Determining the Concentration of CuInS2 Quantum Dots from the Size-Dependent Molar Extinction Coefficient

The size-dependent nature of the molar extinction coefficient of highly photoluminescent copper indium sulfide (CuInS2) quantum dots (CIS-QDs) is presented. We determined the extinction coefficients at both high photon energy (3.1 eV) and at the first excitonic transition band for CIS-QDs, ranging in size from 2.5 nm to 5.1 nm. Both coefficient trends displayed a power-law size dependence for the QDs. These data allow the in situ assessment of the CIS-QD concentration via routine optical absorption measurements, which is an important parameter for many applications. The formation of ZnS on the surface of CIS-QDs dramatically increases the photoluminescence quantum yield, while also blue-shifting the photoluminescent emission. Importantly, we conclude that the concentration of core/shell CIS/ZnS-QD dispersions can be determined using the molar extinction coefficient data of core CIS-QDs. The experimental uncertainties in the solution concentrations determined from the molar extinction coefficient data are in the range of 10%-15%.