AFM-IR / Photo-induced Force Microscopy (PiFM)

 

The future of materials science is nano. Many new products are based on nanomaterials and use their specific properties to obtain new or improved functionality. But how do you know your nanomaterial is exactly as you want it? Nanoscale imaging is not enough; what is required is nanoscale chemical imaging. Photo-induced Force Microscopy (PiFM) is a powerful AFM-IR technique to get infrared spectra at sub-10 nm resolution. ST Instruments offers a PiFM instrument designed by Molecular Vista, which can also be equipped to perform scattering scanning near-field optical microscopy (s-SNOM).

 

 

How does AFM-IR work?

 

In nanoIR or AFM-IR, the atomic force tip scans the sample while an infrared laser excites the sample to facilitate IR spectroscopy. Thus, both the topography and the chemical spectra are recorded with high resolution. There are several ways to measure the IR absorption spectra through the AFM tip. In PiFM, the photoinduced force is the result of a dipole-dipole force attraction between the imaging tip and the sample when illuminated with a monochromatic, coherent light source.

Through imaging at multiple IR wavelengths, corresponding to absorption peaks of different chemical species, PiFM has the ability to spatially map nanometre-scale patterns of the individual chemical components. A schematic diagram of PiFM is shown below:

 

Nanoscale Infrared AFM-IR Photo-induced Force Microscopy

 

Why use AFM-IR / PiFM?

 

Compared to competing AFM-IR techniques, photo-induced force microscopy has several advantages. For example, in photo-thermal AFM-based techniques, the thermal expansion in the sample can reduce the spatial resolution and surface sensitivity. For scattering SNOM type AFM-IR techniques the spatial resolution and surface sensitivity are good but important challenges remain in the interpretation of the images, the extraction of useful information from the background and the presence of artefacts in the optical image.

PiFM has an excellent signal to noise ratio and a small interaction volume, leading to increased spatial resolution. It is for example possible to resolve the different chemical blocks of a PS-b-PMMA block copolymer sample with a pitch of about 22 nm (the rise of the signal / resolution  measures less than 7 nm).

 

Spatial resolution PiFM

 

Excellent surface sensitivity is demonstrated in the DNA origami measurement below. Especially the image at 1000 cm-1 which highlights Mica shows that the technique does not measure the Mica under the 3nm DNA patch.

 

Surface sensitivity PiFM

 

Finally, PiFM does not suffer from background interference and shows a good correlation with conventional IR-spectra from bulk materials without sample thickness effects.

 

Correlation PiFM FTIR

 

A powerful way to analyse unknown samples is hyperspectral AFM-IR imaging (hyPIR), providing images of all the resonant modes in a structure with a single scan. A 128 by 128 pixel grid can be produced in about one hour.

 

hyPIR PiFM hyperspectral

 

Some PiFM examples

 

Using Infrared photo-induced force microscopy you will be able to acquire IR spectra while simultaneously mapping topography. Infrared wavelengths specific to different chemical entities resolve the nanometre-scale distribution of each chemical species in diverse multi-phase and multi-component systems.

 

AFM-IR PiFM image blend

 

For example, hyperspectral infrared PiFM will give you a quick idea about the chemical makeup of perovskite photovoltaic films, which have a complicated domain structure. The scan shows different chemical phases and chemical transitions across phase boundaries.

 

DNA origami molecules (~3 nanometre in height) deposited onto mica are readily imaged via both topography and PiFM. The signal of the DNA molecules clearly shows the amide I vibrational mode.

 

For many semiconductor applications, PiFM is a viable tool for, amongst others, the visualization and analysis of area selective deposition (ASD / Image below), local strain, the cross-section of a trench, the cross-section of a multilayer stack, chemical mechanical polishing (CMP), buried conductive layers and defect analysis.

 

AFM-IR PiFM image ASD

 

As the future of materials science is nano, photo-induced force microscopy (AFM-IR) is an indispensable tool to analyse the topology and chemical makeup of new materials in exquisite detail.