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Researchers Succeed in Using Doping To Shift Nanocrystals for Electronics

by Editor1 last modified July 20, 2011 - 14:40

A team of researchers from Tel Aviv University and Hebrew University has successfully used doping to change the electronic properties of semiconductor nanocrystals. The work would allow nanocrystals to be used as conductors in microelectronics and electro-optics. The work was done by Prof. Eran Rabani of Tel Aviv University's School of Chemistry in collaboration with Pros. Uri Banin and Prof. Oded Millo from Hebrew University.

Researchers Succeed in Using Doping To Shift Nanocrystals for Electronics

A team from Tel Aviv University and Hebrew University has successfully used doping to change the electronic properties of semiconductor nanocrystals.

In traditional bulk semiconductor production, doping intentionally introduces impurities into an extremely pure (or intrinsic) semiconductor to modulate its electrical properties. Lightly and moderately doped semiconductors are referred to as extrinsic. A semiconductor doped to such high levels that it acts more like a conductor than a semiconductor is referred to as degenerate.

But, doping at the nanoscale has proved elusive. The effort to electrically dope nanocrystals has been “an uphill battle,” Prof. Rabani said. In large part, this is because nanocrystals have the capacity to self-purify, cleansing themselves of dopants. A further hurdle is that some synthetic doping methods are problematic on the nanoscale because the nanocrystals were unable to withstand doping techniques used on larger bulk semiconductors, he said.

The novel doping technique for nanocrystals could prove extremely valuable for devices made using pn junctions, such as solar panels.

When these pn junctions absorb light, they separate negatively charged electrons and positively charged holes. This produces an electrical current. "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,” Prof. Rabani said.

The key to the research teams’ work was in finding a doping method for nanocrystals that would succeed without "bleaching" the crystals’ optical properties. Losing these properties would nullify the absorption capabilities. "Whatever you can do with nanocrystals, you can do with doped nanocrystals – and more by controlling their electronic properties," Prof Rabani noted.

The team’s approach was to apply room temperature diffusion-controlled reactions to the nanocrystals. In specific, the team bathed nanocrystals in a solution that included the dopants, which allowed a slow diffusion to occur, whereby the impurities could enter the nanocrystals.

To measure the success of their approach, researchers used a scanning tunneling microscope (STM), which images surfaces at an atomic level. Measurements indicated a change in the nanocrystals’ Fermi energy upon doping -- a key feature in controlling the electronic properties of electronic devices.

The team reported that the results indicated the nanocrystals have been doped with both n-type and p-type dopants, which would allow for their use in electronics that require a pn junction, used by solar panels, light emitting diodes, and more. In addition, the team was able to control the optical properties or the color range that the nanocrystals produce. Once doped, the nanocrystal particles could change color, becoming more red or blue.

“A big portion of future electronics or optics is going to be based on doping nanoparticles," he added.

Doped' nanocrystals are the future of technology, says Tel Aviv University researcher

The team’s research appears in the journal Science.