Controlling the Nucleation and Growth Kinetics of Spheroidal Lead Halide Perovskite Quantum Dots

Colloidal lead halide perovskites LHP (LHP) nanocrystals (NCs) have recently become popular light-emissive materials, of practical interest for LEDs, LCDs, lasers, as well as single photon light sources.^1,2 Most studies on LHP NCs focus on relatively large cuboidal NC exceeding 10 nm in size, pointing out the inherent chalange of producing small (sub 10 nm), stable and monodisperse LHP quantum dots (QDs). This problem directly originates from the highly ionic lattice of LHPs, generally resulting in sub second reaction dynamics, making it very challenging to control their growth on an atomic level. Consequently, the current generation of LHP QDs (especially the hybrid organic-inorganic ones) show significantly less excitonic absorption landscapes compared to conventional QDs such as CdSe, even though LHPs have more simplistic band structure. This thus hinders studies into the size-quantization of excitons in LHPs (and possible practical use) as well as understanding of the mechanism of LHP QD formation, which still significantly lacks behind compared to conventional CdSe and PbS QDs.<br/>&lt;!--![endif]----&gt;<br/>&lt;!--[endif]----&gt;To solve this, we developed a room-temperature synthesis, in which the overall QD formation occurred on a time scale of up to 30 min, slowing down the reaction kinetics by several orders of magnitudes compared conventional LHP QD syntheses.³ The size of these QDs were tunable between 3 and 13 nm range and exhibited a rhombicuboctahedral (spheroidal) shape. These CsPbBr₃ QDs, as well as FAPbBr₃ and MAPbBr₃ exhibited up to four well-resolved excitonic transitions, finally bringing them on par with the highly excitonic absorption landscapes of CdSe and PbS QDs. This slow growth method also allowed for the first time to direct in-situ study the illusive reaction mechanism of LHP QDs, demonstrating the effective separation of the nucleation and growth stages due to the self-limiting formation of an Cs[PbBr₃] intermediate precursors. The slow growth approach was further extended by using an additional in-situ anion exchange step, resulting also in spheroidal CsPb(Cl:Br)₃ QDs with any Cl:Br ration and sizes from 4-10 nm.^3,4 These quaternary QDs still exhibited up to five sharp excitonic absorption transitions, further demonstrating the versatility of the slow growth method. Finally, we used such spheroidal 6 nm CsPbBr₃ QDs to demonstrate that their respective excitons are confined with respect to their center-of-mass motion. Optical pumping of the lowest confined exciton transition with femtosecond laser pulses therefore not only bleaches all exciton resonances as measured with transient absorption spectroscopy, but also reveals a series exciton-to-biexciton transitions, which is further supported by their temporal dynamics.<br/><br/><b>References:</b><br/>1. Protesescu, L.<i> et al.</i> Nanocrystals of Cesium Lead Halide Perovskites (CsPbX₃, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. <i>Nano Lett.</i> <b>15</b>, 3692-3696 (2015).<br/>2. Akkerman, Q. A., Rainò, G., Kovalenko, M. V. & Manna, L. Genesis, Challenges and Opportunities for Colloidal Lead Halide Perovskite Nanocrystals. <i>Nat. Mater.</i> <b>17</b>, 394-405 (2018).<br/>3. Akkerman, Q. A.<i> et al.</i> Taming the Nucleation and Growth Kinetics of Lead Halide Perovskite Quantum Dots. <i>Research Square</i> <b>Preprint</b>, https://doi.org/10.21203/rs.21203.rs-1236393/v1236391 (2022).<br/>4. Akkerman, Q. A<i>.</i> Monodisperse Spheroidal Cesium Lead Chloride Bromide Quantum Dots, and a Fast Determination of Their Size and Halide Content. <b>Submitted.</b><br/>5. Barfüßer, A.<i> et al.</i> Confined Excitons in Spherical-like Halide Perovskite Quantum Dots. <b>Submitted.</b>