Single-Particle Organolead Halide Perovskite Photoluminescence as

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Single-Particle Organolead Halide Perovskite Photoluminescence as a Probe for Surface Reaction Kinetics Juvinch R. Vicente, Ali Rafiei Miandashti, Kurt Waldo E. Sy Piecco, Joseph R. Pyle, Martin Kordesch, and Jixin Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03822 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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ACS Applied Materials & Interfaces

Single-Particle Organolead Halide Perovskite Photoluminescence as a Probe for Surface Reaction Kinetics

Juvinch R. Vicente,1,4 Ali Rafiei Miandashti,1 Kurt Waldo E. Sy Piecco,1,4 Joseph R. Pyle,1 Martin E. Kordesch,2,3 Jixin Chen1,3* 1Department

of Chemistry and Biochemistry, 2Department of Physics and Astronomy, and Quantum Phenomena Institute, Ohio University, Athens, OH 45701. 4Department of Chemistry, University of the Philippines Visayas, Miagao, Iloilo 5023, Philippines. 3Nanoscale

ABSTRACT The photoluminescence (PL) of organolead halide perovskites (OHPs) is sensitive to OHPs’ surface conditions and an effective way to report surface states. Literature has reported that at the ensemble level, the PL of photoexcited OHP nanorods declines under inert nitrogen (N2) atmosphere and recovers under subsequent exposure to oxygen (O2). At the single-particle level, we observed that OHP nanorods photoblink at rates dependent on both the excitation intensity and the O2 concentration. Combining the two sets of information with the charge trapping/detrapping mechanism, we are able to quantitatively evaluate the interaction between a single surface-defect and a single O2 molecule using a new kinetic model. The model predicts that the photodarkening of OHP nanorods in N2 atmosphere has a different mechanism than conventional PL quenching, which we call photo-knockout. This model provides fundamental insights into the interactions of molecular O2 with OHP materials, and help design a suitable OHP interface for a variety of applications in photovoltaics and optoelectronics. Keywords: Nanorod photoblinking, photo-knockout, adsorption and desorption, super-resolution optical imaging, Monte Carlo simulation

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INTRODUCTION Organolead halide perovskites (OHP) have emerged as a promising material for photovoltaics and optoelectronic devices.1–6 This group of materials offers the simplicity of solution-processing methods and excellent optoelectronic properties.7 Under light excitation, OHP films and nanoparticles are highly photoluminescent.8,9 This photoluminescence (PL) has been used to evaluate the charge carrier dynamics, photostability, and other properties of the OHP materials.10–12 Strikingly, the PL behaviors of OHP thinfilms, microcrystals, and nanocrystals are sensitive to various atmospheres.13–15 For example, exposure of OHPs to molecular O2 under illumination enhances the PL, which is quenched under inert gases such as Ar or N2.15 Similar PL enhancement is also observed in both all-organic and all-inorganic perovskite nanocrystals.14,16–18 The exact mechanism of the PL enhancement is still actively explored, but the annihilation of nonradiative relaxation pathways by molecular O2 has been suggested.14,15,17 In addition, the instability of OHP materials under illumination has been shown to be catalyzed by ambient H2O and O2.11,13,19–21 In both examples, the significance of interface- and surface-mediated processes is evident.17,21–26 Despite its outmost importance, surprisingly, a detailed kinetic model for these surface reactions on OHP materials is still lacking. At the single-particle level, OHP nanoparticles photoblink, i.e., the PL emission intensity fluctuates between bright- and dark-states under constant illumination.14,17,27 Although the exact mechanism of the photoblinking is still vague, several models have been proposed.14,16,17 Early reports suggest the involvement of surface trap-states.17,27 When an electron-hole pair is separated between the bulk and a surface trap-state, the charge gets trapped for a given time. It has been under debate that the trapped charge on the surface can activate the defect that effectively quenches the PL via a suspected trap-assisted exciton splitting process (the nanocrystal returns neutral after


