NCAFM2023 Programme Booklet

FLAT PLATE AFM FOR FORCE MEASUREMENTS

Eugene Soh (Presenting) , Sean O’Shea*

1 Institute of Materials Research and Engineering, Agency for Science, Technology and Research, Singapore, 2 Fusionopolis Way, Singapore 138634

Email: s-oshea@imre.a-star.edu.sg

We are exploring methods to create AFM tips consisting of large flat plates (with dimensions of order 100 µm) adhered to the end of the cantilever. Various approaches will be described such as forming detachable connections by “gluing” liquid metal tips on plates via capillary forces (see Figure 1a,b) or welding liquid metal tips to gold coated plates [1], and forming permanent connections using conventional gluing with adhesives. The problem of the relative tilt between the surfaces is not yet resolved, but feasible approaches and ongoing experiments to solve this issue are discussed. The rational of the work is there are several areas that can utilize the technology for enhanced force measurements. We focus on three topics, namely Casmir forces [2], friction and surface forces in liquid environments [3]. To date the measurement of Casimir forces is done in a sphere-on-flat geometry but if a flat plate geometry could be implemented the force sensitivity would increase by around two orders (see Fig. 1c) enabling considerable precision in the measurement of this weak, long range force, both in vacuum and liquid environments. Our aim of using flat plate AFM in friction and liquid surface force experiments is to address the issue of bridging data from the nanoscale i.e., the single AFM tip, to real world meso-macroscale objects.

Fig. 1 Optical images of an AFM cantilever and 2µm thick Si sled (~250x250µm) (a) on a Si substrate and (b) after picking up/ lifting off the Si substrate using a Gallium coated tip. c) Simple calculations of the long range forces between two 100µm metal surfaces in vacuum showing the flat-flat forces are much larger at similar distance compared to the sphere-flat case. References [1] E. J. H. Soh et al., “AFM Manipulation of EGaIn Microdroplets to Generate Controlled, On-Demand Contacts on Molecular Self-Assembled Monolayers,” ACS Nano, vol. 16, no. 9, pp. 14370–14378, 2022, doi: 10.1021/acsnano.2c04667. [2] J. N. Munday and F. Capasso, “Precision measurement of the Casimir-Lifshitz force in a fluid,” pp. 1–6, 2007. [3] P. J. Van Zwol and G. Palasantzas, “Repulsive Casimir forces between solid materials with high-refractive-index intervening liquids,” Phys. Rev. A - At. Mol. Opt. Phys., vol. 81, no. 6, pp. 1–9, 2010, doi: 10.1103/PhysRevA.81.062502.

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