# Scanning tunnel microscope

Scanning tunnel microscope
The first scanning tunneling microscope from Binnig and Rohrer

The scanning tunneling microscope (hereinafter abbreviated RTM , English scanning tunneling microscope , STM) is one of the techniques of scanning probe microscopy (engl. Scanning probe microscopy , SPM), which make it possible to map the surfaces of equal electron density of states of surfaces. The functional principle of the RTM is based on the quantum mechanical tunnel effect . If a voltage is applied between a fine tip and a surface, this leads to a measurable tunnel current if the distance is sufficiently small. The basic requirements for this type of scanning probe microscopy are an electrically conductive sample and an electrically conductive tunnel tip. If you scan the surface line by line and measure the tunnel current at each measuring point, you get a topography of constant electron density. This allows conclusions to be drawn about the actual surface structure down to atomic resolution.

## functionality

Functional principle of the RTM

With the scanning tunnel microscopic measurement, an electrically conductive probe, in the form of a fine tip, is systematically moved in a grid over the object to be examined, which is also conductive. The distance between the tip and the object is extremely small ( nanometers ), but not zero. An electrical voltage is applied between the surface and the tip. Because of the distance there is still a potential barrier that the electrons cannot overcome. Due to the tunnel effect that occurs, a small current can still be measured. This is very sensitive to the smallest changes in distance, since the tunnel probability decreases exponentially with the distance. If there is a topographical increase on the surface, this can be registered by an increase in the tunnel current. By scanning the sample, a two-dimensional image can be generated.

The tunnel effect between two metals, which are separated by a thin oxide layer, was explained in 1961 by John Bardeen with the help of the time-dependent first-order perturbation theory ( Fermi's golden rule ). If this theory is transferred to scanning tunneling microscopy, then an atomically precise knowledge of the tip is necessary in order to interpret the measured images. The so-called Tersoff-Hamann theory represents a significant simplification, which neglects the influence of the tip on the measurement and provides information about the electronic structure of the sample (essentially about the local electronic density of states in the surface area). The tip is assumed to be a metal atom with a linear electronic density of states and spherically symmetric s- wave functions . C. Julian Chen , who calculated more complex point geometries, provided an extension of this theory . A really three-dimensional theory of the scanning tunneling microscope can be set up analytically, but it is usually hardly solvable and is therefore of secondary importance. Three-dimensional systems can only be calculated approximately numerically, usually with the help of several estimated parameters. Simulations of STM images of organic molecules on surfaces are possible by superimposing occupied or unoccupied molecular orbitals of the molecules in a vacuum. B. obtained from density functional theory .

As a first approximation, the tunnel current at the tip results from the following equation:

${\ displaystyle I _ {\ scriptstyle {\ text {tun}}} \ propto \ exp {(-1 {,} 025 \ cdot {\ sqrt {\ Phi}} \ cdot d)}}$

The tunneling current depends on the distance between the sample and the tip and the work function of the electrons. The location dependence of the tunnel current reflects the folding of the real topography with electronic properties. A three-dimensional plot suggests a view of the surface topography, but precisely depicts the height topography of constant electron density.${\ displaystyle I _ {\ scriptstyle {\ text {do}}}}$${\ displaystyle d}$ ${\ displaystyle \ Phi}$

Video explanation of the scanning tunneling microscope (English)

The RTM can e.g. B. can be operated in the following two modes:

• When scanning the sample surface, the height of the tip is regulated by means of precision mechanics ( piezo elements ) so that the tunnel current remains constant. In this way, the tip follows a "height profile" of the surface, with the height control signal being used to display the sample surface. The lateral resolution depends on the radius of curvature of the tip. Ideally, the tunnel current only flows over a single, exposed atom at the tip. The images obtained by constant tunnel current do not necessarily correspond to the surface. Rather, the electronic structure of the surface is primarily scanned, see below .
• The height of the tip can also be kept constant and a reconstruction of the surface can be recorded due to the different distances to the sample surface and the associated variation of the tunnel current. The latter method is more sensitive to electronic surface effects than to geometrical ones, and by comparing the two images, the deviation from topography to electronic structure can be estimated.

