Doping InAs quantum dots opens up possibilities
“Nanocrystal doping” developed by Hebrew University researchers results in semiconductor nanocrystals with enhanced electrical function.
Researchers at the Hebrew University of Jerusalem have achieved a breakthrough in the field of nanoscience by successfully altering nanocrystal properties.
By doping free-standing InAs nanocrystals using a solution-phase synthesis, they say they have opened up the way for the manufacture of improved semiconductor nanocrystals.
Semiconductor nanocrystals consist of tens to thousands of atoms and are 10,000 times smaller than the width of a human hair. These tiny particles have uses in a host of fields, such as in LEDs, solar cells and bio-imaging.
However, these semiconductors are poor electrical conductors, and in order to improve electrical conductivity so they can be used in electronic circuits, their conductivity must be tuned by the addition of impurities (or doping).
Due to the importance of doping to the semiconductor industry, researchers worldwide have made continuing attempts at doping nanocrystals in order to achieve ever greater miniaturisation and to improve production methods for electronic devices.
Unfortunately, these tiny crystals are resistant to doping, as their small size causes the impurities to be expelled. An additional problem is the lack of analytical techniques available to study small amounts of dopants in nanocrystals. Due to this limitation, most of the research in this area has focused on introducing magnetic impurities, which can be analysed more easily. However, the magnetic impurities don’t really improve the conductivity of the nanocrystal.
Uri Banin in his lab (Hebrew University photo)
Now, Uri Banin and his graduate student, David Mocatta, of the Hebrew University Centre for Nanoscience and Nanotechnology, have achieved a breakthrough in their development of a straightforward, room- temperature chemical reaction to introduce impurity atoms of metals into the semiconductor nanocrystals.
In their novel solution-phase synthesis of metallically doped, free-standing InAs nanocrystals, they claim to have seen new observations not previously reported. When the researchers tried to explain the results, however, they found that the physics of doped InAs nanocrystals was not very well understood.
Bit by bit, in collaboration with Oded Millo of the Hebrew University and with Guy Cohen and Eran Rabani of Tel Aviv University, they built up a comprehensive picture of how the impurities affect the properties of nanocrystals. The initial difficulty in explaining this process proved to be a great opportunity, as they discovered that the impurity affects the nanocrystal in unexpected ways, resulting in new and intriguing physics.
“We had to use a combination of many techniques that when taken together make it obvious that we managed to dope the nanocrystals. It took five years but we got there in the end,” said Mocatta.
This breakthrough sets the stage for the development of many potential applications with nanocrystals, ranging from electronics to optics, from sensing to alternative energy solutions. Doped nanocrystals can be used to make new types of nanolasers, solar cells, sensors and transistors, meeting the exacting demands of the semiconductor industry.
This work is further described in the paper “Heavily Doped Semiconductor Nanocrystal Quantum Dots”, by Mocatta et al,Science, Vol. 332 no. 6025 pp. 77-81, published online 1 April 2011 ; DOI: 10.1126/science.119632.