Organic–Inorganic FAPbBr3 Perovskite Quantum Dots as a Quantum

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Organic−Inorganic FAPbBr3 Perovskite Quantum Dots as a Quantum Light Source: Single-Photon Emission and Blinking Behaviors Cong Tai Trinh,†,‡ Duong Nguyen Minh,§,‡ Kwang Jun Ahn,⊥ Youngjong Kang,§ and Kwang-Geol Lee*,† †

Department of Physics, Hanyang University, Seoul 04763, Republic of Korea Department of Chemistry, Research Institute for Natural Sciences, Institute of Nano Science and Technology, Hanyang University, Seoul 04763, Republic of Korea ⊥ Department of Physics and Department of Energy Systems Research, Ajou University, Suwon 16499, Republic of Korea

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ABSTRACT: In the past decade, lead halide perovskite nanocrystals or quantum dots (QDs) have attracted keen interest due to their potential applications in many optoelectronic systems. In addition, all-inorganic (CsPbX3) perovskite QDs are suggested to be efficient single photon emitting centers. Herein, we study the photon emission properties of recently synthesized organic− inorganic FAPbBr3 QDs. Our results show that individual FAPbBr3 QDs can act as good single-photon sources with very low multiphoton emission probability achieved by extremely fast nonradiative Auger recombination. However, they exhibit photodegradation and fluorescence intensity intermittency, called blinking. By analyzing the ON(OFF) duration time distribution, particularly the OFF duration times, we suggest that two types of blinking (type-A and type-B) simultaneously contribute to the blinking behavior of FAPbBr3 QDs. In type-A and type-B blinking, the ON/OFF periods are attributed to charged/discharged states and to activation/deactivation of fast nonradiative recombination centers, respectively. By analyzing the ON/OFF duration cutoff time as a function of the excitation intensity, we verify that type-A blinking is caused mainly by diffusion-controlled electron transfer, partially accompanied by Auger ionization processes. KEYWORDS: perovskite, FAPbBr3, quantum dot, blinking, DCET, single-photon emitter, organic−inorganic

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study the photon emission properties of single FAPbBr3 QDs to reveal their photon emission properties including the quantum nature of single-photon emission and blinking behaviors. Our results show that an individual FAPbBr3 QD can act as a good single-photon emitter with very low multiphoton emission probability. This is caused by extremely fast nonradiative Auger recombination, as reported in allinorganic perovskite QDs.15 Nevertheless, photochemical degradation still occurs, particularly at much higher excitation intensity and/or longer illumination time. In regard to the blinking behaviors, two approaches have been widely accepted in the current understanding of this response in QDs: type-A is the charging/discharging model and type-B considers activation/deactivation of nonradiative recombination centers to explain the ON/OFF periods.21−23 In our experimental studies on FAPbBr3 QDs, both the ON and OFF duration time statistics exhibit a power-law relation followed by exponential truncation cutoff. This is a signature of photoionization (type-A: charging/discharging) in QD blinking.15,21 Interestingly, the OFF duration time distribution

luorescent nanoparticles are key components in current nano, bio, and quantum optical science and technology.1−3 Among them, lead halide perovskites and their nanocrystallines (or quantum dots, QDs) have attracted keen interest in recent years for their potential applications in numerous optoelectronic systems including solar cells, lightemitting diodes, photodetectors, and lasers.4−8 In particular, an impressive improvement in light harvesting efficiency has been a driving force in studies of perovskite materials.6,9,10 This exotic performance of perovskites in solar cells originates from their suitable energy gap, high absorption cross-section, high carrier mobility, long diffusion length, and slow radiative recombination of photoexcited charge carriers.11−14 In addition, all-inorganic perovskite QDs have been demonstrated to behave as single-photon emitters.15−17 However, irreversible photoinduced degradation and the photon emission intermittency, often called blinking, can restrict the optical performance of QDs.15 Many aspects of the photon emission mechanism of perovskites are still not well-understood. Recently, formamidinium lead bromide perovskite (FAPbBr3) QDs have been shown to exhibit high stability in air at high temperatures,18−20 as a possible alternative to semiconductor QDs in optoelectronic applications. Herein, we © XXXX American Chemical Society