## Experimental boundary conditions

RTM measurement of a reconstructed (100) face of a Au - monocrystal
RTM image of a graphite surface in atomic resolution
RTM image of self-assembled molecular chains
Scanning tunnel microscope image of impurities on an iron crystal surface with chromate atoms (small tips)

Since the principle of scanning tunneling microscopy is based on the measurement of a current flow between the sample and the tip of the scanning tunneling microscope, only electrically conductive samples ( metals , semiconductors or superconductors ) can be examined directly. Non-conductive samples also show tunneling phenomena, but the tunnel current cannot pass through the sample to the opposite cathode and cannot be measured. Therefore, they must first be vaporized with a fine electrically conductive layer ( graphite , chrome or gold ), which is in contact with the sample holder at the edge of the sample. Another possibility is to examine very thin layers of a dielectric on a conductive substrate .

Since a very small tunnel current (typically 1 pA to 10 nA) reacts sensitively to changes of hundredths of a nanometer, the tip-sample distance of typically 0.5–1 nm must be stabilized to less than 1% deviation. Therefore, depending on the desired precision, different techniques are used for isolation:

Thermal insulation: Due to the different thermal expansion coefficients of the materials used, temperature variations lead to disruptive modulations of the tip-sample distance. In CCM (constant current mode) these lead to distortions. In modes in which no control loop is used to regulate the distance, such as in CHM (constant height mode) or during spectroscopy, tip-sample contacts that destroy the surface can also take place.

Acoustic isolation: Mechanical parts can be excited to vibrate by sound waves, which either modulate the tip-to-sample distance or disrupt the measurement signal through changes in capacitance in the live cable. This influence can be reduced by using sound insulation or sound boxes.

Mechanical isolation: Mechanical vibrations that are carried through the building via the structure into the system are a major disruptive factor. These vibrations can be reduced through the use of active and passive (air spring) feet, through spring suspension or a magnetic eddy current brake . Combinations of different techniques are often used because the filter properties vary. Eddy current brakes are suitable for damping fast vibrations (> 1 kHz), while actively controlled damping structures are suitable for minimizing low-frequency (<1 Hz) interference caused by building vibrations, especially for structures on higher floors of high buildings.

The tunnel voltages between the tip and the sample are usually a few millivolts up to a few volts. The lower limit is determined by the temperature or by the thermal noise (room temperature approx. 50 mV). At room temperature, the maximum voltage is also determined by gaseous particles that can enter the tunnel barrier. Short-term current peaks occur from approx. 2 V, which permanently destroy the surface. At low temperatures and within a vacuum system, however, very high voltages beyond 100 V can also be applied without any problems, and the transition of the tunnel current into a field emission current can be observed.

Both the surface to be examined and the tip used must be electrically conductive on the surface. If the surface to be examined consists of metals that can oxidize in air (e.g. copper , silicon or silver ), scanning tunneling microscopy must be carried out in an ultra-high vacuum , which means a technical effort that should not be underestimated. The surfaces that can be used under normal conditions , on the other hand , are conductive layer crystals such as graphite or representatives of the layer-crystalline transition metal dichalcogenides such as molybdenum disulfide (MoS 2 ), tantalum (IV) sulfide (TaS 2 ) or tantalum (IV) selenide (TaSe 2) ) in question. A fresh, atomically smooth surface can be achieved with these layer crystals simply by peeling off the top layers with an adhesive tape, since the individual layers are only connected by relatively weak van der Waals interactions .

Piezoelectric ceramics are used to move the tip relative to the sample surface . These allow high-precision control on a sub-nanometer scale via applied electrical voltages .

The probe itself is usually made of tungsten , platinum alloys or gold , with the tip made by electrolytic etching.

## Measurement modes

A scanning tunneling microscope works with a distance between the tip and the sample or with a resolution that is less than the wavelength of the tunnel electrons (compare matter wave ). If an electrical voltage ( bias or tunneling bias ) is applied between the examination subject and the tip, a current , the so-called tunnel current, can flow.

The three methods described below (CHM, CCM and STS images with the exception of point spectroscopy) have in common that the measuring tip is moved linearly over the sample before it detects an adjacent line offset laterally. This results in a grid of lines on the surface.

### Constant altitude mode

The height of the peak is kept constant and the variation in the tunnel current is recorded. There is a risk of crashes due to large structures.

With the CHM ( constant height method ), the tip follows a predetermined height profile without the sample-tip distance being readjusted by a control loop. In parallel, the tunnel current is recorded for each raster point. This enables direct conclusions to be drawn about the height dependencies of the tunnel current and measurement artifacts can be avoided through the feedback loop in the CCM.

