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 [<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mn>2</a:mn><a:mo>+</a:mo><a:mn>2</a:mn></a:mrow></a:math>] 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:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msub><c:mrow><c:mi mathvariant="normal">C</c:mi></c:mrow><c:mrow><c:mn>60</c:mn></c:mrow></c:msub></c:mrow></c:math> with tunable thermal expansion. These findings shed light on the fundamental mechanisms governing thermal expansion in molecular networks towards rational materials design.