Perovskites are among the most researched topics in materials science. Recently, a research team led by Prof. LOH Kian Ping, Chair Professor of Materials Physics and Chemistry and Global STEM Professor of the Department of Applied Physics of The Hong Kong Polytechnic University (PolyU), Dr Kathy LENG, Assistant Professor of the same department, together with Dr Hwa Seob CHOI, Postdoctoral Research Fellow and the first author of the research paper, has solved an age-old challenge to synthesise all-organic two-dimensional perovskites, extending the field into the exciting realm of 2D materials. This breakthrough opens up a new field of 2D all-organic perovskites, which holds promise for both fundamental science and potential applications. This research titled “Molecularly thin, two-dimensional all-organic perovskites” was recently published in the journal Science.
Perovskites are named after their structural resemblance to the mineral calcium titanate perovskite, and are well known for their fascinating properties that can be applied in wide-ranging fields such as solar cells, lighting and catalysis. With a fundamental chemical formula of ABX3, perovskites possess the ability to be finely tuned by adjusting the A and B cations as well as the X anion, paving the way for the development of high-performance materials.
While perovskite was first discovered as an inorganic compound, Prof. Loh’s team has focused their attention on the emerging class of all-organic perovskites. In this new family, A, B, and X constituents are organic molecules rather than individual atoms like metals or oxygen. The design principles for creating three-dimensional (3D) perovskites using organic components have only recently been established. Significantly, all-organic perovskites offer distinct advantages over their all-inorganic counterparts, as they are solution-processible and flexible, enabling cost-effective fabrication. Moreover, by manipulating the chemical composition of the crystal, valuable electromagnetic properties such as dielectric properties, which finds applications in electronics and capacitors, can be precisely engineered.
Traditionally, researchers face challenges in the synthesis of all-organic 3D perovskites due to the restricted selection of organic molecules that can fit with the crystal structure. Recognising this limitation, Prof. Loh and his team proposed an innovative approach: synthesising all-organic perovskites in the form of 2D layers instead of 3D crystals. This strategy aimed to overcome the constraints imposed by bulky molecules and facilitate the incorporation of a broader range of organic ions. The anticipated outcome was the emergence of novel and extraordinary properties in these materials.
Validating their prediction, the team developed a new general class of layered organic perovskites. Following the convention for naming perovskites, they called it the “Choi-Loh-v phase” (CL-v) after Dr Choi and Prof. Loh. These perovskites comprise molecularly thin layers held together by forces that hold graphite layers together, the so-called van der Waals forces — hence the “v” in CL-v. Compared with the previously studied hybrid 2D perovskites, the CL-v phase is stabilised by the addition of another B cation into the unit cell and has the general formula A2B2X4.
Using solution-phase chemistry, the research team prepared a CL-v material known as CMD-N-P2, in which the A, B and X sites are occupied by CMD (a chlorinated cyclic organic molecule), ammonium and PF6− ions, respectively. The expected crystal structure was confirmed by high-resolution electron microscopy carried out at cryogenic temperature. These molecularly thin 2D organic perovskites are fundamentally different from traditional 3D minerals, they are single crystalline in two dimensions and can be exfoliated as hexagonal flakes just a few nanometres thick — 20,000 times thinner than a human hair.
The solution-processibility of 2D organic perovskites presents exciting opportunities for their application in 2D electronics. The Poly U team conducted measurements on the dielectric constants of the CL-v phase, yielding values ranging from 4.8 to 5.5. These values surpass those of commonly used materials such as silicon dioxide and hexagonal boron nitride. This discovery establishes a promising avenue for incorporating CL-v phase as a dielectric layer in 2D electronic devices, as these devices often necessitate 2D dielectric layers with high dielectric constants, which are typically scarce. Team member Dr Leng successfully addressed the challenge of integrating 2D organic perovskites with 2D electronics. In their approach, the CL-v phase was employed as the top gate dielectric layer, while the channel material consisted of atomically thin Molybdenum Sulfide. By utilising the CL-v phase, the transistor achieved superior control over the current flow between the source and drain terminals, surpassing the capabilities of conventional silicon oxide dielectric layers.
Prof. Loh’s research not only establishes an entirely new class of all-organic perovskites but also demonstrates how they can be solution-processed in conjunction with advanced fabrication technique to enhance the performance of 2D electronic devices. These developments open up new possibilities for the creation of more efficient and versatile electronic systems.
