An Introduction to Scanning Tunneling Microscopy (STM) and its Applications

STM, otherwise known as Scanning Tunneling Microscope, was invented by Gerd Binnig and Heinrich Rohrer at Zurich’s IBM laboratories, in the year 1981. The discovery of STM earned Binnig and Rohrer the 1986 Nobel Prize in Physics, and thereby ushered in a new era of sub-nanometer imaging.

Providing a resolution of 0.1 – 0.01 nm, STM can accurately visualize individual atoms and molecules within various materials – such as thin films, ionic liquids, and semiconductors. Further, the STM can examine structures at resolutions that far exceed conventional light microscopy, which faces restrictions by the diffraction limit. However, STM faces a singular limitation – that is, the surface being examined must be conductive or semi-conductive.

Concept

STM functions by the help of the physical phenomenon of quantum tunneling, which describes the ability of an electron to penetrate an energy barrier, even when its energy is below the height of the barrier. Using STM, it is possible to scan a metal probe with a sharp tip over the conducting surface. While the tip scanning occurs close to the surface, application of a bias voltage is undertaken between the tip and surface.
Thus, electrons are allowed to tunnel through a vacuum in the space between the surface and the tip. Moreover, the current produced in tunneling directly corresponds to the tip position above the surface of the sample. An image can be produced by monitoring the current as the tip scans across the surface.

Usually, the tip is scanned in two alternative ways – firstly, by maintaining a constant current or secondly by ensuring a constant height from the surface. In each case, the probe-sample distance depends on a sensitive piezoelectric mechanism. When in constant current mode, the image is subject to variations in charge density. In contrast, when in constant height mode, the image is a result of current variations across the scanned surface, since the voltage and height are held constant.

Limitations and Advantages

In the initial stages, STM was carried out on highly clean and stable conducting surfaces, in an ultra-high vacuum. Moreover, STM required damped equipment for vibration control and the use of sophisticated, shielded electronic measurement devices. While many of the above requirements are still valid, STM’s utility and use has expanded. Today, it is not only used in a vacuum but can also be performed in ambient air, water, and other liquid or gas atmospheres. What’s more, STM can also be applied at temperatures ranging from near 0 K to 1000 °C.

Further, the use of STM in the production of 3D maps of surfaces also finds widespread use, as it can also be focused on areas that are a single atom. Conversely, however, STM requires skilled operators and precision equipment. It must also be used on conductive surfaces that are not likely to be easily oxidized. Closely related to atomic force microscopy (AFM), STM is also a probe technique like AFM, and thus both come under the descriptive umbrella of scanning probe microscopy (SPM).

AFM vs STM

It is evident that AFM and STM are complementary techniques that find use in the study of sample surfaces. Similar to AFM imaging, STM has the advantages of exceptional stability, low drift and high-quality shielded electronics and data acquisition systems. Asylum Research has focussed on the development of AFM methodology. Further, its Cypher and MPH-3D systems also offer the ability to produce STM images using the same advanced technology.

The ability to switch between modes while conducting ultrahigh-resolution imaging of conductive samples is highly useful and valued by researchers. For instance, the nanoworld’s recent publication note about imaging ionic liquids compared STM and AFM in this application. Thus far, STM has been the gold standard in analytical techniques to examine ionic liquids. However, latest AFM studies have uncovered new detail about interfacial structures. This is due to the fact that when AFM is used as an ambient technique, it avoids the formation of artifacts that are made by freezing monolayers of an ionic liquid under ultra-high vacuum conditions.

While STM is only possible with conductive samples, high-resolution AFM has the added capability of allowing for a new mode of characterization for semiconductor and insulator nanomaterials. Thus, the days when sub-nanometer molecular resolution was the province of STM only are now long gone.

It is evident that next-generation AFM instruments – such as the Cypher family of instruments – are fast approaching the resolution that was previously the exclusive domain of STM alone. Thus, AFM is equipped to reveal nanoscale features that might not have been previously seen using STM imaging.

References

  1. Binnig, G.; Rohrer, H. (1986). "Scanning tunneling microscopy". IBM Journal of Research and Development. 30 (4): 355–69.
  2. N Kirchhofer, AFM vs. STM for Molecular Resolution Imaging, https://afm.oxinst.com/learning/view/article/afm-vs-stm-for-molecular-resolution-imaging
  3. D. A. Bonnell & B. D. Huey (2001). ‘Basic principles of scanning probe microscopy’. In D. A. Bonnell. Scanning probe microscopy and spectroscopy: Theory, techniques, and applications (2 ed.). New York: Wiley-VCH
  4. Tersoff, J.: Hamann, D. R.: Theory of the scanning tunneling microscope, Physical Review B 31, 1985, p. 805 - 813

This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.

For more information on this source, please visit Asylum Research - An Oxford Instruments Company.

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