In various application scenarios such as optical measurement analysis, bioimaging, counterfeit detection, liquid crystal displays, and three-dimensional display technologies, linearly polarized light is a crucial technology.

Among them, anisotropic quantum dots with a single asymmetric shape have been proven to emit strong linearly polarized light, such as nanowires, nanorods, etc. The linear polarization ability of these asymmetric single quantum dots is greater than 70%.

However, due to reasons such as the non-uniformity of size, how to orderly arrange them in thin films and devices remains a problem to be solved.

Based on this, a group of researchers from the United Kingdom and the United States proposed that by controlling the evaporation pressure of the solvent, anisotropic perovskite nanosheet quantum dots can self-assemble to form a controllable-oriented superlattice.

The researchers demonstrated the formation of the self-assembled superlattice and the oriented arrangement through structural characterization (grazing incidence wide-angle X-ray diffraction) and optical characterization (angle-resolved momentum space Fourier microscopy).The orderly arrangement of the transition dipole moments allows the characteristic of high linear polarization of a single quantum dot to be preserved in the device, achieving an electroluminescence polarization degree higher than 70%.

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In other words, researchers have achieved a very narrow luminescence half-peak width by controlling the perovskite cations and reaction temperature, ultimately realizing high linear polarization electroluminescence.

It is expected that this achievement can first be applied to the display industry, where quantum dot materials (QLED, Quantum Dot Light Emitting Diode) have always been considered a strong competitor to current organic light-emitting materials (OLED, Organic Light Emitting Diode).

Currently, QLED display backlights are already on the market, including brands such as Samsung and TCL that have QLED electronic products.

Quantum dots with polarization properties can further improve the energy efficiency of these products and reduce energy loss caused by polarization filter screening.At the same time, for emerging display industries such as virtual reality and augmented reality headset products, polarized light sources and luminescent devices with spatial information can accelerate product iteration. By directly emitting polarized light, the use of polarizing filters can be reduced, thereby reducing the product volume and weight.

Additionally, it is expected to be used in medical imaging as well. Traditional medical detection light requires a high degree of linear polarization to reduce the interference of non-polarized background radiation light, which necessitates high-quality linear polarizers, leading to inevitable energy loss and restricting the spatial management of light polarization.

The use of materials and devices that can directly emit polarized light not only improves energy utilization efficiency but also allows for customized spatial layout, achieving control of light polarization at different spatial positions. This has great application prospects for imaging diagnosis and other applications.

In summary, this work provides pioneering exploration of materials and devices for these potential applications.

Recently, the related paper was published in Nature Photonics with the title "Direct linearly polarized electroluminescence from perovskite nanoplatelet superlattices" [1].Dr. Junzhi Ye from the University of Oxford is the first author, while Professor Robert Hoye from the University of Oxford, Professor Akshay Rao from the University of Cambridge, and Dr. Linjie Dai from the Massachusetts Institute of Technology serve as the co-corresponding authors.

This research topic initially started in the second year of Dr. Ye's doctoral studies (2020), with the primary aim to address the issue of electroluminescence stability in hybrid halide perovskite red-light quantum dot devices.

Perovskite quantum dots that emit red light in accordance with the REC.2020 standard (a luminance standard in the display industry) typically rely on the mixing of iodide (I) and bromide (Br) ions to regulate the bandgap and luminance position.

However, under the working conditions of an applied electric field bias, Br and I can lead to a phase separation problem, forming regions of iodide ion aggregation and bromide ion aggregation. This non-uniform mixture can cause a shift and instability in the emission spectrum.

To tackle this issue, the use of a single iodide halide perovskite quantum dot with a high quantum confinement effect has become one of the hot research directions. Among these, the iodide ion nanosheet is a material system worth considering.Before this work, no literature had realized the LED (Light Emitting Diode, Light Emitting Diode) devices of iodide nanosheets, mainly because the synthesis of this material is not stable enough, and the size uniformity is difficult to control.

Based on this, researchers first optimized the synthesis method, achieving highly uniform and stable CsPbI3 nanosheets, with a longitudinal thickness of only 2.5nm, far smaller than the exciton Bohr radius of this material, meeting the requirements of strong quantum confinement.

