More characterization channels facilitate recovering the secret of nanoscale properties. The method of PFIR-KPFM is an SPM method with multimodal characterization capabilities. It integrates the method of PFIR with PF-KPFM: chemical imaging through PFIR, surface mapping through PF-KPFM, and mechanical mapping with peak force tapping, all with comparable spatial resolutions of ~ 10 nm in a single AFM scan. The detailed construction of the method is described in our paper Angewandte Chemie Int. Ed., 59, 16083  (2020)

The peak force tapping mode has a characteristic trait. The resonant profile of the AFM cantilever dynamically switches between two sets: one is when the tip is detached from the sample surface; the other is when the tip is in momentary contact with the sample. The former is dominated by the cantilever free-space oscillation; the latter contains contact resonance and a shift of the free space oscillation frequency. PFIR operates when the tip and sample are in contact; the PF-KPFM operates when they are detached. The two signal read-out channels are in parallel without interference. Integrating PFIR with PF-KPFM can be done without compromising the capability of each modality.

In PFIR-KPFM, the IR laser emission is synchronized to tip-sample contact, and the FET switching is synchronized to the tip-sample detachment. The photothermal response and the Coulombic force-induced cantilever oscillations

Panel (a) the experimental apparatus. (b) Scheme of temporal domain excitation of IR and KPFM. (c) Cantilever response in PFIR-KPFM.

As a demonstration, we have applied PFIR-KPFM to the characterization of perovskite materials. In one AFM scan, the topography, surface potential, modulus, and PFIR @ 1480 cm-1 . PFIR Spectrum and CPD values can be obtained from the individual locations. Correlation analysis can be obtained from simultaneously obtained image channels.

Another application of the PFIR-KPFM is on amyloid fibrils. Both surface potential and IR signals are collected. Surface potential reveals the presence of local charges; an IR signal is indicative of the secondary structures. The correlative analysis reveals the surface potential signal with the anti-parallel beta stacking.

PFIR-KPFM measurement on FapC fibrils.

Panel (c) shows a strong correlation of surface potential with antiparallel beta stacking.