Characterization and growth of doped-PbS in situ with Bi3+, Cd2+ and Er3+ ions by chemical bath

The growth of PbS nanocrystals doped in situ simultaneously with Er3+-Cd2+-Bi3+ solutions at different proportions was examined with the aim of investigating the morphological, optical and structural changes of the nanocrystals obtained. A brief summary of the chemical equilibria plus the Gibbs free energy changes are presented. Fourier transform infrared spectroscopy (FTIR) spectra showed an absorption band located at ~ 3356 cm−1 attributed to stretching of -OH groups and a sharp stretching band at ~ 1932 cm−1 assigned to vibration mode of CO32− ions.This absorption bands are generally due to the decomposition of thiourea in alkaline medium. X-ray diffraction (XRD) patterns exhibited a cubic phase in all films and the grain size (GS) of the undoped samples was ~6.57 nm, whereas for doped films was ~ 6.40-4.16 nm. Transmission Electron Microscopy (TEM) confirmed the structural characteristics of the nanocrystals. Absorbance spectra of PbS films exhibited five absorption bands located at ~ 346, ~ 585, ~ 615, ~ 670 and ~ 775 nm due to strong confinement effect. This multiplex of bands presented in PbS is induced by the simultaneous incorporation of the ions. The shift of the band gap energy is associated with the decrease of crystalline size in PbS films. On the other hand, a small absorption band located at ~ 446 nm, corresponding to 4F7/2 → 4I5/2 (4f → 4f transitions of Er3+ ions) was also observed. A small blue emission band centered at ~ 446 nm was originated from the allowed 3P1 → 1S0 transition of Bi3+ ions. Band gap energy of nanocrystals showed a shift in the ~ 1.4-2.6 eV range. The structural experimental results are associated with the optical changes confirming the gradual incorporation of the dopant ions in the crystallites network. Raman spectra showed two bands located at ~ 450 cm−1 due to the first overtone of the LO phonon (2LO) and other band located at ~ 200 cm−1 which has been attributed to the LO (Γ) phonon,confirming the existence of doped-PbS nanocrystals.