Negative and Positive Anisotropic Thermal Expansion in 2D Fullerene Networks.

We find a design principle for tailoring thermal expansion properties in nearly spherical molecular networks. Using 2D fullerene networks as a representative system, we realize positive thermal expansion along intermolecular [2+2] cycloaddition bonds and negative thermal expansion along intermolecular C-C single bonds by varying the structural frameworks of molecules. The microscopic mechanism originates from a combination of the framework's geometric flexibility and its transverse vibrational characteristics. Based on this insight, we find molecular networks beyond C_{60} with tunable thermal expansion. These findings shed light on the fundamental mechanisms governing thermal expansion in molecular networks towards rational materials design.