Scanning Force Microscopy (SFM)

AFM — Figure 1: Schematic view of the operating principle of the SFM. The deflection of the cantilever spring is read out via laser beam focused on the apex of the cantilever and reflected on a position sensitive detector (PSD). At the apex an atomically sharp tip is scanned laterally by an actuator (magnifying glass). The force between tip and sample surface is held constant by an electronic feedback loop regulating the tip sample distance.

Figure 1: Schematic view of the operating principle of the SFM. The deflection of the cantilever spring is read out via laser beam focused on the apex of the cantilever and reflected on a position sensitive detector (PSD). At the apex an atomically sharp tip is scanned laterally by an actuator (magnifying glass). The force between tip and sample surface is held constant by an electronic feedback loop regulating the tip sample distance.

AFM — Figure 1: Schematic view of the operating principle of the SFM. The deflection of the cantilever spring is read out via laser beam focused on the apex of the cantilever and reflected on a position sensitive detector (PSD). At the apex an atomically sharp tip is scanned laterally by an actuator (magnifying glass). The force between tip and sample surface is held constant by an electronic feedback loop regulating the tip sample distance.

Figure 1: Schematic view of the operating principle of the SFM. The deflection of the cantilever spring is read out via laser beam focused on the apex of the cantilever and reflected on a position sensitive detector (PSD). At the apex an atomically sharp tip is scanned laterally by an actuator (magnifying glass). The force between tip and sample surface is held constant by an electronic feedback loop regulating the tip sample distance.

A scanning force microscope (SFM) is a device that detects the forces between a sample surface and a nanoscale tip (alternatively a colloidal particle or a droplet) with a sensitivity in the order of some 10 pN. These forces originate from the interaction of tip apex with the sample surface, e.g. via electrostatic and van-der-Waals forces. By raster-scanning the topography of the sample can be obtained down to molecular or atomic scales. Beside topography, specific operating modes are be used to characterize lateral frictional, elastic, thermal, and electrical material properties (Figure 1). In combination with infrared light (IR) illumination, we record local IR spectra with a lateral resolution <50 nm. The use of this nano-IR method allows us to locally detect the presence of chemical species in samples. The latter technique can be used for characterizing polymer blends, block-copolymers, coatings etc.

SFM is one of the core facility labs of the MPI-P. We teach students the use of SFM methods and we provide measurements for students on request.

Kelvin Probe Microscopy

Figure 2: *Photograph of the SFM in the glove box. An upright standing sample is illuminated with white light.* in the photosensitive scanning probe microscope, two types of sample illumination are available: white light (top) and laser light at 488 nm (bottom).

Figure 2: *Photograph of the SFM in the glove box. An upright standing sample is illuminated with white light.*
in the photosensitive scanning probe microscope, two types of sample illumination are available: white light (top) and laser light at 488 nm (bottom).

Figure 2: *Photograph of the SFM in the glove box. An upright standing sample is illuminated with white light.* in the photosensitive scanning probe microscope, two types of sample illumination are available: white light (top) and laser light at 488 nm (bottom).

Figure 2: *Photograph of the SFM in the glove box. An upright standing sample is illuminated with white light.*
in the photosensitive scanning probe microscope, two types of sample illumination are available: white light (top) and laser light at 488 nm (bottom).

As molecular electronics advances, the characterization of electrical properties becomes more and more important. In research and industry elements have already reached structure sizes down to a few nanometers. Therefore, electrical modes of SFM play an increasing role for material characterization. Kelvin probe force microscopy (KPFM), for example, measures the contact potential difference. Thereby, work function variations or Galvani potentials across the sample surface can be mapped with 10 nm lateral resolution. In particular for photovoltaic materials, light-induced changes in the measured contact potentials are associated to charge transport. We have built a SFM inside an Ar gas filled glove box (Figure 2). We use this SFM to investigate all-solid-state battery materials while charging and discharging (operando measurements).

Zhu, C.; Fuchs, T.; Weber, S. A. L.; Richter, F. H.; Glasser, G.; Weber, F.; Butt, H.-J.; Janek, J.; Berger, R.: Understanding the evolution of lithium dendrites at Li_6.25Al_0.25La₃Zr₂O₁₂ grain boundaries via operando microscopy techniques. Nature Communications 14, 1300 (2023) , DOI: 10.1038/s41467-023-36792-7
Zhu, C.; Kobayashi, S.; Sugisawa, Y.; Weber, F.; Lin, K.-H.; Kitamura, M.; Horiba, K.; Kumigashira, H.; Nishio, K.; Shimizu, R. et al.: Space Charge Layer Evolution in All-Solid-State Batteries Probed via Operando Kelvin Probe Force Microscopy and Nuclear Reaction Analysis. ACS Nano 19 (45), S. 39062 - 39075 (2025), DOI: 10.1021/acsnano.5c10125

Colloidal Probe Technique

Colloidal probe prepared by sintering a PS sphere to cantilever

The colloidal probe technique uses micropheres as probes instead of sharp tips. The spheres are attached to the end of tipless SFM cantilevers. The well defined contact geometry allows quantitative analysis of SFM force spectroscopy experiments to obtain information on surfaces forces, contact mechanics and mechanical properties on the micro- and nanoscale. In addition, almost any probe material can be used. Attachment of the spheres to the AFM cantilever is achieved under an optical microscope with the help of a hydraulic micromanipulator. Spheres can either be glued to the cantilever using epoxy or UV curable glues or can be sintered to he cantilever at elevated temperatures.

Chen, W.; Song, S.; Samanta, A.; Sethi, S.; Drees, C.; Kappl, M.; Butt, H.-J.; Walther, A.: Growing functional artificial cytoskeletons in the viscoelastic confinement of DNA synthetic cells. Nature Chemical Engineering 2, S. 627 - 639 (2025), DOI: 10.1038/s44286-025-00289-5

Drop Friction Forces

Drop friction image — Figure 3: Using a shadow mask, we made an M-shaped pattern in the surface of high (outside the "M") and low contact angle hysteresis (inside the "M"). By means of DoFFI it is possible to localize and visualize the "M".

Figure 3: Using a shadow mask, we made an M-shaped pattern in the surface of high (outside the "M") and low contact angle hysteresis (inside the "M"). By means of DoFFI it is possible to localize and visualize the "M".

In rain, drops form on windows. If the drops are large enough, they will run down the window. In order to measure drop sliding forces, we have built a drop friction force instrument (DoFFI). In DoFFI a sessile drop sticks to a force sensor by capillary action. When the sample moves laterally, the force sensor deflects until the drops starts sliding over the surface. DoFFI is a 2D characterization tool, which allows imaging of local variations in surface wetting properties by plotting the measured friction force at each drop position. We were able to resolve the wetting features from centimeter to submillimeter sizes (Figure 3). Even surface features having sizes much smaller than the drop diameter are visible and can be characterized. The sDoFFI is not limited to laboratory-based samples but is also suited for characterization of biological and technological surfaces.
Currently, we cooperate with KRÜSS with the aim to commercialize a DoFFI setup.

Hinduja, C.; Laroche, A.; Shumaly, S.; Wang, Y.; Vollmer, D.; Butt, H.-J.; Berger, R.: Scanning Drop Friction Force Microscopy. Langmuir 38 (48), S. 14635 - 14643 (2022), DOI: 10.1021/acs.langmuir.2c02046
Hinduja, C.; Butt, H.-J.; Berger, R.: Slide electrification of drops at low velocities. Soft Matter 20 (15), S. 3349 - 3358 (2024), DOI: 10.1039/d4sm00019f