Engineering of colloidal nanocrystal thin films and their optoelectronic properties: A simple and effective route

This work investigates the effect of heat treatments on CdSe/CdS core–shell nanocrystals assemblies with different sizes and capping ligands. Nanostructured thin films, consisting of an array of CdSe quantum dots embedded in a continuous matrix of CdS, have been obtained by mild thermal treatments of CdSe/CdS core–shell nanocrystals deposited on glass substrates from colloidal solution. These materials are regarded as promising photon upconverters, or absorbers for high-efficiency photovoltaic devices. The treatments were conducted in the range 300–550 °C for up to 150 min. The nanoscale evolution during the heat treatment was investigated by monitoring optical properties of the films. Initially, the well-defined photoluminescence band, corresponding to the first quantum dots exciton state, rapidly shifts towards lower energy, indicating delocalization of the exciton and suggesting that nanocrystal CdS shells are merging by sintering mechanisms, thus forming a continuous matrix. Control over this phase of the heat treatment leads to the desired nanoscale morphology and properties. The kinetics of sintering has been characterized, and the procedure has been systemically applied to CdSe/CdS nanocrystals of different sizes and ligand types. This research provides novel values for the activation energy of the sintering process and strongly support the general applicability of such process to control and optimize optoelectronic properties of nanostructured thin films for applications like light-emitting diodes, tunable emitters and photoconductors among others.