Nano Lett. http://doi.org/bn8f (2016)

Exciting semiconductor nanocrystals with laser light of the appropriate energy creates electron–hole pairs, also known as excitons, which after a certain time recombine, emitting light (photoluminescence, PL). The PL is due to the decay of single excitons and to the sequential decay of multiexciton complexes. The ideal ratio between these two types of emission varies according to the type of device the nanocrystals are used in. Now, Sadahiro Masuo and colleagues from Kwansei Gakuin University and Hokkaido University in Japan have demonstrated how a plasmonic nanostructure can be used to control the type of PL emitted by a single nanocrystal.

The researchers used an atomic force microscope (AFM) tip coated with silver as the plasmonic nanostructure. By decreasing the distance between the AFM tip and the nanocrystal they observed an increase in PL intensity, which they ascribed to an increased population of multiexciton complexes due to the interaction of electrons and holes in the nanocrystals with the surface plasmon on the tip. This hypothesis was confirmed by evaluating the second-order correlation function of the PL, which gives a direct measure of the number of single photons emitted, and showed an increased ratio of multiphoton to single-photon emission when the tip approaches the nanocrystal.