Diverse Applications of Surface and Colloid Chemistry in Science and Industry - Surface and Colloid Chemistry: Principles and Applications - page 216

some interaction between the probe and the surface. The tip is controlled to within
0.1 Å (1 nm).
In SPM, the various interactions between the tip and the substrate are as follows:

STM: The tunneling current between a metallic tip and a conducting substrate
that are in very close proximity but not actually in physical contact. This is
controlled by piezomotors in a stepwise method.
AFM (Atomic force microscopy): The tip is brought closer to the substrate
while the van der Waals force is monitored. At a given force, the piezomotor
controls this setting while the surface is scanned in the x-y direction.
FFM (Freeze Fracture Microscopy): This is a modification of AFM, where
force is measured (Birdi, 2002a).

A schematic description of SPM with the tip (dimension: 0.2 mm) and sample is
shown in Figure 10.2.
The most significant difference between SPM and x-ray diffraction studies has
been that the former can be carried out both in the air and water (or any other fluid).
Corrosion and similar systems have been investigated using STM. The tip is
covered by a plastic material, and this allows one to operate STM under a fluid
environment.
STM has been used to study the molecules adsorbed on solid surfaces. The Langmuir-
Blodgett (LB) films have been extensively investigated by both STM and AFM.
AFM has been used to study surface molecules under different conditions.
Colloidal system studies by AFM: AFM has allowed scientists to be able to study
molecular forces between molecules at very small (almost molecular size) distances.
Further, it is a very attractive and sensitive tool for such measurements. In a recent
study, the colloidal force as a function of pH of SiO 2 immersed in the aqueous phase
was reported using AFM. The force between an SiO 2 sphere (ca. 5 mm diameter) and
a chromium oxide surface in the aqueous phase of sodium phosphate were measured
(pH from 3 to 11). The SiO 2 sphere was attached to the AFM sensor as shown in
Figure 10.3.

Laser

Diode

FB

Sample

Scanner X and Y

FIGure 10.2 A schematic drawing of the sensor (tip/cantilever/optical/magnetic device)
movement over a substrate in x/y/z direction with nanometer sensitivity controlled by piezo-
motor at the solid-gas or solid-liquid interface.