109 research outputs found
Selecting a single orientation for millimeter sized graphene sheets
We have used Low Energy Electron Microscopy (LEEM) and Photo Emission
Electron Microscopy (PEEM) to study and improve the quality of graphene films
grown on Ir(111) using chemical vapor deposition (CVD). CVD at elevated
temperature already yields graphene sheets that are uniform and of monatomic
thickness. Besides domains that are aligned with respect to the substrate,
other rotational variants grow. Cyclic growth exploiting the faster growth and
etch rates of the rotational variants, yields films that are 99 % composed of
aligned domains. Precovering the substrate with a high density of graphene
nuclei prior to CVD yields pure films of aligned domains extending over
millimeters. Such films can be used to prepare cluster-graphene hybrid
materials for catalysis or nanomagnetism and can potentially be combined with
lift-off techniques to yield high-quality, graphene based electronic devices
In situ observation of stress relaxation in epitaxial graphene
Upon cooling, branched line defects develop in epitaxial graphene grown at
high temperature on Pt(111) and Ir(111). Using atomically resolved scanning
tunneling microscopy we demonstrate that these defects are wrinkles in the
graphene layer, i.e. stripes of partially delaminated graphene. With low energy
electron microscopy (LEEM) we investigate the wrinkling phenomenon in situ.
Upon temperature cycling we observe hysteresis in the appearance and
disappearance of the wrinkles. Simultaneously with wrinkle formation a change
in bright field imaging intensity of adjacent areas and a shift in the moire
spot positions for micro diffraction of such areas takes place. The stress
relieved by wrinkle formation results from the mismatch in thermal expansion
coefficients of graphene and the substrate. A simple one-dimensional model
taking into account the energies related to strain, delamination and bending of
graphene is in qualitative agreement with our observations.Comment: Supplementary information: S1: Photo electron emission microscopy and
LEEM measurements of rotational domains, STM data of a delaminated bulge
around a dislocation. S2: Movie with increasing brightness upon wrinkle
formation as in figure 4. v2: Major revision including new experimental dat
Ambipolar charge injection and transport in a single pentacene monolayer island
Electrons and holes are locally injected in a single pentacene monolayer
island. The two-dimensional distribution and concentration of the injected
carriers are measured by electrical force microscopy. In crystalline monolayer
islands, both carriers are delocalized over the whole island. On disordered
monolayer, carriers stay localized at their injection point. These results
provide insight into the electronic properties, at the nanometer scale, of
organic monolayers governing performances of organic transistors and molecular
devices.Comment: To be published in Nano Letter
Europium Underneath Graphene on Ir(111): Intercalation Mechanism, Magnetism, and Band Structure
The intercalation of Eu underneath Gr on Ir(111) is comprehensively
investigated by microscopic, magnetic, and spectroscopic measurements, as well
as by density functional theory. Depending on the coverage, the intercalated Eu
atoms form either a or a R superstructure with respect to Gr. We investigate the
mechanisms of Eu penetration through a nominally closed Gr sheet and measure
the electronic structures and magnetic properties of the two intercalation
systems. Their electronic structures are rather similar. Compared to Gr on
Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger
binding energies resulting in n-doping. The hybridization of the Ir surface
state with Gr states is lifted, and the moire superperiodic potential is
strongly reduced. In contrast, the magnetic behavior of the two intercalation
systems differs substantially as found by X-ray magnetic circular dichroism.
