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

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.

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 Transport in 2D Materials with Strong Spin-orbit Coupling

Artem Pulkin

The thesis describes the computational study of structural, electonic and transport properties of monolayer transition metal dichalcogenides (TMDs) in the stable 2H and the metastable 1T' phases. Several aspects have been covered by the study including the electronic properties of the topological quantum spin Hall (QSH) state in the 1T' monolayer phase as well as the effects of strain, periodic line defects, interfaces and edges of monolayer TMDs. The electronic properties of the bulk monolayer phases were described by the ab-initio density functional theory framework while the electronic and transport properties of 1D defects were calculated using the non-equilibrium Green's function formalism. A specific focus was made on the transport of spin-polarized charge carriers across line defects in the monolayer 2H phase. Subject to energy, pseudomomentum and spin conservation, the size of the transport gap is governed by both bulk properties of a material and symmetries of a line defect. Outside the transport gap energy region, the charge carriers are discriminated with respect to their spin resulting in the spin polarization of the transmitted current. Next, the properties of the metastable monolayer 1T' phase were studied. The presence of a sufficiently large band gap is crucial to observe the QSH phase in the family of materials by probing the topological boundary states. The meV-order band gaps of the 1T' phase of monolayer TMDs were found to be sensitive to materials' lattice constants suggesting the control of the band gap size by strain. In particular, the electronic band structure and the size of the band gap in monolayer 1T'-WSe2 were found to be in agreement with spectroscopy studies. The topologically protected states at the edges of the monolayer 1T' phase as well as at the boundaries between the topological 1T' phase and the trivial 2H phase of monolayer TMDs were studied. Specific atomic structure configurations were suggested to observe experimentally the topological protection of the charge carrier transport against back-scattering. Finally, in the context of lateral semiconducting device engineering, the electronic and transverse transport properties of 2H-1T' phase boundaries as well as the dimerization defects in the 1T' phase were investigated. Both kinds of defects considered exhibit a relatively large transmission probability for the charge carriers crossing the defects. However, the differences between the shapes of bulk bands of the two phases open a sizeable transport gap for charge carriers crossing periodic domain boundaries between the monolayer 2H and 1T' phases. The calculated formation energies of dimerization defects were found to be relatively low suggesting their high concentration in real samples of monolayer 1T'-TMDs. Additionally, the thesis includes studies of magnetic dopants on the surface of Bi2Te3 and atomic vacancies in monolayer 2H-MoSe2 where the electronic properties of point defects were calculated and compared to experimental results. The two possible adsorption sites of Fe on the surface of Bi2Te3 both show a large out-of-plane magnetic anisotropy in agreement with experiments. The calculated local electronic properties of Se vacancies in monolayer 2H-MoSe2 show the presence of in-gap states which are not observed in experiment. Nevertheless, the combination of theoretical and experimental scanning tunneling microscopy images allowed the unambiguous identification of the vacancy defect.

EPFL2017

Localized electronic states at grain boundaries on the surface of graphene and graphite

Gabriel Albert Autes, Oleg Yazyev

Recent advances in large-scale synthesis of graphene and other 2D materials have underscored the importance of local defects such as dislocations and grain boundaries (GBs), and especially their tendency to alter the electronic properties of the material. Understanding how the polycrystalline morphology affects the electronic properties is crucial for the development of applications such as flexible electronics, energy harvesting devices or sensors. We here report on atomic scale characterization of several GBs and on the structural-dependence of the localized electronic states in their vicinity. Using low temperature scanning tunneling microscopy. and spectroscopy, together with tight binding and ab initio numerical simulations we explore GBs on the surface of graphite and elucidate the interconnection between the local density of states and their atomic structure. We show that the electronic fingerprints of these GBs consist of pronounced resonances which, depending on the relative orientation of the adjacent crystallites, appear either on the electron side of the spectrum or as an electron-hole symmetric doublet close to the charge neutrality point. These two types of spectral features will impact very differently the transport properties allowing, in the asymmetric case to introduce transport anisotropy which could be utilized to design novel growth and fabrication strategies to control device performance.

Iop Publishing Ltd2016