Luminescent Quantum Clusters of Gold as Bio-Labels - Advanced Fluorescence Reporters in Chemistry and Biology - page 348

7.5 Photon Antibunching
Photon antibunching is a quantum phenomenon that occurs during luminescence, in
which the emission of one photon reduces the probability that another photon will
be emitted immediately afterward. Unlike single atoms, metal nanoparticles exhibit
collective oscillations of free electrons and produce bunched photons. On the one
hand, antibunched photons cannot be observed from metallic nanoparticles since
they possess continuous-band structure. On the other hand, since the electronic
nature of quantum clusters is molecule-like and also possesses discrete energy
levels, it is possible to observe antibunched photons from them. Antibunched
luminescence from a single Au 23 cluster was observed when excited at 632.8 nm
[ 1 ]. Photons arriving at each of two detectors show a decreased probability of two
photons arriving simultaneously as shown in Fig. 9 . The rise time of the antibunched
signal matches the Au 23 lifetime.
7.6 Photoreactivity at Single-Cluster Level
Reactivity of metal clusters can be studied using spectroscopic techniques. By
means of single molecule luminescence spectroscopy, we can observe individual
clusters. Photoreactivity of gold clusters synthesized using a photochemical method
Fig. 9 Antibunched luminescence from a single Au 23 cluster, excited at 632.8 nm. Photons
arriving at each of two detectors show a decreased probability of two photons arriving simulta-
neously (at zero interphoton delay) [ 1 ]