Diamond Annual Review 2021/22

68 69 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 Bright light emission from composite glasses Related publication : Hou J., Chen P., Shukla A., Krajnc A.,Wang T., Li X., Doasa R., Tizei L. H. G., Chan B., Johnstone D. N., Lin R., Schülli T. U., Martens I., Appadoo D., Ari M. S.,Wang Z.,Wei T., Lo S.-C., Lu M., Li S., Namdas E., Mali G., Cheetham A. K., Collins S. M., ChenV.,Wang L.-Z., Bennett T. D. Liquid-phase sintering of lead halide perovskites and metal-organic framework glasses. Science 374 , 621–625 (2021). DOI: 10.1126/science.abf4460 Publication keywords: Metal-organic framework glass; Metal halide perovskite; Sintering L ead halide perovskites are a family of synthetic semiconductor materials that can be used for solar panels, LEDs, displays, sensors, and many other applications. However, thesematerials have limited stability over time and can leach toxic chemicals into the environment. CsPbI 3 , for example, is sensitive to temperature, air, light, and commonly encountered chemicals like water, limiting its practical applications. An international team of researchers has addressed this challenge by encapsulating CsPbI 3 in a matrix made of a new type of glass derived froma class ofmaterials knownasmetal-organic frameworks. The composite showedorders ofmagnitudehigher efficiency for light emission and significantly enhanced stability, providing high-quality light formore than a year. They turned to advanced tools to understand howand why this newmaterial works the way it does. Using the E02microscope at the electron Physical Science Imaging Centre (ePSIC) made it possible to see individual nanometre scale crystals and determine the type of crystal within the glass. They used a type of transmission electronmicroscopy that takes diffraction data from less than 5 nm of material without damagingmaterials that are otherwise very easily changed when exposed to high energy electrons. Microscopic observations enabled the team to document how the glass preserves and locks in the correct, light-emitting perovskite crystals. Together with many other measurements with collaborators worldwide, the results explain how to make long-lasting light-emitting glass composites, preventing toxic lead leaching while simultaneously improving themechanical properties of thematerial to prevent breakage. Extensive research in the last 10-15 years on lead halide perovskites (LHPs) has resulted in record developments for photovoltaics, light-emitting diodes (LEDs), radiation detection, and thermometry. LHPs exhibit tunable band gaps, highcharge carriermobilities, andbright, narrow-bandphotoluminescence (PL), but the widespread industrial use of LHPs is still hampered by several factors, including (i) their decomposition when exposed to polar solvents, oxygen, heat, and light, (ii) their inherent polymorphism, (iii) the presence of deep trap states, and (iv) the phase segregation and leaching of toxic heavy metal ions. The high-temperature pseudo-cubic ‘black’ phases (α-, β- and γ-phases) of CsPbI 3 , for example, exhibit strong optical absorptivity and direct band gaps, making them excellent for photovoltaics and red-light emitting LEDs. Under ambient circumstances, however, they rapidly change into a thermodynamically stable, non-perovskite ‘yellow’δ-phase, leading to almost complete loss of their optoelectronic properties (Fig. 1). 1 The creation of LHP composites offer a route to address these difficulties. The ionic character of LHPs, however, is not generally favourable to composite production. A critical step is to create low-cost, processable, stable, environmentally friendly, and scalable LHP composites that can be used in real- world applications. Zeolitic imidazolate frameworks (ZIFs) are a kind of metal- organic framework (MOF) that consists of metal tetrahedra (e.g., Zn2+/Co2+) coordinated by imidazolate ligands. Recent improvements in ZIF materials have given access to stable ZIF liquids that can be quenched to produce microporous glasses. These ZIF glasses are a promising host for many other functional materials because the ZIF glasses can be modified to offer many desirable properties, from chemical control to mechanical properties. 