Slippery ferroelectricity in the crystal: the crowd is looking for him for a thousand hundred degrees

Recently, Professor Zhang Yi’s team cooperated with Professor Dong Shuai’s team to discover the slip ferroelectricity in a large band gap and material stable organic-inorganic hybrid system (15-crown ether-5-tri-(cadmium chloride)), and obtained certain evidence such as its hysteresis loop.

On August 4, 2022, the work was published in the journal Nature Materials under the title “Direct observation of geometric and sliding ferroelectricity in an amphidynamic crystal.” Professor Zhang Yi and Professor Dong Shuai are co-corresponding authors; Dr. Miao Leping, Ding Ning and Dr. Wang Na are co-first authors. Associate Professor Shi Chao, Professor Ye Hengyun, Associate Researcher Li Linglong and Professor Yao Yefeng participated in this work.

With the rapid development of micro-nanoelectronics technology, the requirements for ferroelectric materials are becoming more and more demanding, and lower power consumption, higher integration, higher speed, and more fatigue resistance have become the standard, and it is best to have both flexibility, environmental protection, and compatibility with semiconductor processes, etc., which has provoked ferroelectric people to continue to explore. Especially for perovskite ferroelectric oxides, when the thickness of the film is reduced to several atomic sizes, the iron polarization of the material is usually greatly weakened or even disappeared, also known as the “dead layer”. This is mainly due to the surface destruction of the crystal structure of the perovskite, so it is difficult to avoid.

Therefore, researchers began to turn their attention to van der Waals layered materials, because van der Waal layered materials naturally have a passivated surface, even if torn into a single layer, they can retain the properties of the block material to the greatest extent. However, at present, the vast majority of two-dimensional ferroelectric materials do not seem to have surpassed the original knowledge system of ferroelectric physics from the physical mechanism.

Is there a two-dimensional ferroelectric origin that is completely different from the traditional iron motor? There really is! In 2017, Professor Wu Menghao of Huazhong University of Science and Technology proposed the concept of slip-shift ferroelectric power[ACS Nano 11, 6382 (2017)]which is a new ferroelectric mechanism. He pointed out that two-dimensional materials, such as hexagonal boron nitride, molybdenum disulfide, etc., although their single layers are central symmetrical structures, and ferroelectricity has no edge, but in its double or multi-layer, some stacking methods can destroy the symmetry of space inversion and produce electrodeization of extra-surface iron. What’s more, this surface polarization can be flipped by horizontal slippage between layers, hence the name sliding ferroelectricity, as shown in Figure 1. Further, in addition to horizontal slippage between layers, interlayer corners can also play a similar effect, that is, Moire ferroelectricity.

Figure 1: Schematic diagram of slip-shift ferroelectric power. Schematic diagram of the AB and BA stacking structure and iron polarization of bilayer BN, in which the nitrogen atoms and boron atoms distort the 2pz orbitals of the nitrogen atoms vertically, resulting in extra-surface iron polarization. Source: Science 372, 1458 (2021).

Since the 1.0 era of ferroelectricity, the most standard (traditional + direct + decisive) experimental evidence for ferroelectricity is the hysteresis loop, which characterizes the core element of ferroelectricity: reversible spontaneous polarization. But since the 3.0 era of Ferroelectric, this standard seems to be becoming more and more difficult to meet. Existing ways of characterizing ferroelectricity, whether it is electric transmission, or second harmonics, or atomic force microscopy, can only give indirect evidence of ferroelectricity, which is strictly speaking a necessary and insufficient evidence of ferroelectricity. The core evidence of ferroelectricity, that is, the standard hysteresis loop, is not yet available. In fact, in addition to P. Professor Jarillo-Herrero has given a polarized valuation of the BN bilayer[Science 372, 1458 (2021)]and no other experimental work has given them how much sliding iron polarization is. These double-layer or small-layer materials are so thin, the general band gap is very small or even metal at all, the leakage current must be very large, the influence of the surrounding substrate is difficult to avoid, and the polarization of the slip ferroelectric itself is surprisingly small (the theoretical value is about 0.1-1 μC/cm2 order of magnitude), so it is not easy to get its ferroelectric loop. This is an experimental confirmation of a nascent concept such as skidding ferroelectricity, and everything seems to be ready, and only the east wind is owed.

