What is PiFM? (Photo-induced Force Microscopy)
Think of PiFM as supercharged nano-FTIR imaging with ~5 nm spatial resolution and superfast nanoscale IR spectroscopy (as fast as 0.1 second for full spectrum.) PiFM acquires both topography and chemical signature at the nanometer scale and demonstrates excellent sensitivity (monolayer), good correlation to bulk FTIR spectra, and universal applicability to a wide range of organic and inorganic materials.
What is special about PiFM
(Photo- induced Force Microscopy)?
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Exceptional Spatial Resolution in Chemical Mapping
Exceptional Spatial Resolution in Chemical Mapping of PS-b-PMMA Block Copolymer with PiFM.
A block copolymer, with its well-defined distribution of chemical components, demonstrates well the spatial resolution of nano-IR instruments. In this sample of a polystyrene-polymethylmethacrylate block copolymer (PS-b-PMMA), each polymer lamella is ~11 nm wide. By setting the excitation laser to the 1493 cm-1 absorption band of PS, PiFM highlights just the PS components. Likewise, by setting the excitation laser to the 1733 cm-1 absorption band of PMMA, PiFM highlights just the PMMA components. The two domains are complementary, easily visualised in the region in the white circle. The cross-section shows the ~5 nm lateral 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.
Triple helix collagen, left consists of three twisted chains of a repeat amino acid sequence with a diameter of ~2 nm. A PiFM image taken at the amide I band’s 1666cm-1 absorption displays strong signals from the individual triple helices.
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.
PiFM spectra generally correlate well with conventional IR spectra from bulk samples. Below PiFM spectrum recorded on a thin polymer (PES) film (blue trace) agrees well with FTIR spectrum recorded from bulk polymer (black trace).
It is not unusual for peak amplitudes to shift on going from bulk FTIR images to surface PiFM images, a manifestation of the different molecular environments experienced in the two cases.
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.
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 delithiation of LiFePO4 submicron crystals.
The middle image qualitatively shows local SiO2 stress based on the magnitude of the 1113 cm-1 absorption line (Si-O bond). The dark vertical lines are areas composed of SiGe and the red areas are composed of SiO2. The cross-section shows that the Si-O bond gradually recovers to the unstrained condition about 400 nm away from the last SiGe line. The cross-section also shows that in between the SiGe lines, strain reduces the Si-O band amplitude.
On the right, contrast in the PiFM image of a flake of hexagonal boron nitride follows the contours of the surface phonon polariton.
Hyperspectral Nano-IR Imaging
Hyperspectral Nano IR Imaging – hyPIR (hyperspectral infrared PiFM) hyPIR (hyperspectral infrared PiFM) images consist of (n x n) pixels of PiFM spectra. Due to the sensitivity of PiFM, a nanoscale hyPIR image with 128 x 128 pixels can be generated in about an hour. It is a powerful way to analyze unknown samples. See the hyPIR movie of surface phonon polariton (SPP) on hexagonal boron nitride (hBN).
Exceptional ease of setup and use
When it comes to usability of PiFM in comparison with sSNOM and TERS the ability of PiFM to align the system only to the tip for detection and to get data in only 10 minutes is seen by customers as a huge advantage. With sSNOM and TERS there is a dependence on both the excitation alignment and the detector alignment that must be maintained raising the technical ability and experience required of any user allowed to use the instrument.
Moreover, both TERS and sSNOM have a dependence on the quality of the tip which will lower the reproducibility of good Signal-2-Noise ratio for all measurements. PiFM does not suffer from these issues. Once the system is set up properly, it is exceptionally simple to change tips and obtain reliable and repeatable results. We use cantilevers with alignment grooves so that it takes less than 10 minutes to align the laser, approach the sample, and acquire the first PiFM spectra.
Furthermore, customers comment on how easy it is to learn and use the instrument. Speed of acquisition compared with other techniques is also a frequent observation.
Nanoscale Identification of MoS2 Layers and Defects / Monolayer Detection on MoSe2 / Confocal Raman Mapping and PiFM on GrapheneRead More