Publikationsserver der RWTH Aachen University
Epitaxial graphene and cluster lattices on Iridium(111)
Abstract
dc:descriptionThe growth and the structure of epitaxial graphene on Ir(111) and its use as a template for metal cluster superlattices are treated. Experiments have mainly been carried out with STM and LEEM in UHV. Graphene was grown by two basic methods:(i) Temperature programmed growth (TPG), which is adsorption of a carbon supplier, e.g. ethylene, at room temperature and the subsequent pyrolytic cleavage The morpholotgy is determined by the annealing temperatuer, the amount of graphene is determined by the hydrocarbon which is used as a precursor. For ethylene, it is 0.22ML of graphene.(ii) Chemical vapor deposition (CVD) of ethylene at elevated temperatures.The nucleation density is 1-3 orders of magnitude lower and the structures are much larger then with TPG. Islands originating from the same nucleus can have small angle grain boundaries in them. The higher the deposition temperature, the less grain boundaries form.It is possible to grow a high quality graphene film on the whole sample with CVD. The CVD method has also been transferred to a high vacuum system where large graphene flakes can be grown, just as in UHV. LEEM measurements have shown that full layers of graphene grown by CVD comprise a noticeable fraction of graphene which is rotated by a large angle with respect to the substrate. This phase can be erased by intermittent cycles of oxygen etching or surpressed by a combined TPG and CVD growth. Due to the difference in thermal contraction of graphene and Ir, compressive strain builds up during cooling down to room temperature. This strain is relieved by the formation of wrinkles at a cool down of approx. 400K. The formation of wrinkles and the accompanying strain relief can be observed. The process of wrinkle formation has also been modeled as an interplay of compressive energy, bending energy and binding energy. The structure of graphene on Ir(111) has been precisely determined with STM and LEED. The graphene lattice forms a moiré with the Ir(111) surface. With the help of the moiré, the graphene lattice constant has been very accurately determined to be (2.452 plusminus 0.004) Angström. The moiré repeat distance is (25.3 plusminus 0.4)Angström The rotation of neighbouring domains is accommodated for by the periodic insertion of heptagon-pentagon defects, which constitute a small angle grain boundary. Graphene covers the step edges like a carpet with atomic coherency. Making use of the magnifying effect of the moiré one can assess the bending radius. It lies in the range of 0.3,nm as in very small single wall carbon nanotubes. Finally we show, the versatility of the graphene moiré as a template for metal cluster superlattices and address the properties of these cluster superlattices. Superlattices of various metal clusters (Ir, Pt, W, Re, Au, IrFe, IrAu) were grown by physical vapor deposition. The clusters are perfectly ordered in a hexagonal array. The average number of atoms in a cluster can be tuned by the amount of deposited metal, for Ir from 4.5 -- 130 atoms. The clusters exhibit a layer by layer growth and thus are epitaxial. While small Au lusters are only stable at temperatures around 100K, clusters made from Ir are stable above 500K. The decay of the areal density of clusters can be described as a thermally activated hopping of clusters from one moiré cell to the other. The coalescence occurs through two different types of atomic process: complete coalescence for two small clusters results in one cluster occupying a single moiré cell, while for large clusters a sintering takes place. The resulting cluster spans multiple moiré cells. For some materials, which do not form ordered cluster arrays at room temperature, the formation of regular cluster superlattices can be enhanced by a lower substrate temperature. This method is demonstrated for Au and Re. The obtained Re clusters remain stable at room temperature. We also seeded a superlattice with small Ir clusters and in a second step used these well ordered arrays of Ir clusters to anchor additional Fe or Au. Finally it is shown, that using the TPG+CVD growth process for the graphene template, the entire sample can be covered with a cluster superlattice.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2010
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- N'Diaye, Alpha T.
- Contributors dc:contributor
-
- Michely, Thomas
Subjects
dc:subject × 12Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng