Microwave-Enhanced Chemistry at Solid-Liquid Interfaces: Synthesis of All-Inorganic CsPbX Nanocrystals and Unveiling the Anion-Induced Evolution of Structural and Optical Properties.

We demonstrate how microwaves could enhance the chemistry at interfaces of heterogeneous reactions involved in the microwave-solvothermal (MW-ST) synthesis of all-inorganic CsPbX (X = Cl, Br, I) perovskite nanocrystals (PNCs) within 6 min, unlike a conventional hot-injection method that requires 3 h. The enhanced MW-ST reaction rate was quantitatively analyzed by the Eyring equation, and it has been observed that the decreased activation free energy (Δ) and increased activation entropy (Δ) are caused by changes in the relative energies of reactants at their solid-liquid interfaces, leading to the formation of "hot spots", where microwave energy absorption is at its maximum. This rapid and homogeneous microwave heating could facilitate the self-assembly of uniformly distributed CsPbX nanocubes with precise control over the stoichiometric ratio, as confirmed by high-resolution transmission electron microscopy and energy-dispersive X-ray analyses. X-ray diffraction and Raman results indicate that lattice contraction and expansion in CsPbBrX have occurred because of an increase in the metal-halide bond length upon moving down the groups Cl → Br → I, as further ascertained by the Rietveld refinement studies. These anion-induced structural variations accordingly affected the electronic properties of MW-ST-synthesized CsPbX PNCs, which is apparent from the shifts in their conduction-band (CB) and valence-band (VB) positions. Consequently, the optical properties were also altered, resulting in a color-tuned emission from blue to red, with excellent photoluminescence quantum yields (up to 92%) and narrow emission line widths, as is evident from UV-vis and photoluminescence spectroscopy. The MW-ST-synthesized CsPbX PNCs were used as color-conversion layers for the fabrication of light-emitting diodes (LEDs) with commercial 456 nm UV-LED chips.