Colloidal Nanocrystals Regrowth-Assisted Synthesis of Perovskite Microwire Lasers for Integrated Optoelectronics

Abstract

Colloidal perovskite nanocrystals (NCs) are a well-proven platform for growing anisotropic structures. Nanowires (NWs) exhibiting a quantum confinement phenomenon and microwires (MWs), which enable lasing, are of particular interest for optoelectronic devices. Synthesis of the latter is challenging. Herein, we report a straightforward access to high-quality CsPbBr3 MW lasers. We utilize a diphenyl ether (DPE) solvent for the hot-injection synthesis. DPE coordinates strongly to Pb2+ and allows to reduce an excess of oleic acid/oleylamine ligand pair well established for PbBr2 dissolution and inhibition of as-formed NCs regrowth. Therefore, a rapid injection of Cs-oleate into the PbBr2-containing solution yields lead-depleted Cs4PbBr6 NCs which slowly release perovskite precursors and produce CsPbBr3 counterparts. The latter transform into NWs through an oriented-attachment mechanism, which in turn evolve into laser MWs. To demonstrate spectrally tunable lasing in MWs we employ YCl3 for ion exchange in perovskite lattice. Resultant CsPb(Cl,Br)3 MWs show high-Q coherent emission in the 485-540 nm range. To highlight the potential of synthesized MWs for integrated optoelectronics, we assemble a device comprising a CsPb(Cl,Br)3 MW laser coupled to MoO3 lossless nanowaveguide, which delivers coherent light to a CsPbBr3 MW photodetector. The device exhibits a nonlinear optoelectronic response applicable for on-chip neuromorphic computing.

Figures (7)

• Figure 1: Schematic illustration for the synthesis of laser MWs. Injection of CsOA into PbBr$_2$/ligands solution at 140 $^o$C and stirring the formed colloid for 2 h gives a stock solution with NCs and NWs. Heating the stock solution up to 180 $^o$C at constant stirring and taking aliquots every 10 min makes it possible to track the evolution of colloidal particles.
• Figure 2: (a) XRD patterns of colloidal particles from aliquots collected at the first (red, yellow, and green lines) and second (purple line) stages of the synthesis. Square and rhombus symbols identify diffraction peaks belonging to rhombohedral Cs$_4$PbBr$_6$ (space group R-3c) NCs and orthorhombic CsPbBr$_3$ (space group Pbnm) particles, respectively. Three panels (from top to bottom) show the transformation of the non-perovskite phase to the perovskite one during the incubation at the first stage. The bottom panel shows an increase in crystallinity of 1D anisotropic CsPbBr$_3$ particles presented by doublets with major peaks (110) and (220) in the diffraction pattern of the product obtained at the end of the second stage. (b) Schematic illustration of colloidal particles evolution. (c) EDX mapping of a single MW showing an even spatial distribution of Cs, Pb, and Br atoms in the crystal lattice and nearly stoichiometric chemical content. (d-g) HAADF-STEM images of the particles taken at the first stage. Large-sized Cs$_4$PbBr$_6$ NCs (d) serve as a source of precursor species for middle- [7 nm, (e)] and standard-sized [11-12 nm, (f)] CsPbBr$_3$ NCs. Atomically resolved image of a single middle-sized NC (e) allows for determining positions of Cs, Pb, and Br atoms in the crystal lattice [inset image in (e)]. Standard-sized NCs regrow along [110] direction (f) and yield NWs which attach each other through (001) faces (g). (h-k) Images of the particles taken at the second stage. Arrays of NWs form needle-like structures (h), which assemble bunches (i). Bunches undergo internal alignment (j) and chipping of the uneven ends to give Fabry-Pérot laser microcavities (microwires) with reflective end facets (k). (l,m) High-resolution images visualizing a small gap between lattice-matched 30 nm thick NWs (boundaries are highlighted with dashed lines) within a single microcavity [inset image in (l)]. Selected area image and its FFT confirm high crystallinity of the microcavity.
• Figure 3: (a) A picture of the aliquots (samples) 1-7 diluted in hexane, where 1 is collected at the beginning of the second stage, and 7 is the target product. Greenish fluorescence in sample 1 stems from colloidal NCs, whereas bright yellow sediment is NWs arrays. The size of particles in the aliquots increases from left to right that manifests in the gradual change in the sediment color. The dark orange color of the sample 7 is caused by substantial light absorption in bulky CsPbBr$_3$ MWs. (b) Optical absorption and PL spectra for 1 and 7. (c-i) Fluorescence microimages illustrating the stepwise synthetic formation of laser MWs, including the key steps: assembly of NWs arrays into netting, its crumbling into needles, bunching of the needles, their alignment within the bunches, and chipping of MWs with uneven ends. (j) Bright-field image of an isolated laser MW. (k) Dark-field image of the same MW excited by focused white light at the end facet and spectrum of transmitted light collected from the opposite end. Modulation in the spectrum is caused by Fabry-Pérot optical modes, indicating that reflective end facets of the MW enable standing waves [inset picture in (k)].
• Figure 4: (a) Schematic illustration of alloyed CsPbCl$_x$Br$_{3-x}$ MWs synthesis. (b) Fluorescence images and PL spectra of MWs with various content $x$ of chlorine in the perovskite lattice. (c) HAADF-STEM image and EDX mapping of CsPbCl$_{0.49}$Br$_{2.51}$ perovskite particles, revealing a uniform distribution of Cs, Pb, Cl, and Br elements.
• Figure 5: (a) TRPL kinetics for MWs for pure bromine and mixed halide MWs, where each of them was measured for the same amount of time t = 110 s. The inset image demonstrates normalized PL decay kinetics. (b) Photoluminescence spectra of MWs at excitation fluence equal to 1.2 of laser generation threshold fluence F$_{th}$. (c) Dependencies of PL intensity and FWHM value on the pumping fluence.