CAMBRIDGE, Mass., July 28, 2021 – A custom-built near-field infrared (IR) nanoscope and spectroscope by an MIT research team can quickly and economically probe the characteristics of various materials at the nanoscale. Also known as a scattering-type scanning near-field optical microscope, or s-SNOM, the tool can identify internal properties of a material, such as how its optical activity changes over tiny distances. It can also provide a nanoscale view of individual molecules, the researchers showed.
The research team led by Professor Long Ju focused an IR laser positioned on the tip of an atomic force microscope (AFM), turning it into an antenna to amplify the interaction between light and matter. AFM scans the surface of the material and creates a high resolution map of the material topography. The 20nm AFM tip is able to locate physical characteristics of the material that are less than 1nm in height or depth.
By analyzing the backscattered light collected from the light-material interaction, the researchers found that they could learn more about the material’s surface than with conventional AFM. “You can get an image of your sample with a spatial resolution three orders of magnitude greater than conventional infrared measurements,” Ju said.
Depending on their needs and the material samples, users can scan the tip of the nanoscope on the surface of the material while the tip is irradiated with a single wavelength, or they can park the tip on a specific area and probe the area. with different wavelengths of light.
Close-up of the nanoscale material characterization tool. Infrared (red) light is focused on a metal tip. Scattered light can be analyzed for a variety of properties. Courtesy of Long Ju.
In previous work published by Ju and colleagues, the group published images of graphene taken with the AFM and with the new tool. The near-field image taken with the near-field nanoscope developed by MIT included the walls of the domain between two different sections of the material, which were not visible in the image taken with the AFM. The ability to see these domain walls has contributed to a more complete understanding of the structure and properties of the material.
Images comparable in detail to those taken with the MIT nanoscope can be captured by transmission electron microscopy (TEM), but the TEM must be operated in ultrahigh vacuum, which limits the experimental throughput. Additionally, TEM samples must be extremely thin to hang on film or membrane – a requirement, Ju said, which is incompatible with most materials. In contrast, the near-field nanoscope “can be used in air, does not require sample suspension, and you can work on most solid substrates,” Ju said.
The near-field nanoscope provided high-resolution images of surface features; analysis of the light backscattered from the tip of the nanoscope can further provide meaningful information on the internal properties of the sample material. Finally, the device can distinguish between metals and insulators, and between materials that have the same chemical composition but different internal structures, such as diamond versus pencil lead.
The image to the left of a graphene surface was taken using atomic force microscopy. The more detailed image on the right was taken by adding infrared light to the setup by nanoscopy and near-field infrared spectroscopy. Assistant Professor Long Ju designed custom versions of this tool for MIT. Courtesy of Long Ju.
Ju said the nanoscope could even be used to observe the material as it transitions from insulator to superconductor, in response to a change in temperature. It could also be used to monitor chemical reactions at the nanoscale.
Ju’s team completed a second, more advanced version of their near-field nanoscope in May 2021.
The team continues to add functionality to the instrument.