Halide perovskites are increasingly exploited as semiconducting materials in diverse optoelectronic applications, including light emitters, photodetectors, and solar cells. The halide perovskite can be easily processed in solution, making microfluidic synthesis possible.
1. Introduction
The microfluidic chip that confines fluids in micron channels can scale the chemical reactions from extensive batch synthesis down to the microscale, exploiting the physical and chemical properties of liquids and gases at a microscale, significantly reducing the synthesis and analysis of volume reagents
[1][2][3][4][5]. In nanocrystal (NC) synthetic processes, the batch synthesis strategies of NCs are almost always challenging due to rapid perovskite crystallization, the extensive precursor preparation, the difficulties associated with product purification, and the need for particle post-synthesis. It is envisioned that a microreactor platform consisting of flow-focusing microfluidics might be suitable to synthesize high-crystallinity and narrow-size-distribution NCs due to the ultrafast mixing and phase separation during the crystal nucleation and growth. The microfluidic chemical reactions can be precisely detected and explored by in situ spectroscopy
[6][7][8][9][10] and more sufficient and continuous during the reaction on the micron scale. Therefore, there are at least two advantages to microfluidic synthesis. On the macroscopic level, a microreactor can be considered a powerful and effective platform for the mass synthesis of semiconductor nanomaterials. On the microscopic level, the microfluidic technique facilitates the simultaneous collection of both absorption and photoluminescence (PL) spectra of various luminescent materials synthesized in the liquid states, particularly that of halide perovskite nanocrystals.
Quantum dot (QD) semiconductors are promising materials for various applications ranging from light-emitting diode (LED) displays to solar cells, biological sensing, and imaging
[6][7][8]. Specifically as optoelectronic materials, perovskite nanocrystals have attracted much more attention due to their high PL quantum yields, high absorption/emission efficiency, long carrier lifetime, and tunable emission color over the entire visible region
[9][10][11]. Lead halide perovskite structure can be characterized by the general formula ABX
3 (X = Cl, Br, or I), where A and B represent two different cations. A-site cations can be inorganic or organic ions, such as cesium (Cs), formamidinium (FA), and methylammonium (MA), while B-site cation (Pb
2+) could potentially be exchanged by dopant ions (Mn
2+, Fe
2+, Ce
3+, Eu
2+)
[12][13][14][15][16][17][18]. Therefore, the hybrid organic−inorganic lead halide perovskite, such as CH
3NH
3PbX
3; and all inorganic lead halide perovskite, such as CsPbX
3, in the form of nanocrystals, thin films, microcrystals, and bulk single-crystals, show promising properties in LEDs
[9][19], lasers
[20], solar cells
[21][22][23], gas sensors
[24], etc.
2. Microfluidic Synthesis of Halide Perovskite
Generally, microfluidic devices have microchannels ranging from submicron to a few millimeters
,. For as shown in Figure 1various microfluidic syntheses of perovskite nanostructures and composite, which can move or analyze the tiny amount of liquid (droplet) in a single- or multi-phase flow.
Figure 1. Various microfluidic syntheses of perovskite nanostructures and composite. (a) Microfluidic setup with a U.S. dime coin for comparison. Adapted with permission from Ref. [25]. Copyright 2019 Elsevier B.V. (b) Synthesis of perovskite composite microparticles. Adapted with permission from Ref. [26]. Copyright 2021 Wiley-VCH GmbH. (c) Formation of MAPbBr3 PQDs in nanofiber. Adapted with permission from Ref. [27]. Copyright 2022 Wiley-VCH GmbH. (d) Schematic of the PL dynamics of microfluidic droplet. Adapted with permission from Ref. [28]. Copyright 2020 American Chemical Society. (e) Microfluidic chips for synthesizing CsPbBr3. Adapted with permission from Ref. [29]. Copyright 2021 American Chemical Society. (f) QD anion exchange reaction in a continuous flow. Adapted with permission from Ref. [30]. Copyright 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.
The microfluidic reaction has been recognized to be more controllable and continuous during the nanostructure’s synthesis on the microscale. As shown in
Figure 1 schematic of the PL dynamics of microfluidic d
roplet, the QDs were synthesized from multiphase (liquid, gas) in microfluidic channels. Compared to conventional flask synthesis under gas protection at high temperatures, the continuous-flow microfluidic approach benefits the alignment of the quantum-confined perovskite nanocrystals and can promote crystal growth orientation to form long nanowires (NWs) at room temperature.
In Figure 1a–c,Re
,f, reagent precursor solution is injected into the microchannel. After combination in the channel, different types of nanomaterials can be achieved (
Table 1)
[25][26][27][29][31][32][33].
Table 1. Microfluidic synthesis of nanostructured halide perovskite.