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Atomic Force Microscopy (AFM)

AFM Basics

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In atomic force microscopy (AFM), a probe consisting of a sharp tip (nominal tip radius on the order of 10 nm) located near the end of a cantilever beam is raster scanned across the sample surface using piezoelectric scanners. Changes in the tip-sample interaction are often monitored using an optical lever detection system, in which a laser beam is reflected off of the cantilever and onto a position- sensitive photodiode. During scanning, a particular operating parameter is maintained at a constant level, and images are generated through a feedback loop between the optical detection system and the piezoelectric scanners. For a scanning stylus atomic force microscope, the probe tip is scanned above a stationary sample, while in a scanning sample design, the sample is scanned below a fixed probe tip. The discussion to follow is general to either design, as the relative motion of the tip to the sample is used to generate topographic images. Applications of AFM and other types of scanning probe microscopy continue to grow rapidly in number and include biological materials (e.g., studying DNA structure), polymeric materials (e.g., studying morphology, mechanical response, and thermal transitions), and semiconductors (e.g., detecting defects). In particular, AFM can be utilized to evaluate the surface quality of products such as contact lenses, optical components (mirrors, beamsplitters, etc.), and semiconductor wafers after various cleaning, etching, or other manufacturing processes.

Three imaging modes, contact mode, non-contact mode, and intermittent contact or tapping mode, can be used to produce topographic images of sample surfaces.  In contact mode, the probe is essentially dragged across the sample surface. During scanning, a constant bend in the cantilever is maintained.  A bend in the cantilever corresponds to a displacement of the probe tip, zt, relative to an undeflected cantilever, and the applied normal force, P = kzt, where k is the cantilever spring constant.  As the topography of the sample changes, the z-scanner must move the relative position of the tip with respect to the sample to maintain this constant deflection.  Using this feedback mechanism, the topography of the sample is thus mapped during scanning by assuming that the motion of the z-scanner directly corresponds to the sample topography.  To minimize the amount of applied force used to scan the sample, low spring constant (k < 1 N/m) probes are normally used.  However, significant deformation and damage of soft samples (e.g., biological and polymeric materials) often occurs during contact mode imaging in air because significant force must be applied to overcome the effects of surface contamination (e.g., adsorbed moisture).  The combination of a significant normal force, the lateral forces created by the dragging motion of the probe tip across the sample, and the small contact areas involved result in high contact stresses that can damage either the sample or the tip or both.  To overcome this limitation, contact mode imaging can be performed within a liquid environment, which essentially eliminates problems due to surface moisture such that much lower contact forces can be used.   In fact, the ability to image samples in a liquid environment is often a desirable capability of AFM, but in some cases it might not be practical or feasible.  Also, working with liquid cells for many commercial AFM systems can be tricky, particularly avoiding spills and leaks that introduce liquid into the scanners.

To reduce or eliminate the damaging forces associated with contact mode, the cantilever can be oscillated near its first bending mode resonance frequency (normally on the order of 100 kHz) as the probe is raster scanned above the surface in either non-contact mode or tapping mode.  In non-contact mode, both the tip-sample separation and the oscillation amplitude are on the order of 1 nm to 10 nm, such that the tip oscillates just above the surface contamination layer, essentially imaging the surface of, for example, the adsorbed surface moisture.  The resonance frequency and amplitude of the oscillating probe decrease as the sample surface is approached due to interactions with van der Waals and other long-range forces extending above the surface.  These types of forces tend to be quite small relative to the repulsive forces encountered in contact mode.  Either a constant amplitude or constant resonance frequency is maintained through a feedback loop with the scanner and, like contact mode, the motion of the scanner is used to generate the topographic image.  To reduce the tendency for the tip to be pulled down to the surface by attractive forces, the cantilever spring constant is normally much higher compared to contact mode cantilevers. The combination of weak forces affecting feedback and large spring constants causes the non-contact AFM signal to be small, which can lead to unstable feedback and requires slower scan speeds than either contact mode or tapping mode.  Also, the lateral resolution in non-contact mode is limited by the tip-sample separation and is normally lower than that in either contact mode or tapping mode.

