Atomic force microscope
The atomic force microscope , and atomic force microscope or atomic force microscope ( english atomic / scanning force microscope , called Abbreviations AFM or SFM, rare RKM) is a special scanning probe microscope . It is an important tool in surface chemistry and is used for the mechanical scanning of surfaces and the measurement of atomic forces on the nanometer scale . The atomic forces bend a leaf spring with a nanoscopic needle at the end. The force acting between the atoms of the surface and the tip can then be calculated from the measured deflection of the spring. Since no current flows between the sample and the tip, non-conductive samples can also be examined.
During the measurement, a nanoscopic needle attached to a leaf spring - the so-called cantilever - is guided over the surface of a sample line by line in a defined grid . This process is known as scanning ( English to scan : raster, scan). Due to the surface structure of the sample, the leaf spring bends to different degrees depending on the position. This bending or deflection of the tip can be measured with capacitive or typically optical sensors and is a measure of the atomic forces acting between the tip and the surface . In addition to the attractive, long-range van der Waals and capillary forces , strong repulsive forces with a short range occur. On the one hand, these are repulsions based on the Pauli principle based on quantum mechanics, and on the other hand, a Coulomb repulsion of the nuclear charge, which becomes more important when the electron shells overlap. The superposition of these forces is often described with the Lennard-Jones potential .
By recording the deflections or forces point by point, an image of the sample surface can be generated like a digital photo . Each individual pixel then stands for a specific physical or chemical measured variable (see below ). As with profilometers , the possible resolution of the image is mainly determined by the radius of curvature of the tips; it is usually 10 to 20 nm, which, depending on the roughness of the sample surface , allows lateral resolutions of 0.1 to 10 nm. This is sufficient to ideally even be able to depict individual atoms. This means that the atomic force microscope together with the scanning tunneling microscope (RTM or STM) has the highest resolution of all microscopic techniques. Piezo control elements are used to precisely move the needle over the sample , with the aid of which scan areas from 1 µm × 1 µm to 150 µm × 150 µm can be examined. The scanning speed is typically between 0.5 and 10 lines per second (there and back). With normal image resolutions of 256 × 256 to 512 × 512 pixels, this results in a measurement time of approximately 1 to 20 minutes per image.
Modern systems have a so-called “tip box” which can contain different types of measuring tips. The device then automatically switches to the desired measuring tip. With the AFMs used in the semiconductor industry, there is also the option of using a polonium source, which is intended to avoid incorrect measurements by counteracting the electrostatic charging of the sample and the measuring device.
A measuring tip ( English tip ), which is located on an elastically flexible lever arm ( English cantilever ), is guided as a measuring probe ( English probe ) at a small distance over the sample surface. A piezoelectric scanner moves 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 tip can be measured with high resolution, generally by directing a laser beam at the top and is collected, the reflected beam by a photodetector (light pointer principle). Alternatively, the deflection of the lever arm can be measured interferometrically. The bending of the lever arm provides information about the surface properties of the sample. An important element of an atomic force microscope is the controller, which controls the movement of the scanner and the sample or tip and evaluates the signals. The operation of the device is made easier if the positioning of the laser and the tip are supported by a light-optical microscope.
An atomically fine tip can be achieved by using a single carbon monoxide molecule as the tip.
The atomic force microscope can be operated in different operating modes. The operating modes can be classified according to three systems, depending on the situation
- whether imaging occurs:
- which interactions are used for the measurements:
- Contact mode
- Non-contact mode
- Intermittent mode
- how the movement of the needle is regulated:
- Constant-height mode
- Constant force / amplitude mode
In all contact measuring methods, the measuring tip is in direct mechanical contact with the surface to be measured. A strong electrostatic repulsion occurs between the electron shells of the atoms on the surface and the measuring tip touching them .
- Unregulated: The constant height mode (English for: 'mode with constant height') is the oldest measuring method of the atomic force microscope, since only very low demands are made on the control technology. When the sample is scanned, the stylus bends according to the structure of the surface. Since the greater the unevenness on the surface, the greater the forces, this method is particularly suitable for very smooth and hard surfaces, such as the cleavage surfaces of crystals. Since no regulation has to be carried out perpendicular to the sample surface, relatively high measurement speeds of up to 10 lines per second can be achieved with this method. All information about the topography of the surface is contained in the deflection signal of the leaf spring.
