Atomic Force Microscopy: application in nanoscience

About the author: Rui Filipe Serra Maia is a PhD student in Geosciences at Virginia Tech. Check out his profile on the VTSuN student page.

AFM_Dust
Figure 1. First AFM image ever taken in another planet. Dust is everywhere in Mars. Such dust particles give the characteristic pink color to the sky, feed storms that regularly cover the planet and form its distinctive soil. Understanding these particles (shape, size, and physico-chemical properties) is essential to understand the climate of Mars.

Today I am going to tell you about the importance of Atomic Force Microscopy in nanoscience – I am sure you are very keen to learn about it, right? Well, if you are not, let me tell you that Atomic Force Microscopy (AFM) is so important that in 2008 NASA incorporated and AFM instrument in the Phoenix Mars Lander. This AFM was used to study the role of dust particles and nanoparticles in the climate of Mars.

AFMs are instruments that measure the shape, or morphology of a sample at very high resolution, even in the order of fractions of a nanometer – that is approximately 1,000,000 times smaller than the diameter of your hair! Just as a blind person uses a white cane to map their surroundings, an AFM uses a probing tool (cantilever) to measure the morphology of a sample. In order for that to work, AFM instruments use three essential parts: a cantilever with a tip, a laser, and a photodiode (detector of movement).

How does it work?

In order to produce an image, the tip (attached to the cantilever) scans a sample surface very closely causing the cantilever to move up or down according to the sample’s morphology (Figure 2). At the same time, a laser is reflected at the back of the cantilever into a photodiode (detector). The vertical movements of the cantilever change the angle of incidence of the laser on the cantilever which consequently changes the angle of the outgoing beam, therefore changing the location of the photodiode that is hit by the beam. With some fun trigonometry involved, a computer can calculate the surface morphology, giving rise to an image similar to that shown in Figure 1.

AFM_Schematic
Figure 2. Schematic of AFM functioning. The photodiode records variations in the reflection angle of the laser due to movements of the cantilever. The recorded signal is then used by a computer to build the image.

 

Why is it important in nanoscience?

The ability to “see” extremely small particles, which are often sensitive to other destructive imaging techniques, is a huge advantage per se. Furthermore, in an AFM instrument particles can be studied at the atomic resolution (“seeing” atom by atom) in environmental conditions or even immersed a liquid, which cannot be achieved with other imaging methods of comparable resolution, such as scanning and transmission electron microscopy.

graphene
Figure 3. Atomic resolution image of graphene, proving its uniform honey comb structure.

For example, AFM can be used to study biological materials and even living organisms. Additionally, AFM instruments are very handy and efficient tools that can be easily moved to the field. They are for example very useful to inspect the fuselage of airplanes, allowing the investigators to find defects in the fuselage even at the micro scale. Thankfully they do not need to take the airplanes to the lab, but instead they take the AFM instrument to the airplane!

Are you impressed with the capabilities of Atomic Force Microscopy? If you think that imaging is all that it can do, you had better keep reading.

AFM_interactions
Figure 4. Schematic interaction between the AFM tip and the sample surface for identification of individual surface atoms. Adapted from Sugimoto et al. (2007).

Another major application of AFM is called force spectroscopy. Force spectroscopy allows scientists to measure the interaction between the tip of the probe (cantilever) and the sample, and has allowed scientists to measure the hardness of small materials, such as the membrane of a cell. It can also be used for more refined chemical investigations, such as bond energies, Van der Walls forces and dissolution forces – all this at a scale where materials often times show very odd properties, compared to their macro-scale properties. Indeed, measurement of some of these properties used to be a challenge even in macro-scale materials before AFM instruments were invented, let alone in nanomaterials!

Interestingly some AFM instruments with properly developed cantilever tips (atomic scale tips) can even be used to identify individual surface atoms. Atoms can have different electronic properties due to the number and arrangement of their electrons, therefore the electronic interaction of those atoms with the tip will be also different.

By now you should have a good idea of the potential applications of AFM in nanoscience. The study of all these properties is essential to understand the behavior of nanoparticles not only in the environment but also in industrial applications. AFM has allowed scientists to gain new and important knowledge about the physico-chemistry of innumerous nanomaterials over the years!

 

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