Atomic force microscopy

The atomic force microscope, rare (to avoid confusion with the term nuclear power wrongly used mostly in the physical sense ) atomic force microscope (german atomic / scanning force microscope; Abbreviations AFM or SFM, rare RKM ) is a special scanning probe microscope. It is an important tool in surface chemistry and is used for mechanical scanning of surfaces and the measurement of the atomic forces on the nanometer scale.

The microscope was developed in 1986 by Gerd Binnig, Calvin Quate and Christoph Gerber.

• 3.2.1 force - distance curves
• 3.2.2 Single-molecule force spectroscopy

Principle of measurement

The so-called cantilever - - fortified nanosized needle line by line in a defined grid over the surface of a sample during the measurement, a to a leaf spring. This process is called scanning ( engl.to scan: scan, scan ) refers. The surface structure of the sample, while the leaf spring flexes differently depending on the position far. This deformation or deflection of the tip can be measured with a capacitive or optical sensors and is typically a measure of force acting between the tip and the surface atomic forces. In addition to the attractive, long-range van der Waals and capillary forces occur strong repulsive forces at short range. These are for a rejection based in quantum mechanics due to the Pauli principle, on the other hand a Coulomb repulsion of the nuclear charge, which is gaining support in the overlap of the electron shells. The overlay of these forces is often described by the Lennard -Jones potential.

By the pointwise recording the deflections and forces can be like a digital photo an image of the sample surface produce. Each individual pixel is then available for a particular physical or chemical variable ( see below). The possible resolution of the image, as with profilometers, mainly determined by the radius of curvature of the tips, it is usually 10 to 20 nm, which allows lateral resolutions of 0.1 to 10 nm, depending on the roughness of the sample surface. This is sufficient to be able, even in the ideal case mapping individual atoms. Thus, the atomic force microscope combined with a scanning tunneling microscope (STM or STM), the highest resolution of any microscopic techniques. For the precise movement of the needle on the sample are piezoelectric actuating elements, by means of which scanning areas of up to 150 microns x 150 microns can be examined. The scanning speed is typically between 0.5 and 10 lines per second (back and forth ). In normal image resolutions of 256 × 256-512 × 512 pixels, this results in a measurement time of about 1 to 20 minutes per image.

Modern systems feature a so-called " Tip Box ", which can contain different types of probes. The unit then automatically switches to the desired measurement point. In the AFM used in the semiconductor industry, it is also possible to use a polonium source which is intended to prevent erroneous measurements by counteracting the electrostatic charging of the sample and the measuring device.

3000 -fold magnification

50,000 -fold magnification

Construction

A probe tip (german tip), which is located on an elastically flexible arm (English cantilever ) is ( sample English) as Probe out a small distance above the sample surface. A piezoelectric scanner moves this either the tip over the sample or the sample under the fixed tip. The bending of the lever arm, caused by forces between the sample (English sample) and peak can be measured with high resolution, usually by a laser beam directed at the tip and the reflected beam is collected by a photodetector ( light pointer principle). Alternatively, the measurement of the deflection of the lever arm can be interferometrically. The bending of the lever arm provide information about the surface properties of the sample. An important element of an atomic force microscope is the controller that controls the movement of the scanner and the sample or top and evaluates the signals. The operation of the device is facilitated if the positioning of the laser and the top is supported by a light optical microscope.

An atomically fine tip can be achieved by by using a single carbon monoxide molecule as a point.

Modes of operation

The atomic force microscope can be operated in different operating modes. The operating modes can be classified according to three classifications, depending on

• Imaged
• Spectroscopically

• Contact mode
• Non- contact mode
• Intermittent mode

• Constant -height mode
• Constant-force/amplitude-Modus

Imaging methods

Contact mode

In all contact measurement methods is the probe tip in direct mechanical contact with the surface to be measured. Thereby creates a strong electrostatic repulsion between the electron shells of atoms at the surface and the probe tip touching it.

