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

Abstract

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 nearultraviolet 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.

Analysis of Blinking SERS by a Power Law with an Exponential Function

“…The power laws can be explained by the distribution of the first-passage time required for the random walker to return to its starting point for a one-or two-dimensional random walk (Figure 6.2) [55]. For the one-dimensional random walk model, the power law exponent is derived to be −1.5, and many experimental power law exponents in the bright and dark fluorescence from a single QD are consistent with −1.5 [55][56][57]. For a QD, a photo-excited electron in a conduction band, which recombines with a hole in a valence band and then emits fluorescence, is restrained by Coulomb attraction.…”

“…The off was reported to be −1.56, −1.51, and −1.46, like blinking fluorescence from a single QD [55][56][57][58], although the analysis for TC derived off = −1.42 and −1.12 from Figure 6.5b [19]. Figure 6.9 shows semi-logarithm plots of the probability distribution of dark SERS events from Fe-protoporphyrin IX [16].…”

“…In the case of the probability distribution of the dark and bright events in blinking fluorescence from a single QD plotted against the duration times, the log-log plots for dark and bright fluorescence events display a line and a curve truncated at the tail, respectively [56][57][58][59]. Fluorescence from a single QD originates from recombination of a photo-excited electron (e − ) in a conduction band and a hole (h + ) in a valence band, as illustrated in Figure 6.7a.…”

“…The maxima in the histograms of the truncation times were observed to be ∼30 s at 514 nm and ∼50 s at 568 nm, as shown in Figure 6.15a,b, respectively. By inducing excitation at a shorter wavelength than an absorption band of TCC dimer at 505 nm and that of the monomer at 550 nm [76,77], photo-induced desorption may be accelerated, thus shortening the truncation time in a different manner to the excitation wavelength dependence of blinking fluorescence from a single QD, in which the truncation times were similar except for near-ultraviolet excitation [57].…”

“…However, the autocorrelation function was not reproduced by any simple function, suggesting a complex process in the blinking SERS [15]. The blinking SERS from a single Ag nanoaggregate with an adsorbed dye molecule can be analyzed by a power law in a manner similar to that employed in studies of blinking fluorescence from a single semiconductor quantum dot (QD) [55][56][57][58][59]. Power law statistics have been widely applied to the analysis of long-range ordered non-exponential behaviors, especially "self-similarity" or where a feature of objects does not change if length scales are expanded [60].…”