Application: Ultrafast cation doping of perovskite quantum dots in flow using Chemyx Fusion 4000

Flow synthesis of Mn-doped CsPbCl₃ QDs was carried out in an automated modular microfluidic platform previously developed in our group. The assembly of the microfluidic platform involves three structural core modules of the support structure, flow cell, and sampling tracks (custom machined in aluminum, Stratasys Direct Manufacturing). All the fluid-streaming components (tubing, fluidic connections, and fittings) were purchased from IDEX-Health & Sciences. An in-house LabVIEW script was utilized to centralize control of different modules of the automated microfluidic platform, including a dual syringe pump (Chemyx, Fusion 4,000), a mass flow controller (Bronkhorst, EL-FLOW Select), a 30 cm translational stage (Thorlabs, LTS300, with a maximum linear velocity of 5 cm s⁻¹), a high-power LED (Thorlabs, M365LP1), fiber-coupled light source (Ocean Insight, DH-2000BAL), and a fiber-coupled photospectrometer (Ocean Insight, Ocean HDX Miniature Spectrometer).

Continuous metal cation doping of CsPbCl₃ QDs was carried out at room temperature in the modular microfluidic platform shown in Figure 2A. The washed CsPbCl₃ QDs and MnCl₂ precursor were loaded in gas-tight stainless steel (SS) syringes (Chemyx, 50 mL) under inert conditions. To achieve in-flow concentration tuning of the dopant precursor, the dopant solvent (ODE) with the same ligand-to-solvent ratio (OAm:ODE) was prepared and loaded in another SS syringe under inert conditions. Fluorinated ethylene propylene (FEP) tubing (0.02″ ID, 1/16″ OD) was utilized to connect the precursor syringes to off-the-shelf T-junctions and a custom-designed polyether ether ketone (PEEK) four-way junction. The FEP tubing (0.01″ ID, 1/16″ OD) was also used to fabricate two inline static micromixers with a dead volume of 2 μL for intensifying the microscale mixing efficiency of the chemical precursors.

The concentrated MnCl₂ precursor and dilution solution streams were directed to an off-the-shelf T-junction and then passed through a braided micromixer to continuously adjust the concentration of the dopant precursor before the cation-doping process. Next, the diluted stream of MnCl₂ and washed CsPbCl₃ QDs were mixed in another in-series T-junction and braided micromixer to achieve a homogeneous reactive mixture before entering the modular microfluidic reactor. The pre-mixed cation-exchange reaction solution and the inert carrier fluids (i.e., Ar and PFO) were then directed to the modified PEEK cross-junction to form a three-phase flow throughout the flow reactor (FEP tubing, 1/16″ ID, 1/8″ OD). The in-flow cation-doping process was accurately and thoroughly monitored at different residence times along the flow reactor without changing the precursor flowrates (i.e., mixing timescale). The mobile three-port flowcell installed on the translational stage enabled monitoring of the fast cation-doping reaction through time-to-space transformation at 22 distinct optical ports. Concentrations of the dopant precursor and OAm were tuned on the fly by adjusting the volumetric flow rates of the washed CsPbCl₃ QDs (Q₁) and the diluted MnCl₂ precursor (Q₂ = Q_2-1 + Q_2-2; where Q_2-1 and Q_2-2 are the volumetric flow rates of the concentrated MnCl₂ precursor and the dilution solution, respectively) to be Q₁:Q₂ = 1:1. To establish a stable three-phase flow at high axial flow velocity (Video S1) for reaction-limited kinetic and mechanistic studies of the cation-doping process, the flow rates of Ar and PFO streams were set at 5 and 1 mL/min, respectively. Mathematical correction factors were calculated and applied to ensure identical absorption and photoluminescence results for one unique sample across all optical monitoring ports along the flow reactor.