Graphene nanoribbon | EPFL Graph Search

Graphene nanoribbons (GNRs, also called nano-graphene ribbons or nano-graphite ribbons) are strips of graphene with width less than 100 nm. Graphene ribbons were introduced as a theoretical model by Mitsutaka Fujita and coauthors to examine the edge and nanoscale size effect in graphene. Production Nanotomy Large quantities of width-controlled GNRs can be produced via graphite nanotomy, where applying a sharp diamond knife on graphite produces graphite nanoblocks, which can then be exfoliated to produce GNRs as shown by Vikas Berry. GNRs can also be produced by "unzipping" or axially cutting nanotubes. In one such method multi-walled carbon nanotubes were unzipped in solution by action of potassium permanganate and sulfuric acid. In another method GNRs were produced by plasma etching of nanotubes partly embedded in a polymer film.

Fabrication and Optoelectronic Properties of Graphene Nanoribbons

Eberhard Ulrich Stützel

Graphene, a single layer of carbon atoms, exhibits excellent charge transport properties. However, due to the absence of a band gap in this two-dimensional carbon nanostructure, graphene-based field effect transistors cannot be turned off. One strategy to increase the on/off ratio relies on patterning graphene into narrow stripes, so-called graphene nanoribbons (GNRs). The present thesis aimed at developing novel fabrication and chemical functionalization methods for GNRs. Along these lines, electrical transport studies and spectroscopic investigations were envisioned as a major tool to monitor the changes brought about by the functional groups attached to the GNRs. GNRs were fabricated with the aid of V2O5 nanofibers or CdSe nanowires as an etching mask during the plasma etching of graphene. The resulting GNRs exhibited good electrical conductivity for ribbon widths as small as 10 nm. Moreover, the transport gap in the GNRs was found to scale inversely with the ribbon width. Scanning tunneling microscopy and surface enhanced Raman spectroscopy testified a good structural quality of the GNRs. Electrical characterization of GNRs revealed a pronounced hysteresis, which was exploited for the fabrication of electrically switchable GNR memory cells. Dynamic pulse response measurements demonstrated reliable switching between two conductivity states for clock frequencies of up to 1 kHz and pulse durations as short as 500 ns for >10^7 cycles. As the most likely switching mechanism, charge trapping within a water layer in the GNR surrounding could be identified. Furthermore, the optoelectronic properties of individual GNRs were studied by scanning photocurrent microscopy. The pronounced photocurrent signal close to the nanoribbon/metal contacts was found to be directly proportional to the conductance of the devices, suggesting that a local voltage source is generated at the nanoribbon/metal interface by the photo-thermoelectric Seebeck effect. The dominance of this mechanism over charge separation via built-in electrical fields was attributed to strong local heating of the GNRs by the laser spot, combined with the reduced thermal conduction capability of the nanoribbons in comparison to extended graphene sheets. Chemical functionalization of graphene and GNRs was attempted via different gas- and liquid-phase approaches. While electrical transport and Raman measurements revealed the presence of significant doping effects, it was not possible to unequivocally prove the covalent attachment of atoms at the GNR edges, neither through changes in the charge transport characteristics, nor via scanning microscopy studies.

EPFL2013

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