AFM

Atomic Force Microscopy (AFM)

Technique:

Atomic Force Microscopy (AFM), which can be operated in air or water, uses a fine tip (apex radius ~10 nm) to map surface morphology and mechanical and chemical properties through an interaction between the tip and surface. Almost all materials can be measured without specific sample preparations. The AFM at Surface Science Western is used extensively to study polymer surface structures and properties. Topographic images at a resolution on the nanometer scale, and surface energy changes due to surface modification by UV/ozone treatment, can be measured using AFM.

In contact mode AFM, the tip is mechanically contacted with the sample surface at an applied force. Soft cantilevers (e.g., with a spring constant ~0.1 N/m) are used in contact AFM. Because the tip mechanically contacts the surface in contact mode AFM, many surface properties such as friction force distribution can be measured from the interaction between the tip and surface when the tip scans across the surface. Lateral force imaging is based on measuring the torsional movement of the cantilever when the tip is scanning the surface. The difference in the bi-directional lateral force images corresponds to the friction force image. Friction force images enable us to identify regions of higher hydrophilicity, based on the increased interaction with the AFM tip.

By bringing the tip into contact with the surface, followed by retracting the tip from the surface, force-distance curves are obtained. In the retracting cycle, because of the adhesion properties between the tip and surface, the tip will not disengage from the surface until the force used to pull the tip from the surface exceeds the adhesion force between them. This pull-off force can be considered as a measure of the adhesion force between the tip and surface.

Dynamic force (tapping) mode AFM was developed to measure soft surfaces where the contact mode AFM could result in degradation on the surface due to the large applied force. Tapping mode is operated with a stiff cantilever (e.g., ~40 N/m), which is oscillated near its resonant frequency. In the dynamic force mode, the tip is oscillated at around its resonant frequency by applying a driving AC voltage to a bimorph on which the cantilever is attached. When the tip gets close to the sample surface, the tip-sample interaction forces cause the oscillation amplitude to reduce. This damped amplitude is used as the feedback parameter for imaging the topography of the sample. Another quantity measurable is the phase shift angle of the oscillating cantilever, which is the phase difference between the driving voltage and the oscillation of the cantilever as detected by the photodetector. It has been found that the phase shift angle is sensitive to tip-sample interaction, making phase shift imaging a powerful technique to distinguish materials having different mechanical, chemical and adhesion properties.

Instrument:
Park Systems XE-100

System Capabilities:

  • Topographic images with a height resolution of ~0.1nm and lateral resolution down to nanometers
  • Friction force images to distinguish different materials, phases, and chemical properties
  • Adhesion forces on surfaces that can be a measure of surface energy (especially useful in revealing surface modifications)

Selected Applications in Industry:

  • Mapping surface chemistry
  • Eliminating artifacts in AFM images using a biaxially-oriented polypropylene (BOPP) film as a standard to evaluate AFM tips and to verify tip estimation algorithms; the BOPP film can even be used to clean contaminated tips
  • Different phases detected by friction force measurement (e.g., top surface of a laser-cured polymer)
  • Modifications of morphology and surface energy (e.g., UV/ozone treated polypropylene (PP) film)
  • Enhancement of topographic features of “rough” laser-cured polymer surfaces by lateral force imaging