- Regulated: in constant force mode , on the other hand, the suspension point of the leaf spring is controlled with the aid of a piezo actuator so that the deflection of the cantilever and thus the force between the tip and the sample remains as constant as possible. To achieve this, the deflection signal of the leaf spring is fed into a control loop as a controlled variable , which determines the movement of the leaf spring suspension. Since control loops only have a finite speed, this measuring method is limited to lower speeds. With the atomic force microscopes commercially available today, a maximum measuring speed of around 3 to 4 lines per second is currently possible. Although the control can reduce the forces exerted on the surface, a residual load is still retained. With good control, the information about the topography of the surface is contained in the manipulated variable of the piezo actuator.
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, whereby the cantilever is excited to vibrate by an external periodic force. Some devices have an additional piezo element that is attached directly to the cantilever. In the non-contact mode in particular, the principle of self-excitation is used: the vibration signal from the cantilever is fed back directly to the excitation element with a phase shift of 90 °, i.e. a closed resonant circuit is created. This means that the bar always vibrates at its resonance frequency. If forces now occur between the tip of the cantilever and the sample surface to be examined, the resonance frequency of the oscillating circuit changes. This frequency shift is a measure of the force interaction and is used as a control signal when scanning the surface. The cantilever can also be excited with a fixed frequency; the shift in the resonance frequency then results in a phase shift between excitation and oscillation. The non-contact mode is usually used in a vacuum or ultra-high vacuum, where it achieves the highest resolutions compared to the other operating modes of the atomic force microscope. In contrast to the high-resolution scanning tunneling microscope , which achieves atomic resolution on electrically conductive samples, it can even be used to visualize individual atoms and molecules on electrically insulating surfaces.
The intermittent mode ( English intermittent contact mode , also called tapping mode ) also belongs to the family of dynamic excitation modes . In contrast to the non-contact mode, in this case the excitation is carried out externally at a fixed frequency close to the resonance frequency of the cantilever. Interaction forces between the tip of the cantilever and the sample surface change the resonance frequency of the system, which changes the oscillation amplitude and the phase (between excitation and oscillation). The oscillation amplitude is usually used as a control signal when scanning the sample, i.e. a control loop tries to keep the amplitude constant by adjusting the distance, and thus the force interaction, between the tip of the beam and the sample. This mode is usually used for measurements under ambient conditions or in liquids and is therefore widely used.
In addition to simply measuring the surface topography, the AFM can also be used to investigate other physical properties. However, all measurement principles are based on one of the measurement modes listed above:
- Magnetic force microscopy ( English Magnetic Force Microscopy , MFM)
- It is used to investigate the local magnetic strength in the sample and is z. B. used in the development of hard disk drives . The measurement takes place in the non-contact mode, whereby the stylus used is additionally coated with a ferromagnetic material. The measurement itself then takes place in two runs for each image line: In the first run, the height profile of the sample is determined using one of the measurement modes described above. Then, in the second run, this surface profile of the sample is traversed again in such a way that the measuring needle is at a constant distance from the surface (typically less than 100 nm). The information collected is no longer generated by a mechanical deflection of the measuring needle tip, but by the magnetic forces of attraction that have different strengths depending on the local field strength .
- Frictional force measurement ( English latera or friction force measurement , LFM or FFM)
- The measurement is carried out in constant force contact mode. While scanning the surface, the tilt signal of the cantilever is also recorded. Depending on the friction between the stylus and the surface, the cantilever rotates to different degrees. In this way, areas of different friction can be differentiated and statements about the material composition in the sample surface can be made.
- Chemical force microscopy ( English chemical force microscopy , CFM)
- Chemical force microscopy (Baden-Württemberg Innovation Prize 2003) enables topographical and specific chemical imaging of any surface with nanometer accuracy, using chemically uniformly modified probe tips and various liquid imaging media, so that only a very specific interaction with the surface occurs.
