Quantum mazes: luminescent labyrinthine semiconductor nanocrystals having a narrow emission spectrum.

We exploit the polytypism of group II-VI semiconductors and the long-range dipolar interactions typical of CdSe nanoparticle formation to modulate the geometrical structure and the optical emission properties of novel branched CdSe nanocrystals through shape-dependent quantum confinement effects. X-ray diffraction confirms that these materials incorporate crystalline domains of cubic zinc-blende and hexagonal wurtzite within a polycrystalline growth form whose geometry can be controlled by varying thermodynamic conditions. In particular, labyrinthine-shaped nanoparticles of tunable dimensions are reproducibly synthesized based on a heterogeneous reaction between cadmium acetate in a solution in hexadecylamine and trioctylphosphine with Se as a solid precursor at a relatively low temperature (110 degrees C). The resulting highly branched CdSe structures resemble labyrinthine patterns observed in magnetic fluids and superconductors films in magnetic fields, and in lipid films and other materials where strong dipolar interactions "direct" large-scale pattern formation. Surprisingly, these novel maze-like structures emit light within a narrow bandwidth (full-width at half-maximum approximately equal to 33-42 nm) of the visible spectrum (508 nm < lambda < 563 nm), so the regular dimensions of the core regions of these branched structures govern their emission characteristics rather than overall nanoparticle size. This property should make these materials attractive for applications where luminescent materials having tunable emission characteristics and a narrow emission frequency range are required, along with the insensitivity of the particles' luminescent properties to environmental conditions.