Scanning tunneling microscope

Scanning tunneling microscopy (hereinafter abbreviated RTM, English scanning tunneling microscopy, STM ) is one of the techniques of scanning probe microscopy (English scanning probe microscopy, SPM), which differ only in the composition of the probe and its interaction with a to the surface under examination. In STM, the probe is electrically conductive. The interactive process is that of the quantum mechanical tunneling effect. At an applied voltage between a tip and a surface, this leads to a measurable tunnel current.

As a scanning tunneling microscope is referred to the corresponding experimental setup, in which the tip is scanned line by line over a surface and the tunneling current is the principal measure. The local coordinates (x, y) of the grid are unregulated sizes while the tunneling current is dependent on each other is connected by the z-position and the applied voltage and usually via a control loop. The dependence from each other, forms from various physical properties of the surface. at a constant level tunneling spectroscopy is called and gives access to the work function. The application of the spatial dependence of different sizes is called mapping. In this case, a very high resolution is achieved down to the atomic scale, as the tunnel effect the RTM is based is sensitive to changes in the sub- angstrom range. The spatial dependence of the tunneling current reflects the convolution of the real topography with electronic properties. A three-dimensional plot suggests keeping a view of the surface topography, but is exactly enough, the level topography of constant electron density.

Differences to other methods

The scanning tunneling microscopy such as optical microscopy and scanning electron microscopy imaging in real space, a technique that differs only in the reach of this unused physical processes. Therefore, the scanning tunneling microscopy is particularly suitable to make atomic processes of surface physics and surface chemistry (Nobel Prize in Chemistry 2007, Gerhard Ertl ) accessible.

Scanning tunneling microscopy is significantly different from previous techniques of 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 the helium scattering. The latter is limited by the wavelength of the particles used to form by constructive and destructive interference only periodic structures. This is particularly the effects on access to non-periodic structures, in particular defects in defects or atomic steps as they play an important role, especially for catalytic processes, very incomplete.

Operation

In the scanning tunneling microscopy measurements, an electrically conductive probe is moved systematically in a grid over the study also conducting object. The distance is extremely small (nanometer ) and not zero. There is a potential barrier between them, not overcome, probably can be tunneled through but, resulting when a small voltage to a tunneling current. This is very sensitive to the slightest changes in distance, since the tunneling probability decreases exponentially with distance. The RTM can, for example, be operated in the following two modes:

• During scanning of the sample surface, the height of the tip is controlled by a precision engineering (piezo elements ), so that the tunnel current remains constant. So that the tip moves to a "High profile " of the surface, wherein said height control signal is used for displaying the specimen surface. The lateral resolution depends on the curvature radius of the tip. Ideally, the tunnel current flows only through a single atom exposed at the top. The images obtained by constant tunneling current are not related to forcibly of the topography of the surface. Rather, in this case the electronic structure of the surface is primarily sampled, see below.
• Alternatively can be kept constant, the height of the peak, and a reconstruction of the surface recorded by the different distances to the sample surface and the resulting variation of the tunneling current. The latter method is more sensitive to surface electronic effects rather than geometric, and by comparing the two images can be estimated the electronic structure, the deviation of topography.

Since the principle of scanning tunneling microscopy based on the measurement of current flow between the sample and the tip of the scanning tunneling microscope, only conductive samples ( metals, semiconductors or superconductors ) can be studied directly. Although non-conductive samples also show tunnel phenomena, however, the tunneling current can not pass through the sample to the other cathode and measured. Therefore, they must first be vaporized with a fine electrically conductive layer ( graphite, chrome or gold), which, on the edge of the sample contact to the sample holder. A further possibility is to examine a very thin layers of an insulator on a conductive substrate.

