Edge Disorder in Bottom-Up Zigzag Graphene Nanoribbons: Implications for Magnetism and Quantum Electronic Transport

We unveil the nature of the structural disorder in bottom-up zigzag graphene nanoribbons along with its effect on the magnetism and electronic transport on the basis of scanning probe microscopies and first-principles calculations. We find that edge-missing m-xylene units emerging during the cyclodehydrogenation step of the on-surface synthesis are the most common point defects. These "bite" defects act as spin-1 paramagnetic centers, severely disrupt the conductance spectrum around the band extrema, and give rise to spin-polarized charge transport. We further show that the electronic conductance across graphene nanoribbons is more sensitive to "bite" defects forming at the zigzag edges than at the armchair ones. Our work establishes a comprehensive understanding of the low-energy electronic properties of disordered bottom-up graphene nanoribbons.

Magnetism in disordered graphene and irradiated graphite

Oleg Yazyev

The magnetic properties of disordered graphene and irradiated graphite are systematically studied using a combination of mean-field Hubbard model and first-principles calculations. By considering large-scale disordered models of graphene, I conclude that only single-atom defects can induce ferromagnetism in graphene-based materials. The preserved stacking order of graphene layers is shown to be another necessary condition for achieving a finite net magnetic moment of irradiated graphite. Ab initio calculations of hydrogen binding and diffusion and of interstitial-vacancy recombination further confirm the crucial role of stacking order in pi-electron ferromagnetism of proton-bombarded graphite.

2008

Electron Spin Resonance Study of Graphene

Luka Ciric

Graphene, a honeycomb lattice of a single atom thick plane of carbon atoms has captivated the attention of physicists, materials scientists, and engineers because of its extraordinary physical properties. These include exceedingly high charge carrier mobility, current-carrying capacity, thermal conductivity, and mechanical strength. These properties qualify it as an excellent candidate for new electronic technologies both within and beyond metal oxide semiconductors. The attractive properties of graphene for future applications are observed only in high quality graphene produced by mechanical exfoliation of graphite by using the so-called "scotch tape method". However, the yield is low and it satisfies only the needs of basic research. For applications one has to have material which is produced on large scale but preserves the quality of graphene. There are many production routes proposed in the scientific community and their number is still growing. One of the tasks of this PhD work was to test by Electron Spin Resonance (ESR) the quality of graphene produced by various methods. ESR is a suitable technique for looking for ferromagnetic interactions in graphene predicted by theory, and to determine the intrinsic spin relaxation time? These features are important for integration of graphene in spintronics applications. The main results are obtained on a large assembly of three differently prepared samples: 1. Mechanically exfoliated graphite (MEG), 2. Reduced graphene oxide (RGO) and 3. Liquid phase exfoliated (LPE) graphite. Attempts were done to study graphene synthesized by chemical vapor deposition (CVD) and by epitaxial growth on SiC. The mother compound graphite, as a reference sample was also thoroughly investigated by ESR. The LPG sample, obtained by heavy sonication of graphitic powder in NMP solvent, contains more nanographitic particles than graphene. The overall response resembles that of graphite with the exception of detecting a strong magnetic interaction below 26 K. This unambiguously shows the possibility of creating a strong magnetic response in carbon based material, but the challenge to inducing it graphene still remains. The RGO sample, prepared by chemical methods provides a large quantity of sample, suffers from the harsh oxidative and reduction steps involved in the synthesis. Both steps can be incomplete, leaving behind a large number of defects which Anderson-localize the extended states necessary for electronic applications. The best characteristics are obtained on MEG graphene. The temperature dependence of the magnetic susceptibility follows that predicted by theory, and it is distinctly different from that of graphite. Furthermore, the 10-8 s spin lifetime deduced from the ESR linewidth is by two orders of magnitude longer than that obtained in spin-valve experiments and it is encouraging for spintronics applications.

EPFL2011

The role of disorder in the electronic and transport properties of monolayer and bilayer graphene

Fernando Gargiulo

This thesis is devoted to the computational study of the electronic and transport properties of monolayer and bilayer graphene in the presence of disorder arising from both topological and point defects. Among the former, we study grain boundaries in monolayer graphene and stacking domain boundaries in bilayer graphene, whereas among the latter we study hydrogen atoms covalently bound on the graphene crystal lattice. The electronic spectrum of disordered graphene has been studied within a tight-binding framework, which has been coupled to the Landauer-Büttiker theory and Green¿s function techniques in order to have access to the properties of coherent transport of graphene charge carriers. We assess the low-energy equilibrium structures of defective graphene by a combination of ab initio density functional theory, classical potentials, and Monte Carlo methods. We study periodic grain boundaries in monolayer graphene and individuate two classes of defects with opposite effects in terms of scattering of low-energy charge carriers. One class, unexpectedly, is highly reflecting in the limit of low defect density, whereas another is highly transparent. Subsequently, we study disordered grain boundaries in order to predict the intrinsic conductance of realistic polycrystalline graphene samples. In two related works, conducted in collaboration with experimentalists, we identify the atomic structure of periodic grain boundaries imaged by scanning tunneling microscopy, and discuss the valley-filtering capabilities of a line defect of graphene that can be grown in a controllable manner. Next, we investigate the electronic transport of graphene with realistic hydrogen adsorbates, whose equilibrium configurations are obtained by means of Monte Carlo simulations. We find that the conductance of graphene dramatically increases upon formation of cluster adatoms, which we predict to happen spontaneously at room temperature. This is due to the non- resonant nature of a large fraction of hydrogen clusters in the room-temperature distribution, which we further elucidate by means of an analytically solvable model. Finally, we study the behavior, in terms of structural and electronic properties, of twisted bilayer graphene in the limit of zero twist angle. We find a critical angle below which the system arranges in a triangular superlattice of Bernal-stacking domains, separated by a hexagonal network of stacking domain boundaries. The presence of stacking domain boundaries is at the base of our interpretation of an experiment reporting oscillations in the electrical conductance of bilayer graphene subjected to mechanical indentation.