Elemental Semimetal Ferroelectricity in Buckled Carbon Monolayers: Implications for Flexible Field-Effect Transistors

The discovery of ferroelectricity in elemental bismuth monolayer motivated us to look for elemental ferroelectricity in other monolayers. Based on two experimentally synthesized two-dimensional (2D) carbon structures, biphenylene and net-graphene, we used first-principles calculations to predict three carbon monolayers with a buckled phase: 4–6–12 biphenylene, 4–6–8–16 biphenylene-yne, and 4–6–7–5 net-graphene. The total energy of 4–6–7–5 net-graphene is −8.84 eV/atom, much lower than that of experimental biphenylene (−8.75 eV/atom), suggesting its feasibility for experimental synthesis. Hybrid orbital theory analysis shows that the three monolayers exhibit a buckled structure characterized by quasi- sp 2 and quasi- sp - sp 2 hybridization, different from the previous linear/planar phase with sp / sp 2 hybridization. The buckling breaks the symmetry of the monolayer, resulting in out-of-plane ferroelectricity with an experimentally observable polarization value. Furthermore, we show that the reversal of the polarization direction can be induced by applying an external uniaxial strain, which can act as electromechanical coupling in device applications. Interestingly, the three monolayers are semimetals, which is in contrast to the traditional view that metallicity and ferroelectricity are antagonistic. These findings highlight the potential of 2D elemental ferroelectric carbon materials, which combine the unique properties of ferroelectricity with the versatility of carbon elements. Therefore, such materials hold promise as potential candidates for ferroelectric field-effect transistors.