Photo‐Induced Bandgap Engineering of Metal Halide Perovskite Quantum Dots In Flow

Abstract

Over the past decade, lead halide perovskite (LHP) nanocrystals (NCs) have attracted significant attention due to their tunable optoelectronic properties for next‐generation printed photonic and electronic devices. High‐energy photons in the presence of haloalkanes provide a scalable and sustainable pathway for precise bandgap engineering of LHP NCs via photo‐induced anion exchange reaction (PIAER) facilitated by in situ generated halide anions. However, the mechanisms driving photo‐induced bandgap engineering in LHP NCs remain not fully understood. This study elucidates the underlying PIAER mechanisms of LHP NCs through an advanced microfluidic platform. Additionally, the first instance of a PIAER, transforming CsPbBr₃ NCs into high‐performing CsPbI₃ NCs, with the assistance of a thiol‐based additive is reported. Utilizing an intensified photo‐flow microreactor accelerates the anion exchange rate 3.5‐fold, reducing material consumption 100‐fold compared to conventional batch processes. It is demonstrated that CsPbBr₃ NCs act as photocatalysts, driving oxidative bond cleavage in dichloromethane and promoting the photodissociation of 1‐iodopropane using high‐energy photons. Furthermore, it is demonstrated that a thiol‐based additive plays a dual role: surface passivation, which enhances the photoluminescence quantum yield, and facilitates the PIAER. These findings pave the way for the tailored design of perovskite‐based optoelectronic materials.

This work presents a photochemical synthetic route for bandgap engineering of metal halide perovskite quantum dots. Utilizing a material‐efficient microfluidic platform, the underlying mechanism of the photo‐induced anion exchange reaction (PIAER) of metal halide perovskite quantum dots is unveiled and first instance of the PIAER‐enabled synthesis of high‐performing CsPbI₃ NCs is reported.

1. Introduction

Lead halide perovskite (LHP) nanocrystals (NCs), with their remarkable optoelectronic properties, have gained significant interest in recent years, particularly for applications in photonic and optoelectronic devices.^{[ 1} , ² , ³ , ^{4 ]} Precise bandgap engineering of LHP NCs over the entire ultraviolet/visible (UV/Vis) electromagnetic spectrum is crucial for device applications.^{[ 5} , ^{6 ]} The ionic nature of LHPs and the labile nature of halide anions make them amenable to post‐synthetic bandgap engineering via anion exchange reactions.^{[ 7} , ⁸ , ⁹ , ^{10 ]} To initiate the anion exchange reaction of LHP NCs, halide anions can either be introduced directly to the solution via halide salts or generated in situ (such as via a photochemical route) using a haloalkane.^{[ 10} , ¹¹ , ^{12 ]} Although successful and widely used, the direct‐addition strategy of halide anions via halide salts often exhibits imprecise bandgap tunability, limiting the suitability of this approach for LHP NCs–based devices.^{[ 11 ]} Conversely, the in situ halide generation strategy may offer a scalable path for the precision synthesis of custom LHP NCs with desired optoelectronic properties but remains underdeveloped. Despite the vast potential of photo‐induced anion exchange reaction (PIAER), the lack of a fundamental understanding of the underlying reaction mechanism and the roles of photon flux, haloalkane photocatalytic activity, and LHP NCs’ surface chemistry on the overall process have hindered the progress of this facile bandgap engineering technique.^{[ 13} , ¹⁴ , ¹⁵ , ¹⁶ , ^{17 ]} For example, mechanisms such as reducing halide anions and defect–facilitated processes have been reported but remain poorly understood.^{[ 11} , ^{12 ]}

Traditionally, studying LHP NC photo‐induced anion exchange synthesis reactions involves manual flask‐based experiments with ex situ characterization.^{[ 11} , ^{18 ]} This conventional approach, however, faces significant challenges—notably, high material consumption and non‐uniform photon exposure, which negatively impacts reproducibility.^{[ 19} , ^{20 ]} In contrast, microscale flow reactors (microfluidics) offer a promising alternative to overcome these engineering challenges. Automated microfluidic‐based platforms provide superior reaction control and reproducibility of experimental conditions.^{[ 21} , ^{22 ]} These platforms ensure precise control over reaction conditions, provide uniform mixing, and facilitate real‐time reaction monitoring. Access to high‐throughput, real‐time, in situ experimental data is crucial for gaining deeper insights into the kinetics and mechanisms of PIAER and revealing effects of photochemical reaction parameters. In microscale flow reactors, the shorter and uniform light absorption path length results in at least a threefold increase in photon flux per reaction volume compared to batch reactors.^{[ 23 ]} This effective photon flux intensification considerably accelerates the rate of photochemical reactions and prevents side reactions and precipitation issues (Figure 1 ).^{[ 20} , ²³ , ²⁴ , ²⁵ , ^{26 ]}

Figure 1.

Schematic illustration of the batch versus in‐flow bandgap engineering of LHP NCs via PIAER featuring the reaction time and volume differences.

This paper presents a microfluidic approach for accelerated fundamental and applied studies of PIAER of LHP NCs. The automated microfluidic platform is 3.5× faster and requires 100× less material (by volume) than batch approaches. Using this microfluidic approach, we systematically investigate and unveil the effects of reaction parameters—including photon flux, halide source, and solvent concentration—on the kinetics of PIAER and subsequently on the quality—i.e., photoluminescence quantum yield (PLQY), and emission linewidth—of an exemplary LHP, cesium lead halide (CsPbX₃, X = Cl, Br, I) NCs. By utilizing photons to initiate the anion exchange reaction, the precise bandgap engineering of CsPbX₃ NCs is achieved—enabling the colloidal synthesis of NCs spanning the entire UV/Vis spectrum. Moreover, for the first time, we report a photochemical route for anion exchange from CsPbBr₃ to cesium lead iodide (CsPbI₃) NCs with the help of the additive 1‐dodecanethiol (DSH). Finally, utilizing multi‐modal in situ and ex situ characterization data, we investigate and propose a mechanism for the PIAER of LHP NCs. The enhanced understanding and control of photo‐induced LHP NCs bandgap engineering contribute to developing advanced optoelectronic materials with customizable, application‐driven optical properties.^{[ 27} , ²⁸ , ^{29 ]}

