1,789 research outputs found
Aharonov-Bohm interferences from local deformations in graphene
One of the most interesting aspects of graphene is the tied relation between
structural and electronic properties. The observation of ripples in the
graphene samples both free standing and on a substrate has given rise to a very
active investigation around the membrane-like properties of graphene and the
origin of the ripples remains as one of the most interesting open problems in
the system. The interplay of structural and electronic properties is
successfully described by the modelling of curvature and elastic deformations
by fictitious gauge fields that have become an ex- perimental reality after the
suggestion that Landau levels can form associated to strain in graphene and the
subsequent experimental confirmation. Here we propose a device to detect
microstresses in graphene based on a scanning-tunneling-microscopy setup able
to measure Aharonov-Bohm inter- ferences at the nanometer scale. The
interferences to be observed in the local density of states are created by the
fictitious magnetic field associated to elastic deformations of the sample.Comment: Some bugs fixe
Electronic structure and bonding properties of Si-doped hydrogenated amorphous carbon films
[[abstract]]This work investigates the C K-edge x-ray absorption near-edge structure (XANES), valence-band photoelectron spectroscopy (PES), and Fourier transform infrared (FTIR) spectra of Si-doped hydrogenated amorphous carbon films. The C K-edge XANES and valence-band PES spectra indicate that the sp2/sp3 population ratio decreases as the amount of tetramethylsilane vapor precursor increases during deposition, which suggest that Si doping% enhances sp3 and reduces sp2-bonding configurations. FTIR spectra show the formation of a polymeric sp3 C–Hn structure and Si–Hn bonds, which causes the Young’s modulus and hardness of the films to decrease with the increase of the Si content.[[incitationindex]]SCI[[booktype]]紙
STM Spectroscopy of ultra-flat graphene on hexagonal boron nitride
Graphene has demonstrated great promise for future electronics technology as
well as fundamental physics applications because of its linear energy-momentum
dispersion relations which cross at the Dirac point. However, accessing the
physics of the low density region at the Dirac point has been difficult because
of the presence of disorder which leaves the graphene with local microscopic
electron and hole puddles, resulting in a finite density of carriers even at
the charge neutrality point. Efforts have been made to reduce the disorder by
suspending graphene, leading to fabrication challenges and delicate devices
which make local spectroscopic measurements difficult. Recently, it has been
shown that placing graphene on hexagonal boron nitride (hBN) yields improved
device performance. In this letter, we use scanning tunneling microscopy to
show that graphene conforms to hBN, as evidenced by the presence of Moire
patterns in the topographic images. However, contrary to recent predictions,
this conformation does not lead to a sizable band gap due to the misalignment
of the lattices. Moreover, local spectroscopy measurements demonstrate that the
electron-hole charge fluctuations are reduced by two orders of magnitude as
compared to those on silicon oxide. This leads to charge fluctuations which are
as small as in suspended graphene, opening up Dirac point physics to more
diverse experiments than are possible on freestanding devices.Comment: Nature Materials advance online publication 13/02/201
Ripple Texturing of Suspended Graphene Atomic Membranes
Graphene is the nature's thinnest elastic membrane, with exceptional
mechanical and electrical properties. We report the direct observation and
creation of one-dimensional (1D) and 2D periodic ripples in suspended graphene
sheets, using spontaneously and thermally induced longitudinal strains on
patterned substrates, with control over their orientations and wavelengths. We
also provide the first measurement of graphene's thermal expansion coefficient,
which is anomalously large and negative, ~ -7x10^-6 K^-1 at 300K. Our work
enables novel strain-based engineering of graphene devices.Comment: 15 pages, 4 figure
Developmental changes in the role of different metalinguistic awareness skills in Chinese reading acquisition from preschool to third grade
Copyright @ 2014 Wei et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and source are credited.The present study investigated the relationship between Chinese reading skills and metalinguistic awareness skills such as phonological, morphological, and orthographic awareness for 101 Preschool, 94 Grade-1, 98 Grade-2, and 98 Grade-3 children from two primary schools in Mainland China. The aim of the study was to examine how each of these metalinguistic awareness skills would exert their influence on the success of reading in Chinese with age. The results showed that all three metalinguistic awareness skills significantly predicted reading success. It further revealed that orthographic awareness played a dominant role in the early stages of reading acquisition, and its influence decreased with age, while the opposite was true for the contribution of morphological awareness. The results were in stark contrast with studies in English, where phonological awareness is typically shown as the single most potent metalinguistic awareness factor in literacy acquisition. In order to account for the current data, a three-stage model of reading acquisition in Chinese is discussed.National Natural Science Foundation of China and Knowledge Innovation Program of the
Chinese Academy of Sciences
Production of Giant Unilamellar Vesicles and Encapsulation of Nematic Lyotropic Liquid Crystals
