Happy 30th birthday to atomic force microscopy!
Atomic force microscopy is celebrating its 30th birthday this year after its invention in 1986 by Gerd Binnig, Christoph Gerber, and Calvin Quate, who were scientists at the IBM Zurich Laboratory (Binnig and Gerber) and Stanford University (Quate) with the publication in Physical Review Letters:
The abstract from Phiscal Review Letters
Today the AFM is a common tool for material characterization alongside optical and electron microscopy with resolution down to the nanometer scale and beyond. Any characterization facility I have encountered either in a university or at an industrial research organization now contains AFMs, even multiple instruments. It really has become a common microscopic method that cuts across all disciplines from physics and chemistry to biology and materials science.
What is its power and attraction? Obviously its nanoscale resolution is key - but SEM and TEM have comparable or even better resolution. One key differentiator of AFM is the nature of the interaction between the probe and the surface where the probe literally touches the surface, in contrast to interactions with electrons or photons. You can learn a lot about a material by touching it! You can also move molecules on the surfaces and pull on them and measure forces. A second differentiator for the power and allure of AFM – in addition to resolution and nature of probe interaction – is the environment in which it operates. AFM can achieve atomic resolution in real-world environments such as ambient or fluid conditions – no vacuum necessary!
The instrument that pioneered this field – the scanning tunneling microscope (STM) – was actually invented first in 1982 and garnered its inventors Gerd Binnig and Heinrich Rohrer the Nobel Prize in Physics in 1986. The STM produced exquisite images early on such as the structure of a silicon (111) surface [see image below].
An image presented in Phys Rev Lett
Although its success at providing atomic scale resolution was appreciated, STM was limited in its utility as it was a technique that was limited to imaging conducting or semiconducting surfaces. Unfortunately, the vast majority of surfaces – especially in ambient conditions – are insulating or grow very thin insulating layers (e.g. metals growing insulating metal oxide layers very quickly in air) making them impossible to image by STM without removing these layers.
By operating under a totally different scientific principle based on measuring forces between a probe interacting with a tip, the AFM has become the instrument of choice for nanoscale measurements of materials. I experienced this firsthand in my own career. As a graduate student, I worked with STM to image molecules in self-assembled monolayers. Although the work and results were very exciting, they were not the most practical in being able to extend to other systems. After landing at an oil company for my postdoctoral research, there were no STM’s to be found since they were not practical to solving real-world problems. But there were multiple AFMs onsite that were being used to investigate a variety of problems such as corrosion and lubrication. And so voila, I switched to being an AFMer! Fortunately, the technologies are related and there is significant instrumental overlap and manufacturers between STM and AFM, so the switch wasn’t too dramatic.
Now thirty years after its invention, there are dozens of ways (termed AFM modes) that have been invented for the cantilever to interact with the surfaces. But no matter the mode, the AFM today probes a wide variety of properties of the surface including mechanical, mechanical, electrical, and optical. Aside from imaging applications, the AFM probe has also found significant uses for surface and molecular manipulation and lithography on the nanoscale, including a popular use of pulling on biological molecules such as proteins and measuring the forces as they unfold. The AFM’s resolution continues to improve with sophisticated equipment design and probes that have produced unparalleled images of single molecules and chemical reactions, such as recent work that imaged a popular organic chemistry reaction known as the Bergman reaction. [see below] To be fair, these recent measurements are done with custom, home-built equipment using advanced modes operating at very low temperatures and in ultra-high vacuum, but they give us a tantalizing taste of what is possible with AFM.
Images reproduced from Schuler et al., Nature Chemistry 8, 220-224 (2016)
In honor of its birthday, Physical Review Letters has put together free access to a wonderful collection of articles following the major findings of STM and AFM here. I still cannot believe that the AFM technology is only 30 years old. For such a “young” and comparatively immature technology compared with its electron and optical-based microscopy colleagues, it has already become an invaluable tool for scientists seeking nanoscale information. It has come a long way from imaging a pristine silicon surface in ultra high vacuum - who knows what the next 30 years have in store for this field?
By Dalia Yablon, Ph.D.;
SurfaceChar LLC, Sharon, Massachusetts.