A universal challenge facing the development of electrochemical materials is our lack of understanding of physical and chemical processes at local length scales in 10-100 nm regime, and acquiring this understanding requires a new generation of imaging techniques that can resolve local chemistry and fast dynamics in electrochemical materials at the time and length scales relevant to strongly coupled reaction and transport phenomena. In this article, we introduce the scanning thermo-ionic microscopy (STIM) technique for probing local electrochemistry at the nanoscale, utilizing Vegard strain induced via thermal stress excitations for imaging. We have implemented this technique using both resistive heating through a microfabricated AFM thermal probe, as well as photo-thermal heating through a 405 nm laser, and have applied it to a variety of electrochemical materials. The dynamics of ionic motion can be captured from point-wise spectroscopy studies, while the spatial inhomogeneity can be revealed by STIM mapping. Since it utilizes thermal stress-induced oscillation as its driving force, the responses are insensitive to the electromechanical, electrostatic, and capacitive effects, and is immune to global current perturbation, making in-operando testing possible. In principle, STIM can provide a powerful tool for probing local electrochemical functionalities at the nanoscale.

1. Scanning Thermo-ionic Microscopy
Ehsan Nasr Esfahani
Prof. Jiangyu Li
Multifunctional Material Lab
University of Washington

3. Population prospect
Broader sources of sustainable energy are needed.
Source: eia.gov (U.S. energy information administration)

4. Electrochemical materials
(Ormerod 2003)
(Bruce, Scrosati et al. 2008)
Li-ion Battery
Electrochemical process governed at nanoscale:
•Ionic defect formation
•Interfacial chemistry and charge transfer
•Phase nucleation and separation

5. Why Nanoscale?
(Chen, Adler, and Li 2014)
500 nm

6. Current methodologies
• I-V curves
• Scanning electrochemical microscopy
• Measuring associated Vegard strain

7. Why Atomic Force Microscope?
• High spatial resolution
• All information gathered at the volume of tip-surface junction
• Access to wide range of bias, thermal and pressure induced phenomena
• Chemistry and physics merge at the nanoscale
Fundamentals of scanning probe microscopy, 2004
Thermal
Stimulus
Potential
Stimulus

8. Electrochemical Strain Microscopy
𝜕𝑐
𝜕𝑡
= 𝛻. 𝐷𝛻𝑐 + 𝛻.
𝐷𝐹𝑧
𝑅𝑇
𝑐𝛻𝜙
(Kalinin and Balke 2010)
PZT LFP Glass
(Chen et al. 2014)

9. Scanning Thermo-ionic Microscopy
• Ionic diffusion model:
𝜕𝑐
𝜕𝑡
= 𝛻. 𝐷𝛻𝑐 + 𝛻.
𝐷𝐹𝑧
𝑅𝑇
𝑐𝛻𝜙
Light
Source
Photo
Detector
Localized heating
9
−𝛻.
𝐷𝛺
𝑅𝑇
𝑐𝛻𝜎ℎ
• Heating:
𝛥𝑇𝐴𝐶[𝜔] = 𝛥𝑇𝑒 𝑖𝜔𝑡
• Concentration
Δc ω = −𝛻.
𝐷𝛺𝑐0
𝑅𝑇0
𝛻𝜎ℎ0 𝑒 𝑖𝜔𝑡 Δc 2ω = 𝛻.
𝐷𝛺𝑐0
𝑅𝑇0
2 𝛥𝑇 𝛻𝜎ℎ0 𝑒 𝑖2𝜔𝑡
• Thermal expansion
𝛥𝛆∗
𝜔 = 𝛼𝛥𝑇𝑒 𝑖𝜔𝑡
𝐈
𝛥𝜎ℎ 𝜔 =
1
3
𝑡𝑟𝐂 𝛥𝛆 𝜔 − 𝛥𝛆∗ 𝜔 = 𝛥𝜎ℎ0 𝑒 𝑖𝜔𝑡

11. Heated AFM probe
Heated probe using solid state resistor Photo-thermal excitation using blueDrive

12. Point-wise STIM and mother nature amplifier

13. Implementation

14. STIM Mappings
Topography Topography Frequency

15. Blue drive STIM
Height Q
Freq.PhaseThermal exp.

16. Spectroscopy

17. Summary
• A technique based on utilizing Vegard strain induced via thermal stress excitations
• Tip is electrically insulated from surface→ allows in-operando
• Electrochemical and thermomechanical properties mapped:

18. Acknowledgment
 National Science Foundation, USA
 Clean Energy Institute, UW
 Chinese Academy of Science, SIAT
References:
• Esfahani, Ehsan Nasr, et al. "Scanning Thermo-ionic Microscopy: Probing Nanoscale Electrochemistry via Thermal
Stress-induced Oscillation." arXiv preprint arXiv:1703.06184 (2017)
• Eshghinejad, Ahmadreza, et al. "Scanning thermo-ionic microscopy for probing local electrochemistry at the
nanoscale." Journal of Applied Physics 119.20 (2016): 205110.