ZnSe nanocrystals (NCs) of size about 1.8 nm were prepared in aqueous media and were grown using microwave irradiation, which is simple and inexpensive method in which it needs very short reaction time. In this method, using selenium powder and NaBH4 the source of Se ions has been provided and then, the present solution has been injected to Zn ions source. Also solutions of sodium hydroxide (NaOH) and thioglycolic acid were used to adjust the pH of samples and capping agent, respectively. To study the effect of transition elements on the properties of these nanoparticles, Cu ions were used as impurity. After the completion of nucleation and formation of nanoparticles, the ultimate solution was microwave irradiated for 6 min. To investigate the nanoparticles morphology and their optical properties, X-ray diffraction (XRD) and UV–Visible (UV–Vis) optical spectroscopy were used. UV–Vis spectra were recorded at different ambient temperatures. XRD analysis demonstrated cubic zinc blende NCs and field emission scanning electron microscopy (FESEM) image indicated round shape NCs. Using the UV–Vis outputs, the optical band gap ( Eg DASF) and also the exact nature of charge carriers optical transitions were studied and evaluated in ZnSe and ZnSe:Cu nanocrystals, by employing an accurate new method [called in literatures as derivation of absorption spectrum fitting (DASF)]; this method only requires the measurement of the absorbance spectrum of the sample, avoiding any needs to film thickness or any other parameters. According to the UV–Vis analysis, band gap of the nanoparticles was determined and was in the range of 3.350–3.652 eV depending on the temperature and Cu dopant, showing the decreasing trend of energy gap with increasing ambient temperature and impurity. The decreasing trend of Eg DASF with ambient temperature, confirm the semiconducting nature of the present samples. Also, the direct gap nature of their optical transitions was justified. moreover, the band taili
