The search for new two-dimensional (2D) materials with novel optical and electronic properties is always desirable for material development. Herein, we report a comprehensive theoretical prediction of 2D SiC compounds with different stoichiometries from C-rich to Si-rich. In addition to the previously known hexagonal SiC sheet, we identified two types of hitherto-unknown structural motifs with distinctive bonding features. The first type of 2D SiC monolayer, including t-SiC and t-Si₂C sheet, can be described by tetragonal lattice. Among them, t-SiC monolayer sheet is featured by each carbon atom binding with four neighboring silicon atoms in almost the same plane, constituting a quasi-planar four-coordinated rectangular moiety. More interestingly, our calculations demonstrate that this structure exhibits a strain-dependent insulator–semimetal transition, suggesting promising applications in strain-dependent optoelectronic sensors. The second type of 2D SiC sheet is featured by silagraphyne with acetylenic linkages (–CC–). Silagraphyne shows both high pore sizes and Poisson's ratio. These properties make it a potentially important material for applications in separation membranes and catalysis. Moreover, one of the proposed structures, γ-silagraphyne, is a direct-band-gap semiconductor with a bandgap of 0.89 eV, which has a strong absorption peak in the visible-light region, giving a promising application in ultra-thin transistors, optical sensor devices and solar cell devices.

We identify two B–C bonding patterns: pyramidal-geometry tetra-coordinated and hexa-coordinated sp²carbon moiety; B₄C₃has an ultrahigh Young's modulus that can even outperform graphene; the B₂C sheet is metallic with a relatively high superconducting transition temperature (T_c≈ 21.20 K).

Significance: Artificial diamond plays a vital role in the manufacturing industry, jewelry, and future photoelectronic devices, but it is a key challenge to prepare the required large-area diamonds. A distinctive way to solve this problem possibly hides in the undiscovered formation mechanism of thermodynamically metastable diamond compared to graphite in low-pressure chemical vapor deposition. We design a series of short-term growth on the margins of cauliflower-like nanocrystalline diamond particles and find that diamond is formed by the transformation from graphite, not by the piling up of sp ³ carbon. Atomically dispersed Ta atoms let the transition spontaneously occur. This subverts the general knowledge and supplies a way to prepare large-area diamonds based on large-sized graphite under normal pressure.

By means of ab initio computations and the global minimum structure search method, we have investigated structural, mechanical, and electronic properties of D-carbon, a crystalline orthorhombic sp³ carbon allotrope (space group Pmma [${{\mathrm{D}}_{2h}}^5$ ] with 6 atoms per cell). Total-energy calculations demonstrate that D-carbon is energetically more favorable than the previously proposed T₆ structure (with 6 atoms per cell) as well as many others. This novel phase is dynamically, mechanically, and thermally stable at zero pressure and more stable than graphite beyond 63.7 GPa. D-carbon is a semiconductor with a bandgap of 4.33 eV, less than diamond’s gap (5.47 eV). The simulated X-ray diffraction pattern is in satisfactory agreement with previous experimental data in chimney or detonation soot, suggesting its possible presence in the specimen.