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Lennard-Jones potential curve to show the interaction areas where the three AFM imaging modes apply.
The use of an AFM can be done according to different modes of operation that are chosen according to the desired analysis or the nature of the sample. In order to understand the origin of these modes of operation, one must be interested in the force of interaction between two atoms (deriving from the Lenard-Jones potential presented in the previous paragraph). Figure 1 shows the evolution of this force and in particular the existence of an attractive zone and a repellent zone, depending on the distance between the tip and the sample. From this behavior we can extract three classes of operation modes:
Sample tip contact (repulsive force): the contact or static mode.
Small distance between sample tip (maximum attractive force): intermittent contact mode or tapping mode
· Slightly greater distance (attractive force): non-contact mode. The working area is in the field of attractive forces without ever having contact between the tip and the surface of the sample.
In this section, we will detail these three imaging methods by indicating their characteristics in the following sections.
Contact mode
Contact mode
Potential energy between the AFM tip and the sample surface as a function of the distance between them
Contact mode is the easiest way to use the AFM. It makes it possible to obtain the topography (a map of the height of the sample according to the position) with a resolution of some nanometer. The AFM tip is brought into contact with the surface of the sample that is placed on the piezoelectric column. The user chooses a value of the vertical deflection, thus of the force with which the tip pressed on the sample (fig.a). The point is then swept zig-zig on the surface. When the topography changes, the flexible lever on which the point is fixed is deformed and the position of the laser on the photodetector changes (Fig.b) The AFM electronics then adjusts the column extension in real time to bring the laser back to vertical deflection choose by the user (fig.c). The electronics try to keep a constant force between the tip and the sample. An image of the topography can then be formed by recording the extension of the column at each point in the image.
Tapping mode
Tapping mode
Diagram of the microscope operating in tapping mode.
Another way to get this information is to use the Tapping mode, also known as the intermittent contact mode in the literature in this mode, a small piezoelectric element under the tip is used to oscillate the lever at a frequency close to its resonant frequency.
When the tip is approached from the sample surface, the interactions between the tip and the sample change the amplitude of the oscillation. if the tip is close enough, the tip taps the sample at each cycle, causing the amplitude to decrease. In this mode, the user chooses an oscillation amplitude that the control electronics will try to maintain by changing the column extension. As with the contact mode, the topography is then reconstructed from the column extension.
No-contact mode
No-contact mode
non-contact operation
In the attractive regime on the potential curve of the point-to-surface interaction, we are in the non-contact mode (NC-AFM). The tip is oscillated by a piezoelectric modulator at a frequency close to the resonance and at low free amplitudes (less than 5nm). However, the point-to-sample distance remains in the order of the nanometer, so that the detection mode is based on the modification of the oscillation frequency. The tip does not touch the surface. Developed mainly for an ultravacuum application where the AM-AFM mode does not work.
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