432 research outputs found
Thermoelectric properties of the bismuth telluride nanowires in the constant-relaxation-time approximation
Electronic structure of bismuth telluride nanowires with the growth
directions [110] and [015] is studied in the framework of anisotropic effective
mass method using the parabolic band approximation. The components of the
electron and hole effective mass tensor for six valleys are calculated for both
growth directions. For a square nanowire, in the temperature range from 77 K to
500 K, the dependence of the Seebeck coefficient, the electron thermal and
electrical conductivity as well as the figure of merit ZT on the nanowire
thickness and on the excess hole concentration are investigated in the
constant-relaxation-time approximation. The carrier confinement is shown to
play essential role for square nanowires with thickness less than 30 nm. The
confinement decreases both the carrier concentration and the thermal
conductivity but increases the maximum value of Seebeck coefficient in contrast
to the excess holes (impurities). The confinement effect is stronger for the
direction [015] than for the direction [110] due to the carrier mass difference
for these directions. The carrier confinement increases maximum value of ZT and
shifts it towards high temperatures. For the p-type bismuth telluride nanowires
with growth direction [110], the maximum value of the figure of merit is equal
to 1.3, 1.6, and 2.8, correspondingly, at temperatures 310 K, 390 K, 480 K and
the nanowire thicknesses 30 nm, 15 nm, and 7 nm. At the room temperature, the
figure of merit equals 1.2, 1.3, and 1.7, respectively.Comment: 13 pages, 7 figures, 2 tables, typos added, added references for
sections 2-
Intramolecular bonds resolved on a semiconductor surface
Noncontact atomic force microscopy (NC-AFM) is now routinely capable of obtaining submolecular resolution, readily resolving the carbon backbone structure of planar organic molecules adsorbed on metal substrates. Here we show that the same resolution may also be obtained for molecules adsorbed on a reactive semiconducting substrate. Surprisingly, this resolution is routinely obtained without the need for deliberate tip functionalization. Intriguingly, we observe two chemically distinct apex types capable of submolecular imaging. We characterize our tip apices by “inverse imaging” of the silicon adatoms of the Si(111)−7×7 surface and support our findings with detailed density functional theory (DFT) calculations. We also show that intramolecular resolution on individual molecules may be readily obtained at 78 K, rather than solely at 5 K as previously demonstrated. Our results suggest a wide range of tips may be capable of producing intramolecular contrast for molecules adsorbed on semiconductor surfaces, leading to a much broader applicability for submolecular imaging protocols
Simulated structure and imaging of NTCDI on Si(1 1 1)-7 × 7 : a combined STM, NC-AFM and DFT study
The adsorption of naphthalene tetracarboxylic diimide (NTCDI) on Si(1 1 1)-7 × 7 is investigated through a combination of scanning tunnelling microscopy (STM), noncontact atomic force microscopy (NC-AFM) and density functional theory (DFT) calculations. We show that NTCDI adopts multiple planar adsorption geometries on the Si(1 1 1)-7 × 7 surface which can be imaged with intramolecular bond resolution using NC-AFM. DFT calculations reveal adsorption is dominated by covalent bond formation between the molecular oxygen atoms and the surface silicon adatoms. The chemisorption of the molecule is found to induce subtle distortions to the molecular structure, which are observed in NC-AFM images
A connection between stress and development in the multicelular prokaryote Streptomyces coelicolor
Morphological changes leading to aerial mycelium formation and sporulation in the mycelial bacterium Streptomyces coelicolor rely on establishing distinct patterns of gene expression in separate regions of the colony. sH was identified previously as one of three paralogous sigma factors associated with stress responses in S. coelicolor. Here, we show that sigH and the upstream gene prsH (encoding a putative antisigma factor of sH) form an operon transcribed from two developmentally regulated promoters, sigHp1 and sigHp2. While sigHp1 activity is confined to the early phase of growth, transcription of sigHp2 is dramatically induced at the time of aerial hyphae formation. Localization of sigHp2 activity using a transcriptional fusion to the green fluorescent protein reporter gene (sigHp2–egfp) showed that sigHp2 transcription is spatially restricted to sporulating aerial hyphae in wild-type S. coelicolor. However, analysis of mutants unable to form aerial hyphae (bld mutants) showed that sigHp2 transcription and sH protein levels are dramatically upregulated in a bldD mutant, and that the sigHp2–egfp fusion was expressed ectopically in the substrate mycelium in the bldD background. Finally, a protein possessing sigHp2 promoter-binding activity was purified to homogeneity from crude mycelial extracts of S. coelicolor and shown to be BldD. The BldD binding site in the sigHp2 promoter was defined by DNase I footprinting. These data show that expression of sH is subject to temporal and spatial regulation during colony development, that this tissue-specific regulation is mediated directly by the developmental transcription factor BldD and suggest that stress and developmental programmes may be intimately connected in Streptomyces morphogenesis
Physisorption controls the conformation and density of states of an adsorbed porphyrin