2. Results

To intensify (accelerate), miniaturize (reduce chemical consumption and waste generation), and capture (monitor in situ) the PIAER of LHP NCs, we designed, developed, and deployed an automated and material‐efficient microfluidic platform using a single‐droplet photo‐flow microreactor (Figure 2A). Two critical features of the developed photo‐flow microreactor are: i) a tunable high‐power light‐emitting diode (LED) integrated with the single‐droplet photo‐flow microreactor (2–10 µL), which enables process intensification, and ii) a custom‐designed multi‐port flow cell, which facilitates multi‐modal in situ monitoring of the PIAER of LHP NCs (Figure S1, Supporting Information). The single‐droplet photo‐flow microreactor is composed of UV‐transparent Teflon tubing onto which collimated and tunable UV light of 365 nm (0–4500 mA) is directed, ensuring uniform photon flux within the entire microreactor. The absorbed photon flux by the reactive droplet within the single‐droplet photo‐flow microreactor was measured using chemical actinometry (Figure S2A–D, Supporting Information).^{[ 24 ]} The bespoke flow cell incorporates two parallel ports, serving (alternatingly) as both the inlet and outlet of the photo‐flow microreactor (Figure 2B). This flow cell design functions as a droplet detector, triggering oscillatory motion, and multi‐modal in situ reaction monitoring probe, acquiring UV/Vis absorption and photoluminescence (PL) spectra of LHP NCs at each half cycle of the reactive droplet moving within the photo‐flow microreactor (Figure 2C,D). Each experiment begins with the automatic formulation and formation of a 10 µL droplet containing LHP NCs, solvent, and halide source (Figure 2C). Once the droplet is formed, it enters the photo‐flow microreactor, initiating oscillatory motion, ensuring continuous mixing (axisymmetric recirculation)^{[ 21} , ³⁰ , ³¹ , ^{32 ]} within the reaction droplet throughout the entire PIAER (see Video S1, Supporting Information). This constant mixing within the reaction droplet ensures uniform exposure of all NCs to photons, intensifying the photochemical process through homogeneous light absorption.

Figure 2.

Schematic illustration of the developed microfluidic platform for accelerated in situ studies of the PIAER of LHP NCs. A) Schematic diagram of the developed microfluidic platform designed for accelerated high‐throughput in situ studies of PIAER in LHP NC synthesis. B) Overview of the photo‐flow microreactor, featuring an enlarged view of the single‐droplet microreactor and an enlarged view of the flow cell with three optical ports for in situ absorption and PL spectral monitoring of the PIAER. This multi‐modal, in situ reaction monitoring probe comprises a fiber‐coupled broadband light source, a fiber‐coupled 365 nm UV LED excitation source, and a fiber‐coupled miniaturized spectrometer (detection port). The setup includes two openings connected with fluorinated ethylene‐propylene (FEP) tubing, utilized for in situ spectral collection and triggering the oscillatory motion of the reaction droplet. C) Step‐by‐step illustration of the reaction droplet formulation: i. Dichloromethane (DCM)/1‐Iodoporpane infusion; ii. Toluene infusion; iii. The reaction droplet passes through the microreactor; iv. CsPbBr₃ NC infusion, making the final reaction droplet (10 µL). D) Step‐by‐step illustration of the reaction droplet operation within the photo‐flow microreactor (uncoiled for visual clarity) under UV light: i. Droplet entering the microreactor; ii. Droplet moving clockwise within the microreactor; iii. Droplet detected at the microreactor inlet by the flow cell; iv. Droplet entering the microreactor counterclockwise; v. Droplet circulating counterclockwise within the microreactor; vi. Droplets are detected at the microreactor outlet by the flow cell. In panel D, the color transition of the reaction droplet from green to red (stages i to vi) represents PIAER of CsPbBr₃ to CsPbI₃ NCs.

The UV/Vis absorption and PL spectra of LHP NCs at each reaction condition were acquired in situ during each oscillation cycle (i.e., every 90 s) and processed automatically to extract the relevant optical features shown in Figure 2A (inset on computer). The optical features extracted from the UV/Vis absorption spectra included absorbance at 365 nm (A ₃₆₅) and the first excitonic peak wavelength (λ _exc). From the PL spectra, the peak emission wavelength (λ _em), emission linewidth (full‐width‐at‐half‐maximum; FWHM), PL intensity (PL _I), and PL peak area (PL _A) were extracted at each reaction condition. Process automation of the experimental platform using LabVIEW and MATLAB (Figure S3A,B, Supporting Information), reaction miniaturization, in situ characterization, and automated data analysis enabled accelerated in situ monitoring of LHP NC optical property evolution under different photon fluxes, thereby facilitating mechanistic studies of the underlying mechanism controlling the PIAER.

Following the development of the automated photo‐flow microreactor platform, we studied its performance and reproducibility. For these tests, we performed five replicates of the PIAER of CsPbX₃ NCs from bromide to chloride and from bromide to iodide. The λ _em and PL _A were monitored throughout the entire PIAER (Figure S4A–D, Supporting Information), with average relative standard deviations of 0.77% and 0.02% for A ₃₆₅ and λ _em, respectively, indicating high precision and reliability of the developed microfluidic platform.

Control experiments were conducted to assess the need for high‐energy photons to initiate the anion exchange reaction and the photostability of CsPbBr₃ NCs. The experiments were divided into two parts: one without haloalkane and one with. In the control experiments with a haloalkane present, we evaluated whether an anion exchange reaction could occur without UV light. In the dark control experiment (0 pmol s⁻¹), haloalkane was introduced to CsPbBr₃ NCs in the reaction droplet, and the UV/Vis absorption and PL spectra of the NCs were monitored in situ for 65 min. Both PL_I and A₃₆₅ remained constant, demonstrating the colloidal stability of CsPbBr₃ NCs. No physicochemical or optical changes of the NCs in the absence of high‐energy photons were observed for the DCM (Figure 3A–D) or 1‐iodopropane cases (Figure S5A–D, Supporting Information).

Figure 3.

Control experiments of CsPbBr₃ NCs in the photo‐flow microreactor. Time‐evolution of A) PL and B) UV/Vis spectra as well as C) λ_em and D) A₃₆₅ of CsPbBr₃ NCs in the reaction droplet in the presence of DCM and toluene oscillating in the photo‐flow microreactor with no UV illumination (dark experiment). Experimental conditions: [NC] = 4.2 µm; [DCM] = 3.10 m; UV LED: Off; Residence time = 65 min; T = 23 °C.

To study the photostability of CsPbBr₃ NCs in the absence of haloalkane, photon intensity (at 365 nm) was varied (0–120 pmol s⁻¹) for 10 µL droplets of 4.2 µm CsPbBr₃ NCs in toluene (Figure S6, Supporting Information). With no UV photon flux (dark experiment), CsPbBr₃ NCs remained colloidally stable with no observable change in the UV/Vis absorption and PL spectra as a function of time. However, at 10 pmol s⁻¹ illumination, CsPbBr₃ NCs began to decompose, resulting in a blue shift (510  to 506 nm) in 60 min.^{[ 33 ]} At 120 pmol s⁻¹ illumination, CsPbBr₃ NCs agglomerated quickly, causing an initial increase in A ₃₆₅ and λ _em, and complete NC degradation occurred within 17 min under continuous UV exposure—evidenced by the loss of PL as shown in Figure S6 (Supporting Information). It is well established that charges within the NCs can quickly migrate to the NC surface during photon absorption.^{[ 34 ]} This migration can influence the existing ligands already bound to the surface. Specifically, the charges may form or strengthen ionic interactions with these surface‐bound ligands, increasing their vulnerability to displacement. This displacement can lead to a drastic drop in PL_I, as it exposes surface defects that promote non‐radiative recombination of charges.

To gain insight into the mechanism behind PIAER of CsPbBr₃ NCs, we utilized the developed microfluidic platform to systematically investigate the effect of experimental parameters, including photon flux and haloalkane concentration, on the rate and extent of PIAER.