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each cycle, type II),28,29 or the opposite charge left in the bulk creates a trion that actively quenches any newly generated electron-hole pair via a suspected Auger-assisted process (the nanocrystal stays charged before charge recombination, type I).29,30 In both cases, the charge-separated nanocrystal is in a dark-state. The crystal goes back to a bright-state after the separated charges recombine. In OHPs, these trap-states are likely related to surface-defects such as undercoordinated Pb2+ ions, CH3NH3+ vacancies, and/or X- vacancies24,31,32 that are known to act as effective PL-quenching sites on OHP nanoparticles and thin films.17,24,27,31,33 The significance of the surface traps in this mechanism is consistent with the sensitivity of photoblinking to its surrounding atmosphere.13,24 Molecular motions and chemical reactions on a single-molecule basis have been long sought interests in many fields.34 Surface reactions at single-molecule level has been studied using scanning probe microscopy (STM and AFM),35–37 near-field optical microscopy,34 and far-field optical methods such as surface-enhance Raman Scattering (SERS)38 and super-resolution fluorescence microscopy.39–41 We hypothesize that the PL of single OHP nanoparticle will be very sensitive to oxygen adsorbed to the surface-defects. In order to see the individual oxygen binding, we have to reduce the number of surface oxygen molecules. One way is to lower down the O2 concentration in the atmosphere and the other is to passivate the OHP nanoparticle surface to expose very few surface-defects. Herein, we used single-particle PL microscopy to explore the kinetics of the interactions between ligand-protected OHP nanorods and ambient O2. We synthesized colloidal OHP nanorods whose surfaces were fully protected by surfactant ligands. Then, we partially exposed their surface by solvent-washing. Ligand-protection was partially kept to control the number of surface-defects and maintain a single-particle PL response. We then evaluated the time-dependent PL of the OHP


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nanorods (1) under N2 atmosphere with varied excitation laser power densities, (2) under constant excitation power density with varied O2 concentration, and (3) under different laser power densities with alternating N2 and O2 atmosphere. The single-particle PL time-trajectories of >500 nanoparticles were then summed up to represent the ensemble PL behavior of the nanorods. For each nanoparticle, we analyzed its photoblinking kinetics and spatially super-localized its photoblinking centers. We found that the single-particle and ensemble PL behaviors of OHP nanorods under the controlled conditions were highly consistent with the charge trapping and detrapping mechanism. Based on the combined information from the ensemble and the singleparticle PL behaviors of OHP nanorods, a self-consistent kinetic model is proposed which fits the ensemble kinetic data with >95% confidence level. In the end, we used the proposed model to simulate the time trajectories of ~5000 individual OHP nanorods using Monte Carlo method that was able to reconstruct the observed single-particle and ensemble PL behavior.

RESULTS AND DISCUSSION Nanorod synthesis and characterization. We synthesized methylammonium lead iodide (MAPbI3) nanorods with an oleylamine/oleic acid ligand-assisted colloidal method (supporting information SI, S1.1). Optical and morphological characterizations confirm that we have synthesized the same MAPbI3 OHP nanorods as reported in the literature (Figure 1a).24,42 SEM images show that our OHP nanorods have typical lengths of 1400 ± 400 nm and widths of 125 ± 50 nm. Extinction measurement of the colloidal suspension of OHP nanorods shows a band-edge at ~760 nm, with a PL peak at 775 nm. These values are comparable to reported spectra for bulk OHP films,42 which suggests that quantum confinement effects in the OHP nanorods are not relevant. A representative PL image of OHP nanorods immobilized on a clean glass coverslip is


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also shown in Figure 1b. The image shows that the nanorods are sparsely distributed throughout the imaging area, with a few very bright spots that are most likely clusters of a few OHP nanorods (Figure 1b inset). The PL of the individual nanorods is tracked by taking a video over time and the sum of all particles (>500) of a few videos taken at different areas are used to represent the ensemble measurements. The atmosphere of the nanorods is controlled by a ~10×3.0×1.0 mm3 flow cell sealed on the glass coverslip and connected to N2 and O2 gas cylinders (Figure S3).

Figure 1. (a) Extinction spectrum (blue) of as-synthesized MAPbI3 OHP nanorods with ligand protection in toluene suspension and the typical PL spectrum (red) of a single OHP nanorod immobilized on a glass coverslip. B) A typical single frame PL image. The inset shows an SEM image of aggregated nanorods.

Removal of surface-ligand results to PL quenching in N2. In our first measurement, we evaluated the impact of ligand-passivation on the time-dependent PL of the nanorods at both the ensemble and the single-particle level (Figure 2). Ligand-passivation has been shown to be an effective means of improving the PL of lead halide perovskites.9,17,43 It is expected that the assynthesized nanorods (as-) are fully protected by ligands when dried.17 We introduced imperfect ligand-protection by sequential solvent-washing (sw-) in order to partially break the nanorods or


