167 research outputs found
Phonon Knudsen flow in nanostructured semiconductor systems
We determine the size effect on the lattice thermal conductivity of nanoscale
wire and multilayer structures formed in and by some typical semiconductor
materials, using the Boltzmann transport equation and focusing on the Knudsen
flow effect. For both types of nanostructured systems we find that the phonon
transport is reduced significantly below the bulk value by boundary scattering
off interface defects and/or interface modes. The Knudsen flow effects are
important for almost all types of semiconductor nanostructures but we find them
most pronounced in Si and SiC systems due to the very large phonon mean-free
paths. We apply and test our wire thermal-transport results to recent
measurements on Si nanowires. We further investigate and predict size effects
in typical multilayered SiC nanostructures, for example, a
doped-SiC/SiC/SiO layered structure that could define the transport channel
in a nanosize transistor. Here the phonon-interface scattering produces a
heterostructure thermal conductivity smaller than what is predicted in a
traditional heat-transport calculation, suggesting a breakdown of the
traditional Fourier analysis even at room temperatures. Finally, we show that
the effective thermal transport in a SiC/SiO heterostructure is sensitive
to the oxide depth and could thus be used as an in-situ probe of the SiC
oxidation progress.Comment: 29 pages, 9 figures. (Submitted to Journal of Applied Physics
Theory for structure and bulk-modulus determination
A new method for direct evaluation of both crystalline structure, bulk
modulus B_0, and bulk-modulus pressure derivative B'_0 of solid materials with
complex crystal structures is presented. The explicit and exact results
presented here permit a multidimensional polynomial fit of the total energy as
a function of all relevant structure parameters to simultaneously determine the
equilibrium configuration and the elastic properties. The method allows for
inclusion of general (internal) structure parameters, e.g., bond lengths and
angles within the unit cell, on an equal footing with the unit-cell lattice
parameters. The method is illustrated by the calculation of B_0 and B'_0 for a
few selected materials with multiple structure parameters for which data is
obtained by using first-principles density functional theory.Comment: 7 pages, 2 figures, submitted to Phys. Rev.
Thermal transport in SiC nanostructures
SiC is a robust semiconductor material considered ideal for high-power
application due to its material stability and large bulk thermal conductivity
defined by the very fast phonons. In this paper, however, we show that both
material-interface scattering and total-internal reflection significantly limit
the SiC-nanostructure phonon transport and hence the heat dissipation in a
typical device. For simplicity we focus on planar SiC nanostructures and
calculate the thermal transport both parallel to the layers in a
substrate/SiC/oxide heterostructure and across a SiC/metal gate or contact. We
find that the phonon-interface scattering produces a heterostructure thermal
conductivity significantly smaller than what is predicted in a traditional
heat-transport calculation. We also document that the high-temperature heat
flow across the metal/SiC interface is limited by total-internal reflection
effects and maximizes with a small difference in the metal/SiC sound
velocities.Comment: 15 pages, 4 figure
van der Waals density functional calculations of binding in molecular crystals
A recent paper [J. Chem. Phys. 132, 134705 (2010)] illustrated the potential
of the van der Waals density functional (vdW-DF) method [Phys. Rev. Lett. 92,
246401 (2004)] for efficient first-principle accounts of structure and cohesion
in molecular crystals. Since then, modifications of the original vdW-DF version
(identified as vdW-DF1) has been proposed, and there is also a new version
called vdW-DF2 [ArXiv 1003.5255], within the vdW-DF framework. Here we
investigate the performance and nature of the modifications and the new version
for the binding of a set of simple molecular crystals: hexamine, dodecahedrane,
C60, and graphite. These extended systems provide benchmarks for computational
methods dealing with sparse matter. We show that a previously documented
enhancement of non-local correlations of vdW-DF1 over an asymptotic atom-based
account close to and a few A, beyond binding separation persists in vdW-DF2.
