I clearly remember the day I observed, so called, "ultrasonic magic" in a lab (1). It was an ordinary afternoon in November of 2017 and I was testing different ideas to make better quality lead halide perovskite (APbX3, A=Cs+, CH3NH3+, X=Cl-, Br-, I-) microcrystals for micro-laser applications. One of the conventional methods for perovskite microcrystals is dissolution-relocation growth (2) consisting of two consecutive steps: 1. Deposit PbX2 salts on the glass substrate, 2. Immerse the substrate into AX solution. It takes about 20 hours for growing complete micron-size nanowires and microplates. For further use, the detachment of these structures from the substrate is necessary, but unfortunately, this may generate structural damages, which are potentially vulnerable to efficient lasing. 

I tested the idea of injecting highly concentrated precursors (CsBr and PbBr2) to a bad solvent for high-quality perovskites microcrystals. To do this, I aimed to dissolve the same concentration of both CsBr and PbBr2 salts in polar aprotic DMF solvent in a high concentration. Contrary to my initial expectation, salts did not dissolve completely and the dissolution process seemed to be very slow. At that moment, the idea of dissolving more salts came to me unexpectedly; if I put this vial into the ultrasonic bath, ultrasonic force will break the salts into small pieces to accelerate the dissolution process. Finally, I may have highly concentrated precursor solution. This idea brought me to put a small vial having undissolved salts and DMF into the ultrasonic bath. While waiting for few minutes, I left the lab space and took a sip of coffee and chatted with my colleagues shortly about the last Tottenham hotspur's football games. 

After 10 minutes, I came back to check the vial and readily realized something happened in the vial because the transparent vial turned into orange. Since our lab has optical microscopy lab, I placed the solution under the microscope to check what was inside. When the first view of microcrystals appeared on the microscope screen, I proclaimed loudly, "What the heck is this!". 

I ran into the office of my advisor, Prof. Seok-Hyun (Andy) Yun, to discuss on what I observed. He and I put nickname on it as "ultrasonic magic". Few questions were arisen: 1. if this particle is CsPbBr3 perovskite, how can these particles stay stable in polar DMF solvent even after ultrasonication since CsPbBr3 is well known to be dissolved in a polar solvent?  2. what will happen if we mix different concentration ratios of salts? After investigation, we understood this phenomena as the spontaneous nucleation and growth of CsPbBr3 in supersaturation by the help of ultrasonic bubbles. Also, by adjusting precursor concentrations, we could build a two-dimensional concentration phase diagram of different perovskites materials including high-luminescent perovskite-embedded dual-phase microcrystals (Photoluminescence quantum yield > 40 %) . Our new synthesis method enabled us to visualize the lattice alignments in the dual-phase composites and supports the claim of CsPbBr₃ nanocrystals being the photoluminescent sites, which has been in confusion (3). 

Finally, we tested lasing performance of sonochemically prepared CsPbBr3 microcrsytals. Above certain threshold energy, bright scattering spots, narrow linewidth emission, nonlinear light-in-light-out curve proved lasing of CsPbBr3 microcrsytals. High structural quality enables us to demonstrate lasing of CsPbBr₃ microcrystals as small as 2 µm in size. In summary, we discovered our novel sonochemical method to produce a large quantity (10 billions/L) of high-quality perovskite micro-laser particles in a polar solvent within 2 minutes, and provides insights into the unique structure of perovskite- embedded dual-phase microcrystals.

Using such high-quality micro-lasers, we can perform high-contrast and super-resolution imaging. We called this novel image technique as laser particle based stimulated emission microscopy (LASE)(4). This new class probe, referred as "laser particle", enables to generate spectrally narrow emission (<1 nm in full width half maximum (FWHM)) comparing to conventional fluorescent probes (~30 nm in FWHM), which would be beneficial for multiplexed studies(5) and tissue imaging. However, the size of current micro-lasers is still larger than the size of intracellular organelles such as mitochondria (> 0.5 μm). For better spatial mapping, nano-lasers are definitely necessary. Now, we devote ourselves to reduce the size of laser particles to go beyond the diffraction-limit. Stay tuned for our continuing magic story!

For more information, the full article can be found here.

Reference

(1) Cho, S., Yun, S.H. Structure and optical properties of perovskite- embedded dual-phase microcrystals synthesized by sonochemistry. Commun. Chem. 3, 15 (2020).

(2) Fu, Y. et al. Solution growth of single crystal methylammonium lead halide perovskite nanostructures for optoelectronic and photovoltaic applications. J. Am. Chem. Soc. 137, 5810–5818 (2015).

(3) Akkerman, Q. A., Rainò, G., Kovalenko, M. V. & Manna, L. Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals. Nat. Mater. 17, 394–405 (2018). 

(4) Cho, S., Humar, M., Martino, N. & Yun, S. H. Laser particle stimulated emission microscopy. Phys. Rev. Lett. 117, 193902 (2016).

(5) Martino, N., Kwok, S.J.J., Liapis, A.C. et al. Wavelength-encoded laser particles for massively multiplexed cell tagging. Nat. Photonics 13, 720–727 (2019).