Graphene edges: a review of their fabrication and characterization

A graphene nanoribbon network and its biosensing application

Graphene oxide nanoribbons (GONRs) have been prepared by chemically unzipping multiwalled carbon nanotubes (MWCNTs). Thin-film networks of GONRs were fabricated by spray-coating, followed by a chemical or thermal reduction to form reduced graphene oxide nanoribbons (rGONRs). Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) characterizations indicate that the thermal reduction in the presence of ethanol vapor effectively restores the graphitic structure of the GONR as compared to chemical reduction with hydrazine vapor. Electrical measurements under a liquid-gate configuration demonstrates that rGONR network field-effect transistors exhibit much higher on/off ratios than a network of microsized reduced graphene oxides (rGOs) or a continuous film of single-layered pristine or chemical vapor deposited (CVD) graphene. Furthermore, we demonstrated the potential applications of rGONR networks for biosensing, specifically, the real-time and sensitive detection of adenosine triphosphate (ATP) molecules.

Nanoscale chemical imaging of single-layer graphene

Electronic properties in different graphene materials are influenced by the presence of defects and their relative position with respect to the edges. Their localization is crucial for the reliable development of graphene-based electronic devices. Graphene samples produced by standard CVD on copper and by the scotch-tape method on gold were investigated using tip-enhanced Raman spectroscopy (TERS). A resolution of <12 nm is reached using TERS imaging with full spectral information in every pixel. TERS is shown to be capable of identifying defects, contaminants, and pristine graphene due to their different spectroscopic signatures, and of performing chemical imaging. TERS allows the detection of smaller defects than visible by confocal Raman microscopy and a far more precise localization. Consecutive scans on the same sample area show the reproducibility of the measurements, as well as the ability to zoom in from an overview scan onto specific sample features. TERS images can be acquired in as few as 5 min with 32 × 32 pixels. Compared to confocal Raman microscopy, a high sensitivity for defects, edges, hydrogen-terminated areas or contaminated areas (in general for deviations from the two-dimensional structure of pristine graphene) is obtained due to selective enhancement as a consequence of the orientation in the electromagnetic field.

Unidirectional arrays of vertically standing graphenes in reactive plasmas

The possibility for the switch-over of the growth mode from a continuous network to unidirectional arrays of well-separated, self-organized, vertically oriented graphene nanosheets has been demonstrated using a unique, yet simple plasma-based approach. The process enables highly reproducible, catalyst-free synthesis of arrays of graphene nanosheets with reactive open graphitic edges facing upwards. Effective control over the nanosheet length, number density, and the degree of alignment along the electric field direction is achieved by a simple variation of the substrate bias. These results are of interest for environment-friendly fabrication of next-generation nanodevices based on three-dimensional, ordered self-organized nanoarrays of active nanostructures with very large surface areas and aspect ratios, highly reactive edges, and controlled density on the substrate. Our simple and versatile plasma-based approach paves the way for direct integration of such nanoarrays directly into the Si-based nanodevice platform.

Zigzag graphene nanoribbons: bandgap and midgap state modulation

We study zigzag graphene nanoribbons with periodic edge roughness and report significant band gap opening. Interestingly, such nanoribbons have a near-midgap state with a small band width. We extensively study the electronic structure and the electric-field modulation of the conduction/valence bands and the near-midgap state. We summarize the important electronic-structure features like the band gap, the band width and the effective mass. We show that by applying an external electric field in the width direction, the band width of the near-midgap state varies linearly due to the edge localization, whereas the band gap remains almost constant. Additionally, the effective mass of these states can switch polarity from negative (hole-like) to positive (carrier-like) at the Γ-point with the field modulation.

Emergence of atypical properties in assembled graphene nanoribbons

Graphitic nanowiggles (GNWs) are periodic repetitions of nonaligned finite-sized graphitic nanoribbon domains seamlessly stitched together without structural defects. These complex nanostructures have been recently fabricated [Cai et al., Nature (London) 466, 470 (2010)] and are here predicted to possess unusual properties, such as tunable band gaps and versatile magnetic behaviors. We used first-principles theory to highlight the microscopic origins of the emerging electronic and magnetic properties of the main subclasses of GNWs. Our study establishes a road map for guiding the design and synthesis of specific GNWs for nanoelectronic, optoelectronic, and spintronic applications.

Quantum transport in graphene nanonetworks

The quantum transport properties of graphene nanoribbon networks are investigated using first-principles calculations based on density functional theory. Focusing on systems that can be experimentally realized with existing techniques, both in-plane conductance in interconnected graphene nanoribbons and tunneling conductance in out-of-plane nanoribbon intersections were studied. The characteristics of the ab initio electronic transport through in-plane nanoribbon cross-points is found to be in agreement with results obtained with semiempirical approaches. Both simulations confirm the possibility of designing graphene nanoribbon-based networks capable of guiding electrons along desired and predetermined paths. In addition, some of these intersections exhibit different transmission probability for spin up and spin down electrons, suggesting the possible applications of such networks as spin filters. Furthermore, the electron transport properties of out-of-plane nanoribbon cross-points of realistic sizes are described using a combination of first-principles and tight-binding approaches. The stacking angle between individual sheets is found to play a central role in dictating the electronic transmission probability within the networks.

Low-energy termination of graphene edges via the formation of narrow nanotubes

We demonstrate that free graphene sheet edges can curl back on themselves, reconstructing as nanotubes. This results in lower formation energies than any other nonfunctionalized edge structure reported to date in the literature. We determine the critical tube size and formation barrier and compare with density functional simulations of other edge terminations including a new reconstructed Klein edge. Simulated high resolution electron microscopy images show why such rolled edges may be difficult to detect. Rolled zigzag edges serve as metallic conduction channels, separated from the neighboring bulk graphene by a chain of insulating sp(3)-carbon atoms, and introduce van Hove singularities into the graphene density of states.

Electrochemistry of folded graphene edges

There is enormous interest in the investigation of electron transfer rates at the edges of graphene due to possible energy storage and sensing applications. While electrochemistry at the edges and the basal plane of graphene has been studied in the past, the new frontier is the electrochemistry of folded graphene edges. Here we describe the electrochemistry of folded graphene edges and compare it to that of open graphene edges. The materials were characterized in detail by high-resolution transmission electron microscopy, Raman spectroscopy, high-resolution X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy and cyclic voltammetry. We found that the heterogeneous electron transfer rate is significantly lower on folded graphene edges compared to open edge sites for ferro/ferricyanide, and that electrochemical properties of open edges offer lower potential detection of biomarkers than the folded ones. It is apparent, therefore, that for sensing and biosensing applications the folded edges are less active than open edges, which should then be preferred for such applications. As folded edges are the product of thermal treatment of multilayer graphene, such thermal procedures should be avoided when fabricating graphene for electrochemical applications.