Formation of quantum-dot quantum-well heteronanostructures with large lattice mismatch: ZnS/CdS/ZnS

Two-dimensional heterostructures have been exploited extensively in the syn thesis of optoelectronic devices. Structures with small lattice mismatch ca n be synthesized readily. Large lattice mismatch in II-VI film heterostruct ures makes synthesis of devices with these materials more difficult. Howeve r, these large mismatch heterostructures usually have useful optical proper ties. One such heterostructure is the ZnS/CdS system with a large exciton b inding energy and a large band gap useful for blue-green emitting devices. In this work, small II-VI nanoparticles are studied. We show that II-VI het erostructures can be made in quantum dots, despite the large bulk lattice m ismatch. Two well-known techniques are combined to synthesize first very sm all ZnS and CdS seed nanoparticles and then do nanoepitaxy on them to produ ce ZnS/CdS core/shell quantum-dot quantum-well heteronanostructures. These structures are characterized by UV visible absorbance. Measured spectra are compared with electronic level structures calculated for the fabricated he teronanostructures with a tight-binding model. The consistency of the obser ved spectra with the predicted transitions indicates that the desired core/ shell and core/shell/clad structures were grown. The metastability of the Z nS/CdS/ZnS heteronanostructures is attributed to low-temperature constructi on and small crystal size (<3 nm). The small particle size should produce l arge surface forces and ZnS core contraction. Also, the small particle size should accommodate strain, as a result of the ZnS/CdS interfacial curvatur e, which is not possible for planar systems. Furthermore, this new structur e is kinetically stabilized against alloying by the large size difference b etween the Cd2+ ion and Zn2+ ions. We suggest that all of these factors con tribute to the formation of quantum-dot quantum-well ZnS/CdS/ZnS heteronano structures. (C) 2001 American Institute of Physics.