Figure 4 shows two exemplary in‐flow PIAER of CsPbBr₃ NCs with DCM and 1‐iodopropane, demonstrating photo‐activated anion exchange processes with in situ generated halide anions. In these two exemplary reactions, the λ _em shifted from 510  to 410 nm (green to blue; CsPbBr₃ to CsPb(Br/Cl)₃) in 30 min with DCM and from 510  to 667 nm (green to red; CsPbBr₃ to CsPb(Br/I)₃) in 70 min with 1‐iodopropane. These two examples illustrate the efficient bandgap tunability of LHP NCs with haloalkanes under UV exposure in an intensified photo‐flow microreactor over the entire UV/Vis spectrum (Figure 4A–D). Additionally, the results shown in Figure 4 demonstrate real‐time spectral monitoring capability of the developed microfluidic platform throughout the PIAER. Access to such high‐throughput in situ generated experimental data facilitates mechanistic studies of the PIAER of LHP NCs. By continuously monitoring the optical properties of LHP NCs in situ, we can better understand the dynamics of the PIAER toward precise bandgap engineering of high‐performing LHP NCs for device applications. We estimate that the current microfluidic setup produces ≈18 mL h⁻¹ of NCs solution, corresponding to 75.6 nmol h⁻¹ of CsPbBr₃ NCs. By scaling the microreactor volume to 1 mL and switching to continuous flow operation mode at 1 mL min⁻¹, production can be increased to 252 nmol h⁻¹ (Table S12, Supporting Information). Further scalability can be achieved by operating multiple flow reactors in parallel, enabling output rates of over 1000 nmol h⁻¹. These modifications provide a clear pathway for high‐throughput NC synthesis while maintaining precision and control.

Figure 4.

In situ monitoring of the in‐flow PIAER of LHP NCs. The central micrograph depicts images of the reaction droplet under UV illumination inside the single‐droplet photo‐flow microreactor throughout the PIAER. Time‐evolution of A) PL and B) UV/Vis spectra of CsPbBr₃ NCs during the in‐flow PIAER with DCM under a photon flux of 10 pmol.s⁻¹. Time‐evolution of C) PL and D) UV/Vis spectra of anion exchange reactions of a CsPbBr₃ NCs solution with 1‐iodopropane under a photon flux of 26 pmol s⁻¹. Experimental conditions: [CsPbBr₃] = 4.2 µm; [DCM] = 4.7 m; [1‐iodopropane] = 0.13 m; T = 23 °C.

The microscale flow reactor offers distinct advantages over conventional batch systems for capturing time‐dependent PL spectra during anion exchange reactions. Notably, the kinetics of ionic salt‐mediated anion exchanges reactions are rapid in both batch and microfluidic systems but exhibit superior uniformity and precision in flow, whereas photo‐induced anion exchange reactions, characterized by slower kinetics, display transient PL features such as double peaks and gradual lattice reorganization.^{[ 7} , ¹¹ , ^{12 ]} These unique dynamics, observed more effectively in microfluidic systems, emphasize the role of controlled mixing and photon flux, which enhance homogeneity and reduce reaction variability, as evidenced by faster PL shifts and narrower emission linewidths. Furthermore, the PL emission peak uniformity during the in‐flow PIAER indicates a homogeneous anion exchange reaction of all LHP NCs within the reaction droplet without the occurrence of separate populations with different compositions.

To highlight the efficiency and precision of the developed microfluidic platform, we compared it with traditional batch experiments for a PIAER of CsPbBr₃ NCs in the presence of DCM and 1‐iodopropane under similar conditions (Figure S7, Supporting Information). In a batch reactor, the PL spectra of CsPbBr₃ NCs underwent blue‐ and red‐shifts in 60  and 100 min with DCM and 1‐iodopropane, respectively.

To explain the faster anion exchange reaction in the microfluidic setup compared to the batch apparatus, we refer to the analytical calculations from Hamed et al.,^{[ 24 ]} which evaluated the photon flux enhancement factor for a vial (batch) versus a single‐droplet photo‐flow microreactor. Using the Beer‐Lambert law and geometric relationships, these calculations show that the photon flux absorbed per reaction droplet is 3.5× higher in the single‐droplet photo‐flow microreactor (0.76 mm internal diameter) compared to a glass vial (15.24 mm internal diameter).^{[ 24 ]}

To further characterize the effectiveness of the single‐droplet photo‐flow microreactor, we utilized an actinometer (4,4′‐dimethylazobenzene, DMAB) to measure the effective photon flux experienced by LHP NCs within the reaction droplet. By monitoring the UV/Vis absorption spectra of DMAB during the in‐flow photo‐isomerization we quantified the photon flux as a function of the LED power (Figure S2A–D, Supporting Information). The in situ photon flux measurement enables tuning the anion exchange reaction kinetics.

Next, we utilized the developed automated microfluidic platform with the single‐droplet photo‐flow microreactor to investigate the effect of photon flux and NC‐to‐haloalkane ratio on the PIAER of LHP NCs. We divided the anion exchange reactions into two separate studies: i) CsPbBr₃ to CsPb(Br/Cl)₃ and ii) CsPbBr₃ to CsPb(Br/I)₃. In both investigations, we varied the photon flux and evaluated the anion exchange reaction kinetics and its impact on the optical properties of the resulting NCs. Control (dark) experiments were performed before each set of experiments, confirming no physiochemical change or contamination in the reaction droplet containing LHP NCs.

2.1. PIAER of CsPbBr₃ NCs to CsPbCl₃ NCs

Figure 5 presents the effect of photon flux on the optical properties of LHP NCs during PIAER of CsPbBr₃ NCs (Br→Cl) in the presence of DCM. As expected, increasing the photon flux accelerated the anion exchange reaction (Figure 5A–C). It has been reported until now that the photo‐excited electron from the excited NC reduces the DCM,^{[ 11} , ¹⁸ , ³⁵ , ³⁶ , ^{37 ]} but our hypothesis is that there is an oxidative bond cleavage of DCM that generates chloride anions in situ. In this process, the photon flux is crucial in stimulating the NCs to initiate the anion exchange with DCM. The anion exchange reaction is a surface diffusion phenomenon,^{[ 13 ]} and due to the smaller size of chloride anions compared to other halides, there is lower diffusion resistance—resulting in a faster anion exchange reaction than other halides.^{[ 7 ]}

Figure 5.

Effect of photon flux on the photo‐induced CsPbBr₃ to CsPbCl₃ anion exchange reaction. Time‐evolution of A) λ_em, B) A₃₆₅, and C) FWHM of the same reaction droplet (LHP NCs in toluene+DCM). D) Time‐implicit FWHM versus λ_em. Experimental conditions: [NC] = 4.2 µm; [DCM] = 3.1 m; T = 23 °C.

