Conducting Energy on a Nano Scale 15 July 2011

Conducting energy on a nano scale 15 July 2011

Modern electronics as we know them, from method for doping the nanocrystals without televisions to computers, depend on conducting "bleaching" their optical properties - and therefore materials that can control electronic properties. As nullifying their absorption capabilities. If you can technology shrinks down to pocket sized dope nanocrystals in this way, he says, it opens the communications devices and microchips that can door to many practical applications based on fit on the head of a pin, nano-sized conducting nanocrystalline materials. "Whatever you can do materials are in big demand. with nanocrystals, you can do with doped nanocrystals - and more by controlling their Now, Prof. Eran Rabani of Tel Aviv University's electronic properties." School of Chemistry at the Raymond and Beverly Sackler Faculty of Exact Sciences, in collaboration These challenges were circumvented with the use with Profs. Uri Banin and Oded Millo at the Hebrew of room temperature diffusion controlled reactions. University, has been able to demonstrate how The crystals were bathed in a solution that included semiconductor nanocrystals can be doped in order the dopants, where slow diffusion allowed for to change their electronic properties and be used impurities to find their way into the nanocrystal. as conductors. This opens a world of possibilities, says Prof. Rabani, in terms of applications of small The researchers used a scanning tunneling electronic and electro-optical devices, such as microscope (STM), a device that images surfaces diodes and photodiodes, electric components used at an atomic level, in order to determine the in cellular phones, digital cameras, and solar success of their doping procedure. These panels. measurements indicated how the Fermi energy of the nanocrystals changed upon doping, a key Solar panels are typically made from a pn junction. feature in controlling the electronic properties of When they absorb light, the junction separates the electronic devices. The results, notes Prof. Rabani, negatively charged electrons and the positively indicate that the nanocrystals have been doped charged holes, producing an electrical current, with both n-type dopants, indicating the presence of explains Prof. Rabani. "With this new method for excess electrons in the nanocrystals, and p-type, doping nanocrystals to make them both p and n which contribute positively charged holes to the type, we hope that solar panels can be made not semiconductors. This will allow for their use in only more efficient, but cheaper as well," he says. electronics that require a pn junction, such as solar This research has been published recently in the panels, light emitting diodes, and more. journal Science. Broadening the nanocrystal spectrum Crystal-clear progress Not only did Prof. Rabani and his fellow According to Prof. Rabani, the quest to electrically researchers succeed in doping nanocrystals dope nanocrystals has been an uphill battle. The without bleaching their optical properties, but they crystals themselves have the capacity to self- also were able to control the optical properties, purify, which means that they cleanse themselves namely, the color range that the nanocrystals of dopants. Also, he adds, some of the synthetic produce. Once doped, the nanocrystal particles methods for doping were problematic on the nano- could change in color, becoming more red or blue. scale - the crystals were unable to withstand Prof. Rabani and his colleagues were able to doping techniques applicable to bulk develop a theory to explain these observations. semiconductors. Prof. Rabani says that this technology can go a The key, explains Prof. Rabani, was to find a long way. Doping semiconductors, he explains, has

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been essential for the development of technology. "Parallel to this, we also know we want to make electrical components very small. A big portion of future electronics or optics is going to be based on doping nanoparticles."

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