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AFM: Exploring Tapping Mode and AM-FM

Atomic force microscopy (AFM) is an advanced tool used to characterize sample surfaces with outstanding resolution. This eschews optical methods of microscopy in favor of a scanning probe, which comprises an extremely sharp probe tip mounted on a cantilever. This is raster-scanned across a surface to measure pre-defined quantities in a serial fashion. Analysts utilizing atomic force microscopy can measure discrete locations for topography, tribology, nanomechanical and thermal properties, electronic characteristics, and more.

Contact mode atomic force microscopy measures interactions between a sample and a probe tip by continuously tracking how the free end of the cantilever is displaced during raster-scanning. The tip is brought into contact with the sample and an electronic feedback loop ensures the deflection remains constant during scanning, recording sample surface topography as a function of vertical motion.

This can be problematic as the high-lateral forces between the tip and the sample can cause damage to the sample surface and dulling of the probe tip. New modes of atomic force microscopy have been developed to overcome this issue.

Tapping Mode Atomic Force Microscopy

In tapping mode afm, the sharp probe tip is not scanned across the sample surface while in constant contact. Instead, the cantilever is vibrated near its resonance frequency causing the tip to oscillate up and down. This means the probe only comes into close contact with the surface intermittently; hence the title.

Contact mode atomic force microscopy measures the tip-sample interaction force as a function of cantilever deflections, which is detected using a photodiode. In tapping mode, interactions are measured as changes in amplitude. Both use a controlled feedback loop to ensure constant tip-sample interaction forces during raster scanning to acquire images with the highest resolutions possible. The primary benefit of tapping mode atomic force microscopy is the mitigation of damaging lateral forces.

One drawback of tapping mode is that it cannot measure direct forces, as the lock-in amplifier can only report an averaged response pertaining to the tip-surface interactions as a function of force. This creates an unstable feedback situation which limits the information that can be acquired and reliably related to a specific sample property. However, it remains the dominant imaging mode of atomic force microscopy. This is due primarily to the not insignificant benefit of lateral force-free imaging.

Tapping mode is subsequently suited to imaging fragile samples that could not withstand the high lateral forces of conventional contacting probes. Measuring the topographical features of soft solids and organic tissues accurately and non-invasively is extremely problematic for conventional operating modes. Normal tapping mode facilitates rapid high-resolution surface imaging of complex samples with little-to-no surface interference due to the drag of the sharp probe tip.

AM-FM Viscoelastic Mapping with Asylum Research

Asylum Research has combined the inherent benefits of normal tapping mode atomic force microscopy with amplitude and frequency modulation (AM-FM) to provide a unique dual mode capable of fast raster scanning of complex samples. This provides topographical imaging with quantitative estimations of both storage and loss moduli to facilitate extremely accurate viscoelastic mapping.

This has already added new layers of understanding for thin film research and development, with a plethora of additional application areas. If you would like to learn more, feel free to contact a member of the team directly. We are happy to discuss tapping mode atomic force microscopy with you in more detail.

Learn more in our white paper!

Application note focusing on AFM-FM mode. Download the White Paper