Photo Induced Force Microscopy (PiFM)

Photo-induced force microscopy combines Atomic Force Microscopy (AFM) and Infra-red (IR) spectroscopy principles, to perform chemical mapping of photo-polarizable molecular species, at nano-scale. It is applicable to both organic and inorganic molecules. In this case study, a polymer blend sample of Poly Lactic acid (PLA), Acrylic rubber (ACM) and a third component, was analysed by PiFM to identify and map polymer distribution across the surface.

Photo-induced Force Microscopy (PiFM), which combines principles of AFM (Atomic Force Microscopy) and IR (infra-red) spectroscopy, allows species identification as well as spatial distribution, at the level of individual molecules. When polarizable molecules in a sample are subjected to incident radiation of varying wavelengths from a pulsed laser source, photo-induced molecular dipoles are created. This induces a mirror dipole in a metal coated scanning probe tip coupled to a cantilever and optical position detector system. Electrical dipole-dipole interaction between the polarized molecule and its mirror image in the probe proportionately displaces the tip and cantilever system. This displacement is captured in the optical position detector system. Further, IR absorption spectra of molecules, obtained using a pulsed laser IR source allow species identification. The ability to scan topography as well as obtain absorption spectra at less than 10 nm resolution has many applications in advanced and nano-scale materials.

Chemical mapping is a promising application of PiFM, applicable to nano-scale chemical distribution analysis of many organic and inorganic materials. In this case study, a polymer blend sample of Poly Lactic acid (PLA), Acrylic rubber (ACM) and a third component, was analysed by PiFM to identify and map polymer distribution across the surface. In a multi-step process, AFM was used to first scan the sample topography as shown in Figure 1 (a).  Based on the AFM scan, a few prominent features were targeted for scanning by PiFM to obtain PiF spectra as shown in Figure 1 (f). In this case, since the spectral peaks are in the range of 1072 to 1763 cm^-1., three representative scanning wavelengths were chosen for further PiFM imaging and IR absorption spectra. The figures 1(b), 1(c) and 1 (d) show PiFM images at 1750 cm^-1 , 1070 cm^-1 and 1720 cm^-1 , for PLA (blue), third component (red), and ACM (green) respectively. Absorption spectra can also be obtained at these locations at the same IR wavelengths. Composite images of the PiFM scans and the associated absorption spectra are shown in figures 1 (e) and 1 (f). The composite image (e) can be called a chemical map as it shows the location of different molecular species.