Nanoscale mechanical manipulation of ultrathin SiN membranes enabling infrared near-field microscopy of liquid-immersed samples

Abstract: Scattering scanning near-field optical microscopy (s-SNOM) is a powerful technique for mid-infrared spectroscopy at nanometer length scales. By investigating objects in aqueous environments through ultrathin membranes, s-SNOM has recently been extended towards label-free nanoscopy of the dynamics of living cells and nanoparticles, assessing both the optical and the mechanical interactions between the tip, the membrane and the liquid suspension underneath. Here, we report that the tapping AFM tip induces a reversible nanometric deformation of the membrane manifested as either an indentation or protrusion. This mechanism depends on the driving force of the tapping cantilever, which we exploit to minimize topographical deformations of the membrane to improve optical measurements. Furthermore, we show that the tapping phase, or phase delay between driving signal and tip oscillation, is a highly sensitive observable for quantifying the mechanics of adhering objects, exhibiting highest contrast for low tapping amplitudes where the membrane remains nearly flat. We correlate mechanical responses with simultaneously recorded spectroscopy data to reveal the thickness of nanometric water pockets between membrane and adhering objects. Besides a general applicability of depth profiling, our technique holds great promise for studying mechano-active biopolymers and living cells, biomaterials that exhibit complex behaviors when under a mechanical load.