Fullerene-based low-density superhard materials with tunable bandgaps

Highlights: • Four carbon allotropes built from tetrahedral symmetrical fullerenes C₂₈ and C₄₀ are predicted to be superhard materials with mass density around that of water, and all of them are porous semiconductors. • Both the bandgaps and hardness decrease with increasing ratio of sp² hybridized carbon atoms from diamond, T-carbon, C₂₈- and C₄₀-based allotropes. • It is suggested that a low-density hard material with tunable bandgaps can be obtained by substituting the carbon atom in diamond with different T_d-symmetrical non-IPR fullerene C_n. Four carbon allotropes built from tetrahedral symmetrical fullerenes C₂₈ and C₄₀ are predicted to be superhard materials with mass density around that of water, and all of them are porous semiconductors. Both the bandgaps and hardness decrease with increasing ratio of sp² hybridized carbon atoms. The mechanical and thermodynamic stabilities of C₂₈- and C₄₀-based allotropes at zero pressure are confirmed by a variety of state-of-the-art theoretical calculations. The evolution trend of bandgap found here suggests that one can obtain low-density hard materials with tunable bandgaps by substituting the carbon atom in diamond with different T_d-symmetrical non-IPR fullerene C_n.