Solution-Based Processing of Monodisperse Two-Dimensional Nanomaterials

Exfoliation of single-layer graphene from bulk graphite and the subsequent discovery of exotic physics and emergent phenomena in the atomically thin limit has motivated the isolation of other two-dimensional (2D) layered nanomaterials. Early work on isolated 2D nanomaterial flakes has revealed a broad range of unique physical and chemical properties with potential utility in diverse applications. For example, the electronic and optical properties of 2D nanomaterials depend strongly on atomic-scale variations in thickness, enabling enhanced performance in optoelectronic technologies such as light emitters, photodetectors, and photovoltaics. Much of the initial research on 2D nanomaterials has relied on micromechanical exfoliation, which yields high-quality 2D nanomaterial flakes that are suitable for fundamental studies but possesses limited scalability for real-world applications. In an effort to overcome this limitation, solution-processing methods for isolating large quantities of 2D nanomaterials have emerged. Importantly, solution processing results in 2D nanomaterial dispersions that are amenable to roll-to-roll fabrication methods that underlie lost-cost manufacturing of thin-film transistors, transparent conductors, energy storage devices, and solar cells. Despite these advantages, solution-based exfoliation methods typically lack control over the lateral size and thickness of the resulting 2D nanomaterial flakes, resulting in polydisperse dispersions with heterogeneous properties. Therefore, post-exfoliation separation techniques are needed to achieve 2D nanomaterial dispersions with monodispersity in lateral size, thickness, and properties. In this Account, we survey the latest developments in solution-based separation methods that aim to produce monodisperse dispersions and thin films of emerging 2D nanomaterials such as graphene, boron nitride, transition metal dichalcogenides, and black phosphorus. First, we motivate the need for precise thickness control in 2D nanomaterials by reviewing thickness-dependent physical properties. Then we present a succinct survey of solution-based exfoliation methods that yield 2D nanomaterial dispersions in organic solvents and aqueous media. The Account subsequently focuses on separation methods, including a critical analysis of their relative strengths and weaknesses for 2D nanomaterials with different buoyant densities, van der Waals interactions, and chemical reactivities. Specifically, we evaluate sedimentation-based density gradient ultracentrifugation (sDGU) and isopycnic DGU (iDGU) for post-exfoliation 2D nanomaterial dispersion separation. The comparative advantages of sedimentation and isopycnic methods are presented in both aqueous and nonaqueous media for 2D nanomaterials with varying degrees of chemical reactivity. Finally, we survey methods for forming homogeneous thin films from 2D nanomaterial dispersions and emerging technologies that are likely to benefit from these structures. Overall, this Account provides not only an overview of the present state-of-the-art but also a forward-looking vision for the field of solution-processed monodisperse 2D nanomaterials.