Tutorials - Force spectroscopy

Introduction

Information about the sample is also available from measuring the changes while the separation from the surface is varied at a single point, rather than by scanning the lateral position of the tip. In this mode the base of the cantilever is moved in the vertical direction towards the surface using the piezo and then retracted again. During the motion, the deflection of the cantilever and other signals, such as the amplitude or phase in dynamic AFM modes, can be measured. This is usually called force spectroscopy.

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The basic force spectroscopy curves can be understood by thinking about the example of a cantilever in air approaching a hard, incompressible surface such as glass or mica. As the cantilever approaches the surface, initially the forces are too small to give a measurable deflection of the cantilever, and the cantilever remains in its undisturbed position. At some point, the attractive forces (usually Van der Waals, and capillary forces) overcome the cantilever spring constant and the tip jumps into contact with the surface.

Once the tip is in contact with the sample, it remains on the surface as the separation between the base and the sample decreases further, causing a deflection of the tip and an increase in the repulsive contact force.

As the cantilever is retracted from the surface, often the tip remains in contact with the surface due to some adhesion and the cantilever is deflected downwards. At some point the force from the cantilever will be enough to overcome the adhesion, and the tip will break free.

Many interesting samples are not hard and incompressible, and a more general pair of approach and retract curves will include sample compression, hysteresis and more complex adhesion between the tip and surface. In liquid, there may not be an obvious snap to contact in the approach curves even over a hard surface such as mica.

Over a soft, compressible sample in liquid, the force curve often shows a gradual increase in force, without the sharp onset of the interactions seen in air. It is often difficult to define a single point where the tip and sample come into "contact", since the initial compression of the surface causes very little deflection of the cantilever.

The gradient of the repulsive contact region changes as the sample is indented and the apparent stiffness may change as the structure is compressed. For thin samples on a hard surface, the linear repulsive contact regime may be seen at large deflections, as the tip may indent the sample enough to feel the supporting surface below. The contact area will change as the tip indents a soft surface, so the actual interactions involved in compression can be hard to quantify, and different points within the region will experience different levels of compression.

When the tip is retracted from the surface, there is often a hysteresis seen, if the sample is not perfectly elastic, and many different adhesion responses can be observed. In some cases, the cantilever pulls the tip free in stages, such as when there are long molecules in the sample or on the tip.

Extendable contacts are made between the tip and sample, so that as the base of the cantilever retracts, the tip is deflected down towards the sample until the force is strong enough to break the contacts. Different molecules or parts of the sample may adhere and each part may be broken separately, or together. These situations produce a variety of adhesion events, and successive force curves can show very different responses.