Science of 2.5 dimensional materials: paradigm shift of materials science toward future social innovation

Abstract

The past decades of materials science discoveries are the basis of our present society – from the foundation of semiconductor devices to the recent development of internet of things (IoT) technologies. These materials science developments have depended mainly on control of rigid chemical bonds, such as covalent and ionic bonds, in organic molecules and polymers, inorganic crystals and thin films. The recent discovery of graphene and other two-dimensional (2D) materials offers a novel approach to synthesizing materials by controlling their weak out-of-plane van der Waals (vdW) interactions. Artificial stacks of different types of 2D materials are a novel concept in materials synthesis, with the stacks not limited by rigid chemical bonds nor by lattice constants. This offers plenty of opportunities to explore new physics, chemistry, and engineering. An often-overlooked characteristic of vdW stacks is the well-defined 2D nanospace between the layers, which provides unique physical phenomena and a rich field for synthesis of novel materials. Applying the science of intercalation compounds to 2D materials provides new insights and expectations about the use of the vdW nanospace. We call this nascent field of science ‘2.5 dimensional (2.5D) materials,’ to acknowledge the important extra degree of freedom beyond 2D materials. 2.5D materials not only offer a new field of scientific research, but also contribute to the development of practical applications, and will lead to future social innovation. In this paper, we introduce the new scientific concept of this science of ‘2.5D materials’ and review recent research developments based on this new scientific concept.

Twist Angle-Dependent Molecular Intercalation and Sheet Resistance in Bilayer Graphene

“…Another STEM study indicated that new FeCl 2 and FeOCl phases form when BLG is intercalated with FeCl 3 . Therefore, crystalline structures and phase transitions that have never been reported are expected in the confined 2D nanospace of BLG, providing the opportunity to extend a field of materials science …”

“…Intercalation of Na ions, which is expected to be the basis for a new type of battery, has been demonstrated using stacks of functionalized graphene sheets . Thus, the use of the interlayer nanospace further expands the potential applications of the multilayers of 2D materials …”

“…In addition to these twist angle-dependent phenomena, BLG has a well-defined two-dimensional (2D) interlayer nanospace that can be used to intercalate molecules or ions. 8 Intercalation can drastically change the electronic and magnetic properties of the host BLG while maintaining high optical transparency and mechanical flexibility. Initially, the FeCl 3 molecule was intercalated into mechanically exfoliated flakes of few-layer graphene containing BLG regions.…”

“…19 Thus, the use of the interlayer nanospace further expands the potential applications of the multilayers of 2D materials. 8 Confinement of molecules in the interlayer nanospace of BLG has revealed ordered structures not seen in their bulk counterparts. Scanning transmission electron microscopy (STEM) observation revealed that AlCl 3 molecules intercalated in BLG have three ordered structures, differing from the crystal structure of bulk AlCl 3 .…”

Metal Hydroxide Salt Monolayer Nanoparticles: Synthesis, Redox Characterization, and Electrochemical Catalytic Performance

“…Furthermore, surface modification of two-dimensional materials by using low molecular weight species was found to enable tuning of the physical properties . These designs of two-dimensional-based materials have started to be investigated widely …”