The effect of synthesis situation on the structural, dielectric, linear and nonlinear optical properties of thiol-capped water-soluble lead sulfide (PbS) quantum dots

Abstract

In this study, lead sulfide quantum dots (PbS QDs) have been prepared by an aqueous method with a mixture of TGL and DTG as a capping agent with different situations of synthesis. The XRD study defends mono-phase formation of PbS having a cubic system with a crystallite size of 2.9–3.6 nm. The SEM images show that the particles are at the nanoscale. The band gap energies of these samples were obtained 2.47–2.53 eV using UV–Vis absorption spectra. In addition, the optical limiting (OL) method was used to obtain linear absorption coefficient, α. The results of Kramers-Kronig (KK) relations show by increasing pH, the intensity increased in complex refractive index as well as complex dielectric constant. Moreover, regarding the transverse optical and longitudinal optical frequencies of PbS QDs, by increasing synthesized environment pH, both frequencies had significantly blue shifts. Owning the high dielectric constant of the sample, PbS QDs were revealed an appropriate candidate to employ in smart materials. Besides, standard single-beam Z-scan techniques with various laser powers have been employed for detecting nonlinear optical (NLO) characteristics. This research reports the relaxation time and pH-dependent third-order NLO properties of PbS QDs. The consequences have revealed that the nonlinear refractive (NLR) and nonlinear absorption (NLA) coefficients of PbS QDs have been improved by increasing laser power except at 55 mW, reaching values of 1.68 × 10−7 cm2/W and 6.8 × 10−4 cm/W in pH of 9.2 and relaxation time of 1 h at 45 mW, respectively.