A unique optical feedback mechanism lies at the center of the operating mode of HS-LMFM [2,3]. Under this configuration, the position of a vertically oriented probe (VOP) is tracked by measuring the scattering of the evanescent wave of a laser impinging under total internal reflection from the back of an optically transparent electrode. As the tip approaches the surface oscillating close to its resonant frequency, the oscillation frequency is affected by the visco-elastic properties of the various hydration layers. Changes in the VOP oscillations are optically detected by the scattering evanescent field. This approach enables monitoring interaction with individual hydration layers simultaneously with, but independent of, the optical feedback.
In these experiments, the tip is positioned at a distance in which shear force interactions with hydration layers of an In-doped SnO2 electrode (approximately 2.9 nm roughness level) become negligible [4]. As nucleation events are triggered, the hydration layers around individual nuclei move into a detectable range of the VOP, leading to changes in the oscillation frequency. Shear force maps uncover a highly dynamic landscape prior to the formation of stable nucleus. At constant potential, the stochastic formation and dissolution of nuclei can be observed in the sub-second time scale. The growth of a stable nucleus is also followed in real-time, illustrating the potential of this technique for probing complex phase formation phenomena at electrode surfaces.
References
- Matsushima, H. et al. Faraday Discuss. 193, 171-185 (2016).
- Fletcher, J. M. et al. Science 340, 595-599 (2013).
- Antognozzi, M. et al. Nat. Phys. 12, 731-735 (2016).
- Harniman, R.L. et al. Nat. Commun. 8, 971 (2017)