(color online) (a) A unit cell cut directly into the (0001) plane of optimized bulk wurtzite AlN (a = 3.126 Å and c = 5.017 Å) structure. (b) Schematic view of structure for 4× 4× 1 AlN nanosheet surpercells, and the distance (Δd) between X atom and AlN nanosheet plane. (c) Schematic view of structure for 6× 6× 1 AlN nanosheet supercells. Pink and dark blue spheres represent Al and N atoms, respectively. We use i to indicate the dopant pair, X (0, i), depicted in Fig. 1(c), in which the first dopant atom occupies the fixed Al position, labeled 0 in Fig. 1(c), and the second dopant atom occupies the positions indicated by i = 1–4.