Scientists are exploring new ways to artificially stack two-dimensional (2D) materials, introducing so-called 2.5D materials with unique physical properties. Japanese researchers reviewed the latest advances and applications of 2.5D materials in the journal Advanced Materials Science and Technology.
“The 2.5D concept symbolizes freedom from composition, materials, angles, and space typically used in 2D materials research,” says nanomaterials scientist and lead author Hiroki Ago of the University of Kyushu in Japan.
2D materials, like graphene, are made of a single layer of atoms and are used in applications such as flexible touch panels, integrated circuits and sensors.
Recently, new methods have been introduced to allow 2D materials to be artificially stacked vertically, in-plane or at twisted angles, regardless of their compositions and structures. This is thanks to the ability to control van der Waals forces: weak electrical interactions between atoms and molecules, similar to the attraction of dust to a microfiber cloth. It is also now possible to integrate 2D materials with other dimensional materials, such as ions, nanotubes and bulk crystals.
A common method of making 2.5D materials is chemical vapor deposition (CVD), which deposits one layer, one atom or molecule at a time, onto a solid surface. Commonly used building blocks for 2.5D materials include graphene, hexagonal boron nitride (hBN) (a compound used in cosmetics and aerospace), and transition metal dichalcogenides (TMDC) (a semiconductor to nanosheets).
Using the CVD method, the researchers selectively synthesized a bilayer of graphene, the simplest form of a 2.5D material, using copper-nickel foil with a relatively high nickel concentration as a catalyst. The nickel makes the carbon highly soluble, giving researchers more control over the number of graphene layers. When an electric field was applied vertically across the graphene bilayer, it opened a band gap, meaning its conductivity can be turned on and off. This is a phenomenon that is not observed in single-layer graphene because it has no band gap and stays on all the time. By tilting the stacking angle by one degree, the scientists discovered that the material became superconducting.
Similarly, another group in the UK and US found that a layer of graphene and hBN drives the quantum Hall effect, a conduction phenomenon involving a magnetic field that produces a potential difference. Others have shown that the stacking of TMDCs traps excitons (paired electrons with their associated holes in a bound state) in the overlapping lattice patterns. This may lead to applications in information storage devices. New robotic assembly techniques have also made it possible to build more complex vertical structures, including a stacked heterostructure composed of 29 alternating layers of graphene and hBN, for example.
Other research has used the nanospaces that form between layers of a 2.5D material to insert molecules and ions to improve the electrical, magnetic and optical properties of the host material.
So far, for example, researchers have found that graphene stabilizes iron chloride when inserted between its stacked layers, while the insertion of lithium ions leads to a faster rate of diffusion (the speed with which molecules propagate in an area) than that of graphite, an electrical conductor. used in batteries. This implies that the material could be used in high performance rechargeable batteries.
Additionally, the researchers found that inserting aluminum chloride molecules between two sheets of graphene results in the formation of new crystal structures that are completely different from the bulk aluminum chloride crystal. Further research is needed to understand why this happens and what applications it might have.
“There are many opportunities to explore with this new 2.5D concept,” says Ago.
Future applications for 2.5D materials include solar cells, batteries, flexible devices, quantum devices, and ultra-low power consumption devices.
The next steps should integrate machine learning, deep learning, and materials computing to advance 2.5D materials design and synthesis.
News Article: New Study Reveals Unexpected Softness of Bilayer Graphene
Hiroki Ago et al, 2.5-Dimensional Materials Science: Paradigm Shift from Materials Science to Future Social Innovation, Advanced Materials Science and Technology (2022). DOI: 10.1080/14686996.2022.2062576
Provided by Kyushu University
Quote: A new age of 2.5D materials (May 6, 2022) retrieved May 6, 2022 from https://phys.org/news/2022-05-age-25d-materials.html
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