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partially remove the capping ligands on the surface (SI, S1.1).44,45 To further characterize our swOHP nanorods, we carried out solution 1H-NMR measurements in each solvent-wash cycle. The same approach has been used to investigate the surface ligand binding on CsPbX3 perovskites in recent reports.28,46,47 It has been observed that the surface ligands can be washed away or ligandexchanged from the surface of the perovskite nanocrystals, which introduces surface defects of halide vacancies. Our 1H-NMR measurements are consistent with these reports (SI Fig. S5). To minimize sample-variation, we used the same batch of washed nanorods in all our experiments. The increasing degree of surface exposure results in increasing PL decline in N2 (Figure S6). For the as-synthesized nanorods, their PL appears stable in N2, but has an initial decay in O2 and then gets stable at both the ensemble and the single-particle level (Figure 2a, 2b). We further found out that this initial decay in O2 is irreversible under dark in N2 (Figure 2c, top), neither reversible in O2 or N2 under relatively weak light (Figure 2c, middle). However, this initial decay in O2 is, surprisingly, reversible under stronger illumination in N2 (Figure 2c, bottom).


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Figure 2. The significance of surface-ligand and different atmosphere on the PL-behaviors of OHP nanorods. The (a) Ensemble PL trajectories of as-synthesized (as-) and solvent-washed (sw-) nanorods under N2 and O2. The inset shows the scheme of a flow cell. (b) Representative singleparticle PL from (a) (left) and corresponding PL-intensity distribution (right). (c) Ensemble PL of as-nanorods under alternating gas with different laser intensities. The black arrow (top) indicates when the laser was turned off, and the red arrow indicates when it was turned on for ~2 s to check the PL. Red and blue lines in the middle figure guide the average photocounts. (d) Ensemble (top) and representative single-particle PL (bottom) of sw-nanorods under alternating O2 (red) and N2 (blue) gas. For solvent-washed nanorods, the PL declines rapidly upon illumination in pure N2 but remains relatively stable in O2 (Figure 2a). The PL decline in N2 cannot be explained by photodegradation of CH3NH3PbI3 because the PL quickly recovers in O2 (Figure 2d). With an almost complete loss of PL in N2, if degradation is the reason, significant degradation should have occurred which would have shifted the emission peak. However, the emission peak stays unchanged over the illumination time (Figure S1). At the single-particle level in N2 or O2, no photoblinking is observed for as-nanorods but are clearly observed for sw-nanorods (Figure 2b, 2d). For the sw-nanorods, the dark state is more favored in N2 than in O2. The photoblinking of nanorods under O2 atmosphere persists over the measurement time (Figure 2b, O2 curve). However, surprisingly, a sudden loss of PL to a photobleaching-like state is observed in N2 (Figure 2b red arrow). The quenched PL in N2 recovers by sitting the nanorods in the dark for some time (Figure S6a) which is consistent with the literature report,48 or recovers immediately after flowing-in O2 (Figure 2d, Movie S1).


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Mechanistic discussion. The PL responses of both the as-synthesized and solvent-washed nanorods are consistent with those reported in the literature with the charge trapping/detrapping mechanism.17,24,27,49,50 Ligand is known to protect the surface-defect and prevent the charges from getting trapped on the surface-defects.9,17,51 When the surface is partially opened, the surfacedefect is able to trap charges that quench the PL via the nonradiative trap-assisted or Auger-assisted charge recombination processes.28,29,49,50,52 The mechanism why the nanorod has a higher PL in O2 than in N2 is still unclear and motivated this work. There are a few mechanisms proposed in the literature involving passivation of deep traps by adsorbed O2 molecules,18,32,53 and removal of trapped charges from the surface by molecular O2.17,19,20 The reversibility of this process agrees well with the weak physicochemical interactions of O2 with OHPs.32,53–55 We attribute the initial PL decay of as-nanorods in O2 to the penetration of oxygen molecules to the perovskite surface and replace the ligands which are better passivating agents than oxygen. Under illumination, the ligand shell is disturbed, and oxygen intercalates, resulting to slight quenching of the OHP nanorods, and possibly oxidize the surface under relatively strong laser intensities.54–56 In the as-perovskite, this negative effect of oxygen dominates because there are no exposed surface defects for the oxygen to bind. However, in sw-perovskite, the PL enhancement effect of oxygen by the passivation of the surface defects dominates the negative effect of oxygen. Under relatively strong light and inert gas N2, oxygen is removed from the OHP surface resulting in a recovery of the PL of the as-perovskite nanorods (Fig. 1c bottom). This assignment is supported by a kinetic model proposed later. Since no permanent photodegradation of sw-nanorods is observed in N2, we name the temporary loss of PL in N2 “photo-knockout” to distinguish it from other PL quenching mechanisms such as photodegradation and photoblinking. According to the recent


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literature,15,17,24,48 the sudden loss of total PL signal in N2 is associated with the accumulation of the trapped charges on the surface. In macroscopic films and ensemble level, this behavior could be observed as a gradual, smooth decline in PL and simply referred to as “PL quenching” or “PL decline”. However, at single particle/domain level with sizes the same magnitude as the quenching volume of a single quenching site in OHPs (~7 x 104 nm3 as estimated by Tian et al.),27 the “photoknockout” effect is observed as a step-wise loss of PL, as in the case of our OHP nanorods. This phenomenon can be observed from quantum dots 0.50 W/cm2).