The time‐implicit FWHM versus λ _em data under different photon fluxes (Figure 5D) suggest all PIAER follow the same path from CsPbBr₃ to CsPbCl₃. This result indicates that the PIAER of the same reaction droplet (LHP NCs in the presence of DCM) exposed to different photon fluxes converge toward a similar final state of chloride‐rich NCs. The consistent FWHM trends provide evidence that the pathways for achieving chloride‐rich NCs are uniform across different photon fluxes of the same PIAER. The initial increase of the NCs emission linewidth during the photo‐induced Br→Cl exchange reaction from 110 meV (at a λ _em of 510 nm) to 126 meV (at a λ _em of 475 nm), Figure S8A (Supporting Information) is attributed to the rapid initial surface anion exchange events, leading to homogeneous broadening as halide compositions change simultaneously.^{[ 7 ]} The decrease of the emission linewidth after a λ _em of 475 nm can be attributed to the formation of homogenous CsPbCl₃.

The observed decrease in FWHM during the PIAER from CsPbBr₃ to CsPbCl₃ NCs, along with a reduction in proxy PLQY $(\widehat{PLQY})$ (Figure S9A, Supporting Information) can be attributed to several factors. CsPbCl₃ has a smaller lattice constant than CsPbBr₃ due to the smaller ionic radius^{[ 38 ]} of Cl⁻ (1.81 Å) compared to Br⁻ (1.95 Å), resulting in reduced phonon interactions and weaker electron‐phonon coupling, leading to narrower emission linewidth.^{[ 39 ]} Additionally, CsPbCl₃ has a larger bandgap energy, reducing the density of states at the band edges and further narrowing the emission peak.^{[ 39 ]} However, the decrease in $\widehat{PLQY}$, suggests a higher density of intrinsic defects and surface trap states in CsPbCl₃ NCs, increasing non‐radiative recombination, and thereby reducing the efficiency of radiative recombination.^{[ 8} , ^{40 ]} Transmission electron microscopy (TEM) images of the final reaction product (CsPbBr_3−xCl_x NCs) after complete anion exchange reaction (Figure S10A–H, Supporting Information) revealed no change in the NCs morphology and an average NC size decrease from 8.94 ± 0.98 nm CsPbBr₃ to 8.50 ± 1.30 nm CsPbBr_3−xCl_x. X‐ray diffraction (XRD, Figure S12A, Supporting Information) analysis of the starting and final NC samples revealed a shift of the characteristic (100) peak position from 15.06 to 15.80 (2θ) mainly due to the decrease in atomic size from Br⁻ to Cl⁻. Furthermore, the presence of the characteristic planes of cubic phase CsPbX₃ [(100), (110), (200)] in both CsPbBr₃ and CsPbBr_3−xCl_x NCs samples confirmed no change in the crystal structure upon photo‐induced anion exchange. The well‐resolved lattice fringes observed in high‐resolution transmission electron microscopy (HRTEM) images further validate the high crystallinity (Figure S33A,B, Supporting Information).

We observed a similar trend for PL_A and λ _em when increasing the haloalkane concentration (Figure S13, Supporting Information). As the DCM concentration increases, the anion exchange reaction accelerates due to the increase in local halide anion concentration around the NCs. For converting CsPbBr₃ NCs to CsPbCl₃ NCs, maintaining a high photon flux preserves a high $\widehat{PLQY}$. This phenomenon can be attributed to a localized NCs surface healing process. Higher photon flux facilitates the faster generation of chloride anions, which promptly replace vacancies in the NCs lattice.

2.2. PIAER of CsPbBr₃ NCs to CsPbI₃ NCs

In transitioning from CsPbBr₃ NCs to CsPbI₃ NCs through a PIAER using only 1‐iodopropane, the process proved unsuccessful, leading to quenching of the PL spectra at 510 nm and the solution turning reddish‐brown, with no anion exchange observed (see Figure S14, Supporting Information). To overcome this issue, we explored thiol‐based additives, which have been reported to enhance the PLQY by passivating surface defects and mitigating non‐radiative recombination pathways.^{[ 41} , ^{42 ]} In particular, the nucleophilic nature of thiol aids in forming hydrogen iodide (HI)^{[ 43 ]} in situ with the iodine anion or radical being formed in the solution, which drives the Br→I anion exchange reaction. Upon screening two well‐known thiol‐based additives, i.e., DSH and thiophenol, we identified DSH as the most effective in promoting the Br→I anion exchange reaction, leading to the successful formation of CsPbI₃ NCs. In contrast, other thiol‐based additives, such as thiophenol (7 mm), caused immediate quenching of CsPbBr₃ NCs (Figure S15, Supporting Information).

During the Br→I PIAER, we observed that photon flux notably increases the anion reaction kinetics (Figure 6A). For example, increasing the photon flux from 10  to 100 pmol s⁻¹, decreases the PIAER time from 200 to 20 min. Increasing the photon flux during the photo‐induced Br→I exchange reaction of CsPbBr₃ NCs increases the anion exchange reaction rate while promoting the degradation of iodide‐rich CsPb(Br/I)₃ to the non‐perovskite phase,^{[ 44 ]} evidenced by the observed NCs degradation at a λ _em> 667 nm. Figure 6B shows the increase in A ₃₆₅ values of LHP NCs when transitioning from the bromide‐ to iodide‐rich NCs due to the higher absorption coefficient and bandgap structure differences.^{[ 45 ]} The decrease in the bandgap and increased UV/Vis absorption cross‐section of LHP NCs from chloride‐ to iodide‐rich crystals arises from increased ionic size and polarizability.^{[ 46 ]}

Figure 6.

Effect of photon flux on the photo‐induced CsPbBr₃ to CsPbI₃ anion exchange reaction. Time‐evolution of A) λ_em, B) A₃₆₅, and C) FWHM of the same reaction droplet (LHP NCs in toluene+1‐iodopropane). D) Time‐implicit FWHM versus λ_em. Experimental conditions: [NC] = 4.2 µm; [1‐iodopropane] = 0.13 m; [DSH] = 7 mm; T = 23 °C.

