Implantation and atomic scale investigation of self-interstitials in graphene

Crystallographic defects play a key role in determining the properties of crystalline materials. The new class of two-dimensional materials, foremost graphene, have enabled atomically resolved studies of defects, such as vacancies, grain boundaries, dislocations, and foreign atom substitutions. However, atomic resolution imaging of implanted self-interstitials has so far not been reported in any three- but also not in any two-dimensional material. Here, we deposit extra carbon into single-layer graphene at soft landing energies of ~1 eV using a standard carbon coater. We identify all the self-interstitial dimer structures theoretically predicted earlier, employing 80 kV aberration-corrected high-resolution transmission electron microscopy. We demonstrate accumulation of the interstitials into larger aggregates and dislocation dipoles, which we predict to have strong local curvature by atomistic modeling, and to be energetically favourable configurations as compared to isolated interstitial dimers. Our results contribute to the basic knowledge on crystallographic defects, and lay out a pathway into engineering the properties of graphene by pushing the crystal into a state of metastable supersaturation

Chemistry on Graphene

Das hohe Auflösungsvermögen moderner Transmissions-Elektronenmikroskope (TEM) unter einem Angstrom ermöglicht die Untersuchung von Materialien auf atomarer Ebene. Mit dem TEM können sowohl die atomare Struktur als auch dynamische Prozesse direkt beobachtet und damit Rückschlüsse auf physikalische und chemische Eigenschaften gezogen werden. Diese Arbeit beschäftigt sich mit der Untersuchung und Charakterisierung von Graphen - einer einlagigen Kohlenstoffschicht mit herausragenden physikalischen und chemischen Eigenschaften - mit Hilfe unterschiedlicher TEM Verfahren. Einerseits konnte die Qualität von unterschiedlich hergestellten Graphen-Proben an Hand von Strukturuntersuchungen verglichen werden. Andererseits wurden die Einsatzmöglichkeiten von Graphen als Träger- und Schutzschicht, Ausgangsmaterial, Substrat, sowie als Nano-Behälter untersucht. Als Trägermaterial wurde Graphen für Nanoteilchen verwendet, welche für biologische Anwendungen konzipiert wurden. Um geeignete Proben für TEM Untersuchungen herzustellen, waren Oberflächenpräparation und Optimierung der Transfermethode entscheidend. Die TEM Untersuchungen an Nanoteilchen (Au NCs, QDs und Nano-Diamanten mit atomaren Fehlstellen) auf Graphen ermöglichten eine direkte Beschreibung ihrer atomaren Struktur, Größe und Größenverteilung. Untersuchungen von DNA auf Graphen zeigten, dass die Abbildung von biologischen Proben auf Graphen-Trägermaterialen mittels TEM möglich ist. Des Weiteren konnte nachgewiesen werden, dass Graphen auch als Schutzschicht für strahlempfindliche Materialien, wie z.B. C3N4 oder MoS2, geeignet ist und damit die Abbildung dieser Proben in ihrem ursprünglichen Zustand erlaubt. In Bestrahlungexperimenten konnten einwandige Kohlenstoff-Nanoröhrchen aus einer Graphen-Doppellage geformt werden. In einem weiteren Experiment gelang es aus den Adsorbaten auf Graphen eine weitere Graphen-Lage (in-situ) unter Elektronenbeschuss zu wachsen. Weitere Experimente an Wasser, das zwischen zwei Graphen-Lagen eingeschlossen wurde (nano-confinement), erlaubten erstmals die direkte Beobachtung und Charakterisierung einer neuen Modifikation von Eis bei Zimmertemperatur: dem "square ice". Nicht zuletzt wurde im Rahmen dieser Arbeit eine neue Methode zur Säuberung von Graphen-Oberflächen von Adsorbaten mit Hilfe von Adsorptionsmitteln entwickelt, dem sogenannten "dry-cleaning".State-of-the-art transmission electron microscopes (TEMs) are capable to achieve sub-Angstrom resolution. Therefore matter can be studied at the atomic level, i.e., with a TEM the atomic structures and processes can be observed, consequently physical and chemical properties can be derived. In this work, graphene, one atom thick material with outstanding physical and chemical properties, has been thoroughly characterised by different TEM techniques. The structural description of graphene allowed us to compare graphene samples fabricated by different methods and to assess their quality. Furthermore, graphene has been used as a substrate, protective layer, raw material, surface template and nano-confiner.Graphene substrates were used to support nano-objects which were designed for biological applications. Treatments of the graphene substrates prior to sample deposition as well as sample deposition techniques provided the means to obtained samples suitable for TEM investigations. The TEM studies in nano-objects (Au NCs, QDs, nanodiamond with NV centres), deposited on graphene, resulted in the characterisation of their structure, size and dispersion. DNA deposited on graphene was also investigated by TEM. The results showed that the approach of using graphene as substrate can be used to image the structure biological samples. It is presented also in this thesis that graphene can protect radiation sensitive materials such as C3N4 and MoS2 from the electron beam, allowing imaging these materials in their pristine state. By using the electron beam to nano-engineer bilayer graphene it was possible to create single-walled carbon nanotubes. In another experiment, graphene served as surface template where an adlayer graphene grew from residual contamination during imaging. Experiments with water trapped between graphene layers (nano-confinement) resulted in the detection, observation and characterisation of a new form of ice at room temperature, i.e. square ice. Additionally, atomically clean graphene was obtained by the development of a new cleaning method using adsorbents - dry-cleaning -

