Water-Triggered Chemical Transformation of Perovskite Nanocrystals

Amino Acid-Driven Dimensional Reduction of CsPbBr3 Nanocrystals

Inspired by biomineralization, the recent incorporation of organic molecules into inorganic lattices shows interesting optical properties and tunability. We functionalize all inorganic CsPbBr3 perovskite nanocrystals (PNCs) with amino acid (AA) cysteine using the water-hexane interfacial approach. Along with the AA cysteine, we added AuBr3 salt into the aqueous phase, leading to the formation of a Au-cysteine thiolate complex to activate the aqueous to nonaqueous phase transportation of the AA via a molecular shuttle, oleylamine. The interaction between CsPbBr3 PNCs and the Au-cysteine thiolate complex is probed using optical spectroscopy, which reveals dimensional reduction of the parent PNCs to form CsPbBr3 nanoplatelets (NPls) and subsequent phase transformation to CsPb2Br5 NPls. X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy conclusively support the above chemical transformation reaction via interfacial chemistry. We propose a mechanistic insight into the dimensional growth in one direction in the presence of AAs via preferential ligand binding to specific facets, leading to transformation from 3D cubes to 2D NPls, while, presumably, the phase transformation occurs via the CsBr stripping mechanism upon prolonged interaction with water. Since AAs are building blocks for several redox-active complex biological moieties, including proteins, investigation of the interaction of AAs with PNCs may be advantageous since the latter can act as a fluorescent probe for bioimaging application.

Obtaining Ligand-Free Aqueous Au-Nanoparticles Using Reversible CsPbBr3 ↔ Au@CsPbBr3 Nanocrystal Transformation

Water-hexane interfacial preparation of photostable Au@CsPbBr3 (Au@CPB) hybrid nanocrystals (NCs) from pure CsPbBr3 (CPB) NCs is reported, with the coexistence of exciton and localized surface plasmon resonance with equal dominance. This enables strong exciton-plasmon coupling in these plasmonic perovskite NCs where not only the photoluminescence is quenched intrinsically due to ultrafast charge separation, but also the light absorption property increases significantly, covering the entire visible region. Using a controlled interfacial strategy, a reversible chemical transformation between CPB and Au@CPB NCs is shown, with the simultaneous eruption of larger-size ligand-free aqueous Au nanoparticles (NPs). An adsorption-desorption mechanism is proposed for the reversible transformation, while the overgrowth reaction of the Au NPs passes through the Au aggregation intermediate. This study further shows that the plasmonic Au@CPB hybrid NCs as well as ligand-free Au NPs exhibit clear surface enhanced Raman scattering (SERS) effect of a commercially available probe molecule. Overall, the beautiful interfacial chemistry delivers two independent plasmonic materials, i.e., Au@CPB NCs and ligand-free aqueous Au NPs, which may find important implications in photocatalytic and biomedical applications.

Size-Tunable Manganese-Doped Spheroidal CsPbCl3 Quantum Dots

Manganese doping has been demonstrated as a versatile tool to tune the emission of CsPbCl3 nanocrystals (NCs). Although this has been demonstrated in nanocubes and nanoplatelets, strategies for doping Mn2+ in size-tunable, excitonic CsPbCl3 quantum dots (QDs) remain absent. In this work, we demonstrate the synthesis of size-tunable spheroidal CsPbCl3:Mn2+ QDs, which can be obtained by a water-hexane interfacial combined anion and cation exchange strategy starting from CsPbBr3 QDs. Interestingly, the QDs exhibit a fast 0.2 ms Mn2+ photoluminescence (PL) lifetime and an energy transfer (ET) time of approximately 100 ps from the excitonic state of the QD to the atomic state of the Mn2+ ion. The size dependence observation of the manganese PL efficiency and the slow ET rate suggest that Mn2+ mainly gets incorporated at the QD's surface, highlighting the importance of strategies chosen for the incorporation of Mn2+ into perovskite QDs.

Reversible CsPbBr3 ↔ CsPb2Br5 Transformation via Reverse Micellar Aqueous Solution

Lead halide perovskites suffer from water and moisture instability due to the highly ionic nature of the crystal structures, though a few groups took advantage of it for chemical transformation via water-assisted strategy. However, direct exposure of the perovskite to bulk water leads to uncontrolled chemical transformation. Here, we report a controlled chemical transformation of CsPbBr3 to CsPb2Br5 triggered by nanoconfined water by placing CsPbBr3 in the nonpolar phase within a reverse micelle. The chemical transformation reaction is probed by using steady-state and time-resolved optical spectroscopy. We observe absorption and photoluminescence in the UV region stemming clearly from the CsPb2Br5 phase upon interaction with the reverse micellar aqueous solution. Transmission electron microscopy and X-ray diffraction measurements further provided the structure and morphology. Our results direct the formation of CsPbBr3-CsPb2Br5 nanocomposite under dry conditions while the chemically transformed CsPb2Br5 phase exists only in moist conditions, which we explain via the CsBr-stripping mechanism.