The main strength of CHM lies in the high sampling rate, which is no longer limited by the bandwidth of the feedback loop, but by the bandwidth for reading out the tunnel current.

The use of CHM is advantageous when studying the thermally induced mobility of individual atoms, chemical processes, or molecules at high process speeds. In all these cases, the local geometry changes and can easily be identified within differential images of recordings in rapid succession (video RTM).

The disadvantage is that higher demands are placed on the experimental setup with regard to long-term stability. Slow mechanical disturbances (e.g. building vibrations, creep and drift of the piezo drive) can lead to significant peaks in the tunnel current, right up to tip-sample contacts that locally disturb the surface.

### Constant tunnel current mode

The tunnel current is kept constant, the tip follows the surface.

Another method of mapping ( constant current method , abbreviated to CCM or constant gap width mode , abbreviated to CGM) is based on continuously changing the height of the tip so that the current remains constant. This is done via an electronic control circuit for distance control. Thus, the three-dimensional image of the surface can now be determined directly via the position of the tip. The resolution in this process is so high that the atomic electronic structure of the surface becomes visible. The image contrast should not be understood directly as an atomic structure. In the meantime, at least nine different contrast mechanisms are known which influence the formation of the image and which must be taken into account during interpretation. However, the measuring speed of the method is limited by the control loop; the recording of an image usually takes several times from ten seconds to hours. In practice this mode is mostly used.

### Spectroscopy mode

Since one first measures the local electronic structure of the sample surface with the scanning tunneling microscope, conveyed via the tunnel effect, it can also be used directly to determine this. For example, a single oxygen atom on a surface of the semiconductor material gallium arsenide appears sometimes as a depression and sometimes as a hill, depending on whether a positive or negative voltage is applied between the tip and the sample.

This can be used to either determine the energetic positions of the surface states at one location of the sample (TS spectra at one location, so-called point spectroscopy ) or the locations where electrons are allowed to stay at a certain energy (corresponds to the tunnel voltage ) (STS images at constant tunnel voltage). To do this, a small high-frequency alternating voltage must be superimposed on the tunnel voltage and the so-called density of states can then be calculated from the derivation of the current according to the voltage. Scanning tunnel spectroscopy is often carried out at low temperatures of a few Kelvin , as the energetic resolution depends on the temperature via the Fermi distribution . The spectroscopy mode is further divided into various sub-modes.

### Video scanning tunneling microscopy

At scan rates of one image per second, one speaks of video scanning tunneling microscopy. The frame rate ranges up to 50 Hertz. With this method, diffusion processes or surface reactions can be observed in real time , depending on the system .

### Line scan

When scanning in a line over a phase boundary or atomic level that is in dynamic equilibrium with its environment, so-called pseudo images (also: Xt scan) can be measured. From these measurement data, in which the x-axis is a time specification and the y-axis is a location specification, the step correlation function can in turn be calculated, from which information about the diffusion processes at the corresponding point is obtained.

## Image errors

A number of influences can impair or limit the imaging quality of the scanning tunneling microscope images. Particular attention must be paid to avoiding external vibrations , for which, for example, vibration isolation can be used. But the actuators for the raster can also cause internal vibrations, which can be reduced by a suitable choice of natural frequencies . Furthermore, the piezoelectric materials used tend to creep as well as to hysteresis , which causes inaccuracies in determining the position. As a rule, these materials also have a relatively high temperature drift , so that the temperature should be kept as constant as possible during a measurement. The noise of the tunnel current limits the accuracy of the height determination. For this reason, current-voltage converters that are as noise-free as possible are used with the required bandwidth for the frequencies that occur. The deflection voltages of the actuators must also have the required accuracies in terms of linearity and delay time .

Ghost image in a scanning tunnel microscope image of a 75 nm × 75 nm copper surface

When using double and multiple tunnel tips, the point at which the tunnel current flows can switch between individual tips, which can then lead, for example, to multiple, but offset, scanning of the same sample area. The ghost images that may arise are characterized by parallel structures.

## manipulation

RTM nanomanipulation of a self-assembled PTCDA molecular
layer on graphite in which the logo of the Center for NanoScience (CeNS) was written.

Another application of the scanning tunneling microscope is the targeted modification of an object.

