In order to obtain data on the formation process of CdSe/CdS core/shell semiconductor nanocrystals, we subjected samples of nanocrystals to long-term heat treatment at a temperature of 80 $^\circ$C. After a certain time, the absorption and the emission spectra were measured using parts of the samples. As the heat treatment time was increased, the Stokes shift decreased until the growth of the CdSe nanocrystals stopped. The time when the Stokes shift increases after a certain time can be regarded as the time when the surfaces of nanocrystals change. A method for distinguishing whether CdSe/CdS has core-shell structure or is an alloy was studied by analyzing the Raman spectra and the X-ray diffraction (XRD) patterns of the samples at the time when the surfaces of the nanocrystals changed. When CdSe/CdS core-shell semiconductor nanocrystals are synthesized, a chance exists that interfaces will be observed in CdS$_x$Se$_{1-x}$ alloys. Thus, distinguishing whether the synthesized nanocrystals have a core-shell structure or are CdS$_x$Se$_{1-x}$ alloys is possible. If a new phonon is observed between phonon L01 and phonon L02 in the Raman spectra, the material is an alloy. A more precise core-shell structure can be identified by looking at the XRD pattern and the Stokes shift.

Keywords: Nanocrystals, Core/shell, Alloy, Stokes shift, Raman spectrum