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Figure 3. The influence of excitation intensity on the PL decline of OHP sw-nanorods under pure N2 atmosphere. (a) Normalized ensemble PL trajectories of sw-nanorods exposed to pure N2 under various laser intensities. Each halo in (a) represents the standard deviation between >3 replicate measurements. (b) Initial PL-decay rates (first ~10 s) as a function of excitation intensity with a trendline shown for the lower densities. (c) A series of representative single-particle PL trajectory for each corresponding ensemble measurement in (a). (d) Corresponding PL intensity distribution for the single-particle PL trajectory in (c). (e) Shows the PL intensity distribution for all the particles analyzed. The color of the curves in (c), (d) and (e) correspond to that of (a).

At the single-particle level, a clear correlation between the photo-knockout rate and the excitation intensity is observed (Figure 3c), that is, the higher the laser intensity, the faster the nanorods are knocked out into the dark state. Accordingly, the probability distribution of PL


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intensity shifts towards darker-state as the excitation intensity is increased for the single-particle (Figure 3d) and the ensemble level (Figure 3e). The similarity observed between Figure 3d and Figure 3e suggests that we have picked a typical trajectory to be displayed in Figure 3c among the single-particle trajectories that are not shown. The average photo-knockout times for all particles studied in each excitation intensity will be further analyzed later. The laser excitation intensity-dependence of the PL decay in N2 is consistent with the charge trapping/detrapping mechanism, i.e. the stronger the laser, the faster the charges accumulate on the surface when N2 cannot help in detrapping them. We exclude the multiphoton process, i.e. multiple electron-hole pairs in the same nanorod at a given time that diffuse close enough to interact with each other, which could affect the PL-laser power dependence (Figure S8). Thus, assuming each electron-hole pair is statistically equivalent, a higher rate of charge trapping is observed for the nanorods in N2 under stronger laser radiation. When multiple charges are accumulated on the nanorod surface in N2, the nanorod turns dark by the photo-knockout effect. The saturation of the dependence in Figure 3b suggests that a limited number of charges can accumulate on a single nanorod surface. The trapping of multiple charges is consistent with the literature,27 and are also supported later by the super-resolution analysis on the PL of nanorods. Alternatively, the PL-decline could be explained by the photo-assisted degradation of perovskite. However, our extended measurements showed that photodegradation is not significant under N2 even at an excitation of 1000 mW/cm2 (Figure S7b). At the same excitation of 1000 mW/cm2, the onset of photodegradation in the air is only significant at t > 500 seconds which is far beyond the timeframe of our experiments (Figure S7b – c). For this reason, photodegradation has been ruled out as the significant factor dictating the observed laser-dependent PL decline in OHP nanorods.


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Exposure of OHP sw-nanorods to molecular O2 retards their PL decline rate. Under the same laser intensity, a clear anti-correlation between the ensemble PL decay rate and the O2 concentration is observed (Figure 4a, Movie S3), i.e., the higher the O2 concentration, the slower the decay. The initial slope of the decay curves vs the O2 concentration is shown in Figure 4b. The relationship also appears to be non-linear, following a Langmuir adsorption-like curve. At higher O2 concentration (>2.0% [O2]), the O2 effect saturates and no significant difference on the decay rate was observed between 2.0% and 100% pure oxygen (Figure S9). However, we are able to lower down to relatively low O2 concentration 0.0-1.0%. In this regime, a linear approximation can be applied.

Figure 4. The effect of O2 concentration on the PL decline of OHP nanorods under constant excitation intensity. (a) Normalized ensemble PL trajectories of sw-nanorods exposed to constant excitation intensity of 400 mW/cm2 under a series of O2 concentrations. Each halo in (a) represents the standard deviation between replicate measurements. (b) Initial PL-decay rates (first ~10 s) as a function of O2 concentration with a trendline show for lower O2 concentrations. (c) A series of


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representative single-particle PL trajectory for each corresponding ensemble measurement in (a). (d) Corresponding PL intensity distribution for the single-particle PL trajectory in (c). (e) Shows the PL intensity distribution for all the particles analyzed. The color of the curves in (c), (d) and (e) correspond to that of (a).

