Electronic properties of graphene

Graphene is a semimetal whose conduction and valence bands meet at the Dirac points, which are six locations in momentum space, the vertices of its hexagonal Brillouin zone, divided into two non-equivalent sets of three points. The two sets are labeled K and K'. The sets give graphene a valley degeneracy of gv = 2. By contrast, for traditional semiconductors the primary point of interest is generally Γ, where momentum is zero. Four electronic properties separate it from other condensed matter systems. Electronic spectrum Electrons propagating through graphene's honeycomb lattice effectively lose their mass, producing quasi-particles that are described by a 2D analogue of the Dirac equation rather than the Schrödinger equation for spin- particles. Dispersion relation When atoms are placed onto the graphene hexagonal lattice, the overlap between the pz(π) orbitals and the s or the px and py orbitals is zero by symmetry. The pz electrons

Electronic transport in graphene nanoribbon junctions

Kristians Cernevics

Graphene nanoribbons (GNRs) - one-dimensional strips of graphene - share many of the exciting properties of graphene, such as ballistic transport over micron dimensions, strength and flexibility, but more importantly, they exhibit a tunable band gap that depends on the atomic structure. Recent advances of fabricating atomically precise bottom-up GNRs with unprecedented control over their atomic structure have attracted interest from the field of nanoelectronics. However, a big part of the future success of GNRs depends on the ability to produce and integrate GNR junctions into complex next-generation devices. Much more effort is needed in both perfecting the production techniques and improving the theoretical understanding of these exciting nanostructures. This thesis is, therefore, devoted to exploring electronic transport properties in graphene nanoribbon junctions and unraveling their underlying structure-property relationships. Using tight-binding models, density functional theory and Green's function method, we determine the electronic properties of both experimentally synthesized and theoretically proposed junctions.In the first part of the thesis, we examine width-modulated GNR nanostructures and discover a subtle interplay between the localized states in the scattering region and the continuum of states in the leads. We show that depending on the size of the scattering region, we observe contrasting behavior on the electronic transport properties. Next, we expand the width-modulated region and show that a width-dependent transport gap opens in the presence of a quantum dot, thereby yielding built-in one-dimensional metal-semiconductor-metal junction. Part II of this thesis is dedicated to a joint experimental and theoretical effort in order to reveal the detrimental effect of

bite'' defects, resulting upon the cleavage of phenyl groups of precursor molecules. We explore their effect on the electronic transport from first-principles calculations and show how conduction is disrupted at the band edges. We then generalize our theoretical findings to other nanoribbons in a systematic manner, thus establishing guidelines to minimize the detrimental role of such defects. Later, we show that strategically placed

bite'' defects can selectively modify electronic transport properties and apply this concept to construct two prototypical components for nanoelectronics. Whereas, in Part III we employ high-throughput screening of over 400000 angled junctions in order to find potential candidates for interconnects in logic circuits and determine design rules based on structure-property relationships. We discover that the bipartite symmetry of graphene lattice and the presence of resonant states, localized at the junction, play an important role in determining the transport properties of angled junctions. Besides, we also provide a web application that allows easy design and calculation of electronic properties of GNR junctions. Finally, the last chapter of the thesis involves developing a more realistic model for transport calculations by including finite length and contact effects in order to reduce the gap between the experimental and theoretical results.

EPFL2022

Electronic properties of nano-crystalline titanium dioxide thin films

Alain Bally

Titanium dioxide is a cheap, chemically stable and non-toxic material. However its electrical properties are unstable and it is a modest semiconductor and a mediocre insulator. For several applications it would be interesting to make it either more insulating or more conducting. The goal of this work was to modify the electrical properties of nano-crystalline TiO2 thin films deposited by reactive sputtering and to understand the mechanism leading to these modifications. The principal factors that influence the electrical conductivity are on the one hand the concentration and nature of the chemical impurities incorporated in TiO2, and on the other hand the morphology of the thin films. The study was split into two parts. The fist part describes the modifications of the sputtered material obtained by chemical doping of the TiO2 thin films. Niobium, cerium, iron, and fluorine were incorporated successfully in TiO2. The second part describes the modifications obtained by modifying the reactive gas used during the sputtering process: thus oxygen was substituted with water vapor. Several analysis techniques have been used to characterize the TiO2 thin films. They are essentially divided in four categories. The chemical analyses included electron probe microanalyses, x-ray photoemission spectroscopy, and secondary ion mass spectrometry. The structure and morphology analyses of thin films were carried out with x-ray diffraction and atomic force microscopy. The electrical properties in dc or ac mode were measured between room temperature and 350°C. Finally optical transmission provided information on the electronic states and morphology of the thin films. With an appropriate choice of impurities, titanium dioxide can be made more insulating (TiO2:Ce), more conducting with an n-type electrical conductivity (TiO2:Nb) or p-type electrical conductivity (TiO2:Fe). Chemical doping is also the cause of important structure and morphology changes, for example it can force the transformation from the anatase to the rutile structure. It is shown in particular that dopant atoms generate defects such as oxygen vacancies. These defects impede the variation of the electrical conductivity produced by the impurities, thus, in spite of dopant concentration in the percent range, the variation of the electrical conductivity is an increase or a decrease of three order of magnitude at most when compared with undoped TiO2. Thin films deposited with water vapor as the reactive gas present an increase in the electrical conductivity by height order of magnitude compared to samples prepared with oxygen in similar conditions. No atom other than titanium or oxygen could be detected in the thin films in significant concentration. The dramatic conductivity increase is due to the injection inside TiO2 grains of electrons donated by unsaturated titanium atoms lying on the grain surface. A nanometric grain size is a requirement to make such high doping levels possible. Depending on the impurities or reactive gas selected, titanium dioxide can be made either into a good insulator with a high breakdown field and a high permittivity or into a reasonably conducting, transparent film. The electrical conductivity of the samples prepared for this study cover the range from 10-10 to 103 S m-1.

EPFL1999

Electronic transport in graphene nanoribbon junctions

Kristians Cernevics

bite'' defects, resulting upon the cleavage of phenyl groups of precursor molecules. We explore their effect on the electronic transport from first-principles calculations and show how conduction is disrupted at the band edges. We then generalize our theoretical findings to other nanoribbons in a systematic manner, thus establishing guidelines to minimize the detrimental role of such defects. Later, we show that strategically placed

EPFL2022

Electronic properties of nano-crystalline titanium dioxide thin films

Alain Bally

EPFL1999

Electronic properties of graphene

Electronic transport in graphene nanoribbon junctions

Structural and electronic properties of metal contacts to Ga1-xAlxAs

Electronic properties of nano-crystalline titanium dioxide thin films

Electronic transport in graphene nanoribbon junctions

Structural and electronic properties of metal contacts to Ga1-xAlxAs

Electronic properties of nano-crystalline titanium dioxide thin films