There are a number of AFM techniques sensitive to local mechanical 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.

For the selective quantitative characterization of the properties, a number of techniques have been developed. The measurements of the force curves in the contact mode provide information about local mechanical properties. The experiments can be focused on exploring stretching and rupture of individual macromolecules.

Force volume

Force Volume imaging is an automated collection of force-curves data over a certain area. Practically, this is done by pulling the probe from a surface after the probe apex adheres to macromolecules or larger species lying on the substrate. A pulling experiment usually involves two types of probes: regular Si probes and coated probes promoting the adhesion process. These experiments are conducting at different strain rates and require a large statistics due to low yield of successful events. A rational analysis of the pulling force-curves is performed in terms of energy of individual chemical bonds.

Nanoindention

Another use of the force curves is for indentation purposes. Indentation techniques are known in materials science for many years being applied for examination of hardness of various materials ranging from ceramics and metals to polymers and polymer composites. AFM-related nanoindentation complements these studies by providing the experimental data at smaller scale and at lower indentation forces.

The general routine for AFM nanoindentation experiments looks simple but it has a lot of hurdles to obtained quantitative mechanical properties. The force-curves, which are recorded on one or multiple locations, can be obtained for different load levels at which the probe-induced sample deformation can be elastic, viscoelastic or plastic. After proper calibration of optical sensitivity of a microscope, a precise determination of the cantilever stiffness and geometry of the tip apex, the force curves can recalculated into deformation versus force dependencies. The latter are used for evaluation of elastic modulus, plastic yield deformation and other mechanical parameters.

This procedure is not-trivial because it depends on choice of appropriate model used for describing local deformation of different type. The existing models are mostly rational approximations than rigorously obtained solutions for AFM-relevant measurements. At the current stage of AFM-based nanoindentation reliable experimental data are badly needed. The use of well-characterized probes (i.e. probes with precisely determined cantilever stiffness and tip apex geometry) are an important pre-requisite of such experiments. Definite advantage is offered by LS probes with rounded tip apex because the latter can be relatively easy characterized with scanning electron microscopy and they are less susceptible for damage during indenting.

To make the measurements quantitative, one may choose cantilevers with calibrated spring constants. Probes of the 14 series are optimal in terms of sensitivity and accuracy of the calibration method.

Conventional probe can be replaced by colloidal particles of glass, silica, polystyrene, different metals, etc., which can also be further functionalized. The applications include investigations of different phenomena like surface charging in fluids, polarization, hydrophobicity, and chemical forces. Use of special tipless cantilevers may be convenient for attaching colloidal particles.

Young modulus measurement

This pioneering approach utilizes a probe with a T-shaped cantilever carrying a tip on one of its "wings". The offset of the tip from the cantilever longitudinal axis provides a very sensitive torsional response of the probe when the tip intermittently strikes a sample. A large number of harmonics are generated in the tip-sample contact that can be detected by broadband controllers.

Instead of imaging of any of detected harmonics, one can perform a conversion of harmonics data into time domain for reconstruction of tip-sample force curves practically in each oscillatory cycle of tip-sample interaction (Fig.1). Therefore, one can obtain topography information, phase image and map of any properties derived from the force curves (elastic modulus, adhesion, etc) in a single scan (Fig.2). Such a way provides unique extension of nanoindentation in low-force (10 pN), high-resolution (1 nm) and high-speed (10 sec) domains.

Most samples that can be imaged in tapping mode in air can be investigated with TL300 probe. TL180 is for ultra high resolution mechanical measurements. This probe can perform sensitive mechanical measurements with peak tipsample forces less than 1 Nano Newtons as well as reliable and ultra-low force topographical imaging.