Temperature Dependence of the Band Gap of Semiconducting Carbon Nanotubes

The temperature dependence of the band gap of semiconducting single-wall carbon nanotubes (SWNTs) is calculated by direct evaluation of electron-phonon couplings within a ``frozen-phonon'' scheme. An interesting diameter and chirality dependence of $E_g(T)$ is obtained, including non-monotonic behavior for certain tubes and distinct ``family'' behavior. These results are traced to a strong and complex coupling between band-edge states and the lowest-energy optical phonon modes in SWNTs. The $E_g(T)$ curves are modeled by an analytic function with diameter and chirality dependent parameters; these provide a valuable guide for systematic estimates of $E_g(T)$ for any given SWNT. Magnitudes of the temperature shifts at 300 K are smaller than 12 meV and should not affect $(n,m)$ assignments based on optical measurements.Comment: To appear in Phys. Rev. Let

The Asthma-COPD Overlap Syndrome: A Common Clinical Problem in the Elderly

Many patients with breathlessness and chronic obstructive lung disease are diagnosed with either asthma, COPD, or—frequently—mixed disease. More commonly, patients with uncharacterized breathlessness are treated with therapies that target asthma and COPD rather than one of these diseases. This common practice represents the difficulty in distinguishing these disorders clinically, particularly in patients with a history that does not easily differentiate asthma from COPD. A common clinical scenario is an older former smoker with partially reversible or fixed airflow obstruction and evidence of atopy, demonstrating “overlap” features of asthma and COPD. We stress that asthma-COPD overlap syndrome becomes more prevalent with advancing age as patients respond less favorably to guideline-recommended drug therapy. We review the similarities and differences in clinical characteristics between these disorders, and their physiologic and inflammatory profiles within the context of the aging patient. We underscore the difficulties in differentiating asthma from COPD in current or former smokers, share our institutional experience with overlap syndrome, and highlight the need for new research to better characterize and investigate this important clinical phenotype

Oxidative stress and antioxidants at biosurfaces: plants, skin, and respiratory tract surfaces.

Atmospheric pollutants represent an important source of oxidative and nitrosative stress to both terrestrial plants and to animals. The exposed biosurfaces of plants and animals are directly exposed to these pollutant stresses. Not surprisingly, living organisms have developed complex integrated extracellular and intracellular defense systems against stresses related to reactive oxygen and nitrogen species (ROS, RNS), including O3 and NO2. Plant and animal epithelial surfaces and respiratory tract surfaces contain antioxidants that would be expected to provide defense against environmental stress caused by ambient ROS and RNS, thus ameliorating their injurious effects on more delicate underlying cellular constituents. Parallelisms among these surfaces with regard to their antioxidant constituents and environmental oxidants are presented. The reactive substances at these biosurfaces not only represent an important protective system against oxidizing environments, but products of their reactions with ROS/RNS may also serve as biomarkers of environmental oxidative stress. Moreover, the reaction products may also induce injury to underlying cells or cause cell activation, resulting in production of proinflammatory substances including cytokines. In this review we discuss antioxidant defense systems against environmental toxins in plant cell wall/apoplastic fluids, dead keratinized cells/interstitial fluids of stratum corneum (the outermost skin layer), and mucus/respiratory tract lining fluids

Anomalous Quasiparticle Lifetime in Graphite: Band Structure Effects

We report ab initio calculation of quasiparticle lifetimes in graphite, as determined from the imaginary part of the self-energy operator within the GW aproximation. The inverse lifetime in the energy range from 0.5 to 3.5 eV above the Fermi level presents significant deviations from the quadratic behavior naively expected from Fermi liquid theory. The deviations are explained in terms of the unique features of the band structure of this material. We also discuss the experimental results from different groups and make some predictions for future experiments.Comment: 4 pages, 4 figures, submitted PR

Superlattices Consisting of "Lines" of Adsorbed Hydrogen Atom Pairs on Graphene

The structures and electron properties of new superlattices formed on graphene by adsorbed hydrogen molecules are theoretically described. It has been shown that superlattices of the (n, 0) zigzag type with linearly arranged pairs of H atoms have band structures similar to the spectra of (n, 0) carbon nanotubes. At the same time, superlattices of the (n, n) type with a "staircase" of adsorbed pairs of H atoms are substantially metallic with a high density of electronic states at the Fermi level and this property distinguishes their spectra from the spectra of the corresponding (n, n) nanotubes. The features of the spectra have the Van Hove form, which is characteristic of each individual superlattice. The possibility of using such planar structures with nanometer thickness is discussed.Comment: 5 pages, 4 figure

Metal-semiconductor (semimetal) superlattices on a graphite sheet with vacancies

It has been found that periodically closely spaced vacancies on a graphite sheet cause a significant rearrange-ment of its electronic spectrum: metallic waveguides with a high density of states near the Fermi level are formed along the vacancy lines. In the direction perpendicular to these lines, the spectrum exhibits a semimetal or semiconductor character with a gap where a vacancy miniband is degenerated into impurity levels.Comment: 4 pages, 3 figure