2 In order to translate these principles to LHPs, a technique known as liquid phase sintering was applied to combine a preformed ZIF glass powder and LHP materials to create a single material in a carefully controlled heat treatment. This is the first demonstration of liquid phase sintering, an industrial powder processing technique, applied to the material systems of LHPs and ZIF glass. More specifically, ZIF glass {agZIF-62 (Zn[(Im)1.95(bIm)0.05] (Im: imidazolate; bIm: benzimidazolate))} and CsPbI 3 were synthesised via ball milling directly from the precursors, and then homogenised at a weight ratio of 3:1. The mixture was pelletised and then sintered at a rate of 20 °C/min at different temperatures under the protection of an inert Argon atmosphere. To minimise any undesirable thermal decomposition, the sintering process was quenched using liquid nitrogen. The fabricated composites were referred to as for (CsPbI 3 )0.25(agZIF-62)0.75. The (CsPbI 3 )0.25(agZIF-62)0.75 started to show intense and narrow red PL emission after being heated to 175 °C (Fig. 2). The narrow-band emission is a strong indication that the metastable, optoelectronically active phase of CsPbI 3 was created and preserved within the composite. Further increasing the sintering temperature enhanced the PL intensity, with the strongest PL achieved by sintering at 275 °C. Notably, the PL intensity of the sintered composite was over 200 times higher than the sample consisting of the mixed powder pellet prior to sintering, referred to as (CsPbI 3 )(agZIF-62)(25/75). To understand the detailed microstructure and crystal phase information, Scanning Transmission Electron Microscopy (STEM) was used to examine the (CsPbI 3 )0.25(agZIF-62)0.75 composite with nanometre spatial resolution, the critical length scale for CsPbI 3 nanocrystals. Using annular dark field STEM (ADF-STEM) imaging clearly distinguished the atomic number contrast between the two phases (Fig. 3), and the predicted elemental distribution was further corroborated by mapping the composite using X-ray Energy- Dispersive Spectroscopy (STEM-EDS). By taking an electron diffraction pattern at every point in the image, a technique known as Scanning Electron Diffraction (SED), maps were constructed of the crystalline areas corresponding to the CsPbI 3 nanocrystals (Fig. 3b). Classification in terms of the active and optically inactive CsPbI 3 phases was retrieved from the SED data through a machine learning approach, using a convolutional neural network. This classification demonstrated that the optoelectronically active phase was the main crystalline component in the composite (Fig. 3c). To look inside the three-dimensional structure of the composite, tomography in ADF-STEM (Fig. 3d) unveiled the interfacial contact as well as voids characteristic of the liquid phase sintering process. Electrondiffraction captured fromsingle CsPbI 3 crystal grains confirmed that active γ-CsPbI 3 crystals were found in the regions of the composite with the most interfacial contact and dense packing of the ZIF glass around the CsPbI 3 grains. Subsequently, to examine the practicality of these materials, the stability of the composites was tested by soaking in water for over 10,000 h, sonicating in various nonpolar, polar protic, and polar aprotic organic solvents, and under exposure to constant laser excitation (ca. 57 mW/cm 2 ) for over 5,000 seconds. The composite showed satisfactory stability in all of these challenging environments. The ZIF glass composite approach was further extended across the inorganic lead halide perovskite family (Cl, Br, I and mixed halide ions), generating emission from blue to red. Through suitable selection of colour combinations, these materials were ultimately demonstrated in a white light LED device. References: 1. Steele J. et al . Thermal unequilibrium of strained black CsPbI 3 thin films. Science 365 , 679–684 (2019). DOI: 10.1126/science.aax3878 2. Hou J. et al . Metal-organic framework crystal-glass composites. Nature Communications 10 , 2580 (2019). DOI: 10.1038/s41467-019-10470-z Funding acknowledgement: The authors acknowledge financial support from the Australian Research Council (DE190100803, DP180103874, DE190101152 and FL190100139); The University of Queensland (UQECR2057677); the Royal Society and Leverhulme Trust for a University Research Fellowship (UF150021) and Philip Leverhulme Prize (2019). The authors gratefully acknowledge the Leeds EPSRC Nanoscience and Nanotechnology Facility (LENNF) for support & assistance in this work, and the Diamond Light Source for access and support in the use of the electron Physical Sciences Imaging Centre (MG21980, MG25140). Corresponding author: Dr. Jingwei Hou, University of Queensland, Jingwei.hou@uq.edu.au Imaging andMicroscopy Group ePSIC Figure 1: Phase transition of CsPbI 3 in its pure phase and within the composites. Figure 2: Photoluminescence spectra for (CsPbI 3 )0.25(agZIF-62)0.75 composites fabricated at different sintering temperatures and cryogenic quenching. Figure 3: Phase distribution for the (CsPbI 3 )0.25(agZIF-62)0.75 composite fabricated with 300 °C sintering. (a) ADF-STEM image, (b) SED-STEMmapping and (c) CsPbI 3 crystal phase classification results for (CsPbI 3 )0.25(agZIF-62)0.75 composite. (d) Volume rendering of a tomographic reconstruction of (CsPbI 3 )0.25(agZIF-62)0.75 and a single cross-sectional plane extracted from the volume. Colour-coded arrows highlight the regions where electron diffraction data were collected. Scale bars in (a-d) are 250 nm.