  We adopt a global optimization method to predict two-dimensional (2D)
nanostructures through the particle-swarm optimization (PSO) algorithm. By
performing PSO simulations, we predict new stable structures of 2D boron-carbon
(B-C) compounds for a wide range of boron concentrations. Our calculations show
that: (1) All 2D B-C compounds are metallic except for BC3 which is a magic
case where the isolation of carbon six-membered ring by boron atoms results in
a semiconducting behavior. (2) For C-rich B-C compounds, the most stable 2D
structures can be viewed as boron doped graphene structures, where boron atoms
typically form 1D zigzag chains except for BC3 in which boron atoms are
uniformly distributed. (3) The most stable 2D structure of BC has alternative
carbon and boron ribbons with strong in-between B-C bonds, which possesses a
high thermal stability above 2000K. (4) For B-rich 2D B-C compounds, there is a
novel planar-tetracoordinate carbon motif with an approximate C2v symmetry.
