Atomic and molecular adsorption on transition-metal carbide (111) surfaces from density-functional theory: A trend study of surface electronic factors

This study explores atomic and molecular adsorption on a number of early transition-metal carbides (TMC's) by means of density-functional theory calculations. Trend studies are conducted with respect to both period and group in the periodic table, choosing the substrates ScC, TiC, VC, ZrC, NbC, delta-MoC, TaC, and WC and the adsorbates H, B, C, N, O, F, NH, NH2, and NH3. Trends in adsorption strength are explained in terms of surface electronic factors, by correlating the calculated adsorption energy values with the calculated surface electronic structures. The results are rationalized with use of a concerted-coupling model (CCM), which has previously been applied succesfully to the description of adsorption on TiC(111) and TiN(111) surfaces [Solid State Commun. 141, 48 (2007)]. First, the clean TMC(111) surfaces are characterized by calculating surface energies, surface relaxations, Bader charges, and surface-localized densities of states (DOS's). Detailed comparisons between surface and bulk DOS's reveal the existence of transition-metal localized SR's (TMSR's) in the pseudogap and of several C-localized SR's (CSR's) in the upper valence band on all considered TMC(111) surfaces. Then, atomic and molecular adsorption energies, geometries, and charge transfers are presented. An analysis of the adsorbate-induced changes in surface DOS's reveals a presence of both adsorbate--TMSR and adsorbate--CSR's interactions, of varying strengths depending on the surface and the adsorbate. These variations are correlated to the variations in adsorption energies. The results are used to generalize the content and applications of the previously proposed CCM to this larger class of substrates and adsorbates. Implications for other classes of materials, for catalysis, and for other surface processes are discussed

Nature of Versatile Chemisorption on TiC(111) and TiN(111) Surfaces

Density-functional calculations on the polar TiX(111) (X = C, N) surfaces show (i) for clean surfaces, strong Ti3d-derived surface resonances (SR's) at the Fermi level and X2p-derived SR's deep in the upper valence band and (ii) for adatoms in periods 1-3, pyramidic trends in atomic adsorption energies, peaking at oxygen (9 eV). A concerted-coupling model, where adatom states couple to both kinds of SR's in a concerted way, describes the adsorption. The chemisorption versatility and the general nature of the model indicate ramifications and predictive abilities in, e.g., growth and catalysis.Comment: 5 pages, 4 figures, submitted to Physical Review Letters (2006

Trends in Atomic Adsorption on Titanium Carbide and Nitride

Extensive density-functional calculations on atomic chemisorption of H, B, C, N, O, F, Al, Si, P, S, and Cl on the polar TiC(111) and TiN(111) yield similar adsorption trends for the two surfaces: (i) pyramid-like adsorption-energy trends along the adatom periods; (ii) strongest adsorption for O, C, N, S, and F; (iii) large adsorption variety; (iv) record-high adsorption energy for O (8.4-8.8 eV). However, a stronger adsorption on TiN is found for elements on the left of the periodic table and on TiC for elements on the right. The results support that a concerted-coupling model, proposed for chemisorption on TiC, applies also to TiN.Comment: 5 pages, 4 figures, 2 tables, conference proceeding presented at ECOSS-23 (Berlin, 2005), submitted to Surf. Sci. (2005

Nature of Chemisorption on Titanium Carbide and Nitride

Extensive density-functional calculations are performed to understand atomic chemisorption on the TiC(111) and TiN(111) surfaces, in particular the calculated pyramid-shaped trends in the adsorption energies for second- and third-period adatoms. Our previously proposed concerted-coupling model for chemisorption on TiC(111) is tested against new results for adsorption on TiN(111) and found to apply on this surface as well, thus reflecting both similarities and differences in electronic structure between the two compounds.Comment: 7 pages, 4 figures, conference proceeding presented at IWSP-2005 (Polanica Zdoj, Poland, 2005), submitted to Surf. Sci. (2005

From electronic structure to catalytic activity: A single descriptor for adsorption and reactivity on transition-metal carbides

Adsorption and catalytic properties of the polar (111) surface of transition-metal carbides (TMC's) are investigated by density-functional theory. Atomic and molecular adsorption are rationalized with the concerted-coupling model, in which two types of TMC surface resonances (SR's) play key roles. The transition-metal derived SR is found to be a single measurable descriptor for the adsorption processes, implying that the Br{\o}nsted-Evans-Polanyi relation and scaling relations apply. This gives a picture with implications for ligand and vacancy effects and which has a potential for a broad screening procedure for heterogeneous catalysts.Comment: 5 pages, 3 figure

The Availability and Consistency of Dengue Surveillance Data Provided Online by the World Health Organization

