Layered metal carbo-selenide Nb₂CSe₂ with van der Waals interlayer coupling

The stacking structure of Nb₂CSe₂, a newly synthesized layered metal carbo-selenide, is elucidated by scanning transmission electron microscopy. Nb₂CSe₂ features Se-Nb-C-Nb-Se quintuple atomic layers. These layers stacked in Bernal mode. According to the mode, Nb₂CSe₂ crystallizes in a trigonal symmetry (space group P3(_)m1, No. 164), with lattice parameters of a = 3.33 Å, and c = 18.20 Å. Electronic structure calculations indicate that the metal carbo-selenide has Fermi energy crossing the bands where they touch to give a zero gap, showing it is an electronic conductor. As evidenced experimentally, the electrical conductivity is as high as 6.6 ´ 10⁵ S m^-1, outperforming the counterparts in the MXene family. Due to the layered structure, the bonding in Nb₂CSe₂ with an ionic formula of (Nb^1.48+)₂(C^1.74-)(Se^0.61-)₂ is highly anisotropic with metallic–covalent–ionic bonding in intralayers while weak bonding between interlayers. The layered nature is further evidenced by elastic properties, interlayer energy, and friction coefficient determination. These characteristics recognize that Nb₂CSe₂ is exactly the analogue of MoS₂ that is the typical binary van der Waals solid (vdW). Moreover, vibrational properties are reported, which may offer an optical identification standard to the new ternary vdW solid in spectroscopic studies including Raman scattering and infra-red absorption.