A composition and method for fabricating and tuning a dopant based core-shell semiconductor having a quantum dot core with an excitation band-gap are provided. A quantum dot core composed of an alloy of cadmium sulfide (CdS) and zinc sulfide (ZnS) as semi-conductor materials include a dopant of manganese (Mn) added to the core and an outer shell of zinc sulfide (ZnS). The dopant based core/shell quantum dot semiconductor of the present invention allows the fine tuning of an excitation band-gap, covering a wide range (from 2.4 eV to .about.4 eV). When doped with Mn, these alloy Qdots emit bright yellow/orange light. Tuning of the excitation band is accomplished by changing the alloy composition of the core. Based on photophysical studies a new core/shell/shell model is provided, in place of the traditional core/shell model. Due to the interfacial diffusion of the cations from the core and shell an intermediate alloy layer is formed providing an inner shell; this inner shell layer is the real host of the dopant ions.