18 research outputs found
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 <sub>x</sub>-Ni(OH)<sub>2</sub>Oxygen 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