PiFM – best of Nano IR and scanning near-field optical microscopy
Nano IR (for nanoscale infrared spectroscopy) refers to a combination of AFM and IR spectroscopy in one instrument to acquire both topography and chemical signature from a nanoscale region. Among the currently available Nano IR techniques, Photo-induced Force Microscopy (PiFM) enjoys an exceptional spatial resolution (~ 5 nm), excellent sensitivity (monolayer), good correlation to bulk FTIR spectrum, and universal sample applicability (organic, inorganic, and 2D materials).
Exceptional Spatial Resolution in Chemical Mapping
Ps-b-PMMA Block Coploymer, L0 = 22 nm
A block copolymer (BCP), with its well-defined distribution of chemical components, is a good sample to demonstrate the spatial resolution of nano IR instruments. In this PS-PMMA BCP, each chemical component is about 11 nm wide. By setting the excitation laser to 1493 cm-1 (1733 cm-1), a PS (PMMA) absorption band, PiFM highlights just the PS (PMMA) molecules. The area defined by the white circles shows that PS and PMMA molecules occupy complementary regions as BCP should. The cross-section clearly shows ~ 5 nm spatial resolution. Other nano IR techniques have yet to show separate images of distinct chemical components imaged at the associated IR bands on such tightly packed BCP.
Excellent Sensitivity (Monolayer)
triple helix collagen
Sample courtesy of Jinhui, PNNL
A triple helix collagen consists of three chains of repetitious amino acid sequence, with a diameter of ~ 2 nm. PiFM image taken at the amide I band (1666 cm-1) demonstrates excellent signal from individual collagen helix molecules. Examining the area defined by the dashed line rectangle, one can see that PiFM provides more details than the topography.
Good Correlation between PiFM and Conventional IR Spectra
PiFM spectra generally correlate with conventional IR spectra recorded from bulk samples. PiFM spectrum on polyethersulfone (PES) agrees exceptionally well with FTIR spectrum (shown above). More typically, differences in amplitude ratios are observed in PiFM spectra, due to a much smaller population of molecules probed.
Universal Sample Applicability
PiFM works equally well with inorganic and 2D materials. The PiFM image on the left combines two images at 941 cm-1 (green for FePO4) and 1054 cm-1 (red for LiFePO4) to show how lithium is escaping from LiFePO4 micro-crystals upon delithiation. The image in the middle shows the strength of Si-O band in SiO2 in-between and near SiGe layers (shown as minimum signal.) The cross-section shows that the Si-O bond gradually recovers to the unstrained condition about 400 nm away from the last SiGe line, and that in between the SiGe lines, strain reduces the Si-O band amplitude. The right image shows the surface phonon polariton on 2D hexagonal boron nitride flake.