Received: August 13, 2018

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DOI: 10.1021/acsphotonics.8b01130 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 1. (a) TEM micrograph of the synthesized FAPbBr3 QDs. The inset shows the unit cell structure of FAPbBr3 perovskite. (b) PL lifetime of a solution sample. (c) Three selected PL spectra of single FAPbBr3 QDs. Solid lines are guides to the eye. Inset is an EMCCD image of the fluorescent signals from individual particles. (d) PL peak statistics of 30 individual particles. Dashed line is the PL spectrum of a solution sample. (e) fwhm histogram of PL spectra in (d). (f) PL saturation curve of a QD fitted with the function (const)×

(

σj 1 / τx + σj

), where σ, j, and τ are the x

absorption cross-section, the input excitation photon flux, and the fluorescence lifetime, respectively. The inset shows the absorption coefficient of FAPbBr3 QDs in solution.

containing a low concentration of colloidal FAPbBr3 QDs was spin-coated onto a clean cover glass substrate so that individual QDs could be separated from each other. A continuous wave (CW)-mode laser at 405 nm was used as an excitation source in a home-built confocal microscope system. The stream of emitted photons was collected using an objective lens (Nikon Plan Apo VC, numerical aperture = 1.4) and guided to an EMCCD camera, spectrometer, or Hanbury Brown and Twiss (HBT) setup for measuring the second-order correlation function and the time trace of emitted photons. Spatial filtering was applied to only select signal from regions near the QD position, thus reducing spurious background noise. A small amount of background contribution was carefully determined and subtracted in the second-order correlation measurements. All the measurements were performed at room temperature. A TEM micrograph of the synthesized FAPbBr3 QDs is shown in Figure 1a, and the unit cell structure of FAPbBr3 perovskite is shown in the inset. The photoluminescence (PL) lifetime of a solution sample is measured as shown in Figure 1b and fitted by a triple-exponential function:

shows unprecedented features of a tailing power-law distribution after the truncation time. This suggests that another mechanism, possibly type-B, simultaneously contributes to the blinking mechanism of FAPbBr3 QDs.21 Our simple but intuitive model well produces our experimental data demonstrating that the relatively small contribution of type-B compared to type-A can easily be hidden in the ON time distribution but rather easily be deciphered from the OFF time distribution. Many theoretical models have been introduced including direct or thermally activated tunneling, diffusion-controlled electron transfer (DCET), and multiexciton-involved Auger ionization, in addition to many other scenarios to explain the charging/discharging mechanisms of the type-A model.24−30 However, there is no master key explanation of the blinking behavior of QDs yet. Furthermore, the suggested explanations seem to be valid only for specific samples. In particular, the discharging (neutralization) process is still a deep mystery.22,23 In this study, we demonstrate that DCET and Auger processes simultaneously contribute to type-A blinking of FAPbBr3 QDs by analyzing the ON(OFF) duration time distribution, especially the ON(OFF) duration cutoff time TON(OFF) as a C function of the excitation intensity IP. Our results show that the −ν ON duration time TON C decreases depending on IP (where 1 < ν < 2) and saturates at a higher excitation intensity, while the OFF duration time TOFF slightly sublinearly decreases. This C suggests that the DCET is the dominant mechanism in the type-A process of FAPbBr3 QDs, partially accompanied by Auger ionization processes.

( ) + A exp(− ) + A exp(− ). The three t

A 0 + A1exp − τ

1

2

t τ2

3

t τ3

decay time constants are obtained as τ1 = 12.5 ns, τ2 = 44.9 ns, and τ3 = 173 ns to give the average lifetime value of 80.2 ns with their relative contributions of 22.6%, 44.2%, and 33.2%, respectively. The PL quantum yield was measured to be 75% using a fluorescein standard, as reported in our previous work.31 The three decay times of the perovskite QDs in solution seem to result from their large size distribution, which is sensitively influenced by the used synthetic processes.11,31,32 Furthermore, the PL lifetime of fluorophores sensitively depends on the local environment where the fluorophores are located.32,33 In the later part of our studies, we performed measurements for selectively chosen single QDs with a relatively uniform size (∼11 nm). The PL spectra of three single FAPbBr3 QDs are presented in Figure 1c. Due to the slight inhomogeneous size distribution, the PL peak wavelength varies from 513 to 533 nm (with an average of 524 ± 5

