Studying a single nanoparticle provides insight in the distribution of properties, static and dynamic, of the nanoparticles in the ensemble. Single particles represent also the limit of integration of potential devices. Single particles of quantum dots or metallic nanoantenna dispersed on microscope slides can easily be observed with optical microscopy by looking at the fluorescence or scattered light in dark field mode.

Emission of Single Charged Quantum Dots

We investigate the optical signal emitted by a single nanoparticle. When excited by blue light, colloidal quantum dots emit luminescence that can be filtered from the blue light and seen by eye in the objective of an optical microscope. The light can also be recorded with a camera. Since the amount of light is small, we use dedicated high-sensitivity and low-noise cameras. With plasmonic nanoparticles, there is little luminescence but a lot of scattering, and we record instead the scattered light using a dark field imaging mode.

A single dot is on an electrode in an electrochemcial cell. The photoluminescence (PL) switches periodically with an applied sawtooth potential that raises the Fermi level and injects electrons on a periodic basis. JPC 2010

The emission histogram of a single CdSeS/ZnS nanoparticles in the negative trion state (left panel, blue curve) shows no off-events indicating supressed blinking. In contrast the histogram for the traditional CdSe/ZnS shows no resolved signal in the trion state. Why the alloy is brighter in the charged state than the unalloyed core of the same size is not fully understood, but it may point the way towards bright tunable and charged emitters which will be useful for sensing, LEDs and laser applications. ACS Nano 2012

Metal nanoparticles larger than ~ 50 nm are easy and beautiful to look at with a dark field optical microscope. When the sizes and shapes are quite different, they all have different colors and intensity. By making two laser beams interfere on one particle, one measures its optical nonlinearity. PRB 2006