Graphene edge structures: Folding, scrolling, tubing, rippling and twisting

Conventional three-dimensional crystal lattices are terminated by surfaces, which can demonstrate complex rebonding and rehybridisation, localised strain and dislocation formation. Two dimensional crystal lattices, of which graphene is the archetype, are terminated by lines. The additional available dimension at such interfaces opens up a range of new topological interface possibilities. We show that graphene sheet edges can adopt a range of topological distortions depending on their nature. Rehybridisation, local bond reordering, chemical functionalisation with bulky, charged, or multi-functional groups can lead to edge buckling to relieve strain, folding, rolling and even tube formation. We discuss the topological possibilities at a 2D graphene edge, and under what circumstances we expect different edge topologies to occur. Density functional calculations are used to explore in more depth different graphene edge types.Comment: Additional figure in published versio

A comparative study of density functional and density functional tight binding calculations of defects in graphene

The density functional tight binding approach (DFTB) is well adapted for the study of point and line defects in graphene based systems. After briefly reviewing the use of DFTB in this area, we present a comparative study of defect structures, energies and dynamics between DFTB results obtained using the dftb+ code, and density functional results using the localised Gaussian orbital code, AIMPRO. DFTB accurately reproduces structures and energies for a range of point defect structures such as vacancies and Stone-Wales defects in graphene, as well as various unfunctionalised and hydroxylated graphene sheet edges. Migration barriers for the vacancy and Stone-Wales defect formation barriers are accurately reproduced using a nudged elastic band approach. Finally we explore the potential for dynamic defect simulations using DFTB, taking as an example electron irradiation damage in graphene

Prevailing Negative Soil Biota Effect and No Evidence for Local Adaptation in a Widespread Eurasian Grass

Soil biota effects are increasingly accepted as an important driver of the abundance and distribution of plants. While biogeographical studies on alien invasive plant species have indicated coevolution with soil biota in their native distribution range, it is unknown whether adaptation to soil biota varies among populations within the native distribution range. The question of local adaptation between plants and their soil biota has important implications for conservation of biodiversity and may justify the use of seed material from local provenances in restoration campaigns.We studied soil biota effects in ten populations of the steppe grass Stipa capillata from two distinct regions, Europe and Asia. We tested for local adaptation at two different scales, both within (ca. 10-80 km) and between (ca. 3300 km) regions, using a reciprocal inoculation experiment in the greenhouse for nine months. Generally, negative soil biota effects were consistent. However, we did not find evidence for local adaptation: both within and between regions, growth of plants in their 'home soil' was not significantly larger relative to that in soil from other, more distant, populations.Our study suggests that negative soil biota effects can prevail in different parts of a plant species' range. Absence of local adaptation points to the possibility of similar rhizosphere biota composition across populations and regions, sufficient gene flow to prevent coevolution, selection in favor of plasticity, or functional redundancy among different soil biota. From the point of view of plant--soil biota interactions, our findings indicate that the current practice of using seeds exclusively from local provenances in ecosystem restoration campaigns may not be justified

Different approaches in grassland analysis – Editorial to the 11th EDGG Grassland Special Feature

This Special Feature is the eleventh in a series devoted to grasslands and edited by members of the Eurasian Dry Grassland Group (EDGG; http://www.edgg.org) or its predecessors European Dry Grassland Group and Arbeitsgruppe Trockenrasen that has been published in Tuexenia since 2005. In this Special Feature we present six papers about a broad range of Manuscript received 12 July 2016, accepted 13 July 2016 288 grassland types ranging from serpentinite grasslands to alpine grasslands and dealing with questions of syntaxonomy, plant functional traits, experimental recruitment, disturbance, and vegetation dynamics of grasslands. Study regions were the Balkan peninsula (i.e. Serbia, Bosnia and Herzegovina), Belgium, Germany, Hungary, and Italy. A total of 38 authors from nine countries contributed to this Special Feature, including authors from Hungary (14 authors), Germany (11), Serbia (6), Belgium (3), Czech Republic (1), Italy (1), Slovakia (1), and Slovenia/Croatia (1), with 19 of the 38 authors publishing in Tuexenia for the first time

Mechanical properties of nanosheets and nanotubes investigated using a new geometry independent volume definition

Cross-sectional area and volume become difficult to define as material dimensions approach the atomic scale. This limits the transferability of macroscopic concepts such as Young's modulus. We propose a new volume definition where the enclosed nanosheet or nanotube average electron density matches that of the parent layered bulk material. We calculate the Young's moduli for various nanosheets (including graphene, BN and MoS2) and nanotubes. Further implications of this new volume definition such as a Fermi level dependent Young's modulus and out-of-plane Poisson's ratio are shown

Ripple edge engineering of graphene nanoribbons

It is now possible to produce graphene nanoribbons (GNRs) with atomically defined widths. GNRs offer many opportunities for electronic devices and composites, if it is possible to establish the link between edge structure and functionalisation, and resultant GNR properties. Switching hydrogen edge termination to larger more complex functional groups such as hydroxyls or thiols induces strain at the ribbon edge. However we show that this strain is then relieved via the formation of static out-of-plane ripples. The resultant ribbons have a significantly reduced Young's Modulus which varies as a function of ribbon width, modified band gaps, as well as heterogeneous chemical reactivity along the edge. Rather than being the exception, such static edge ripples are likely on the majority of functionalized graphene ribbon edges.Comment: Supplementary Materials availabl

