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.
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 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 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 tip artefact resulting in similar outline of all the surface features, especially the smallest ones. This tip artefact might be probably caused by tip damage due to high loads exerted in LFM.For this reason, LFM may require DLC coated tips because they ensure higher durability and larger tip-sample interaction area producing a stronger lateral deflection of the cantilever. The larger cantilever deflection must be balanced against the lower lateral resolution delivered by the blunter 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 a tip or a sample is driven into an oscillation by a piezoelement or a broadband transducer. In FM the cantilever is forced to oscillate at the resonance frequency of the piezoelement (typically ~ 5 - 10 kHz) and the deflections of the cantilever are measured at the same frequency. This deflection (or its amplitude) is large when the tip hits a stiffer location and the related contrast in the FM Amplitude image provides a compositional map of the sample. Typically, Si probes with stiffness between 0.5 N/m and 5 N/m are applied for this purpose.
In the CR technique, the information is gained from the frequency spectrum of the probe staying in contact with a sample. The shifts of the resonance probe frequency and phase due to mechanical interactions with the sample are measured and applied for an estimation of its mechanical properties. Si probes with stiffness from 0.1 N/m to 5 N/m can be used in these experiments. From a viewpoint of quantitative mechanical measurements, it can be attractive to use probes with tips that have larger apex with well-defined dimensions.
To get a high-contrast image in Force Modulation, cantilever and sample should be matched in terms of stiffness. Different cantilevers can be used to find the best match. For samples with unknown mechanical properties, it is recommended using a probe of the 18 series featuring an intermediate spring constant of ~ 5 N/m.
Resolution of the scans depends on the type of the 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 probe (or the phase of the oscillating piezoelectric transducer that drives the probe) and the phase of the 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 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 a probe of the 14 series which features an intermediate spring constant of ~ 5 N/m.
"Hard" tapping (large amplitudes and low set point ratios) is usually required to achieve strong material-sensitive contrast in phase imaging.
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Higher sensitivity to lateral forces
Hardened probes with larger tip radius
HQ:NSC probes with medium spring constant
HQ:CSC probes with low spring constant
HQ:NSC probes with high spring constant
HQ:NSC probes with a medium spring constant
High resolution imaging
Hi'Res-C probes with medium spring constant
Hi'Res-C probes with low force constant
HQ:NSC probes with high spring constant