Architecture of Nanocrystal Building Blocks

Crystals consist of a periodic alternation of specific repeating molecules. The individual repeating molecules have quantized electronic structures while crystals have continuous electronic band structures that result from the overlap and combination of molecular orbitals of repeating molecules. Therefore, isolated molecules exhibit quantum mechanical properties, while the chemical and physical properties of bulk crystals obey the laws of classical mechanics. However, when the crystal size decreases into the nano-scale regime (1 ∼ 100 nm), the electronic band of the crystals starts to be quantized and the resulting nanocrystals behave as an intermediate between molecules and crystals.2–5 These nanocrystals exhibit novel properties which differ from both molecular and bulk properties. For example, CdSe semiconductor crystals on the 10 nm scale, the characteristic red luminescence is no longer observed but the luminescence can be continuously tuned from red to blue by varying the crystal size. The melting temperature of nanocrystals simultaneously decreases when the nanocrystal size is reduced.6,7 In the nanoscopic world, crystal properties are highly dependent on the size, shape, and surface state of the crystals.