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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 to 0.6 eV and the system can be a direct or an indirect semiconductor or even a half-metal depending on surface coverage. (2) Unlike graphene, the hydrogenation of BN sheet is endothermic. (3) Unlike graphene, BN sheet has heteroatomic composition. When codecorated with H and F, it can lead to anisotropic structures with rich electronic and magnetic properties. (4) Unlike graphene, BN …