A specially prepared measuring probe (possible probe radius less than 3 nm) from atomic force microscopy is densely covered with a single chemical end group, such as -OH, -CH 3 , -CF 3 , -NH 2 or -COOH. Due to the now chemically uniform surface of the probe and the use of water, buffered solutions or solvents such as hexadecane as imaging medium , it is achieved that - in contrast to normal atomic force microscopy - only very specific interactions occur between the CFM probe and the surface to be imaged. A chemical selectivity of the CFM probe is achieved in this way. The strength of the specific interaction measured at a location on the surface allows conclusions to be drawn about the density of the specifically detected chemical end groups on the surface. When scanning the surface line by line, the chemical probe is brought into contact with the surface at each measuring point and then separated again (digital pulsed force mode). During this physical process, the strength of the interaction, the rigidity of the surface and other chemical or physical parameters are determined and assigned to each pixel.
- Electrochemical AFM ( English electrochemical scanning microscopy , EC-AFM)
- Electrochemical atomic force microscopy enables the imaging of the topography while at the same time checking the electrochemical potential of the sample. It is thus possible to simultaneously record topographical and electrochemical properties of electrode surfaces.
- Current-voltage microscopy ( English current sensing atomic force microscopy , CS-AFM)
- In contact mode, a voltage is applied between the sample and the tip and the resulting current is output in addition to the topographical information. A special measuring tip coated with a conductive material is required for this measuring technique. In general, different tips can be used for this, silicon nitride tips with a platinum coating are often used.
- Scanning Kelvin microscopy ( English kelvin force microscopy , KFM or surface potential imaging )
- A measuring tip ( Kelvin probe ) made of conductive material is also used in this mode . The measuring tip and the sample generally have a different work function. As a result, if there is a conductive contact between the sample surface and the measuring tip, a voltage occurs which can be used to measure the electrostatic characteristics of the surface. Thus the KFM technique allows a statement about the work function and the spatially resolved voltage curve compared to the topography.
Here the AFM is not used to take an image, but to examine the elasto-plastic properties of the sample at a predefined location.
To measure force-distance curves, the cantilever is lowered one or more times onto the sample, pressed on with a defined force and removed from the sample again. The force acting on the measuring needle is recorded as a function of the tip position. Conclusions can then be drawn from the resulting curves about various properties of the material and the surface, such as the adhesive forces and elasticity. In order to increase the measurement accuracy and avoid artifacts e.g. B. to be eliminated by noise, normally not a single curve, but a set of curves, a so-called force volume, is recorded. An average curve is then formed from these and evaluated. Figure 5 shows typical force-distance curves that can result from such a measurement. The blue curve represents the approach process, the red curve the withdrawal of the tip.
In the picture on the right (a) shows the ideal case of the measurement on a purely elastic sample. The horizontal section in the right half of the picture represents the zero line (force curves are usually always read from the zero line) before the tip comes into contact with the surface. When the tip approaches the sample, the tip finally leaps onto the surface, which is caused by short-range attractive forces. Then the force increases proportionally with the further approach (so-called "contact regime"). After the movement has been reversed at the maximum, the curve drops again in the same linear manner, but remains stuck to the surface until the spring force of the cantilever becomes greater than the adhesive force of the surface and the cantilever jumps back to its zero position.
In the picture on the right (b) shows a typical force curve on many sample types. While the zero line and the jump into contact do not deviate from Figure a, one recognizes in the contact regime that the line is no longer linear, but is initially flatter and then becomes steeper. On the one hand, this can be caused by hardening of the material during the indentation (elasto-plastic behavior) or, on the other hand, because the harder specimen base influences the measurement as the indentation increases. The work done on the sample can be calculated from the hysteresis between the approach and withdrawal curves .
Finally, (c) in the picture demonstrates the most common artifact in force-distance measurements. In contrast to images a and b, the retraction curve in the contact regime is above the approach curve, that is, the forces appear to be higher when the tip is withdrawn than when it is approaching. The artifact is mostly caused by nonlinearities of the piezo actuators in the force microscope.
