Pedro Guerra Demingos

Due to the strong in-plane but weak out-of-plane bonding, it is relatively easy to separate nanosheets of two-dimensional (2D) materials from their respective bulk crystals. This exfoliation of 2D materials can yield large 2D nanosheets, hundreds of micrometers wide, that can be as thin as one or a few atomic layers thick. However, the underlying physical mechanisms unique to each exfoliation technique can produce a wide distribution of defects, yields, functionalization, lateral sizes, and thicknesses, which can be appropriate for specific end applications. The five most commonly used exfoliation techniques include micromechanical cleavage, ultrasonication, shear exfoliation, ball milling, and electrochemical exfoliation. In this review, we present an overview of the field of 2D material exfoliation and the underlying physical mechanisms with emphasis on progress over the last decade. The beneficial characteristics and shortcomings of each exfoliation process are discussed in the context of their functional properties to guide the selection of the best technique for a given application. Furthermore, an analysis of standard applications of exfoliated 2D nanosheets is presented including their use in energy storage, electronics, lubrication, composite, and structural applications. By providing detailed insight into the underlying exfoliation mechanisms along with the advantages and disadvantages of each technique, this review intends to guide the reader toward the appropriate batch-scale exfoliation techniques for a wide variety of industrial applications.

2D materials are well-known for their low-friction behavior by modifying the interfacial forces at atomic surfaces. Of the wide range of 2D materials, MXenes represent an emerging material class but their lubricating behavior has been scarcely investigated. Herein, the friction mechanisms of 2D Ti3C2Tx MXenes are demonstrated which are attributed to their surface terminations. We find that Ti3C2Tx MXenes do not exhibit the well-known frictional layer dependence of other 2D materials. Instead, the nanoscale lubricity of 2D MXenes is governed by the termination species resulting from synthesis. Annealing the MXenes demonstrate a 7% reduction in OH termination which translates to a 16–57% reduction of friction in agreement with DFT calculations. Finally, the stability of MXene flakes is demonstrated upon isolation from their aqueous environment. This work indicates that MXenes can provide sustainable lubricity at any thickness which makes them uniquely positioned among 2D material lubricants.

Carbon nanothreads (NTs) are ultrathin materials synthesized by solid-state reaction of crystalline benzene or pyridine under high pressure. Recent experimental studies show that the sp2–sp3 conversion in C–C or C–N bonds toward NT formation is not always complete, typically resulting in samples constituted by a mixture of both partially and fully saturated structures. The objective of this study is to use density functional theory calculations to compute the mechanical and electronic properties of partially saturated carbon and carbon nitride nanothreads and analyze how they differ from those of conventional fully saturated NTs. The results show that partially saturated NTs have lower ideal strengths and stiffness compared to their fully saturated versions, but they are still remarkably strong. The electronic behavior varies from semiconducting to insulating, with band gaps in the range ∼1.8–4.0 eV, while fully saturated NTs usually have wider gaps (>4.0 eV). These results show that partially saturated nanothreads can be used for the same applications previously suggested for fully saturated NTs on the basis of their outstanding mechanical strength, and novel applications may be envisioned due to their wider range of possible band gaps.