New combination of quantum dots
doi:10.1038/nindia.2011.68 Published online 16 May 2011
New research has shown that coupling two different types of semiconductor quantum dots can produce a composite quantum dot that has tunable electronic properties. Such coupled quantum dots could be potentially useful for quantum information processing and optoelectronic devices.
Semiconductors and insulators exhibit an energy difference between the top of their valence band and the bottom of their conduction band. In this energy gap, known as the 'bandgap' in solid-state physics, no electron states can exist. Electrons excited by heat or light jump from the valence band to the conduction band, thereby generating electric current.
Semiconductor quantum dots are useful because they have small bandgaps. Unfortunately, however, single semiconductor quantum dots cannot exhibit such electron transition over a broad range of wavelengths. To overcome this, the researchers produced coupled quantum dots from zinc selenide (ZnSe) and cadmium sulphide (CdS) quantum dots in a two-step process.
The ZnSe dots had diameters of around 2 nm and the CdS dots had diameters of around 3 nm, resulting in coupled-dot diameter of 5 nm. The researchers observed that electrons excited by visible light moved from the ZnSe conduction band to the conduction band edge of CdS. This provides a method of tuning the visible fluorescence of the coupled dots. The results also suggest that the ZnSe and CdS dots retain their original states even after coupling.
"Such asymmetric structures of semiconductors may offer a diverse set of building blocks for optoelectronic devices, ultrahigh density memories and quantum bits for quantum information processing," says lead researcher Somobrata Acharya.
The authors of this work are from: Centre for Advanced Materials and Department of Solid State Physics, Indian Association for the Cultivation of Science, Kolkata, and Solid State and Structural Chemistry Unit & Centre for Condensed Matter Theory, Indian Institute of Science, Bangalore, India.