Stable and monodisperse Au nanoparticles smaller than 2 nm are easily prepared by the reduction of HAuCl4 center dot 4H(2)O in DMF/H2O in the presence of a series of bidentate ligands, 2,6-bis(1'(n-thioalkyl)-benzimidazol- 2-yl)pyridine (TCnBIP, n = 3, 6, 8, 10, 12). The TCnBIP ligands afford stronger coordination ability than alkanethiols due to their bidentate nature. These small nanoparticles form hexagonal close-packed (hcp) two-dimensional (2D) superlattices with tunable interparticle spacings (from 1.2 to 2.5 nm), produced by changing the length of the ligand at both the hydrophobic amorphous carbon and the air-water interface. Long-range-ordered hcp 2D superlattices were fabricated through the cleavage and construction of interligand pi-pi interactions formed via an annealing process at the air-water interface.