Simulation model of ideal primal 2D silicon with hexagonal lattice passivated by hydrogen atoms (white balls), showing (a) laterally cross-cut structure of ideal primal model in 2D structure with hexagonal lattice, (b) overlook structure of ideal primal model in 2D structure with hexagonal lattice, (c) silicene structure with hexagonal lattice after optimizing process in simulating calculation, in which convex bond angle of silicon atom is about 115°, and (d) TEM image of silicene crystal with hexagonal lattice observed in experiment.