Stability of Emissive Cubic Phase of Double Perovskite Nanocrystals with Li Compositions

Saar Shaek^a^,^b^,^c^,^d, Offir Zachs^a^,^b^,^c^,^d, Emma Massasa^a^,^b^,^c^,^d, Rachel Lifer^a^,^b^,^c^,^d, Lotte Kortstee^e, George Dosovitskiy^a^,^b^,^c^,^d, Boaz Pokroy^a, Ivano Castelli^e, Yehonadav Bekenstein^a^,^b^,^c^,^d

^aTechnion - Israel Institute of Technology, Materials Science and Engineering Faculty

^bTechnion - Israel Institute of Technology, Solid State Institude

^cTechnion - Israel Institute of Technology, the Helen Diller Quantum Center

^dTechnion - Israel Institute of Technology, Grand Technion Energy Program - GTEP

^eDepartment of Energy Conversion and Storage, Technical University of Denmark (DTU)

Proceedings of Emerging Light Emitting Materials 2024 (EMLEM24)

La Canea, Greece, 2024 October 16th - 18th

Organizers: Grigorios Itskos, Sohee Jeong and Jacky Even

Oral, Saar Shaek, presentation 038
DOI: https://doi.org/10.29363/nanoge.emlem.2024.038
Publication date: 13th July 2024

Lithium-ion technology is leading the market of energy storage. Direct monitoring of free lithium ions is critical for safety and improved efficiency. We report for the first-time a synthesis of emissive nanocrystals that are sensitive to lithium-ion concentration in their surroundings. These double perovskite nanocrystals with compositions of Cs₂Li_(1-x)Na_(x)InCl₆ can serve as such indicators due to their broad emission spectrum in the visible range and facile cation exchange schemes. Using nanocrystals, we can synthesize a stable cubic phase (with a band gap of 2.78eV). However, for bulk Cs₂LiInCl₆, this phase was not reported, but a trigonal phase (with a band gap of 3.23eV) is known.

Here, we demonstrate two strategies for stabilizing the emissive cubic phase.

First, we employ the well-documented size-stabilizing effect for colloidal nanocrystals, which asserts that a high surface-to-volume ratio will have a stabilizing surface effect due to the passivating of organic surface ligands. In our case, we can control the size by varying the reaction temperature.

Second, we show another stabilizing mechanism for the cubic phase by alloying between the B site cation (Li and Na). This alloying is possible with both direct synthesis and post-synthesis treatment upon exposure to a free Li or Na ions environment. The evident effect of alloying of the B site, which directly corresponds to a shift in the emission wavelength, suggests that these materials and mechanisms could be used as indicators for monitoring Li-ion content in their surroundings.

We report a colloidal synthesis for nanoparticles of Cs₂LiInCl₆ with a narrow size distribution of ~10nm with access to all Na-Li alloy ratios. The structure and size of the nanocrystals are determined by X-ray diffraction (XRD), a synchrotron high-resolution powder XRD (HRPXRD), and an atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). We show topotactical stability of the cubic phase (100%Na) to the incorporation of Li ions, which does not change the cubic double perovskite structure for the nanoparticles, while for bulk materials at these Li concentrations, only the trigonal phase is stable. We confirm the composition of our resulting NCs via inductively coupled plasma mass spectrometry (ICP-MS), time-of-flight ion mass spectroscopy (TOF-SIMS), and electron energy loss spectroscopy (EELS).

The intrinsic cubic phase is emissive; however, it is still low, and its emission can be further enhanced. The PLQY of many double perovskites was shown to increase with the transfer of free exciton into self-trapped exciton propagated by doping. In previous studies, Sb-doping in Cs₂NaInCl₆ double perovskite was highlighted as a candidate for notable PLQY and controlled emission wavelength through B-site alloying (Shaek et al., 2023). We hypothesize that this doping scheme is also optimal for our Cs₂LiInCl₆ NCs.

The vision is to allow direct detection and monitoring of Li-ion content in energy storage technologies through visible changes of a coupled NCs-based passive indicator.

Acknowledgements:

We thank the EuroTech (DTU and Technion) alliance program for funding and support and the opportunity for co-supervision by Prof. Ivano E. Castelli from DTU. We acknowledge Dr. Y. Kauffmann from our microscopy center (MIKA) and Dr. Maria Koifman for the continuous support and helpful advice. We thank the Technion Helen Diller Quantum Center and the Grand Technion Energy Program - GTEP for their generous support. This work is supported by the Israel Science Foundation grant number 890015. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 949682-ERC-HeteroPlates.

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