Background: The use of high quality disease surveillance data has become increasingly important for public health action against new threats. In response, countries have developed a wide range of disease surveillance systems enabled by technological advancements. The heterogeneity and complexity of country data systems have caused a growing need for international organizations such as the World Health Organization (WHO) to coordinate the standardization, integration, and dissemination of country disease data at the global level for research and policy. The availability and consistency of currently available disease surveillance data at the global level are unclear. We investigated this for dengue surveillance data provided online by the WHO. Methods and Findings: We extracted all dengue surveillance data provided online by WHO Headquarters and Regional Offices (RO’s). We assessed the availability and consistency of these data by comparing indicators within and between sources. We also assessed the consistency of dengue data provided online by two example countries (Brazil and Indonesia). Data were available from WHO for 100 countries since 1955 representing a total of 23 million dengue cases and 82 thousand deaths ever reported to WHO. The availability of data on DengueNet and some RO’s declined dramatically after 2005. Consistency was lacking between sources (84% across all indicators representing a discrepancy of almost half a million cases). Within sources, data at high spatial resolution were often incomplete. Conclusions: The decline of publicly available, integrated dengue surveillance data at the global level will limit opportunities for research, policy, and advocacy. A new financial and operational framework will be necessary for innovation and for the continued availability of integrated country disease data at the global level

Ab initio structure modeling of complex thin-film oxides: thermodynamical stability of TiC/thin-film alumina

We present an efficient and general method to identify promising candidate configurations for thin-film oxides and to determine structural characteristics of (metastable) thin-film structures using ab initio calculations. At the heart of this method is the complexity of the oxide bulk structure, from which a large number of thin films with structural building blocks, that is motifs, from metastable bulk oxide systems can be extracted. These span a coarse but well-defined network of initial configurations for which density functional theory (DFT) calculations predict and implement dramatic atomic relaxations in the corresponding, resulting thin-film candidates. The network of thin-film candidates (for various film thicknesses and stoichiometries) can be ordered according to their variation in ab initio total energy or in ab initio equilibrium Gibbs free energy. Analysis of the relaxed atomic structures for the most favored structures gives insight into the nature of stable and metastable thin-film oxides. We investigate ultrathin alumina nucleated on TiC as a model system to illustrate this method.Comment: Submitted to PRB; 16 pages, 11 figure

Adhd-like symptoms in children affected by obstructive sleep apnea syndrome: A case-control study

Introduction: ADHD is characterized by inattention, hyperactivity, impulsivity, or a combination of these symptoms. Sleep disorders may be considered as a not secondary underlying cause of ADHD and growing evidence evidenced that obstructive sleep apnea syndrome (OSA) symptoms may overlap ADHD's ones. Aim of the present study is verifying the presence of ADHD-like symptoms in other frequent condition such as sleep-related breathing disorders in pediatric age. Materials and methods: 34 children (19 males and 15 females) aged 6-10 years (mean age 9.706 \ub1 3.434) with polysomnographic diagnosis of OSA, according to ICSD-3 criteria, were recruited. Control group was composed by 89 typical developing children (47 males, 42 females) (mean age 9,528 \ub1 3,351). The Conners' Rating Scale- Revised (CRS-R) test was administered to mothers to assess their children's behavior. Results: The two groups are comparable for age (p = 0.794) and sex distribution (p = 0.918). Subjects affected by OSA present higher scores in pathological range in all scales of CRS-R than controls (Table 1). Conclusions: Scientific and clinical evidences tend to stress the similarities between ADHD and OSA effects/comorbidities in pediatric age, suggesting sleep screening as mandatory before starting stimulant drugs treatment

Internalizing symptoms in children affected by childhood absence epilepsy: A preliminary study

Introduction: Childhood absence epilepsy (CAE) is a common type of pediatric idiopathic generalized epilepsy, characterized by multiple seizures of typical absence, with typical EEG pattern consisting in bilateral synchronous and symmetrical discharges of generalized 3 Hz spike-wave (SWDs). Recently, some researchers have suggested that the underlying epileptogenic mechanism of absence seizures selectively involves the frontal cortical circuits, also supported by video-electroencephalography data(3). These data may be considered as a new window in CAE comprehension and management, particularly about symptoms different from seizure that children affected may present. In this light, aim of the present study is evaluating the presence of internalizing problems in prepubertal children affected by CAE. Material and methods: 18 patients (10 females, 8 males) ranging age from 8-11 years (mean age 9.36 ± 1.32) affected by typical CAE were recruited. Control group was composed by 43 subjects (32 females, 11 males) (mean age 8.54 ± 2.01). All subjects were screened for internalizing symptoms with SAFA-A scale and CDI test. Results: The two groups were comparable for age (p=0.117) and sex distribution (p=0.251). CAE children showed significantly higher score than controls for anxiety (p<0.001) and depressive symptoms screening tests (p<0.001) (Table 1). Conclusions: Results of present study suggest the importance of screening for anxiety and depressive signs in CAE children, in order to optimize their management beyond the exclusive idea to control and stop epileptic seizure only