At the single-particle level, the anti-correlation between oxygen concentration and photoknockout time is evident at lower O2 concentrations (Figure 4c). Consequently, as the concentration of O2 increases, the probability distribution of the PL intensities is gradually shifted towards the bright-states at both the single-particle and the ensemble level (Figure 4d and e). The average photo-knockout lifetime of the single-nanorod will be further analyzed later. The O2 dependence is also consistent with the charge trapping/detrapping mechanism. 15,32,53

Similar effect of O2 on the PL intermittency of MAPbBr3 nanocrystals has been observed

very recently.14 When the surface is passivated by O2, it no longer has the ability to trap the electrons and holes, or the trapped charges can be effectively removed by the oxygen molecules. The Langmuir-like surface adsorption curve and the saturation of the PL decline rate at ~2.0% (v/v) O2 suggests a diffusion-limited surface adsorption/reaction mechanism.

Photoblinking and Photo-knockout Analysis To get a quantitative idea on the observed PL intermittency in the individual OHP nanorod from each experimental condition discussed above, we extracted their photoblinking and photoknockout statistics using a typical method in the literature.58,59 The scheme of the analysis is shown in Figure 5a-c and details are provided in the supporting information (SI, S2.2). Briefly, we extracted the probability distributions of ON-state and OFF-state dwell-times from the single-


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particle PL trajectories, from here-on we refer as tON and tOFF, respectively. We also defined photoknockout time tKO in each curve as the total time it takes for an individual particle to turn dark and fail to recover from an OFF-state to the ON-state until the end of our observation window. According to our extended measurements, the probability of these nanorods to get back to ONstate under the same condition is negligible.

Figure 5. Photoblinking statistics of individual OHP sw-nanorods exposed to different excitation intensities and O2 concentrations. (a-c) A scheme of photoblinking analysis. (d-f), The effective average dwell-times of ON, OFF, and photo-knockout states under different excitation intensities and (h-i) O2 concentrations within our measuring window.

In our analysis, we simply assume that the OFF-state is solely ascribed to trapping of a charge in a surface-defect and the ON-state to the subsequent detrapping of the charge. This way, we can relate the average lifetimes of the ON-states (τON) and OFF-states (τOFF) on the rates of charge trapping (kT) and detrapping (kDT) according to Eq. 1 and Eq. 2, respectively. That is, an increased rate of trapping is manifested by a reduced average ON-state lifetime, and vice versa.


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The same correspondence could be said, too, for average OFF-state lifetime and the detrapping rate. We are going to refer to this concept to bridge our single-particle analysis results to their observed ensemble PL behavior in the next section. 𝑘T =

𝑘DT =

1 𝜏ON

(1)

1

(2)

𝜏OFF

The probability distribution P(t) of both ON and OFF-state duration obeys power-law behavior at short ON-duration events with –m power, and crosses over to exponential distribution for long-time events (Figure S10, S11), the cross-over time is indicated by the truncation time, tc (Eq 3).58,60 𝑃(𝑡) ∝ 𝑡

―𝑚

𝑒

―

𝑡 𝑡c

(3)

These distributions are consistent with the photoblinking behaviors observed in colloidal quantum dots including a variety of perovskite nanoparticles.14,16,24,29,58 SI, Figure S10 summarizes the qualitative effect of excitation intensity and oxygen concentration on the probability distribution of tON and tOFF of OHP nanorods. While we recognize that an average is not well-defined for power-law distributions, we believe that it’s reasonable to get an effective τON and τOFF values within our experimental time frame, which are summarized in Figure 5d-e, and 5g-h, which we will elaborate in the following paragraphs. OHP photoblinking is extremely sensitive to surface reactions. One trapped charge is capable to quench ~106 electron-hole pairs cm2 W-1 s-1 under N2 (SI Figure S12), and a single oxygen molecule is able to remove the trapped charge. As such, the surface adsorption/reaction signal of a single oxygen molecule is amplified a million times and becomes easy to detect.


