Semiconductor quantum dots are tiny pieces of semiconducting material containing just a few hundreds or thousands of atoms. They could be used to make a new generation of optoelectronics devices such as quantum-dot LEDs and quantum-dot lasers that could one day replace all present-day light sources.

Dark and bright states
The optical properties of quantum dots – for example, their ability to emit visible light at room temperature – are controlled by electron–hole exchange interactions. These quantum mechanical interactions split the quantum-dot electronic excited states into "dark" and "bright" states. If the exchange interaction is too large, the dots remain in the dark state – that is, they do not emit light.

Now, Alex Zunger and colleagues have shown how the exchange interaction depends on the electronic structure of the quantum-dot semiconductor material. The researchers calculated that dots made of a direct-bandgap semiconductor (such as InAs) have both long-range and short-range exchange interactions while dots made of an indirect-gap semiconductor (such as silicon) have only short-range interactions.

Manipulating optical properties of nanoparticles
The result is important from a fundamental viewpoint because it reveals the origin of electron–hole exchange interactions and helps explain light emission in these materials on the nanoscale. The calculations also suggest that the optical properties of nanoparticles can be manipulated by changing just their size. "We predict that certain types of direct-gap semiconductors, such as GaAs, can change their electronic identity to indirect-gap as they approach the nanoscale," team member Alberto Franceschetti told nanotechweb.org. "In such a case, our calculations suggest a highly unusual, non-monotonic dependence of optical properties on size."

The NREL team obtained its results using atomistic quantum-mechanical calculations performed on large supercomputers. The scientists calculated the interactions between electrons and holes using pseudopotential wave functions that accurately describe the localization of charge carriers within the quantum dot.

The work was published in Nano Letters.