image: Fig. 1. a, Crystal structure of (Bmpip)2SnBr4. b, Photographs of (Bmpip)2SnBr4 powder under illustrations of daylight and UV light (365 nm). c, XRD spectrum of (Bmpip)2SnBr4 powder. d, PL spectrum of (Bmpip)2SnBr4 excited by a 266 nm laser at room temperature. A low-energy Peak 1 and a high-energy Peak 2 can be observed. e, PLE spectra of Peak 1 and Peak 2.
Credit: ©Science China Press
Zero-dimensional organic-inorganic metal halides (0D-OIMH) are an emerging category of materials that have garnered significant attention in the field of optoelectronics, due to their high photoluminescence quantum efficiency (PLQE) and unique optoelectronic properties. These materials typically consist of large organic cations and inorganic anions. By adjusting their organic and inorganic components, a wide range of semiconductor materials with different optoelectronic properties can be synthesized. As they are composed of organic and inorganic components, they inherently possess the advantages of both. Additionally, they can be prepared through a simple low-temperature solution method, which is cost-effective, making them a highly promising class of luminescent materials. Moreover, due to their intrinsic zero-dimensional electronic structure characteristics, the physical mechanisms behind their optical emission are also diverse and fascinating. While, a unified theoretical framework has not yet been established. There has been ongoing debate regarding the electronic structure and spin configuration behind the common dual-peak emission phenomenon observed in 0D-OIMH materials. Traditionally, researchers suggest that the low-energy peak originates from the triplet dark state, while the high-energy peak comes from the singlet bright state. This point of view is directly borrowed from the experience with organic semiconductors, which lacks strong evidence.
Recently, team of Professor Jianpu Wang from the School of Flexible Electronics (Future Technology) at Nanjing Tech University conducted a systematic study on a typical 0D-OIMH material. With the application of low-temperature magneto-optical measurements, they successfully revealed the spin configuration of excited states in the material. This discovery not only provides a new perspective for understanding the electronic structure of 0D-OIMH materials but also has significant implications for the development of new optoelectronic devices, such as light-emitting diodes (LEDs), spin optoelectronic devices, and quantum technologies.
They start from a typical 0D-OIMH material, (Bmpip)2SnBr4. By systematically combining multi-physical field measurements, they found that the (Bmpip)2SnBr4 exhibits an unusual photoluminescence quench at low temperatures, which may result from the emission from a low-energy dark state. To verify the existence of the dark state, the team introduced an external magnetic field perturbation at low temperatures. With the magnetic field disturbance, the team observed a significant brightening of the dark state, which is attributed to the mixing of bright and dark state oscillator strengths, thereby enhancing the emission of the dark state. The accelerated decay dynamics of the dark state also confirmed its "dark exciton" nature. Furthermore, through measuring the degree of polarization under the application of magnetic field, the researchers further clarified the spin configurations of the dark and bright states, proving that the high-energy peak emission is from a pure bright state, while the low-energy peak includes both bright and dark states, and the dark state is a singlet.
These findings refuted the previous assignment of the dual-peak attribution and provided a new perspective for understanding the optical transition mechanism of 0D-OIMH materials. With an in-depth understanding of the optical transition mechanism of these materials, it is expected that more innovations and breakthroughs will be achieved in the field of optoelectronics in the future, which pave the way for exploring their applications in illumination and display as well as in spin optoelectronics.