Atomic-Scale Dynamics of the Formation and Dissolution of Carbon Clusters in SiO2

Oxidation of SiC produces SiO2 while CO is released. A `reoxidation' step at lower temperatures is, however, necessary to produce high-quality SiO2. This step is believed to cleanse the oxide of residual C without further oxidation of the SiC substrate. We report first-principles calculations that describe the nucleation and growth of O-deficient C clusters in SiO2 under oxidation conditions, fed by the production of CO at the advancing interface, and their gradual dissolution by the supply of O under reoxidation conditions. We predict that both CO and CO2 are released during both steps.Comment: RevTex, 4 pages, 2 ps figures, to appear in Phys. Rev. Lett. (June 25, 2001

Theory-assisted determination of nano-rippling and impurities in atomic resolution images of angle-mismatched bilayer graphene

Ripples and impurity atoms are universally present in 2D materials, limiting carrier mobility, creating pseudo–magnetic fields, or affecting the electronic and magnetic properties. Scanning transmission electron microscopy (STEM) generally provides picometer-level precision in the determination of the location of atoms or atomic 'columns' in the in-image plane (xy plane). However, precise atomic positions in the z-direction as well as the presence of certain impurities are difficult to detect. Furthermore, images containing moiré patterns such as those in angle-mismatched bilayer graphene compound the problem by limiting the determination of atomic positions in the xy plane. Here, we introduce a reconstructive approach for the analysis of STEM images of twisted bilayers that combines the accessible xy coordinates of atomic positions in a STEM image with density-functional-theory calculations. The approach allows us to determine all three coordinates of all atomic positions in the bilayer and establishes the presence and identity of impurities. The deduced strain-induced rippling in a twisted bilayer graphene sample is consistent with the continuum model of elasticity. We also find that the moiré pattern induces undulations in the z direction that are approximately an order of magnitude smaller than the strain-induced rippling. A single substitutional impurity, identified as nitrogen, is detected. The present reconstructive approach can, therefore, distinguish between moiré and strain-induced effects and allows for the full reconstruction of 3D positions and atomic identities

Direct Observation of an Interface Dipole between Two Metallic Oxides Caused by Localized Oxygen Vacancies

Oxygen vacancies are increasingly recognized to play a role in phenomena observed at transition-metal oxide interfaces. Here we report a study of SrRuO3/La0.7Sr0.3MnO3 (SRO/LSMO) interfaces using a combination of quantitative aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and density-functional calculations. Cation displacements are observed at the interface, indicative of a dipole-like electric field even though both materials are nominally metallic. The observed displacements are reproduced by theory if O vacancies are present in the near-interface LSMO layers. The results suggest that atomic-scale structural mapping can serve as a quantitative indicator of the presence of O vacancies at interfaces

Atomic-resolution visualization and doping effects of complex structures in intercalated bilayer graphene

Molecules intercalating two-dimensional materials form complex structures that have been characterized primarily by spatially averaged techniques. Here we use aberration-corrected scanning transmission electron microscopy and density-functional-theory (DFT) calculations to study the atomic structure of bilayer graphene (BLG) and few-layer graphene (FLG) intercalated with FeCl3. In BLG, we discover two distinct intercalated structures that we identify as monolayer FeCl3 and monolayer FeCl2. The two structures are separated by atomically sharp boundaries and induce large free-carrier densities on the order of 1013cm−2 in the graphene layers. In FLG, we observe multiple FeCl3 layers stacked in a variety of possible configurations with respect to one another. Finally, we find that the microscope's electron beam can convert the FeCl3 monolayer into FeOCl monolayers in a rectangular lattice. These results reveal the need for a combination of atomically resolved microscopy, spectroscopy, and DFT calculations to identify intercalated structures and study their properties

ApoE−/− PGC-1α−/− Mice Display Reduced IL-18 Levels and Do Not Develop Enhanced Atherosclerosis

BACKGROUND: Atherosclerosis is a chronic inflammatory disease that evolves from the interaction of activated endothelial cells, macrophages, lymphocytes and modified lipoproteins (LDLs). In the last years many molecules with crucial metabolic functions have been shown to prevent important steps in the progression of atherogenesis, including peroxisome proliferator activated receptors (PPARs) and the class III histone deacetylase (HDAC) SIRT1. The PPARγ coactivator 1 alpha (Ppargc1a or PGC-1α) was identified as an important transcriptional cofactor of PPARγ and is activated by SIRT1. The aim of this study was to analyze total PGC-1α deficiency in an atherosclerotic mouse model. METHODOLOGY/PRINCIPAL FINDINGS: To investigate if total PGC-1α deficiency affects atherosclerosis, we compared ApoE(-/-) PGC-1α(-/-) and ApoE(-/-) PGC-1α(+/+) mice kept on a high cholesterol diet. Despite having more macrophages and a higher ICAM-1 expression in plaques, ApoE(-/-) PGC-1α(-/-) did not display more or larger atherosclerotic plaques than their ApoE(-/-) PGC-1α(+/+) littermates. In line with the previously published phenotype of PGC-1α(-/-) mice, ApoE(-/-) PGC-1α(-/-) mice had marked reduced body, liver and epididymal white adipose tissue (WAT) weight. VLDL/LDL-cholesterol and triglyceride contents were also reduced. Aortic expression of PPARα and PPARγ, two crucial regulators for adipocyte differentiation and glucose and lipid metabolism, as well as the expression of some PPAR target genes was significantly reduced in ApoE(-/-) PGC-1α(-/-) mice. Importantly, the epididymal WAT and aortic expression of IL-18 and IL-18 plasma levels, a pro-atherosclerotic cytokine, was markedly reduced in ApoE(-/-) PGC-1α(-/-) mice. CONCLUSIONS/SIGNIFICANCE: ApoE(-/-) PGC-1α(-/-) mice, similar as PGC-1α(-/-) mice exhibit markedly reduced total body and visceral fat weight. Since inflammation of visceral fat is a crucial trigger of atherogenesis, decreased visceral fat in PGC-1α-deficient mice may explain why these mice do not develop enhanced atherosclerosis