Growth and Properties of Graphene-Based Materials

In this thesis, I have focused on graphene-based nanostructures as a versatile means of manipulating the electronic properties of graphene, while working with objects perfect at the atomic level. This is the nanotechnological approach, where we exploit the infinite possibilities of making small things with new materials. For these reasons, I concentrated my research efforts to graphene-based nanomaterials, because graphene is one of the most exciting materials we have to date, and because manipulation of surfaces at the nano-level is what allows us to make new materials today. In this thesis, I will show how we have created and studied new graphene-based nanostructures by employing cutting-edge surface science techniques. Most of the experimental data we have acquired has been given a new light by powerful Density Functional Theory calculations, that allow for an approach where hardly accessible data (experimentally) becomes indirectly known through numerical calculations, while providing valuable feedback for further aimed calculations. I will show how we have undertaken a route that takes us from a detailed study of how carbon monomers, the building blocks of graphene, come to exist on an Ir(1 1 1) surface after ethylene dissociation. Next, simple nanostructures have been ex- ploited, so that the properties of a preexisting graphene layer are manipulated by intercalating different metals between graphene and the substrate. Then I will discuss an experiment where graphene was grown on a highly anisotropic substrate, Ru(1 0 1 0), which proved to be an extremely rich system, giving rise to several self-assembled graphene nanostructures, including nanoribbons and one-dimensional quasi free-standing graphene waves. Then, we will progress to what are commonly perceived as being proper graphene-based nanostructures. We have, in fact, managed to create size selected graphene nanodomes on Ir(1 1 1) using coronene as a precursor, and we have understood many details of the dynamics in the formation of these carbon-based nanostructures, discovering that in certain steps of the reaction they lift from the surface and rotate, before settling in the definitive adsorption position. Furthermore, while performing similar experiments on pentacene (a semiconducting molecule, used the fabrication of molecular FETs) on Ir(1 1 1), we have discovered that the molecules exhibit a reversible dehydrogenation, allowing for a switch between semiconducting molecules and minimalistic graphene nanoribbons, only one aromatic ring wide. Finally, a size-selected nanocluster source system will be described. In parallel with my research activity, I have been profoundly involved in the commissioning of such a machine that is currently capable of producing size selected nanoclusters

Growth and Properties of Graphene-Based Materials

In this thesis, I have focused on graphene-based nanostructures as a versatile means of manipulating the electronic properties of graphene, while working with objects perfect at the atomic level. This is the nanotechnological approach, where we exploit the infinite possibilities of making small things with new materials. For these reasons, I concentrated my research efforts to graphene-based nanomaterials, because graphene is one of the most exciting materials we have to date, and because manipulation of surfaces at the nano-level is what allows us to make new materials today. In this thesis, I will show how we have created and studied new graphene-based nanostructures by employing cutting-edge surface science techniques. Most of the experimental data we have acquired has been given a new light by powerful Density Functional Theory calculations, that allow for an approach where hardly accessible data (experimentally) becomes indirectly known through numerical calculations, while providing valuable feedback for further aimed calculations. I will show how we have undertaken a route that takes us from a detailed study of how carbon monomers, the building blocks of graphene, come to exist on an Ir(1 1 1) surface after ethylene dissociation. Next, simple nanostructures have been ex- ploited, so that the properties of a preexisting graphene layer are manipulated by intercalating different metals between graphene and the substrate. Then I will discuss an experiment where graphene was grown on a highly anisotropic substrate, Ru(1 0 1 0), which proved to be an extremely rich system, giving rise to several self-assembled graphene nanostructures, including nanoribbons and one-dimensional quasi free-standing graphene waves. Then, we will progress to what are commonly perceived as being proper graphene-based nanostructures. We have, in fact, managed to create size selected graphene nanodomes on Ir(1 1 1) using coronene as a precursor, and we have understood many details of the dynamics in the formation of these carbon-based nanostructures, discovering that in certain steps of the reaction they lift from the surface and rotate, before settling in the definitive adsorption position. Furthermore, while performing similar experiments on pentacene (a semiconducting molecule, used the fabrication of molecular FETs) on Ir(1 1 1), we have discovered that the molecules exhibit a reversible dehydrogenation, allowing for a switch between semiconducting molecules and minimalistic graphene nanoribbons, only one aromatic ring wide. Finally, a size-selected nanocluster source system will be described. In parallel with my research activity, I have been profoundly involved in the commissioning of such a machine that is currently capable of producing size selected nanoclusters