We describe a modified microfluidic method for making Giant Unilamellar Vesicles (GUVs)viawater/octanol-lipid/water double emulsion droplets. At a high enough lipid concentration we show that thede-wetting of the octanol from these droplets occurs spontaneously (off-chip) without the need to useshear to aid the de-wetting process. The resultant mixture of octanol droplets and GUVs can beseparated by making use of the buoyancy of the octanol. A simpler microfluidic device and pumpsystem can be employed and, because of the higher flow-rates and much higher rate of formation ofthe double emulsion droplets (B1500 s 1compared to up toB75 s 1), it is easier to make largernumbers of GUVs and larger volumes of solution. Because of the potential for using GUVs thatincorporate lyotropic nematic liquid crystals in biosensors we have used this method to make GUVs thatincorporate the nematic phases of sunset yellow and disodium chromoglycate. However, the phasebehaviour of these lyotropic liquid crystals is quite sensitive to concentration and we found that there isan unexpected spread in the concentration of the contents of the GUVs obtained
Textures of Nematic Liquid Crystal Cylindric-Section Droplets Confined by Chemically Patterned Surfaces
The director fields adopted by nematic liquid crystals (LCs) that are confined by the surface to form long, thin droplets are investigated using polarising optical microscopy. Samples are produced by de-wetting of the LC on a surface patterned with alternating high-surface energy and low-surface energy stripes of 10–30 μm width. The droplets obtained are expected to adopt a profile which is that of a longitudinal section of a cylinder and, as this suggests, the director fields observed are variants in the case where the LC is constrained in a cylindrical capillary or fibre. Hence, when there is normal anchoring at the air interface, the textures observed are related to the well-known escaped radial texture (for the nematic LC mixture E7) or plane polar texture (for the LC mixture MLC6609). More surprising is the observation that the nematic LC mixture MLC7023, which is anchored in a planar or tilted manner at the air interface, also gives what appears to be an escaped radial director field. As an exploration of the possibility of using these systems in creating sensors, the effects of adding a chiral dopant and of adding water to the substrates are also investigated
Lack of correlation of stem cell markers in breast cancer stem cells
BACKGROUND: Various markers are used to identify the unique sub-population of breast cancer cells with stem cell properties. Whether these markers are expressed in all breast cancers, identify the same population of cells, or equate to therapeutic response is controversial. METHODS: We investigated the expression of multiple cancer stem cell markers in human breast cancer samples and cell lines in vitro and in vivo, comparing across and within samples and relating expression with growth and therapeutic response to doxorubicin, docetaxol and radiotherapy. RESULTS: CD24, CD44, ALDH and SOX2 expression, the ability to form mammospheres and side-population cells are variably present in human cancers and cell lines. Each marker identifies a unique rather than common population of cancer cells. In vivo, cells expressing these markers are not specifically localized to the presumptive stem cell niche at the tumour/stroma interface. Repeated therapy does not consistently enrich cells expressing these markers, although ER-negative cells accumulate. CONCLUSIONS: Commonly employed methods identify different cancer cell sub-populations with no consistent therapeutic implications, rather than a single population of cells. The relationships of breast cancer stem cells to clinical parameters will require identification of specific markers or panels for the individual cancer
The Control of Director Fields in Phospholipid-Coated Liquid Crystal Droplets
In liquid crystal (LC) droplets, small changes in surface anchoring energy can produce large changes in the director field which result in readily detectable optical effects. This makes them attractive for use as biosensors. Coating LC droplets with a phospholipid monolayer provides a bridge between the hydrophobic world of LCs and the water-based world of biology and makes it possible to incorporate naturally occurring biosensor systems. However, phos-pholipids promote strong perpendicular (homeotropic) anchoring that can inhibit switching of the director field. We show that the tendency for phospholipid layers to promote perpendicular anchoring can be suppressed by using syn-thetic phospholipids in which the acyl chains are terminated with bulky tert-butyl or ferrocenyl groups; the larger these end-group(s), the less likely the system is to be perpendicular/radial. Additionally, the droplet director field is found to be dependent on the nature of the LC, particularly its intrinsic surface properties; but not (apparently) on the sign of the dielectric anisotropy, the proximity to the melting/isotropic phase transition, the surface tension (in air) or the values of the Frank elastic constants
Explosive Nucleosynthesis: What we learned and what we still do not understand
This review touches on historical aspects, going back to the early days of
nuclear astrophysics, initiated by BFH and Cameron, discusses (i) the
required nuclear input from reaction rates and decay properties up to the
nuclear equation of state, continues (ii) with the tools to perform
nucleosynthesis calculations and (iii) early parametrized nucleosynthesis
studies, before (iv) reliable stellar models became available for the late
stages of stellar evolution. It passes then through (v) explosive environments
from core-collapse supernovae to explosive events in binary systems (including
type Ia supernovae and compact binary mergers), and finally (vi) discusses the
role of all these nucleosynthesis production sites in the evolution of
galaxies. The focus is put on the comparison of early ideas and present, very
recent, understanding.Comment: 11 pages, to appear in Springer Proceedings in Physics (Proc. of
Intl. Conf. "Nuclei in the Cosmos XV", LNGS Assergi, Italy, June 2018
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