Research Topics of Rüdiger Berger

Currently, my research is focussing on four topics: (1) Using electrical modes of scanning force microscopy to understand charge extraction in perovskite solar cells. (2) Understanding and controlling of lateral adhesion forces of liquid drops that slide over surfaces. (3) Developping ultra-sensitive sensors based on micromechanical cantilevers and (4) Measuring forces at high hydrostatic pressures.

Examples below in the boxes.

Lateral force measurements of liquids on surfaces

We built up a method that allows the measurement of forces required to slide sessile drops over surfaces. The forces were measured by means of a deflectable glass capillary stuck in the drop (schematic image close by). Deflections of this capillary are detected by a laser beam that is reflected from the glas capillary surface and falls onto a postion sensitive detector. The drop adhesion force instrument (DAFI) allowed the investigation of the dynamic lateral adhesion force of water drops of 0.1 to 2 µL volume at defined velocities. The movement of the drop relative to the surfaces enabled us to resolve the pinning of the three-phase contact line to individual defects [1].
The DAFI is a suitable tool for characterizing lateral adhesion and thus hydrophobicity. In particular, the relative movement of the drop on surfaces allows us to resolve different wetting phenomena of surfaces spatially. Thus, the DAFI could even be applied for quality control of surfaces made by large-scale industrial processes. We used the DAFI to investigate the "static" and a "kinetic" regime of sliding drops [2].
We are using the setup to characterize omniphobic surfaces and study the interaction between transport and wetting processes within the collaborative reserach center 1194. Recently, we investigated how contamintations are removed from surfaces by sliding drops. We monitored the removal of individual contaminant particles on the micron scale by confocal microscopy while drops are kept in position by DAFI. We correlate the space- and time-resolved information with measurements of the lateral friction force of the sliding drop [3].

References:
[1] Dynamic Measurement of the Force Required to Move a Liquid Drop on a Solid Surface, D. W. Pilat et al., Langmuir 28, 16812-16820 (2012).
[2] How drops start sliding over solid surfaces, Nan Gao et al., Nature Physics, 14, 191 - 196 (2018).
[3] When and how self-cleaning of superhydrophobic surfaces works, F. Geyer et al., Science Advances 6, eaaw9727 (2020).

Local electrical current transport through interfaces

Interfaces play a major role in electrical devices. In case interfaces are heterogenous the electrical current that can flow through the interface locally varies. Such variations can be probed by conductive mode (cSFM). In cSFM the surface is typically scanned in contact mode. In addition an electrical potential (Us) is applied and the local current between tip and sample surface (Itip) is measured. However the operation of contact mode SFM is often destructive for soft matter surfaces. Therefore, we have developped and applied non-destructive modes based on torsion force microscopy [1],[2] and force distance based modes [3] (peak force tapping or quantitative imaging).
These modes were used to quantitify the current flow along individual nano-pillars made from a thermally cross-linked triphenylamine-derivate semiconductor [1] and made from P3HT and PCBM [3] (see also image). Furthermore, we studied TiO2 anatase thin films which were UV-ozone treated [4]. We found that the latter is an efficient method to increase the conductance through the film by more than one order of magnitude. The increased conductance of TiO2 anatase thin films results in a 2 % increase of the average power conversion efficiency (PCE) of methylammonium lead iodide based perovskite solar cells. PCE values up to 19.5 % for mesoporous solar cells are realized. Using cSFM we probed the conductance of homo- and blended conjugated polymers in confined in nanostructures. The resulting structures lead to high charge mobility along vertical direction for both homo- and blended conjugated polymers. We found a more than two orders of magnitude enhanced charge mobility along vertical direction [5].

References:
[1] Mapping of Local Conductivity Variations on Fragile Nanopillar Arrays by Scanning Conductive Torsion Mode Microscopy, Stefan A.L. Weber et al., Nano Letters, 10, 1194 - 1197 (2010). Langmuir 28, 16812-16820(2012).
[2] Electrical tip-sample contact in scanning conductive torsion mode, Stefan Weber, Rüdiger Berger, Applied Physics Letters 102, 163105 (2013).
[3] Controlled Mutual Diffusion between Fullerene and Conjugated Polymer Nanopillars in Ordered Heterojunction Solar Cells, Jongkuk Ko et al., Advanced Materials Interfaces, 1600264 (2016).
[4] Removal of Surface Oxygen Vacancies increases Conductance through TiO2 Thin Films for Perovskite Solar Cells, Alexander Klasen et al., Journal of Physical Chemistry C (2019).
[5] Enhanced Vertical Charge Transport of Homo- and Blended Semiconducting Polymers by Nanoconfinement, Jongkuk Ko et al., Advanced Materials xxx, xx, xxx - xxx (2019).

