Excitation Energy Dependence of Fluorescence Intermittency in CdSe/ZnS Core-Shell Nanocrystals

We report measurements of the excitation energy dependence of the fluorescence intermittency of single CdSe/ZnS core/shell nanocrystals (NCs), using two different sizes of NCs and three different excitation energies. The lowest excitation energy corresponds to exciting the smaller size NC at its optical band gap, so we examine both excitation at the band gap and at varying energies above the band gap. We find that off-time probability distributions follow a power law with exponent roughly - 1.5 regardless of NC size or excitation energy. The on-time probability distributions follow a truncated power law with a power-law exponent that again does not depend on size or energy. With comparable absorption rate, the truncation times for the larger nanocrystals are very similar when excited either 270 or 480 meV above the band gap but shorten for near-ultraviolet excitation 1 eV above the band gap. For the smaller nanocrystals, the truncation times are comparable whether excited at the band gap or 270 meV above. Our findings support a spectral diffusion-controlled model for blinking and are consistent with the existence of a quasicontinuous manifold of excited states with altered emission dynamics above the 1P(e) state; they also indicate a change in emission dynamics at very high excess energy.