MXenes are attracting attention due to their rich chemistry and intriguing properties. Here a new type of metal-carbon-based sheet composed of transition metal centers and C2 dimers rather than individual C atom is designed. Taking the Ti system as a test case, density functional theory calculations combined with a thermodynamic analysis uncover the thermal and dynamic stability of the sheet, as well as a metallic band structure, anisotropic Young's modulus and Poisson's ratio, a high heat capacity, and a large Debye stiffness. Moreover, the TiC2 sheet has an excellent Li storage capacity with a small migration barrier, a lower mass density compared with standard MXenes, and better chemical stability as compared to the MXene Ti2C sheet. When Ti is replaced with other transition metal centers, diverse new MC2 sheets containing C=C dimers can be formed, the properties of which merit further investigation.

The phase controlled synthesis of iron carbide nanoparticles was proposed through a thermodynamical and dynamical manner by introducing hetero-halide ions.

Going beyond conventional hexagonal sheets, pentagonal 2D structures are of current interest due to their novel properties and broad applications. Herein, for the first time, we study a ternary pentagonal BCN monolayer, penta-BCN, which exhibits intrinsic piezoelectric properties. Based on state-of-the-art theoretical calculations, we find that penta-BCN is stable mechanically, thermally, and dynamically and has a direct band gap of 2.81 eV. Due to its unique atomic configuration with noncentrosymmetric and semiconducting features, penta-BCN displays high spontaneous polarization of 3.17 × 10 −10 C/m and a prominent piezoelectricity with d 21 = 0.878 pm/V, d 22 = −0.678 pm/V, and d 16 = 1.72 pm/V, which are larger than those of an h-BN sheet and functionalized pentagraphene. Since B, C, and N are rich in resources, light in mass, and benign to the environment, the intrinsic polarization and piezoelectricity make the penta-BCN monolayer promising for technological applications. This study expands the family of 2D pentagonal structures with new features.

Super-alkalis are clusters of atoms. With ionization potentials smaller than those of the alkali atoms, they are playing an increasing role in chemistry as highlighted by recent applications in solar cells as well as in Li-ion batteries. For the past 40 years superalkalis were designed using inorganic elements with the sp orbital character. Here, we show that a large class of superalkalis composed of only simple metal atoms, transition metal complexes as well as organic molecules can be designed by making use of electron counting rules beyond the octet rule. Examples include Al, Mn(BNH), BCH, and CNH which obey the jellium shell closure rule, the 18-electron rule, the Wade-Mingos rule, and Hückel's aromatic rule, respectively. We further show that the ability of superalkalis to transfer an electron easily can be used to activate a CO molecule by transforming it from a linear to a bent structure. These results, based on density functional theory with generalized gradient approximation for exchange-correlation potential, open the door to a new class of catalysts for CO activation.