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Here, recent advances in exfoliation and modification of layered two‐dimensional (2D) materials in various supercritical fluids (SCFs) are highlighted. The motivating factors for enhancing exfoliation efficiency and product quality in SCFs, mechanisms for exfoliation and dispersion in SCFs, as well as general metrics applied to assess quality and processability of exfoliated 2D materials are critically discussed.

Abstract

Since the first intercalation of layered silicates by using supercritical CO₂ as a processing medium, considerable efforts have been dedicated to intercalating and exfoliating layered two‐dimensional (2D) materials in various supercritical fluids (SCFs) to yield single‐ and few‐layer nanosheets. Here, recent work in this area is highlighted. Motivating factors for enhancing exfoliation efficiency and product quality in SCFs, mechanisms for exfoliation and dispersion in SCFs, as well as general metrics applied to assess quality and processability of exfoliated 2D materials are critically discussed. Further, advances in formation and application of 2D material–based composites with assistance from SCFs are presented. These discussions address chemical transformations accompanying SCF processing such as doping, covalent surface modification, and heterostructure formation. Promising features, challenges, and routes to expanding SCF processing techniques are described.

Highly crystallized 2,9‐didecyldinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene (C₁₀‐DNTT) monolayer crystal with large‐area uniformity is obtained by an ultraslow shearing method. This monolayer organic semiconductor thin film is used as the template for thermally evaporated upper C₁₀‐DNTT film. The organic thin films deposited by this hybrid approach show an interesting coherence structure with a copied molecular orientation of the templating crystal.

Abstract

Solution‐processed 2D organic semiconductors (OSCs) have drawn considerable attention because of their novel applications from flexible optoelectronics to biosensors. However, obtaining well‐oriented sheets of 2D organic materials with low defect density still poses a challenge. Here, a highly crystallized 2,9‐didecyldinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene (C₁₀‐DNTT) monolayer crystal with large‐area uniformity is obtained by an ultraslow shearing (USS) method and its growth pattern shows a kinetic Wulff's construction supported by theoretical calculations of surface energies. The resulting seamless and highly crystalline monolayers are then used as templates for thermally depositing another C₁₀‐DNTT ultrathin top‐up film. The organic thin films deposited by this hybrid approach show an interesting coherence structure with a copied molecular orientation of the templating crystal. The organic field‐effect transistors developed by these hybrid C₁₀‐DNTT films exhibit improved carrier mobility of 14.7 cm² V⁻¹ s⁻¹ as compared with 7.3 cm² V⁻¹ s⁻¹ achieved by pure thermal evaporation (100% improvement) and 2.8 cm² V⁻¹ s⁻¹ achieved by solution sheared monolayer C₁₀‐DNTT. This work establishes a simple yet effective approach for fabricating high‐performance and low‐cost electronics on a large scale.

Nature Reviews Materials, Published online: 30 July 2019; doi:10.1038/s41578-019-0126-z

Synthetic nanopores in 2D materials are an emerging platform for power harvesting from the controlled mixing of fresh and salty water. This Review surveys their physics and materials properties and the progress in the design of new, high-density, ion-selective membrane materials.

Nature Communications, Published online: 31 July 2019; doi:10.1038/s41467-019-11497-y

2D electronic spectroscopy found experimental indications of coherently interacting excitons and trions in doped transition metal dichalcogenides (TMDCs). Here, the authors perform simulations of 2D spectra of monolayer TMDCs based on a many-body formalism, allowing to relate exciton-trion coherence to quantum beats based on microscopic principles.