A distinction must be made here between various changes, between the displacement (lateral and vertical manipulation) and the modification of objects (dissociation and structural modification, especially for molecular systems). The following processes are used: Breaking bonds through local heating and shifting through a change in potential:

Local heating: Especially in systems with covalent bonds, e.g. B. within molecules or silicon-hydrogen bonds, vibration modes can be excited by the tunneling electrons. Through the accumulation of this energy, a bond can ultimately be broken (or closed). Since the lifespan of corresponding excitations is usually very short (fs-ms), an energy accumulation can be achieved through correspondingly high currents (note 1 nA ~ 0.1 ns time interval between two tunneling electrons).

Change in potential: The attractive or repulsive interaction between the tip and the object through its potential is sufficient to move objects. The potential can also be modulated by the applied tunnel voltage. Correspondingly, objects can be dragged when the interaction is attractive, and pushed when the interaction is more repulsive. When the tip approaches the object sufficiently, the object can also be transferred from the sample surface to the tip. In some cases, the back transfer is also possible via an additional use of the tunnel voltage and this is called vertical manipulation.

With the help of these methods, the so-called atomic writing was carried out, which depicts lettering such as IBM , logos of individual universities or map sketches with individual atoms on surfaces.

In the field of magnetic data storage , IBM has developed a scanning tunneling microscope that works at very low temperatures (≈ 4  K ). Successful attempts are said to have been made to change the spin (magnetic) alignment of individual atoms in a magnetic layer and to influence them in a targeted manner. The method is spin-excitation spectroscopy ( spin-excitation spectroscopy called).

## history

The first successful experiment to detect a distance-dependent tunnel current was carried out on March 18, 1981 in the IBM research laboratory in Rüschlikon (CH). The two physicists Gerd Binnig (Germany) and Heinrich Rohrer (Switzerland), who carried out the experiment and ultimately also made the scanning tunneling microscope a useful instrument, received the 1986 Nobel Prize in Physics for this . Christoph Gerber and Edmund Weibel were also involved in the development.

However, there is earlier work in this area in which the essential aspects of an RTM / RTM were demonstrated - in particular the occurrence of a tunnel current. This device was developed by Russel Young, John Ward and Fredric Scire in the late 1960s as a so-called topographer . However, there were bureaucratic and technical difficulties, for example the vibration of the air conditioning interfered with the measurements. However, the Nobel Prize Committee later recognized her achievements.

The scanning tunneling microscope is the father of all other scanning probe microscopes . In the following period were primarily the atomic force microscope ( atomic force microscope , AFM) and optical Rasternahfeldmikroskop ( scanning near-field optical microscope , SNOM) developed which make use of another atomic interaction. The development of all these scanning probe microscopes was a major step in the direction of nanosciences , as they can be used to observe and manipulate nanoscopic objects (objects that are smaller than the light wavelength of 400 to 800 nm) in a very simple and comparatively inexpensive way can.

Furthermore, scanning tunneling microscopy has made a significant contribution to the illustration of quantum mechanics . So-called quantum corrals were produced and measured in the early 1990s . Quantum Corrals are simple geometric quantum systems on surfaces. On the basis of these Quantum Corrals, the analogy between electron waves and water waves could be shown extremely clearly , which is a direct confirmation of quantum mechanics in real space that was not previously available. The images of these Quantum Corrals are now going around the world: They are the most frequently displayed RTM images in books and also in daily newspapers. Such images, their interpretation and effect are now even the subject of research in visual science (compare Horst Bredekamp ) and art history .

Scanning tunnel microscopy, like optical microscopy or scanning electron microscopy, is a real space imaging technology that only differs in the range of the physical processes used. Scanning tunnel microscopy is therefore particularly suitable for making atomic processes in surface physics and surface chemistry (Nobel Prize for Chemistry 2007, Gerhard Ertl ) accessible.

Scanning tunneling microscopy differs significantly from previous techniques in surface physics and chemistry, which were dependent on scattering processes, such as the scattering of electrons ( RHEED -scattering of high-energy electrons, Low Energy Electron Diffraction -backscattering of low-energy electrons) or helium scattering. The latter are limited by the wavelength of the particles used and only reproduce periodic structures due to constructive and destructive interference. In particular, access to effects on non-periodic structures, in particular defects at impurities or atomic levels, as they play an essential role in catalytic processes, is very incomplete.

