Professor Feng Ding's team at UNIST developed a single-crystal two-dimensional material.
Presentation of a theory of substrate cross-section shapes favorable for synthesis
Helps in the synthesis of two-dimensional materials, including graphene


Two-dimensional new materials like graphene are synthesized on a metal substrate, using it as a "field." This process involves attaching vapor-state raw materials to the substrate to form a thin film. This method requires careful substrate selection, as substrate characteristics impact the quality of the two-dimensional material.

On the 2nd, a research team led by Professor Feng Ding of the Department of Materials Science and Engineering at Ulsan National Institute of Science and Technology (UNIST) theoretically presented criteria for selecting a substrate when synthesizing two-dimensional materials in the form of single crystals.

Even for the same material, single-crystal forms are superior in quality to polycrystalline forms. Theoretical calculations suggest that high-Miller-index substrates, with their denser step-like structures across their cross-sections, are suitable for synthesizing two-dimensional materials in single-crystal form.

The Miller index is a mathematical representation of the cross-sectional shape of a material. Just as a watermelon has different cross-sectional shapes when cut lengthwise or cross-sectionally, a metal substrate like copper, with its atoms neatly arranged, also has different cross-sectional shapes depending on the cut direction. A high index surface, which is a diagonally cut cross-section, has a structure in which atoms are stacked one layer at a time like steps.

Professor Ding's research team previously discovered that high-Miller-index substrates are more suitable for synthesizing two-dimensional single-crystal materials than readily available low-Miller-index substrates. This time, they discovered that the denser the step-like structure, the easier it is to synthesize high-Miller-index substrates.

Contrary to theory, actual substrates are not perfectly flat and have uneven cross-sections. When the surface is uneven, the step edges are curved rather than straight. However, when the step structure is dense, the edge shape becomes closer to a straight line, which is advantageous for single crystal synthesis.

The research team discovered this by conducting computer simulations of synthesizing hexagonal boron nitride on high-Miller index copper substrates with various cross-sectional shapes. The simulation results also closely mirrored previous experimental results from two-dimensional material synthesis. “The results of this theoretical study demonstrate that high Miller index substrates should be actively utilized in the synthesis of two-dimensional single-crystal materials,” said first author Leining Zhang, a researcher.

Professor Fung Ding explained, “While the technology for synthesizing single-crystal silicon used in semiconductors has been studied for decades and a clear manufacturing method has been established, the technology for synthesizing newly emerged two-dimensional materials such as graphene and hexagonal boron nitride in single-crystal form is still in the early stages of research.”

He continued, “With the help of theoretical research that reduces the trial and error required to establish manufacturing technology for single-crystal two-dimensional materials, we expect to be able to develop technology to synthesize single-crystal two-dimensional materials in large quantities at a commercial level within 5 to 10 years.”

Two-dimensional materials are flat, flat materials just one atom thick. Representative examples include graphene, which is stronger than steel but also bends easily, and hexagonal boron nitride, which is gaining attention as a semiconductor insulator. The process of synthesizing two-dimensional materials as described above is called epitaxial synthesis.

This process involves three stages: first, the creation of crystal seeds; then, the growth of these single crystal seeds into small crystal fragments; and finally, the growth of these crystal islands, which then meet to form a single crystal. Polycrystalline materials are formed when the crystal islands grow in different directions.

The research was supported by the Institute for Basic Science (IBS), and the results were published on July 16 in the international academic journal Advanced Functional Materials.