Quantum Dots

Quantum dots are crystals containing only a few hundred atoms and when illuminated with UV light, for example, will fluoresce at only one specific wavelength of light. Why does the dot emit only one wavelength of light when excited?

Answer

Quantum dots are crystals, essentially metal or semiconductor boxes, containing only a few hundred atoms and a well-defined number of electrons. The number of electrons can be controlled by the electrostatic environment. The trick is to adjust how many electrons end up in each quantum dot.

An electron in a 3-D box is constrained to have a quantum mechanical wave function that matches the boundary conditions for the Schrodinger wave equation, producing discrete energy levels that are inversely proportional to the square of the wavelength. As the box is made smaller, the energy levels become farther apart. If the quantum dot diameter that is, box diameter is made small enough in fabrication, only a few energy levels will exist inside for the electron. Hence one can make quantum dots small enough to allow only one fluorescence transition possible in the visible part of the spectrum.

The data from the first quantum dot spectrum showed a rich harmonic series of transitions between electron energy levels. Subsequent tweaking of the electrostatic potential was shown to reduce the dot size and increase the energy spacings. Later researchers have been able to magnetically link together quantum dots with the hope of making arrays of them for quantum computing.