Traditional electrochemical characterization microscopy methods based on charge measurement do not provide nanometer spatial resolution. The recently developed electrochemical scanning microscopy (ESM) which is based on atomic force microscopy (AFM) provides nanoscale measurements, however, the electrochemical measurements consist of other effects such as electromechanical and electrostatic effects.
We developed a scanning thermo-ionic microscopy (STIM) to probe local electrochemical activities at the nanoscale regime. The microscopy mechanism is based on imaging the Vegard strain induced by thermally driven stress and temperature oscillation. The Vegard strain linearly correlates with material lattice constant and can be used as a measure of ionic species concentration. Through theoretical analysis and experimental validation, we have demonstrated that second and fourth harmonic components of the AFM deflection signal contains information about species concentration. It is demonstrated that the second harmonic response predominantly correlates with the local thermal expansion information, while the fourth harmonic one is characteristic of local transport activities that is presented only in ionic systems. All our measurements are resonance enhanced and since the tip-sample resonance varies during scanning, four lock-in units and a PID controller are integrated with the AFM to track the resonance frequency. The technique has been applied to probe Sm-doped nanocrystalline Ceria and LiFePO4, both of which exhibit higher STIM amplitude near grain boundaries as expected.
The STIM is an innovative tool to study local electrochemistry with high sensitivity and spatial resolution for a wide range of systems, without any electrical cross-talk which makes it suitable to be applied in operando.
Ehsan Nasr Esfahani and Jiangyu Li, "Scanning thermo-ionic microscopy (Conference Presentation)," Proc. SPIE 10165, Behavior and Mechanics of Multifunctional Materials and Composites 2017, 101650H (Presented at SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring: March 27, 2017; Published: 10 May 2017); https://doi.org/10.1117/12.2266971.5427412054001.
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