White photoluminescence emission using CdS + CdCO3 composite thin films

The optical properties of CdS + CdCO3 composite thin films prepared by the chemical bath method on glass substrates, at different bath-temperatures (Tb) in the interval 20–80 °C, are reported. Chemical bath method allows to produce CdS + CdCO3 composite films controlling their composition through bath temperature. For Tb = 80 °C CdS is mainly produced, for 20 °C ≤ Tb ≤ 70 °C a composite of CdS + CdCO3 is formed in the films, while at Tb = 20 °C a predominantly CdCO3 in the bulk of the films along with nanostructured CdS (5 ± 2 nm size) is obtained. X ray diffraction patterns shown that CdS grows in the cubic zinc blende phase and CdCO3 in the rhombohedral structure; CdS and CdCO3 grow simultaneously and independently, but Raman measurements indicate how the proportion between CdS and CdCO3 depends on Tb. Optical absorbance spectra allow to assign an indirect transition for CdCO3 at 3.10 ± 0.03 eV when Tb = 20 °C and 2.80 ± 0.04 eV for 70 °C. Direct band gap values in the 2.50–2.80 eV interval were calculated for the seven CdS samples prepared across the entire Tb range. The CdS average crystallite size varied from 5 nm to 35 nm as Tb changed from 20 to 80 °C. The PL spectra show similar bands in the 1.6–2.9 eV (775 - 430 nm) interval indicating PL emissions are originated in CdS nanocrystals and probable defect states in CdCO3. The production of composite material with a major proportion of CdCO3 promotes the nucleation of CdS nanocrystals, at relative low temperatures, with an effective coupling improving emission properties of the films.