Green synthesis and optical properties of ZnSe:Cu@ZnS core/shell nanocrystals fabricated by new photochemical microwave-assisted colloidal method

Abstract

Abstract Growth of a secondary material-shell on a core of another material and so, formation of a core-shell system has been a successful route in the surface modification of nanostructured samples. In this research, ZnSe:Cu and ZnSe:Cu@ZnS core-shell nanocrystals (NCs) with different contents of Cu-dopant under microwave irradiation times (MWIRTs) = 0 & 6 min were synthesized by a new and rapid photochemical microwave-assisted method at home temperature. Crystal structure, crystallite size (by XRD analysis), morphology and elemental analysis (by TEM, FESEM, EDX and Map), strain, dislocation density, absorption edge, energy band gap (by exact method of DASF), the exact nature of charge carrier optical transition (m), Urbach energy ( E tail ), Refractive index (n), dielectric constant (e) at the absorption edge and third order non-linear optical susceptibility (χ(3)) were determined. The influences of various experimental variables, including formation of ZnS shell on ZnSe:Cu core, Cu content and MWIRT on their opto-structural features were systematically investigated. The obtained NCs present high degree of crystallinity and have a cubic zinc blende structure which their diameters varies from 1.81 to 2.26 nm. The formation of core-shell structures was confirmed TEM. For ZnSe:Cu@ZnS core-shell NCs, XRD diffraction peaks shifted slightly to higher angles. Band gap of ZnSe NCs was obtained about 3.45 eV, which was tunable by formation of ZnS shell on ZnSe:Cu NCs core and also by altering the synthesis conditions. However, the band gap values of ZnSe:Cu@ZnS systems were generally smaller than that of ZnSe:Cu NCs; also, in each cases of bare ZnSe:Cu NCs and ZnSe:Cu@ZnScore/shell NCs, increase in MWIRT results to the red shift in band gap. For all samples, optical charge carrier transition index of m were found to be nearly around 3/2 (correspond to the direct non-allowed transition gap type), except for ZnSe:Cu(1.5%)@ZnS with MWIRT = 6 min with m-index nearly around 1/2 (correspond to the direct allowed transition gap type); the recent exception may be due to its different structure, which has been caused to its different structure certified by TEM. Upon the all opto-structural performed analysis, ZnSe:Cu(0.75%)@ZnS and ZnSe:Cu(0.1%)@ZnS NCs synthesized photochemically under MWIR = 6 min, have the highest (χ(3)), lowest band gap and lowest lattice suggesting them as promising and potentially materials in nano-optoelectronic applications such as solar cells, fibers or even in strong coating materials.