Bringing Mosquito Net Patterns to Nanomaterials: UNIST Develops MOF-Based Nano Patterning Technology

Precise Control of Moire Patterns Achieved Using MOFs
Unlocking Tunable Quasiperiodic Structures for Twistronics and Quantum Materials

If you overlap mosquito nets in a crisscross pattern, you will see undulating patterns appear. These are called Moire patterns, created by interference as the grids of the upper and lower mosquito nets are misaligned.

While this is simply an optical illusion in everyday life, in the nanoscale world of materials like graphene, such patterns can actually alter the pathways through which electrons move. By controlling these patterns, it is possible to develop new nanoscale devices and quantum materials. However, because atoms are fixed in place, it has been difficult to change these patterns.

Now, for the first time in the world, a Korean research team has developed a system that can precisely control the length of Moire patterns at the molecular level using metal-organic frameworks (MOFs). This achievement provides a level of tunability that was difficult to attain in conventional graphene-based structures, and the team succeeded in creating even complex symmetric patterns with great precision.

On August 18, a team led by Professor Wonyoung Choi of the Department of Chemistry at UNIST, in collaboration with Professor Jihan Kim of KAIST and Professor Seona Park of POSTECH, announced that they had used MOFs to precisely control the periodicity of Moire patterns at the molecular level.

Research team: Professor Wonyoung Choi of UNIST (corresponding author), Professor Jihan Kim of KAIST, Professor Seona Park of POSTECH, and Dr. Jiyeon Kim of UNIST (first author). Provided by UNIST

The research team took advantage of the high degree of design freedom offered by MOFs. MOFs are nanomaterials in which metal ions and organic molecules are combined in a mesh-like structure. By changing the type and length of the organic molecules, the size and spacing of the mesh can be controlled.

The researchers created paper-thin layers of zirconium (Zr)-based MOFs, stacking them at different angles so that the upper and lower MOF layers were misaligned. Transmission electron microscopy revealed that the shape and periodicity of the patterns changed depending on the misalignment angle between the two MOF layers and the length of the organic molecules.

Mathematical analysis also revealed the presence of quasiperiodic symmetric structures hidden within the Moire patterns. When the two MOF layers were rotated by 30 degrees, a dodecagonal symmetric structure-known as the 'Stampfli tiling' pattern, similar to the stained glass of Gothic cathedrals or the arabesque decorations of mosques-emerged. Although these patterns may appear regular at first glance, they are in fact quasiperiodic, meaning that no arrangement is ever exactly repeated.

Dr. Jiyeon Kim (currently a postdoctoral researcher at the Max Planck Institute), the first author, explained, "Quasiperiodic patterns can induce subtle changes in electron movement, allowing for more precise design of the electronic and optical properties of Moire structures."

Comparison of Quasiperiodic Length Scales in Various Systems.

The research team also theoretically verified that these patterns are not accidental but are, in fact, stable structures. Professor Jihan Kim's team at KAIST performed molecular dynamics (MD) simulations to calculate the relative energy stability of the MOF bilayer structures formed at various rotation angles. Some of these structures were found to be the most energetically stable, supporting the experimental results.

Professor Wonyoung Choi emphasized, "Because MOFs can be structurally designed at the molecular level, it is possible to freely control the Moire periodicity as if turning a dial. This achievement paves the way for the development of twistronics and new types of quantum materials."

Twistronics is a technology that manipulates electronic properties by stacking two layers of material at a twist to create Moire patterns.

The research results were published in the August 13 issue of the international journal Nature Communications.

This research was supported by the National Research Foundation of Korea and the Institute for Information & Communications Technology Planning & Evaluation, under the Ministry of Science and ICT.

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