High-quality graphite is one of the most significant advanced materials for many purposes, including being utilized as the light thermal conductor in cell phones. It also possesses good mechanical strength, thermal stability, high flexibility, and very high in-plane thermal and electric conductivities. For instance, Highly Ordered Pyrolytic Graphite (HOPG), a particular kind of graphite, is one of the most often used lab materials. These exceptional qualities come from the layered structure of graphite, where the strong covalent bonds between carbon atoms in a graphene layer contribute to the material’s excellent mechanical, thermal, and electrical conductivities, and the material’s extremely weak interactions with neighboring graphene layers cause it to be very flexible.
The quality of graphite samples, whether they come from natural sources or artificially created ones, is far from ideal, despite the fact that graphite has been known to exist in nature for more than 1000 years and that its artificial synthesis has been studied for more than 100 years. In contrast to the size of many crystals, like quartz single crystals and silicon single crystals, which can approach meter scale, the size of the biggest single crystalline graphite domains in graphitic materials is typically less than 1 mm. Due to the weak contact between graphite layers and the difficulty in maintaining the flatness of a graphene layer during the growing process, single-crystalline graphite is very tiny and easily breaks into a few single crystals with disordered grain boundaries.
A method to create single-crystalline graphite films that are orders of magnitude larger, up to inch scale, has been proposed by the Distinguished Professor of Ulsan National Institute of Science and Technology (UNIST) and his collaborators, Professor Kaihui Liu and Professor Enge Wang of Peking University, among others, to address the crucial problem. According to their method, caron atoms are supplied from the back side of single-crystalline Ni foils using an “isothermal dissolution-diffusion-precipitation process.” They decide to use solid carbon materials to feed the growth of graphite rather than a gas-phase carbon source. This innovative technique enables the production of 35 m-thick, or more than 100,000 graphene layers, single-crystalline graphite films that are 1 inch thick in a matter of days. In comparison to all other known graphite samples, the single crystalline graphite has the lowest layer distances, the lowest impurity concentrations, and a measured thermal conductivity of 2,880 Wm-1K-1.
“This accomplishment truly focuses on a few key aspects of the experimental design:
- Disorders in the manufactured graphite can be prevented by successfully synthesizing massive single-crystalline Ni sheets, which act as an ultra-flat substrate;
- The isothermal growth of 100,000 layers of graphene over a period of about 100 hours enables the synthesis of every layer of graphene in the exact same chemical environment and temperature, ensuring the homogeneity of the graphite quality;
-  “The contiguous growth of graphene layers at a very high growth rate—one layer every five seconds—is made possible by continuous carbon feeding via the reverse side of the Ni foil. Prof. Ding gave an explanation.”
The results of this study have been published in Nature Nanotechnology’s October 2022 edition. Professor Kaihui Liu and Professor Enge Wang from Peking University collaborated on this project.
