Characterization and device applications of II-VI nanocomposites

Abstract

This thesis describes the synthesis and characterization of CdSe core - ZnS shell (CdSe)ZnS nanocrystals, which behave as quantum dots (QDs), their incorporation into transparent polymers and ZnS, and possible device applications of the resulting luminescent materials. The crystalline structure of these quantum dots is examined using XRD. The nature of the core-shell interface is inferred by comparison of the XRD spectra with calculated spectra and in consideration of TEM images of the QDs. The influence the addition of a shell material over the CdSe core has on the QD exciton energy is examined by comparing the measured and calculated change in band edge emission energy when ZnS or CdS are used as overcoat materials. Methods of incorporating the QDs into various polymers are considered and the emission of a GaN LED/ QD-containing polymer composite structure is simulated and compared with the output of actual devices. The simulation is also used as a design tool in the production of a white-light LED. Embedding the QDs into ZnS films via an electrospray technique is carried out and the resulting films characterized with x-ray diffraction and photoluminesence measurements. QD film / ZnS film structures are also considered, with atomic layer deposition used as the method for depositing the ZnS to improve the film uniformity as well as to maintain high quality deposition at temperatures low enough to be compatible with the presence of QDs. Finally, thin film devices are synthesized and the behavior of the QD photoluminescence when an electric field is applied is studied.

(cont.) Significant levels of photoluminescence quenching is observed when a voltage is applied across an ITO/ZnS/QD/ZnS/Al, and similar, devices. Various reasons for this quenching are considered.