Room-temperature ferromagnetism and piezoelectricity in metal-free 2D semiconductor crystalline carbon nitride

Two-dimensional (2D) materials that combine ferromagnetic, semiconductor, and piezoelectric properties hold significant potential for both fundamental research and spin electronic devices. However, the majority of reported 2D ferromagnetic-semiconductor-piezoelectric materials rely on d-electron systems, which limits their practical applications due to a Curie temperature lower than room temperature (RT). Here, we report a high-crystallinity carbon nitride (CCN) material based on sp-electrons using a chemical vapor deposition strategy. CCN exhibits a band gap of 1.8 eV and has been confirmed to possess substantial in-plane and out-of-plane piezoelectricity. Moreover, we acquired clear evidences of ferromagnetic behavior at room temperature. Extensive structural characterizations combined with theoretical calculations reveal that incorporating structural oxygen into the highly ordered heptazine structure causes partial substitution of nitrogen sites, which is primarily responsible for generating room-temperature ferromagnetism and piezoelectricity. As a result, the strain in wrinkles can effectively modulate the domain behavior and piezoelectric potential at room temperature. The addition of RT ferromagnetic-semiconductor-piezoelectric material based on sp-electrons to the family of two-dimensional materials opens up numerous possibilities for novel applications in fundamental research and spin electronic devices.