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.

Twin boundaries have been exploited to stabilize ultrafine grains and improve mechanical properties of nanomaterials. The production of the twin boundaries and nanotwins is however prohibitively challenging in carbide ceramics. Using a scanning transmission electron microscope as a unique platform for atomic-scale structure engineering, we demonstrate that twin platelets could be produced in carbides by engineering antisite defects. The antisite defects at metal sites in various layered ternary carbides are collectively and controllably generated, and the metal elements are homogenized by electron irradiation, which transforms a twin-like lamellae into nanotwin platelets. Accompanying chemical homogenization, α-Ti₃AlC₂ transforms to unconventional β-Ti₃AlC₂. The chemical homogeneity and the width of the twin platelets can be tuned by dose and energy of bombarding electrons. Chemically homogenized nanotwins can boost hardness by ~45%. Our results provide a new way to produce ultrathin (< 5 nm) nanotwin platelets in scientifically and technologically important carbide materials and showcase feasibility of defect engineering by an angstrom-sized electron probe.