A team of researchers have demonstrated how semiconductor nanocrystals can be doped in order to change their electronic properties and be used as conductors. This opens a world of possibilities for applications of small electronic and electro-optical devices, such as diodes and photodiodes, electric components in cellular phones, digital cameras, and solar panels.

Professor Eran Rabani of Tel Aviv University's School of Chemistry collaborated with professors Uri Banin and Oded Millo at the Hebrew University on the research.

Solar panels are typically made from a pn junction. When they absorb light, the junction separates the negatively charged electrons and the positively charged holes, producing an electrical current, says Rabani. "With this new method for doping nanocrystals to make them both p and n type, we hope that solar panels can be made not only more efficient, but cheaper as well," he says.

According to Rabani, the quest to electrically dope nanocrystals has been an uphill battle. The crystals themselves can self-purify, which means that they cleanse themselves of dopants. Also, some of the synthetic methods for doping were problematic on the nano-scale -- the crystals were unable to withstand doping techniques applicable to bulk semiconductors.

The key was to find a method for doping the nanocrystals without "bleaching" their optical properties and therefore nullifying their absorption capabilities. If you can dope nanocrystals in this way, Rabani says, it opens the door to many practical applications based on nanocrystalline materials. "Whatever you can do with nanocrystals, you can do with doped nanocrystals — and more by controlling their electronic properties."

These challenges were circumvented with the use of room-temperature, diffusion-controlled reactions. The crystals were bathed in a solution that included the dopants, where slow diffusion allowed for impurities to find their way into the nanocrystal.

The team used a scanning tunneling microscope to determine the success of their doping procedure. These measurements indicated how the Fermi energy of the nanocrystals changed upon doping, a key feature in controlling the electronic properties of electronic devices. The results indicate that the nanocrystals have been doped with both n-type dopants, indicating the presence of excess electrons in the nanocrystals, and p-type, which contribute positively charged holes to the semiconductors. This will allow for their use in electronics that require a pn junction, such as solar panels and LEDs.

The team was also able to control the optical properties, namely, the color range that the nanocrystals produce. Once doped, the nanocrystal particles could change in color, becoming more red or blue. Rabani and his colleagues were able to develop a theory to explain these observations.

(Tel Aviv University)

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