In further literature reading and experiments, they realized that this type of asymmetrically shaped nanosheet quantum dot has a higher exciton fine structure splitting, which is expected to achieve highly linear polarization.

However, how to orderly arrange these quantum dots on the thin film and maintain the polarization light-emitting properties is the scientific problem they need to solve.

During this period, Ye Junzhi, in discussion with this paper's co-corresponding author Dr. Dai Linjie, referred to some previous work of traditional CdSe nanosheets and found that solvent evaporation pressure can affect the lying and standing arrangement of nanosheets.Based on these preliminary works, they successfully achieved oriented control and superlattice arrangement on perovskite nanoplates. Moreover, they verified their method through optical and structural characterization, and obtained experimental results in polarization tests of thin films and devices that met expectations.

To complete the exploration of the theoretical part, Ye Junzhi, while preparing to submit his doctoral thesis in 2022, conducted a large number of low-temperature and transient spectroscopy tests to explore the connection between exciton fine structure and polarized luminescence, and contacted the French theoretical calculation team to verify the exciton fine splitting energy observed in their experiments.

The results showed that the low-temperature experimental data highly matched the theoretical values calculated by the quantum physics model. Therefore, while revising his doctoral thesis, Ye Junzhi also completed the revision of the manuscript submitted to Nature Photonics. Finally, after the doctoral thesis was passed, the Nature Photonics paper was also accepted.

However, the current linear luminescent devices are limited by the stability of materials and device efficiency, and there is still a considerable gap before they can be fully commercialized. To this end, their subsequent work mainly focuses on the stability control of anisotropic asymmetric quantum dots and the stability optimization of devices.

By understanding the mechanisms of surface chemistry and ligand chemistry, they design more stable synthetic ligands and device structures, improving device efficiency and stability while maintaining polarized luminescence.Secondly, the exploration of the loss mechanism for polarized luminescence is still in its early stages. The highest linear polarization degree that can be achieved by a single quantum dot is greater than 90%, but the experimental values they can observe for the device are only 70%, leaving room for improvement in the linear polarization degree.

Based on this, the research team will conduct more detailed spectral studies to investigate the loss mechanism of polarization, thereby further enhancing the efficiency of polarized luminescence.

In addition, during the synthesis and optimization of anisotropic quantum dots, there are a large number of synthesis parameters to be adjusted, including the type of ligands, precursors, concentration, temperature, reaction time, and purification methods. Traditional manual optimization is time-consuming and labor-intensive, and it is difficult to achieve precise size control.

At present, Ye Junzhi has begun to try to introduce artificial intelligence-assisted high-throughput synthesis into the optimization process of quantum dot synthesis. Through high-throughput synthesis platforms such as robotic arms, a large amount of synthesis parameter data is accumulated, and a model that conforms to the laws of thermodynamics is established through machine learning.

The related work is in collaboration with Professor Zhao Haitao from the Shenzhen and Wenzhou Advanced Research Institutes of the Chinese Academy of Sciences. The preliminary work was also published in the January 2024 issue of "Angewandte Chemie International Edition" [2].Ye Junzhi said: "In the future, there will be more collaborations on AI for Science to accelerate the synthesis and stability optimization of anisotropic quantum dots. Personally, I believe this can help establish effective and precise machine learning models constrained by physical models, provide database references for the synthesis of quantum dots, and thus accelerate their research and development in the field of optoelectronics."

It is also reported that Ye Junzhi graduated from the Department of Chemical and Materials Engineering at the University of Auckland in New Zealand. After graduation, he came to the Cavendish Laboratory of the Physics Department at the University of Cambridge in the UK to pursue a direct Ph.D.

Currently, Ye Junzhi is conducting postdoctoral research in the Inorganic Chemistry Laboratory of the Department of Chemistry at the University of Oxford. He is mainly engaged in the research of non-lead, non-toxic anisotropic inorganic quantum dots, and is also studying the growth mechanism of anisotropic quantum dots and their impact on optoelectronic properties.