Effect of the size-induced structural transformation on the band gap in CdS nanoparticles

The interrelation between particle size, crystal structure and optical prop erties in semiconductor quantum dots has elicited widespread interest. We r eport the first attempt at relating the size-induced transformation from a hexagonal to a cubic structure in CdS nanoparticles to a change in the band gap. CdS nanoparticles with particle size in the 0.7-10 nm range were prep ared by chemical precipitation using thiophenol as a capping agent. Whereas the band gap for bulk hexagonal CdS is about 2.5 eV, that for 1 nm cubic C dS nanoparticles was found to be almost 3.9 eV. We also suggest a simple me chanism (based on the periodic insertion of stacking faults) for the transf ormation from the cubic zinc blende structure to the hexagonal wurtzite str ucture.