Preparation and photoluminescence of SiO2-CdTe multilayer film by layer-by-layer self-assembly

Preparation parameters of layer-by-layer self-assembled films consisting of functional SiO2 layers and emitting CdTe quantum dots (QDs) layers were investigated. A surface-treated plate was successively dipped into three different solutions: surfactant (thioglycolic acid, TGA), colloidal QDs, and functional silicon alkoxide. Since the free TGA molecules in the first solution became linkers between the two kinds of layers, the condition of the first solution (TGA and Cd2+ ion concentrations and pH) affected the properties of the film. Under optimal conditions, the QDs were dispersed at a high concentration (0.01 M) in a closely packed manner without agglomeration. This resulted in a narrowing of the photoluminescence (PL) spectrum and a red shift of the peak wavelength due to reabsorption of the emission. The PL efficiency of films thus prepared with ten double layers (ca. ∼50 nm thick) was 25%. This high efficiency together with the high concentration resulted in a brightness much higher than that of a commercial phosphor when their thickness was same. However, when there were excess free TGA molecules in the first solution, the film had a higher concentration (0.02 M) and lower PL efficiency due to agglomeration of the QDs.