The optical properties of CdS/CTS were evaluated using UV-Vis spectroscopy (Figure ) and DRS (Figure ). UV-Vis spectrum of CdS/CTS was recorded, as shown in Figure . The UV-Vis spectrum of the CdS/CTS QDs shows shoulder absorption onset at 550 nm (Figure ). Based on the empirical equation (Eq. 2), the CdS/CTS nanocrystals have an average particle size of 3.1 nm, which agrees with the particle size range obtained from TEM and HR-TEM (Figure ). Kubelka-Munk plot for CdS/CTS using Tauc's equation is plotted as (αhν) 2 versus hν (Figure ). The bandgap energy (Eg) value for CdS/CTS is 2.25 eV (Figure ). The optical properties of CdS with and without CTS were evaluated using DRS (Figure ). DRS spectra of CdS and CdS/CTS show absorbance in the 200-570 nm (Figure ). There are apparent differences in the case of CdS/CTS compared to bare CdS. The difference in the DRS is due to the interaction between CdS and CTS's functional groups. Tauc's plot of CdS and CdS/CTS indicates an optical band gap of 2.47 and 2.30 eV, respectively (Figure ). CTS decreases the optical band gap for the core of CdS.