Two-dimensional van der Waals C60 molecular crystal

Two-dimensional (2D) atomic materials, such as graphene, hexagonal boron nitride, silicene, phosphorene, transition metal dichalcogenides and layered oxides, have drawn considerable attention recently due to their unique crystalline structures and fascinating properties1,2,3,4,5. In these 2D atomic materials, covalent bonds make them strong within their monolayers; while relatively weak van der Waals (vdW) forces across these monolayers enable great flexibility for design and assembly. To fully utilize their various properties in different 2D atomic materials, it was proposed to stack different 2D materials via van der Waals forces to achieve extraordinary properties and novel functionalities5,6. On the other hand, in molecular solids, molecules are also held together in bulk form by weak vdW forces7. These solids are of low density and low stiffness compared to their atomic counterparts7,8. Remarkably, the 2D form of molecular solids has been synthesised through self-assembly on various substrates9,10,11,12,13,14,15. It is noted that the self-assembly of the vdW molecular crystals on substrate depends on many factors, such as surface properties, adhesion energies, molecular size and shape, orientations, pressure, temperature and solvents used to disperse the molecules9,10,11,12,13. Currently, forming a well-defined free-standing molecular monolayer remains a daunting task due to the weak binding forces between its constituent molecules9. Recently, 2D organic molecular crystals have been synthesized and several interesting applications in novel devices have been demonstrated14,15,16,17,18,19,20,21. It is noted that the C60 molecules have been used as a basic building block in many recent applications, for example, in organic photovoltaics14,16, molecular electronics15,17, cosmetics and healthcare18,19,20,21,22,23. Although the C60 molecules have been self-assembled into a stable monolayer thin film on substrates,11,14 the existence of the free-standing van der Waals molecular monolayer crystal, that is, without the support of a substrate, remains unknown. Before the seminal work by Geim and co-workers24, it was commonly believed that free-standing 2D atomic materials are thermodynamically unstable. Their work, however, proved the existence of the free-standing monolayer atomic crystal and thus busted the previous belief24. Therefore, a few interesting questions arise: Can the C60 molecules form a stable free-standing 2D molecular monolayer crystal? If the answer is yes, then, up to what temperature? What are the mechanical and failure behaviour of the monolayer? What are its electronic characteristics? Clearly, answers to these questions are not only of scientific significance but also of technological impact. Thus, studying the stability of the C60 molecular monolayer and revealing their mechanical and electronic properties constitute the subject of the present study.