Because of these and other artefacts that occur, a great deal of care and experience is required both when calibrating the device and when evaluating the force curves.
Single molecule force spectroscopy
A method similar to that used for force-distance curves can also be used to measure binding forces in individual molecules such as proteins . It is z. B. With the help of special molecules, the molecule to be measured is covalently bound to a sample carrier and to the measuring tip and then stretched by pulling back the measuring tip. Since the folding of proteins occurs through hydrogen bonds or even weaker bonds, the molecule is initially completely unfolded before one of the covalent bonds in the molecule or on the surface breaks. In the associated force-distance curve, the unfolding can be recognized by a sawtooth-like structure. An understanding of the measurement results cannot be achieved without at least basic molecular knowledge.
Disturbances during the measurement
The evaluation of the data obtained during the measurements requires a detailed analysis, since disturbances can occur during each measurement and the data are also superimposed by system-related errors. A fundamental problem with all images with a finite-sized measuring tip is that the measurement 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 system-related errors, various malfunctions can occur during the measurement:
- Vibrations: On the one hand, these are caused by building vibrations or impact sound . AFM measuring stations are therefore often set up on vibration-isolated tables, mostly consisting of thick marble slabs on damping compressed air feet, or on tables actively damped with piezo elements . In addition, during measurements under normal pressure, the acoustic sound, which is transmitted directly to the cantilever via the air, represents a strong source of interference. The closer the resonance frequency of the cantilever is to the frequency range of normal noises, the more this is. For this reason, it makes sense to operate AFMs in special soundproof boxes. If it is possible from the point of view of the sample to be examined, devices that work under vacuum conditions can also be used.
- Thermal drift: As a result of thermal expansion between the sample and the cantilever, displacements of a few nanometers can occur in the course of a measurement interval, which becomes visible as distortion in images with high resolution.
- Interference phenomena: With strongly reflective samples it can happen that part of the laser beam is reflected by the sample surface and interferes in the photodetector with the part that comes from the cantilever . This becomes noticeable in stripes running perpendicular to the scanning direction, which are superimposed on the actual height image.
- Static charges: Especially with MFM measurements of non-metallic samples, electrical charges that are collected by the tip can falsify the measurements or make them completely impossible. To avoid these charges, the sample and cantilever should have the same ground potential. For this purpose, non-metallic samples can be vapor-deposited with a fine gold layer. Where this is not possible, the air can also be ionized with a radioactive source, which effects a potential equalization of the undesired electrical charges. If the charges are constant over the measuring surface, they can also be compensated for using the control software or the control circuit of the measuring arrangement.
In professional AFMs, evaluation software is usually integrated in the control program of the hardware . The data formats are mostly manufacturer-dependent, since in addition to pure image data, the settings for the respective measurement such as B. the scan speed should be saved. In addition, the created measurement images can also be converted into known data formats such as BMP or JPEG files . For Macintosh computers there is the proprietary measurement software Image SXM , based on NIH Image , which, among other things, is able to process the raw data of many atomic force and scanning tunneling microscopes. The free analysis software Gwyddion is available for GNU / Linux, Microsoft Windows, Mac OS X and FreeBSD and can also import a variety of different raw data formats. In addition to extensive built-in functionalities, it also offers the option of being flexibly expanded using modules in various programming languages.
- R. Wiesendanger : Scanning Probe Microscopy and Spectroscopy - Methods and Applications . Cambridge University Press, Cambridge 1994, ISBN 0-521-42847-5 (English).
- B. Parkinson: Procedures in Scanning Probe Microscopies . John Wiley and Sons Ltd, 1997 (English).
- B. Cappella, G. Dietler: Force-distance curves by atomic force microscopy . In: Surface Science Reports . tape 34 , no. 1–3 , 1999, pp. 1-104 , doi : 10.1016 / S0167-5729 (99) 00003-5 .
- Franz Josef Giessibl : Advances in atomic force microscopy . In: Reviews of Modern Physics . tape 75 , no. 3 , 2003, p. 949-983 , doi : 10.1103 / RevModPhys.75.949 .