The calculation and analysis of the binding in molecular crystals requires
appropriate computational tools. In this paper, we also present details on our
real-space parallel implementation of the vdW-DF correlation and on the method
used to generate asymptotic atom-based pair potentials based on vdW-DF.Comment: 5 pages, 4 figure
Diffusive Charge Transport in Graphene on SiO2
We review our recent work on the physical mechanisms limiting the mobility of
graphene on SiO2. We have used intentional addition of charged scattering
impurities and systematic variation of the dielectric environment to
differentiate the effects of charged impurities and short-range scatterers. The
results show that charged impurities indeed lead to a conductivity linear in
density in graphene, with a scattering magnitude that agrees quantitatively
with theoretical estimates [1]; increased dielectric screening reduces
scattering from charged impurities, but increases scattering from short-range
scatterers [2]. We evaluate the effects of the corrugations (ripples) of
graphene on SiO2 on transport by measuring the height-height correlation
function. The results show that the corrugations cannot mimic long-range
(charged impurity) scattering effects, and have too small an
amplitude-to-wavelength ratio to significantly affect the observed mobility via
short-range scattering [3, 4]. Temperature-dependent measurements show that
longitudinal acoustic phonons in graphene produce a resistivity linear in
temperature and independent of carrier density [5]; at higher temperatures,
polar optical phonons of the SiO2 substrate give rise to an activated, carrier
density-dependent resistivity [5]. Together the results paint a complete
picture of charge carrier transport in graphene on SiO2 in the diffusive
regime.Comment: 28 pages, 7 figures, submitted to Graphene Week proceeding
O adsorption and incipient oxidation of the Mg(0001) surface
First principles density functional calculations are used to study the early
oxidation stages of the Mg(0001) surface for oxygen coverages 1/16 <= Theta <=
3 monolayers. It is found that at very low coverages O is incorporated below
the topmost Mg layer in tetrahedral sites. At higher oxygen-load the binding in
on-surface sites is increased but at one monolayer coverage the on-surface
binding is still about 60 meV weaker than for subsurface sites. The subsurface
octahedral sites are found to be unfavorable compared to subsurface tetrahedral
sites and to on-surface sites. At higher coverages oxygen adsorbs both under
the surface and up. Our calculations predict island formation and clustering of
incorporated and adsorbed oxygen in agreement with previous calculations. The
calculated configurations are compared with the angle-scanned x-ray
photoelectron diffraction experiment to determine the geometrical structure of
the oxidized Mg(0001) surface.Comment: 10 pages, 5 figure
Strain uses gap junctions to reverse stimulation of osteoblast proliferation by osteocytes
Identifying mechanisms by which cells of the osteoblastic lineage communicate in vivo is complicated by the mineralised matrix that encases osteocytes, and thus, vital mechanoadaptive processes used to achieve loadβbearing integrity remain unresolved. We have used the coculture of immunomagnetically purified osteocytes and primary osteoblasts from both embryonic chick long bone and calvariae to examine these mechanisms. We exploited the fact that purified osteocytes are postmitotic to examine both their effect on proliferation of primary osteoblasts and the role of gap junctions in such communication. We found that chick long bone osteocytes significantly increased basal proliferation of primary osteoblasts derived from an identical source (tibiotarsi). Using a gap junction inhibitor, 18Ξ²βglycyrrhetinic acid, we also demonstrated that this osteocyteβrelated increase in osteoblast proliferation was not reliant on functional gap junctions. In contrast, osteocytes purified from calvarial bone failed to modify basal proliferation of primary osteoblast, but long bone osteocytes preserved their proproliferative action upon calvarialβderived primary osteoblasts. We also showed that coincubated purified osteocytes exerted a marked inhibitory action on mechanical strainβrelated increases in proliferation of primary osteoblasts and that this action was abrogated in the presence of a gap junction inhibitor. These data reveal regulatory differences between purified osteocytes derived from functionally distinct bones and provide evidence for 2 mechanisms by which purified osteocytes communicate with primary osteoblasts to coordinate their activity
Premature Osteoblast Clustering by Enamel Matrix Proteins Induces Osteoblast Differentiation through Up-Regulation of Connexin 43 and N-Cadherin
In recent years, enamel matrix derivative (EMD) has garnered much interest in the dental field for its apparent bioactivity that stimulates regeneration of periodontal tissues including periodontal ligament, cementum and alveolar bone. Despite its widespread use, the underlying cellular mechanisms remain unclear and an understanding of its biological interactions could identify new strategies for tissue engineering. Previous in vitro research has demonstrated that EMD promotes premature osteoblast clustering at early time points. The aim of the present study was to evaluate the influence of cell clustering on vital osteoblast cell-cell communication and adhesion molecules, connexin 43 (cx43) and N-cadherin (N-cad) as assessed by immunofluorescence imaging, real-time PCR and Western blot analysis. In addition, differentiation markers of osteoblasts were quantified using alkaline phosphatase, osteocalcin and von Kossa staining. EMD significantly increased the expression of connexin 43 and N-cadherin at early time points ranging from 2 to 5 days. Protein expression was localized to cell membranes when compared to control groups. Alkaline phosphatase activity was also significantly increased on EMD-coated samples at 3, 5 and 7 days post seeding. Interestingly, higher activity was localized to cell cluster regions. There was a 3 fold increase in osteocalcin and bone sialoprotein mRNA levels for osteoblasts cultured on EMD-coated culture dishes. Moreover, EMD significantly increased extracellular mineral deposition in cell clusters as assessed through von Kossa staining at 5, 7, 10 and 14 days post seeding. We conclude that EMD up-regulates the expression of vital osteoblast cell-cell communication and adhesion molecules, which enhances the differentiation and mineralization activity of osteoblasts. These findings provide further support for the clinical evidence that EMD increases the speed and quality of new bone formation in vivo
Impaired Carbohydrate Digestion and Transport and Mucosal Dysbiosis in the Intestines of Children with Autism and Gastrointestinal Disturbances
Gastrointestinal disturbances are commonly reported in children with autism, complicate clinical management, and may contribute to behavioral impairment. Reports of deficiencies in disaccharidase enzymatic activity and of beneficial responses to probiotic and dietary therapies led us to survey gene expression and the mucoepithelial microbiota in intestinal biopsies from children with autism and gastrointestinal disease and children with gastrointestinal disease alone. Ileal transcripts encoding disaccharidases and hexose transporters were deficient in children with autism, indicating impairment of the primary pathway for carbohydrate digestion and transport in enterocytes. Deficient expression of these enzymes and transporters was associated with expression of the intestinal transcription factor, CDX2. Metagenomic analysis of intestinal bacteria revealed compositional dysbiosis manifest as decreases in Bacteroidetes, increases in the ratio of Firmicutes to Bacteroidetes, and increases in Betaproteobacteria. Expression levels of disaccharidases and transporters were associated with the abundance of affected bacterial phylotypes. These results indicate a relationship between human intestinal gene expression and bacterial community structure and may provide insights into the pathophysiology of gastrointestinal disturbances in children with autism
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