ZnSe Quantum Dots: Determination of Molar Extinction Coefficient for UV to Blue Emitters

The focus on heavy metal-free visible emitters in optoelectronic devices has increased interest in ZnSe semiconductor quantum dots (QDs) over the past decade. Empirical fit equations correlating the lowest energy electron transition to their size and molar extinction coefficients (ε) are often used to determine the concentration of suspensions containing QDs. This is essential to the design and successful synthesis of complex semiconductor nanoparticles including core/shell and dot-in-rod heterostructures as well as consistent device fabrication. While these equations are known and heavily used for CdSe, CdTe, CdS, PbS, etc., they are not well established for ZnSe nanocrystals; the only two reports of ε in the literature differ by over an order of magnitude. In this study, a series of ZnSe QDs with diameters ranging from 2 to 6 nm were characterized with small angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and UV-Vis spectroscopy. SAXS-based size analysis enabled practical inclusion of small particles in the evaluation. Elemental analysis with microwave plasma atomic emission spectroscopy (MP-AES) yields a non-stoichiometric Zn:Se ratio consistent with zinc-terminated spherical ZnSe QDs. Using these combined results, molar extinction coefficients for each QD sample were calculated. Empirical fit equations correlating QD size with its lowest energy electron transition (i.e., 1S peak position) and molar extinction coefficients for both 1S peak and high energy wavelengths are reported. These results will enable the consistent and reliable use of ZnSe core particles in complex heterostructures and devices.