- Alex de Lozanne: Sensors for Proximal Probe Microscopy . Encyclopedia of Sensors (EOS), 2005, ( EOS-Online ).
- B. Voigtländer: Atomic Force Microscopy . Springer, 2019, ISBN 978-3-03013653-6 , doi : 10.1007 / 978-3-030-13654-3 .
- Nobel laureate and inventor of the atomic force microscope Gerd Binnig on functionality, development process and applications from August 18, 2010
- Overview article by the German Patent and Trademark Office
- Link catalog on the subject of manufacturers of atomic force microscopes (English) at curlie.org (formerly DMOZ )
- ImageSXM - The free measurement software for Apple Macintosh from Dr. Steve Barrett
- Gwyddion - The free evaluation software for Windows and Unix systems.
- Nanotec Electronica - free evaluation software WSXM for post-processing of force microscopy images
- FAFM First AFM on Mars, contains animations on how the AFM works
- Atomic force microscopy in the school laboratory : simple explanations of the structure and functionality of an AFM
- G. Binnig, CF Quate, Ch. Gerber: Atomic Force Microscope . In: Physical Review Letters . tape 56 , no. 9 , 1986, pp. 930-933 , doi : 10.1103 / PhysRevLett.56.930 .
- Bild der Wissenschaft 4/2011.
- Franz Josef Giessibl , p Hembacher, H. Bielefeldt, J. Mannhart: Subatomic features on the silicon (111) - (7 × 7) surface-observed by atomic force microscopy. In: Science. 289, 2000, pp. 422-425, doi: 10.1126 / science.289.5478.422 ( PDF (PDF)).
- M. Schneider, M. Zhu, G. Papastavrou, S. Akari, H. Möhwald: Chemical pulsed-force microscopy of single polyethylenemine molecules in aqueous solution. In: Langmuir. 18, 2002, pp. 602f.
- M. Nonnenmacher, MP O'Boyle and HK Wickramasinghe, Appl. Phys. Lett. 58 (1991) 2921.
- M. Nonnenmacher, MP O'Boyle and HK Wickramasinghe, Ultramicroscopy 42-44 (1992) 268.
- B. Cappella, P. Baschieri, C. Frediani, P. Miccoli, C. Ascoli: Force-distance curves by AFM. In: IEEE Engineering in Medicine and Biology. 16, No. 2, 1997, pp. 58-65.
- GU, DA Kidwell, RJ Colton: Sensing discrete streptavidin-biotin interactions with atomic force microscopy . In: Langmuir. 10, No. 2, 1994, pp. 354-357.
- VT Moy, E.-L. Florin, HE Gaub : Intermolecular forces and energies between ligands and receptors . In: Science. 266, No. 5183, 1994, pp. 257-259.
- RH Eibl , VT Moy: Atomic force microscopy measurements of protein-ligand interactions on living cells. In: G. Ulrich Nienhaus (Ed.): Protein-Ligand Interactions. , Humana Press, Totowa, NJ 2005, ISBN 1-58829-372-6 , pp. 437-448.
- KL Westra, AW Mitchell, DJ Thomson: Tip Artifacts in Atomic-Force Microscope Imaging of Thin-Film Surfaces. In: Journal of Applied Physics. 74, No. 5, 1993, pp. 3608-3610, doi: 10.1063 / 1.354498 .
- KL Westra, DJ Thomson: Atomic Force Microscope Tip Radius Needed for Accurate Imaging of Thin Film Surfaces. in: Journal of Vacuum Science and Technology B. 12, No. 6, 1994, pp. 3176-3181, doi: 10.1116 / 1.587495 .
- L. Emerson, G. Cox: Charging artefacts in atomic force microscopy. In: Micron. 25, No. 3, 1994, pp. 267-269, doi: 10.1016 / 0968-4328 (94) 90032-9 .
- Gwyddion Features. Retrieved October 29, 2018 .
- I. Horcas, R. Fernández, JM Gómez-Rodríguez, J. Colchero, J. Gómez-Herrero, AM Baro: WSXM: A software for scanning probe microscopy and a tool for nanotechnology . In: Review of Scientific Instruments . tape 78 , 2007, p. 013705 .