Two-dimensional silicon crystals with sizable band gaps and ultrahigh carrier mobility

Due to their compatibility in the well-developed Si-based semiconductor industry, exploring two-dimensional (2D) silicon crystals with both sizable band gaps and high carrier mobility is important to develop high-performance electronic and optoelectronic devices on the nanoscale. Here, eleven new 2D silicon crystals are reported based on the strategy of mixing 3-fold and 4-fold coordinated silicon atoms in 2D confined phases and first-principles calculations. We establish that these 2D silicon crystals can be obtained by functionalizing silicene with silicon atoms, dimers, or chains, which exhibit lower formation energy than that of silicene. Their dynamic stability and thermal stability are confirmed by phonon calculations and Born-Oppenheimer molecular dynamic simulation at temperatures up to 700 K. Electronic structure calculations reveal that these 2D silicon crystals are semiconductors with sizable and tunable band gaps, ranging from 1.12 to 1.67 eV, and four of them are direct or quasi-direct band gap semiconductors with strong absorption in the visible-light frequency. The calculated Young's stiffness of 2D silicon crystals ranges from 31 to 88 N m^-1, which are comparable to phosphorene, but remarkably smaller than those of MoS₂ monolayer and graphene. Remarkably, C_z-P2/c-Si₁₂ possesses a negative Poisson's ratio with a maximum value of -0.055. In particular, 2D silicon crystals possess ultrahigh carrier mobility of up to 1.7 × 10⁵ and 1.3 × 10⁴ cm² V^-1 s^-1 at room temperature for electrons and holes, respectively, suitable for high-speed electronic and optoelectronic applications on the nanoscale.