Tapping mode tends to be more applicable to general imaging in air, particularly for soft samples, as the resolution is similar to contact mode while the forces applied to the sample are lower and less damaging.  In fact, the only real disadvantages of tapping mode relative to contact mode are that the scan speeds are slightly slower and the AFM operation is a bit more complex, but these disadvantages tend to be outweighed by the advantages.  In tapping mode, the cantilever oscillates close to its first bending mode resonance frequency, as in non-contact mode.  However, the oscillation amplitude of the probe tip is much larger than for non-contact mode, often in the range of 20 nm to 200 nm, and the tip makes contact with the sample for a short duration in each oscillation cycle.  As the tip approaches the sample, the tip-sample interactions alter the amplitude, resonance frequency, and phase angle of the oscillating cantilever.  During scanning, the amplitude at the operating frequency is maintained at a constant level, called the set-point amplitude, by adjusting the relative position of the tip with respect to the sample.  In general, the amplitude of oscillation during scanning should be large enough such that the probe maintains enough energy for the tip to tap through and back out of the surface contamination layer.

Contrast Imaging

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One recent development in tapping mode is the use of the changes in phase angle of the cantilever probe to produce a second image, called a phase image or phase contrast image.  This image often provides significantly more contrast than the topographic image and has been shown to be sensitive to material surface properties, such as stiffness, viscoelasticity, and chemical composition.  In general, changes in phase angle during scanning are related to energy dissipation during tip-sample interaction and can be due to changes in topography, tip-sample molecular interactions, deformation at the tip-sample contact, and even experimental conditions.  Depending on the operating conditions, different levels of tapping force might be required to produce accurate, reproducible images on different samples.  These changes in tapping force will often affect the phase image, particularly with regard to whether local tip-sample interactions are attractive or repulsive.  Similar contrast images can be constructed concurrently with the topographic image in contact mode.  One example is lateral force or friction force imaging, in which torsional rotation of the probe is detected while the probe is dragged across the surface in a direction perpendicular to the long axis of the cantilever.  Friction force imaging with a chemically modified probe (i.e., a probe that has been coated with a monolayer of a specific organic group) is often referred to as chemical force microscopy.  Another example of contrast imaging is force modulation, which combines contact mode imaging with a small oscillation of the probe tip at a frequency far below resonance frequency.  This oscillating force should deform softer regions more than harder regions of a heterogeneous sample such that contrast between these regions is observed.  In practice, however, the difference between the elastic modulus of the different regions usually has to be substantial (e.g., rubber particles in a plastic, carbon fibers in an epoxy) for contrast to be realized.  Thus far, these types of AFM contrast images are purely qualitative due to inaccurate or unknown spring contants, unknown contact geometry, and contributions from different types of tip-sample interactions.

References:

Chemically Functionalized Tips

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illustration of tip modification

For many AFM studies, controlling the surface chemistry of the probe tip can be advantageous, particularly with regard to enhancing contrast in a friction or phase contrast image.  Traditionally, chemically modified tips have been used in lateral force or friction imaging in contact mode, both in fluids and in air.  This mode of imaging is sometimes referred to as a chemical force microscopy (CFM).  The AFM (silicon or silicon nitride) tip is functionalized with a particular chemical species and scanned over the sample to detect differences in interaction forces between the species on the tip and those on the sample surface.  The tip modification process includes the controlled deposition of very thin metallic films (normally a 5 nm thick chromium layer followed by a 50 nm thick gold layer) onto the probe followed by immersion of the probe in a solution of organic thiol.  One end of the thiol covalently bonds to the gold surface, forming a self-assembled monolayer (SAM), and the other end contains the appropriate functional group.  This functionalization process is shown schematically to the left. Applications include mapping the functional group microstructure in polymers and binding/recognition interactions in biological systems. Examples of recent results are shown in the image gallery.

Image Gallery

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Typically, we utilize AFM contrast imaging to map heterogeneity in multi-component and multi-phase polymeric material systems, although other types of samples are also studied.  Recent research efforts have included:

Example images currently available in our image gallery include:

Interfaces and Interphases

Degradation of Organic Coatings

Phase Imaging of Polymer Blends

Self-Assembled Monolayers (SAMs)

Chemically Functionalized Tips

Fiber-Reinforced Polymer (FRP) Composites

Polymer Nanocomposites

Other Construction Materials

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Date created: 4/01/2000
Last updated: 2/27/2003