Theoretical Study of One-dimensional Chains of Metal Atoms in Nanotubes

Using first-principles total-energy pseudopotential calculations, we have studied the properties of chains of potassium and aluminum in nanotubes. For BN tubes, there is little interaction between the metal chains and the tubes, and the conductivity of these tubes is through carriers located at the inner part of the tube. In contrast, for small radius carbon nanotubes, there are two types of interactions: charge-transfer (dominant for alkali atoms) leading to strong ionic cohesion, and hybridization (for multivalent metal atoms) resulting in a smaller cohesion. For Al-atomic chains in carbon tubes, we show that both effects contribute. New electronic properties related to these confined atomic chains of metal are analyzed.Comment: 12 pages + 3 figure

First principles study of strain/electronic interplay in ZnO; Stress and temperature dependence of the piezoelectric constants

We present a first-principles study of the relationship between stress, temperature and electronic properties in piezoelectric ZnO. Our method is a plane wave pseudopotential implementation of density functional theory and density functional linear response within the local density approximation. We observe marked changes in the piezoelectric and dielectric constants when the material is distorted. This stress dependence is the result of strong, bond length dependent, hybridization between the O $2p$ and Zn $3d$ electrons. Our results indicate that fine tuning of the piezoelectric properties for specific device applications can be achieved by control of the ZnO lattice constant, for example by epitaxial growth on an appropriate substrate.Comment: accepted for publication in Phys. Rev.

Structural evidence for the partially oxidized dipyrromethene and dipyrromethanone forms of the cofactor of porphobilinogen deaminase: structures of the Bacillus megaterium

The enzyme porphobilinogen deaminase (PBGD; hydroxymethylbilane synthase; EC 2.5.1.61) catalyses an early step of the tetrapyrrole-biosynthesis pathway in which four molecules of the monopyrrole porphobilinogen are condensed to form a linear tetrapyrrole. The enzyme possesses a dipyrromethane cofactor, which is covalently linked by a thioether bridge to an invariant cysteine residue (Cys241 in the Bacillus megaterium enzyme). The cofactor is extended during the reaction by the sequential addition of the four substrate molecules, which are released as a linear tetrapyrrole product. Expression in Escherichia coli of a His-tagged form of B. megaterium PBGD has permitted the X-ray analysis of the enzyme from this species at high resolution, showing that the cofactor becomes progressively oxidized to the dipyrromethene and dipyrromethanone forms. In previously solved PBGD structures, the oxidized cofactor is in the dipyromethenone form, in which both pyrrole rings are approximately coplanar. In contrast, the oxidized cofactor in the B. megaterium enzyme appears to be in the dipyrromethanone form, in which the C atom at the bridging α-position of the outer pyrrole ring is very clearly in a tetrahedral configuration. It is suggested that the pink colour of the freshly purified protein is owing to the presence of the dipyrromethene form of the cofactor which, in the structure reported here, adopts the same conformation as the fully reduced dipyrromethane form

Receptor-targeted iron oxide nanoparticles for molecular MR imaging of inflamed atherosclerotic plaques

In a number of literature reports iron oxide nanoparticles have been investigated for use in imaging atherosclerotic plaques and found to accumulate in plaques via uptake by macrophages, which are critical in the process of atheroma initiation, propagation, and rupture. However, the uptake of these agents is non-specific; thus the labeling efficiency for plaques in vivo is not ideal. We have developed targeted agents to improve the efficiency for labeling macrophage-laden plaques. These probes are based on iron oxide nanoparticles coated with dextran sulfate, a ligand of macrophage scavenger receptor type A (SR-A). We have sulfated dextran-coated iron oxide nanoparticles (DIO) with sulfur trioxide, thereby targeting our nanoparticle imaging agents to SR-A. The sulfated DIO (SDIO) remained mono-dispersed and had an average hydrodynamic diameter of 62 nm, an r_1 relaxivity of 18.1 mM^(−1) s^(−1), and an r_2 relaxivity of 95.8 mM^(−1) s^(−1) (37 °C, 1.4 T). Cell studies confirmed that these nanoparticles were nontoxic and specifically targeted to macrophages. In vivo MRI after intravenous injection of the contrast agent into an atherosclerotic mouse injury model showed substantial signal loss on the injured carotid at 4 and 24 h post-injection of SDIO. No discernable signal decrease was seen at the control carotid and only mild signal loss was observed for the injured carotid post-injection of non-sulfated DIO, indicating preferential uptake of the SDIO particles at the site of atherosclerotic plaque. These results indicate that SDIO can facilitate MRI detection and diagnosis of vulnerable plaques in atherosclerosis