Solution-processed networks of 2D nanosheets are promising for a range of applications in the field of printed electronics. However, the electrical performance of these networks — represented, for example, by the mobility — is almost always inferior to that of the individual nanosheets. In this Review, we highlight the central role that the inter-sheet junctions play in determining the electrical characteristics of such networks. After briefly reviewing ink formulation and printing methods, we use a selection of electronic applications as examples to demonstrate the dependence of network conductivity on network morphology. We show the network morphology to be heavily influenced by the deposition method, the post-treatment regime and the nanosheet properties. In turn, the morphology of the network fundamentally determines the properties of the inter-sheet junctions, which, ultimately, control the electrical performance of the network. We use reported electrical data to show that three main conduction regimes exist: the network conductivity can be limited by the junctions, by a combination of junction and material properties or, very rarely, by the material properties. Using a meta-analysis of published data, we propose simple models relating network conductivity and mobility to the junction resistance. Solution-processed networks of 2D materials are promising for applications in printed electronics. This Review examines how these networks are often electrically limited by the junctions between nanosheets, a phenomenon rarely reported and poorly understood, and surveys the macroscopic electrical properties of printed 2D networks, with a focus on inter-sheet junctions.

The electrical conductivity of solution-processed nanosheet networks

Advances in Liquid-Phase and Intercalation Exfoliations of Transition Metal Dichalcogenides to Produce 2D Framework

Graphene is a two-dimensional nanomaterial with a monolayer of atomic thickness. Due to its high specific surface area, weak interlayer interaction and good chemical stability, graphene has shown remarkable tribological properties as a lubricant additive. This review focuses on the research progress on graphene-based additives witnessed in recent years. Various synthesis methods of graphene nanomaterials have been displayed, and the review especially highlights the preparation processes of graphene using as a lubricant additive. The dispersion stability of graphene in lubricants is a key concern that has been presented. Besides physical and chemical modifications, a new dispersion method of structural regulation, which has obvious self-dispersed effect, is also discussed in detail. Furthermore, the lubrication mechanisms of graphene additives have been summarized, which will be highly beneficial to optimize the synthesis processes and to regulate the microstructures of graphene for achieving better lubrication. Finally, the challenges and outlook of the future studies on graphene additives are presented in the field of lubrication.

Two-dimensional (2D) graphene nanosheets as advanced lubricant additives: A critical review and prospect

Graphene is an excellent 2D material that has extraordinary properties such as high surface area, electron mobility, conductivity, and high light transmission. Polymer composites are used in many applications in place of polymers. In recent years, the development of stable graphene dispersions with high graphene concentrations has attracted great attention due to their applications in energy, bio-fields, and so forth. Thus, this review essentially discusses the preparation of stable graphene–polymer composites/dispersions. Discussion on existing methods of preparing graphene is included with their merits and demerits. Among existing methods, mechanical exfoliation is widely used for the preparation of stable graphene dispersion, the theoretical background of this method is discussed briefly. Solvents, surfactants, and polymers that are used for dispersing graphene and the factors to be considered while preparing stable graphene dispersions are discussed in detail. Further, the direct applications of stable graphene dispersions are discussed briefly. Finally, a summary and prospects for the development of stable graphene dispersions are proposed.

https://www.mdpi.com/2073-4360/13/14/2375/pdf

Recent Studies on Dispersion of Graphene-Polymer Composites.

On the relationship between morphology and conductivity in nanosheet networks

Graphene and other graphitic materials are suggested as a route to cheap, high‐performance, environmentally‐sustainable electronic devices owing to their almost unique combination of properties. Liquid‐phase exfoliation is a family of shear‐based techniques that produce dispersions of nanosheets from bulk layered material crystallites. High‐quality nanosheets of graphene can be produced in solvents or surfactant dispersions; however the lateral size of these sheets limits the network transport properties observed in printed films. A high‐throughput, industrially‐scalable aqueous process for the production of graphene and related layered nanomaterials is presented. By considering not only the exfoliation process, but also the size selection and deposition processes, printable graphitic nanoparticulate materials with conductivities up to 50 000 S m−1 are demonstrated. This value is ten times larger than is typically obtained for few‐layer graphene produced by liquid‐phase exfoliation. The size selection process is critical to obtaining the maximum conductivity of deposited films, with an optimized nanographite having greater performance than few‐layer graphene or graphite that is processed and used without size selection. Building on these results a radio‐frequency antenna application is demonstrated, which is competitive with the state‐of‐the‐art, and a route to recycling of such printed short‐lifetime electronic devices to lower the environmental impact is discussed.

Large‐Scale Surfactant Exfoliation of Graphene and Conductivity‐Optimized Graphite Enabling Wireless Connectivity

Laser-deposited carbon aerogel is a low-density porous network of carbon clusters synthesized using a laser process. A one-step synthesis, involving deposition and annealing, results in the formation of a thin porous conductive film which can be applied as a chemiresistor. This material is sensitive to NO2 compared to ammonia and other volatile organic compounds and is able to detect ultra-low concentrations down to at least 10 parts-per-billion. The sensing mechanism, based on the solubility of NO2 in the water layer adsorbed on the aerogel, increases the usability of the sensor in practically-relevant ambient environments. A heating step, achieved in tandem with a microheater, allows the recovery to the baseline making it operable in real world environments. The operability at room temperature, its low cost and scalable production makes it promising for Internet-of-Things air quality monitoring.

/pdf/laser-deposited-carbon-aerogel-derived-from-graphene-oxide-3xnzqnntp3.pdf

Thomas Waters

Papers

Large‐Scale Surfactant Exfoliation of Graphene and Conductivity‐Optimized Graphite Enabling Wireless Connectivity

Laser-Deposited Carbon Aerogel Derived from Graphene Oxide Enables NO2-Selective Parts-per-Billion Sensing.