Soft Intermittent Contact

Soft Intermittent Contact : 3rd AFM mode (Soft IC)

The advantages of contact and resonant AFM modes without the drawbacks
Mechanical properties : Adhesion, Stiffness, Young’s Modulus
 Constant force = quantitative measurement
Compatible with : ResiScope, SThM, PFM, C-AFM…

PDMS – Soft IC mode – 20µm
Topography (left) & Stiffness (right)

The advantages of contact and resonant AFM modes without the drawbacks

Concept Scientific Instruments has developed an alternative measurement mode called Soft Intermittent Contact mode (or Soft IC) that combines the advantages of contact mode and force spectroscopy but presentencing from their inconvenient like friction forces or intrinsic slowness. Soft IC mode works as follows (Fig. 2) : The tip is separated some safe distance (referred as «lift height») from the surface and moved fastly to the next measurement point (determined  by the number of pixels of the image), when it reaches the position the tip is stopped and it performs a force spectroscopy curve as described in the previous section. The maximum force applied is set by the user by a setpoint value in a similar manner as in contact mode. Another advantage of Soft IC respect to contact mode is to avoid the instabilities due to changes in the adhesion force during the scan as it happens frequently in contact mode. By setting a lift height higher than the adhesion force, tip can be totally disengaged from the surface. This also has the advantage that even softer cantilevers (typically they experience  very high adhesion forces) can be used routinely with this mode. In addition, stiffness and adhesion can be obtained directly from every measured point. The  stiffness (i.e., the ratio of the applied force and the deformation of the sample) can be used in combination with Soft Mechanic software module to calculate the  Young’s modulus.

Principle

Soft IC mode works as follows  : The tip is separated some safe distance (referred as « lift height ») from the surface and moved fastly  to the next measurement point (determined by the number of pixels of the image), when it reaches the position the tip is stopped and it performs a force spectroscopy curve as described in the previous section. The maximum force applied is set by the user by a setpoint value in a similar manner as in contact mode. Another advantage of Soft IC respect to contact mode is to avoid the unstabilites due to changes in the adhesion force during the scan as it happens frequently in contact mode. By setting a lift height higher than the adhesion force, tip can be totally disengaged from the surface. This also has the advantage that even softer cantilevers (tipically they experience very high adhesion forces)  can be used routinely with this mode. In addition, stiffnes and adhesion can be obtained directly from every measured point. The stiffness (i.e., the ratio of the applied force and the deformation of the sample) can be used in combination with Soft Meca software module to calculate the Young modulus.

An example of the comparison of the advantages of Soft IC is illustrated in Figure on left. A PS/PMMA blend sample whas first imaged in contact mode with a stiff cantilever (k = 37 N/m, ACT from AppNano. The surface shows two clear separated phases of PS islands (yellow domains) embebed in PMMA matrix (brown domains). Although the applied force (45 nN) is not high enough to produce a permanent deformation, both adhesion and friction forces produced during the scan drag parts of PMMA on both the PS islands and on the sides of the scanned area (white spots along blue-dashed square on Figure 2). Same force was applied using Soft IC mode (red square) while increasing the lift height upto 300 nm to completely release the tip from the surface. The image in Soft IC mode does not show permanent damage on the sample. The bottom part of the image corresponds with measurement in resonant mode (green-dashed square) showing no permanent damage either. However, in resonant mode is not straightforward to provide the value of the applied force. For the experimental conditions used, we estimate the applied average force on 38 nN.

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