How much light can CdSe quantum dots emit?

Quantum dots are artificial light sources with a diameter of a few nanometers, that can be fabricated to have any color in the visible spectrum. Because of their small size and flexibility these dots are well suited for applications. However, for effective use of these lightsources it is important to know how many photons are emitted per second.

Researchers from the FOM institute for Atomic and Molecular Physics, together with colleagues from the Danish Technical University and the University of Twente have now succeeded in determining this number of photons per second. This is an important step for further insight into the behavior of these quantum dots. The results have been published online in Physical Review B on the fifth of january. 

Most of the light we can see is created by a process called spontaneous emission. For instance the yellow street lights along the highway have sodium atoms that use this process. The atom is placed in an excited state by adding energy. This energy is lost by emitting light.

Not just atoms are capable of spontaneous emission. There also exist artificial sources called quantum dots. Quantum dots are clusters of semiconductor material of a few nanometer in diameter. Depending on the size of this cluster, the light that is emitted by spontaneous emission changes color. This flexibility makes quantum dots ideal for applications. For example in biology, where these light emitting particles are linked to proteins to make these proteins visible. But future LEDs or quantum computers are also very likely to contain quantum dots.

To make full use the potential of quantum dots it is important to know how much light the quantum dots can emit per second. A quantum dot can be excited, which leads to a decay event. The energy is lost either to creation of a photon or heat. The total decay rate is a sum of the radiative decay rate, how many photons are created per second, and the nonradiative decay rate, or how much heat is created per second. Unfortunately, in a direct measurement, only the total decay rate can be measured, while we are interested in the radiative component.

Researchers from the FOM institute AMOLF, together with colleagues from the Danish Technical University and the University of Twente have now succeeded in determining this radiative decay rate. They used for this a special property of the decay rate: The rate is not just dependent on the source but is also changed by the properties of the direct environment of the emitter. By placing quantum dots very close to a silver mirror, their own emitted light is reflected back to the quantum dot. However this only affects the radiative decay rate, while the nonradiative decay rate remains unchanged. In this way the two components of the decay rate can be seperated.

The most surprising result is that the radiative decay rate hardly depends on the size of the quantum dot. Irrespective of the size, quantum dots can emit about 60 million photons per second. Since we now know this value, it becomes possible to do new calculations on the behavior of the quantum dots, to better understand these important light sources.

Reference: Physical Review B 79, 045301 (2009)
URL: link.aps.org/doi/10.1103/PhysRevB.79.045301

A transmission electron micrograph of a CdSe quantum dots. The scale bar is 2 nanometer