Researchers at LMU Munich have overcome two long-standing obstacles to the practical use of perovskite quantum dots - their instability in solution and the difficulty of controlling their growth with precision. The advances, reported across two papers in the Journal of the American Chemical Society and ACS Energy Letters, open new pathways for applying these materials in LEDs, photocatalysis, and future quantum light sources.

Perovskite quantum dots are semiconductor crystals just a few nanometers in size, composed of perovskite materials typically combining metals and halides. At such small scales, quantum effects dominate, strongly altering the optical and electronic properties of the material and enabling it to absorb and re-emit light with high efficiency. Despite their relative ease of manufacture in solution, perovskite quantum dots have a significant weakness: their soft ionic crystal lattices make them vulnerable to many solvents, particularly polar solvents such as alcohols, in which they rapidly disintegrate.

To address this, Dr. Quinten Akkerman and his team at the Nano-Institute Munich and the Faculty of Physics developed a stabilization strategy using Gemini ligands - molecules that bind through their charged groups to the surface of the quantum dots while simultaneously presenting a polar outer surface. This allows the quantum dots to disperse stably in polar solvents including ethanol. The ligand shell remains exceptionally thin at around 0.7 nanometers, preserving the optical properties of the underlying material. The stabilized dots retain high photoluminescence quantum yields over extended periods in solution and can now be processed using green solvents, an advantage for future optoelectronic manufacturing.

The second study tackled the problem of growth control. The size and structure of perovskite quantum dots determine the color and intensity of the light they emit, making precise control of these parameters essential for device applications. Akkerman's team developed a method that suppresses the formation of new seed crystals, instead directing material to grow onto existing quantum dots in a controlled manner. By carefully coordinating reaction conditions and the ligands used - which influence reaction kinetics - the researchers implemented a multi-stage injection strategy that allowed growth to be guided over extended timeframes. The approach achieved sub-unit-cell precision, meaning growth was controlled to a scale smaller than a single crystal lattice cell.

The resulting quantum dots exhibit narrow size distribution and stable optical properties - preconditions for reliable use in LEDs or quantum light applications. "While the new ligand chemistry improves their processing and stability, the precise control of their growth enables precise tuning of their optical properties," Akkerman said. "Together, the two studies provide new approaches for solving challenges relating to perovskite quantum dots."

Research Report:Polar Opposites: Ligand-Mediated Polarity Inversion for Perovskite Quantum Dots with Sub-Nanometer Ligand Shells