The emission linewidth of LHP NCs increases with higher iodide content (Figure 6C,D). CsPbI₃ NCs exhibit broader FWHMs than bromide‐ and chloride‐rich NCs^{[ 47 ]} due to larger iodide ions, increasing the phonon interactions. At a λ _em of 590–600 nm, the distribution of bromide and iodide anions in the NCs is balanced to minimize lattice strain.^{[ 47 ]} This reduction in lattice strain leads to fewer defects in the crystal structure, resulting in a relatively constant emission linewidth.^{[ 48 ]} At the photon flux of 120 pmol s⁻¹, there is an abrupt change in the emission linewidth before reaching a steady state. Figure S8B (Supporting Information) helps to comprehend the change in the emission linewidth in eV during the photo‐induced Br→I exchange reaction of CsPbBr₃ NCs; in which, the increase in FWHM can be attributed to homogeneous broadening, while the continuous change in the halide compositions followed by a decrease at a λ _em of 590–600 nm signifies the stabilization of the crystal structure. At a λ _em > 600 nm, the NC's emission linewidth increased as it became enriched in iodide, indicating increased static disorder in the NCs (Figure S8B, Supporting Information).^{[ 49 ]} We compared the FWHM values with literature data in Table S11 (Supporting Information), demonstrating that the PIAER method offers superior control over FWHM. TEM imaging (Figure S11A–H, Supporting Information) revealed no change in the NC morphology of iodide‐rich LHP NCs with an average size increase from 8.94 ± 0.98  to 10.05 ± 0.98 nm. XRD patterns of iodide‐exchanged LHP NCs (Figure S12B, Supporting Information) revealed a slight peak position shift from 15.03⁰ to 14.4⁰ (2θ) due to an increase in atomic radius from Br^– to I^– with no change in the crystal phase post– PIAER. The clear lattice fringes observed in HRTEM images provide further confirmation of the NCs high crystallinity (Figure S33A,C, Supporting Information). We observed a similar trend with λ_em, FWHM, and PL_A versus time with different concentrations of 1‐iodopropane (Figure S16, Supporting Information). Next, we studied the effect of the haloalkane bond dissociation energy on the photo‐induced Br→I exchange reaction by using 2‐iodopropane instead of 1‐iodopropane. In the case of 2‐iodopropane, the anion exchange reaction transitioning from CsPbBr₃ NCs to CsPbI₃ NCs took less than 20 min (Figures S17 and S18, Supporting Information) mainly because of the formation of more stabilized counter radical (CH₃–CH^•–CH₃) through hyperconjugation. Interestingly, the $\widehat{PLQY}$ of iodide‐rich LHP NCs in the presence of 2‐iodopropane was higher than with 1‐iodopropane, despite both maintaining a similar emission linewidth (Figures S9B and S17D, Supporting Information). This result suggests that the faster in situ generation of iodide radicals at lower photon flux is crucial for enhancing the emission properties of CsPbI₃ NCs after the anion exchange reaction. Stable radical formation in 2‐iodopropane under UV illumination facilitates rapid in situ iodine generation, enabling effective anion exchange reaction with minimal defects or NC degradation. For the conversions of CsPbBr₃ NCs to CsPbI₃ NCs using 1‐iodopropane and 2‐iodoropane, a lower photon flux results in a higher $\widehat{PLQY}$ (Figures S9B and S17D, Supporting Information). The reverse correlation of the photon flux and $\widehat{PLQY}$ is attributed to the more sensitive nature of the haloalkane and NCs to UV light, necessitating careful photon flux optimization to obtain the highest‐performing LHP NCs. Therefore, tuning the photon flux is essential to optimize anion exchange reactions, ensuring high $\widehat{PLQY}$ and efficient photoconversion in both scenarios.

3. Discussion

We leveraged access to the multi‐modal in situ spectra data during the reaction to understand the mechanism(s) behind PIAER of CsPbBr₃ NCs. During the PIAER from CsPbBr₃ to CsPbCl₃, the PL_A decreases initially due to ligand desorption caused by UV light irradiation^{[ 34 ]} and the polarity of the DCM (polarity index 3.5) compared to toluene (polarity index 2.4) (Figure 7A). It is well‐known that more polar solvents can negatively impact the optical properties and stability of all‐inorganic LHP NCs.^{[ 50 ]} Specifically, polar solvents can partially replace surrounding ligands and solvate PbX₂ and Cs‐complexes, forming strong coordinative complexes with Pb²⁺ and creating charge trap states that result in PL quenching.^{[ 51 ]} The quenching effect, chloride doping, and defect generation by the DCM solvent during the anion exchange were confirmed via transient PL experiments (Figure S19A,B, Supporting Information). Under a different series of control experiments, we investigated the effect of direct UV irradiation on CsPbBr₃ NCs in toluene (Figure S6C, Supporting Information), where PL_A drastically decreased within 5 min under a high photon flux (120 pmol s⁻¹).

Figure 7.

Mechanistic study of Br→Cl. A) Time‐evolution and B) time‐implicit (versus λ_em) variation of PL_A with [DCM] = 3 .1m. C) S‐V analysis of CsPbBr₃ NCs in toluene and different concentrations of DCM. D) The ratio of the initial (I₀) to the final (I) PL intensity from the S‐V experiments for different DCM concentrations. E) EPR measurement of NCs in DCM (3.1 m) when irradiated with 370 nm UV light (black) versus a simulated spectrum (red). F) In situ ¹H‐NMR spectra of the reaction product after 10 min irradiation with 365 nm UV light. All experiments used [NC] = 4.2 µm; T = 23 °C.

DCM generates chloride anions in situ under 365 nm UV radiation via hole transfer from the excited NCs. Cl⁻ anions occupy the available bromide vacancies on the surface of the CsPbBr₃ NCs,^{[ 40 ]} resulting in an initial increase (until a λ _em of 470 nm) followed by a decrease in PL_A for all the photon fluxes examined (Figure 7B). Initially, as the PIAER progresses, the NCs surface defect passivation by chloride anion doping results in enhancement of the emission intensity through the passivation of non‐radiative recombination centers. The initial emission intensity enhancement of CsPb(Br/Cl)₃ NCs is followed by a continuous decrease (λ _em < 470 nm) due to a reduction in the Goldschmidt tolerance factor.^{[ 52 ]} Furthermore, the increased chloride content of CsPb(Br/Cl)₃ NCs exacerbates lattice strain, leading to a higher likelihood of intrinsic defects such as vacancies and interstitial defects.^{[ 53 ]} Such intrinsic defects can act as non‐radiative recombination centers, thereby reducing the $\widehat{PLQY}$ of chloride‐rich LHP NCs (Figure S9A, Supporting Information).

To further investigate the photo‐induced anion exchange mechanism of LHP NCs, we performed Stern–Volmer (S‐V) analysis, shown in Figure 7C,D. The S‐V analysis probes the PL quenching of CsPbBr₃ NCs by DCM, shedding light on the interaction between the chromophore (CsPbBr₃) and the quencher (DCM). Upon photoexcitation of the NCs, an electron‐hole pair is generated. As illustrated in Figure 7C, increasing DCM concentrations in toluene‐dispersed NCs systematically decreases the PL_I, exhibiting a linear relationship that scales with DCM concentration. This linearity confirms the applicability of the S‐V analysis. From these observations, we can infer that the excited state of the NCs is quenched by DCM. Electron transfer from CsPbBr₃ to DCM is not feasible due to the energy mismatch between the conduction band (CB) of CsPbBr₃ and the reduction potential of DCM. Specifically, the CB of CsPbBr₃ is positioned at −1.3 V versus SCE (saturated calomel electrode), while the reduction potential of DCM is at −2.7 V versus SCE.^{[ 45} , ⁵⁴ , ⁵⁵ , ^{56 ]} Consequently, it is hypothesized that the hole generated upon photoexcitation is likely responsible for the oxidative bond cleavage of a C–Cl bond in DCM, ultimately leading to the formation of ^•CH₂Cl and Cl^•. Following H‐atom abstraction, HCl is produced and is the likely halide source resulting from photoexcitation. To further validate the hole oxidation mechanism during the PIAER, methanol was used as a hole scavenger^{[ 57 ]} to assess its impact on the reaction rate (Figure S20, Supporting Information). The PIAER data with and without the hole scavenger demonstrates that methanol inhibits the anion exchange reaction by competing with DCM for photogenerated holes, reducing oxidative bond cleavage.