## literature

• Russell Young, John Ward, Fredric Scire: The Topografiner. An Instrument for Measuring Surface Microtopography . In: Review of scientific instruments, with physics news and views . American Institute of Physics, Lancaster PA, 43, 1972, , p. 999.
• Patent CH643397 : Scanning apparatus for surface analysis using vacuum-tunnel effect at cryogenic temperatures (device for grid-like surface analysis using the vacuum tunnel effect at cryogenic temperatures). Registered on September 20, 1979 , applicant: IBM, inventor: Gerd Binnig , Heinrich Rohrer .
• G. Binnig, H. Rohrer, Ch Gerber, E. Weibel: Tunneling through a controllable vacuum gap . In: Applied Physics Letters . tape 40 , no. 2 , January 15, 1982, p. 178-180 , doi : 10.1063 / 1.92999 .
• G. Binnig, H. Rohrer, Ch. Gerber, E. Weibel: Surface Studies by Scanning Tunneling Microscopy . In: Physical Review Letters . tape 49 , no. 1 , July 5, 1982, p. 57-61 , doi : 10.1103 / PhysRevLett.49.57 .
• G. Binnig, H. Rohrer, Ch. Gerber, E. Weibel: 7 × 7 Reconstruction on Si (111) Resolved in Real Space . In: Physical Review Letters . tape 50 , no. 2 , January 10, 1983, p. 120-123 , doi : 10.1103 / PhysRevLett.50.120 .
• C. Hamann, M. Hietschold: Scanning tunnel microscopy . Akademie Verlag, Berlin 1991, ISBN 3-05-501272-0 .
• C. Julian Chen: Introduction to Scanning Tunneling Microscopy . Oxford University Press, Oxford 1993, ISBN 0-19-507150-6 . (English)
• Roland Wiesendanger : Scanning Probe Microscopy and Spectroscopy - Methods and Applications . Cambridge University Press, Cambridge 1994, ISBN 0-521-42847-5 . (English)
• B. Voigtländer: Scanning Probe Microscopy . Springer, 2015, ISBN 978-3-662-45239-4 , doi : 10.1007 / 978-3-662-45240-0 .

Commons : Scanning Tunneling Microscope  - collection of images, videos and audio files

## Individual evidence

1. ^ J. Bardeen: Tunneling from a Many-Particle Point of View . In: Physical Review Letters . tape 6 , no. 2 , January 15, 1961, p. 57-59 , doi : 10.1103 / PhysRevLett.6.57 .
2. J. Tersoff, DR Hamann: Theory of the scanning tunneling microscope . In: Physical Review B . tape 31 , no. 2 , January 15, 1985, p. 805-813 , doi : 10.1103 / PhysRevB.31.805 .
3. ^ C. Julian Chen: Origin of atomic resolution on metal surfaces in scanning tunneling microscopy . In: Physical Review Letters . tape 65 , no. 4 , July 23, 1990, pp. 448-451 , doi : 10.1103 / PhysRevLett.65.448 .
4. ^ Wandelt, K. (Klaus), 1944-: Encyclopedia of interfacial chemistry: surface science and electrochemistry. Volume 1, 1.1 experimental methods, 1.2 surface science under environmental conditions . Amsterdam, Netherlands, ISBN 978-0-12-809894-3 ( elsevier.com [accessed January 10, 2020]).
5. High-speed scanning tunneling microscopy (video RTM) ( Memento from June 11, 2007 in the Internet Archive )
6. Thomas Waldmann, Daniela Künzel, Harry E. Hoster, Axel Groß, R. Jürgen Behm: Oxidation of an Organic Adlayer: A Bird's Eye View . In: Journal of the American Chemical Society . tape 134 , no. 21 , May 30, 2012, p. 8817-8822 , doi : 10.1021 / ja302593v .
7. Markus Bautsch: Scanning tunnel microscopic investigations on metals atomized with argon. Chapter 3.5: Image errors. Verlag Köster, Berlin 1993, ISBN 3-929937-42-5 .
8. ^ Russell Young, John Ward, Fredric Scire: The Topografiner: An Instrument for Measuring Surface Microtopography . In: Review of Scientific Instruments . tape 43 , no. 7 , July 1972, p. 999-1011 , doi : 10.1063 / 1.1685846 .
 This version was added to the list of articles worth reading on March 17, 2007 .