The rapid, solvent-less, straightforward reactions under autogenic pressure at elevated temperature (RAPET) approach is explored for the fabrication of core-shell nanomaterials. Carbon-encapsulated ZnS and ZnSe nanoparticles are synthesized by a one-step thermal decomposition of diethyl zinc in the presence of either S or Se powders in a closed reactor at 700°C for 30 min under their autogenic pressure in an inert atmosphere. The Raman spectroscopy measurements provide the structural evidence for the formation of pure ZnS or ZnSe particles and ensure the nature of the coated carbon. The X-ray diffraction measurements confirm the hexagonal phase of ZnS and the face-centered cubic crystal structure of ZnSe. The high-resolution transmission electron microscopy technique distinguishes core and shell morphologies. Interestingly, core-shell nano-structures of ZnS and ZnSe coated with carbon show a single electron spin resonance signal, whereas the bare ZnS and ZnSe nanoparticles demonstrate several signals. The nitrogen gas adsorption on the surface of core-shell nanostructures is determined by Brunauer-Emmett-Teller surface area analysis. The plausible mechanism for the formation of semiconducting cores (ZnS or ZnSe) with a carbon shell is elucidated.