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Layered metal carbo–selenide Nb2CSe2 with van der Waals interlayer coupling
Journal of Advanced Ceramics 2025, 14(1): 9221008
Published: 17 January 2025
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The stacking structure of Nb2CSe2, a newly synthesized layered metal carbo-selenide, was elucidated by scanning transmission electron microscopy. Nb2CSe2 features Se−Nb−C−Nb−Se quintuple atomic layers. These layers are stacked in Bernal mode. In this mode, Nb2CSe2 crystallizes in a trigonal symmetry (space group P 3¯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 it touches to give a zero gap, indicating that 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. Owing to the layered structure, the bonding in Nb2CSe2 with an ionic formula of (Nb1.48+)2(C1.74−)(Se0.61−)2 is highly anisotropic, with metallic–covalent–ionic bonding in intralayers and weak bonding between interlayers. The layered nature is further evidenced by elastic properties, interlayer energy, and friction coefficient determination. These characteristics indicate that Nb2CSe2 is an analog of molybdenum disulfide (MoS2), which is a typical binary van der Waals (vdW) solid. Moreover, vibrational properties are reported, which may offer an optical identification standard for new ternary vdW solids in spectroscopic studies, including Raman scattering and infrared absorption.

Open Access Research Article Issue
Electron-irradiation-facilitated production of chemically homogenized nanotwins in nanolaminated carbides
Journal of Advanced Ceramics 2023, 12(6): 1288-1297
Published: 05 June 2023
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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.

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