Synthesis and structural characterization of ZnS quantum dots (< 2 nm) vis-a-vis studies on their spectroscopic and dielectric properties

In this work, zinc sulfide (ZnS) QDs were synthesized using green technique with the help of Tabernaemontana divaricata flower extract as capping/stabilizing agent, characterized by different physical techniques, like XRD, FESEM, EDX, TEM, FT-IR, XPS, UV-Vis-NIR, and PL, and were compared with their pristine analogue. From XRD analysis, it is observed that the size of the synthesized ZnS QDs lie in 1.79-1.46 nm range and with addition of flower extract particle size decreases. FT-IR analysis indicated the functional groups present in the synthesized QDs. The direct energy band gap (3.21-3.51 eV) values as well as the refractive index values (2.28-2.33) of the QDs were calculated utilizing the absorption data. Due to various defect levels present in the ZnS QDs within the energy band gap, four peaks (three in visible and the other in NIR region) were observed in the PL spectra. Surface morphology and elemental composition of the particles were understood from FESEM and EDX studies. Dielectric constant, loss, and ac and dc conductivity of the materials were studied. At room temperature, all synthesized ZnS materials exhibited high dielectric constant (epsilon r) values in the low-frequency region. It is found that epsilon(r) values for pristine ZnS QD (similar to 23,000) are much higher than the green ZnS QDs (similar to 100-800) where overall dielectric loss is less than 2 which decreases with QD size. Results obtained from the characterization studies conducted on pristine and green-synthesized ZnS QDs elucidate that it is possible to effectively control the size, optical, and dielectric properties of the potentially important ZnS QDs through green synthesis technique.