Force Curves


1.Force Curve Measurements

In addition to these topographic measurements, the AFM can also provide much more information. The AFM can also record the amount of force felt by the cantilever as the probe tip is brought close to - and even indented into - a sample surface and then pulled away. This technique can be used to measure the long range attractive or repulsive forces between the probe tip and the sample surface, elucidating local chemical and mechanical properties like adhesion and elasticity, and even thickness of adsorbed molecular layers or bond rupture lengths.

Force curves( force-versus-distance curve) typically show the deflection of the free end of the AFM cantilever as the fixed end of the cantilever is brought vertically towards and then away from the sample surface. Experimentally, this is done by applying a triangle-wave voltage pattern to the electrodes for the z-axis scanner. This causes the scanner to expand and then contract in the vertical direction, generating relative motion between the cantilever and sample. The deflection of the free end of the cantilever is measured and plotted at many points as the z-axis scanner extends the cantilever towards the surface and then retracts it again. By controlling the amplitude and frequency of the triangle-wave voltage pattern, the researcher can vary the distance and speed that the AFM cantilever tip travels during the force measurement.

Similar measurements can be made with oscillating probe systems like TappingMode and non-contact AFM. This sort of work is just beginning for oscillating probe systems, but measurements of cantilever amplitude and/or phase versus separation can provide more information about the details of magnetic and electric fields over surfaces and also provide information about viscoelastic properties of sample surfaces.

2.Anatomy of a Force Curve

A:The cantilever starts not touching the surface. In this region, if the cantilever feels a long-range attractive (or repulsive) force it will deflect downwards (or upwards) before making contact with the surface.

B: As the probe tip is brought very close to the surface, it may jump into contact if it feels sufficient attractive force from the sample.

C: Once the tip is in contact with the surface, cantilever deflection will increase as the fixed end of the cantilever is brought closer to the sample. If the cantilever is sufficiently stiff, the probe tip may indent into the surface at this point. In this case, the slope or shape of the contact part of the force curve can provide information about the elasticity of the sample surface.

D:After loading the cantilever to a desired force value, the process is reversed. As the cantilever is withdrawn, adhesion or bonds formed during contact with the surface may cause the cantilever to adhere to the sample some distance past the initial contact point on the approach curve (B).

E:A key measurement of the AFM force curve is the point at which the adhesion is broken and the cantilever comes free from the surface. This can be used to measure the rupture force required to break the bond or adhesion.

One of the first uses of force measurements was to improve the quality of AFM images by monitoring and minimizing the attractive forces between the tip and sample. Force measurements were also used to demonstrate similarly reduced capillary forces for samples in vacuum and in reduced humidity environments.

3.Measuring Fundamental Forces

Concern with the fundamental interactions between surfaces extends across physics, chemistry, materials science and a variety of other disciplines. With a force sensitivity on the order of a few piconewtons (pN = 10 to -12 N), AFMs are excellent tools for probing these fundamental force interactions. Force measurements in water revealed the benefits of AFM imaging in this environment due to the lower tip-sample forces. Some of the most interesting force measurements have also been performed with samples under liquids where the environment can be quickly changed to adjust the concentration of various chemical components. In liquids, electrostatic forces between dissolved ions and other charged groups play an important role in determining the forces sensed by an AFM cantilever. The liquid environment has become an important stage for fundamental force measurement because researchers can control many of the details of the probe surface force interaction by adjusting properties of the liquid. Experimentally, the electrostatic tip-sample forces depend strongly on pH and salt concentration. In fact, it is often possible to adjust the pH or salt concentration such that the attractive Van der Waals forces are effectively negated by repulsive electrostatic forces. This has been an important discovery because it can allow tuning of the liquid environment to minimize adhesive tip-sample forces that can damage the sample during imaging.

Atomic Force Microscopy has made its mark on a wide variety of applications as a topographic measurement and mapping tool. Now AFM force measurements are providing information on atomic- and molecular-scale interactions as well as nano-scale adhesive and elastic response. These measurements are beginning to revolutionize the way we quantitatively observe and, indeed, think about our chemical, biological and physical world. The vast majority of AFM force measurements have been performed with NanoScope systems and we are actively adding functionality to make these measurements increasingly accurate, repeatable and easier to perform.


Author: Hong-Qiang Li: email: hli@uoguelph.ca
Curator: Dan Thomas email: <thomas@chembio.uoguelph.ca>
Last Updated: Thursday, April 24 1997