Structural and optical properties depending on tuning of Cd:S ratio in the CdS nanocomposite via SILAR technique
摘要
This study systematically investigated the structural, morphological, and optical properties of CdS nanocomposites synthesized by the successive ionic layer absorption and reaction (SILAR) technique at different Cd:S ratios (1:0.1, 1:0.25, 1:0.5, 1:0.75, and 1:1) in polyvinyl alcohol (PVA) polymer matrix. X-ray diffraction (XRD) analysis revealed a phase change from hexagonal (1:0.1) to mixed phase (cubic + hexagonal for 1:0.25 and 1:0.5) with increasing S2− content, and finally to amorphous structure when the stoichiometric ratio was equal (1:1). Atomic force microscopy (AFM) observed that the nucleation rate changed depending on the cation anion ratio and the smallest particles (5–25 nm) were formed at equal stoichiometric ratios. UV–Visible (UV–Vis.) spectroscopy showed a systematic decrease in the direct and indirect bandgaps-Eg (from 3.32 eV to 2.41 eV) with increasing sulfur content up to 1:0.75, followed by an increase in the amorphous phase (2.52 eV). AFM results show that the increase in the S2− content of the particles up to 1:0.75 is due to the increase in the ion balance and sulfur concentration, which is associated with a strong quantum size effect, which occurs with a decrease of the bandgap value with increasing particle size and, conversely, an increase by decreasing of particle size at a 1:1 ratio. From FTIR analysis, a red shift was observed with increasing S2− content, which confirms the strong interaction between the CdS nanoparticles and the PVA matrix. Raman spectroscopy reveals changes in phonon behavior at different Cd:S ratios. According to the results, although the typical vibration modes of first-order longitudinal optical (1LO ~ 298 cm⁻1) and second-order longitudinal optical (2LO ~ 600 cm⁻1) phonon modes for CdS were not observed at different ratios, these phonon vibrations begin to appear at a ratio of 1:0.5 and are most intense at a ratio of 1:0.75. This is due to the formation of its well-ordered and stable hexagonal phase. Photoluminescence (PL) spectra of the CdS nanocomposites showed strong emission peaks at 450 nm (band-to-band recombination), 486 nm (defect-related to sulfur vacancies or Cd interstitials), 564 nm (deep-level sulfur vacancies), and 669 nm (deep trap states), showing strong dependence on the cation-to-anion ratio. The PL intensity depended strongly on the cation-to-anion ratio, with the 1:0.75 sample exhibiting the highest exciton recombination efficiency. Additionally, a weak emission at about 669 nm indicates the presence of deep traps or surface defects, likely related to sulfur vacancies and lattice disorder in CdS. The results indicate that tuning the Cd:S ratio is an effective strategy to control the structural and optoelectronic properties of CdS-based nanocomposites, which makes specific ratio (Cd:S = 1:0.75) of CdS nanocomposites ideal materials for photovoltaic applications such as solar cells.