One-dimensional electronic states in a natural misfit structure

Misfit compounds are thermodynamically stable stacks of two-dimensional materials, forming a three-dimensional structure that remains incommensurate in one direction parallel to the layers. As a consequence, no true bonding is expected between the layers, with their interaction being dominated by charge transfer. In contrast to this well-established picture, we show that interlayer coupling can strongly influence the electronic properties of one type of layer in a misfit structure, in a similar way to the creation of modified band structures in an artificial moir\'e structure between two-dimensional materials. Using angle-resolved photoemission spectroscopy with a micron-scale light focus, we selectively probe the electronic properties of hexagonal NbSe$_2$ and square BiSe layers that terminate the surface of the (BiSe)$_{1+\delta}$NbSe$_2$ misfit compound. We show that the band structure in the BiSe layers is strongly affected by the presence of the hexagonal NbSe$_2$ layers, leading to quasi one-dimensional electronic features. The electronic structure of the NbSe$_2$ layers, on the other hand, is hardly influenced by the presence of the BiSe. Using density functional theory calculations of the unfolded band structures, we argue that the preferred modification of one type of bands is mainly due to the atomic and orbital character of the states involved, opening a promising way to design novel electronic states that exploit the partially incommensurate character of the misfit compounds

In-situ exfoliation method of large-area 2D materials

The success in studying 2D materials inherently relies on producing samples of large area, and high quality enough for the experimental conditions. Because their 2D nature surface sensitive techniques such as photoemission spectroscopy , tunneling microscopy and electron diffraction, that work in ultra high vacuum (UHV) environment are prime techniques that have been employed with great success in unveiling new properties of 2D materials but it requires samples to be free of adsorbates. The technique that most easily and readily yields 2dmaterials of highest quality is indubitably mechanical exfoliation from bulk grown samples, however as this technique is traditionally done in dedicated environment, the transfer of these samples into UHV setups requires some form of surface cleaning that tempers with the sample quality. In this article, we report on a simple and general method of \textit{in-situ} mechanical exfoliation directly in UHV that yields large-area single-layered films. By employing standard UHV cleaning techniques and by purpusedly exploiting the chemical affinity between the substrate and the sample we could yield large area exfoliation of transition metal dichalcogenides. Multiple transition metal dichalcogenides, both metallic and semiconducting, are exfoliated \textit{in-situ} onto Au and Ag, and Ge. Exfoliated flakes are found to be sub-milimeter size with excellent crystallinity and purity, as evidenced by angle-resolved photoemission spectroscopy, atomic force microscopy and low-energy electron diffraction. In addition, we demonstrate exfoliation of air-sensitive 2D materials and possibility of controlling the substrate-2D material twist angle

In Situ Exfoliation Method of Large-Area 2D Materials

2D materials provide a rich platform to study novel physical phenomena arising from quantum confinement of charge carriers. Many of these phenomena are discovered by surface sensitive techniques, such as photoemission spectroscopy, that work in ultra-high vacuum (UHV). Success in experimental studies of 2D materials, however, inherently relies on producing adsorbate-free, large-area, high-quality samples. The method that yields 2D materials of highest quality is mechanical exfoliation from bulk-grown samples. However, as this technique is traditionally performed in a dedicated environment, the transfer of samples into vacuum requires surface cleaning that might diminish the quality of the samples. In this article, a simple method for in situ exfoliation directly in UHV is reported, which yields large-area, single-layered films. Multiple metallic and semiconducting transition metal dichalcogenides are exfoliated in situ onto Au, Ag, and Ge. The exfoliated flakes are found to be of sub-millimeter size with excellent crystallinity and purity, as supported by angle-resolved photoemission spectroscopy, atomic force microscopy, and low-energy electron diffraction. The approach is well-suited for air-sensitive 2D materials, enabling the study of a new suite of electronic properties. In addition, the exfoliation of surface alloys and the possibility of controlling the substrate-2D material twist angle is demonstrated.</p

Ethylene Dissociation on Ni3Al(111)

Combining density functional theory, Nudged Elastic Band and high-energy resolution x-ray photoelectron spectroscopy experiments, we study the early stages and reaction pathways whereby ethylene molecules decompose on Ni3Al (111) prior to graphene nucleation and growth. After characterizing stable configurations of ethylene on the surface, and of all intermediate products leading to carbon species, we calculate energy barriers for all relevant processes, including dehydrogenation, isomerization, C-C cleavage and their respective inverse reactions. This quantitative analysis helps in identifying the most probable reaction pathways. The combination of temperature dependent C 1s core level photoelectron spectroscopy measurements and of core level shift calculations for all the different species investigated allow us to understand the temperature evolution of the surface species, and to identify the whole reaction mechanism. Combined analysis of this kind is useful for understanding which species are present on the surface at various temperatures during chemical vapor deposition graphene growth experiments