Microcantilever Sensors

An essential part of scanning force microscopy is a micromechanical cantilever sensor (MCS) which transduces a force acting on the tip into a deflection. Forces of pico newtons can be measured which correspond to a sub-nanometer deflection of the MCS. However, not only forces acting on the tip lead to a deflection, also expansive or contractive forces acting on one side of the cantilever surface result in a bending. For example this is the case for a swelling or a phase change of thin polymer films, which have been deposited on one side of a MCS. In addition, tensile and compressive surface stress changes arise when molecules specifically adsorb on one side of the cantilever surface. A review of MCS operating modes and their applications can be found in an article published in Materials Today.

In particular, the surface stress changes can be measured in liquids which is a requirement for most biochemistry applications. In the field of biotechnology, DNA hybridisation between self complementary strands leads to conformational changes which result in a cantilever sensor bending. In addition, polymer materials are very attractive as responsive coatings for various sensing application [1], [2]. Beside sensing, the MCS technique can be applied for material characterization. In cooperaton with Prof. Dr. Akiko Itakura (NIMS) and Prof. Dr. Masaya Toda (Univ. of Tohoku), we analyzed the response of MCS that are coated with polymers in different solvents. Hereby the Young's module of the polymer can be calculated from the bending response [3].
The cantilever sensors are very small (typically 0.5 µm thick, 50 µm wide and 500 µm long). This offers the possibility to arrange several of these cantilever sensors in an array on a single chip. Hereby, experimental noise can be reduced by averaging signals or the response of an analyte (e.g. mixture of substances) to different coated sensors can be studied. The cantilever sensor technique is useful for studying tiny amounts of materials that are expensive to be produced in large quantities.
Currently, we develop methods to measure mass changes of samples that are attached to the cantilever's end. Two phenomena can be investigated: (a) changes in mass upon heating of the sample. Such a method is called (micromechancial) thermogravimetry and we aim to analyze biominerals within a DFG project in cooperation with Prof. Dr. Filipe Natalio from the Weizmann Institute [4]. The movie displays a heated zeolite crystal that is filled with a yellow dye. The dye excapes at 200 °C (i.e. at about 15 seconds in the movie) and leaves a transparent zeolite behind after cooling down. (b) changes in mass of polymers upon exposure of the samples to solvents. Here we would like to develop a fast and reliable method to determine the mechanical properties of soft matter materials [5].


References:
[1] Thin Polyelectrolyte Multilayers Made by Inkjet Printing and Their Characterization by Nanomechanical Cantilever Sensors, Masaya Toda et al., Journal of Physical Chemistry C 118, 8071 - 8078 (2014).
[2] Simplifying cantilever sensors: Segmental analysis, a way to multiply your output, Jannis W. Ochsmann et al., Sensors and Actuators B 177, 1142 - 1148 (2013).
[3] Effective Young's Modulus Measurement of Thin Film Using Micromechanical Cantilever Sensors, Akiko N. Itakura et al., Japanese Journal of Applied Physics, 52, 110111 (2013).
[4] Pico-thermogravimetric material properties analysis using a diamond cantilever beam, Ioana Voiculescu et al., Sensors and Actuators A, 271, 356-363 (2018).
[5] Young's modulus of plasma-polymerized allylamine films using micromechanical cantilever sensor and laser-based surface acoustic wave techniques, Masaya Toda et al., Plasma Process Polym., e1800083 (2018).

Forces at High Hydrostatic Pressure

We have designed a optical trapping setup that allowed us to explore the interaction of a micrometer-sized glass bead and a solid glass wall in water at hydrostatic pressures of up to 1 kbar. The setup allowed us to measure the distance between bead and wall with a subnanometer resolution [1].

References:
[1] Surface forces between colloidal particles at high hydrostatic pressure, D. W. Pilat, B. Pouligny, A. Best, T. A. Nick, R. Berger*, and H.-J. Butt, Physical Review E, 022608 (2016).

The projects are integrated in the research at the Max Planck Institute for Polymer Research in particular in the science perfromed in the department Physics of Interfaces of Prof. Dr. H.-J. Butt.