Images and force measurements taken by an atomic‐force microscope (AFM) depend greatly on the properties of the spring and tip used to probe the sample’s surface. In this article, we describe a simple, nondestructive procedure for measuring the force constant, resonant frequency, and quality factor of an AFM cantilever spring and the effective radius of curvature of an AFM tip. Our procedure uses the AFM itself and does not require additional equipment.

Calibration of atomic‐force microscope tips

http://experimentationlab.berkeley.edu/sites/default/files/AFMImages/butt_AFM.pdf

Force measurements with the atomic force microscope: Technique, interpretation and applications

Solid lipid nanoparticles: Production, characterization and applications

SINCE the invention of the scanning tunnelling microscope1, the value of establishing a physical connection between the macroscopic world and individual nanometre-scale objects has become increasingly evident, both for probing these objects2–4 and for direct manipulation5–7 and fabrication8–10 at the nanometre scale. While good progress has been made in controlling the position of the macroscopic probe of such devices to sub-angstrom accuracy, and in designing sensitive detection schemes, less has been done to improve the probe tip itself4. Ideally the tip should be as precisely defined as the object under investigation, and should maintain its integrity after repeated use not only in high vacuum but also in air and water. The best tips currently used for scanning probe microscopy do sometimes achieve sub-nanometre resolution, but they seldom survive a 'tip crash' with the surface, and it is rarely clear what the atomic configuration of the tip is during imaging. Here we show that carbon nanotubes11,12 might constitute well defined tips for scanning probe microscopy. We have attached individual nanotubes several micrometres in length to the silicon cantilevers of conventional atomic force microscopes. Because of their flexibility, the tips are resistant to damage from tip crashes, while their slenderness permits imaging of sharp recesses in surface topography. We have also been able to exploit the electrical conductivity of nanotubes by using them for scanning tunnelling microscopy.

Nanotubes as nanoprobes in scanning probe microscopy

Single-molecule force spectroscopy has emerged as a powerful tool to investigate the forces and motions associated with biological molecules and enzymatic activity. The most common force spectroscopy techniques are optical tweezers, magnetic tweezers and atomic force microscopy. Here we describe these techniques and illustrate them with examples highlighting current capabilities and limitations.

Single-molecule force spectroscopy: optical tweezers, magnetic tweezers and atomic force microscopy

The atomic force microscope (AFM) can be used to image the surface of both conductors and nonconductors even if they are covered with water or aqueous solutions. An AFM was used that combines microfabricated cantilevers with a previously described optical lever system to monitor deflection. Images of mica demonstrate that atomic resolution is possible on rigid materials, thus opening the possibility of atomic-scale corrosion experiments on nonconductors. Images of polyalanine, an amino acid polymer, show the potential of the AFM for revealing the structure of molecules important in biology and medicine. Finally, a series of ten images of the polymerization of fibrin, the basic component of blood clots, illustrate the potential of the AFM for revealing subtle details of biological processes as they occur in real time.

https://science.sciencemag.org/content/sci/243/4898/1586.full.pdf

Imaging crystals, polymers, and processes in water with the atomic force microscope.

A new atomic force microscope, which combines a microfabricated cantilever with an optical lever detection system, now makes it possible to measure the absolute force applied by a tip on a surface. This absolute force has been measured as a function of distance (=position of the surface) in air and water over a range of 600 nm. In the absolute force versus distance curves there are two transitions from touching the surface to a total release in air caused by van der Waals interaction and surface tension. One transition is due to lifting off the surface; the other is due to lifting out of an adsorbed layer on the surface. In water there is just one transition due to lifting off the surface. There is also a transition in air and water when the totally released tip is pulled down to touch the surface as the surface and tip are brought together. Based on the force versus distance curves, we propose a procedure to set the lowest possible imaging force. It can now be as low as 10−9 N or less in water and 10−7 N...

Forces in atomic force microscopy in air and water

Understanding the forces such as adhesion, attraction, and repulsion between surfaces and liquids is the key not only to understanding phenomena such as lubrication and indentation but also the key to understanding how best to operate an atomic-force microscope (AFM). In this paper, we examined the cases of an insulating tip on an insulating sample (silicon nitride tip on mica) and of a conducting tip on a conducting sample (tungsten carbide tip on a gold or platinum foil). The force-versus-distance curves for these two limiting systems were very different in different liquids. In ethanol, the curve is just what one would expect theoretically: a slightly attractive force before contact, a jump into contact, then a small pull-out force, about 0.2 nN for an insulating tip on the insulating surface and about 0.5 nN for two conducting surfaces. In pure water, the behavior is complex and variable. Pull-out forces vary from 0.2 to 1.5 nN for two insulating surfaces. For two conducting surfaces the force-versus-distance curves show large pull-out forces of order of 10 nN. These large forces are probably due to adsorption contamination layers on the metal surfaces that are not removed by the solvent action of the pure water. These forces, however, can be reduced to less than one-hundredth of the original value by adding ethanol to the water. This makes ethanol a useful liquid for routine imaging of macromolecules such as DNA, proteins, and polymers, that have been adsorbed to a substrate and that must be imaged at low force. In formamide, we observed a predicted repulsive interaction in the nontouching regime for insulating surfaces as predicted by Hartmann. In different concentrations of KCl aqueous solution, we observed again a repulsive interaction in the nontouching regime due to double-layer repulsion of charged surfaces across ionic solutions. The measured Debye length agrees well with the theoretical prediction. And last, the dependence of the pull-out force on the indentation in water has been investigated. The more the tip indents the sample surface in a force-versus-distance cycle, the larger the pull-out force will be. This shows also the usefulness of the AFM for investigations of micromechanical properties.

https://link.aps.org/pdf/10.1103/PhysRevB.45.11226

Measuring adhesion, attraction, and repulsion between surfaces in liquids with an atomic-force microscope.

SUMMARY

The scan speed limit of atomic force microscopes has been calculated. It is determined by the spring constant of the cantilever k, its effective mass m, the damping constant D of the cantilever in the surrounding medium and the stiffness of the sample. Techniques to measure k, k/m and D/m are described. In liquids the damping constant and the effective mass of the cantilever increase. A consequence of this is that the transfer function always depends on the scan speed when imaging in liquids. The practical scan speed limit for atomic resolution in vacuum is 0·1 μm/s while in water it increases to about 2 μm/s due to the additional damping of cantilever movements. Sample stiffness or damping of cantilever movements by the sample increase these limits. For soft biological materials imaged in water at a desired resolution of 1 nm the scan speed should not exceed 2 μm/s.

Scan speed limit in atomic force microscopy

The atomic force microscope (AFM) can now bridge the gap from imaging objects that can be seen with an optical microscope to imaging atoms: a range in magnification of 104. High magnification images of germanium show single atoms separated by 0.4 nm while low magnification images of entire cells and portions of an integrated circuit chip provide lateral and vertical information over a range of 25 μm.

A. L. Weisenhorn

Papers

Imaging crystals, polymers, and processes in water with the atomic force microscope.

Forces in atomic force microscopy in air and water

Measuring adhesion, attraction, and repulsion between surfaces in liquids with an atomic-force microscope.

Scan speed limit in atomic force microscopy

From atoms to integrated circuit chips, blood cells, and bacteria with the atomic force microscope