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In N2, photoexcited charge carriers promote activation of surface traps but do not affect the detrapping rate. The gradual decrease in effective τON (the lifetime of the state with no trapped charges) with the increase of excitation intensity suggests that the increase in laser intensity results in faster charge trapping and a reduced tON time (Figure 5d). The effective τOFF values appear relatively consistent over the range of excitation intensities (Figure 5e), indicating that the generated photoexcited charges have no significant effects on the detrapping kinetics of charges from surface-defects. Molecular O2 facilitates relaxation of an active trap and inhibits their reactivation. The distribution of tON and tOFF between pure N2 and 2.0% O2/N2 atmosphere is summarized in Figure S10c-d. Qualitatively, it is evident that the distribution of tON shifts towards higher values in the presence of molecular O2 (Figure 5g). At the same time, the distribution of tOFF shifts towards lower values (Figure 5h). This clear positive correlation between effective τON and increasing O2 concentration suggests that adsorbed molecular O2 is capable of passivating the surface-defects to prevent their activation, i.e. hinder charge-trapping (Eq. 1). Possible mechanisms responsible for this have been reported in the literature.18,32,53 In addition, the effective τOFF is anti-correlated with the increase in O2 concentration. This indicates that besides passivation of surface-defects to prevent a charge trapping, molecular O2 also facilitates detrapping of a charge from them after a charge has already been trapped on the surface (Figure 5h and Eq. 2). The charge-detrapping by molecular O2 is consistent with recent reports.17,19,20 The photo-knockout analysis also supports the same charge trapping/detrapping mechanism. When at any given period of time, there is one or more than one charge trapped by the surface of a nanorod, the nanorod stays the “OFF-state”, the “photo-knockout” effect. Stronger lasers accumulate the surface charges faster than weaker lasers (Figure 5f) and higher oxygen


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concentration slows down this process (Figure 5i). The presence of multiple charges is further supported by the super-resolution localization analysis in the next section.

Super-resolution analysis of single-nanorod PL The surface-defects are randomly distributed over the sw-nanorod and stochastically activated/deactivated under illumination. To get an idea on the spatial distribution of the surface-defects over the nanorod structure, we performed super-resolution localization analysis of multiple individual nanorods. Similar analysis has been done previously in the literature.27,33 The details of the analysis are discussed in the supporting information (S2.4) and the scheme is shown in Figure 6a. Briefly, the frame-by-frame emission profile of each identified particles is fitted with a rotating 2D-Gaussian equation to determine its center position. This position represents the average position of the emissive portions of an individual nanorod. Thus, for a completely emissive OHP nanorod in its ON-state, this position is right at the center of the particle (Figure 6a, red). In the same scheme, it is also shown how a quenched/non-emissive part of a particle in OFF-state shifts the average position of the localization. The presence of PL localization “hotspots” for ON and OFF states suggests that only a few “fixed” defects are responsible for the blinking behavior observed in each nanorod (Figure 6b). This observation agrees well with recent reports.24,27,33 In this study, we subjected the nanorods to several cycles of alternating pure O2/N2 atmosphere while monitoring the localization of ON and OFF-state PL (Figure 6c-d). More examples are shown in Figure S13. In all cases, the ON-state localization stayed practically constant, however, it appears that the OFF-state localization fluctuates between cycles. We ascribe this spatial variation from the stochastic activation and deactivation of different surface-defects in each cycle. This implies


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that in most OHP nanorods, there are more than enough surface traps capable of bringing it to photo-knockout state when activated. Due to the heterogeneity of our particle size, we could not get a reliable estimate of this value, however it has been suggested that it is in the magnitude of ~1016 cm-3.27,61 Another possibility is that some surface-defects were permanently oxidized by O2 in earlier cycles, hence, they cannot be reactivated anymore in the later cycles. However, within our experimental conditions, we think this is less likely due to the high reversibility of O2 passivation as shown above (Figure 2d).

Figure 6. Spatio-temporal analysis of ON and OFF-states using super-resolution localization microscopy. (a) A scheme of the super-resolution analysis. The intensity profile of each particle is fitted to a 2D-Gaussian function. (b) Spatial distribution of ON-state (red) and OFF-state (blue) from a representative OHP nanorod for a 2000-frame video. The overlaid rectangle dashed-line is the estimated size of the OHP nanorod from the diffraction-limited image (c) The PL-trajectory (left) of an OHP particle under several cycles of O2 (red) and N2 (blue) atmosphere. Distribution of PL-intensities from the trajectory (left). Note that three distinct states are arbitrarily identified.


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(d) The spatial distribution of the three distinct states identified in (c) over 4 cycles of O2/N2 atmosphere. Scale bar is 100 nm. The inset diffraction-limited image is ~2×2 µm.