From Graphene constrictions to single carbon chains

We present an atomic-resolution observation and analysis of graphene constrictions and ribbons with sub-nanometer width. Graphene membranes are studied by imaging side spherical aberration-corrected transmission electron microscopy at 80 kV. Holes are formed in the honeycomb-like structure due to radiation damage. As the holes grow and two holes approach each other, the hexagonal structure that lies between them narrows down. Transitions and deviations from the hexagonal structure in this graphene ribbon occur as its width shrinks below one nanometer. Some reconstructions, involving more pentagons and heptagons than hexagons, turn out to be surprisingly stable. Finally, single carbon atom chain bridges between graphene contacts are observed. The dynamics are observed in real time at atomic resolution with enough sensitivity to detect every carbon atom that remains stable for a sufficient amount of time. The carbon chains appear reproducibly and in various configurations from graphene bridges, between adsorbates, or at open edges and seem to represent one of the most stable configurations that a few-atomic carbon system accomodates in the presence of continuous energy input from the electron beam.Comment: 12 pages, 4 figure

Site specific and localized structural displacements in open structured multimetallic oxides

The structures of solids can locally differ from the macroscopic picture obtained by structural averaging techniques. This difference significantly influences the performance of any functional material. Measurements of these local structures are challenging. Thus, the description of defects is often disregarded. However, in order to understand the functionality, such irregularities have to be investigated. Here, we present a high resolution scanning transmission electron microscopic (STEM) study revealing local structural irregularities in open structured oxides using catalytically active orthorhombic (Mo,V,Te,Nb)Ox as a complex example. Detailed analysis of annular dark field- and annular bright field-STEM images reveal site specific local structural displacements of individual framework and channel sites in the picometer range. These experimental observables can be considered as an important structural addendum for theoretical modelling and should be implemented into the existing data in order to quantify site specific potential energies and stresses. This information can further be used to describe the impact of the structure on the catalytic performance in greater detail

Competitive Management of Sugarcane Waste and Reduction of CO2 Emissions from Harvest Burning in Supply Regions

Sugarcane is an important crop in more than 100 countries around the world. Their burning is a cultural activity before and after the harvest; however, pollutants and greenhouse gases emitted to the atmosphere can affect the human health and weather, respectively. The aim of this research is to report the CO2 emissions of the main countries dedicated to the cane production and explain their relevant relation with the dry matter available to the burn and how it can affect their alternative uses. The methodology used in this study identifies the relation between biomass burned (dry matter) and CO2 emissions, estimated by the Food and Agriculture Organization of the United Nations with the techniques of the Intergovernmental Panel on Climate Change. The study was carried out for the period of 1990–2014. The results show an important positive trend in the increase in the annual production levels and the biomass burned during the harvest period. The high correlation between harvested area and yield per hectare in countries such as Brazil and the United States allows to have more biomass available for alternative uses. Countries such as Mexico and Colombia have a low correlation between both the parameters due to the increase in the harvested hectares and reduction of their performance per hectare

Nematic suspension of a microporous layered silicate obtained by forceless spontaneous delamination via repulsive osmotic swelling for casting high-barrier all-inorganic films