Experimental boundary conditions

Since a very small tunneling current ( typically 1 pA to 10 nA) sensitive already responded to changes in hundredths of nanometers, the tip-sample distance of typically 0.5-1 nm to less than 1 % deviation must be stabilized. Therefore, depending on the desired precision can find different techniques for isolation using:

Thermal insulation: temperature variations, because of the different thermal expansion coefficients of materials used to disturbing modulation of the tip-sample distance. In the CCM, this distortion. In modes where no loop is used to regulate the distance as in CHM or during spectroscopy, surface- destructive tip-sample contacts can take place.

Acoustic Insulation: With sound waves mechanical parts can be excited to oscillate, which modulate either the tip-sample distance, or disturbed by changes in the capacity of conductor cables, the measuring signal. By the use of sound insulation or sound boxes of this influence can be reduced.

Mechanical Isolation: A highly disruptive influence are mechanical vibrations that are carried into the system through the construction of the building. These vibrations can be reduced through the use of active and passive ( air spring ) feet, with spring suspension, or a magnetic eddy current brake. Often combinations of different techniques are used, as the filter characteristics vary. Eddy current brakes are suitable for damping vibrations faster (> 1 kHz) while actively controlled damping structures especially in constructions in higher floors of tall buildings are suitable, low-frequency (< 1 Hz) to minimize interference from building vibrations.

The bias voltages between the tip and sample are generally from a few millivolts to a few volts. The lower limit is due to the temperature and determined by the thermal noise ( room temperature, about 50 mV). At room temperature, the maximum voltage is also determined by gaseous particles, which may occur in the tunnel barrier. So kick from about 2 V to brief current spikes that destroy the surface term. At low temperatures and in a vacuum system as well as very high voltages beyond 100 V can be easily applied, the transition of the tunnel current can be observed in a field emission current.

Both the examined surface and the tip must be used on the surface electrically conductive. If there is to be examined surface of metals that can oxidize in air (eg, copper, silicon or silver), the scanning tunneling microscopy is carried out in an ultrahigh vacuum, resulting in a not to be underestimated technical complexity. As surfaces that can be used, however, under normal conditions, conductive layer crystals such as graphite or representative of the layer- crystalline transition metal dichalcogenides such as molybdenum disulfide ( MoS2), tantalum disulfide ( TaS2 ) or tantalum diselenide come ( TASE2 ) in question. A fresh, atomically smooth surface and is easily accessible by removing the top layers with adhesive tape, since the individual layers are only connected by relatively weak van der Waals interactions in this layer crystals.

The movement of the tip relative to the sample surface is accomplished by means of the piezoelectric ceramics. These allow high-precision control in the sub -nanometer scale via applied electric voltages.

The probe itself is usually made of tungsten, platinum or gold alloy, wherein the tip is produced by electrolytic etching.

Theory

The tunnel effect between two metals separated by a thin oxide layer, was declared in 1961 by John Bardeen using the time-dependent perturbation theory of first order ( Fermi's golden rule). Applying this theory to the scanning tunneling microscopy, as an atomically precise knowledge of the tip is necessary in order to interpret the measured images. A major simplification is the so-called Tersoff -Hamann theory is that neglected the influence of the tip on the measurement and statements about the electronic structure of the sample supplies (primarily through the local electronic density of states in the surface region ). The tip is assumed to be a metal atom with a linear electronic density of states and spherically symmetric s- wave functions. An extension of this theory yielded C. Julian Chen, the complex tip geometries calculated. A truly three-dimensional theory of the scanning tunneling microscope is indeed analytically opened, but usually hardly solvable and therefore of minor importance. Three-dimensional systems can only be approximately calculated numerically, usually with the aid of several estimated parameters. Simulations of STM images of organic molecules on surfaces are possible by superpositions of occupied and unoccupied molecular orbitals of the molecules in vacuum, obtained for example from the density functional theory ( quantum physics ).

Measurement modes

A scanning tunneling microscope employs a distance of the tip of the sample and with a resolution that is less than the wavelength of the tunneling electrons (see material and de Broglie wave equation). When an electrical voltage (English bias or tunneling bias) applied between the study object and the top, so can a current flow so-called tunneling current.

