A novel noninvasive spectroscopic scanning probe method called Peak Force Infrared Microscopy (PFIR) has been developed by the Researchers at the Lehigh University’s Department of Chemistry, in collaboration with New Jersey Institute of Technology’s Department of Chemistry and Environmental Science.
The PFIR microscopy technique, recently published in Science Advances, allows for simultaneous chemical imaging, collection of broadband infrared (IR) spectra and mechanical mapping at a high spatial resolution of 10 nanometers (nm)1. This new nondestructive microscopy technique has the potential to be used in a wide variety of imaging applications including rough, sticky and very tiny delicate samples1.
Most materials in nature occur as heterogeneous samples that have different chemical composition. For example, cell membranes are embedded with proteins that have specific functions therefore nanoscale defects present within the materials affect the chemical and mechanical properties of the material1.
While research communities can benefit greatly from having the ability to non-destructively obtain details about the chemical composition, mechanical properties and geometric shapes of a sample at a high resolution, present-day microscopy techniques are not capable of performing this multimodal characterization. The IR absorption spectrum of a material depends on the vibrational modes of the functional groups present in the material1. Due to the optical diffraction limit corresponding to several micrometers in Fourier transform infrared (FTIR) microscopy, it is not possible to achieve nanoscale spatial resolution1.
Infrared scattering type scanning near field optical microscopy (s-SNOM) utilizes a sharp metal-coated probe controlled by atomic force microscope (AFM) to locally improve the optical field and provide spatial information of the sample by exciting the vibrational, phonon or polaritonic resonances in the sample1, 2.
The spatial resolution of this technique depends on the curvature of the probe, which usually measures between 10 – 20 nm. While this microscopy could overcome the diffraction limit at mid IR frequencies, there are a few limitations associated with s-SNOM. These limitations include:
- Because of the scattering of the incident light from the surrounding of the sharp tip of the probe, a large far-field background is present. The interferometric detection and lock-in signal demodulations used here to minimize background noise and improve the contrast, adds bulk to the equipment.
- The incorporation of Fourier Transform (FT) type detection required to collect a broad spectrum requires very expensive light sources such as femtosecond lasers or synchrotron IR radiation1.
Due to its spatial resolution of only 50 – 100 nm, Atomic Force Microscopy techniques (AFM-IR) are also not adequate for detailed nanoscale characterization1. Although photoinduced force microscopy (PiFM), which is based on the dipole-dipole interactions between the metallic probe and the dipoles induced in the sample can technically perform IR imaging of polymer surfaces with a spatial resolution of approximately 10 nm, this technique requires a special AFM probe with two well behaved mechanical resonances with appropriate frequency difference1.
To overcome these challenges, Xiaoji G. Xu’s team developed an action based spectromicroscopy technique known as the PFIR microscopy, that combined synchronized IR pulse excitations with time-gated detection of the laser-induced mechanical responses in the sample1. Due to its ability to operate in the peak force tapping mode of AFM, the PFIR microscopy could be used in a analyze a variety of samples, including rough, sticky, fine and intricate samples, that cannot be analyzed using previous techniques1.
While s-SNOM requires separate narrowband and broadband laser sources to perform single wavelength imaging and nano-FTIR measurements, the PFIR microscopy uses a simple equipment with one simple laser source, with which both chemical imaging and nanoscale point microscopy can be simultaneously performed1, 2. In the present study, Xiaoji G. Xu’s team used three types of representative materials including soft polymers, perovskits crystals and boron nitride nanotubes to demonstrate the suitability of PFIR microscopy to a variety of materials1. It is possible to obtain fuller IR spectrum and a sharper image with 6 nm spatial resolution, all with using a light source that only costs one third of the cost of light sources used in existing methods1.
Materials which have different chemical compositions and varying rigidity properties can also be analyzed by this technique. Through the integration of two types of measurements in one device, PFIR equipment is compact, requiring much less space as compared to previous microscopy equipment that has been used for similar purposes.
Although the PFIR technique is unsuitable for liquid samples, the properties of dried biological samples, such as cell walls and other protein aggregates, can be analyzed at 10 nm spatial resolution without requiring staining or any genetic modification1.
Overall, the novel PFIR microscopy developed by Xiaoji G. Xu’s team is a powerful tool that compact and less expensive, yet suitable for the simultaneous analyzes of chemical imaging, collection of broadband infrared (IR) spectra, and mechanical mapping at a high spatial resolution of 10 nanometers of a variety of materials1.
- “Nanoscale simultaneous chemical and mechanical imaging via peak force infared microscopy” L. Wang, H. Wang, et al. Science Advances. (2017). DOI: 10.1126/sciadv.1700255.
- “Scanning Near-Field Microscopy (SNO) – Principles and Modes of Operation by NT-MDT”