Exploiting the full potential of layered materials for a broad range of applications requires delamination into functional nanosheets. Delamination via repulsive osmotic swelling is driven by thermodynamics and represents the most gentle route to obtain nematic liquid crystals consisting exclusively of single-layer nanosheets. This mechanism was, however, long limited to very few compounds, including 2:1-type clay minerals, layered titanates, or niobates. Despite the great potential of zeolites and their microporous layered counterparts, nanosheet production is challenging and troublesome, and published procedures implied the use of some shearing forces. Here, we present a scalable, eco-friendly, and utter delamination of the microporous layered silicate ilerite into single-layer nanosheets that extends repulsive delamination to the class of layered zeolites. As the sheet diameter is preserved, nematic suspensions with cofacial nanosheets of ≈9000 aspect ratio are obtained that can be cast into oriented films, e.g., for barrier applications

Influence of sugarcane burning on soil carbon and nitrogen release under drought and evapotranspiration conditions in a Mexican sugarcane supply zone

"Sugarcane cultivation is an agricultural activity of worldwide importance. This crop has been cultivated in Mexico for centuries, impacting important productive areas like the study region called Huasteca Potosina which is located at the Central-East part of Mexico. The relationship between soil conditions, weather and production per hectare allows identifying the edaphological and climatic conditions (aptitude levels) for cultivating sugarcane in the study area. The objective of this research work is to analyze the relation between the cultivated hectares and production of sugarcane. Likewise, to explain its behavior and interpret the contribution of N and C released to the atmosphere for burning sugar cane in association with events and meteorological parameters involved in the flux of water between soil and atmosphere such as drought, evapotranspiration and rainfall. The methods and indicators for burning waste provided by the Intergovernmental Panel on Climate Change (IPCC), the value of potential evapotranspiration as Thornthwaite and drought severity index were applied in order to understand the correlations between the drought severity index and potential evapotranspiration, and between C and N release and potential rainfall availability. The results show the increment of harvested hectares and the loss of water from the soil by the increase of periods of drought events and evapotranspiration. The volatilized nutrients in soil varied from 1.32 x 105 to 2.17 x 105 t for C and from 1.32 x 103 t to 2.17 x 103 t for N during the burning of sugarcane, affecting production levels with values fluctuating between 38 t/ha to 77 t/ha for the term of 1990-2010.""El cultivo de caña de azúcar es una actividad agrícola de importancia mundial llevada a cabo en México durante siglos, impactando importantes áreas productivas como la región de estudio llamada Huasteca Potosina, localizada al este central de México. La relación entre las condiciones del suelo, clima y producción por hectárea permite identificar las condiciones edafológicas y climáticas (niveles de aptitud) para cultivar la caña de azúcar en el área de estudio. Bajo este contexto, el objetivo fue analizar la relación entre las hectáreas cultivadas y la producción de caña de azúcar y explicar su comportamiento al interpretar las contribuciones del N y C liberados a la atmósfera por la quema de caña asociados a eventos y parámetros meteorológicos involucrados en el flujo de agua entre el suelo y la atmósfera como sequía, evapotranspiración y lluvia. Los métodos e indicadores para la quema de residuos provistos por el Panel Intergubernamental del Cambio Climático (IPCC. por sus siglas en inglés), el valor de la evapotranspiración potencial por Thornthwaite y el índice de severidad de la sequía fueron aplicados para comprender las correlaciones entre el índice de severidad de la sequía y la evapotranspiración potencial, así como entre el C y el N liberados y la disponibilidad potencial de lluvia. Los resultados muestran el incremento en las hectáreas cosechadas y la pérdida de agua del suelo por el aumento de los periodos de sequía y la evapotranspiración. Los nutrientes del suelo volatilizados, varían de 1.32 x 105 ton a 2.17 x 105 ton para C y 1.32 x 103 ton a 2.17 x 103 ton para N durante la quema de caña de azúcar, lo que afecta sus niveles de producción con valores que fluctúan entre 38 t/ha y 77 t/ha durante el periodo 1990-2010.

Diseño de adobes urbanos para construcción de vivienda en México

Debido a la escasez de documentación sobre técnicas y materiales de construcción tradicional para entornos urbanos, se proponen bloques de suelo compactado que se adapten a las viviendas urbanas de la región y que sea rentables y con bajo impacto ambiental. Se busca la eliminación del proceso de cocción del ladrillo rojo debido a su alta contribución de gases de efecto invernadero y contaminantes, además de su fácil reintegración al ciclo de viva de la vivienda y capacidad térmica. Esto se logra a través de elementos aglutinantes y estabilizantes para cumplir con las normas de construcción de México. Este trabajo presenta distintas dosificaciones de aglutinantes y estabilizantes con materiales de la región y las capacidades de carga obtenidas. Los resultados son la base para normas específicas de adobe para uso urbano