Localised strain and doping of 2D materials.

There is a growing interest in 2D materials-based devices as the replacement for established materials, such as silicon and metal oxides in microelectronics and sensing, respectively. However, the atomically thin nature of 2D materials makes them susceptible to slight variations caused by their immediate environment, inducing doping and strain, which can vary between, and even microscopically within, devices. One of the misapprehensions for using 2D materials is the consideration of unanimous intrinsic properties over different support surfaces. The interfacial interaction, intrinsic structural disorder and external strain modulate the properties of 2D materials and govern the device performance. The understanding, measurement and control of these factors are thus one of the significant challenges for the adoption of 2D materials in industrial electronics, sensing, and polymer composites. This topical review provides a comprehensive overview of the effect of strain-induced lattice deformation and its relationship with physical and electronic properties. Using the example of graphene and MoS2 (as the prototypical 2D semiconductor), we rationalise the importance of scanning probe techniques and Raman spectroscopy to elucidate strain and doping in 2D materials. These effects can be directly and accurately characterised through Raman shifts in a non-destructive manner. A generalised model has been presented that deconvolutes the intertwined relationship between strain and doping in graphene and MoS2 that could apply to other members of the 2D materials family. The emerging field of straintronics is presented, where the controlled application of strain over 2D materials induces tuneable physical and electronic properties. These perspectives highlight practical considerations for strain engineering and related microelectromechanical applications.

Mid-Infrared Electrochromics Enabled by Intraband Modulation in Carbon Nanotube Networks.

Tuneable infrared properties, such as transparency and emissivity, are highly desirable for a range of applications, including thermal windows and emissive cooling. Here, we demonstrate the use of carbon nanotube networks spray-deposited onto an ionic liquid-infused membrane to fabricate devices with electrochromic modulation in the mid-infrared spectrum, facilitating control of emissivity and apparent temperature. Such modulation is enabled by intraband transitions in unsorted single-walled carbon nanotube networks, allowing the use of scalable nanotube inks for printed devices. These devices are optimized by varying film thickness and sheet resistance, demonstrating the emissivity modulation (from ∼0.5 to ∼0.2). These devices and the understanding thereof open the door to selection criteria for infrared electrochromic materials based on the relationship between band structure, electrochemistry, and optothermal properties to enable the development of solution-processable large-area coatings for widespread thermal management applications.

Nanosheet-Stabilized Emulsions: Near-Minimum Loading and Surface Energy Design of Conductive Networks.

Here, we develop a framework for assembly, understanding, and application of functional emulsions stabilized by few-layer pristine two-dimensional (2D) nanosheets. Liquid-exfoliated graphene and MoS2 are demonstrated to stabilize emulsions at ultralow nanosheet volume fractions, approaching the minimum loading achievable with 2D materials. These nanosheet-stabilized emulsions allow controlled droplet deposition free from the coffee ring effect to facilitate single-droplet devices from minute quantities of material or assembly into large-area films with high network conductivity. To broaden the range of compositions and subsequent applications, an understanding of emulsion stability and orientation in terms of surface energy of the three phases is developed. Importantly, this model facilitates determination of the surface energies of the nanosheets themselves and identifies strategies based on surface tension and pH to allow design of emulsion structures. Finally, this approach is used to prepare conductive silicone emulsion composites with a record-low loading level and excellent electromechanical sensitivity. The versatility of these nanosheet-stabilized emulsions illustrates their potential for low-loading composites, thin-film formation and surface energy determination, and the design of functional structures for a range of segregated network applications.

Size selection and thin-film assembly of MoS2 elucidates thousandfold conductivity enhancement in few-layer nanosheet networks.

Printed electronics based on liquid-exfoliated nanosheet networks are limited by inter-nanosheet junctions and thick films which hinder field-effect gating. Here, few-layer molybdenum disulfide nanosheets are assembled by Langmuir deposition into thin films, and size selection is shown to lead to a thousandfold conductivity enhancement with potential applicability to all nanosheet networks.

Surfactant-free liquid-exfoliated copper hydroxide nanocuboids for non-enzymatic electrochemical glucose detection.

