Highly Organized Monolayer Arrangement of 2D Materials and Its Applications

2D materials, also termed 2D nanosheets, have attracted significant interest due to their unique molecularly thin 2D structure to exhibit various attractive properties. They include a diverse range of materials, such as graphene, chalcogenide, oxide, hydroxide, and carbide. Such 2D materials can be produced via the delamination of their precursor layered compounds. Different from graphite and van der Waals layered compounds, there are a wide range of layered materials accommodating interlayer counterions, serving as a trigger for delamination upon exchange with suitable species. Since interlayer galleries swell evenly and infinitely, single-layer nanosheets can be obtained in high yield in the form of a colloidal suspension.

The arrangement of unilamellar 2D nanosheets on a substrate surface, avoiding large gaps and overlaps, is crucial for fully harnessing their performance. A resulting monolayer film of neatly tiled 2D nanosheets can provide a molecularly thin interface and a well-defined crystalline surface, leading to the development of unique properties and reactivities. Consequently, considerable efforts have been focused on developing solution-based assembly techniques, including electrostatic self-assembly, the Langmuir–Blodgett (LB) method, and spin coating, to produce highly organized monolayer films.

In the electrostatic self-assembly process, a substrate with an oppositely charged surface is immersed in the nanosheet suspension, and nanosheets are adsorbed on the substrate through electrostatic attraction, forming a monolayer film of nanosheets in a self-assembly fashion. In the case of LB and spin coating methods, nanosheets trapped at the air–liquid interface are densely packed in a lateral direction to achieve neat monolayer tiling on a solvent surface, which is then transferred onto a substrate surface. Compared to the electrostatic self-assembled film, the LB method yields a higher-quality monolayer film of nanosheets without large gaps or overlaps thanks to the surface compression. Similar neat tiling has been achieved by using the spin coating method with optimized deposition parameters. The advantage of this method is its ability to fabricate the film in a shorter period (∼a few minutes), making it most suitable for practical use.

Neatly tiled monolayer films of nanosheets have been applied to modify the surface and interface properties of materials, as exemplified by the performance enhancement of batteries and epitaxial growth of crystalline thin films. Furthermore, the precise monolayer tiling serves as the fundamental step for constructing multilayer films of each nanosheet or even artificial lattice-like films, where nanosheets are stacked in a designed sequence, allowing for the evolution of sophisticated functionalities via synergetic coupling between constituent nanosheets. It has been demonstrated that heterostructured films, composed of various types of nanosheets, can enhance the individual properties of components and introduce novel functions. The integration of nanosheets with different properties using the methods outlined in this Account will lead to the realization of various next-generation devices.