Seminário Extra: Exploring the rhombohedral phase in natural graphite

Graphite is a 3D material which consists of a stacking of graphene planes (carbon atoms forming regular honeycomb hexagons) and with sp2 hybridizations. Crystalline graphite finds itself in two forms, the synthesized and widely reported HOPG (highly oriented pyrolytic graphite) and the natural graphite of mines, that shows also crystallinity. Particularly, the natural graphite from Nacional do Grafite in Minas Gerais (Brazil), have a very high quality, not only due to the crystallinity but also to the purity. The research group from Manchester in UK has used as their graphite source the natural graphite from these mines in Minas Gerais, where they were able to synthesize for the first time the two-dimensional (2D) system called Graphene. By use of a scotch tape and the micromechanical exfoliation technique, it was possible to separate the graphene monolayers, depositing them on several substrates. This 2D material showed an impressive physical property, like high electronic mobility and quantum effects at room temperature, which gave to the group of Prof. Geim and Novoselov the Nobel Prize in physics in 2010. Since then, several groups have tried to reproduce the graphene on an industrial scale. However the synthesis techniques do not allow obtaining an uniform graphene films in the micrometer order, the graphene properties are reproducible, depending directly on the type of graphite used, as reported by the group of Manchester. Therefore, we will try to exploit the importance of the structural properties of the natural graphite in several samples from Nacional do Grafite, with which we are developing a scientific technical cooperation. Recently, in collaboration with the University of Leipzig (Germany) has shown interesting new results regarding a possible superconducting phase in these natural graphite samples, providing hints that the effect would be associated with localized interfaces, that seems to act like a 2D granular superconductor with localized superconducting regions, connected by Josephson junctions with the critical temperature above the room temperature. These interfaces occur due to the presence of two crystalline regions at the internal structure of graphite, being one possibility the hexagonal (Bernal) stacking order (ABABAB…) and the other one the Rhombohedral (ABCABC…). The nature behind these interfaces with the rhombohedra stacking order were until now not well exploited by the scientific community as the possible origin of the superconducting behavior on graphite, since the rhombohedral phase appears more in natural graphite and with a maximum of 30% in relation with the Bernal phase. In this direction, an attempt to exploit these two present phases in graphite would help to understand better the nature of graphite and its behavior in a thermodynamic stable system.