There are many microscopy techniques out there, each different to the last. However, there is a new form of microscopy available on the marketplace known as photo-induced force microscopy (PiFM), which shows great similarities, and can be used in conjunction, with atomic force microscopy (AFM).
What is AFM?
AFM is a type of scanning probe microscopy (SPM). AFM uses a cantilever tip to measure the local environment of and measure the local properties exhibited by a sample, such as the height, friction and magnetism.
In a basic sense, an AFM measures a sample by firing a laser towards a sharp cantilever tip (which is scanning the surface) and is reflected back to a position sensitive detector, which measures the relative position of tip and backs out the topology of the surface.
AFMs indirectly measure the force between the probe, i.e. the sharp pyramidal cantilever tip, and the sample, as a function of its relative position. To obtain a high image resolution, AFMs measure both the vertical and lateral deflections of the cantilever using an optical lever. The optical lever is operated by reflecting the laser beam of the cantilever.
The reflected laser beam then hits the position-sensitive photo-detector. The photodetector in an AFM consists of four sections, where the difference between the sections indicates the position where the laser hit the detector, and deduces the angular deflections of the cantilever. The cantilever is kept in place using piezo ceramics, which expand or contract under a voltage bias, and is the main reason why AFM can operate in 3-dimensions.
AFM can investigate samples through a technique called contact mode. This mode employs a feedback loop that both regulates and measures the force on the sample, and allows for samples to be imaged at very low forces.
Tapping mode, which is often applied, works along a similar principle as contact mode but the cantilever ‘taps’ the surface. This mode is often used as it causes less damage to the surface of the sample and a better image can be generated through measuring the differences in the oscillation when the tip is close to the surface and when it is removed.
The feedback loop within an AFM machine normally consists of tube scanner, which controls the height of the tip through the cantilever and optical lever. Upon measuring of the local height, the feedback circuit tries to keep the cantilever deflection constant by adjusting the voltage applied to the scanner. To achieve a high resolution and AFM performance, an efficient and well-constructed feedback loop is essential.
AFM also has a non-contact mode, in which the tip vibrates above the resonance frequency surface of the sample. However, long range interaction forces, such as van der Waals forces, decrease the resonant frequency and this mode is not commonly used. Non-contact mode is more useful for delicate and soft samples, but is not as effective.
Benefits of AFM
AFM possess many advantages over other microscopy techniques, including other established techniques such as scanning electron microscopy (SEM). For instance, unlike other microscopy techniques that back-out a 2D profile, AFM produces a clear 3D image. This is in addition to providing an enhanced resolution over many techniques.
AFM also has an added advantage in that the samples don’t require any special pre-treatment which could damage the sample and produce inaccurate results. AFM can also work under ambient and moisture-rich conditions, so, unlike other microscopy techniques, AFM can be used to study biological and live samples.
Applications of AFM
AFM is a well-established technique that has been used across the biological, chemical and physical sciences for many years. As such it has embedded itself as a staple characterisation technique across many areas, too many to mention specifically.
However, the areas in general in which it commonly finds widespread use are in biomaterials and biomolecules, cells and tissues, energy storage, food science, graphene and other 2D materials, magnetics and data storage, nanomaterial (including property) characterisation, piezo and ferroelectrics, polymers, semiconductors, microelectronics, photovoltaics and solar cells, thin films and coatings.
Within these areas, AFM is used extensively, and these are just some of the main areas in which AFM is employed today.
What is Photo-induced Force Microscopy (PiFM)?
Produced by Molecular Vista, PiFM is a relatively new microscopy technique which detects the photo-induced molecular polarizability of features at the molecular level. It is generally achieved by detecting the force gradient between the optically driven dipole and its mirror image, using a metal-coated AFM tip.
PiFM uses a different approach to most microscopy techniques, including that of AFM. PiFM still uses a cantilever tip to analyse the sample, but in this instance, is designed and optimised to detect the electromagnetically induced force on the tip.
As PFIM couples both AFM and optical excitations, the output(s) are in the form of PiFM and topography spectra. The internal components of a PiFM-machine include 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.
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In terms of a mechanistic approach, PiFM detects the photo-induced molecular polarizability at the molecular level by mechanically detecting the force gradient between the optically driven molecular dipole and its mirror image. The force gradient between the two energies is greatest when the frequency of the external laser is the same as the excitation energy of the sample, producing a measurable attractive force.
Benefits of Photo-Induced Force Microscopy (PiFM)
PiFM has already shown that is possesses multiple benefits. With its ability to detect samples within the near field, it offers an alternative approach to what many other microscopy techniques offer- which is far field resolution. As such, PiFM is known to possess an easy and robust operational approach, unseen by other methods.
One key benefit of PiFM, is its lack of background noise which allows for an excellent signal-to-noise ratio to be produced, even in tricky environments such as low excitation powers and ultra-thin sample experiments.
PiFM, is a great choice for soft materials, such as polymers, because PiFM uses both non-contact and light tapping AFM modes, rather than a constant tapping mode. The use of these modes also allows for an increased spatial resolution against AFM methods.
One other benefit of PiFM is its ability to image over a range of excitation frequencies extending from the visible to the mid-infrared range. This wide berth operating frequency gives PiFM the ability to image contrast at the nanoscale as a function of the electronic and/or vibrational transitions in the sample.
Applications for Photo-Induced Force Microscopy (PiFM)
The ability to produce both the resolution seen with AFM with an optical excitation of the sample, makes PiFM an exciting technique for the future. As it has not been around for a long time, PiFM is only currently used for spectroscopic imaging of samples.
Within this area though, the imaging potential has been tested on a wide range of materials, including polymer blends, nano-fibers, homopolymers, polypeptides, block copolymers (BCP) and 2D materials.
There are many areas where PiFM could be implemented in the future, and show great promise, from further BCP research, to process improvement, defect reduction in materials and in lithographic processes. But only time will tell how many come to fruition.
“Nanoscale chemical imaging by photoinduced force microscopy”- Nowak D., et al,
Science Advances, 2016, DOI:10.1126/sciadv.1501571
“Gradient and scattering forces in photoinduced force microscopy”- Jahng J., et al,
Physical Review B, 90, DOI: 10.1103/PhysRevB.90.155417
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