In addition to surface topography, AFM allows probing different mechanical properties of materials. This type of experiment can be utilized to enhance the imaging contrast of topography scans and to map the materials of heterogeneous samples.
|Fig.1. Height (a) and phase (b) images of a biaxially oriented high-density polyethylene film obtained in tapping mode. Scan size 400nm. Images courtesy of S. Magonov.
Compositional mapping of mechanical properties can be conducted in contact or tapping mode. In contact mode, usually one of two imaging modes are employed: Lateral Force Microscopy (LFM) to probe local friction or Force Modulation (FM) to map material elasticity. Phase imaging is one of the imaging modes of tapping mode AFM, which is sensitive to the local viscoelasticity, adhesion, and friction properties of a sample.
However, it is still challenging to distinguish between the different properties of materials like stiffness, hardness, adhesion, and viscoelasticity on these compositional maps.
Lateral Force Microscopy
Lateral Force Microscopy (LFM), in which lateral force images are detected, is more sensitive to variations of stiffness and adhesion in heterogeneous samples such as for example organic layers prepared from a mixture of components. For LFM measurements AFM probes with the same stiffness as those applied in the contact mode studies can be used.
LFM images of gold evaporated on mica with an organic layer are presented in Fig. 2. One might note the AFM tip artefact resulting in similar outline of all the surface features, especially the smallest ones. This AFM tip artefact might be probably caused by AFM tip damage due to high loads exerted in LFM.
|Fig.2. Gold evaporated on mica with organic layer. Gold islands reveal lower friction. Trace (a) and retrace (b) images, left to right and right to left. Scan size 1micron. Image courtesy of L.V. Kulikova and I.V. Yaminsky, MSU&ATC.
For this reason, LFM may require DLC coated AFM tips because they ensure higher durability and larger tip-sample interaction area producing a stronger lateral deflection of the cantilever. The larger AFM cantilever deflection must be balanced against the lower lateral resolution delivered by the blunter AFM tip.
Force modulation and contact resonance
There are two other techniques: Force Modulation (FM) and Contact Resonance (CR), which are aimed at studies of local mechanical properties with the AFM probe staying in permanent contact with a sample. They are actually oscillatory methods in which an AFM tip or a sample is driven into an oscillation by a piezo element or a broadband transducer. In FM the AFM cantilever is forced to oscillate at the resonance frequency of the piezo element (typically ~5-10kHz) and the deflections of the AFM cantilever are measured at the same frequency. This deflection (or its amplitude) is large when the AFM tip hits a stiffer location and the related contrast in the FM Amplitude image provides a compositional map of the sample. Typically, Si AFM probes with stiffness between 0.5N/m and 5N/m are applied for this purpose.
In the CR technique, the information is gained from the frequency spectrum of the AFM probe staying in contact with a sample. The shifts of the AFM probe's resonance frequency and phase due to mechanical interactions with the sample are measured and applied for an estimation of its mechanical properties. Si AFM probes with stiffness from 0.1N/m to 5N/m can be used in these experiments. From a viewpoint of quantitative mechanical measurements, it can be attractive to use AFM probes with AFM tips that have larger apex with well-defined dimensions.
To get a high-contrast image in Force Modulation, the AFM cantilever and the sample should be matched in terms of stiffness. Different AFM cantilevers can be used to find the best match. For samples with unknown mechanical properties, it is recommended using an AFM probe of the 18 series featuring an intermediate spring constant of ~5N/m.
Resolution of the scans depends on the type of the AFM probe tip.
The development of phase imaging was another pivotal event that made AFM a recognized characterization technique especially for the investigation of heterogeneous materials. The difference between the phase of the free-oscillating AFM probe (or the phase of the oscillating piezoelectric transducer that drives the AFM probe) and the phase of the AFM probe oscillation while it interacts with the sample appeared to be very sensitive to variations of the mechanical, adhesive and electromagnetic properties. The contrast of the phase images of multicomponent materials reveals the distribution of individual components. Imaging at elevated forces with AFM probes whose stiffness is close to that of the components is particularly important for getting high-contrast phase images. The fact that contemporary materials include components with a very broad spectrum of mechanical properties means that the optimization of imaging conditions requires a proper selection of the cantilever stiffness.
For successful imaging of soft samples such as polymers or biomaterials, the cantilever spring constant should match the effective spring constant of the tip-surface contact area. For samples with unknown mechanical properties, it is recommended using an AFM probe of the HQ:NSC14 series which features an intermediate spring constant of ~5N/m.
"Hard" tapping (large amplitudes and low set point ratios) is usually required to achieve strong material-sensitive contrast in phase imaging.