Editorial Feature

An Introduction to Photo-Induced Force Microscopy (PIFM)

There are many types of microscopy out there on the marketplace today, and many are now widely used across all scientific branches. Established microscopy techniques commonly used in research are scanning electron microscopy (SEM), scanning tunnelling microscopy (STM), optical microscopy (OM) and atomic force microscopy (AFM).

However, a new microscopy technique has reached the market in the form of photo-induced force microscopy (PiFM). In this article, we take a look at this technique.

As it is a relatively new technique, PiFM is not known by many scientists. In short, PiFM is a scanning probe (spectroscopic probing) method which allows for the imaging of optical near-fields in nanoscale structures. The technique can map their morphology and obtain spectroscopic information, all with high spatiotemporal resolution of less than 10 nm and broadband spectral sensitivity.

PiFM operates using principles of AFM, and even contains some of the same components. The technique offers the high resolution seen with AFM coupled with a high spectroscopic sensitivity provided through optically exciting the sample. It can also be used, in principle, in non-contact mode.

As PiFM couples both AFM and optical excitations, the output(s) are PiFM and topography spectra. The machine uses an AFM laser, a metal-coated cantilever tip, a stage to hold the sample, a pulsed QCL, alignment optics, a parabolic mirror and a photosensitive detector, so is not too dissimilar to AFM.

Generally, there are two ways to use a scan probe to detect optically excited molecules. The first, uses near-field methods to detect the optical field by transferring near-field information to a far-field photo-detector. The second, measures the expansion of the sample due to thermal heating after optical excitation and requires a cantilevered tip that is sensitive to minute changes.

PiFM is slightly different to the norm in its approach. Whilst it still uses a cantilever tip, it is designed and optimized to detect the electromagnetically induced force on the tip, rather than sensing heat. PiFM detects the photo-induced molecular polarizability at the molecular level by mechanically detecting the force gradient arising from the interaction between the optically driven molecular dipole and its mirror image.

PiFM not only excites the sample near-field, but also detects the response near-field. This occurs by reading out the time-integrated force between the tip and the sample, and is a novel feature of PiFM that is not seen with any other tip-enhanced optical microscopy techniques.

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The near-field detecting ability enables a much easier and more robust operation compared to techniques that rely on far-field detection. PiFM also possesses another significant advantage in that there is no far-field background signal. The absence of this signal produces an excellent signal-to-noise ratio, even when used at a low excitation power, or with very thin samples.

PiFM does not rely on the constant tapping mode exhibited by other techniques, which is an advantage. PiFM uses a non-contact or light taping AFM mode, which not only protects the softest of samples from damage, but also facilitates a higher spatial resolution of that produced by AFM- this is mainly due to AFM possessing a steeper dependence of the dipole-dipole force on the tip-sample distance.

Despite being a relatively new approach, PiFM has already been found to be compatible with multiple excitation frequencies, ranging from the visible to mid-infrared regions. Such a range has produced a nanoscale imaging contrast, based around the electronic and/or vibrational transitions in the sample.

It’s compatibility with various materials, coupled with its beneficial properties (compared to other microscopy techniques), has already allowed PiFM to establish itself as an attractive method for the visualisation and spectroscopic characterisation of nanomaterials. At the moment, this ranges from semi-conducting nanoparticles, to polymer thin films, to sensitive measurements of single molecules, but will no doubt increase in the future as the techniques becomes more widely known and widely used across varying applications.

Sources:

Molecular Vista- http://molecularvista.com/technology/pifm/

University of Michigan- https://events.umich.edu/event/34588

“Photo-induced force for spectroscopic imaging at the nanoscale”- Jahng J., et al, Proc. SPIE 9764, Complex Light and Optical Forces X, 97641J, 2016

“Photoinduced Force Mapping of Plasmonic Nanostructures”- Tumkur T. U., et al, ACS Nanoletters, 2016

https://molecular-plasmonics.de/molplasmon17/pdf/abstracts/Meyer.pdf

Image Credit: https://www.youtube.com/watch?v=xrnYImhxeT4https://www.youtube.com/watch?v=xrnYImhxeT4

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