Graphene nanoribbons – an exercise in control

Graphene nanoribbons offer a range of useful properties. Now, with clever use of water and incredibly precise slicing, a research team from Rice University thinks they have mastered their difficult fabrication

Graphene nanoribbons (GNRs) are a form of graphene that are stable and possess high carrier mobility and high conductivity¹. With a width of a few nanometers, GNRs are coveted for their quantum confinement effect, in which electrons are confined to the material’s restricted dimensions. In this quantum regime, their properties strongly depend on their widths, defects and edge states².The edges of GNRs are significant. Imagine drawing a very thin line in a Power Point presentation. The colour you choose for the line’s edge dominates the colour you select for the line. Similarly, the nature of a GNR edge dominates its electronic properties at small widths. The quantum effect does not play significant role at large widths.

The physical characteristics of GNRs open up plenty of room to manipulate the electronic, optical and magnetic properties at the nanoscale. And GNRs exhibit excellent mechanical properties. But harnessing these remarkable qualities requires atomically precise material synthesis, modification and control.

GNRs have been prepared by lithographic patterning of graphene, by splitting multiwalled carbon nanotubes (MWCNTs) with plasma etching and by bulk longitudinal unzipping of MWCNTs in solution¹. Their edges have also been chemically modified to incorporate specific functionality. In fact, GNRs have started to gain leverage in practical devices and applications, offering a glimpse of their potential.

Researchers are interested in GNRs as components in nanoelectronics, supercapacitors, lithium-ion batteries, conductive electrodes, reinforced composites and fibres. Some applications need them to be semiconducting while others require conductive behaviour, and still others their mechanical robustness.

The holy grail for semiconducting GNRs would be for use in nanoelectronics, where the ability to place or generate nanoscale devices at specific locations, reproducibly on a massive scale, is highly desirable. Here, lithographic patterning remains the most cost-effective and practical technique. Straightforward use of lithography to pattern GNRs from graphene sheets would solve this problem, but the material's technological advantage in semiconductor applications depends on its confined geometry. In order to show quantum confinement, GNR widths would need to be below 20 nanometers – a requirement at the cutting-edge of the microelectronic semiconductor industry’s capabilities. Furthermore, GNRs' semiconducting properties strongly depend on their width at this scale, much more so than the properties of conventional semiconductor nanostructures. Thus, limited lithographic resolution will result in non-reproducible device fabrication and intractable device characteristics even across device elements on a chip.

Our research team at Rice University has developed a technique of top-down GNR fabrication³ that is fully compatible with modern planar microelectronics technology. By turning to something as ubiquitous as water, the researchers have demonstrated a high-throughput method of fabricating GNRs of the desired shape and at the desired locations on a chip. A tiny water meniscus around the base of metallic structures deposited onto a graphene sheet turned out to be the key, serving as a mask for reproducibly patterning graphene.

This incredibly reproducible mask is formed in an ambient, moist-air environment. Even in high vacuum, the water meniscus is sufficiently stable to mask sections of a graphene sheet during plasma etch. Although bulk water cannot exist in a vacuum, the last few molecular layers remain and accumulate in concave features. In turn, very narrow and consistent GNRs were produced along the edge of the patterns.

The structure that masked the graphene depends exclusively on the surface properties of the two media within which it is confined and the properties of the adsorbate itself. This allows the width of the resulting GNR to remain unbound from conventional lithographic limitations; it also keeps the ribbons' positions strictly defined. This method takes the conventional lithographic patterning method and reinforces its capability for fine resolution with a water mask. Despite its simplicity, it worked the same way for very thin layers of many other materials. This is not surprising for a method so amenable to fabricating demanding structures like GNRs.

Having produced such large scale, narrow, long GNRs at desired positions on a chip, we are set to tackle the other long-standing challenge of GNR production. The chemical state and structure of GNR edges are not well-defined. Since they affect the GNRs' electronic properties, their control would represent another milestone in producing functional and tunable electronic devices.

By turning to something as ubiquitous as water, the researchers have demonstrated a high-throughput method of fabricating GNRs of the desired shape

In addition to on-chip GNR fabrication, our research team has developed bulk GNR synthesis based on chemically longitudinally unzipping MWCNTs, much like slicing a straw with a razor blade down its long axis to yield a ribbon structure. This approach has had a technological impact since its discovery in 2009. GNRs obtained from solution-based synthesis through intercalating and unzipping of MWCNTs with potassium permanganate¹ (KMnO₄) were non-conductive and dispersible only in water-based media. Subsequent improvements were made to produce highly conductive GNRs by intercalating MWCNTs with potassium (K) in the gas phase,⁴ but they were hard to disperse in both aqueous and organic media.

