Principle of Scanning probe microscopy

General
Modern microscopes, such as the so-called Scanning Tunneling Microscope (STM), can image the surfaces of materials with unparalleled magnification. The magnification is so extreme, that individual atoms become visible. With its ultimate resolution, this remarkable instrument forms the basis of an enormous development within physics. But also in the fields of chemistry and biology, the STM and derived microscopes have conquered an important position within a very short time. With an STM one can only image surfaces of materials that conduct electrical currents. But the principle of the STM is so flexible, that with relatively modest changes in the technology also non-conductive materials can be imaged. In this way, also oxides and even biological materials, such as DNA, can be investigated on the sub-nanometer scale. By now, there is a family of some twenty different types of microscopes, derived from the STM, which are referred to as Scanning Probe Microscopes (SPM).

In 1986, very soon after their first publications about the STM in 1981, the inventors of this marvelous instrument, Gert Binnig and Heinrich Rohrer from the IBM Research Laboratory in Rüschlikon (Switzerland), were awarded the Nobel Prize in Physics. Nowadays, SPMs can be found in many academic and industrial physics, chemistry and biology laboratories. They are used both as standard analysis tools and as high-level research instruments. There is an impressive literature about SPM-technology and application. The following books provide a good introduction [1,2,3,4,5,6].

The STM works like a record player… 
The principle of the STM is remarkably simple, and can be compared best with that of an old-fashioned record player. Just like in a record player, the instrument uses a sharp needle, referred to as the ‘tip’, to interrogate the shape of the surface. But in contrast with a normal record player, the STM tip does not touch the surface. This is done by the method, indicated in the schematic picture. A voltage is applied between the metallic tip and the specimen, typically between a few milliVolts (mV) and a few Volts (V). When the tip touches the surface of the specimen, the voltage will, of course, result in a current. When the tip is far away from the surface, the current is zero. The STM operates in the regime of extremely small distances between the tip and the surface of only 0.5 to 1.0 nm, i.e. 2 to 4 atomic diameters. At these distances, the electrons can ‘jump’ from the tip to the surface or vice versa. This ‘jumping’ is a quantum mechanical process, known as ‘tunneling’. Hence the name of this microscope: tunneling microscope. The tunneling process is very difficult, which implies that the tunneling current is always very low. STMs usually operate at tunneling currents between a few picoAmperes (pA) and a few nanoAmperes (nA).

The tunneling current depends very critically on the precise distance between the last atom of the tip and the nearest atom or atoms of the underlying specimen. When this distance is increased only a little bit, the tunneling current decreases strongly. As a rule of thumb, for every extra atom diameter that is added to the distance, the current becomes a factor 1000 lower! This means that the tunneling current provides a highly sensitive measure of the distance between the tip and the surface.

Feedback        
The STM tip is attached to a piezo-electric element. This is a piece of material with the useful property that it changes its length a little bit, when it is put under an electrical voltage. By adjusting the voltage on the piezo element, the distance between the tip and the surface can be regulated. In most STMs, the voltage on the piezo element is adjusted such, that the tunneling current always has the same value, for example 1 nA. In this way, the distance between the last atom on the tip and the nearest atoms in the surface is being kept constant. This distance regulation is performed automatically, by so-called feedback electronics, which continually measure the deviation of the tunneling current from the desired value – e.g. 1 nA – and retract the tip when the current is too high or advance it when the current is too low.

Scanning
While this feedback system is active, two other parts of the piezo element are used to move the tip in the X- and Y-directions, parallel to the surface, to scan over the surface, line by line, similar to the way a television or computer screen image is built up. The combination of small displacements in the X-, Y-, and Z-directions, parallel and perpendicular to the surface, is obtained with suitable combinations or geometries of piezo-electric elements, as indicated below.

In most modern scanning probe microscopes, one uses a tube geometry (see picture on the right). Each of the four indicated sections can be made longer or shorter individually. If all four sections are made longer or shorter by the same amount, the tip moves in the z-direction. If the X+ side is made longer, and at the same time the X- side is made shorter by the same amount, the tube deforms a little bit, as indicated. For small deformations, this makes the tip move primarily in the X-direction. The same can be done in the Y-direction.

