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 (T_b) 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 T_b = 80 °C CdS is mainly produced, for 20 °C ≤ T_b ≤ 70 °C a composite of CdS + CdCO₃ is formed in the films, while at T_b = 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 T_b. Optical absorbance spectra allow to assign an indirect transition for CdCO₃ at 3.10 ± 0.03 eV when T_b = 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 T_b range. The CdS average crystallite size varied from 5 nm to 35 nm as T_b 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.