Conformational changes caused by adsorption can dramatically affect a molecule’s properties. Despite extensive study, however, the exact mechanisms underpinning conformational switching are often unclear. Here we show that the conformation of a prototypical flexible molecule, the freebase tetra(4-bromophenyl) porphyrin, adsorbed on Cu(111), depends critically on its precise adsorption site and that, remarkably, large conformational changes are dominated by van der Waals interactions between the molecule and the substrate surface. A combination of scanning probe microscopy, single-molecule manipulation, DFT with dispersion density functional theory, and molecular dynamics simulations show that van der Waals forces drive significant distortions of the molecular architecture so that the porphyrin can adopt one of two low-energy conformations. We find that adsorption driven by van der Waals forces alone is capable of causing large shifts in the molecular density of states, despite the apparent absence of chemical interactions. These findings highlight the essential role that van der Waals forces play in determining key molecular properties
Photochemistry of framework-supported M(diimine)(CO)₃X complexes in 3D Lithium-Carboxylate metal−organic frameworks: monitoring the effect of framework cations
The structures and photochemical behaviour of two new metal-organic frameworks are reported. Reaction of Re(2,2ʹ-bipyʹ-5,5ʹ-dicarboxylic acid)(CO)₃Cl or Mn(2,2ʹ-bipyʹ-5,5ʹ- dicarboxylic acid)(CO)₃Br with either LiCl or LiBr, respectively, produces single crystals of {Li₂(DMF)₂[(2,2ʹ-bipyʹ-5,5ʹ-dicarboxylate)Re(CO)₃Cl]}n (ReLi) or {Li₂(DMF)₂[(2,2ʹ-bipyʹ- 5,5ʹ-dicarboxylate)Mn(CO)₃Br]}n (MnLi). The structures formed by the two MOFs comprise one-dimensional chains of carboxylate-bridged Li(I) cations that are cross-linked by units of Re(2,2ʹ-bipyʹ-5,5ʹ-dicarboxylate)(CO)₃Cl (ReLi) or Mn(2,2ʹ-bipyʹ-5,5ʹ- dicarboxylate)(CO)₃Br (MnLi). The photophysical and photochemical behaviour of both ReLi and MnLi are probed. The rhenium-containing MOF, ReLi, exhibits luminescence and the excited state behaviour, as established by time-resolved infra-red measurements, are closer in behaviour to that of unsubstituted [Re(bipy)(CO)₃Cl] rather than a related MOF where the Li(I) cations are replaced by Mn(II) cations. These observations are further supported by DFT calculations. Upon excitation MnLi forms a dicarbonyl species which rapidly recombines with the dissociated CO, in a fashion consistent with the majority of the photoejected CO not escaping the MOF channels
Broken symmetry and the variation of critical properties in the phase behaviour of supramolecular rhombus tilings
The degree of randomness, or partial order, present in two-dimensional
supramolecular arrays of isophthalate tetracarboxylic acids is shown to vary
due to subtle chemical changes such as the choice of solvent or small
differences in molecular dimensions. This variation may be quantified using an
order parameter and reveals a novel phase behaviour including random tiling
with varying critical properties as well as ordered phases dominated by either
parallel or non-parallel alignment of neighbouring molecules, consistent with
long-standing theoretical studies. The balance between order and randomness is
driven by small differences in the intermolecular interaction energies, which
we show, using numerical simulations, can be related to the measured order
parameter. Significant variations occur even when the energy difference is much
less than the thermal energy highlighting the delicate balance between entropic
and energetic effects in complex self-assembly processes
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Evolutionary Ecology of Prokaryotic Immune Mechanisms.
Published onlineJournal ArticleReviewBacteria have a range of distinct immune strategies that provide protection against bacteriophage (phage) infections. While much has been learned about the mechanism of action of these defense strategies, it is less clear why such diversity in defense strategies has evolved. In this review, we discuss the short- and long-term costs and benefits of the different resistance strategies and, hence, the ecological conditions that are likely to favor the different strategies alone and in combination. Finally, we discuss some of the broader consequences, beyond resistance to phage and other genetic elements, resulting from the operation of different immune strategies.S.V.H. received funding from the European Union's Horizon 2020 research and innovation program under Marie Skłodowska-Curie grant agreement no. 660039. We also acknowledge the NERC, the BBSRC, the Royal Society, the Leverhulme Trust, the Wellcome Trust, and the AXA research fund for funding
Minority-carrier effects in poly-phenylenevinylene as studied by electrical characterization
Electrical measurements have been performed on poly[2-methoxy, 5 ethyl (2' hexyloxy) paraphenylenevinylene] in a pn junction with silicon. These included current-voltage measurements, capacitance-voltage measurements, capacitance-transient spectroscopy, and admittance spectroscopy. The measurements show evidence for large minority-carrier injection into the polymer possibly enabled by interface states for which evidence is also found. The shallow acceptor level depth (0.12 eV) and four deep trap level activation energies (0.30 and 1.0 eV majority-carrier type; 0.48 and 1.3 eV minority-carrier type) are found. Another trap that is visible at room temperature has point-defect nature. (C) 2001 American Institute of Physics
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