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

Two-dimensional heterostructures have been exploited extensively in the synthesis of optoelectronic devices. Structures with small lattice mismatch can be synthesized readily. Large lattice mismatch in II-VI film heterostructures makes synthesis of devices with these materials more difficult. However, these large mismatch heterostructures usually have useful optical properties. One such heterostructure is the ZnS/CdS system with a large exciton binding 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 heterostructures can be made in quantum dots, despite the large bulk lattice mismatch. Two well-known techniques are combined to synthesize first very small ZnS and CdS seed nanoparticles and then do nanoepitaxy on them to produce 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 heteronanostructures with a tight-binding model. The consistency of the observed spectra with the predicted transitions indicates that the desired core/shell and core/shell/clad structures were grown. The metastability of the ZnS/CdS/ZnS heteronanostructures is attributed to low-temperature construction and small crystal size (<3 nm). The small particle size should produce large surface forces and ZnS core contraction. Also, the small particle size should accommodate strain, as a result of the ZnS/CdS interfacial curvature, which is not possible for planar systems. Furthermore, this new structure is kinetically stabilized against alloying by the large size difference between the Cd2+ ion and Zn2+ ions. We suggest that all of these factors contribute to the formation of quantum-dot quantum-well ZnS/CdS/ZnS heteronanostructures. (C) 2001 American Institute of Physics.