Synthesis of Chalcogenide Perovskite Nanoparticles by Hot Injection Method

Chalcogenide perovskites (IIA-IVB-VIA₃: IIA = Ca, Sr ,Ba; IVB = Zr, Hf; VIA = S, Se, Te) are predicted to have excellent optical properties, such as high absorption coefficients exceeding 10⁵ cm^-1 and sharp absorption edges. ^[1] They have a tunable bandgap from visible to near-infrared regions and are robust to heat, oxygen, and water. Furthermore, they don’t contain toxic elements. These characteristics make them potential materials for optoelectronic devices that could solve problems of conventional lead halide perovskites.^[2] They are expected to a light absorber for thin film solar cells and an efficient light emitter for luminescent devices. Notably, SrHfS₃ has been reported as a green luminescent material.^[3] The key challenge in using these in optoelectronics devices is the high synthesis temperature for the film deposition, which limits substrate and electrode selection.^[4]<br/>One of solution for this problem is to synthesize nanoparticle (NP) of chalcogenide perovskites: instead of the high-temperature deposition process, their thin films can be obtained by coating pre-synthesized nanocrystals dispersed in organic solvents. Furthermore, the luminescent efficiency improves due to quantum confinement effect, and the emission peak wavelength can be controlled via the quantum size effect.<br/>We have found the difficulties to synthesize chalcogenide perovskite NPs: the common raw materials for liquid phase synthesis (carboxylates, alkoxides, halides) can't be used due to the robustness of the oxides of IVB(ZrO₂, HfO₂) preventing sulfide formation, and the formation of IIA-VIIA (such as BaCl₂) having poor solubility in organic solvents. Therefore, it was clarified that the precursor materials excluding oxygen or halogens are needed.<br/>We have developed a liquid-phase synthesis method for chalcogenide perovskite NPs. This "hot injection" method utilizes metal-dithiocarbamate (DTC) complexes as precursors and oleylamine as the reaction initiator and surface ligand. DTC complexes are stable up to around 400°C but decompose to sulfide at around 120°C with primary amines.^[5] Utilizing this reactivity difference, BaZrS₃ nanoparticles were successfully synthesized by heating Ba- and Zr-DTC in hydrocarbon solvent with injecting oleylamine.<br/>The XRD pattern of synthesized NPs matched the calculated data of distorted orthorhombic BaZrS₃. The crystallite size calculated from the FWHM of the main peak at 25.2° using Scherrer's formula was 11 nm. This is equivalent to the average particle size obtained from STEM images of the synthesized BaZrS₃ NPs, suggesting that each individual NP is a single crystal. However, they have a broad size distribution ranging from a few nm to 20 nm, and irregular shapes. Rietveld analysis of the XRD pattern suggested a sulfur deficiency, and an elemental analysis also confirmed a sulfur deficient composition compared to stoichiometric ratios. Absorption spectroscopy revealed that the synthesized BaZrS₃ NPs have a higher absorption coefficient within the visible range compared to commercially available InP quantum dots. This suggests that they exhibit greater light absorption with lesser quantities, thereby indicating its potential applicability as a color conversion material. On the other hand, fluorescence spectroscopy revealed a broad emission spectrum and poor PLQY. The PLQY could be enhanced by forming a core-shell structure or passivating surface defects via surface ligands. To narrow the luminescence peak, the emission via defect states must be suppressed, and the bandgap maintained constant by the controlling the particle size distribution. We are now focusing on optimizing reaction conditions and suppressing defect formation.<br/><br/>[1] S. Niu et al., Adv. Mater. 2017, 1604733<br/>[2] K. V. Sopiha et al., Adv. Optical Mater. 2021, 2101704<br/>[3] K. Hanzawa et al., J. Am. Chem. Soc. 2019, 141, 5343-5349.<br/>[4] X. Wei et al., Nano Energy 68 (2020) 104317<br/>[5] N. Hollingsworth et al., Chem. Mater. 2014, 26, 6281−6292