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 -

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

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

Preparation of Solid Solution and Layered IrO _x-Ni(OH)₂Oxygen Evolution Catalysts:Toward Optimizing Iridium Efficiency for OER

Minimizing iridium loading in oxygen evolution reaction (OER) catalysts, without impairing electrocatalytic activity and stability is crucial to reduce the cost of water electrolysis. In this work, two Ir0.5Ni0.5Ox mixed oxide catalysts with layered and solid solution morphologies were prepared by modifying a facile hydrothermal methodology. The catalytic OER activity and stability of the Ir-Ni catalyst with a homogeneous distribution (IrNi-HD) was seriously compromised compared to pure IrOx due to the high concentration of surface nickel prone to corrosion under reaction conditions. However, the design of layered IrOx-Ni(OH)x (IrNi-LY) with Ir at the exposed surface allowed a 50% reduction in the molar concentration of the precious metal on the electrode compared to IrOx without impairing the catalytic activity or stability. As a result, IrNi-LY outperformed IrOx in activity when normalized to the Ir mass. </p

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

Lithium-directed transformation of amorphous iridium (oxy)hydroxides to produce active water oxidation catalysts

The oxygen evolution reaction (OER) is crucial to future energy systems based on water electrolysis. Iridium oxides are promising catalysts due to their resistance to corrosion under acidic and oxidizing conditions. Highly active iridium (oxy)­hydroxides prepared using alkali metal bases transform into low activity rutile IrO2 at elevated temperatures (>350 °C) during catalyst/electrode preparation. Depending on the residual amount of alkali metals, we now show that this transformation can result in either rutile IrO2 or nano-crystalline Li-intercalated IrO x . While the transition to rutile results in poor activity, the Li-intercalated IrO x has comparative activity and improved stability when compared to the highly active amorphous material despite being treated at 500 °C. This highly active nanocrystalline form of lithium iridate could be more resistant to industrial procedures to produce PEM membranes and provide a route to stabilize the high populations of redox active sites of amorphous iridium (oxy)­hydroxides

Electron radiation damage mechanisms in 2D MoSe2

The contributions of different damage mechanisms in single-layer MoSe2 were studied by investigating different MoSe2/graphene heterostructures by the aberration-corrected high–resolution transmission electron microscopy (AC–HRTEM) at 80 keV. The damage cross–sections were determined by direct counting of atoms in the AC–HRTEM images. The contributions of damage mechanisms such as knock–on damage or ionization effects were estimated by comparing the damage rates in different heterostructure configurations, similarly to what has been earlier done with MoS2. The behaviour of MoSe2 was found to be nearly identical to that of MoS2, which is an unexpected result, as the knock-on mechanism should be suppressed in MoSe2 due to the high mass of Se, as compared to S

Dry-cleaning of graphene

Algara-Siller G, Lehtinen O, Turchanin A, Kaiser U. Dry-cleaning of graphene. Applied Physics Letters. 2014;104(15): 153115.Studies of the structural and electronic properties of graphene in its pristine state are hindered by hydrocarbon contamination on the surfaces. Also, in many applications, contamination reduces the performance of graphene. Contamination is introduced during sample preparation and is adsorbed also directly from air. Here, we report on the development of a simple dry-cleaning method for producing large atomically clean areas in free-standing graphene. The cleanness of graphene is proven using aberration-corrected high-resolution transmission electron microscopy and electron spectroscopy. (C) 2014 AIP Publishing LLC