2.2.1 Cantilever-sample interaction potential. AFM operation modes

Introduction.

Sample investigation is available thanks to the forces acting between a cantilever and a surface. They are quite different. One or another force dominate at different tip-sample separations.

• During contact and the surface deformation by the cantilever, the elastic repulsion force dominates; this approximation is called the Hertz model and is considered in the chapter "Elastic interactions. The Hertz problem".
• At tip-sample separations of the order of several tens of angstrom the major interaction is the intermolecular interaction called the Van der Waals force (see chapter "The Van der Waals force").
• At the same distance between the tip and the sample and in the presence of liquid films, the interaction is influenced much by capillary and adhesion forces. The range of capillary forces considered in the chapter "Capillary forces" is determined by the liquid film thickness.
• At larger separations the electrostatic interaction starts to dominate.
• At separations of the order of a thousand of angstroms magnetic forces considered in the chapter "Magnetic force microscopy" prevail.

Probe-surface interaction potential.

"Joining" potentials of forces acting at various tip-sample separations one can construct the curve shown in Fig. 1 that allows to classify operation modes of an atomic force microscope.

Fig. 1. Probe-sample interaction potential.

Atomic force microscope operation modes.

Depending on tip-sample separation during scanning, three modes of an atomic force microscope are available (see Fig. 1): 1) contact, 2) non-contact, 3) "semicontact" which is intermediate between contact and non-contact.

In the contact mode the probe tip directly touches the sample surface during scanning. In the non-contact mode the probe is far enough and do not touch the surface. In the semicontact mode the intermittent contact occurs. The last two AFM modes are needed to implement modulation (or vibration) techniques.

Each mode is used to solve particular problems, some investigations being conducted using various techniques in different modes. This gives the researcher a wide scope of opportunities and allows to operate a microscope in a most appropriate and effective mode under given experimental conditions.

For example, three AFM modes of the surface relief measurements exist:

• contact atomic force microscopy – surface topography imaging in the contact mode.
• non-contact atomic force microscopy – surface topography imaging in the non-contact mode based on vibrating technique.
• "semicontact" atomic force microscopy (or intermittent-contact atomic force microscopy) – in this case the vibrating technique is used; the oscillating tip barely taps the sample surface.

Experimental techniques based on various AFM modes will be reviewed in respective chapters. The instrument operation in the contact AFM and "semicontact" AFM is the basis for another AFM techniques. Correct combining of measurements in three modes permits to acquire new interesting results.

Summary.

• AFM-investigation of surfaces is based on detection of forces acting between the tip and the sample. Depending on the distance between the tip and the sample different forces prevail.
• Three basic modes of topography imaging exist depending on the tip-sample separation.