• Unregulated: The constant height mode (English for: mode of constant height ' ) is the oldest method of measurement of the atomic force microscope, since you only need very little requirements are placed on the control technology. When scanning the sample, the stylus bends according to the structure of the surface. Since this all the larger forces, the greater are the bumps on the surface, this method is suitable especially for very smooth and hard surfaces, such as cleavage planes of crystals. Since no control has to be done perpendicular to the sample surface, relatively high measurement speed can be up to about 10 lines per second, obtained with this method. The complete information about the topography of the surface is included in the displacement signal of the leaf spring.
• Regulated: in the constant force mode (English for: constant force mode ") however, the suspension point of the leaf spring using a piezo actuator element is controlled so that the deflection of the cantilever and thus the force between tip and sample is the same as possible. To achieve this, the deflection signal of the leaf spring is fed as a control variable in a control loop which determines the movement of the leaf spring suspension. Since loops have only a finite speed, this measurement method is limited to lower speeds. With today's commercially available atomic force microscopes is currently a maximum measurement rate of about 3 to 4 lines per second are possible. Although the forces exerted on the surface can be reduced by the control, although a residual stress is retained. With a good system, the information about the topography of the surface in the control variable of the piezo actuator element is included.

Non- contact mode (NC - AFM)

The non - contact mode (English: non -contact, nc -mode or dynamic mode ) belongs to the family of dynamic excitation modes, the cantilever is excited by an external periodic force oscillations. Some devices have to specially an additional piezo element, which is mounted directly to the cantilever. Especially in non- contact mode while the principle of self-excitation is utilized The oscillation signal of the cantilever is directly related to a phase shift of 90 ° is fed back again to the excitation element, that is, a closed resonant circuit is formed. Thus, the beam vibrates in principle always at its resonant frequency. If now occur between the tip of the cantilever and the sample surface to be examined forces, so the resonant frequency of the resonant circuit changes. This frequency shift is a measure of the strength of interaction, and is used as a control signal during scanning of the surface. The cantilever can be excited at a fixed frequency; the shift of the resonance frequency then provides a phase shift between the excitation and oscillation. The non- contact mode is used in a vacuum or ultrahigh vacuum, where it achieves the highest resolution as compared to the other modes of operation of the atomic force microscope. In contrast to even high-resolution scanning tunneling microscope, which achieved atomic resolution on electrically conducting samples, even single atoms and molecules can hereby be depicted on electrically insulating surfaces.

Intermittent mode

The intermittent mode ( engl.: intermittent contact mode, tapping mode at Digital Instruments ™ called ) also belongs to the family of dynamic excitation modes. In contrast to the non-contact mode, the excitation is carried out externally at a fixed frequency near the resonance frequency of the cantilever in this case. Interaction forces between the tip of the cantilever and the sample surface, to change the resonant frequency of the system, thus increasing the vibration amplitude and the phase change ( between excitation and vibration ). Usually, the vibration amplitude is used as a control signal during scanning of the sample, that is, a control loop tries to keep the amplitude constant, by increasing the distance, and hence the interaction force is adjusted between the tip and sample beams. This mode is typically used for measurements under ambient conditions or in liquids and thereby has found widespread use.

Other measured variables

Can be studied over the easy measurement of the surface topography by AFM addition, other physical properties. For all measurement principles but is one of the modes listed above are based:

Spectroscopic methods

Here, the AFM is not used to capture an image, but in order to examine the elasto-plastic properties of the sample at a predefined location.

Force - distance curves

For the measurement of force-distance curves of the cantilever is on or more than once lowered onto the sample, pressed with a defined force, and removed from the sample. The force acting on the measuring needle force is recorded as a function of tip position. From the resulting curves then can be obtained such as on the adhesion forces and the resiliency conclusions regarding various properties of the material and the surface. In order to increase the measurement accuracy and artifacts such as to eliminate by noise, is typically not a single curve but a family of curves, called Force Volume was added. From these an average curve is then formed and evaluated. Figure 5 shows typical force - distance curves that can arise in such a measurement. The blue curve represents respectively the approximation process, the red, the retraction of the tip.

Figure 5a shows the ideal case the measurement on a purely elastic sample. The horizontal section in the right half represents the zero line ( force curves are usually always read from the zero line from ) before the tip comes into contact with the surface. Approaching the top of the sample, it eventually comes to a jump of the tip to the surface, which is caused by short-range attractive forces. The force increases proportionally with the further approaching (so-called "contact regime "). After the movement has been reversed at the maximum of the curve falls exactly linear again, but remains adhering to the surface until the spring force of the cantilever is greater than the adhesion of the surface and the cantilever springs back to its zero position.

