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The direct visualization of local events leading to the formation of a critical nucleus remains as one of the most formidable challenges in electrodeposition. Recently, state-of-the-art video-rate scanning tunnelling microscopy has been able to follow lateral growth of Bi deposits with exquisite atomic resolution [1]. However, strong local potential bias between tip and substrate often leads to ‘shielding effects’, significantly decreasing local nucleation rates. Substrate-tip interactions in conventional atomic force microscopy also have a strong effect in local nucleation rate. In this contribution, we shall demonstrate a new approach to monitor individual nucleation events based on tracking local perturbations of hydration shells employing high-speed lateral molecular force microscopy (HS-LMFM). 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) |