Low energy graphene edge termination via small diameter nanotube formation

We demonstrate that free graphene sheet edges can curl back on themselves,reconstructing as nanotubes. This results in lower formation energies than any other non-functionalised edge structure reported to date in the literature. We determine the critical tube size and formation barrier and compare with density functional simulations of other edge terminations including a new reconstructed Klein edge. Simulated high resolution electron microscopy images show why such rolled edges may be difficult to detect. Rolled zigzag edges serve as metallic conduction channels, separated from the neighbouring bulk graphene by a chain of insulating sp$^3$-carbon atoms, and introduce Van Hove singularities into the graphene density of states.Comment: To appear in Phys. Rev. Let

Restoration, monitoring, conservation and phytosociology of semi-natural and natural grasslands in Central Europe : editorial to the 14th EDGG special feature

Seit 14 Jahren werden von Mitgliedern der Eurasian Dry Grassland Group (EDGG) und deren Vorgängerorganisationen Grasland-Sonderteile (Special Features) in Tuexenia herausgegeben. Das diesjährige Special Feature mit dem Titel Wiederherstellung, Überwachung, Schutz und Soziologie des halbnatürlichen und natürlichen Graslands in Mitteleuropa umfasst sechs Artikel: BOCH et al. untersuchen die Eignung von Quasidauerflächen zum Langzeitmonitoring mittlerer Zeigerwerte sowie Zusammensetzung und Turnover der Arten in Trockenrasen. CHYTRÝ et al. beschreiben die Vegetation der Trockenrasen des Tieflands der Transkarpaten und gleichzeitig drei neue Assoziationen für die Ukraine. ERDŐS et al. zeigen Trends in der Artenzusammensetzung und dem Artenreichtum entlang eines Gradienten vom Zentrum zum Rand in Waldsteppen im südlichen Karpatenbecken. GHEZA et al. beschreiben die Syntaxonomie, Ökologie und den Naturschutzwert der Erdflechtengesellschaften bodensaurer Thero-Airion-Pionierrasen in der Poebene in Italien. Mardari et al. untersuchen die Populationstruktur und Lebensraumeigenschaften von Arnica montana in den Karpaten. ROLEČEK et al. berichten über neue Maxima des kleinräumigen Gefäßpflanzenartenreichtums rumänischer und ukrainischer Halbtrockenrasen. Insgesamt haben 40 Autoren zum 14. Special Feature beigetragen

Traditional land use, management and biodiversity of European semi-natural grasslands : editorial to the 15th EDGG Special Feature

Seit 15 Jahren geben Mitglieder der Eurasian Dry Grassland Group (EDGG) und deren Vorgängerorganisationen Grasland-Sonderausgaben (Special Features) in Tuexenia heraus. Das diesjährige Special Feature mit dem Titel Traditionelle Landnutzung, Management und Biodiversität von halb-natürlichem Grasland in Europa umfasst acht Artikel, die viele Aspekte der Graslandforschung beleuchten und verschiedene Organismengruppen miteinbeziehen: JANIŠOVÁ et al. (2020a) untersuchten den Einfluss verschiedener traditioneller Landnutzungsformen auf den Artenreichtum des extensiven Graslands im Apuseni-Gebirge in Westrumänien. LABADESSA et al. (2020) verglichen die Artenzusammensetzung von beweidetem und unbeweidetem Grasland im Rahmen eines Renaturierungsprojektes in Südostitalien. PÁPAY et al. (2020) untersuchten den Einfluss der Gehölzdeckung auf die Artenzusammensetzung des halbnatürlichen Graslands im Mátra-Gebirge in Nordungarn und inwieweit wilde Huftiere die Verbuschung eindämmen können. DAYNEKO et al. (2020) beschrieben den Einfluss antiker Siedlungen aus skythischer und sarmathianischer Zeit auf die Biodiversität von Steppenhabitaten im Dnipro-Becken in der Südukraine. ZANIEWSKI et al. (2020) untersuchten, ob Störungen durch Geländefahrzeuge die Sukzession von Binnendünen in Zentralpolen aufhalten und dadurch die ehemalige Landnutzung ersetzen können, um die Kryptogamendiversität zu erhalten. JANIŠOVÁ et al. (2020b) charakterisierten Diversitätsmuster von Carex humilis-dominierten Felsen-steppen in vier biogeographischen Regionen des östlichen Mitteleuropas entlang eines Höhengradienten über 1240 m. BÜCHLER et al. (2020) gingen der Frage nach, inwieweit der Naturschutzwert von Mesobromion-Halbtrockenrasen im Kanton Zürich in der Schweiz auf unterschiedliche Standortbedingungen zurückgeführt werden kann. CANCELLIERI et al. (2020) gaben einen Überblick über die Pflanzengesellschaften und die ökologischen Bedingungen der Trockenrasen des Nationalparks Abruzzen, Latium und Molise in Mittelitalien. Insgesamt haben 47 Autoren aus zehn Ländern zur 15. Grasland-Sonderausgabe beigetragen