Formation of Quantum-Dot Quantum-Well Heteronanostructures with Large Lattice Mismatch: ZnS/CdS/ZnS
R B. Little, M El-Sayed, Garnett W. Bryant, S Burke
Two-dimensional, layered superlattice structures have been exploited extensively in the synthesis of quality optoelectronic devices. Structures with small lattice-mismatch can be synthesized readily. Large lattice-mismatch in II-VI film superlattice structures has prevented the synthesis of quality optoelectronic devices from these materials. 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 systems. In this work, small II-VI nanoparticles are studied. We show that high quality II-VI superlattice structures 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 nano-epitaxy on them to produce high quality ZnS/CdS core/shell quantum dot quantum well heteronanostructures with extremely curved ZnS/CdS interfaces. The highly curved interfaces allow for relief of the large lattice-mismatch. These structures are characterized by UV-visible absorbance. The measured spectra are compared with electronic level structures calculated by use of a tight-binding model for the different fabricated heteronanostructures. The consistency of the observed spectra and the predicted transitions confirms that the desired core/shell and core/shell/clad structures were grown. Although the CdS/ZnS core/shell structure is thermodynamically stable to alloying, the ZnS/CdS structure is unstable. The observed metastability of the ZnS/CdS/ZnS heteronanostructures is attributed to low temperature construction and small crystal-size (< 3 nm). The small size produces extreme surface forces and ZnS core contraction. The small size of the structure also accommodates strain, as a result of the high ZnS/CdS interfacial curvature with large surface tension, which is not possible for bulk systems. Furthermore, this new structure is kinetically stabilized against alloying by the large size difference between the Cd2+ ion and Zn2+ ions. All of these factors contribute to the formation of high quality quantum-dot quantum-well ZnS/CdS/ZnS heteronanostructures with novel optical properties.
, El-Sayed, M.
, Bryant, G.
and Burke, S.
Formation of Quantum-Dot Quantum-Well Heteronanostructures with Large Lattice Mismatch: ZnS/CdS/ZnS, Journal of Chemical Physics
(Accessed February 21, 2024)