We performed transient PL measurements to determine the nature of the quenching mechanism (Figure S19C,D, Supporting Information). The results, shown in Figure S19 (Supporting Information), suggest that the quenching is static, implying that the quencher (DCM) adsorbs onto the surface of CsPbBr₃ NCs before light excitation, most likely at vacancy sites. These vacancies are electron‐poor, and the dipole moment of DCM can align with the vacancy, facilitating a dipolar interaction. To confirm that CsPbBr₃ acts as a photocatalyst, we conducted photophysics studies using a high repetition rate Ti:S oscillator frequency doubled to 500 , 470 , and 390 nm. We observed that as the excitation wavelength decreased, the anion exchange rate increased in proportion to the absorption oscillator strength, suggesting that the NCs are acting as photocatalysts across their entire absorption profile (Figure S21A, Supporting Information).

To ensure there was no direct cleavage of C–Cl bonds under UV light activation, a control experiment with DCM and toluene (without CsPbBr₃ NCs) was conducted (Figure S22A, Supporting Information) which resulted in no change in absorbance, i.e., there was no direct formation of Cl^• or Cl₂ through the cleavage of DCM under UV light excitation.

To confirm the formation of Cl⁻ and CH₂Cl^•, we performed X‐band electron paramagnetic resonance (EPR) measurements, shown in Figure 7E. When CsPbBr₃ NCs are dissolved in DCM in the presence of a spin trap, such as N‐tert‐butyl‐α‐phenylnitrone (PBN), and photolyzed for 5 min under 365 nm irradiation, a triplet of doublets splitting pattern was observed with hyperfine values of a _N = 14.45 G and a _H = 2.74 G (Figure 7E). These likely correspond to the spin adduct formed by DCM solvent radicals (CHCl₂ ^•/CH₂Cl^•) resulting from selective excitation of the NCs (CHCl₂ ^•/CH₂Cl^•).^{[ 58 ]} To further illustrate radical formation during photo excitation we did a similar EPR experiment using carbon tetrachloride as an additive and captured CCl₃ ^• in spin‐trapping experiments (Figure S23, Supporting Information).

Next, to better understand the role of surface ligands on the PIAER of CsPbBr₃ NCs in the presence of DCM, we performed an in situ Proton nuclear magnetic resonance (¹H‐NMR) experiment with NCs exposed to 365 nm light for 10 min. No change in the ligand environment was observed during and after PIAER. Figure 7F shows a small desorption of oleylamine (OAm) and oleic acid (OA) from the NC surface, evidenced by decreased and narrower alkene peaks, further supporting the decrease in the NC's $\widehat{PLQY}$ due to surface defect formation (Figures S24 and S25, Supporting Information).

The in situ generated chloromethane radical (CH₂Cl^•) plays a crucial role in enabling the PIAER of CsPbBr₃ NCs to CsPbCl₃ NCs. The in situ generated CH₂Cl^• radicals remove bromine atoms from CsPbBr₃ NCs to form bromochloromethane (confirmed via ¹H NMR spectroscopy, Figure S22B, Supporting Information), which likely creates halide vacancies on the NC surface which are synergistically filled by chloride anions. Finally, OAm‐passivated bromide anions on the NC surface perform a halide exchange reaction with chloride anions.

We investigated the underlying mechanism of the PIAER of CsPbBr₃ to CsPbI₃ facilitated by DSH, using complementary in situ and ex situ characterizations utilized for the mechanistic studies of the photo‐induced Br→I exchange reactions. The increase in PL_A shown in Figure 8A for all photon fluxes tested is attributed to the surface passivation by DSH.^{[ 41} , ^{42 ]} The time‐implicit plot of PL_A (Figure 8B) demonstrates that the photo‐induced Br→I exchange reaction (same reaction mixture), under different photon flux, follows the same process. To further investigate the mechanism of the photo‐induced transition of CsPbBr₃ NCs to CsPbI₃ NCs, we performed S‐V experiments with and without the additive DSH (Figure 8C,D). We conducted a S‐V analysis similar to the Br→Cl anion exchange reaction. However, in this case, the conduction band electron likely reduces 1‐iodopropane as its reduction potential lies below the conduction band of CsPbBr₃.^{[ 45} , ⁵⁴ , ⁵⁵ , ^{56 ]} Further, we confirmed a static quenching process akin to the DCM case; here, 1‐iodopropane adsorbs on the surface of CsPbBr₃ (Figure S19E,F, Supporting Information). Upon addition of DSH to CsPbBr₃ NCs in toluene and 1‐iodopropane, the superior defect passivation^{[ 59 ]} of DSH leads to the NC's PLQY enhancement, which dominates any charge transfer effect; thus, we do not see quenching effects with different iodopropane concentrations (Figure 8E,F).^{[ 41 ]}

Figure 8.

Mechanistic study of Br→I. A) Time‐evolution and B) time‐implicit (versus λ_em) variation of PL_A with [1‐iodopropane] = 0.13 m and [DSH] = 7 mm and T = 23 °C. C) S‐V analysis of CsPbBr₃ NCs in toluene and different concentrations of 1‐iodopropane. D) The ratio of the initial (I₀) (no 1‐iodopropane) to the final (I) (with 1‐iodopropane) PL intensity from the S‐V experiments for different 1‐iodopropane concentrations. E) PL spectra of CsPbBr₃ NCs at different 1‐iodopropane concentrations in the presence of DSH. F) Ratios of the initial (I₀) (no 1‐iodopropane) to the final (I) (with 1‐iodopropane) PL intensity.

Next, we conducted a control experiment to study whether the UV light (365 nm) can photo‐dissociate 1‐iodopropane without the CsPbBr₃ NCs being present. UV irradiation of 1‐iodopropane resulted in an absorbance increase at 500 nm due to the formation of iodine molecules (Figure S26A, Supporting Information). An additional control experiment of CsPbBr₃ NCs in toluene with 1‐iodopropane and no DSH, resulted in no anion exchange reactions being observed (λ _em remained constant, Figures S14 and S26D, Supporting Information). However, upon the addition of DSH to the pre‐mixed solution of CsPbBr₃ NCs in toluene with 1‐iodopropane under UV illumination, the anion exchange reaction immediately proceeded (Figure S26E–G, Supporting Information). Therefore, DSH, due to its nucleophilic properties, likely inhibits the formation of iodine molecules. This inhibition occurs because DSH likely reacts with iodine radicals (I^•) generated from the cleavage of the C–I bond in 1‐iodopropane under UV irradiation to form hydrogen iodide (HI) ultimately. To further support this claim, another control experiment demonstrated that DSH stabilizes the iodine radicals produced during the photoexcitation of 1‐iodopropane. The UV/Vis spectra of the UV irradiated mixture of 1‐iodopropane with toluene and DSH did not indicate any formation of iodine molecules at 500 nm (Figure S26B, Supporting Information).