Proposed Kinetic Model. Based on observed ensemble and single-particle PL behaviors of OHP nanorods in N2 and O2, we propose a kinetic model and fitted the sw-nanorod data at a high confidence level (R2≈1, Figure 7). In this model, we hypothesize that the quenching of the PL is solely attributed to the activation of an exposed surface-defect. To begin with, we assume that the series of solvent washing effectively exposes surface-defects that lack ligand-protection, which we label here as, SD, to distinguish them from ligand-protected surface-defects, SDLig. From Figure 2a, we know that exposure of OHP nanorods to O2 results in PL recovery due to passivation of surface-defects with O2, which we now note as SDO2. Evacuating O2 and replacing it with N2 under illumination results to a PL decline (Figure 2d). Hence, illumination (hν) is capable of kicking off O2 molecule from the surface-defects, which provides the basis for a reversible reaction 1 (Figure 7a). The reversible reaction 2 simply illustrates the photoblinking behavior of the nanorods, where SD and SD* represent empty and activated surface-defects, respectively. Since the increasing concentration of O2 results in decreasing average τOFF, we propose that O2 facilitates detrapping of a charge from SD* as described in reaction 3 (Figure 7c). Finally, when the fully ligand-protected nanorods were exposed in O2 under illumination, the PL declined, while no decay was observed under N2 (Figure 2a). These experiments suggest that under illumination, O2 is capable of replacing a surface-ligand even on the fully ligand-protected nanorods while N2 cannot (Figure 2c). When O2 replaces a bound surfactant from the surface of the nanorod (SDlig), the unbound


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surfactant (Lig) stays on the proximity of the nanorod due to the van der Waals force among the ligands. Thus, when an O2 molecule is kicked-off from the surface of the nanorod with the aid of light (hv), unbound surface ligands could take its place (Figure 2c, bottom). This process is described in reaction 4 (Figure 7d).

Figure 7. (a-d), Proposed surface reaction mechanism. e, Global fitting of the two sets of data in Figure 3 and 4 using the same parameters from the reaction model. (f,g) Monte Carlo simulated ensemble accumulated from 5000 individual particles and the representative single-nanorod PLtime trajectories.

The two sets of ensemble PL behavior in Figure 3a and Figure 4a are globally treated in three steps (SI, S3). Briefly, these three steps are (1) analytically fitting the data using approximations to obtain the initial estimates of the parameters shown in Figure 7a-d, followed by (2) numerical fitting, and (3) Monte Carlo single-particle simulations. The goodness of the numerical fitting (Figure 7e) and examples of Monte Carlo simulated trajectories are shown in


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Figure 7f-g. We are able to fit the ensemble results with >95% confidence level and R2≈1. The results of the numerical fitting are summarized in Table S2. Monte Carlo simulation using the parameters from the numerical fitting of the ensemble data is able to reproduce representative (averaged) single-particle PL trajectories that contain equivalent features of typical experimental trajectories. In this kinetic model (Figure 7), under pure N2, all reactions with O2 as a reactant diminish (i.e. reaction 1, reverse and reaction 3, forward). Thus, both SD and SD* accumulate under illumination over time. Eventually, the SD* concentration will reach a point where at the singleparticle level it remains above one per nanorod over time, bringing the nanorod to a photoknockout state (Figure 7f, bottom). In the presence of bulk O2, the O2-associated reactions in reactions 1 and 4 become significant, which effectively reduces the concentrations of SD and SD*. Thus, photo-knockout is seldom observed when O2 is present (Figure 5 and 7g, bottom) and τKO decreases with the increase in O2 concentration (Figure 5i). This model could also kinetically explain the saturation of PL-decline observed in Figure 3b and 4b, that is, multiple carriers or multiple O2 molecules on the nanorod surface create a rate-limiting step. Reaction 4 is suggested to be a rate limiting step for relatively high oxygen concentration. When removing reaction 4 from the model, the remaining three reactions can explain the trend of the laser and oxygen responses but no longer able to fit the curves (SI S3). As such, reaction 4 is necessary which further supports the assignment we have made before for the oxygen-ligandperovskite interactions. However, this assignment requires future investigations. Although the fitted values in Table S2 are limited to the degree of washing of the nanorods and thus are subjected to variations for different batches of nanorods, and the model is developed for conditions under relatively mild laser power and low oxygen concentrations, interesting


