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Research on the Role of PFIR Microscopy in Explorations at the Nanoscale Across Wide Disciplines

According to Xiaoji Xu, aerosol particles may be invisible and tiny, but these particles suspended in gases play a vital role in environmental pollution and cloud formation and can be harmful to human health.

The lower portion of this image by Xiaoji Xu’s group shows the operational scheme of peak force infrared (PFIR) microscopy. The upper portion shows the topography of nanoscale PS-b-PMMA polymer islands on a gold substrate. (Image courtesy of Xiaoji Xu)

Aerosol particles that are detected in dust, haze and vehicle exhaust measure in the microns. One micron is one-millionth of a meter; a thin strand of human hair is about 30 μm thick.

Xu states that the particles are among the many materials whose mechanical and chemical properties cannot be completely measured until Scientists come up with a better method of studying materials at the microscale as well as the much smaller nanoscale (1 nm is one-billionth of a meter).

Xu, an Assistant Professor of Chemistry, has created such a method and employed it for performing noninvasive chemical imaging of a wide range of materials, as well as mechanical mapping along with a spatial resolution of 10 nm.

The technique, known as peak force infrared (PFIR) microscopy, incorporates scanning probe microscopy and spectroscopy. Besides shedding light on aerosol particles, Xu says, PFIR will enable Scientists to study micro and nanoscale phenomena in a wide range of inhomogeneous materials.

Materials in nature are rarely homogeneous. Functional polymer materials often consist of nanoscale domains that have specific tasks. Cellular membranes are embedded with proteins that are nanometers in size. Nanoscale defects of materials exist that affect their mechanical and chemical properties. PFIR microscopy represents a fundamental breakthrough that will enable multiple innovations in areas ranging from the study of aerosol particles to the investigation of heterogeneous and biological materials.

Xiaoji Xu, Assistant Professor of Chemistry

Recently, Xu and his group reported their results in an article titled “Nanoscale simultaneous chemical and mechanical imaging via peak force infrared microscopy.” The article was featured in Science Advances, a journal of the American Association for the Advancement of Science, which also publishes Science magazine.

The article’s main author is Le Wang, a Ph.D. Student at Lehigh. Co-authors include Xu and Lehigh Ph.D. Students Haomin Wang and Devon S. Jakob, as well as Martin Wagner of Bruker Nano in Santa Barbara, Calif., and Yong Yan of the New Jersey Institute of Technology.

“PFIR microscopy enables reliable chemical imaging, the collection of broadband spectra, and simultaneous mechanical mapping in one simple setup with a spatial resolution of ~10 nm,” the group wrote.

The group then added, “We have investigated three types of representative materials, namely, soft polymers, perovskite crystals and boron nitride nanotubes, all of which provide a strong PFIR resonance for unambiguous nanochemical identification. Many other materials should be suited as well for the multimodal characterization that PFIR microscopy has to offer... In summary, PFIR microscopy will provide a powerful analytical tool for explorations at the nanoscale across wide disciplines.”

An article about the use of PFIR to study aerosols was recently published by Xu and Le Wang. Titled “Nanoscale spectroscopic and mechanical characterization of individual aerosol particles using peak force infrared microscopy,” the article featured in an “Emerging Investigators” issue of Chemical Communications, a journal of the Royal Society of Chemistry. Xu was featured as one of the prominent investigators in the issue. The article was coauthored with Researchers from the University of Macau and the City University of Hong Kong, both based out of China.

Xu states that PFIR simultaneously attains mechanical and chemical information. PFIR enables Researchers to examine a material at different places, and to establish its mechanical properties and chemical compositions at each of these places, at the nanoscale.

A material is not often homogeneous. Its mechanical properties can vary from one region to another. Biological systems such as cell walls are inhomogeneous, and so are materials with defects. The features of a cell wall measure about 100 nanometers in size, placing them well within range of PFIR and its capabilities.

Xiaoji Xu, Assistant Professor of Chemistry

Xu points out that PFIR has a number of advantages over scanning near-field optical microscopy (SNOM), which is presently used for measuring material properties. PFIR obtains a fuller infrared spectrum and a sharper image — 6 nm spatial resolution — of  wider variey of materials than does SNOM. SNOM works well with inorganic materials, however, but does not obtain an infrared signal that is as strong as that obtained by the Lehigh technique from softer materials such as biological materials or polymers.

“Our technique is more robust,” says Xu. “It works better with soft materials, chemical as well as biological.”

The second advantage of PFIR is that it is capable of performing what Xu calls point spectroscopy.

“If there is something of interest chemically on a surface,” Xu says, “I put an AFM [atomic force microscopy] probe to that location to measure the peak-force infrared response. It is very difficult to obtain these spectra with current scattering-type scanning near-field optical microscopy. It can be done, but it requires very expensive light sources. Our method uses a narrow-band infrared laser and costs about $100,000. The existing method uses a broadband light source and costs about $300,000.”

Xu states that a third advantage refers to the fact that PFIR obtains a mechanical and also a chemical response from a material.

Xu said, “No other spectroscopy method can do this,” says Xu. “Is a material rigid or soft? Is it inhomogeneous—is it soft in one area and rigid in another? How does the composition vary from the soft to the rigid areas? A material can be relatively rigid and have one type of chemical composition in one area, and be relatively soft with another type of composition in another area. Our method simultaneously obtains chemical and mechanical information. It will be useful for analyzing a material at various places and determining its compositions and mechanical properties at each of these places, at the nanoscale.”

According to Xu, a fourth advantage of PFIR refers to its size.

“We use a table-top laser to get infrared spectra. Ours is a very compact light source, as opposed to the much larger sizes of competing light sources. Our laser is responsible for gathering information concerning chemical composition. We get mechanical information from the AFM. We integrate the two types of measurements into one device to simultaneously obtain two channels of information.” Xu added.

Xu explains that even though PFIR does not work with liquid samples, it is capable of measuring the properties of dried biological samples, including protein aggregates and cell walls, accomplishing a 10 nm spatial resolution without genetic modification or staining.

Xu’s work received start-up funding from Lehigh, a Lehigh Faculty Research Grant, and in-kind equipment support from Bruker Nano.

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