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RESULTS AND DISCUSSION Colloidal FAPbBr3 QDs were synthesized using a ligandassisted reprecipitation method with PbBr2−DMSO (dimethyl sulfoxide) complexes as precursors at room temperature.31 The average size of the FAPbBr3 QDs was 11 ± 3 nm. More details are given in the Methods section. For the photon emission characterization from single QDs, a 3 wt % solution of PMMA (poly(methyl methacrylate)) B

DOI: 10.1021/acsphotonics.8b01130 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 2. (a) Normalized second-order correlation function of photons emitted from a single FAPbBr3 QD at two different pump intensities. (b) Histogram of g(2)(0) for 27 particles. (c) Probability histogram of the biexciton lifetime (τxx) determined from single-exciton lifetime (τx) and g(2)(0).

Figure 3. (a, b) PL time traces of a single QD with a time bin size of 0.2 ms. (c) Histogram of photon numbers per time bin. Red dashed line is the threshold. (d) Change in the PL spectra under continuous illumination. (e) ON time distribution probability derived from the time trace of (b). The red curve is a fitting with eq 1 under the assumption of the truncation cutoff. (f) OFF time distribution probability. The red curve is a fitting with eq 2.

QDs. The excitation input polarization was not fully optimized, especially for the off-plane axis (z-axis). The size of FAPbBr3 QDs is about 11 nm, so the quantum confinement effect is expected.11 To confirm their quantum nature including their single-photon emission property, we measured the second-order correlation function g(2)(τ) using an HBT setup. g(2)(τ) was measured using the start−stop mode of a Pico-Harp (model 300, Pico Quant, 4 ps time resolution). Figure 2a displays the g(2)(τ) curves of a selected QD for two different pump intensities: IP = 34 W/cm2 (red) and 180 W/cm2 (blue). The value of g(2)(0) is determined by fitting the g(2)(τ) curve after properly considering the background noise and the convolution effect caused by the limited time resolution of the detection system.35 The value of g(2)(τ) is normalized to be 1 for a long time delay. In this case, the decay time of g(2)(τ) measured at a sufficiently low pump intensity is defined by the fluorescence lifetime (or the singleexciton lifetime for a QD) τx = 13.8 ns (red curve in Figure 2a), which is consistent with 13 ns in ref 36. The results of g(2)(0) measurements conducted on 27 QDs are summarized in Figure 2b. The majority of the studied QDs showed a g(2)(0) value of less than 0.1 with an average of 0.06. This excellent photon antibunching feature strongly suggests that single FAPbBr3 QDs can serve as a good single-photon source. The strong antibunching (i.e., nearly zero value of g(2)(0)), which was also observed for all-inorganic perovskites and

nm). The histogram of the peak wavelength and the full-width at half-maximum (fwhm) of 30 individual QDs are presented in Figure 1d and e, respectively. The majority of the QDs exhibit a fwhm between 19 and 23 nm with an average value of 21.3 nm, which is slightly below the PL line width of the ensemble QD solution of 25 nm (dashed line in Figure 1d). Contrary to our expectation, we found no correlation between the peak position and the fwhm. In Figure 1f, we plot the PL intensity of single QDs as a function of the laser excitation intensity. As will be shown later, there is negligible fluorescence emission from a multiexciton of which the decay time is much shorter than that of a single exciton. Under this circumstance, in fitting the PL intensity saturation curve, we approximate the target QD as a single-photon emitter with a two-level energy band structure. Then, the PL curve under a CW excitation can be fitted with the function jσ ∼ 1 / τ + jσ ,34 where σ, j, and τx are the absorption cross-section x

of the QDs, the input photon flux at the position of the QD, and the fluorescence lifetime, respectively. From the fitting, σ of the targeted single QD can be estimated as 1.7 × 10−13 cm2. The statistic for 10 different QDs gives the average absorption cross-section σ = (3.1 ± 2.4) × 10−13 cm2, which is comparable to that of the all-inorganic CsPbBr3 QDs reported in ref 16. The large deviation is possibly due to the inhomogeneous size distribution and random absorption dipole orientation of the