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information can be suggested from the fitted rate constants. For example, the rate constant of the light-assisted O2 desorption kd = (k12+k13+k14) is fitted 4.3±0.2×10-2 J-1 cm2. Assuming a small area, A, on the surface absorbs a photon at laser power density P, with 90% probability the O2 desorbed from the surface. This area A is the effective adsorption cross-section that triggers O2 desorption. The photon energy of 473 nm laser is ε = 4.2×10-19 J, and k = kdP = PA/ε. Thus, A = kd ε =1.8×0-20 cm2. This is a very small absorption cross-section, suggesting that only the electron adjacent to or at the adsorbed O2 site, when excited, has the ability to remove the O2 from the surface, while the electrons excited in the bulk, which quickly relax to the conduction band edge within 1 ps,62 has negligible ability to remove the O2. Our kinetic model hypothesizes that the trapped charges quench the PL based on the literature report.29 One mechanism is Auger-assisted quenching,57 when both hole and electron left in the bulk of the perovskite nanocrystals have been believed to form positive and negative trions respectively that quench the PL.63,64 This mechanism has been associated with strong light (multiple photon absorption) and quantum confinement,29,57,65 which is inconsistent with the relatively weak light and large-sized particles we used. The localized PL quenching we observed in Fig. 6 also disagrees with this mechanism because trions should be free to diffuse in the NR, which would have quenched the PL more evenly along the whole NR. Thus, the direct trap-assisted exciton splitting mechanism is more consistent with our observations.28 The mechanism of the trap activation is still under debate, which has been speculated to charge-induced ligand rearrangement, surface chemical reaction, and/or environmental changes.29 The energy level of inactivated traps has been believed to lie within the bandgap, which is associated with halide vacancies.28,29,31 We suspect that oxygen deactivates the surface defects by raising the energy level of the surface defect, which also reduces the probability of the next charge-transfer event.


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CONCLUSIONS In summary, the series of measurements of the PL in N2 and O2 at the single-particle level support the surface charge trapping/detrapping mechanism for OHP nanorods. (1) We have shown that OHP single-particle photoluminescence (PL) has a single-molecule sensitivity to O2, which could be used to probe the surface reactions of OHPs. In particular, the exposure of an OHP nanorod to single O2 molecules under illumination enhances its PL, by facilitating the relaxation of an active trap and effectively passivating the surface-defect from further activation. (2) By systematically measuring PL under controlled excitation intensities and O2 concentrations, we are able to come up with a detailed kinetic model that is consistent with the observed single-particle and ensemble PL behaviors of the OHP nanorods. We proposed the simplest model that can fit the data globally in which each step may not necessary represent the unit reaction. Although successful model fitting cannot confirm the charge trapping/detrapping mechanism, it certainly provides essential support. Monte Carlo simulation of single-particle trajectory suggests that the photoknockout effect observed in inert gas is due to the accumulation of multiple charges in an OHP crystal that effectively quenches the PL of the whole crystal. These findings could provide a potential strategy to design suitable interfacial chemistry for photovoltaic, optoelectronic devices and gas sensors. (3) While bulk measurements on OHP materials surely revealed a great deal of information about their interaction with their ambient atmosphere, we believe that the singleparticle PL approach we carried out in this study offers a new and unique perspective. Particularly it can be used to develop and test sound and quantifiable mechanistic model that could help explain some of the optoelectronic properties of OHPs. We further believe that the approach used for OHPs in this report could be extended to other luminescent materials.


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Supporting Information The Supporting Information is available free of charge on the ACS Publication Website at DOI: XXX. Supporting Information. Experimental Methods - Detailed methods for the preparation of CH3NH3PbI3 nanorods; Detailed Methods and additional data on the characterization of OHP nanorods Sample preparation for PL imaging; Fluorescence microscope and experimental set-up for PL imaging. Additional data on the effect of O2 concentration on PL decay. Data Analysis Single-particle identification and PL-trajectory; Photoblinking analysis; Data on the distributions of tON and tOFF lifetimes; Additional data on the blinking amplitudes as a function of excitation power densities; Super-resolution imaging analysis procedure and additional data. Kinetic Model Development. (PDF) Movie S1. Example single-particle PL trajectories under alternating O2-N2 atmosphere. (AVI) Movie S2. Representative PL-videos from samples in constant N2 atmosphere under 50 mW/cm2 and 1000 mW/cm2 excitation laser intensities. (AVI) Movie S3. Representative PL-videos from samples in constant excitation intensity under pure N2 and 2.0% O2 concentrations. (AVI) Data S1 and Data S2. Simulated PL data for the 4-reaction kinetic model in Figure 7f and 7g, respectively of the main text. (XLSX)

ACKNOWLEDGMENTS We thank Ohio University startup fund; Prof. Hugh Richardson for beneficial discussions; and Dr. Andrew Tangonan for his help with the 1H-NMR measurements.

AUTHOR INFORMATION Author Contributions


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Major experimental design, data analysis, and writing were done by J.C. and J.R.V.; J.R.V collected the major data and the rest of the authors contributed in data collection; all authors read and proved the manuscript. Competing interests The authors declare no competing interests. Corresponding Author *Jixin Chen: [email protected]

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Single-Particle Organolead Halide Perovskite Photoluminescence as

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