The nearly free electron states and the conductivity limited by electron-phonon scattering of an OH-terminated MXene material, a case study of the Hf 2 C(OH) 2 monolayer.

Reducing the electron-phonon scattering is always desirable for realizing high conductivity of actual materials at room temperature. It is seemingly feasible in some OH-terminated MXenes such as the Hf 2 C(OH) 2 monolayer, which hosts the so-called nearly free electron states (NFESs) near the Fermi energy. The NFESs are characterized by a large separation between the major electronic probability distribution and the atomic layer of MXenes. This implies that the NFESs suffer from a very weak electron-phonon scattering, hence the high conductivity at room temperature of these materials. We perform first principles calculations on the conductivity limited by the electron-phonon (e-ph) scattering of the Hf 2 C(OH) 2 monolayer. Our results indicate that the conductivity of the Hf 2 C(OH) 2 monolayer at room temperature is indeed higher than those of most of the MXene materials. However, such a high conductivity cannot be attributed to the existence of the NFESs because of their relatively low electronic band velocity. This conclusion is applicable to other OH-terminated MXene materials such as Zr 2 C(OH) 2 since their band structures around the Fermi energy are highly analogous. Our study suggests that both large band velocity and weak e-ph coupling are important for realizing ultrahigh conductivity facilitated by the NFESs in materials.