Claudia Backes

Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids

To progress from the laboratory to commercial applications, it will be necessary to develop industrially scalable methods to produce large quantities of defect-free graphene. Here we show that high-shear mixing of graphite in suitable stabilizing liquids results in large-scale exfoliation to give dispersions of graphene nanosheets. X-ray photoelectron spectroscopy and Raman spectroscopy show the exfoliated flakes to be unoxidized and free of basal-plane defects. We have developed a simple model that shows exfoliation to occur once the local shear rate exceeds 10(4) s(-1). By fully characterizing the scaling behaviour of the graphene production rate, we show that exfoliation can be achieved in liquid volumes from hundreds of millilitres up to hundreds of litres and beyond. The graphene produced by this method performs well in applications from composites to conductive coatings. This method can be applied to exfoliate BN, MoS2 and a range of other layered crystals.

Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond electronics.

Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to fillers for composite reinforcement.

Sensitive, High-Strain, High-Rate Bodily Motion Sensors Based on Graphene–Rubber Composites

Monitoring of human bodily motion requires wearable sensors that can detect position, velocity and acceleration. They should be cheap, lightweight, mechanically compliant and display reasonable sensitivity at high strains and strain rates. No reported material has simultaneously demonstrated all the above requirements. Here we describe a simple method to infuse liquid-exfoliated graphene into natural rubber to create conducting composites. These materials are excellent strain sensors displaying 10(4)-fold increases in resistance and working at strains exceeding 800%. The sensitivity is reasonably high, with gauge factors of up to 35 observed. More importantly, these sensors can effectively track dynamic strain, working well at vibration frequencies of at least 160 Hz. At 60 Hz, we could monitor strains of at least 6% at strain rates exceeding 6000%/s. We have used these composites as bodily motion sensors, effectively monitoring joint and muscle motion as well and breathing and pulse.

Manipulating single-wall carbon nanotubes by chemical doping and charge transfer with perylene dyes

Single-wall carbon nanotubes (SWNTs) are emerging as materials with much potential in several disciplines, in particular in electronics and photovoltaics. The combination of SWNTs with electron donors or acceptors generates active materials, which can produce electrical energy when irradiated. However, SWNTs are very elusive species when characterization of their metastable states is required. This problem mainly arises because of the polydispersive nature of SWNT samples and the inevitable presence of SWNTs in bundles of different sizes. Here, we report the complete and thorough characterization of an SWNT radical ion-pair state induced by complexation with a perylene dye, which combines excellent electron-accepting and -conducting features with a five-fused ring π-system. At the same time, the perylene dye enables the dispersion of SWNTs by means of π–π interactions, which gives individual SWNTs in solution. This work clears a path towards electronic and optoelectronic devices in which regulated electrical transport properties are important. Using carbon nanotubes in electronic or photovoltaic devices generates active metastable states. These elusive species are hard to characterize because of the polydisperse and aggregate nature of nanotube bundles. A complete characterization of the radical–ion pair state has now been achieved using a range of techniques.

Edge and confinement effects allow in situ measurement of size and thickness of liquid-exfoliated nanosheets

Two-dimensional nanomaterials such as MoS2 are of great interest both because of their novel physical properties and their applications potential. Liquid exfoliation, an important production method, is limited by our inability to quickly and easily measure nanosheet size, thickness or concentration. Here we demonstrate a method to simultaneously determine mean values of these properties from an optical extinction spectrum measured on a liquid dispersion of MoS2 nanosheets. The concentration measurement is based on the size-independence of the low-wavelength extinction coefficient, while the size and thickness measurements rely on the effect of edges and quantum confinement on the optical spectra. The resultant controllability of concentration, size and thickness facilitates the preparation of dispersions with pre-determined properties such as high monolayer-content, leading to first measurement of A-exciton MoS2 luminescence in liquid suspensions. These techniques are general and can be applied to a range of two-dimensional materials including WS2, MoSe2 and WSe2.

Sensitive electromechanical sensors using viscoelastic graphene-polymer nanocomposites

Super sensitive, not so silly, putty Many composites blend stiff materials, such as glass or carbon fibers, into a softer elastic polymer matrix to generate a material with better overall mechanical toughness. Boland et al. added graphene to a lightly cross-linked silicone polymer (also known as Silly Putty). The resulting composite has unusual mechanical properties, allowing the manufacture of strain sensors that can detect respiration and the footsteps of spiders. Science, this issue p. 1257 A composite of graphene and a viscoelastic polymer results in a material with unexpected electromechanical properties. Despite its widespread use in nanocomposites, the effect of embedding graphene in highly viscoelastic polymer matrices is not well understood. We added graphene to a lightly cross-linked polysilicone, often encountered as Silly Putty, changing its electromechanical properties substantially. The resulting nanocomposites display unusual electromechanical behavior, such as postdeformation temporal relaxation of electrical resistance and nonmonotonic changes in resistivity with strain. These phenomena are associated with the mobility of the nanosheets in the low-viscosity polymer matrix. By considering both the connectivity and mobility of the nanosheets, we developed a quantitative model that completely describes the electromechanical properties. These nanocomposites are sensitive electromechanical sensors with gauge factors >500 that can measure pulse, blood pressure, and even the impact associated with the footsteps of a small spider.

