Scanning tunneling microscope (STM)

Scanning tunneling microscope (STM) is a powerful technique for viewing surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer (at IBM Zürich), the Nobel Prize in Physics in 1986. STM probes the density of states of a material using tunneling current. For STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution. The STM can be used not only in ultra high vacuum but also in air and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to a few hundred degrees Celsius. The STM is based on the concept of quantum tunnelling. When a conducting tip is brought very near to a metallic or semiconducting surface, a bias between the two can allow electrons to tunnel through the vacuum between them. For low voltages, this tunneling current is a function of the local density of states (LDOS) at the Fermi level, Ef, of the sample. Variations in current as the probe passes over the surface are translated into an image. STM can be a challenging technique, as it requires extremely clean surfaces and sharp tips. Tunnelling is a functioning concept that arises from quantum mechanics. Classically, an object hitting an impenetrable wall will bounce back. Imagine throwing a baseball to a friend on the other side of a mile high brick wall, directly at the wall. One would be rightfully astonished if, rather than bouncing back upon impact, the ball were to simply pass through to your friend on the other side of the wall. For objects of very small mass, as is the electron, wavelike nature has a more pronounced effect, so such an event, referred to as tunneling, has a measurable probability. The components of an STM include scanning tip, piezoelectric controlled height and x,y scanner, coarse sample-to-tip control, vibration isolation system, and computer. The resolution of an image is limited by the radius of curvature of the scanning tip of the STM. Additionally, image artifacts can occur if the tip has two tips at the end rather than a single atom; this leads to “double-tip imaging,” a situation in which both tips contribute to the tunneling. Therefore it has been essential to develop processes for consistently obtaining sharp, usable tips. Recently, carbon nanotubes have been used in this instance. A closeup of a simple scanning tunneling microscope head at the University of St Andrews scanning MoS2 using a Platinum-Iridium stylus. The tip is often made of tungsten or platinum-iridium, though gold is also used. Tungsten tips are usually made by electrochemical etching, and platinum-iridium tips by mechanical shearing. Both processes are outlined in C. Bai’s book, reference below. Due to the extreme sensitivity of tunnel current to height, proper vibration isolation is imperative for obtaining usable results. In the first STM by Binnig and Rohrer, magnetic levitation was used to keep the STM free from vibrations; now spring systems are often used. Additionally, mechanisms for reducing eddy currents are implemented. Maintaining the tip position with respect to the sample, scanning the sample in raster fashion and acquiring the data is computer controlled. The computer is also used for enhancing the image with the help of image processing as well as performing quantitative morphological measurements.

Further information:

1. G. Binnig, H. Rohrer. Scanning tunneling microscopy IBM Journal of Research and Development 30,4 (1986)
2. C. Bai. Scanning tunneling microscopy and its applications Springer Verlag, 2nd edition, New York (1999)
3. C. Julian. Chen Introduction to Scanning Tunneling Microscopy(1993)
4. A. Bonnell and B. D. Huey. Basic principles of scanning probe microscopy from Scanning probe microscopy and spectroscopy: Theory, techniques, and applications 2nd edition Ed. By D. A. Bonnell Wiley-VCH, Inc. New York (2001)
5. J. Bardeen. Tunneling from a many particle point of view. Phys. Rev. Lett. 6,2 57-59 (1961)
6. K. Oura, V. G. Lifshits, A. A. Saranin, A. V. Zotov, M. Katayama. Surface science: an introduction Springer-Verlag Berlin (2003)
7. R. Wiesendanger, I. V. Shvets, D. Bürgler, G. Tarrach, H.-J. Güntherodt, J.M.D. Coey. Recent advances in spin-polarized scanning tunneling microscopy. Ultramicroscopy 42-44 (1992)
8. R. Young, J. Ward, F. Scire. The Topografiner: An Instrument for Measuring Surface Topography. Rev. Sci. Instrum. 43, 999 (1972)
9. Article  Scanning electron microscope from Wikipedia, the Free Enciclopedia. Available under the license Creative Commons Attribution-Share Alike.

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