Making STM-images   
In the XY-scan, every time that the last atom of the tip is precisely above a surface atom, the tip needs to be retracted a little bit, while it has to be brought slightly closer when the tip atom is between the surface atoms. Thus, the tip automatically follows a bumpy trajectory, which mimics the atomic corrugation of the surface. The information about this trajectory is available in the form of the voltages that have been applied by the feedback electronics to the piezo element during the XY-scan.

The last step is to visualize the tip trajectories. There are several ways to do this, for example in the form of a collection of individual height lines, or in the form of a gray-scale or color-scale representation, or in the form of some three-dimensional perspective view. It is important to remember that the STM is ‘color blind’, and that all gray scales and color scales represent nothing else than heights. Also note that the height scale in most perspective views is grossly exaggerated, in order to bring out the height variations more clearly.

Electronics
The circuit that is used for the feedback electronics schematically looks as follows. The tunneling current I_t, which is typically 1 nA, is converted by a so-called pre-amplifier to an ordinary voltage V_t. Often the conversion is such that a current I_t of 1 nA at the input of the preamplifier results in a voltage V_t of 1 V at the output. In the next step, this voltage V_t is amplified logarithmically. Because the tunneling current – and therefore also the voltage V_t  – is an exponential function of the distance d between the tip and the surface, the logarithm of the voltage V_t is a measure for the distance between the tip and the surface. We refer to the result of the logarithmic conversion as V_log. The relation between V_log and the distance d is then V_log=a + b·d, where a and b are constants. From this voltage V_log we subtract a reference voltage V_ref. This reference value can be chosen at will. It forms a measure for the tip-surface distance that we want to work at. When the difference voltage between V_log and V_ref is equal to zero, the tip-surface distance is precisely right. When the difference is positive, the distance is too small; and when it is negative, the distance is too large. This difference voltage forms the input for a high-voltage amplifier, which amplifies it very strongly and passes it on to the piezo element, with which we control the height of the tip. This closes the feedback loop and makes the system complete. Many extra elements are necessary to make the feedback circuit work in practice. Examples are an absolute value amplifier, which allows the STM to work at both positive and negative tunneling voltages, appropriate filters, to avoid spontaneous ringing of the STM, and combinations of linear, integrating and differentiating amplifiers, in order to obtain a more ideal response.

Why STM works…?    
How it is possible to obtain atomic resolution with a simple instrument as the STM can be understood on the basis of the following simple arguments. First, consider the electronic structure of a metal, as symbolized in the picture on the left. The electrons of the metal occupy all available energy levels up to the energy E_F, at which they precisely compensate the positive charge of the metal ions. For an electron to leave the metal, it needs to acquire an extra amount of energy of F above the Fermi energy. This brings it up to the vacuum level, at which point it is free to move away from the metal. The energy F is known as the work function of the metal. The middle picture shows the situation of a tip and a specimen in close proximity. There is only a narrow region of space between the two, but there is no conductive connection. Classically, electrons still need to have an extra energy F above the Fermi energy to move from specimen to tip or vice versa. 

However, quantum mechanics allows the electrons to go ‘right through’ the barrier, a process known as tunneling. When an electrical voltage V is applied between specimen and tip, this tunneling phenomenon results in a net electrical current, the ‘tunneling current’. This current depends on the tip-surface distance d, on the voltage V, and on the height of the barrier F:

This (approximate) equation shows that the tunneling current obeys Ohm’s law, i.e. the current I is proportional to the voltage V. It depends exponentially on the distance d. The other quantities in the equation are the work function F, the electron charge and mass e and m, and Planck’s constant. For a typical value of the work function F of 4 electronVolt (eV), the tunneling current reduces by a factor 10 for every 0.1 nm increase in d. This means that over a typical atomic diameter of e.g. 0.3 nm, the tunneling current changes by a factor 1000! This is what makes the STM so sensitive. The tunneling current depends so strongly on the distance that it is dominated by the contribution flowing between the last atom of the tip and the nearest atom in the specimen.

