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J. Hafner, D. Canena, D. Platz, M. Schneider, P. Hinterdorfer, U. Schmid:
"Scanning Probes with High Resonance Frequency and Low Stiffness for High-speed AFM Applications in Liquid Environments";
Talk: 8th Multifrequency AFM conference, Madrid; 10-27-2020 - 10-30-2020; in: "8th Multifrequency AFM conference, Book of Abstracts", (2020), 28.

Conventional scanning probes for high-speed AFM applications are free-standing beams
vibrating in one-dimensional modes, i.e. Euler Bernoulli modes. However, in these types of
scanning probes high resonance frequencies and low spring constants are not compatible which
limits their applicability to biological samples. Here, we present a novel concept for high-speed
scanning probes, which combines both high resonance frequencies and low spring constants.
Instead of a narrow beam oscillating in a one-dimensional Euler Bernoulli mode, we use a
micromachined plate oscillating in a two-dimensional mode (see Fig. 1a,b). These non-
conventional plate modes exhibit high resonance frequencies of more than 1 MHz in liquids1,2.
The plate itself is not suitable as probe for high-speed AFM applications of soft or biological
samples, since the spring constant kp of two-dimensional plate modes can be in the order of 104
N/m. To circumvent this problem, we attach a short narrow beam at the centre of the free end
of the plate. The beam consists of a soft material such as gold which leads to a low spring
constant kb of less than 0.1 N/m. Consequently, the effective stiffness keff of the two coupled
microstructures in series is consistently lower than kp or kb. The beam equipped with a tip scans
the surface. While scanning, the plate oscillates at a resonance frequency of a non-conventional
mode and the attached short cantilever quasi-statically follows the movement of the plate. To
demonstrate the performance of proposed probes, we performed high-speed AFM
measurements of a biological sample in a buffer solution, i.e. in a liquid environment. We
demonstrate for the first time high-speed AFM measurements using a scanning probe with a
high resonance frequency of 1.02 MHz in the buffer solution. We imaged an area of 200 nm x
200 nm with 2 frames per second (see Fig. 1c). We anticipate that numerous AFM applications
like high-speed AFM or high-speed force spectroscopy will greatly benefit from the properties
of the novel probe design.