Atomic Force Microscopy: Contact mode (Static mode)

The AFM operates by measuring the intermolecular forces between the tip and sample. The most common method used in imaging AFM is contact mode, where the piezoelectric element slightly touches the tip to the sample. The experimental setup is shown in figure 4. As a result of the close contact, the tip and sample remain in the repulsive regime of the tip-sample interaction shown in figure 5. Thus, the AFM measures repulsive force between the tip and sample. As the raster-scan moves the tip along the sample, the two-panel photodiode measures the vertical deflection of the cantilever, which reveals the local sample height. Each contour of the surface results in a movement of the tip in the xyz-direction, resulting in a change in the deflection angle of the laser beam. This change is measured through the photodiode and translated finally to an image.

1. Tip-sample interaction

The cantilever in the AFM is a critical component [1]. The force produced by a spring always tends to restore the spring to its equilibrium position. When the spring is pushed upward by a distance z, it has to be pulled downward. This restoring force is given by Hooke's Law as:

F(z) = - k * (z - z o)

Where k is a spring constant and depends on the material and dimensions of the cantilever, z is the vertical position of the cantilever, and z o is the equilibrium position. As a cantilever with a spring constant of 0.1 newton/meter (N/m) is moved by 1 nm, the resulting force is 0.1 nanonewton (nN). The first tip used by the inventors of the AFM was made from diamond glued to a lever of gold foil [1] . Micro-fabricated cantilever tips are now commercially used.

Electromagnetic forces determine the properties of solids, liquids, and gases, the behavior of particles in solution, and the organization of biological structures [25]. These forces are also the source of all intermolecular interactions including covalent bonds, Coulomb forces, ionic forces, ion-dipole interaction, and dipole-dipole interaction. In the AFM the intermolecular interactions between the tip and the sample surface include van der Waals forces, electrostatic forces, water capillary force, and material properties including elasticity. The most common force in the tip-sample interaction is the van der Waals force. The force is calculated using the Lennard-Jones potential, which combines the attractive van der Waals and the repulsive atomic potentials [25]. The force depends upon the distance between the tip and the sample as shown in figure 5. This calculated force is an estimate of the van der Waals forces and is usually a few nanonewtons in magnitude.

2. Force-distance curve

Since the invention of the AFM, many researchers have utilized it to measure the tip-sample interaction force on the atomic scale. The AFM records the force as the tip is brought in close proximity to the sample surface, even indented into the surface, and then pulled off. The measured force curve is a plot of cantilever deflection versus the extension of the z-piezoelectric scanner (z-piezo). A force-distance curve is a kind of interpretation of the force measurements. It needs a simple relationship between the cantilever deflection and the tip-sample distance. Thus, the force-distance curve describes the tip-sample interaction force as a function of the tip-sample distance rather than as a function of the z-piezo position. It is difficult to measure the quantitative forces with this technique because the spring constant of the cantilever and the shape of the tip are not accurately known. However, this technique has been used to study adhesion, elasticity, bond rupture length, and even thickness of adsorbed layers. These studies of the fundamental interactions between the sample surfaces have extended across basic science, chemistry, biology, and even material science. The interaction force between tip and sample is typically in the order of tens of pN for biomolecular interactions. Force measurements in solution have the advantages of the AFM due to the lower tip-sample interaction.

3. Constant force and Constant height

In contact mode, the tip is scanned across the surface in contact either at a constant force or at a constant height above the sample. Constant force mode is achieved by use of a z-feedback loop from the deflection signal. The feedback circuits serve to maintain a constant force between the tip and the sample while the tip follows the contours of the surface. The piezoelectric tube can respond to any changes in the cantilever deflection. A computer program acts to keep the cantilever deflection at a constant level. Then the tip-sample interaction can be kept at a predetermined restoring force. This technique is used to observe the precise topography of the sample surface. If the z-feedback loop is switched off, then the z-direction of the piezoelectric tube is kept constant and an image is generated based on the cantilever deflection. Using constant height can be useful for imaging very flat samples.

 4. Lateral force microscopy

Lateral force microscopy (LFM) is an extension of contact mode, where an additional detected parameter is the torsion of the cantilever, which changes according to the friction force [26]. This lateral force induces a torsion of the cantilever which, in turn, causes the reflected laser beam to undergo a change in a perpendicular direction to that resulting from the surface corrugation. The LFM uses a photodiode with four-segments to measure the torsion of the cantilever. When the cantilever is scanned across the surface in contact, differences in friction between tip and sample cause the tip to stick-slip on the surface. This stick-slip behavior creates a characteristic saw-tooth waveform of atomic level in the friction image [27]. The LFM can provide material-sensitive contrast since different components of a composite material exert different friction forces. Researchers often call this operation mode friction force microscopy [27, 28]. Increasing wear with decreasing sliding velocity on the nanometer scale has been observed with this technique. It has been demonstrated with LFM that on the atomic scale, frictional properties are sensitive to changes in surface properties on chemical modification. The LFM can also be applied to chemical force microscopy (CFM) by a modified tip with chemical functionality [29]. It has been demonstrated with CFM that mapping the spatial arrangement of chemical functional groups and their interactions is of significant importance to problems ranging from lubrication and adhesion to recognition in biological systems.

5. Capillary force

The thin surface water layer that exists on the sample surface will form a small capillary bridge between the tip and the sample. The capillary force is important when the AFM is operated in air. Let us examine the effect of surface tension on AFM measurements. At the moment of tip contact with a liquid film on a flat surface, the film surface reshapes producing a ring around the tip. The water layer wets the tip surface because the water-tip contact (if it is hydrophilic) is energetically advantageous as compared to the water-air contact. If the tip radius is 10 nm and the contact angle is small i.e. hydrophilic, we can get a capillary force of about 10 nN. Thus, the capillary force is the same order of magnitude as the van der Waals interaction. An AFM tip has been used to write alkanethiols with a 30 nm line width resolution on a gold thin film in a manner analogous to that of a dip pen [30]. Recently, this dip-pen nanolithography has also been applied to direct nanoscale patterning of biological materials such as DNA, peptides and proteins on glass substrates.

5-VIbration Mode (Dynamic Mode)