A first-principles calculation of the impurity screening in Si and Ge
nanocrystals is presented. We show that isocoric screening gives results in
agreement with both the linear response and the point-charge approximations.
Based on the present ab initio results, and by comparison with previous
calculations, we propose a physical real-space interpretation of the several
contributions to the screening. Combining the Thomas-Fermi theory and simple
electrostatics, we show that it is possible to construct a model screening
function that has the merit of being of simple physical interpretation. The
main point upon which the model is based is that, up to distances of the order
of a bond length from the perturbation, the charge response does not depend on
the nanocrystal size. We show in a very clear way that the link between the
screening at the nanoscale and in the bulk is given by the surface
polarization. A detailed discussion is devoted to the importance of local field
effects in the screening. Our first-principles calculations and the
Thomas-Fermi theory clearly show that in Si and Ge nanocrystals, local field
effects are dominated by surface polarization, which causes a reduction of the
screening in going from the bulk down to the nanoscale. Finally, the model
screening function is compared with recent state-of-the-art ab initio
calculations and tested with impurity activation energies
Disabled women’s attendance at community women’s groups in rural Nepal
There is strong evidence that participatory approaches to health and participatory women’s groups hold great potential to improve the health of women and children in resource poor settings. It is important to consider if interventions are reaching the most marginalized, and therefore we examined disabled women’s participation in women’s groups and other community groups in rural Nepal. People with disabilities constitute 15% of the world’s population and face high levels of poverty, stigma, social marginalization and unequal access to health resources, and therefore their access to women’s groups is particularly important. We used a mixed methods approach to describe attendance in groups among disabled and non-disabled women, considering different types and severities of disability. We found no significant differences in the percentage of women that had ever attended at least one of our women’s groups, between non-disabled and disabled women. This was true for women with all severities and types of disability, except physically disabled women who were slightly less likely to have attended. Barriers such as poverty, lack of family support, lack of self-confidence and attendance in many groups prevented women from attending groups. Our findings are particularly significant because disabled people’s participation in broader community groups, not focused on disability, has been little studied. We conclude that women’s groups are an important way to reach disabled women in resource poor communities. We recommend that disabled persons organizations help to increase awareness of disability issues among organizations running community groups to further increase their effectiveness in reaching disabled women
Long-term clinical impact of permanent pacemaker implantation in patients undergoing transcatheter aortic valve implantation: a systematic review and meta-analysis
AIMS: The aims of this study is to assess by an updated meta-analysis the clinical outcomes related to permanent pacemaker implantation (PPI) after transcatheter aortic valve implantation (TAVI) at long-term (≥12 months) follow-up (LTF). METHODS AND RESULTS: A comprehensive literature research was performed on PubMed and EMBASE. The primary endpoint was all-cause death. Secondary endpoints were rehospitalization for heart failure, stroke, and myocardial infarction. A subgroup analysis was performed according to the Society of Thoracic Surgeon-Predicted Risk of Mortality (STS-PROM) score. This study is registered with PROSPERO (CRD42021243301). A total of 51 069 patients undergoing TAVI from 31 observational studies were included. The mean duration of follow-up was 22 months. At LTF, PPI post-TAVI was associated with a higher risk of all-cause death [risk ratio (RR) 1.18, 95% confidence interval (CI) 1.10-1.25; P < 0.001] and rehospitalization for heart failure (RR 1.32, 95% CI 1.13-1.52; P < 0.001). In contrast, the risks of stroke and myocardial infarction were not affected. Among the 20 studies that reported procedural risk, the association between PPI and all-cause death risk at LTF was statistically significant only in studies enrolling patients with high STS-PROM score (RR 1.25, 95% CI 1.12-1.40), although there was a similar tendency of the results in those at medium and low risk. CONCLUSION: Patients necessitating PPI after TAVI have a higher long-term risk of all-cause death and rehospitalization for heart failure as compared to those who do not receive PPI
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