Holographic polymer nanocomposites with both high diffraction efficiency and bright upconversion emission

By incorporating liquid crystals and core-shell structured upconversion nanoparticles

Polymer nanocomposites have emerged as an efficient alternative material for various applications. Owing to their excellent properties, reliability, and affordability, extensive research on polymer nanocomposites has continued in a pursuit to enhance their properties and achieve desirable functions. Among the available polymer nanocomposites, holographic polymer nanocomposites,[1] particularly those formed by photopolymerization-induced phase separation when exposed to laser interference, have attracted significant attention. They are convenient for storing colored three-dimensional (3D) images, and more functions can be realized by introducing functional components like liquids crystals and by tunning the phase separation structures.[2,3]

Previously, the rod-like lanthanide-doped upconversion nanoparticles (UCNPs) have been introduced in the holographic polymer nanocomposites to attain the upconversion photoluminescence functions.[4] Interestingly, the nanocomposite emits different visible colors upon excitation. Nevertheless, the surface quenching effect of the UCNP results in limited upconversion emission intensity that hinders the nanocomposite application. The proposed solutions to overcome this challenge include coating the UCNP core with inert shell to depress the surface quenching effect and has resulted in a significant increase in the emission intensities. However, this method requires effective patterning of the core-shell structured UCNP through holography, which is a major challenge. Additionally, emerging applications require holographic polymer nanocomposites with both high upconversion emission intensity and high diffraction efficiency, which remains sparsely explored.

In this regard, a team of researchers at the Huazhong University of Science and Technology: Mr. Wen Luo, Dr. Mingli Ni, Professor Xingping Zhou, Professor Haiyan Peng, and Professor Xiaolin Xie developed a viable method for fabricating holographic polymer nanocomposites containing core-shell structured UCNPs.[5] They aimed at achieving bright upconversion emission and high diffraction efficiency simultaneously. Their work is currently published in the research journal, Composites Part B.

In this approach, the polymerization process to fabricate the holographic polymer nanocomposites involved enriching the UCNPs in the constructive regions of laser interference patterns where monomer polymerize and liquid crystal (LC) in the destructive regions. Three core-shell structured UCNPs with uniform sizes but different emission colors were synthesized and characterized through various techniques such as X-ray diffraction and transmission electron microscopy. The critical factors affecting the optical performances of the resulting holographic polymer nanocomposites were evaluated and elucidated.

The authors successfully fabricated holographic polymer nanocomposites with both high diffraction efficiency and bright upconversion emission. The mass ratio of oleic acid to water on the UCNPs surface was identified to be critical for the homogeneous dispersion of UCNPs and subsequent preparation of holographic polymer nanocomposites. Additionally, the high diffraction efficiency was attributed to the well-defined phase separation between the LC phase and polymer/UCNP phase. The demonstrated colored holographic images were observed under room light. The images comprised four encrypted upconversion emission states: red, yellow-green, blue and none. Furthermore, it is worth noting that the holographic images exhibited distinct colors at different viewing angles and could be switched via electric fields.

In summary, the study reported the fabrication of holographic polymer nanocomposites with improved diffraction efficiency and bright upconversion emission. Based on the results, the properties of the resulting holographic polymer nanocomposite were noted to depend on the mass ratio of oleic acid to water as well as the phase separation of the LC and UCNP phases. The synthesized holographic polymer nanocomposites exhibited improved optical performance with enhanced features. In a statant to Advances in Engineering, authors stated that the holographic polymer nanocomposites demonstrated improved properties and features, making them viable candidates for high security level anti-counterfeiting applications.

Xiaolin Xie is a full professor at Huazhong University of Science and Technology (HUST). He received his PhD degree from Sichuan University in 1996, and then conducted 2-year postdoctoral research at Zhejiang University. In December 1997, he joined in HUST. He visited the University of Sydney (2003.04-2004.03, 2006.11-2007.01, 2007.11-2008.01) and University of Colorado Boulder (2016.07-2016.08) as a Visiting Professor. His current research interests focus on liquid crystal chemistry and laser holography, polymer composites, polymer processing and rheology.

Haiyan Peng received his PhD degree from HUST in 2014. He visited the University of Colorado Boulder from 2012 to 2014, sponsored by the China Scholarship Council (CSC). Then he did research as an Assistant Professor at Guangzhou Institute of Advanced Technology, Chinese Academy of Sciences (2014-2016), and conducted postdoctoral research at City University of Hong Kong (2015-2016). He has been an Associate Professor at HUST since 2016. His current research interests focus on holographic polymer nanocomposites, photopolymerization and photorheology.

Mingli Ni received his PhD degree from HUST in 2017. He visited the University of Colorado Boulder as a joint PhD student from 2015 to 2016, sponsored by the CSC. From 2017 to 2018, he worked at BYD Auto Co., Ltd. as a Senior R&D Engineer. He has been a postdoctoral researcher at HUST since 2018. His current research interests focus on holographic photopolymers and luminescent polymer nanocomposites.

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References

  1. Ni, M. L.; Chen, G. N.; Wang, Y.; Peng, H. Y.; Liao, Y. G.; Xie, X. L. (2019). Holographic polymer nanocomposites with ordered structures and improved electro-optical performance by doping POSS. Compos. Pt. B-Eng., 174, 107045.
  2. Zhao, Y.; Zhao, X. Y.; Li, M.-D.; Li, Z. A.; Peng, H. Y.; Xie, X. L. (2020) Crosstalk-free patterning of cooperative-thermoresponse images by the synergy of the AIEgen with the liquid crystal. Angew. Chem., Int. Ed. 59, 10066-10072.
  3. Hu, Y.-X.; Hao, X. T.; Xu, L.; Xie, X. L.; Xiong, B. J.; Hu, Z. B.; Sun, H. T.; Yin, G.-Q.; Li, X. P.; Peng, H. Y.; Yang, H.-B. (2020). Construction of supramolecular liquid-crystalline metallacycles for holographic storage of colored images. J. Am. Chem. Soc. 142, 6285-6294.
  4. Zhang, X. M.; Yao, W. J.; Zhou, X. P.; Wu, W.; Liu, Q. K.; Peng, H. Y.; Zhu, J. T.; Smalyukh, I. I.; Xie, X. L. (2019) Holographic polymer nanocomposites with simultaneously boosted diffraction efficiency and upconversion photoluminescence. Compos. Sci. Technol. 181, 107705.
  5. Luo, W.; Ni, M. L.; Zhou, X. P.; Peng, H. Y.; Xie, X. L. (2020) Holographic polymer nanocomposites with both high diffraction efficiency and bright upconversion emission by incorporating liquid crystals and core-shell structured upconversion nanoparticles. Compos.  Pt. B-Eng. 199, 108290.

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