Rb. Little et al., Formation of quantum-dot quantum-well heteronanostructures with large lattice mismatch: ZnS/CdS/ZnS, J CHEM PHYS, 114(4), 2001, pp. 1813-1822
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.