Amanda L. Higginbotham

Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons

Graphene, or single-layered graphite, with its high crystallinity and interesting semimetal electronic properties, has emerged as an exciting two-dimensional material showing great promise for the fabrication of nanoscale devices. Thin, elongated strips of graphene that possess straight edges, termed graphene ribbons, gradually transform from semiconductors to semimetals as their width increases, and represent a particularly versatile variety of graphene. Several lithographic, chemical and synthetic procedures are known to produce microscopic samples of graphene nanoribbons, and one chemical vapour deposition process has successfully produced macroscopic quantities of nanoribbons at 950 °C. Here we describe a simple solution-based oxidative process for producing a nearly 100% yield of nanoribbon structures by lengthwise cutting and unravelling of multiwalled carbon nanotube (MWCNT) side walls. Although oxidative shortening of MWCNTs has previously been achieved, lengthwise cutting is hitherto unreported. Ribbon structures with high water solubility are obtained. Subsequent chemical reduction of the nanoribbons from MWCNTs results in restoration of electrical conductivity. These early results affording nanoribbons could eventually lead to applications in fields of electronics and composite materials where bulk quantities of nanoribbons are required.

Spontaneous high-concentration dispersions and liquid crystals of graphene

Graphene combines unique electronic properties and surprising quantum effects with outstanding thermal and mechanical properties. Many potential applications, including electronics and nanocomposites, require that graphene be dispersed and processed in a fluid phase. Here, we show that graphite spontaneously exfoliates into single-layer graphene in chlorosulphonic acid, and dissolves at isotropic concentrations as high as approximately 2 mg ml(-1), which is an order of magnitude higher than previously reported values. This occurs without the need for covalent functionalization, surfactant stabilization, or sonication, which can compromise the properties of graphene or reduce flake size. We also report spontaneous formation of liquid-crystalline phases at high concentrations ( approximately 20-30 mg ml(-1)). Transparent, conducting films are produced from these dispersions at 1,000 Omega square(-1) and approximately 80% transparency. High-concentration solutions, both isotropic and liquid crystalline, could be particularly useful for making flexible electronics as well as multifunctional fibres.

Lower-Defect Graphene Oxide Nanoribbons from Multiwalled Carbon Nanotubes

An improved method is described for the production of graphene oxide nanoribbons (GONRs) via longitudinal unzipping of multiwalled carbon nanotubes. The method produces GONRs with fewer defects and/or holes on the basal plane, maintains narrow ribbons <100 nm wide, and maximizes the high aspect ratio. Changes in the reaction conditions such as acid content, time, and temperature were investigated. The new, optimized method which introduces a second, weaker acid into the system, improves the selectivity of the oxidative unzipping presumably by in situ protection of the vicinal diols formed on the basal plane of graphene during the oxidation, and thereby prevents their overoxidation and subsequent hole generation. The optimized GONRs exhibit increased electrical conductivity over those chemically reduced nanoribbons produced by previously reported procedures.

Graphite Oxide Flame-Retardant Polymer Nanocomposites

Graphite oxide (GO) polymer nanocomposites were developed at 1, 5, and 10 wt % GO with polycarbonate (PC), acrylonitrile butadiene styrene, and high-impact polystyrene for the purpose of evaluating the flammability reduction and material properties of the resulting systems. The overall morphology and dispersion of GO within the polymer nanocomposites were studied by scanning electron microscopy and optical microscopy; GO was found to be well-dispersed throughout the matrix without the formation of large aggregates. Mechanical testing was performed using dynamic mechanical analysis to measure the storage modulus, which increased for all polymer systems with increased GO loading. Microscale oxygen consumption calorimetry revealed that the addition of GO reduced the total heat release and peak heat release rates in all systems, and GO-PC composites demonstrated very fast self-extinguishing times in vertical open flame tests, which are important to some regulatory fire safety applications.

Electronic transport in monolayer graphene nanoribbons produced by chemical unzipping of carbon nanotubes

We report on the structural and electrical properties of graphene nanoribbons (GNRs) produced by the oxidative unzipping of carbon nanotubes. GNRs were reduced by hydrazine at 95 °C and further annealed in Ar/H2 at 900 °C; monolayer ribbons were selected for the fabrication of electronic devices. GNR devices on Si/SiO2 substrates exhibit an ambipolar electric field effect typical for graphene. The conductivity of monolayer GNRs (∼35 S/cm) and mobility of charge carriers (0.5–3 cm2/V s) are less than the conductivity and mobility of pristine graphene, which could be explained by oxidative damage caused by the harsh H2SO4/KMnO4 used to make GNRs. The resistance of GNR devices increases by about three orders of magnitude upon cooling from 300 to 20 K. The resistance/temperature data is consistent with the variable range hopping mechanism, which, along with the microscopy data, suggests that the GNRs have a nonuniform structure.

Diameter-Dependent Solubility of Single-Walled Carbon Nanotubes

We study the solubility and dispersibility of as-produced and purified HiPco single-walled carbon nanotubes (SWNTs). Variation in specific operating conditions of the HiPco process are found to lead to significant differences in the respective SWNT solubilities in oleum and surfactant suspensions. The diameter distributions of SWNTs dispersed in surfactant solutions are batch-dependent, as evidenced by luminescence and Raman spectroscopies, but are identical for metallic and semiconducting SWNTs within a batch. We thus find that small diameter SWNTs disperse at higher concentration in aqueous surfactants and dissolve at higher concentration in oleum than do large-diameter SWNTs. These results highlight the importance of controlling SWNT synthesis methods in order to optimize processes dependent on solubility, including macroscopic processing such as fiber spinning, material reinforcement, and films production, as well as for fundamental research in type selective chemistry, optoelectronics, and nanophotonics.

Unzipped graphene nanoribbons as sensitive O2 sensors: Electron spin resonance probing and dissociation kinetics

Electron spin resonance study of unzipped graphene nanoribbons (GNRs) reveals a specific carbon-related signal, denoted GC, at g=2.0032, attributed to GNRs periphery centers. The signal is observed to be readily quenched upon O2 adsorption, which appears to be fully reversible upon room temperature vacuum treatment. Its depassivation behavior, observed from 130 K onward, is well described by first-order kinetics, characterized by the dissociation energy of 0.58±0.04 eV of spread 0.11±0.02 eV. The GC signal is not sensitive to other gases, such as H2, He, N2, and Ar, pointing to a GC–O2 physisorption interaction unique for GNRs, in agreement with theoretical insight. The GC center thus emerges as a highly selective, sensitive, and reversible O2 sensor.

Paramagnetic centers in graphene nanoribbons prepared from longitudinal unzipping of carbon nanotubes

Electron spin resonance (ESR) investigation of graphene nanoribbons (GNRs) prepared through longitudinal unzipping of multi-walled carbon nanotubes indicates the presence of C-related dangling bond centers, exhibiting paramagnetic features. ESR signal broadening from pristine or oxidized GNRs is explained in terms of unresolved hyperfine structure, and in the case of reduced GNRs, the broadening of the ESR signal can be due to enhancement of conductivity upon reduction. The spin dynamics observed from ESR line width-temperature data reflect a variable range hopping mechanism through localized states, consistent with resistance-temperature data.