AFM (atomic force microscopy) is used in a seemingly endless range of disciplines, from biology, chemistry and engineering. This is due to its high resolution and versatility, which make it a very useful and powerful tool for imaging the diverse nanoscale world in liquid, air and vacuum environments. AFM use can be highlighted across the disciplines, here are some prime examples of AFM use in specific discipline areas.
Biology - Nanoscale mechanics of brain abscess: an atomic force microscopy study
A brain abscess is a life-threatening swelling of part of the brain which arises from an infection. In this example, the viscoelastic properties: Young’s modulus and hysteresis (H) of three layers of brain abscess tissue were measured using atomic force microscopy for the very first time.
The measurements were performed immediately after surgical removal where the tissue was rough and inhomogeneous in terms of its thickness. Therefore, an AFM instrument with a large z-range movement had to be used, allowing the researchers to map large areas of the tissue without any interruption. In this way, they were able to avoid the preparation steps such as fixation or freezing of the tissue to facilitate measurement, which could go on to affect the mechanical response of the sample.
Nanomechanical characterisation of the abscess is important for understanding how it responds to surgical instruments and for developing precise mechanical models of the brain.
Chemistry - Fast and controlled fabrication of porous graphene oxide: application of AFM tapping for mechano-chemistry
Porous graphene oxide is a crucial material in applications like energy storage and nanofiltration. Pores are commonly formed by etching the material under oxygen plasma. This example demonstrates how the evolution of pores in the material can be controlled locally by scanning it with an AFM tip in tapping mode.
The mechanism is as follows. Graphene oxide has both graphene and polar hydroxylated domains. During the etching process, oxygen species attach and intermediate structures are formed. The unstable nature of these structures means that their carbon-carbon bonds break, which releases volatile products. When the graphene oxide is removed from the plasma chamber to be scanned with an AFM tip, it is exposed to moisture in the air. The water molecules then react with the intermediate structures of the hydroxylated domains. These then form more stable structures, which leads to a reduction in the etching rate and thus, the ability to control and tune pore formation.
Engineering - Design and realisation of 3D printed AFM probes
AFM probes are typically produced from one base material using a technique called micromachining, which involves many varied lithographic and processing steps. This approach has some limitations in terms of usable material and complexity of the final structure given its time-consuming nature. In order to successfully overcome these aforementioned limitations, the use of additive manufacturing might be a viable option.
Within the example cited above, direct laser writing lithography using two-photon polymerisation is explored as a potential avenue for the fabrication of AFM probes. This will enable the 3D structure of the AFM probes to be produced through a single process. Initially, 3D polymer probes were constructed in a monolithic fashion. Given their low quality-factor, the bandwidth was about ten times larger than that of silicon probes. This meant that they were successfully used to perform high-speed AFM where the image quality remained static after around 200 scans.
Furthermore, given the ability to 3D-print the arbitrary structures of AFM probes using the direct laser writing technique, bisegmented probes have been fabricated. These probes allowed the first and second harmonic resonant frequencies to be tuned independently, enabling new types of tip-sample interactions to be investigated.
Physics - Light emission from plasmonic nanostructures enhanced with fluorescent nanodiamonds
Metallic nanostructures are known to enhance the light emission from fluorescent emitters in a process that is known as surface-enhanced fluorescence. However, the effect of fluorescent emitters on metallic nanostructure light emission has not been widely studied or researched because this emission is weak compared to that of the fluorescent emitters and the signals overlap on a photoluminescence spectrum.
In this example mentioned above, a single gold nanoparticle was manipulated to approach a single fluorescent nanodiamond using an atomic force microscope. Since the gold nanoparticle emits light via the anti-stokes process and the fluorescent nanodiamond does not, the emission from both could easily be separately analysed in the resulting spectra. By measuring emission before and after coupling between the species, it was shown that the emission signal of both was enhanced after the species coupling.
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