Optical, morphological and structural characterization of Er3 +-Bi3+ co-doped PbS nanocrystals grown by chemical bath

PbS nanocrystals co-doped simultaneously in Er3+and Bi3+ solutions were grown and the modification of morphological, structural and some optical properties was investigated. The thicknesses of the undoped and doped PbS films were in the ∼560-400 nm range. The morphological changes of the nanocrystals were analyzed using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) techniques. Fourier transform infrared spectroscopy ((FT-IR)) spectra showed a broad absorption band located at ∼3500 cm−1 attributed to stretching of the -OH groups and a sharp band at ∼1384 cm-1 owing to stretching vibrations mode of CO32- ions. X-ray diffraction displayed a cubic phase in all films and grain size (GS) of the undoped and doped samples were ∼33 and ∼21-17 nm, respectively. The films showed stress, a typical behavior of doped nanocrystals displaying residual strain. The absorbance spectra of PbS film exhibited four absorption bands located at ∼251 nm (∼4.9 eV), ∼610 nm (∼2.0 eV), ∼668 nm (∼1.8 eV) and ∼830 nm (∼1.4 eV) due to strong confinement effect and ∼446 nm (∼2.7 eV), ∼478 nm (∼2.5 eV) corresponding to 4F7/2→4I5/2, 4F3/2→4I15/2 (f→f) transitions of Er3+ ions. Sharp bands were found at ∼287 nm (∼4.3 eV) and 366 nm (∼3.38 eV), corresponding to transitions of Bi3+ions. The band gap energy of films showed a shift in the ∼0.9-1.2 eV range. Raman spectra showed two bands located at ∼450 cm−1 due to the first overtone of the longitudinal optical (LO) phonon (2LO) and a band observed at ∼200 cm−1 which was attributed to the LO (Γ) phonon. A kinetic mechanism using the free energy changes Gibbs is proposed.