An essential part of scanning force microscopy is a micromechanical cantilever sensor (MCS) which transduces forces acting on the tip into a deflection. Forces of pico newtons can be measured which correspond to a sub-nanometer deflection of the micromechanical cantilever sensor end. However, not only forces acting directly on the tip lead to a deflection, also surface energy changes, expansive or contractive forces acting on one side of the cantilever surface result in measurable deflections of micromechanical cantilever sensors. Thus this method provides a means to study surface properties of thin films.

The objective of our research is to further develop the micromechanical cantilever sensor method and apply the technique for the investigation of surface and thin film properties of polymers, swelling and phase change of polymers films, e.g. polymer brush layers. MCS can be easily fabricated in arrays and are suitable measurement procedure for sensing of biomolecules or screening of material properties.

Investigation of colloid monolayers on MCS

We have investigated the film formation from colloids made from polymers by thermal annealing. Monolayers of PS colloids were prepared on MCS surfaces by transferring colloids that are situated at the air water interface (Figure a). Depending on the way the MCS is immersed into the solution we were able to either coat one side of the MCS or both sides with monolayers of PS colloids. One way to obtain information about the film formation process is to vibrate the MCS at its resonance frequency. Upon thermal annealing the film formation results in an increase in resonance frequency at the temperature where film formations starts (~ 150°C at Figure c). This increase is associated to an increase in interaction between colloids and the MCS surface (regime II). We found that the film formation process is finished at a temperature of 180 °C. At this temperature the resonance frequency decreases owing to a decrease in Young’s module of the PS film and the Si MCS (regime III). Subsequent cooling of the MCS revealed a shift in resonance frequency at 20 °C to higher frequencies compared to the starting value (regime V). This shift in resonance frequency reflects the different type of coating on the MCS, i.e. in the beginning of the experiment colloids (regime I) and at the end a continuous film (regime V). The kinks between the curves in regime I and II and IV and V were observed at the same temperature and indicate the glass transition temperature of the film coating at the particular frequency where the MCS is resonated.

Figure 1: (a) Transfer of a colloidal monolayer on micromechanical cantilever sensors. By adjusting the removal procedure we were able to achieve a single side coating of a monolayer of PS colloids having diameters of 300 nm to 500 nm as proven by SEM (b). (c) Change in resonance frequency of the MCS in dependence of temperature.

Investigation of composite materials with MCS

Composite materials made of metallic nanoparticles embedded in an organic matrix are highly promising candidates as coatings for novel chemical sensors. Micromechanical cantilever measurements revealed that the Au-nanoparticle terphenyldithiol composite material swells upon dosing with toluene vapor. Significant differences in the mechanical transduction of a 100 nm thick Au-nanoparticle terphenyldithiol composite material that was prepared on a 3-aminopropyldimethylmonoethoxysilane (APDMES) surface and on a Au-surface were observed. The transduction of swelling of the composite film into a mechanical deflection was found to be more efficient for the composite film prepared on the Au-surface attributed to covalent binding of the terphenyldithiol molecules with the Au-surface. In contrast, the interface of the APDMES layer and the Au-terphenyldithiol composite material is based on electrostatic interaction between the Au-nanoparticles and the amino interface. The analysis of the micromechanical cantilever sensor measurements lead to the conclusion that the composite film at the APDMES interface is more mobile compared to a similar film that was prepared on Au.

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• M. Toda, Y. Joseph and R. Berger: Swelling of Composite Films at Interfaces. Journal of Physical Chemistry C, 114, 2012-2017 (2010).
• S. Lenz, A. Ruehm, J. Major, R. Berger, J. S. Gutmann: Softening of PMMA Brushes upon Collapse/Swelling Transition. A Combined Neutron Reflectivity and Nanomechanical Cantilever Sensor Study, Macromolecules, 2011, 44, 360–367
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