In many field electron emission experiments on single-walled carbon nanotubes (SWCNTs), the SWCNT stands on one of two well-separated parallel plane plates, with a macroscopic field F M applied ...between them. For any given location “L” on the SWCNT surface, a field enhancement factor (FEF) is defined as F L/F M, where F L is a local field defined at “L”. The best emission measurements from small-radii capped SWCNTs exhibit characteristic FEFs that are constant (i.e., independent of F M). This paper discusses how to retrieve this result in quantum-mechanical (as opposed to classical electrostatic) calculations. Density functional theory (DFT) is used to analyze the properties of two short, floating SWCNTs, capped at both ends, namely, a (6,6) and a (10,0) structure. Both have effectively the same height (∼5.46 nm) and radius (∼0.42 nm). It is found that apex values of local induced FEF are similar for the two SWCNTs, are independent of F M, and are similar to FEF values found from classical conductor models. It is suggested that these induced-FEF values are related to the SWCNT longitudinal system polarizabilities, which are presumed similar. The DFT calculations also generate “real”, as opposed to “induced”, potential-energy (PE) barriers for the two SWCNTs, for F M values from 3 V/μm to 2 V/nm. PE profiles along the SWCNT axis and along a parallel “observation line” through one of the topmost atoms are similar. At low macroscopic fields, the details of barrier shape differ for the two SWCNT types. Even for F M = 0, there are distinct PE structures present at the emitter apex (different for the two SWCNTs); this suggests the presence of structure-specific chemically induced charge transfers and related patch-field distributions.
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Corannulene has been a useful prototype for studying C-based nanostructures as well as surface chemistry and reactivity of sp2-hybridized carbon-based materials. We have investigated fluorination and ...hydrogenation of corannulene carrying out density functional theory calculations. In general, the fluorination is energetically more favorable than hydrogenation of corannulene. The substitution of the peripheral H atoms in the corannulene molecule by F atoms leads to a larger cohesive energy gain than when F (or H) atoms are bonded to the hub carbon and bridge carbon sites of this molecule. As expected for doped C-based nanostructures, the hydrogenation or fluorination significantly changes the HOMO–LUMO gap of the system. We have obtained HOMO–LUMO gap variations of 0.13–3.46 eV for F-doped and 0.38–1.52 eV for H-doped systems. These variations strongly depend on the concentration and position of the incorporated F/H atoms, instead of the structural stability of the doped systems. Considering these calculations, we avoid practical difficulties associated with the addition/substitution reactions of larger curved two-dimensional (2D) carbon nanostructures, and we obtain a comprehensive and systematic understanding of a variety of F/H 2D doped systems.
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Abstract
Aim
This historical prospective cohort study of the adult population of Sweden is based on data from a national registry: the primary aim was to evaluate the long‐term survival of teeth ...after periradicular surgery. A secondary aim was to identify factors predictive of extraction within 10 years of registration of periradicular surgery.
Methodology
The cohort consisted of all individuals who had undergone periradicular surgery to treat apical periodontitis, as reported to the Swedish Social Insurance Agency (SSIA) in 2009. The cohort was followed until 31 December 2020. Subsequent registrations of extractions were collected for Kaplan–Meier survival analyses and survival tables. The patients' sex, age, dental service provider and tooth group were also retrieved from SSIA. Only one tooth per individual was included in the analyses. Multivariable regression analysis was used and
p
< .05 was considered statistically significant. The reporting guidelines STROBE and PROBE were followed.
Results
After data cleaning, and exclusion of 157 teeth, 5622 teeth/individuals remained for analysis. The mean age of the individuals at the time of the periradicular surgery was 60.5 years (range 20–97, standard deviation 13.31); 55% were women. At the end of the follow‐up, that is, up to 12 years, a total of 34.1% of the teeth had been reported as extracted. The multivariate logistic regression analysis, based on follow‐up data at 10 years after registration of the periradicular surgery, included 5548 teeth, of which 1461 (26.3%) had been extracted. Significant associations between the independent variables tooth group and dental care setting (both
p
< .001) and the dependent variable extraction were found. The highest odds ratio (OR) for extraction applied to tooth group: compared to maxillary incisors and canines, mandibular molars were at greatest risk of extraction (OR 2.429, confidence interval 1.975–2.987,
p
< .001).
Conclusions
After periradicular surgery in predominantly elderly people in Sweden, approximately three‐quarters of the teeth are retained over a 10‐year period. The type of tooth is associated with extraction: mandibular molars are at greater risk of extraction than maxillary incisors and canines.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Experimental Fowler–Nordheim plots taken from orthodoxly behaving carbon nanotube (CNT) field electron emitters are known to be linear. This shows that, for such emitters, there exists a ...characteristic field enhancement factor (FEF) that is constant for a range of applied voltages and applied macroscopic fields F M. A constant FEF of this kind can be evaluated for classical CNT emitter models by finite-element and other methods, but (apparently contrary to experiment) several past quantum-mechanical (QM) CNT calculations find FEF values that vary with F M. A common feature of most such calculations is that they focus only on deriving the CNT real-charge distributions. Here we report on calculations that use first-principles electronic structure calculations to derive real-charge distributions and then use these to generate the related induced-charge distributions and related fields and FEFs. We have analyzed three carbon nanostructures involving CNT-like nanoprotrusions of various lengths, and have also simulated geometrically equivalent classical emitter models, using finite-element methods. We find that when the first-principles local induced FEFs (LIFEFs) are used, the resulting values are effectively independent of macroscopic field and behave in the same qualitative manner as the classical FEF values. Further, there is fair to good quantitative agreement between a characteristic FEF determined classically and the equivalent characteristic LIFEF generated via first-principles approaches. This is a significant step forward in linking classical and QM theories of CNT electrostatics. It also shows clearly that, for ideal CNTs, the known experimental constancy of the FEF value for a range of macroscopic fields can also be found in appropriately developed QM theory.
