MIM capacitance images of aligned SWNTs on quartz substrates overlaid on top of 3D surface topography produced by atomic force microscopy.
MIM capacitance images of aligned SWNTs on quartz substrates overlaid on top of 3D surface topography produced by atomic force microscopy.

How do you study a material that you cannot see? That is a question that researchers investigating nanomaterials such as quantum dots, nanoparticles and nanotubes are seeking to answer.

Recent discoveries such as a super-resolution microscopy, which won the Nobel Prize in 2014, have greatly enhanced scientists' capacity to use light to learn about these small-scale objects. Nevertheless, the fact that the wavelength of the inspecting radiation is always much larger than the scale of the nano-objects being studied still creates problems.

For example, nanotubes and nanowires – the building blocks of next-generation electronic devices – have diameters that are hundreds of times smaller than visible light can resolve. Researchers must find ways to circumvent this physical limitation in order to achieve sub-wavelength spatial resolution and to explore the nature of these materials.

A group of scientists, including Slava Rotkin from Lehigh University, has now reported an important new method for measuring the properties of nanotube materials using a microwave probe. Their findings are published in a paper in ACS Nano.

The researchers studied single-walled carbon nanotubes (SWNTs), which have electronic properties that make them excellent candidates for use in next-generation electronics technologies. The first prototype of a nanotube computer has already been built by researchers at Stanford University, while the IBM T.J. Watson Research Center is currently developing nanotube transistors for commercial use.

For this study, the scientists grew a series of parallel nanotube lines, similar to the way nanotubes will be used in computer chips, with each nanotube just 1nm wide. To explore the material's properties, they then used microwave impedance microscopy (MIM) to image individual nanotubes.

"Although microwave near-field imaging offers an extremely versatile 'nondestructive' tool for characterizing materials, it is not an immediately obvious choice," explained Rotkin, a professor with a dual appointment in Lehigh's Department of Physics and Department of Materials Science and Engineering. "Indeed, the wavelength of the radiation used in the experiment was even longer than what is typically used in optical microscopy – about 12 inches, which is approximately 100,000,000 times larger than the nanotubes we measured."

He added: "The nanotube, in this case, is like a very bright needle in a very large haystack."

The imaging method they developed used reflected microwaves to show exactly where the nanotubes are on the silicon chip. More importantly, the information delivered by the microwave signal from individual nanotubes revealed which nanotubes were and were not able to conduct electric current. Unexpectedly, the scientists were even able to measure the nanotube quantum capacitance – a very unique property of a nanoscale object – under these experimental conditions.

"We began our collaboration seeking to understand the images taken by the microwave microscopy and ended by unveiling the nanotube's quantum behavior, which can now be measured with atomistic resolution," said Rotkin.

As an inspection tool or metrology technique, this approach could have a tremendous impact on future technologies, allowing optimization of processing strategies like scalable enriched nanotube growth, post-growth purification, and fabrication of better device contacts. Using MMI, Rotken and his colleagues can now distinguish, in one simple step, between semiconductor nanotubes that are useful for electronics and metallic ones that can cause a computer to fail.

This story is adapted from material from Lehigh University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.