So another way of thinking, can it be confirmed in the crystal that slip ferroelectric? If you can, then the traditional ferroelectric laboratory can show its skills in this emerging direction, and there is no need to come to Yuanmuyu again. In fact, in Professor Wu Menghao’s original prediction, the slip ferroelectricity can exist in a quasi-two-dimensional system such as Van Der Waals block.

Along this route, Professor Zhang Yi’s team cooperated with Professor Dong Shuai’s team to discover the slip ferroelectricity in a large band gap and material-stable organic-inorganic hybrid system[15-crown ether-5-tri-(cadmium chloride)]and obtained certain evidence such as its hysteresis loop. As shown in Figure 2, this van der Waal crystal is colorless and transparent (highly insulated). Its monolayer consists of An organic crown ether ring strung together by An inorganic chain of CdCl2, stacked abs in the b direction of the crystal.

Fig. 2: Schematic diagram of the crystal structure of 15-crown ether-5-tri-(cadmium chloride). a, single crystal photo. b, Schematic diagram of the structure. Source: Nat. Mater. (2022)。

As shown in Figure 3, by measuring the second harmonic, dielectric function, etc., it can be proved that the polar phase transition of the system has occurred above room temperature. Pyroelectric measurements, hysteresis loops, and ferroelectric domain characterization further demonstrate their ferroelectricity. Experiments confirmed that its polarization was in the direction of crystal b, that is, in the direction of van der Waals.

Figure 3: Ferroelectric characterization. a, SHG signal. b, the real part of the dielectric constant. c, Pyroelectric current (bright line) and iron polarization (dark line). d, current-electric field loop and polarization-electric field loop. E, PFM characterization and ferroelectric domain walls. Source: Nat. Mater. (2022)。

The above experimental evidence gives strong evidence of the ferroelectricity of the crystal, but the question is, is this ferroelectricity a slip-shift ferroelectric? After all, there are many organic-inorganic hybrid ferroelectric materials, such as the first ferroelectric material Luoshi salt. And their ferroelectric origins are mostly quite traditional.

Let’s clarify the origin mechanism of its ferroelectric power. First of all, in the high temperature paraelectric state, the five oxygen ions in a single crown ether ring are disordered in ten possible positions (50% occupied), and the center is inverted symmetrically, while the oxygen ions in a single crown ether ring in the low-temperature ferroelectric phase are frozen, occupying five positions in an orderly manner. Because the number of oxygen ions is odd, such a freeze leads to the spatial inversion symmetry breaking, resulting in a single layer of ferroelectricity, also known as geometric ferroelectricity (Figure 4a).

Calculating the polarization of the single layer, it was found that in the a and c directions of the crystal, the polarization of the A and B layers was exactly canceled out, and the net polarization synthesized by the polarization vector synthesis of the two layers was along the direction of the crystal b, which coincided with the experiment. However, the polarization value of its synthesis is significantly less than the total iron polarization, which means that in addition to geometric ferroelectricity, there are other contributions of ferroelectric origin.

Combined with the single crystal experimental structure and the first principle calculation, the authors found that there was a significant internal slip in the ac plane between the A and B layers before and after the ferroelectric transition, and the slip led to the polarization of the crystal in the b direction, which was positively superimposed with the geometric iron polarization, and the contribution ratio of the two to the total polarization was 57% (slip) and 43% (geometric), respectively. The calculated total polarization value is highly consistent with the polarization value obtained by the ferroelectric loop, which is about 0.4 μC/cm2.

Figure 4: Schematic diagram of geometry and slip ferroelectricity. a, the relative ratio of the paraelectric phase and ferroelectric phase of the coronether ring ion arrangement. b, Slippage of the CdCl2 skeleton. The solid part is the paraelectric phase, and the dashed part is the ferroelectric phase (±P). All displacements are relative to the Cd1 ion. The green and red arrows in the figure indicate the direction of slippage (not polarization). Source: Nat. Mater. (2022)。

At this point, the origin of ferroelectricity of 15-crown ether-5-tri-(cadmium chloride) has been revealed, which provides a strong support for the new branch of ferroelectric power of slip ferroelectricity. This work has found a simple and reliable way to characterize and utilize the ferroelectricity of skidding, and has also extended the geometric ferroelectricity and sliding ferroelectricity to the field of molecular ferroelectricity, deepening the scientific understanding of molecular ferroelectricity. (Source: Science Network)

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