Atomic Force Microscopy

The principle used in the imaging using an atomic force microscope involves the use of a micro machined cantilever with a sharp tip. This tip is subjected to attractive or repulsive forces/interactions depending on the distance between the atoms and the tip. These forces/interactions are recorded by measuring the degree of deflection of the cantilever. This detection is facilitated by a laser beam that is reflected off the back of the cantilever onto a photo sensitive photo detector. During the scanning of a sample, a tube-shaped scanner located under the sample moves the sample in the horizontal direction (X-Y) and in the vertical direction (Z). This motion causes the sample to be scanned line by line, while the photo sensitive photo detector signal is used to control the vertical movement of the scanner via a feedback loop as the cantilever moves across the sample. The AFM is capable of obtaining measurements from conductors, non-conductors as well as some liquids without delicate sample preparation. The Park XE100 can operate in the following modes; contact mode, non-contact mode, lateral force mode, magnetic force microscopy and scanning tunnelling microscopy.

There are 3 modes of operation possible with the instrument, those being contact mode, non-contact mode and tapping mode. The contact mode causes the tip to be “dragged” across the surface of the sample and the contours of the surface are measured using the feedback signal required to keep the cantilever at a constant position. In non-contact mode, the tip of the cantilever does not contact the sample surface as the name suggests. Instead, it oscillated at either its resonant frequency (frequency modulation) or just above (amplitude modulation) where the amplitude of oscillation is typically a few nanometers (<10 nm) down to a few picometers. The van der Waals forces, which are strongest from 1 nm to 10 nm above the surface, or any other long-range force that extends above the surface acts to decrease the resonance frequency of the cantilever. This decrease in resonant frequency combined with the feedback loop system maintains a constant oscillation amplitude or frequency by adjusting the average tip-to-sample distance. Measuring the tip-to-sample distance at each (x,y) data point allows the scanning software to construct a topographic image of the sample surface. The tapping mode was developed to bypass the problem of keeping the probe tip close enough to the sample for short-range forces to become detectable while preventing the tip from sticking to the surface in ambient conditions. The images in tapping mode are produced by imaging the force of the intermittent contacts of the tip with the sample surface.