To further confirm the electron transfer from the NCs during the photo‐induced Br→I anion exchange reactions of CsPbBr₃ NCs, we conducted a wavelength‐dependent photolysis study due to the limitations of the S‐V analysis in the presence of DSH. Specifically, we used the same tunable titanium sapphire oscillator mentioned earlier and performed wavelength‐dependent photo‐induced anion exchange experiments of CsPbBr₃ NCs in toluene with 1‐iodopropane and DSH (500, 470, and 390 nm) for 1 hr, and recorded the NCs PL spectra every 10 min (Figure S21B, Supporting Information). Under 500 nm excitation, the λ _em remained constant with a peak sustained at 510 nm. Reducing the excitation wavelength to 470 nm initiated the photo‐induced Br→I anion exchange reactions of CsPbBr₃ NCs—exhibiting a slow initial phase (shifting the λ _em from 510 to 520 nm over 40 min) followed by a rapid increase of λ _em to 600 nm over the subsequent 20 min photolysis. As the NCs became more iodide‐rich, the rate of the photo‐induced Br→I anion exchange reaction increased. Under 390 nm excitation, the complete photo‐induced transition of CsPbBr₃ to CsPbI₃ NCs occurred within 1 hr. This result supports the proposed electron transfer mechanism from CsPbBr₃ NCs to 1‐iodopropane under UV light excitation, which reduces 1‐iodopropane, yielding I⁻, consistent with the relevant reduction potentials.^{[ 45} , ^{55 ]}

The additive, DSH, plays multiple roles during the photo‐induced Br→I anion exchange reactions of CsPbBr₃ NCs; it likely coordinates to lead,^{[ 42 ]} facilitates surface passivation, and acts as a nucleophile. It has been observed that the rate of the PLQY change of CsPbBr₃ NCs becomes enhanced using 365 nm light irradiation.^{[ 41} , ^{42 ]} We observed an increase in the PLQY of CsPbBr₃ NCs in the presence of DSH from 60% to near unity (≈98%) after 15 min of light excitation (Figure S26C, Supporting Information). This PLQY enhancement is believed to result from a thiol‐ene reaction with OA and OAm to form a thioether, which has a strong affinity for the stimulated NCs.^{[ 42 ]} Even under ambient room light, thiols can react with the C = C moieties present in OA, OAm, and 1‐octadecene (ODE), resulting in gradual increases of the PLQY.^{[ 42 ]} When exposed to UV light or a radical initiator, thiols (R–SH) undergo homolytic bond cleavage, forming thiol radicals (R–S^•) and hydrogen atoms (H^•), potentially leading to the thiol‐ene reaction^{[ 41 ]} (in situ ¹H‐NMR experiments indicates decreases in the peak area of olefinic species, Figure S27A,B, Supporting Information^{[ 42 ]}).

Finally, we investigated the intrinsic kinetics of both anion exchange reactions as a function of the applied photon flux (Figure S28, Supporting Information). The reaction rate constant |k| (eV.s⁻¹) increases linearly with photon flux for both Br→Cl and Br→I anion exchange reactions. The Br→Cl anion exchange reaction exhibited a higher reaction rate across all photon fluxes compared to the Br→I anion exchange reaction. The higher reaction rate of Br→Cl can be attributed to the smaller ionic radius of chloride versus iodide ions. To further validate the success of PIAER, we conducted compositional analysis of the anion‐exchanged LHP NCs with different λ _em (420  to 667 nm). Energy dispersive spectroscopy (EDS) and X‐ray photoelectron spectroscopy (XPS) were performed on CsPbBr_3−xCl_x and CsPbBr_3−xI_x NCs (Figures S29–S32, Supporting Information). EDS and XPS results confirmed the formation of mixed halide LHP NCs as a result of the PIAER.

4. Conclusion

In summary, we developed, characterized, and deployed an automated and material‐efficient photo‐flow microreactor for accelerated mechanistic studies of the PIAER of LHP NCs. The developed microfluidic platform integrated process intensification (3.5‐fold faster reaction in flow than batch) with reaction miniaturization (300‐fold reduction in precursor volumes in flow than batch) and multi‐modal in situ spectral characterization to unveil the underlying mechanism of the photo‐induced bandgap engineering of LHP NCs over the entire UV/Vis range. Automated microscale photo‐flow chemistry enabled rapid systematic investigation of the effects of photon flux and haloalkane concentration on the kinetics and overall optoelectronic properties of halide‐exchanged LHP NCs.

Complementary to in situ reaction studies, a multitude of ex situ measurements were utilized to understand the role of additives and surface ligands on the formation of anions and radicals during the PIAER of LHP NCs. We demonstrated photo‐induced anion exchange from CsPbBr₃ NCs to CsPbI₃ NCs using DSH as an additive. The proposed anion exchange reaction mechanism and the intensified photo‐flow microreactor have direct implications for scalable precision nanomanufacturing of high‐performing LHP NCs for applications in photocatalysis and photonic devices.

5. Experimental Section

Materials

Lead(II) bromide (≥98%), Cesium carbonate (Cs₂CO₃) (ReagentPlus, 99%), Oleic acid (90%, technical grade), N,N‐Dimethylformamide (99.8%, anhydrous), Dichloromethane (DCM, anhydrous, ≥99.8%), Tetrachloroethylene, 1‐Iodopropane, 2‐Iodopropane, 1‐Dodecanethiol, oleic acid (technical grade, 90%), Oleylamine (OAm, technical grade, 70%), 1‐Octadecene (ODE, technical grade, 90%), phenyl‐tert‐butyl‐α‐phenylnitrone(PBN), carbon tetrachloride (anhydrous, ≥99.5%), and toluene (anhydrous, 99.8%) were purchased from Sigma–Aldrich. Methyl acetate (99%, extra pure) was purchased from Acros Organics. All chemicals were used as received.

Cesium Oleate (Cs‐OA) Precursor Preparation

In a 20 mL septa vial, Cs₂CO₃ (101.7 mg) is added, followed by the addition of 5 mL ODE and 325 µL OA along with a magnetic stir bar. Subsequently, the vial and solution are placed under vacuum for 30 min at 120 °C. After 30 min, the temperature is increased to 150 °C, and the vacuum is replaced with N₂ flow for 10 min. Conditions held until Cesium lead bromine NC synthesis (see below).

Lead Bromide (PbBr₂) Precursor

In a 20 mL septa vial, PbBr₂ (69 mg) is added with 0.5 mL OAm, 0.5 mL OA, and 5 mL ODE. The vial and solution are placed under vacuum for 30 min at 120 °C. Subsequently, the temperature increased to 180 °C under N₂ purge for 10 min. Conditions held until Cesium lead bromine NC synthesis (see below).

Cesium Lead Bromide (CsPbBr₃) NC Synthesis

Once the Cs‐OA precursor temperature reaches 150 °C, the PbBr₂ precursor vial is heated to 180 °C. After maintaining a steady state temperature of the PbBr₂ precursor for 2 min at 180 °C, 0.6 mL of Cs‐OA precursor is injected into the PbBr₂ precursor vial to initiate the synthesis of CsPbBr₃ NCs. After 5 s, the reaction is rapidly quenched in an ice bath.

CsPbBr₃ NCs Purification

3 mL of the crude mixture containing CsPbBr₃ NCs are added to two 14 mL conical falcon centrifuge tubes. In each tube, 3 mL of methyl acetate (methyl acetate: NCs = 1:1 by volume) are added. The centrifuge tubes are then centrifuged for 6 min at 8500 rpm. Subsequently, the supernatant is removed, and the precipitates in each tube are redispersed in 3 mL of Toluene. Next, both samples are centrifuged again at 8000 rpm for 5 min, and a total 6 mL volume of purified CsPbBr₃ NCs is collected.