In order to make GNRs that were both conductive and organic soluble, MWCNTs were intercalated and unzipped with a sodium-potassium alloy (NaK) in the solution phase and selectively modified at the edges with organo-soluble functional groups⁵. They have also been edge-modified with polymers for enhanced capability. Furthermore, a non-destructive, surfactant-free method for the preparation of stable aqueous colloidal GNR solutions was developed by sonication of GNRs in hypophosphorous acid, filtration, washing and subsequent dispersion in water to form the colloidal solutions⁶. This sequence of advancements has enabled rapid application of GNRs for supercapacitors, lithium-ion batteries, conductive films, polymer composites and fiber technologies, where they have outperformed most current carbon-based materials.

GNRs have proven valuable in significantly extending the cycle life of supercapacitors. Supercapacitors are energy-storage devices that have the capacity to deliver energy exceedingly fast. However, they suffer from capacity decay after few cycles, contributing to their limited usefulness. By using GNRs as conductive substrates for the active materials in supercapacitors, significant enhancement was achieved in cycling. This was accomplished by directly growing polyaniline nanorods, a well-known supercapacitor material, from GNR surfaces⁷.

Furthermore, our team recently reported a unique structure of porous manganese oxide (MnO₂) directly grown from surfaces of GNRs. Since GNRs possess high surface area and high electrical conductivity, they serve as a suitable template on which MnO₂ is directly grown by a hydrothermal reaction to form MnO₂-GNRs, which was further wrapped with graphene to produce a robust graphene-MnO₂-GNRs structure⁸. MnO₂ is capable of storing high amounts of energy in lithium-ion batteries but its capacity fades quickly because it is not a good conductor. But placing long, conductive GNRs in good contact with MnO₂ established electrical percolation in the battery. In the hybrid, the internal resistance of the lithium-ion cells was significantly lowered. A graphene sheet was used to provide additional protection for the MnO₂-GNRs to restrict volume changes that contribute to capacity loss in the device and prevent loss of MnO₂ during reaction with Li, thus improving the composite's electrochemical stability performance. This underscores the importance of GNRs as both mechanical and electrical substrates in lithium-ion battery electrodes.

Our research team also developed scalable, robust, flexible and electrically conductive composite thin films made of edge-modified GNRs spray-coated on flexible polymer substrates⁹. The edge-modification enables them to be conveniently dispersed in a suitable solvent for spray-, spin- or blade-coating while preserving electrical conductivity along their basal planes. Through a real-time waveguide transmission experiment, the researchers established that ultrathin films made of long, randomly aligned GNRs are transparent to radiofrequency (RF) waves, because the films’ thicknesses are much smaller than their electromagnetic wave depth or “skin depth”. The film’s conductivity could be tuned depending on its thickness, providing de-icing capability demonstrated through electrical heating of the film by applying a voltage commonly accessible aboard ships and aircraft. This sprayable, RF-transparent, electrically conductive GNR film is an interesting discovery that will have applications in radomes, phased-array antennas and other platforms that need to be resistively heated to remove ice during their outdoor operation.

Unlike pristine MWCNTs that provide poor structural reinforcement, GNRs provide noticeable enhancement in mechanical properties when added to polymer composites. GNRs are able to effectively transfer load because GNRs have better interfacial contact with the polymer matrix, better wetting and interfacial adhesion of polymer chains to GNRs edges. At ~0.3% weight of GNRs in epoxy composite, the Young’s modulus of the GNR-based composite exceeds that based on MWCNTs by 30%. In addition, the ultimate tensile strength for a GNR-based composite at ~0.3% weight fraction is ~22% better than MWCNT-based composites with the same filler amount¹⁰. Thus, using GNRs as an additive in polymers can enable production of lightweight, reinforced polymer composites for structural applications.

Mechanically strong and highly electrically conductive fibres were made of thermally reduced graphene nanoribbons (trGNRs).  trGNR fibres hundreds of meters long were made by heat treatment (at 1,500°C) of fibres spun from liquid crystals of graphene oxide nanoribbons (GONRs) formed in chlorosulphonic acid. The GONRs align into liquid crystal phases in chlorosulphonic acid at high concentrations. The heat treatment restores conductivity by reducing the GONRs into trGNRs. With a tensile strength of 378 MPa, Young's modulus of 36.2 GPa, and electrical conductivity of 285 S/cm, the performance of trGNR fibres surpasses that of graphene-derived fibres. trGNR fibres were also found to have higher specific modulus than fibres based on commercial carbon, as well as steel¹¹.

