A Spectroelectrochemical Method for Locating Fluorescence Trap States in Nanoparticles and Quantum Dots

We here devise an electrochemical method for determining the absolute energetic position of trap levels involved in fluorescence. The method utilizes potentiostatic control of the Fermi level in the material, and thereby also the electronic population of the energy states involved in the fluorescence. The method is especially useful for nanoparticle semiconductor electrodes. Here we exemplify the method by determining the position of the trap levels involved in the green fluorescence in thin films of ZnO quantum dots. The exact mechanism and the absolute positions of these states have been debated in the literature. Here we show that the visible fluorescence is caused by a transition from energy levels slightly below the conduction band edge to a deep trap within the band gap. We further pinpoint the location of the upper trap level to be at 0.35 +/- 0.03 eV below the conduction band edge. Particles between 5 and 8 nm in diameter have been analyzed, which is in the quantum confined region of ZnO. We also show that the position of the upper trap level shifts with the size of the quantum dots in the same way as the conduction band.