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Aggregation, molecular adsorption, and dissociation of water on the Fe(100) surface were investigated using spin-polarized density functional calculations. The preferential sites for H2O, HO, O, and ...H were carefully investigated on this surface. Also, the dissociation of H2O into H + OH species, and further OH into O + H species, was examined. The charge transfer mechanism during these dissociation processes, as well as of small water aggregates at different orientations on the Fe(100) surface, was studied within the Bader charge analysis. The coverage dependence on the adsorption properties was examined by comparing the results of a (2 × 2) with a (3 × 3) supercell. These calculations predicted that H2O is weakly adsorbed (physisorption) on hollow, bridge, and on-top sites, with the on-top site being slightly preferred for both coverages of 0.11 and 0.25 monolayer. As expected, OH was predicted to be strongly adsorbed (chemisorption) on the Fe(100) sites, producing a large charge transfer from the surface to p-orbitals of the O atom. A dissociation barrier of about 1.0 eV, for the dissociation H2O → OH + H, was calculated from the on-top site to the next most stable bridge and hollow sites, respectively. In contrast, a smaller barrier of ca. 0.8 eV was calculated for the dissociation of OH. Regarding the adsorption of small water aggregates on Fe(100), the present study has demonstrated that they are strongly reoriented on the surface in comparison to the isolated structures, leading to stable adsorbates. Most interestingly, the dissociation of a water molecule, after the dimer formation, leads to an energy barrier of 1.25 eV, about 25% higher than the corresponding value of an adsorbed single water molecule.
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We propose hybrid molecular systems containing small carbon atomic chains interconnected by graphene-like flakes, theoretically predicted as true energy minima, as low-dimensional structures that may ...be useful in electronic devices at the limit of the atomic miniaturization. The effects of an external electric field applied along the direction of the carbon chains indicate that it is possible to control energy gap and spin-polarization with sufficiently high strength, within the limit of the structural restoring of the systems. In this sense, by applying electric fields with magnitudes in the 1–5 V/nm range, we obtain semiconductor-to-metallic transitions for all odd-numbered carbon-chain systems proposed here. Furthermore, high-spin-to-low-spin transitions are determined for these systems as a function of the electric-field magnitude. In the case of the even-numbered carbon-chain systems, the overall electric field effect is pushing electron density near the Fermi level, leading to a gapless or metallic regime at 3.0 V/nm. An electric-field control of the spin-polarization of these latter systems is only achieved by doping the extremities of the graphene-like terminations with sulfur atoms. This finding, however, is beneficial for applications of these systems in spin-controlled carbon-based devices connected by gold electrodes, even in the presence of a weak spin–orbit coupling.
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The nature of the multicenter, long bond in neutral phenalenyl dimers is analyzed in detail and compared to the multicenter, long bond in TCNE2 2−. These dimers are prototypes of multicenter, long ...bond in dimers of neutral and anion radicals. This was done by examining the number of electrons (m) and atomic centers (c) involved in the long bond for these dimers, as well as identifying the dominant attractive components of their interaction energy (SOMO−SOMO bonding, dispersion, and the sum of the exchange-repulsion and electrostatic components) in accord with Pauling’s focus on total bond energies. The long bond in TCNE2 2- is a 2e−/4c bond, the electrostatic component is repulsive, and the dominant attractive component is the dispersion component (−27.7 kcal/mol), about two times larger than the bonding component. In phenalenyl dimers the dispersion component (−31.7 kcal/mol) is about 2.5 times stronger (than the SOMO−SOMO bonding component; hence, the multicenter, long bond in these dimers is closer to a van der Waals bond than to a covalent bond. Consequently, it possesses a two-electrons/fourteen center 2e−/14c bond, rather than the 2e−/12c bond suggested by the SOMO−SOMO bonding component. The covalent-like properties in phenalenyl dimers result from the dominant dispersion component that enable the fragments to approach each other so that their SOMOs overlap and produce a qualitative MO diagram identical to that found in conventional covalent bonds.
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We investigate theoretically the electronic and optical absorption properties of two sub-classes of oligosilanes: (i) Si(CH3)4, Si4(CH3)8, and Si8(CH3)8 that contain Si dot, ring and cage, ...respectively, and exhibit typical SiC and SiSi bonds; and (ii) persilastaffanes Si7H6(CH3)6 and Si12H6(CH3)12, which contain extended delocalized σ-electrons in SiSi bonds over three-dimensional Si frameworks. Our modeling is performed within the GW approach up to the partially self-consistent GW0 approximation, which is more adequate for reliably predicting the optical band gaps of materials. We examine how the optical properties of these organosilicon compounds depend on their size, geometric features, and Si/C composition. Our results indicate that the present methodology offers a viable way of describing the optical excitations of tailored functional Si-C-based clusters and molecular optical tags with potential use as efficient light absorbers/emitters in molecular optical devices.
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•Cyclic oligosilanes and persilastaffanes•Dielectric screening of the Coulomb interaction•Quasiparticle calculations within the GWA•Optical properties of oligosilanes
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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