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Scanning Tunnelling Microscope

A scanning tunneling microscope (STM) is a type of 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.[1][2][3] 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.[4]

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.[5][6]

Scanning tunneling microscope operating principle [Video ]

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.[5]

A refinement of the technique known as scanning tunneling spectroscopy consists of keeping the tip in a constant position above the surface, varying the bias voltage and recording the resultant change in current. Using this technique, the local density of the electronic states can be reconstructed.[7] 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.

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.[8]

Procedure

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 (w < 3 Å), but still in the region where attractive interaction exists (3 < w < 10 Å).[5] The tunneling current, being in the sub-nanoampere range, is amplified as close to the scanner as possible. Once tunneling is established, the sample bias and tip position with respect to the sample are varied according to the requirements of the experiment.

As the tip is moved across the surface in a discrete x–y matrix, the changes in surface height and population of the electronic states cause changes in the tunneling current. Digital images of the surface are formed in one of the two ways: in the constant-height mode changes of the tunneling current are mapped directly, while in the constant-current mode the voltage that controls the height (z) of the tip is recorded while the tunneling current is kept at a predetermined level.[5]

In constant-current mode, feedback electronics adjust the height by a voltage to the piezoelectric height-control mechanism. If at some point the tunneling current is below the set level, the tip is moved towards the sample, and conversely. This mode is relatively slow, as the electronics need to check the tunneling current and adjust the height in a feedback loop at each measured point of the surface.

When the surface is atomically flat, the voltage applied to the z-scanner mainly reflects variations in local charge density. But when an atomic step is encountered, or when the surface is buckled due to reconstruction, the height of the scanner also have to change because of the overall topography. The image formed of the z-scanner voltages that were needed to keep the tunneling current constant as the tip scanned the surface thus contain both topographical and electron density data. In some cases it may not be clear whether height changes came as a result of one or the other.

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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.[21] 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.[22]

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;[4] scanning tunneling potentiometry (STP), which measures electric potential across a surface;[4] spin-polarized scanning tunneling microscopy (SPSTM), which uses a ferromagnetic tip to tunnel spin-polarized electrons into a magnetic sample;[23] 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.[4]

This technique has been used to create electron corrals with a small number of adsorbed atoms and observe Friedel oscillations in the electron density on the surface of the substrate. Aside from modifying the actual sample surface, one can also use the STM to tunnel electrons into a layer of electron-beam photoresist on the sample, in order to do lithography. This has the advantage of offering more control of the exposure than traditional electron-beam lithography. Another practical application of STM is atomic deposition of metals (gold, silver, tungsten, etc.) with any desired (pre-programmed) pattern, which can be used as contacts to nanodevices or as nanodevices themselves.

See also

References

Scanning tunneling microscope - Wikipedia

In 1978, Binnig accepted an offer from IBM to join their Zürich research group, where he worked with Heinrich Rohrer, Christoph Gerber and Edmund Weibel. There they developed the scanning tunneling microscope (STM), an instrument for imaging surfaces at the atomic level.[4] The Nobel committee described the effect that the invention of the STM had on science, saying that "entirely new fields are opening up for the study of the structure of matter."[2] The physical principles on which the STM was based were already known before the IBM team developed the STM, but Binnig and his colleagues were the first to solve the significant experimental challenges involved in putting it into effect.[2]

The IBM Zürich team were soon recognized with a number of prizes: the German Physics Prize, the Otto Klung Prize, the Hewlett Packard Prize and the King Faisal Prize.[1] In 1986, Binnig and Rohrer shared half of the Nobel Prize in Physics, the other half of the Prize was awarded to Ernst Ruska.

From 1985–1988, he worked in California. He was at IBM in Almaden Valley where James Burke was shown the letters IBM written in electrons, and was visiting professor at Stanford University.[5]

In 1985, Binnig invented the atomic force microscope (AFM)[6] and Binnig, Christoph Gerber and Calvin Quate went on to develop a working version of this new microscope for insulating surfaces.[7]

In 1987 Binnig was appointed IBM Fellow. In the same year, he started the IBM Physics group Munich, working on creativity[8] and atomic force microscopy.[9]

In 1994 Professor Gerd Binnig founded Definiens which turned in the year 2000 into a commercial enterprise. The company developed Cognition Network Technology to analyze images just like the human eye and brain are capable of doing.[10]

in 2016, Binnig won the Kavli Prize in Nanoscience.[11] He became a fellow of the Norwegian Academy of Science and Letters.[12]

The Binnig and Rohrer Nanotechnology Center, an IBM-owned research facility in Rüschlikon, Zürich is named after Gerd Binnig and Heinrich Rohrer.

Rohrer was born in Buchs, St. Gallen half an hour after his twin sister. He enjoyed a carefree country childhood until the family moved to Zürich in 1949. He enrolled in the Swiss Federal Institute of Technology (ETH) in 1951, where he was student of Wolfgang Pauli and Paul Scherrer. His PhD thesis was supervised by Prof P. Grassmann who worked on cryogenic engineering. Rohrer measured the length changes of superconductors at the magnetic-field-induced superconducting transition, a project begun by Jørgen Lykke Olsen. In the course of his research, he found that he had to do most of his research at night after the city was asleep because his measurements were so sensitive to vibration.

His studies were interrupted by his military service in the Swiss mountain infantry. In 1961, he married Rose-Marie Egger. Their honeymoon trip to the United States included a stint doing research on thermal conductivity of type-II superconductors and metals with Bernie Serin at Rutgers University in New Jersey.

In 1963, he joined the IBM Research Laboratory in Rüschlikon under the direction of Ambros Speiser. The first couple of years at IBM, he studied Kondo systems with magnetoresistance in pulsed magnetic fields. He then began studying magnetic phase diagrams, which eventually brought him into the field of critical phenomena.

In 1974, he spent a sabbatical year at the University of California in Santa Barbara, California studying nuclear magnetic resonance with Vince Jaccarino and Alan King.[citation ^needed]

Until 1982 he worked on the scanning tunneling microscope. He was appointed IBM Fellow in 1986, and led the physics department of the research lab from 1986 until 1988. Rohrer was elected as an honourable member of the Swiss Physical Society in 1990 and an honorary academician of Academia Sinica in 2008.[9]