The three methods described in the following ( CHM, CCM and STS images with the exception of point spectroscopy) is common that the probe tip is moved line by way of the sample before it laterally offset detected an adjacent line. This results in a line raster on the surface.

Constant height mode

By CHM ( engl. constant height method), the tip follows a pre- specified height profile without the sample-tip distance is re-adjusted by a control loop. In parallel, the tunneling current will be recorded at each grid point. This direct conclusions on height dependencies of the tunneling current as well as possible measurement artifacts caused by the feedback loop in the CCM are preventable.

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

Advantageous is the use of CHM for the study of thermally induced mobility of single atoms, in chemical processes, or of molecules at high process speeds. In all these cases, change the local geometry and can be easily identified in difference images of fast consecutive shots ( video RTM).

The disadvantage is that greater demands are placed on the experimental setup in terms of long -term stability. Slow mechanical disturbances (for example, building vibrations, Creep and drift of the piezo actuator) can lead to significant peaks in the tunneling current to the tip-sample contacts, locally disturb the surface.

Constant tunneling current mode

Another method of imaging (constant current method, abbreviated CCM or constant gap width mode, abbreviated CGM) is based on an ongoing basis to change the height of the tip so that the current remains constant. This is done via an electronic control circuit for distance control. Thus, it can now decide on the position of the tip of the three-dimensional image of the surface directly. The resolution is so high that the atomic electronic structure of the surface is visible in this method. The image contrast must not be understood directly as atomic structure. Meanwhile, at least nine different contrast mechanisms are known which affect the image formation and must be considered in the interpretation. However, the method is limited by the control circuit in its measurement speed, the recording of an image usually takes several times of ten seconds to hours. In practice, this mode is usually used.

Spectroscopy mode

See also scanning tunneling spectroscopy ( engl.scanning tunneling spectroscopy STS).

Since it conveys with the scanning tunneling microscope, on the tunnel effect, first measure the local electronic structure of the sample surface, it can also be directly exploited for determining this. For example, a single oxygen atom on a surface of the semiconductor material gallium arsenide appears sometimes as well, and sometimes as a hill, depending on whether you are applying positive or negative voltage between tip and sample.

One can exploit this to either the energetic positions of the surface states in one place the sample ( TS spectra at a place called Point spectroscopy) or the places at which electrons at a given energy ( corresponding to the tunneling voltage ) reside may (STS- images to determine at constant tunneling voltage ). One has to the tunneling voltage, a small high-frequency alternating voltage are superimposed, and then can be calculated from the derivative of the current according to the so-called density of the voltage. Scanning tunneling spectroscopy is often carried out at low temperatures of a few Kelvin, as the energy resolution dependent on the Fermi distribution of the temperature. The spectroscopy mode is further divided into various sub-modes.

Video scanning tunneling microscopy

At scan rates from one frame per second is called video - scanning tunneling microscopy. The frame rate will be 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 across a phase boundary or atomic level, which is in dynamic equilibrium with its environment, can so-called pseudo images ( also: xt scan) measure. From these measurement data, in which the x-axis, a time indication, and the y - axis is a location information, it is possible in turn to calculate the levels of the correlation function, information can be derived from the diffusion process at the appropriate location.

Aberrations

A number of factors may affect or limit the image quality of the scanning tunneling microscope images. Particular care must be taken to avoid external vibrations, including, for example vibration insulation can be used. But also the actuators for the grid can cause internal vibrations may be reduced by suitable choice of the natural frequencies. Further tilting the piezoelectric materials used both for hysteresis and creep, which causes inaccuracies in the position determination. These materials generally have a relatively high temperature drift, so that the temperature should be kept sufficiently constant during the measurement. The noise of the tunneling current limits the accuracy of the height determination. Therefore, noise-free as possible current-voltage converters are used with the required bandwidth for the frequencies occurring. The deflection voltages of the actuators must have the required accuracy in terms of linearity and delay time.

