  First principles calculations based on density functional theory reveal some
unusual properties of BN sheet functionalized with hydrogen and fluorine. These
properties differ from those of similarly functionalized graphene even though
both share the same honeycomb structure. (1) Unlike graphene which undergoes a
metal to insulator transition when fully hydrogenated, the band gap of the BN
sheet significantly narrows when fully saturated with hydrogen. Furthermore,
the band gap of the BN sheet can be tuned from 4.7 eV to 0.6 eV and the system
can be a direct or an indirect semiconductor or even a half-metal depending
upon surface coverage. (2) Unlike graphene, BN sheet has hetero-atomic
composition, when co-decorated with H and F, it can lead to anisotropic
structures with rich electronic and magnetic properties. (3) Unlike graphene,
BN sheets can be made ferromagnetic, antiferromagnetic, or magnetically
degenerate depending upon how the surface is functionalized. (4) The stability
of magnetic coupling of functionalized BN sheet can be further modulated by
applying external strain. Our study highlights the potential of functionalized
BN sheets for novel applications.