Experimental Platform

The key components of the developed microfluidic platform, shown in Figure 2, are a custom‐designed photo‐flow microreactor comprised of a high‐power, collimated light source (365 nm UV LED, Thorlabs, SOLIS‐365C) with automatically tunable power output (Thorlabs, DC2200 LED driver) integrated with a UV‐transparent fluorinated ethylene propylene (FEP) tube—0.762 mm internal diameter (ID), 1.5875 mm outer diameter (OD), 250 µL total fluid volume. The microreactor fluidic path utilized FEP tubing (0.508 mm ID × 1.5875 mm OD). The photo‐flow microreactor is enclosed in a bespoke computer numerical control (CNC)‐machined aluminum plate. The photo‐flow microreactor features parallel channels at the inlet and outlet sides. The photo‐flow microreactor's inlet and outlet channels are placed in a 3D‐printed flow cell made of black polylactic acid, which enables direct access to the spectral monitoring components, including a miniature spectrometer (Ocean Insights, Ocean‐HDXXR), a broadband light source (Ocean Insights, DH‐2000‐BAL), and a 365 nm LED (Thorlabs, M365LP1) connected via fiber optic patch cords (Ocean Insights, QP‐600‐1‐SR). The reaction mixture formulation is automated using computer‐controlled syringe pumps (Fusion 200 Chemyx) connected to a 250 µL glass syringe (SGE) through a T‐junction (IDEX Health & Sciences, polyether ether ketone). The microfluidic platform's inlet and outlet are pressurized using nitrogen (N₂) gas (Airgas, NI UHP300) using two pressure vessels (138 kPa, 20 lbf.in²). A computer‐controlled pump (VICI‐Valco Instruments, M‐6 Series) controls the movement of the reaction mixture microdroplet within the photo‐flow microreactor at a flow rate of 300 µL.min⁻¹ and an oscillation speed of 1.1 cm s⁻¹. After each experiment, the main fluidic path undergoes an automatic washing cycle with DMF, toluene, and DCM, utilizing N₂ as the carrier gas. A washing protocol enables the platform to continuously conduct PIAER experiments with high reproducibility. An automated refill module is used for toluene and haloalkane, while a 5 mL glass syringe (SGE) is employed for dimethylformamide (DMF). The integration and automation of optical instruments and fluidic delivery units are achieved using a custom‐developed process automation and data acquisition code (LabVIEW). A MATLAB code is then used for automated processing of the UV/Vis absorption and PL spectra obtained in situ. The process automation, (implemented in LabVIEW), takes input conditions from an Excel file with three columns: photon flux (mA), haloalkane (Vol%), and NCs (Vol%) for each experiment (Figure S3, Supporting Information). An automated refill module was also implemented to refill toluene and haloalkane syringes.

Batch Anion Exchange Reaction

Benchmarking of the flow‐ PIAER versus a batch reactor is conducted in a 6 mL glass vial with 100 µL of CsPbBr₃ NCs, 150 µL of haloalkane, and 50 µL of 1‐Dodecanethiol (when using iodopropane as the haloalkane) in 2.7 mL of Toluene. The same light source utilized for microfluidic studies of the PIAER (365 nm UV LED, Thorlabs, SOLIS‐365C) is positioned 2 cm away from the glass vial. For the spectroscopy measurements, an ex situ spectrometer is employed to determine absorbance and PLQY.

Ex situ Measurements

The purified CsPbX₃ (X: Cl, Br, I) NCs undergo various structural and surface characterization techniques, including TEM, EDS, ¹H NMR spectroscopy, time‐correlated single photon counting (TCSPC), XPS, and film XRD. TEM images are obtained using an FEI Talos F200X at 200 kV. EDS data are acquired with an FEI Talos F200X. ¹H NMR spectroscopy is conducted on a Bruker Avance NEO 400 MHz instrument at 25 °C using a standard proton pulse, 128 scans, and 1 s delay between the scans. LHP NC samples for NMR measurements are vacuum‐dried, redispersed in toluene‐d₈, and analyzed in 5 mm NMR tubes. For in situ NMR a Prizmatix LED light source coupled is used with optical fiber sanded at the region of the sample with power output 70 mW. Absorbance, PL_I, PLQY, and TCSPC measurements are performed using an Edinburgh Instruments FS5 Spectrofluorometer. Absorbance, PL_I, and TCSPC measurements utilize a standard cuvette holder, with TCSPC specifically using a 375 nm picosecond laser diode. PLQY is measured with an integrating sphere sample holder. XPS measurements were performed using an XPS/UVS‐SPECS system featuring a PHOIBOS 150 analyzer under ultra‐high vacuum conditions (≈3 × 10⁻¹⁰ mbar). The instrument was equipped with both Mg Kα (hν = 1253.6 eV) and Al Kα (hν = 1486.7 eV) X‐ray sources, with data acquisition conducted using the Mg Kα source at 10 kV and 30 mA (300 W). Survey spectra were recorded with a pass energy of 24  in 0.5 eV steps, and the C1s peak at 285 eV was used as an internal reference for calibration. The resolution of the analyzer was <1 eV, and data processing, including peak fitting, was performed using CasaXPS software. Samples for XPS analysis were prepared by drop‐casting 100 µL of sample onto silicon substrates, followed by drying at room temperature for 24 h. Grazing incidence X‐ray refraction (GIXRD) analysis is conducted at room temperature (298 K) using a Rigaku SmartLab X‐ray diffractometer with Cu Kα radiation (Cu Kα source, 1.54 Å, 44 mA, 40 kV) and parallel beam configurations. Samples are drop‐cast (100 µL) onto a microscope glass slide and allowed to evaporate in open air for 24 h. Room temperature electron paramagnetic resonance (EPR) measurements are recorded using a Bruker ELEXSYS E500 X‐Band CW spectrometer. EPR spin trap experiments are performed with N‐phenyl‐tert‐butyl‐α‐phenylnitrone (PBN) as the spin trap. Samples are prepared in an N₂ filled glovebox with 100 µL of CsPbBr₃ NCs, 1 mL of corresponding solvent, and 18 mg of PBN (0.1 m). Samples are then removed from the glovebox and an initial spectrum was acquired. Samples are then irradiated with 370 nm light for the specified amounts of time and spectra then re‐acquired. Simulated spectra are calculated using the MATLAB open‐source toolbox, Easyspin, using the esfit and garlic fitting functions.^{[ 60 ]} Corresponding hyperfine values are taken from the simulated spectra. The laser experiment is performed using a Coherent Chameleon Ultra II laser with a second harmonic generator (SHG HarmoniXX, A.P.E.) with a power 140 mW.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Supplemental Video 1

Jha P., Mukhin N., Ghorai A., Morshedian H., Canty R. B., Delgado‐Licona F., Brown E. E., Pyrch A. J., Castellano F. N., Abolhasani M., Photo‐Induced Bandgap Engineering of Metal Halide Perovskite Quantum Dots In Flow. Adv. Mater. 2025, 37, 2419668. 10.1002/adma.202419668

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.