thumb|Image of [[Surface reconstruction|reconstruction on a clean (100) surface of gold]]
A scanning tunneling microscope (STM) is a type of scanning probe microscope used for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer, then at IBM Zürich, the Nobel Prize in Physics in 1986. STM senses the surface by using an extremely sharp conducting tip that can distinguish features smaller than 0.1 nm with a 0.01 nm (10 pm) depth resolution. This means that individual atoms can routinely be imaged and manipulated. Most scanning tunneling microscopes are built for use in ultra-high vacuum at temperatures approaching absolute zero, but variants exist for studies in air, water and other environments, and for temperatures over 1000 °C.
thumb|Scanning tunneling microscope operating principle
STM is based on the concept of quantum tunneling. When the tip is brought very near to the surface to be examined, a bias voltage applied between the two allows electrons to tunnel through the vacuum separating them. The resulting tunneling current is a function of the tip position, applied voltage, and the local density of states (LDOS) of the sample. Information is acquired by monitoring the current as the tip scans across the surface, and is usually displayed in image form. This is sometimes performed in high magnetic fields and in presence of impurities to infer the properties and interactions of electrons in the studied material, for example from quasiparticle interference imaging.
Scanning tunneling microscopy can be a challenging technique, as it requires extremely clean and stable surfaces, sharp tips, excellent vibration isolation, and sophisticated electronics. Nonetheless, many hobbyists build their own microscopes.
Procedure
thumb|300x300px|Schematic view of an STM
The tip is brought close to the sample by a coarse positioning mechanism that is usually monitored visually. At close range, fine control of the tip position with respect to the sample surface is achieved by piezoelectric scanner tubes whose length can be altered by a control voltage. A bias voltage is applied between the sample and the tip, and the scanner is gradually elongated until the tip starts receiving the tunneling current. The tip–sample separation w is then kept somewhere in the 4–7 Å (0.4–0.7 nm) range, slightly above the height where the tip would experience repulsive interaction but still in the region where attractive interaction exists + U(z).</math>
The electron will have a defined, non-zero momentum p only in regions where the initial energy E is greater than U(z). In quantum physics, however, the electron can pass through classically forbidden regions. This is referred to as tunneling. His model takes two separate orthonormal sets of wave functions for the two electrodes and examines their time evolution as the systems are put close together. used Bardeen's theory and modeled the tip as a structureless geometric point. and consist of six atomic rows that sit on top of five rows of the crystal bulk. Image size is approximately 10 nm by 10 nm.
File:Chiraltube.png|A 7 nm long part of a single-walled carbon nanotube.
File:Silicium-atomes.png|Atoms on the surface of a crystal of silicon carbide (SiC) are arranged in a hexagonal lattice and are 0.3 nm apart.
File:Cens nanomanipulation3d Trixler.jpg|STM nanomanipulation of PTCDA molecules on graphite to inscribe the logo of the Center for NanoScience (CeNS), Munich.
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Early invention
An earlier invention similar to Binnig and Rohrer's, the Topografiner of R. Young, J. Ward, and F. Scire from the NIST, relied on field emission. However, Young is credited by the Nobel Committee as the person who realized that it should be possible to achieve better resolution by using the tunnel effect.
Other related techniques
Many other microscopy techniques have been developed based upon STM. These include photon scanning microscopy (PSTM), which uses an optical tip to tunnel photons; multi-tip scanning tunneling microscopy, which enables electrical measurements to be performed at the nanoscale; and atomic force microscopy (AFM), in which the force caused by interaction between the tip and sample is measured.
STM can be used to manipulate atoms and change the topography of the sample. This is attractive for several reasons. Firstly the STM has an atomically precise positioning system, which enables very accurate atomic-scale manipulation. Furthermore, after the surface is modified by the tip, the same instrument can be used to image the resulting structures. IBM researchers famously developed a way to manipulate xenon atoms adsorbed on a nickel surface.