Quinuclidinium salt ferroelectric thin-film with duodecuple-rotational polarization-directions

Ferroelectric thin-films are highly desirable for their applications on energy conversion, data storage and so on. Molecular ferroelectrics had been expected to be a better candidate compared to conventional ferroelectric ceramics, due to its simple and low-cost film-processability. However, most molecular ferroelectrics are mono-polar-axial, and the polar axes of the entire thin-film must be well oriented to a specific direction to realize the macroscopic ferroelectricity. To align the polar axes, an orientation-controlled single-crystalline thin-film growth method must be employed, which is complicated, high-cost and is extremely substrate-dependent. In this work, we discover a new molecular ferroelectric of quinuclidinium periodate, which possesses six-fold rotational polar axes. The multi-axes nature allows the thin-film of quinuclidinium periodate to be simply prepared on various substrates including flexible polymer, transparent glasses and amorphous metal plates, without considering the crystallinity and crystal orientation. With those benefits and excellent ferroelectric properties, quinuclidinium periodate shows great potential in applications like wearable devices, flexible materials, bio-machines and so on.

This work discovered the new molecular ferroelectrics of quinuclidinium periodate, which can be readily prepared on various substrates without the concern on the film crystallinity and crystalorientation. The multi-axes (six polar axes) molecular ferroelectrics exhibits the dendritic growth pattern and 12-fold polarization direction. As a result, the thin films with switchable polarization possess great application potentials. There are a great many of outstanding points of this work, such as the simple preparation of aqueous solution processing and the flexible structure on ITO-PET electrode substrate. In addition, the micrometer scale thin-film can be fabricated on various substrates, including the flexible polymers with high uniformity. I strongly recommend this work for the publication.
There are two minor comments for the authors' consideration: 1) It may be helpful to add the rational of smaller remnant polarization (currently discussed in the supporting information as compared to the theoretically predicted one) into the main text. 2) The author may consider add a few sentences to discuss the strategies to reduce the coercive field of the films, though six polar axes of quinuclidinium periodate.
The manuscript has been revised appropriately on the basis of two reviewers' comments. The manuscript is worthy for publication after addressing the following issues.

Minor issues
Color and atoms should be correlated in Fig. 1; N (blue) is not included in the revised Figure (Fig. 1).
It would be useful to include the elemental analysis (CHN) of 1 in the Materials section.
Reviewer #2 (Remarks to the Author): The authors have addressed and revised the manuscript, which is ok for the publication. Author's response: We would like to thank the reviewer for his/her positive remark on the importance of our manuscript, and also for their suggestions to help us improve the quality of this manuscript as well as our future research indeed. Our point-to-point response are listed below in blue color.
I have several concerns listed as follows, which should be addressed.
The present study is closely related to the work by Harada et al. (ref.17), who reported directionally tunable ferroelectric crystals in quinuclidinium perrhenate. Therefore, a detailed comparison between the author's and Harada's work is necessary. Note that it would be helpful to readers if the authors clearly state the novelty of their work, especially, when compared with Harada's results.
Author's response: We have added the comparison between those two compounds in the main text (page 4) featuring following points 1) The ferroelectric phase of 1 has a broader temperature range (< 322 K) than that of quinuclidinium perrhenate (345-367 K).
A detailed comparison of ferroelectric properties between 1 and quinuclidinium perrhenate were listed in Table S1 in Supporting Information, also shown below.
Description of the novelty of 1 was also included in the introduction part (page 4). Therefore, the present study should focus more on the characterization of ferroelectric thin films.
Author's response: We do agree that the thin-film study is the significance of our work. The six-fold rotational polar-axis is another major concern. Single crystal analysis shows that the quinuclidinium periodate has a six-fold rotational polar-axis. This suggests that an identical P-E curve should be observed when an electric field is applied from any of the six directions; however, it is not directly demonstrated by experiment. The paper could be strengthened by performing P-E curve measurements using single crystal in at least two different directions.
Author's response: The reviewer made an excellent suggestion on demonstration of the six-fold rotational polar-axes by measuring macroscopic P-E loop in different directions.
However, for measuring single crystal sample, the relative large coercive field (255 kV/cm) limits the dimension of sample to be much smaller than 100 µm, with highest voltage of 2 kV we can supply. Thus it would be extremely difficult to perform the measurement along two different directions on a <100 µm crystal.
On the other hand, since poly-crystalline thin-film samples were prepared without any control of crystallinity, the crystallographic orientations are random over the thin-film. By successfully obtained nice P-E hysteresis loop and observation of polarization reversal on thin-film samples, we have already demonstrated that the macroscopic ferroelectricity can be observed along different crystallographic direction.

Minor issues
Color and atoms should be correlated in Fig. 1.

Figures 2a and 2b
should be plotted for the same temperature range, for example, between 280 and 360 K to enable comparison.
Color in Fig. 6 (Phase) appears to be red, and not purple.
In the Berry phase calculation section, " in Figure 3" should be revised to " in Figure S1." It would be helpful to provide the definition of λ in Fig. S1.
Label for the y-axis should be added to Fig. S2.
"Heigt" in Fig. S7b should be revised to "Height." Author's response: This work discovered the new molecular ferroelectrics of quinuclidinium periodate, which can be readily prepared on various substrates without the concern on the film crystallinity and crystal-orientation. The multi-axes (six polar axes) molecular ferroelectrics exhibits the dendritic growth pattern and 12-fold polarization direction. As a result, the thin films with switchable polarization possess great application potentials. There are a great many of outstanding points of this work, such as the simple preparation of aqueous solution processing and the flexible structure on ITO-PET electrode substrate. In addition, the micrometer scale thin-film can be fabricated on various substrates, including the flexible polymers with high uniformity. I strongly recommend this work for the publication.
Author's response: The authors would like to express our appreciation for reviewer's positive comments on the scientific significance of our work. The reviewer's suggestions are very valuable and helpful for us in the aspect of improving the quality of our manuscript. Below, we response to the comments in blue color.
There are two minor comments for the authors' consideration: 1) It may be helpful to add the rational of smaller remnant polarization (currently discussed in the supporting information as compared to the theoretically predicted one) into the main text.
2) The author may consider add a few sentences to discuss the strategies to reduce the coercive field of the films, though six polar axes of quinuclidinium periodate.