In the last two decades, the wet-chemical synthesis of colloidal semiconductor nanocrystals of II-VI, IV-VI and III-V compounds has taken a high flight. This led to a word wide access to suspensions of nanocrystals with specific size and shape, various surface capping and bright emission. Compared to the typical solid state quantum dots, colloidal quantum dots have several advantages: they are quantum objects that do NOT interact with each other, they are available in high quantities, and they can be used as building blocks for more complex materials.
The optical spectroscopy on ensembles and single colloidal nanocrystals resulted in the demonstration and partial understanding of emission blinking, bi-exciton vs. exciton emission, and type I vs. type II quantum dots. On the other hand, electron-tunnelling spectroscopy on single quantum dots revealed the discrete energy levels of these systems and the degree of quantum coupling for quantum dots in an array.