To facilitate printable sensing solutions particles need to be suspended and stabilised in a liquid medium. Hansen parameters were used to identify that alcohol-water blends are ideal for stabilising colloidal copper hydroxide in dispersion. The suspended material can be further separated in various size fractions with a distinct cuboid geometry which was verified using atomic force microscopy. This facilitates the development of Raman spectroscopic metrics for determining particle sizes. This aspect ratio is related to the anisotropic crystal structure of the bulk crystallites. As the size of the nanocuboids decreases electrochemical sensitivity of the material increases due to an increase in specific surface area. Electrochemical glucose sensitivity was investigated using both cyclic voltammetry and chronoamperometry. The sensitivity is noted to saturate with film thickness. The electrochemical response of 253 mA M-1 cm-2 up to 0.1 mM and 120 mA cm-2 up to 0.6 mM allow for calibration of potential devices. These results indicate suitability for use as a glucose sensor and, due to the surfactant-free, low boiling point solvent approach used to exfoliate the nanocuboids, it is an ideal candidate for printable solutions. The ease of processing will also allow this material to be integrated in composite films for improved functionality in future devices.

Langmuir Films of Layered Nanomaterials: Edge Interactions and Cell Culture Applications.

The application of nanomaterials in technology is limited by challenges in their processing into macroscopic structures with reliable and scalable methods. Herein, it is demonstrated that using scalable fabrication methods such as liquid-phase exfoliation, it is possible to produce dispersions of a wide variety of layered nanomaterials, including the first demonstration of boron nitride, with controllable and standardized size and thickness scaling. These can be used, as-produced, for Langmuir deposition, to create single layer films with tuneable density. Of particular importance, we show that the difference in edge chemistry of these materials dictates the film formation process, and therefore can be used to provide a generic fabrication methodology that is demonstrated for various layered nanomaterials, including graphene, boron nitride, and transition metal dichalcogenides. We show that this leads to controllable cancer cell growth on graphene substrates with different edge densities but comparable surface coverage, which can be produced on a statistically relevant cell study amount. This opens pathways for the generic fabrication of a range of layered nanomaterial films for various applications toward a commercially viable film fabrication technology.

Sonochemical edge functionalisation of molybdenum disulfide.

Liquid-phase exfoliation (LPE) has been shown to be capable of producing large quantities of high-quality dispersions suitable for processing into subsequent applications. LPE typically requires surfactants for aqueous dispersions or organic solvents with high boiling point. However, they have major drawbacks such as toxicity, aggregation during solvent evaporation or the presence of residues. Here, dispersions of molybdenum disulfide in acetone are prepared and show much higher concentration and stability than predicted by Hansen parameter analysis. Aiming to understand these enhanced properties, the nanosheets were characterised using UV-visible spectroscopy, zeta potential measurements, atomic force microscopy, Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and scanning transmission microscopy combined with spatially-resolved electron energy loss spectroscopy. Also, the performance of the MoS2 nanosheets exfoliated in acetone was compared to that of those exfoliated in isopropanol as a catalyst for the hydrogen evolution reaction. The conclusion from the chemical characterisation was that MoS2 nanosheets exfoliated in acetone have an oxygen edge functionalisation, in the form of molybdenum oxides, changing its interaction with solvents and explaining the observed high-quality and stability of the resulting dispersion in a low boiling point solvent. Exfoliation in acetone could potentially be applied as a pretreatment to modify the solubility of MoS2 by edge functionalisation.

Charge Transfer Hybrids of Graphene Oxide and the Intrinsically Microporous Polymer PIM-1.

Nanohybrid materials based on nanoparticles of the intrinsically microporous polymer PIM-1 and graphene oxide (GO) are prepared from aqueous dispersions with a reprecipitation method, resulting in the surface of the GO sheets being decorated with nanoparticles of PIM-1. The significant blueshift in fluorescence signals for the GO/PIM-1 nanohybrids indicates modification of the optoelectronic properties of the PIM-1 in the presence of the GO due to their strong interactions. The stiffening in the Raman G peak of GO (by nearly 6 cm-1) further indicates p-doping of the GO in the presence of PIM. Kelvin probe force microscopy (KPFM) and electrochemical reduction measurements of the nanohybrids provide direct evidence for charge transfer between the PIM-1 nanoparticles and the GO nanosheets. These observations will be of importance for future applications of GO-PIM-1 nanohybrids as substrates and promoters in catalysis and sensing.

Functional liquid structures by emulsification of graphene and other two-dimensional nanomaterials.

Pickering emulsions stabilised with nanomaterials provide routes to a range of functional macroscopic assemblies. We demonstrate the formation and properties of water-in-oil emulsions prepared through liquid-phase exfoliation of graphene. Due to the functional nature of the stabiliser, the emulsions exhibit conductivity due to inter-particle tunnelling. We demonstrate a strain sensing application with a large gauge factor of ∼40; the highest reported in a liquid. Our methodology can be applied to other two-dimensional layered materials opening up applications such as energy storage materials, and flexible and printable electronics.