Scanning Tunneling Microscopes (STM) - History, Overview of Analysis Methods and Future Developments

Topics Covered

Background

How Does the Scanning Tunneling Microscope (STM) Work?  

What is an Atomic Force Microscope (AFM)?

Viewing the Nanoscale in 3D - What the Future Holds for Microscopy

Background

On August 10, 1982, IBM won US patent 4,343,993 for the invention of the Scanning Tunneling Microscope (STM), the first microscope that allowed researchers to “see” at the atomic scale. The invention earned its creators a Nobel Prize in 1986 and opened the door to understanding and manipulating nano-scale phenomena. As physicist Richard Feynman said in his now famous lecture in 1959: “if you want to make atomic-level manipulations, first you must be able to see what’s going on”. The STM is, quite simply, a fundamental tool indispensable to the development of nanotechnology.

How Does the Scanning Tunneling Microscope (STM) Work? 

The STM doesn’t work the way a conventional microscope does - it doesn’t magnify a sample until it is big enough to see with an unaided eye. Instead, a fine needlelike tip that is electrically conductive is scanned just above the surface of an electrically conductive sample. The distance between the tip and the sample is only a few Angstroms (a nanometer is 10 times bigger than an Angstrom). When a tiny voltage is applied, the rules of quantum mechanics allow electrons to jump - or “tunnel” - across the space between tip and sample. Though very small, the flow of electrons can be detected easily. As the tip moves along the surface of the sample, the tip’s position is constantly adjusted to make sure the distance (and hence, the electrical current) remains constant. These adjustments trace the surface features of the sample. When the features are graphically displayed on a computer screen, it is possible to “see” the individual atoms and molecules that make up the sample.

What is an Atomic Force Microscope (AFM)?  

Because the prototype STM relied on electrical flow between tip and sample, it could only be used to examine materials that would conduct at least a small electric current. Since the early 1980s, STMs have evolved into Atomic Force Microscopes (AFMs) that are able to see a wider range of nano-scale samples. The process resembles the original one where a needle-like tip scans across a surface whose topography is “read” and then translated into a graphic image, but the AFM is able to see samples that are not highly conductive, such as biological samples. Rather than maintaining a constant distance between tip and sample, the tip of an AFM is attached to the end of a highly sensitive cantilevered arm and actually touches the surface of the sample to trace it and generate an image.   

Viewing the Nanoscale in 3D - What the Future Holds for Microscopy

Coming soon is a tool to view the nano-realm in three dimensions, which would dramatically increase our understanding of how things work at the molecular level - such as the complex patterns of protein folding. The idea is to combine magnetic-resonance imaging (MRI) with an AFM - the resulting tool is called a magnetic-resonance force microscope (MRFM). The technology is in the earliest stages, but there are already six granted patents referring to MRF Microscopy. Another development achieved by a research team at Zhejiang University in China is an AFM that can be completely immersed in liquid, facilitating imaging of biological samples. The microscope works in a wide range of corrosive solutions as well, which allowed the researches to measure the corrosion of a sample of lead in real time.

Source: ETC Group report entitled ‘Nanotech’s “Second Nature” Patents: Implications for the Global South’, April/May 2005.

For more information on this source please visit the ETC Group.

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