Scientists from the University of California-Riverside have developed a new imaging technology that compresses lamp light such that it can be “held” at the end of a silver nanowire, revealing previously invisible detail at the nanoscale.
Materials scientists have developed materials for next-generation electronics so tiny that they are indistinguishable when closely packed. They are so small that, even under the most powerful optical microscopes, they do not reflect enough light (visible light varies in wavelength from 400-700nm) to show fine details such as colours. For instance, carbon nanotubes merely appear grey under an optical microscope.
The inability to distinguish fine details and differences between individual pieces of nanomaterials makes it difficult for scientists to study their unique properties and discover ways to perfect them for practical applications.
The advance permitted by this new technique improves colour-imaging resolution to an unprecedented 6nm level, helping scientists view nanomaterials in enough detail to make more use of them in electronics and other applications.
Professor Ming Liu and Professor Ruoxue Yan, both from Riverside’s College of Engineering, developed the tool with a superfocusing technique previously used to observe the vibration of molecular bonds at 1nm spatial resolution, without the need for any focusing lens. In this study, they modified the tool to measure signals ranging the entire visible wavelength range; this allows it to be used to render the colour and depict the electronic band structures of the object being observed, instead of just molecule vibrations.
The tool squeezes the light from a tungsten lamp into a silver nanowire with near-zero scattering or reflection, where light is carried by the oscillation wave of free electrons at the silver surface.The condensed light leaves the silver nanowire tip - which has a radius of just 5nm - in a conical path, like the widening light beam from a torch. When the tip passes over an object, its influence on the beam shape and colour is detected and recorded.
“It is like using your thumb to control the water spray from a hose,” explained Liu, “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 then focused into a spectrometer, where it forms a very small ring shape. By scanning the probe over an area and recording two spectra per pixel, the researchers can formulate the absorption and scattering images in full colour. This means that the formerly grey carbon nanotubes can be photographed in colour.
“The atomically smooth sharp-tip silver nanowire and its nearly scatterless optical coupling and focusing is critical for the imaging,” said Yan. “Otherwise, there would be intense stray light in the background that ruins the whole effort. “
The researchers expect that the new technology can be an important tool to help the semiconductor industry make uniform nanomaterials with consistent properties for use in electronic devices. The new full-colour nanoimaging technique could also be used to improve scientific understanding of catalysis, quantum optics, and nanoelectronics. The study was described in a Nature Communications paper.
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