A new view of materials
01/01/2008
A new AFM mode enables improved nanoscale characterization.
By Bede Pittenger, Veeco Instruments
Polymer materials researchers increasingly need to understand the performance characteristics, on a nanoscale level, of composite polymer products. By characterizing what’s happening at the interphase boundaries of composites that blend two or more polymers, it is possible to more accurately predict and control overall material properties such as texture, wear, and shock resistance. And because the dimensions of the component domains and the boundaries are often sub-micron, the atomic force microscope (AFM) naturally lends itself to their study.
“Tapping mode” (also known as intermittent-contact or dynamic-force mode) phase imaging and nanoindentation are two AFM techniques commonly used to study material properties. Phase imaging—which involves low normal forces, lack of lateral force, very high resolution, and relatively high speed—is often used to map variations across a sample. But phase contrast is difficult to interpret since it arises from a combination of material properties that cause dissipation of the cantilever kinetic energy. On the other hand, AFM-based nanoindentation is much simpler to interpret since force curves tell us the deformation of the sample as a function of applied force, as well as the tip-sample adhesion and hysteresis. The disadvantages of nanoindentation are lack of resolution and speed.
HarmoniX material mapping, a new AFM mode of operation developed by Veeco Instruments in partnership with Ozgur Sahin of the Rowland Institute at Harvard University, combines the advantages of both these techniques. It involves measuring torsional amplitude at exact integer multiples (harmonics) of the tapping mode drive frequency and makes it possible to determine the variation in force between the tip and the sample when the tip goes through a period of tapping oscillation.
A correctly chosen single harmonic can provide compositional mapping of a complex composite material while providing the same resolution as a tapping image (down to ~5nm). Multiple harmonics can also be observed and converted back to the time domain, providing force distance data analogous to an averaged force curve. HarmoniX material mapping has high spatial resolution and relatively high speed and can detect a large range of elasticities.
Extracting hidden information
In AFM, when the sensor tip approaches, pushes into and retracts from a sample’s surface, the tip registers a force that varies with its position. This force information can be used to characterize a material’s stiffness, hardness, adhesion, and viscoelasticity, among other properties.
In tapping mode, the tip is attached to a long cantilever that oscillates (with an amplitude of several nanometers) at or near the resonance frequency of the cantilever. A feedback loop is used to maintain the excursion of the oscillating tip at a constant level. The variations in both amplitude and phase of the feedback signal reveal the surface topography, and images obtained with the phase signal exhibit good contrast for different materials.
It can be difficult to interpret the phase signal and relate it to material properties, however, because of multiple dissipation sources such as capillary forces, viscoelasticity of samples, and electronic dissipation. A wealth of information exists in the harmonics generated in AFM tapping mode.
 Three harmonic amplitude images of a polystyrene, PMMA binary polymer film at 145°C, 175°C, and 190°C, captured via HarmoniX material mapping. |
Previously this information has been hidden beneath the noise floor because of the rapid decay of the harmonics’ amplitude. HarmoniX material mapping can capture this hidden information. The key is a newly designed, “T-shaped” torsional harmonic cantilever with the tip offset from the central axis. This offset enhances the signal-to-noise ratio of the harmonics by coupling the twisting motion of the torsional harmonic cantilever with the normal flexural (up-and-down), cantilever motion.
Such harmonic cantilevers enhance the harmonic data because their torsional resonance is 10x to 20x that of the basic flexural resonant frequency; thus, the measurement bandwidth is increased by a factor of 10 to 20. This additional bandwidth allows a near-complete reconstruction of the actual tapping mode tip-sample force interaction. When a torsional harmonic cantilever tip hits the material surface, the force on the tip is approximately a clipped sine wave, with a period equal to the tapping drive period. Shorter pulses correspond to shorter contact times and stiffer materials.
Veeco Instruments has developed several software tools to allow observation of cantilever harmonics.
 Tip-sample force extracted from harmonic data collected on (a) polystyrene and (b) PMMA. |
These include the ability to select a specific harmonic for imaging, as well as simultaneous high-speed data capture of the entire resonant spectrum at a single point on the sample surface.
Once it has been collected, the data can be Fourier transformed to find the amplitude at each harmonic. This is useful for choosing a harmonic for later imaging.
Application example
In an experiment to demonstrate HarmoniX’s ability to map material properties in polymer blends, a 50nm-thick film was imaged as it was heated. The film is a blend of acrylic glass (PMMA) and polystyrene. As with traditional force curves, stiffer samples result in a steeper increase in tip-sample force. Both polymers are stiff and brittle below their glass transition temperatures (about 100°C). At higher temperatures, polystyrene gets softer, while the PMMA has a constant stiffness.
HarmoniX was able to measure the change in the Young’s moduli of the two components of the polymer film near their respective glass transition points, while also imaging the sample’s topography. Combined with standard AFM topography measurements, this made it possible to clearly distinguish elastic and dissipative effects due to adhesion.
As newer, more-complex polymer blends are developed, and smaller filler particles are adopted, quantitative material mapping will become more valuable for polymer R&D.
Bede Pittenger is applications scientist at Veeco Instruments. You can reach him at [email protected].