The surface passivation of luminescent CdS quantum dots (QDs) via epitaxial overgrowth of new alloyed ternary AgZnSe shell layer is reported here. Two synthetic fabrication strategies were used to tune the optical properties of CdS/AgZnSe core/alloyed shell QDs across the visible region. Transmission electron microscopy, powder X-ray diffraction, Raman, UV/vis and fluorescence spectrophotometric techniques were used to characterize the nanocrystals. Analysis of the internal structure of the QDs revealed that homogeneity of the particle reduced as the size increased, thus indicating that the QDs growth transitioned from an interfacial epitaxial homogenous state to a heterogeneous state. The crystal structure of the QDs revealed a distinct zinc-blende diffraction pattern for CdS while CdS/AgZnSe core/alloyed shell QDs kinetically favoured a phase change process from the zinc-blende phase to a wurtzite phase. Analysis of the photophysical properties revealed varying degrees of interfacial defect state suppression in CdS/AgZnSe QDs which was dependent on the QDs size. Specifically, the fluorescence quantum yield (QY) of CdS/AgZnSe QDs was at most ~5-fold higher than the CdS core and varied from 35% to 73%. We found that band gap modulation via the synthetic fabrication strategy employed, influenced the optical properties of the core/alloyed shell QDs. The CdS/AgZnSe QDs produced in this work hold great promise in light-emitting optoelectronic applications.