Determination of the Electronic Energy Levels of Colloidal Nanocrystals using Field-Effect Transistors and Ab-Initio Calculations

We investigate the absolute energy of the LUMO and HOMO energy states, and consequently the electronic bandgap values, for PbS NCs of different sizes, through a combined experimental and theoretical effort. The experimental method relies on the measurement of ambipolar field-effect transistors by using an ionic-liquid-based ion-gel gate. This represents the first successful application of this method to an array of fully quantum-confined systems. This method does not only allow the determination of the bandgap but also the higher energy levels of the NC array, which are characterized by slope variation (going true plateau) in the transfer characteristics. Our experimental results are directly compared with results obtained with advanced ab-initio calculations, performed with density functional theory (DFT) using a fully relativistic approach, which allow a direct determination of absolute energy levels. The agreement between measured and calculated energy values is excellent.