Highly uniform nanocrystals synthesized by liquid crystalline antisolvent

A research team led by Professors Young-Ki Kim and Yong-Young Noh at POSTECH has developed a groundbreaking method for synthesizing perovskite nanocrystals (PNCs), a next-generation semiconductor material, in a more uniform and efficient manner. This study is expected to serve as a key breakthrough in overcoming the complexities of conventional synthesis methods and accelerating the commercialization of various optoelectronic devices, such as light-emitting diodes (LEDs) and solar cells, that utilize nanocrystals.

This study was conducted by Professor Young-Ki Kim and Professor Yong-Young Noh from the Department of Chemical Engineering at POSTECH, along with Ph.D. candidate Jun-Hyung Im, Dr. Myeonggeun Han (Samsung Electronics), and Dr. Jisoo Hong (Princeton University). The research was recently published in ‘ACS Nano’, an international journal in the field of nanotechnology.

PNCs have great potential in next-generation solar cells and high-efficiency displays, as their ability to absorb and emit light can be precisely controlled based on particle size and shape through the ‘quantum confinement effect.’ However, conventional methods used to synthesize PNCs such as ‘hot-injection’ and ‘ligand-assisted reprecipitation (LARP)’ have limitations in producing uniformly sized and shaped particles due to high synthesis temperatures and complex experimental conditions. As a result, additional processing steps were required to obtain particles with the desired properties, which in turn reduced productivity and restricted industrial applications.

The POSTECH research team has developed a synthesis method that precisely controls the size and shape of PNCs using a ‘liquid crystal(LC)’ as an antisolvent in the LARP method. LC is an intermediate phase of matter that possesses both liquid-like fluidity and crystal-like long-range molecular ordering. In LC phases, molecules are aligned to a preferred orientation (defined by the director), which leads to elasticity. Therefore, when an external force is applied to an LC medium, LC molecules are reoriented, producing considerable elastic strains. Inspired by this property, the team precisely controlled the growth of PNCs by simply replacing the antisolvent in the conventional LARP method with LC while maintaining the other synthesis conditions. The elastic strains of LCs restricted the growth of PNCs upon reaching the extrapolation length (ξ) of LCs, enabling mass production of uniformly sized PNCs without the need for additional purification processes.

The research team also discovered that the interaction between ligands binding to the surface of PNCs and LC molecules plays a crucial role in reducing surface defects. Since LC molecules have a long, rod-like structure, ligands can be densely arranged between them. As a result, ligands bind more densely to the surface during nanocrystals formation, thereby minimizing surface defects and enhancing luminescence properties.

Professor Young-Ki Kim explained, “The synthesis method developed by our research team is highly compatible with existing synthesis techniques, such as ligand exchange and microfluidic synthesis, and will enhance the performance of various optoelectronic devices, including LEDs, solar cells, lasers, and photodetectors.” He also stated, “This technology enables the large-scale production of uniform, high-performance nanocrystals at room temperature, and we anticipate it will help accelerate the commercialization of nanocrystal-based optoelectronic devices.”

This research was supported by the Basic Research Program (Hanwoomul-Phagi Basic Research) and the Pioneer Program for Promising Future Convergence Technology of the National Research Foundation of Korea (NRF).