ACUMEN Spring 2022

COLLEGE OF ARTS AND SCIENCES 11 Xiaoji Xu (center) and his team employ spectroscopic microscopy at levels better than the optical diffraction limit. probing the optical near-field in three dimensions at the scale of tens of nanometers. Xu’s PF-KPFM is a robust technique based on the pulsed force method of atomic force microscopy that allows him to routinely map the electrical properties of surface potentials at less than 10 nanometers solution in the lab on a range of samples. “In addition to measuring chemical property or mechanical property of the sample, we can measure the electrical property of the sample. We have achieved very high spatial resolutions, much better than existing technologies. Before our work, others achieved similar resolutions, but not as high, about 50 nanometers to 100 nanometers. In our case, we push it to about 10 nanometers.” Using these practices, Xu’s lab demonstrated not long ago the chemical and mechanical mapping of small indoor aerosol particles. “Because COVID-19 passes through aerosols, we wanted to measure the small particles that may be involved in this transmission [of the virus]. So recently, we focused our research on demonstrating the chemical and mechanical mapping of indoor aerosol particles of less than force microscopy, (PF-KPFM), a new form of imaging tool that tolerates ~ 10 nm spatial resolution for measurement of surface potential under ambient conditions. Researchers are often limited when using optical microscopy, because regular optical microscopies are restricted by the optical diffraction limit. As objects of interest decrease in size, the smallest resolvable distance between two tiny spots using a conventional microscope may never be smaller than half the light wavelength. Nanomaterials often have features smaller than the diffraction limit. To put this nanoscale into perspective, a human hair is approximately 100 micrometers wide, and one nanometer is one thousandth of a micrometer. To overcome this limitation, Xu has developed super-resolution infrared microscopies through the combination of atomic force microscopy with infrared laser radiations. The PFIR microscopy measures the photothermal expansion of materials as needle-like probes interact with a sample. “This needle scans on this surface and follows the sample shape,” says Xu, associate professor of chemistry. “In the meantime, I have a laser shine into this tip and sample. This laser is infrared light, which will excite the sample according to its resonance according to its molecular structures. Then, the sample with absorbed light undergoes photothermal expansion, generating a force that is felt by the sharp tip. By measuring the motion of the sharp tip, I can perform chemical analysis with a high spatial resolution determined by the sharpness of the tip.” Connected to this work, Xu and his team also improved the designs and technologies surrounding scattering-type scanning nearfield optical microscopy. With Xu’s PF-SNOM technique, scattered light from the sample is detected by the metallic tip based on the sample’s optical properties. PF-SNOM permits “B efore our work, others achieved similar resolutions, but not as high, about 50 nanometers to 100 nanometers. In our case, we push it to about 10 nanometers.”