Page 25 - Read Online

Page 10 of 12 Zhang et al. Microstructures 2023;3:2023010 https://dx.doi.org/10.20517/microstructures.2022.39

surface, the electric field converges towards the probe tip, as the inset indicates. The Cu ions migrate to the
probe and decrease the Schottky barrier between the probe and the sample. The effective electric field
directly acting on the CIPS phase is enough to switch the domain, and the yellow domain is switched to the
black domain. On the contrary, when a voltage of +6 V is applied, the direction of the electric field diverges
from the probe, and the Cu ions are driven away from the tip, and the effective electric field acting on the
CIPS phase therefore cannot switch domain. As a result, all domains did not change significantly, and the
overall piezoelectric response decreased. To confirm ion mobility in CIPS-IPS, we characterized the
macroscopically manifested I-V curves, and the results are shown in Figure 4F. In the upward polarization
region, we can find that the forward current gradually increases as the rate of the applied bias cycle
decreases. Likewise, in the downward polarization region, the forward current is also large at lower voltage
sweep speeds. Nevertheless, with the increase of scanning speed, the whole forward current decreases until
zero, while the reverse current gradually increases and tends to be stable. All suggest that Cu ion migration
dominates the current feature, which is consistent with our previous studies on current regulation in pure
CIPS . These results favorably confirm that Cu ions mediate the switching of ferroelectric domains.
[16]

CONCLUSIONS
In conclusion, we accurately characterized the Young’s modulus of the CIPS-IPS two phases for the first
time by various experimental methods (nanoindentation method and atomic force contact resonance
method) in this study, and the Young’s modulus of the CIPS phase was 27.42 ± 0.05 GPa, slightly less than
that of the IPS phase, which was 27.51 ± 0.04 GPa. In addition, we calculated the elastic matrices of the two
phases of CIPS-IPS using the first-principles method, and deduced their respective Young’s modulus, all of
which are in good agreement with our experimental values. Finally, we also discover the asymmetry of
domain switching and propose an ion-mediated nonreciprocal domain switching model, which strongly
explains this interesting phenomenon. Our work provides a reliable experimental reference for the follow-
up study of the elastic properties of CIPS-IPS and the phase field simulation for regulating the domain
structure.

DECLARATIONS
Authors’ contributions
AFM characterization, writing original draft: Zhang X
Review & editing, supervision: Jiang X
Raman characterization: Du G
DFT calculation: Ren Q
EDS characterization: Zhu W
Sample synthesis: Kang J, Deng J
Guidance for experiment and data analysis: Lun Y, Wang T, Bai B, Yu Z
Conceptualization, review, supervision: Hong J, Wang X, Chen Y

Availability of data and materials
The list of elemental atomic percentages of different CIPS flakes obtained from EDS, single-point CRF
measurement results of CIPS-IPS and HOPG by CR-AFM, CR-AFM method, A: frequency and friction
histogram and original PFM data are provided in Supplementary Information.

Financial support and sponsorship
The work at Beijing Institute of Technology is supported by National Natural Science Foundation of China
with Grant Nos. 12172047, 11604011, 92163101, 12202056, National Key Research and Development
Program of China (2019YFA0307900), the Beijing Natural Science Foundation (Z190011), and Beijing
Institute of Technology Research Fund Program for Young Scholars.