The Eu structure displays plain paramagnetic behavior, whereas
for the R structure the large
zero-field susceptibility indicates ferromagnetic coupling, despite the absence
of hysteresis at 10 K. For the latter structure, a considerable easy-plane
magnetic anisotropy is observed and interpreted as shape anisotropy.Comment: 18 pages with 14 figures, including Supplemental Materia
In Vitro and In Vivo Antagonism of a G Protein-Coupled Receptor (S1P3) with a Novel Blocking Monoclonal Antibody
Background: S1P 3 is a lipid-activated G protein-couple receptor (GPCR) that has been implicated in the pathological processes of a number of diseases, including sepsis and cancer. Currently, there are no available high-affinity, subtypeselective drug compounds that can block activation of S1P3. We have developed a monoclonal antibody (7H9) that specifically recognizes S1P3 and acts as a functional antagonist. Methodology/Principal Findings: Specific binding of 7H9 was demonstrated by immunocytochemistry using cells that over-express individual members of the S1P receptor family. We show, in vitro, that 7H9 can inhibit the activation of S1P3mediated cellular processes, including arrestin translocation, receptor internalization, adenylate cyclase inhibiton, and calcium mobilization. We also demonstrate that 7H9 blocks activation of S1P3 in vivo, 1) by preventing lethality due to systemic inflammation, and 2) by altering the progression of breast tumor xenografts. Conclusions/Significance: We have developed the first-reported monoclonal antibody that selectively recognizes a lipidactivated GPCR and blocks functional activity. In addition to serving as a lead drug compound for the treatment of sepsi
Lysophosphatidic acid and sphingosine-1-phosphate promote morphogenesis and block invasion of prostate cancer cells in three-dimensional organotypic models
Normal prostate and some malignant prostate cancer (PrCa) cell lines undergo acinar differentiation and form spheroids in three-dimensional (3-D) organotypic culture. Acini formed by PC-3 and PC-3M, less pronounced also in other PrCa cell lines, spontaneously undergo an invasive switch, leading to the disintegration of epithelial structures and the basal lamina, and formation of invadopodia. This demonstrates the highly dynamic nature of epithelial plasticity, balancing epithelial-to-mesenchymal transition against metastable acinar differentiation. This study assessed the role of lipid metabolites on epithelial maturation. PC-3 cells completely failed to form acinar structures in delipidated serum. Adding back lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) rescued acinar morphogenesis and repressed invasion effectively. Blocking LPA receptor 1 (LPAR1) functions by siRNA (small interference RNA) or the specific LPAR1 inhibitor Ki16425 promoted invasion, while silencing of other G-protein-coupled receptors responsive to LPA or S1P mainly caused growth arrest or had no effects. The G-proteins Gα12/13 and Gαi were identified as key mediators of LPA signalling via stimulation of RhoA and Rho kinases ROCK1 and 2, activating Rac1, while inhibition of adenylate cyclase and accumulation of cAMP may be secondary. Interfering with these pathways specifically impeded epithelial polarization in transformed cells. In contrast, blocking the same pathways in non-transformed, normal cells promoted differentiation. We conclude that LPA and LPAR1 effectively promote epithelial maturation and block invasion of PrCa cells in 3-D culture. The analysis of clinical transcriptome data confirmed reduced expression of LPAR1 in a subset of PrCa's. Our study demonstrates a metastasis-suppressor function for LPAR1 and Gα12/13 signalling, regulating cell motility and invasion versus epithelial maturation
Somatostatin Inhibits Cell Migration and Reduces Cell Counts of Human Keratinocytes and Delays Epidermal Wound Healing in an Ex Vivo Wound Model
The peptide hormone somatostatin (SST) and its five G protein-coupled receptors
(SSTR1-5) were described to be present in the skin, but their cutaneous
function(s) and skin-specific signalling mechanisms are widely unknown. By using
receptor specific agonists we show here that the SSTRs expressed in
keratinocytes are functionally coupled to the inhibition of adenylate cyclase.
In addition, treatment with SSTR4 and SSTR5/1 specific agonists significantly
influences the MAP kinase signalling pathway. As epidermal hormone receptors in
general are known to regulate re-epithelialization following skin injury, we
investigated the effect of SST on cell counts and migration of human
keratinocytes. Our results demonstrate a significant inhibition of cell
migration and reduction of cell counts by SST. We do not observe an effect on
apoptosis and necrosis. Analysis of signalling pathways showed that somatostatin
inhibits cell migration independent of its effect on cAMP. Migrating
keratinocytes treated with SST show altered cytoskeleton dynamics with delayed
lamellipodia formation. Furthermore, the activity of the small GTPase Rac1 is
diminished, providing evidence for the control of the actin cytoskeleton by
somatostatin receptors in keratinocytes. While activation of all receptors leads
to redundant effects on cell migration, only treatment with a SSTR5/1 specific
agonist resulted in decreased cell counts. In accordance with reduced cell
counts and impaired migration we observe delayed re-epithelialization in an
ex vivo wound healing model. Consequently, our experiments
suggest SST as a negative regulator of epidermal wound healing
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