Figure 5b schematically a typical force curve on many sample types. While the zero line and the jump in the contact does not differ from a picture, you can see in the contact regime, that the line is no longer linear, but is initially flat and then becomes steeper. This may on the one hand by a hardening of the material during indentation come about ( elasto- plastic behavior ), or to others in that for thin samples affected with increasing indentation the harder sample pad measurement. Of the hysteresis between the approaching and receding curves, the work done on the sample work can be calculated.

Figure 5c finally demonstrates the most common artifact in force-distance measurements. In contrast to the images a and b is here the retraction curve in the contact regime above the approach curve, that is, apparently, the forces upon retraction of the tip are higher than during the approach. The artifact is usually concluded by nonlinearities of the piezo actuator elements in the force microscope.

Because of these and other artifacts that occur a great deal of care and experience is necessary both for the calibration of the instrument as well as the evaluation of the force curves.

Single-molecule force spectroscopy

A similar procedure as in the force - distance curves can also be used to measure binding forces in individual molecules such as proteins. Here, for example, the molecule to be measured is covalently attached using special molecules on a sample holder and to the tip and then stretched by pulling back the measuring tip. Since the folding of proteins is by hydrogen bonding or even weaker bonds state, characterized the first molecule is fully deployed before coming ultimately to a rupture of one of the covalent bonds in the molecule or surface. In the associated force-distance curve, the unfolding of a sawtooth-like structure can be seen. An understanding of the measurement results can not be achieved without at least basic molecular knowledge.

Disturbances during measurement

The evaluation of the data obtained during the measurements requires a detailed analysis, since during each measurement disturbances may occur and the data will be overlaid by system-related errors. A fundamental problem with all pictures with a finite size measuring tip is that the measured data do not represent the actual sample surface but a convolution of the geometry of the tip with the structure of the surface

In addition to the systemic errors different faults can occur during the measurement:

• Vibrations: These come about firstly by building vibrations or impact noise. AFM measurement stations are therefore often based on vibration-isolated tables, mostly consisting of thick marble slabs on damping feet compressed air, or with piezo elements actively damped tables. In addition, when making measurements under atmospheric pressure, the acoustic sound, which is transmitted over the air directly to the cantilever, a strong source of disturbance dar. This the more so the closer the resonance frequency of the cantilever is the frequency range of normal sound. For this reason, it is useful to operate the AFM in special soundproof boxes. If possible from point of view of the test sample can also be devices that operate under vacuum conditions, can be used.
• Thermal drift: Thermal expansion between sample and cantilever displacements of a few nanometers can occur during a measurement interval, which is visible in images with high resolution and distortion.
• Interference phenomena: the case of highly reflecting samples, it is possible that a portion of the laser beam is reflected from the sample surface and in the photodetector with the proportion that comes from the cantilever interferes. This manifests itself in perpendicular to the scan direction strips noticeable, which are superimposed on the actual height image.
• Static charges: Especially for non-metallic samples MFM measurements, electrical charges which are collected from the top, distort the measurements or make impossible. To avoid these charges, sample and cantilever should be at the same ground potential. Non-metallic samples to be vapor-coated with a fine layer of gold. Where this is not possible, the air can be ionized with a radioactive source, causing a potential compensation of unwanted electric charges. These are the charges of the measurement area constant, this can be compensated using the control software, or the control circuit of the measuring arrangement.

Evaluation software

In professional AFMs an evaluation software driver program the hardware is usually integrated. The data formats are usually manufacturer -dependent, since in addition to pure image data and the settings of each measurement, such as the scanning speed to be saved. In addition, the measurement images created can also convert into known data formats such as BMP or JPEG files. For Macintosh computers, there is the NIH Image -based measurement software free ImageSXM that is able to process, among other things, the raw data of many atomic force and scanning tunneling microscopes. For GNU / Linux, Microsoft Windows, Mac OS X and FreeBSD the free evaluation software Gwyddion is available, which can also import a variety of different raw data formats.