Bulk GNRs are starting to move beyond the laboratory. We are likely to see industrial scale production of GNRs soon, a remarkable step toward technological improvement and widespread use of the wonder material in electronics, optics, energy storage and other applications. AZ Electronic Materials (AZ), a leading global producer and supplier of chemical materials for integrated circuits (ICs) and devices, flat-panel displays (FPDs), light-emitting diodes (LEDs) and photolithographic printing, and Rice University announced in November 2012 a licensing and sponsored-research agreement to study GNRs for advanced electronic and optical device applications¹². Indeed, through their unique and diverse properties, GNRs will make an increasing impact in both engineering and consumer products.

References

1. James, D. K.; Tour, J, M. The Chemical Synthesis of Graphene Nanoribbons—A Tutorial Review. Macromol. Chem. Phys.  2012, 213, 1033-1050.
2. Dutta, S.; Patti, S. K. Novel Properties of Graphene Nanoribbons: A Review. J. Mater. Chem. 2010,20, 8207-8223.
3. Abramova, V.; Slesarev, S.; Tour, J. M. Meniscus-Mask Lithography for Narrow Graphene Nanoribbons. ACS Nano 2013, 7, 6894–6898.
4. Kosynkin, D. V.; Lu, W.; Sinitskii, A.; Pera, G.; Sun, Z.; Tour, J. M. Highly Conductive Graphene Nanoribbons by Longitudinal Splitting of Carbon Nanotubes Using Potassium Vapor. ACS Nano 2011, 5, 968-974.
5. Genorio, B.; Lu, W.; Dimiev, A. M.; Zhu, Y.; Raji, A.-R. O.; Novosel, B.;  Alemany, L. B.;  Tour, J. M. In Situ Intercalation Replacement and Selective Functionalization of Graphene Nanoribbon Stacks. ACS Nano 2013, 6, 4231-4240.
6. Dimiev, A. M.; Gizzatov, A.; Wilson, L. J.; Tour, J. M. Stable Aqueous Colloidal Solutions of Intact Surfactant-Free Graphene Nanoribbons and Related Graphitic Nanostructures. Chem. Commun. 2013, 49, 2613-2615.
7. Li, L.; Raji, A.-R. O.; Fei, H.; Yang, Y. Samuel, E. L. G.; Tour, J. M. Nanocomposite of Polyaniline Nanorods Grown on Graphene Nanoribbons for Highly Capacitive Pseudocapacitors. ACS Appl. Mater. Interfaces 2013, 5, 6622?6627.
8. Li, L.; Raji, A.-R. O.; Tour, J. M. Graphene-Wrapped MnO 2 –Graphene Nanoribbons as Anode Materials for High-Performance Lithium Ion Batteries. Adv. Mater. 2013, DOI: 10.1002/adma.201302915.
9. Volman, V.; Zhu, Y.; Raji, A.-R. O.; Genorio, B.; Lu, W.; Xiang, C.; Tour, J. M. Radiofrequency Transparent, Electrically Conductive Graphene Nanoribbon Thin Films as De-Icing Heating Layers. Under review.
10. Rafiee, M. A.; Lu, W. Thomas, A. V.; Zandiatashbar, A.; Rafiee, J.; Tour, J. M.; Koratkar, N. A.  Graphene Nanoribbon Composites. ACS Nano 2014, 4, 9415-9420.
11. Xiang, C.; Behabtu, N.; Liu, Y.; Chae, H. G.; Young, C. C.; Genorio, B.; Tsentalovich, D. E.; Zhang, C.; Kosynkin, D.V.; Lomeda, J. R.; Hwang, C.-C.; Kumar, S.; Pasquali, M.; Tour, J. M.  Graphene Nanoribbons as an Advanced Precursor for Making Carbon Fiber. ACS Nano 2013, 7, 1628-1637.
12. AZ Electronic Materials Licenses Technology from Rice University for Graphene Nanoribbons. Nazir, M. 2012, http://www.azem.com/en/AboutAZ/News/AZEM%20licenses%20Technology%20from%20Rice% 20University%20for%20Graphene%20Nanoribbons.aspx.

Authors Abdul-Rahman O. Raji, PhD Candidate; Vera Abramova, PhD Candidate; and James M. Tour, T. T. and W. F. Chao Professor of Chemistry, Professor of Computer Science, and Professor of Mechanical Engineering and Materials Science, Rice University