Photo-induced Force Microscopy (PiFM), Molecular Vista

The Photo-induced Force Microscope (PiFM) and Infrared Spectroscopy (PiF-IR) is a revolutionary technique that uses mechanical instead of optical detection to measure the near-field optical interaction between the cantilever tip and sample. PiFM leverages local polarization of the sample by tip-enhanced optical illumination, resulting in highly localized forces acting between the tip and sample which are easily detected by atomic force microscopy (AFM). Coupled to a mid-infrared (MIR) light source and combined with an interferometer, it can perform nanoscale Fourier Transform Infrared Spectroscopy (nano-FTIR) that allows for simultaneous acquisition of 3D topographic data with molecular chemical information at high spatial resolution (~ 5 nm) and excellent spectral resolution (~ 1 cm⁻¹).

In PiFM, the tip-sample interface generates two types of short-range forces: (1) the usual van der Waals (vdW) force that non-contact AFM (NC-AFM) relies on for mapping the topography of the sample surface, and (2) the photo-induced force (PiF) generated by near field illumination. Both forces act over a very short range, coming into play only when the tip is atomically close to the sample surface (~10 nm or less). PiFM utilizes two vibrational modes of the rectangular AFM cantilever to measure the two forces independently – the interatomic vdW force with one mode, and the photo-induced force (PiF) with another. Since the vibration of the two modes is completely independent, motion in one mode generally does not influence the other.

The metal-coated AFM tip creates a highly local enhanced field (yellow) from the PiF excitation laser (red). This field locally polarizes the sample, resulting in tip-sample forces that are detected by PiFM.

Instrumentation

PiFM Vista One at RSES is an original PiF microscope from Molecular Vista for nano‑IR chemical analysis. It is installed and in use since February 2021. Vista One makes nanoscale chemical maps and point spectra with more detail than FTIR or nano-FTIR. It features mono-layer molecular sensitivity and can characterize organic and inorganic materials at the highest resolution.

Sub-10 nm PiF‑IR spectroscopy

• 1 wavenumber spectral resolution with a widely tunable laser for nano-IR experiments
• Tunable IR lasers can sweep a full PiF‑IR spectrum in as little as 100 ms.
• Use existing FTIR spectra to identify materials.

Sub-10 nm PiFM chemical mapping

• Fixed-wavelength PiFM images can be acquired in minutes with a range of up to 100 µm in X and Y.
• hyPIR™ imaging and our automated principle component analysis tools provide complete image and spectral data‑sets with minimal effort.

Sample formats

• Sample Size: less than 50 mm x 50 mm
• Sample Thickness: less than 10 mm
• Sample Shape: PiFM is based on AFM. If AFM measurement cannot be done on the sample, then PiFM measurement cannot be done as well. Therefore, it may be very much worthwhile to try the sample with an AFM to ensure that the region of interest is accessible by AFM.
• Reference Samples: When providing reference samples for analysis of blended samples or for identification of potential contaminants, please provide enough material for both ATR-FTIR measurements and PiFM measurements. These measurements are possible for films if the thickness is over ~ 500 nm. Please use a substrate that is not too conductive (glass, silicon, mica, sapphire, etc. are good choices for substrates).
• Sample Storage/Transport: PiFM is surface sensitive enough to detect monolayer of molecules. When shipping, do not use gelpak sample containers as they will outgas and contaminate the sample surface. Use Fluoroware if available. If not, use wafer carrier trays made out of Natural Polypropylene as they have lower outgassing [1]. If adhesive needs to be used, please use metal tapes with acrylic adhesive.

Data Analysis Software

• SurfaceWorks 3.0

Access

The instrument is available for ANU researchers and must only be used by trained and authorized operators in accordance with proper safety procedures. It is also available as a fee-for-service for external researchers and industry.

• Contact lab manager to discuss options for training (DIY) or service data acquisition.

• Prerequisites for DIY users: Completion of ANU Pulse Online Training Module Laser Safety and WHS induction.