You no longer need to apply a thick layer of castor oil to do B-ultrasound, soft skin-like photoelectric devices, extremely microwave nano-robots... These seem to be full of dreamy imaginations, but because of the world’s first “metal-free perovskite” The advent of "type ferroelectric" material has become a reality.

On July 13th, Southeast University‘s Xiong Rengen team and You Yumeng team made important research progress in the field of "molecular ferroelectric materials". Their relevant research results were titled "metal-free three-dimensional perovskite ferroelectrics" in the world‘s top academic journals "Science" (Science) magazine published online.

Before understanding this result, we must first know what perovskite, ferroelectric and chirality are. Otherwise, you must come in with a bewildered look and go out with a bewildered look.

Perovskite

Perovskite in a broad sense refers to a type of ceramic oxide. The structural formula of perovskite materials is generally ABX3, where A and B are two cations, and X is an anion. This peculiar crystal structure gives it many unique physical and chemical properties, such as light absorption, electrocatalysis, etc., and has many applications in the fields of chemistry and physics. The perovskite family now includes hundreds of substances, ranging from conductors, semiconductors to insulators, and many of them are artificially synthesized. Perovskite is now the new favorite of photovoltaic cells, and the photoelectric conversion efficiency of organic-inorganic hybrid perovskite solar cells exceeds 20%.

In nature, there are mainly two common perovskite materials: inorganic perovskite and organic-inorganic hybrid perovskite. However, both types of perovskite materials contain metal elements, which increases the difficulty of their processing and preparation, and the processed materials have poor flexibility. Some metal elements (such as lead) can cause serious problems. Environmental pollution.

Ferroelectric

Ferroelectric is a material that has a certain "memory" to external electrical stimulation. In fact, this type of material does not contain iron, but has ferroelectricity. The term ferroelectricity comes from ferromagnetism. The “iron” here has nothing to do with the iron in steel and refers to spontaneous magnetization or spontaneous polarization. It was found that the hysteresis loop of ferroelectrics was similar to the hysteresis loop, so it was named ferroelectricity.

In recent years, the application prospects of ferroelectrics have become more and more promising, such as ferroelectric storage. Traditional storage media such as magnetic disks and magnetic cards will fail if exposed to a strong magnetic field. Ferroelectric storage does not have this problem. It can also be used for image display. At present, some transparent ferroelectric ceramic devices have been developed, such as display devices, light valves, holographic devices, etc., which use an external electric field to make ferroelectric domains oriented to a certain extent. In short, in the field of optoelectronics, ferroelectrics are a kind of emerging "potential stocks."

Chirality

The so-called chirality means that an object cannot overlap with its mirror image. The two are extremely similar but never overlap. They may even have different personalities and cognitions. For example, our hands, the left hand and the right hand that are mirror images of each other do not overlap. There are also many compound molecules that exist in nature, with the same atomic composition, and the two molecular structures are no different from the plane. However, if there is a slight change in the three-dimensional structure of the space, the physiological and pharmacological activities may be very different. Even the left-handed molecule can cure diseases, and the right-handed molecule may be fatal.

Chirality can be described as a clever "cryptographic switch" designed by nature. In all aspects of nature, especially in physics and chemistry, there are many symmetrical concepts: negatively charged electrons and positively charged reverses. Electrons, the south and north poles of the magnetic field, as well as the decomposition and combination reactions in chemistry. Even distant galaxies outside the galactic system also have positive and counter-rotating vortex structures. The achievements of Mr. Yang Zhenning and Mr. Li Zhengdao who won the Nobel Prize in 1957 are related to chirality.

Now we already know that the application of perovskite materials is very common, from daily necessities lighters to space shuttles and other advanced equipment need to use such materials. However, both types of perovskite materials contain metallic elements and have natural defects. For many years, people have been looking for the third category in the perovskite family-all organic perovskite materials, that is, metal-free perovskites.

"Let the perovskite material get rid of the metal‘s‘tightening curse‘, in exchange for a more precious freedom." Professor Yu Yumeng of Southeast University introduced that when all the metal cations in the material are replaced by molecules, the material will not only show Some "magic" properties are developed, and the difficulty of preparation and processing will be greatly reduced, which is more suitable for the requirements of the film, flexibility, and biocompatibility of materials in the new era.

As early as 2002, the top international academic journal "Science" predicted the novel properties of metal-free perovskite materials. Among them, the use of chiral molecules to assemble metal-free perovskite materials with chiral enantiomers is clearly proposed. However, scientists from all over the world have worked hard for decades and have failed to make a breakthrough.

The scientific research team of Southeast University cleverly used dynamic crystal engineering and the highly creative "spherical and non-spherical" design theory to make charged molecular groups replace inorganic ions, and successfully prepared a large class of 23 fully organic new types. Perovskite materials. Among them, a total of 17 materials show good ferroelectricity. At the same time, similar to inorganic perovskite ferroelectrics, this new type of metal-free perovskite ferroelectrics also have multiple polarization axes, which also brings huge potential for thin film applications.

What‘s more worth mentioning is that the team synthesized the left-handed enantiomers, right-handed enantiomers and racemic compounds of four materials, and proved their ferroelectricity. In history, there have never been reports of such left-handed, right-handed, and achiral compounds simultaneously having ferroelectricity. Combining the changeable structure and composition of all-organic perovskite materials, unique ferroelectric properties and the difference of chiral enantiomers to light, these materials will also be used in data storage, logic operations, optical quantum communications, optical radar, and energy It has emerged in applications such as conversion.

This research result was published by "Science", which not only turned the prediction made more than ten years ago into reality, and added new members to the important material family of perovskite, but also brought to the research of ferroelectric materials. The new ideas and directions also mark that our country has once again been in the forefront of the world in the field of molecular materials.

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