High Population of Individualized SWCNTs through the Adsorption of Water-Soluble Perylenes

The aqueous dispersion of SWCNTs in the presence of the water-soluble perylene derivatives 1-3 is reported. Significantly, even very low concentrations of the perylenes such as 0.01 wt% of the amphiphilic derivative 3, cause an efficient dissolution of the SWCNTs in water accompanied by a very pronounced individualization. The individualization of SWCNTs in water after ultrasonication in the presence of water-soluble aromatic perylenes was investigated in detail by absorption, emission, and Raman spectroscopy as well as by AFM and cryo-TEM. These studies also revealed that the individualization of the SWCNTs caused by the adsorption of 3 is much more effective than that induced by SDBS, which is the most frequently used surfactant for SWCNT dispersion in water. The pi-pi-stacking interaction and the electronic interaction between the perylene unit and the nanotube surface is reflected, for example, by the distinct absorption and emission features in the UV/vis/nIR, which differ significantly from those observed for SWNTs dispersed in the presence of SDBS and by the quenching of the perylene fluorescence of 3 when being in contact with the tubes.

Nanotube Surfactant Design: The Versatility of Water‐Soluble Perylene Bisimides

The synthesis of perylene-based single-walled carbon nanotube (SWCNT) surfactants and the dispersion and exfoliation of SWCNTs in water by a variety of designed surfactants is investigated. The quality of the nanotube dispersions is evaluated by optical absorption and emission spectroscopy, zeta-potential measurements and statistical atomic force microscopy (AFM). Significantly the dispersion efficiency can be increased at higher pH, as water solubility of the surfactants is ensured by peripheral derivatization with carboxyl-functionalized first- and second-order Newkome dendrimers. Even at very low perylene concentrations of 0.1 g L(-1) and a nanotube-to-surfactant ratio of 1:1, the nanotube supernatant after centrifugation contains up to 73% of the pristine material with exfoliation degrees (the number of fractions of individualized nanotubes N(I)/N(T)) of up to 76%. The adsorption of the perylene core to the nanotube scaffold is indicated by red-shifted perylene-absorption and SWCNT-emission features except for the smallest perylene amphiphile, where solubilization is presumably based on a micellar arrangement. The nanotube fluorescence is significantly altered and reduced in intensity compared to nanotubes dispersed in sodium dodecylbenzene sulfonate (SDBS) being strongly dependent on the structure of the perylene surfactant. We attribute this observation to the homogeneity of the surfactant coverage, e.g., the supramolecular arrangement onto the nanotube backbone. This study represents a step forward in understanding the structure-property relationship of nanotube surfactants. Furthermore high-quality nanotube dispersions with increased degrees of exfoliation are highly desirable, as the efficiency of nanotube separation techniques relies on highly individualized samples.

2D-Crystal-Based Functional Inks

The possibility to produce and process graphene, related 2D crystals, and heterostructures in the liquid phase makes them promising materials for an ever-growing class of applications as composite materials, sensors, in flexible optoelectronics, and energy storage and conversion. In particular, the ability to formulate functional inks with on-demand rheological and morphological properties, i.e., lateral size and thickness of the dispersed 2D crystals, is a step forward toward the development of industrial-scale, reliable, inexpensive printing/coating processes, a boost for the full exploitation of such nanomaterials. Here, the exfoliation strategies of graphite and other layered crystals are reviewed, along with the advances in the sorting of lateral size and thickness of the exfoliated sheets together with the formulation of functional inks and the current development of printing/coating processes of interest for the realization of 2D-crystal-based devices.

Production of Highly Monolayer Enriched Dispersions of Liquid-Exfoliated Nanosheets by Liquid Cascade Centrifugation

While liquid exfoliation is a powerful technique to produce defect-free nanosheets in large quantities, its usefulness is limited by broad nanosheet thickness distributions and low monolayer contents. Here we demonstrate liquid processing techniques, based on iterative centrifugation cascades, which can be designed to achieve either highly efficient nanosheet size-selection and/or monolayer enrichment. The resultant size-selected dispersions were used to establish quantitative metrics to determine monolayer volume fraction, as well as mean nanosheet size and thickness, from standard spectroscopic measurements. Such metrics allowed us to design and optimize centrifugation cascades to enrich liquid exfoliated WS2 dispersions up to monolayer contents of 75%. Monolayer-rich dispersions show relatively bright photoluminescence with narrow line widths (<35 meV) indicating the high quality of the nanosheets. The enriched dispersions display extinction spectra with distinct features, which also allow the direct estimation of monolayer contents.