Sep 19, 2013
Nanogaps intensify terahertz waves
Using a technique known as atomic layer deposition and, surprisingly, a roll of sticky tape, researchers from the US and Korea have come up with a new technique to produce extremely thin gaps in metal layers. Light with frequencies in the optical or near-infrared parts of the electromagnetic spectrum can pass through the gaps, as can terahertz waves with millimetre-scale wavelengths – even though the gaps are thousands or hundreds of thousands of times narrower than these wavelengths. The technology will allow scientists to rapidly manufacture gaps and slits as narrow as 1 nm across, something that was hitherto impossible, and so make devices, such as ultrafast and energy-efficient nanoscale optical switches and miniaturized chips. Sensors that rely on harnessing terahertz waves for diagnostics and chemical sensing could also be envisaged, says the team.
Sophisticated patterning techniques such as electron-beam lithography and ion-beam milling are routinely used to create gaps and slits in metals and semiconductors. However, it is not possible to pattern sub-nanometre-wide vertical gaps in metals using these standard techniques.
The researchers, led by Sang-Hyun Oh of the University of Minnesota in the US instead constructed their nanogaps by layering atomic-scale thin films of aluminium oxide on the sides of metal patterns using a process known as atomic layer deposition (ALD) and then capping the structure with another metal layer. "No expensive patterning tools were needed to form the gaps this way, but we found that it was quite a challenge to remove the excess metals on top and expose the tiny gaps," explains Oh. "Happily, our student Xiaoshu Chen, also at Minnesota, found that she could do this by using simple Scotch tape."
"The sticky tape works nicely, which was unexpected," he said. "The technique is simple yet can create uniform and ultrasmall gaps, the like of which we have never been able to produce before," he told nanotechweb.org. "We hope that the technique will be rapidly adopted by other research teams."
The gaps formed using the new technique are narrower than anything that can be created using conventional patterning techniques, he added. What is more, the thickness of the gaps can be highly uniform over millimetre length scales – which is a million times bigger than the gap thickness itself – and can be controlled with atomic-scale resolution. Another first.
The researchers measured the transmission of different wavelengths of light through the tiny gaps and found that light with frequencies in both the optical and near infrared parts of the electromagnetic spectrum pass through. "Terahertz waves also pass though," said Oh, "but the good thing here is that the intensity of these waves is boosted by an unprecedented factor of up to 600 million." To measure such effects, the team – which includes researchers from Seoul National University and Argonne National Laboratory – had to pattern 1 nm-wide gaps, which extend along millimetre-sized loops, and pattern them over a whole 4-inch wafer. "This is only possible using the atomic layer lithography/sticky tape technique we developed," says Oh.
The method could allow researchers to make nanoscale optical switches and miniaturized chips that process data faster using much less energy, adds team member Hyeong-Ryeol Park. "There is also a growing interest in harnessing THz waves for diagnostics and chemical sensing but the key challenge here has been the size mismatch between their millimetre-scale wavelengths and the molecules being detected. The fact that the THz waves become much more intense inside the nanogap means that they could now be used to sensitively detect small numbers of molecules that enter the gap." This phenomenon could be exploited in applications such as rapid medical diagnostics or for detecting dangerous chemicals in the environment, he said.
More information about the research can be found in Nature Communications doi:10.1038/ncomms3361.
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About the author
Belle Dumé is contributing editor at nanotechweb.org.