Scanning Tunneling Microscopy/Spectroscopy in Analysis of Surfaces
Published Online: 15 SEP 2006
Copyright © 2000 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Analytical Chemistry
How to Cite
Dharmadhikari, C. 2006. Scanning Tunneling Microscopy/Spectroscopy in Analysis of Surfaces. Encyclopedia of Analytical Chemistry. .
- Published Online: 15 SEP 2006
Scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and related techniques with their unique capability to resolve topological and electronic structures at atomic level have revolutionized the power of experimental techniques in high resolution imaging of surfaces. In STM, a sharp conducting tip is mechanically scanned over a conducting or semi-conducting sample at the distance of few angstroms from the surface. A voltage bias applied between the tip and the sample leads to flow of tunnel current, which decreases exponentially as the gap increases. By keeping the tunnel current constant with an electronic feedback controller the tip is maintained at a fixed distance during the scanning by means of a 3-D scanner made of a piezoelectric transducer. The trajectory of the tip then traces out a profile of the surface, including the bumps due to individual atoms. In the context of atomic resolution imaging capability, there are only a few techniques such as scanning transmission electron microscopy (STEM), transmission electron microscopy (TEM) and field ion microscopy (FIM) which perform as well as STM, but they do so under extremely special circumstances. The traditional spectroscopic techniques that can be compared with STS are those of conventional (elastic) electron tunneling spectroscopy (ETS) and inelastic electron tunneling spectroscopy (IETS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPS). But all these techniques detect and average the data from a relatively large area, a few microns to a few millimeters across. The STM/STS, in contrast, can take spectra localized to as small an area as an individual atom. The most important characteristic that distinguishes STM/STS from other techniques is ability to operate in a variety of different environments such as air, reactive gases, liquids, electrolytes and biological fluids, leading to its applications not only in basic research in surface science but also in such diverse fields as lithography, electrochemistry, biology and medicine. An inherent limitation of STM is that it always operates at high resolution and, therefore, works better for atomically flat surfaces.