This image shows how 'white' light from a tungsten lamp is focused into the tip of a silver nanowire to determine the light scattering and absorption of a sample with high fidelity. Image: Ma et. al, 2021.
This image shows how 'white' light from a tungsten lamp is focused into the tip of a silver nanowire to determine the light scattering and absorption of a sample with high fidelity. Image: Ma et. al, 2021.

Many nanomaterials are so tiny that they are not only indistinguishable when closely packed, but don’t reflect enough light to show fine details, such as colors, when studied with even the most powerful optical microscopes. Under an optical microscope, carbon nanotubes, for example, look grayish. The inability to distinguish fine details and differences between individual pieces of nanomaterials makes it hard for scientists to study their unique properties and discover ways to perfect them for industrial use.

Now, in a paper in Nature Communications, researchers from the University of California (UC) Riverside describe a revolutionary imaging technology that compresses lamp light into a nanometer-sized spot. The light is then held at the end of a silver nanowire, like a Hogwarts student practicing the 'Lumos' spell, and used to reveal previously invisible details about nanomaterials, including colors.

This advance, which improves color-imaging resolution to an unprecedented 6nm level, will help scientists see nanomaterials in enough detail to make them more useful in electronics and other applications.

Ming Liu and Ruoxue Yan, associate professors in UC Riverside’s Marlan and Rosemary Bourns College of Engineering, developed this unique tool by taking advantage of a superfocusing technique previously developed by the team. This technique had been used in earlier work to observe the vibration of molecular bonds at 1nm spatial resolution without the need for any focusing lens.

In this new study, Liu and Yan modified the tool to measure signals spanning the whole visible wavelength range, allowing it to render the color and depict the electronic band structures of an object, rather than just observe molecular vibrations. The tool squeezes the light from a tungsten lamp into a silver nanowire with near-zero scattering or reflection; the light is carried by the oscillation wave of free electrons at the silver surface.

The condensed light is then projected from the silver nanowire tip, which has a radius of just 5nm, in a conical path, like the light beam from a flashlight. When the tip passes over an object, its influence on the beam shape and color is detected and recorded.

“It is like using your thumb to control the water spray from a hose,” Liu said, “You know how to get the desired spraying pattern by changing the thumb position, and likewise, in the experiment, we read the light pattern to retrieve the details of the object blocking the 5nm-sized light nozzle.”

The light is focused into a spectrometer, where it forms a tiny ring shape. By scanning the probe over an area and recording two spectra for each pixel, the researchers can formulate the absorption and scattering images with colors. In this way, the first color image of grayish carbon nanotubes can be captured, and an individual carbon nanotube has the chance to exhibit its unique color.

“The atomically smooth sharp-tip silver nanowire and its nearly scatterless optical coupling and focusing is critical for the imaging,” Yan said. “Otherwise, there would be intense stray light in the background that ruins the whole effort. “

The researchers expect that the new technology will become an important tool to help the semiconductor industry make uniform nanomaterials with consistent properties for use in electronic devices. The new full-color nano-imaging technique could also be used to improve understanding of catalysis, quantum optics and nanoelectronics.

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