STM in the Interface Physics Group     
In the Interface Physics Group, Scanning Tunneling Microscopy is used to investigate the structure and dynamic behavior of metal surfaces. We investigate surfaces both in the highly idealized model environment of ultrahigh vacuum (UHV), and in the more realistic context of high gas pressures. The UHV experiments are used to study various types of surface diffusion, growth phenomena, and surface phase transitions. In the high-pressure STM experiments we investigate the influence of high pressures of reactive gas mixtures on the structure of metal surfaces, and the resulting effects on the catalytic activity of these metals. For both types of STM experiments, the group has developed special STM equipment. For the UHV-experiments, this is a high-speed, variable-temperature STM [7,8]. The imaging rate can be as high as 10 images per second (we are presently working on video-rate STM). The STM can keep a single region on the surface ‘in view’ over an appreciable temperature range of up to 300 K. For the high-pressure experiments, we have constructed a special, so-called ‘Reactor-STM’, in which only the STM tip is allowed inside a tiny reactor chamber, together with the metal specimen, while the rest of the STM is kept outside [9]. More about these special instruments and their applications can be found on our research pages about Thermo-STM, surface dynamics, and Reactor-STM. 

Atomic Force Microscopy       
The second scanning probe microscope, which was developed soon after the STM, is the Atomic Force Microscope (AFM). The important difference between the AFM and the STM is that in the AFM, the tip is not kept at a short distance from the surface. Instead, the AFM tip gently touches the surface. The AFM does not record the tunneling current but the small force between the tip and the surface. To this end, the AFM tip is attached to a tiny leaf spring, the cantilever, which has a low spring constant. The bending of this cantilever is detected, often with the use of a laser beam, which is reflected from the cantilever. Thus, rather than to measure contours of constant tunneling current, the AFM measures contours of constant attractive or repulsive force. The detection is made so sensitive that the forces that can be detected can be as small as a few picoNewton. Forces below 1 nanoNewton are usually sufficiently low to avoid damage to either the surface or the tip. Because the AFM does not rely on the presence of a tunneling current, it can also be used on non-conductive materials.
Play/download movie of the principle of AFM (24 Mb).

In the Interface Physics Group, Atomic Force Microscopy is used mainly to investigate the structure and behavior of biological model systems. We are modifying existing microscopes and developing new ones, in order to improve imaging speed, force sensitivity, and increase functionality. More about these special AFM applications can be found on our research pages about bio-AFM.

Friction Force Microscopy      
Soon after the introduction of the AFM, it was realized that the same instrument could be used to also measure forces in the direction(s) parallel to the surface, i.e. the friction forces. For this application, the AFM usually detects not only the deflection of the cantilever perpendicular to the surface, but also the torsion of the cantilever, resulting from one component of the lateral force. Again, the sensitivity can go down to the atomic level, so that this instrument allows us to investigate the atomic-scale origin of the friction force. This field of research is known as nanoTribology. Unfortunately, the geometry of a traditional AFM cantilever is not very sensitive to friction, unless the cantilever is made very thin, in which case it becomes ‘over-sensitive’ to the forces normal to the surface.
Play/download movie of the principle of FFM (8 Mb)

In order to overcome this problem, the Interface Physics Group has designed and constructed a special Friction Force Microscope (FFM), which is highly sensitive to both components of the force parallel to the surface, while it has the regular sensitivity, typical for a traditional AFM, to the perpendicular force [10]. More about atomic-scale friction and our research on friction can be found on the research pages of our group about nanoTribology.

SPM on the market     
Although several academic research groups, such as the Interface Physics Group in Leiden, develop new types of SPMs, or improve existing types of SPM, most SPM investigations in academic and industrial research groups are carried out with commercial SPMs nowadays. There is a wide collection of general-purpose or more specialized scanning probe microscopes, which can be obtained from a growing number of companies. Go to our links page, in order to find links to several SPM-suppliers.

Interface Physics Group Joost Frenken: E-mail Last modification: 13.09.2004