Electronic transport in graphene nanoribbon junctions

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.

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.

EPFL2015

Grain boundaries and quantum transport in monolayer MoS2

Kolyo Marinov

The growing research on two-dimensional materials reveals their exceptional physical properties and enormous potential for future applications and investigation of advanced physics phenomena. They represent the ultimate limit in terms of active channel thickness as they consist only of few atomic layers and hold a great promise for future scaling of electronic devices. Further, their unique band structure provides a platform for the exploration of novel physics in the field of spin and valleytronics. The presence of a large band gap allows fully electrostatic control over doping and current flow, as well as the realization of novel optoelectronic devices. On of the most appealing and readily available 2D material is molybdenum disulphide (MoS2). This thesis is concentrated on the electronic properties of monolayers MoS2 ranging from the investigation of line defects in synthetically grown material to advanced study of the confinement of charge carriers at low temperature as function of magnetic field. A chapter is describing the details of each of the principal experiments. In Chapter 4 we study the properties of epitaxially grown MoS2 monolayers by means of electrical measurements and advanced scanning probe techniques. After confirming the high quality of the grown crystallites, we investigate the predominant types of occurring grain boundaries by means of Kelvin probe force microscopy. Due to the highly oriented growth, we find that the electronic properties of the film are homogeneous even across the line defects. This fact is confirmed by the observation of constant mobility even in devices over large area containing multiple grain boundaries. Chapter 5 presents different approaches for the fabrication of high-quality dual-gated MoS2 field effect transistors (FETs). Dielectric layers deposited by means of atomic layer deposition can guarantee good mobility and low contact resistance, but the observation of low-dimensional effects may be hindered due to the presence of charged impurities. On the other hand the use of hexagonal boron nitride as dielectric and few layer graphene as contact material is shown to yield very high mobility and low contact resistance. Using dual gated FETs with such material configuration, an electrostatically controlled quantum point contact (QPC) is realized in monolayer MoS2 and presented in Chapter 6. We find that the device conductance is quantized in multiples of e^2/h revealing the lifting of spin and valley degeneracies. Further, we perform bias spectroscopy at different magnetic fields and extract subband energy spacing from 0.8 (at 0T) to 2 meV (at 9.8T) and g-factor in the conduction band of 2.12±0.13. Finally, in Chapter 7 we propose several methods aiming the fabrication of a quantum dot in MoS2. The electrostatic definition of a small conductive island, weakly couples to two highly doped charge carrier reservoirs, requires similar advanced engineering of the dielectric material environment. Despite the low stability, we find possible regimes, in which controlled tunneling from and to a well defined quantum dot can be detected. The charging energy measured electrically is in very good agreement with the lithographically defined size of the island. In conclusion, this work provides insights about the electron transport in monolayer MoS2 exploring grain boundaries, dielectric environment and electron confinement. The results contribute to better understanding of its electronic properties.

EPFL2018

Electrical Properties of Chemically Derived Graphene

Ravi Shankar Sundaram

Graphene, an atomically thin sheet of carbon, is the most recent endeavor for the application of carbon nanostructures in conventional electronics. The envisioned creation of devices completely carved out of graphene could lead to the revolution of electronic circuitry. However, the most established technique to obtain high quality graphene sheets, i.e, by micromechanical cleavage cannot be easily upscaled, serving as an impediment towards technological applications. The present thesis is dedicated to the study of graphene prepared via an alternative scalable, high yield and cost effective method which involves the chemical reduction of graphene oxide. The first part of this thesis describes in detail the atomic structure of graphene obtained by this method. Raman spectroscopy was used for this purpose followed by Transmission Electron Microscopy (TEM) and Near Edge X-Ray and Fine Structure (NEXAFS) measurements locally probe these sheets with atomic resolution. This revealed a highly disordered structure of this material, where regions with perfect crystallinity are separated by defect clusters. These defective patches were found to contain remnant oxygenated functional groups. Electrical characterization of chemically derived graphene sheets yielded ambipolar transfer characteristics similar to that of micromechanically cleaved graphene, albeit with moderate performance. The presence of a significant amount of disorder precludes ballistic transport and the charge carriers traverse through the sheet predominantly by thermally assisted variable range hopping in combination with a field driven tunneling mechanism. This is clearly evidenced in the low temperature two-probe conduction measurements performed on devices fabricated from these sheets. Metal contacts in graphene devices play a crucial role in limiting the applicability of these materials. Although the performance of graphene based devices as prepared today is sufficient for a variety of applications, a clear understanding and engineering of contacts is important for utilization of these materials towards high end applications. In the latter part of this thesis a detailed study of contacts is presented; firstly on graphene grown via CVD on copper to understand the physics in play at a simpler carbon-metal interface, followed by a photocurrent microscopy study to understand the more complex interface between metal and reduced graphene oxide and its implications on device characteristics. A strong difference between the two kinds of graphene is found with metal contacts to high quality graphene creating a potential barrier at the interface owing to a charge transfer between them, whereas in the case of the chemically synthesized version contacts are of a non invasive nature stable upto 300 °C. In addition, Raman spectroscopy and Scanning Photocurrent Microscopy investigation of graphene obtained by CVD under the gold, away from the contact edge provided interesting insights towards the phenomena occuring at the gold graphene interface, thus highlighting the importance of understanding and engineering adequate contacts before the successful transition of graphene based devices into the industry. Finally, a novel chemical vapor treatment of graphene oxide was employed to enhance its electrical characteristics. Exposure of reduced graphene oxide to ethylene vapor at 800 °C results in an improvement of conductivities by 2 orders of magnitude lagging behind that of pristine graphene. Electrical and structural characterization revealed that this increase in conductivity occurred inspite of the presence of a considerable amount of defects.

EPFL2011