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insulator with nanoscale metals on top,” he says. “In principle, you could get a piece of plastic and spread a handful of metal powders on top to make the devices, if you do it right. But we were trying to create it in nanoscale, so we chose a nanoscale insulator, boron nitride nanotubes, (or BNNTs) for the substrate.”
Yap’s team had figured out how to make virtual carpets of BNNTs, which happen to be insulators and thus highly resistant to electrical charge. Using lasers, the team then placed quantum dots (QDs) of gold as small as three nanometres across on the tops of the BNNTs, forming QDs-BNNTs.
BNNTs are the perfect substrates for these quantum dots due to their small, controllable, and uniform diameters, as well as their insulating nature. BNNTs confine the size of the dots that can be deposited.
In collaboration with scientists at Oak Ridge National Laboratory (ORNL), they fired up electrodes on both ends of the QDs-BNNTs at room temperature, and something interesting happened. Electrons jumped very precisely from gold dot to gold dot, a phenomenon known as quantum tunnelling.
“Imagine that the nanotubes are a river, with an electrode on each bank. Now imagine some very tiny stepping stones across the river,” says Yap. “The electrons hopped between the gold stepping stones. The stones are so small, you can only get one electron on the stone at a time. Every electron is passing the same way, so the device is always stable.”
Yap’s team had made a transistor without a semiconductor. When sufficient voltage was applied, it switched to a conducting state. When the voltage was low or turned off, it reverted to its natural state as an insulator.
What’s more, there was no “leakage”. In other words, no electrons from the gold dots escaped into the insulating BNNTs, thus keeping the tunnelling channel cool. In contrast, silicon is subject to leakage, which wastes energy in electronic devices and generates a lot of heat.
Other people have made transistors that exploit quantum tunnelling, explains Michigan Tech physicist John Jaszczak, who has developed the theoretical framework for Yap’s experimental research. However, those tunnelling devices have only worked in conditions that would discourage the typical cellphone user.
Jaszczak says, “They only operate at liquid-helium temperatures”.
The secret to Yap’s gold-and-nanotube device is its submicroscopic size: one micron long and about 20
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www.compoundsemiconductor.net July 2013 nanometres wide.
”The gold islands have to be on the order of nanometres across to control the electrons at room temperature,” Jaszczak says. “If they are too big, too many electrons can flow.” In this case, smaller is truly better: “Working with nanotubes and quantum dots gets you to the scale you want for electronic devices.”
“Theoretically, these tunnelling channels can be miniaturised into virtually zero dimension when the distance between electrodes is reduced to a small fraction of a micron,” says Yap.
Yap has filed for a full international patent on the technology.
This work is described in the article “Room Temperature Tunneling Behavior of Boron Nitride Nanotubes Functionalized with Gold Quantum Dots,” by Chee Huei Lee et al, published online on June 17th in Advanced Materials. DOI: 10.1002/adma.201301339
This work was funded by the Office of Basic Energy Sciences of the US Department of Energy (Award # DE- FG02-06ER46294, PI:Y.K.Yap) and was conducted in part at ORNL (Projects CNMS2009-213 and CNMS2012- 083, PI: Y.K.Yap).
Enhance optical and electron microscopy with QDs
A fast, versatile, and high-resolution technique potentially allows surface and subsurface viewing of features 10 nm in size
Researchers working at the National Institute of Standards and Technology (NIST) have developed a new microscopy technique that uses a process similar to how an old tube television produces a picture.
Using cathodoluminescence, the scientists have imaged nanoscale features. Combining the best features of optical and scanning electron microscopy, the fast, versatile, and high-resolution technique allows scientists to view surface and subsurface features potentially as small as 10 nanometres (nm) in size.
Much like in an old tube television where a beam of electrons moves over a phosphor screen to create images, the new microscopy technique works by scanning a beam of electrons over a sample that has been coated with specially engineered quantum dots.
This approach is quantitatively compared with direct
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