Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 182

AFMs operate by measuring the tiny forces (less than ~1 nN) between atoms of a probe in the shape of a sharp tip and those of the specimen surface (Figure 5.22). These tiny forces cause a very small flexible cantilever to bend, which is used to sense the proximity of the tip to the surface and, thus, the topographical features of the surface. A variety of tip materials and shapes are available but commonly used tips include silicon nitride and single‐crystal silicon in the shape of a pyramid several micrometers high and an end radius of ~20 nm or larger. To acquire a three‐dimensional image, the tip mounted on a flexible cantilever is scanned with high precision over the surface and the relative lateral and vertical deflection of the tip due to its interaction with the surface is measured using an optical lever system. In some instruments, the tip is stationary and the specimen is moveable. The resolution of the topographical features depends on the sharpness of the tip and sensitivity of the cantilever system but modern AFMs can have a lateral resolution of less than 20–30 nm and a vertical resolution down to less than 0.1 nm. This makes AFM ideally suited for characterizing nanoscale topography, although it is also used for characterizing microscale features.

Figure 5.22 Schematic illustrating (a) the main components of the AFM technique, (b) imaging in the contact mode, and (c) imaging in the noncontact mode.

AFM imaging of a surface can be performed in three different scanning modes referred to as contact, noncontact, and tapping modes. In contact mode imaging, a tiny constant force is applied to the cantilever and the tip is brought into contact with the surface. Repulsive forces between the atoms of the tip and the surface (Figure 5.23), similar to those between atoms considered in Chapter 2, produce a deflection of the cantilever. This deflection is used in a feedback circuit to move the scanner up or down in the vertical direction in response to the topography by keeping the cantilever deflection constant. Contact mode imaging can damage the surface of soft samples and, consequently, it is commonly used for metals, ceramics, and hard polymers. In noncontact mode imaging, the cantilever is vibrated at its resonant frequency and at a constant amplitude using a piezoelectric device, and the tip is brought to within a few nanometers of the surface but not in contact with the surface. Weak van der Waals forces of attraction produce a deflection of the cantilever that is used to form an image. As the tip does not come into contact with the specimen surface, noncontact AFM is very suitable for soft polymers. Tapping mode imaging can be considered to be somewhat between contact and noncontact mode imaging. The cantilever is vibrated at its resonant frequency but with an amplitude lower than that in contact mode, and the tip slightly taps the surface of the sample. Tapping mode imaging gives a higher resolution than noncontact mode and can be used for a wide range of polymers.

Figure 5.23 Schematic curve of force versus separation between the tip and specimen surface, showing the repulsive and attractive regions corresponding to contact mode and noncontact mode imaging.