The use of double and multiple tunnel tips, the point at which the tunneling current flows between individual switch points, which may lead, for example, for multiple, but offset frames of same sample area. The optionally resulting ghost images are characterized by parallel structures.

Manipulation

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

A distinction is made between different changes between the displacement ( lateral and vertical manipulation) and the modification of objects ( dissociation and structural modification, particularly for molecular systems ). The following processes are used: bond-breaking by local heating and displacement by potential change:

Locally heating: In particular, in systems with, for example, covalent bonds within molecules or silicon -hydrogen bonds, the vibration modes can be excited through the tunneling electrons. Through the accumulation of this energy may ultimately a bond to be broken (or be closed). Since the lifetime of corresponding suggestions are usually very low ( fs- ms), an energy accumulation can be achieved by correspondingly high currents ( Note 1 nA ~ 0.1 ns temporal distance between two of tunneling electrons).

Potential change: To shift of objects but also extends the already attractive or repulsive interaction of the tip to the object by its potential. In this case, the potential can also be modulated by the applied bias voltage. Accordingly, one can draw with attractive interaction objects, pushing in repulsive. With sufficient approach of the tip to the object a transfer of the object from the sample surface can also be generated at the tip. In some cases, the back transfer of an additional use of the bias voltage is possible, and this is called the vertical manipulation.

Using these methods, the so-called atomic writing was performed showing the logos such as IBM, logos individual universities or country sketch maps with single atoms on surfaces.

In the field of magnetic data storage IBM has developed a scanning tunneling microscope, the work at very low temperatures ( ≈ 4 K). In order to successful attempts have been carried out, in its spin ( magnetic ) orientation to change individual atoms in a magnetic layer and selectively influencing. The method is called the spin excitation spectroscopy ( spin excitation spectroscopy).

History of Research

The first successful experiment for detecting a distance-dependent tunneling current could be carried out on 18 March 1981 at the IBM Research Laboratory in Rüschlikon (CH). The two physicists Gerd Binnig ( Germany ) and Heinrich Rohrer ( Switzerland ), who carried out the experiment and the scanning tunneling microscope ultimately also made for purpose instrument, received this 1986 Nobel Prize in Physics. Furthermore, Christoph Gerber and Edmund Weibel were involved in the development.

But there were previous work in this field, in which the essential elements of an RTM / RTM were demonstrated - in particular the occurrence of a tunnel current. This device was developed by Russell Young, John Ward, and Fredric Scire the late 1960s as so-called Topografiner. However, there were bureaucratic and technical difficulties, such as disrupted the vibrations of the air conditioning the measurements. The Nobel Committee recognized, however, later on their achievements.

The scanning tunneling microscope is the father of all other scanning probe microscopes. In the following years mainly the atomic force microscope (atomic force microscope, AFM) and optical Rasternahfeldmikroskop ( scanning near -field optical microscope SNOM ) have been developed which make use of a different atomic interaction. The development of all this scanning probe microscopes was an essential step in the direction of nanoscience, as you watch them in a very simple and relatively inexpensive way nanoscopic objects (objects that are smaller than the wavelength of light from 400 to 800 nm) and beyond also manipulate can.

Furthermore, the scanning tunneling microscopy has contributed significantly to illustrate the quantum mechanics. The early 1990s, so-called quantum corrals were generated and measured. Quantum Corrals are simple geometric quantum systems on surfaces. Based on this Quantum Corrals could be extremely vividly illustrated the analogy between electron waves and water waves, which is a hitherto non-existent direct confirmation of quantum mechanics in real space. The illustrations in this Quantum Corrals now go around the world: you have to find the most STM images shown in books and in addition also in daily newspapers. Such images, their interpretation and effect are now even research topic of Image Science (see Bredekamp ) and of art history.