MPS Content-Dependent Tuning of Optical Properties of MPS-Capped CdS Quantum Dot Thin Films

The precise control of optical and dielectric properties of semiconductor quantum-dot-based thin films remains a critical challenge for advanced optoelectronic applications. In this respect, (3-mercaptopropyl)trimethoxysilane (MPS)-capped CdS quantum dot thin films were prepared with MPS:Cd molar ratios of 0.25, 0.50, 1.00, and 2.00 to investigate the role of MPS content in tuning their optical and dielectric properties. The colloidal quantum dot solutions were deposited onto glass substrates by spin coating and subsequently annealed in air at 350 °C. Optical analyses showed that increasing MPS content enhanced film transmittance, whereas the refractive index, extinction coefficient, dielectric loss tangent, and optical conductivity decreased. At 550 nm, the refractive index decreased from 2.12 to 1.61, while the optical conductivity decreased from 6861 to 644 S m−1 as the MPS ratio increased from 0.25 to 2.00. The Urbach energy exhibited a non-monotonic variation, reaching a maximum value of 415 meV at MPS:Cd = 0.50 and a minimum value of 193 meV at MPS:Cd = 2.00. Wemple–DiDomenico analysis revealed that the single-oscillator energy increased systematically with increasing MPS ratio, and the E0/E1S−1S ratios remained within a narrow range of 1.32–1.46 throughout the MPS:Cd series. The analysis further revealed that the dispersion energy, static refractive index, infinite-wavelength dielectric constant, and linear susceptibility decreased with increasing MPS molar ratio. The estimated third-order nonlinear optical susceptibility and nonlinear refractive index also decreased with MPS content.