ALBERT

Description: Dislocations represent one of the most fascinating and fundamental concepts in materials science. Most importantly, dislocations are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly affect the local electronic and optical properties of semiconductors and ionic crystals. In materials with small dimensions, they experience extensive image forces, which attract them to the surface to release strain energy. However, in layered crystals such as graphite, dislocation movement is mainly restricted to the basal plane. Thus, the dislocations cannot escape, enabling their confinement in crystals as thin as only two monolayers. To explore the nature of dislocations under such extreme boundary conditions, the material of choice is bilayer graphene, the thinnest possible quasi-two-dimensional crystal in which such linear defects can be confined. Homogeneous and robust graphene membranes derived from high-quality epitaxial graphene on silicon carbide provide an ideal platform for their investigation. Here we report the direct observation of basal-plane dislocations in freestanding bilayer graphene using transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. Our investigation reveals two striking size effects. First, the absence of stacking-fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern that corresponds to an alternating AB B[Symbol: see text]AC change of the stacking order. Second, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane that results directly from accommodation of strain. In fact, the buckling changes the strain state of the bilayer graphene and is of key importance for its electronic properties. Our findings will contribute to the understanding of dislocations and of their role in the structural, mechanical and electronic properties of bilayer and few-layer graphene.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Butz, Benjamin -- Dolle, Christian -- Niekiel, Florian -- Weber, Konstantin -- Waldmann, Daniel -- Weber, Heiko B -- Meyer, Bernd -- Spiecker, Erdmann -- England -- Nature. 2014 Jan 23;505(7484):533-7. doi: 10.1038/nature12780. Epub 2013 Dec 18.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Center for Nanoanalysis and Electron Microscopy, Friedrich-Alexander-Universitat Erlangen-Nurnberg, Cauerstrasse 6, 91058 Erlangen, Germany. ; Interdisziplinares Zentrum fur Molekulare Materialien und Computer-Chemie-Centrum, Friedrich-Alexander-Universitat Erlangen-Nurnberg, Nagelsbachstrasse 25, 91052 Erlangen, Germany. ; Lehrstuhl fur Angewandte Physik, Friedrich-Alexander-Universitat Erlangen-Nurnberg, Staudtstrasse 7, 91058 Erlangen, Germany.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/24352231" target="_blank"〉PubMed〈/a〉

Description: Nicotinamide adenine dinucleotide phosphate (NADP) is a critical cofactor during metabolism, calcium signaling, and oxidative defense, yet how animals regulate their NADP pools in vivo and how NADP-synthesizing enzymes are regulated have long remained unknown. Here we show that expression of Nadk, an NAD+ kinase-encoding gene, governs NADP biosynthesis in...

Description: Topological defects in crystalline solids are of fundamental interest in physics and materials science because they can radically alter the properties of virtually any material. Of particular importance are line defects, known as dislocations, which are the main carriers of plasticity and have a tremendous effect on electronic and optical properties. Understanding and controlling the occurrence and behavior of those defects have been of major and ongoing interest since their discovery in the 1930s. This interest was renewed with the advent of two-dimensional materials in which a single topological defect can alter the functionality of the whole system and even create new physical phenomena. We present an experimental approach to directly manipulate dislocations in situ on the nanometer scale by using a dedicated scanning electron microscope setup. With this approach, key fundamental characteristics such as line tension, defect interaction, and node formation have been studied. A novel switching reaction, based on the recombination of dislocation lines, was found, which paves the way for the concept of switches made of a bimodal topological defect configuration.