A method of forming rare earth oxide nanocrystals include the steps of dissolving a rare earth including compound in a solution containing at least one organic solvent, heating the solution to a temperature of at least 160° C., wherein a concentration of the rare earth including compound provided upon decomposition is sufficient to provide critical supersaturation of at least one active intermediate in the solution to nucleate a plurality of rare earth oxide nanocrystals. The plurality of rare earth nanocrystals are then grown, wherein the growing step proceeds at least in part in the absence of critical supersaturation of the active intermediate. The rare earth nanocrystals can assemble into at least one close-packed, ordered nanocrystal superlattice.