C

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ij τ yz −nOFF zz + D τ −mOFF P(τOFF) = C0τOFF expjjj− OFF 0 OFF j T OFF zz k C {

standard colloidal QDs, is attributed to a direct consequence of a highly active nonradiative multiexciton Auger decay channel that dramatically suppresses the multiphoton emission events.15 To estimate the biexciton lifetime (τxx), we used τ the relationship g(2)(0) = 4 τxx ,15,37,38 to determine that the x

N (τON/OFF)

1 . tot NON/OFF δτON/OFF

Here, the number of occur-

rences of a given event, N(τON/OFF), is divided by the total number of all-events, Ntot ON/OFF, and by the average time duration between neighboring pre- and post-events, δτON/OFF.45 The ON duration time distribution shows a clear deviation from a pure power law (Figure 3e) and can be described by a modified power law in the form of15,21,27−29 ij τ yz −nON zz P(τON) = B0 τON expjjj− ON j T ON zz k C {

(2)

Here, we assume that two types of blinking processes are involved in FAPbBr3 QDs. The first term of eq 2 is of type-A blinking, where the ON (OFF) to OFF (ON) transition is due to the charging (discharging) of QDs.21,22 The power laws with exponential truncation cutoff for both the ON and OFF durations are the fingerprint of type-A blinking.21 Fitting this function to the data in Figure 3f yields the exponent power law nOFF = 0.79 and the truncation cutoff time TOFF C = 3.1 ms. The second term of eq 2 is of type-B blinking, where the ON/OFF durations are due to the activation/deactivation of fast nonradiative recombination centers.21,22 For this case, both the ON and OFF times follow a pure power law without the exponential cutoff.21 The power-law exponent is obtained as mOFF = 1.65. Based on successful reproduction of the experiments by our trial fitting functions, especially for the OFF times, we are of the view that both type-A and -B blinking processes simultaneously contribute to FAPbBr3 perovskite QDs. Here, we assume that the ON time duration can be broken into OFF by either type-A or type-B. However, the OFF time should follow type-A (B) to return to the ON state when the OFF duration is initiated by type-A (B). Therefore, the duration statistics of the ON and the OFF times are given respectively as multiples and sum of type-A and type-B. This is why the ON and OFF durations follow different curves as given in Figure 3e,f. From the fitting in Figure 3f, we obtain C0/D0 ≈ 48, which indicates that type-A blinking is dominant over type-B and further supports the quasi exponential cutoff appearing in the ON times. Note that, under this circumstance, the relatively small contribution of type-B blinking can easily be hidden in the ON time distribution, but rather easily be deciphered from the OFF time distribution. In order to further verify the possible mechanisms involved in the blinking of FAPbBr3 QDs, we varied the pump intensity to observe any dependency of the blinking behaviors on this parameter. Previous experimental and theoretical studies of QDs suggested that the ON duration cutoff time TON C has a linear28,46 or quadratic41,47 inverse dependency on the pump intensity with saturation at higher excitation intensities.46,48 This is attributed to photoinduced spectral diffusion (based on the DCET process) or Auger ionization due to biexciton creation, respectively. Figure 4a displays the ON duration time

most probable decay lifetime of the biexciton was τxx ≈ 150 ps, which is slightly less than the 227 ps reported by Eperon et al.39 Further, with a higher excitation intensity, the possibility that a multiexciton in a single QD will be generated should be greatly increased.16,24,40 However, we found almost the same g(2)(0), as proof of a strong nonradiative Auger decay of multiexcitons. For example, the average exciton number per fluorescence lifetime, ⟨N⟩ = σjτx,41 for the blue curve in Figure 2a is already above unity (⟨N⟩ = 1.3). Next, we investigated the blinking mechanism of FAPbBr3 QDs. For this purpose, the PL intensity trajectory of individual FAPbBr3 QD is recorded using the time-tagged time-resolved (TTTR) mode of the Pico-Harp and then reconstructed using an algorithm coded using MATLAB. Figure 3a and b present the PL intensity time trace of a QD recorded under CW laser excitation for a pump intensity of IP = 44 W/cm2. The measured PL intensity fluctuates between two levels, which are clearly resolved in the histogram of Figure 3c. The average PL intensity during the high emissivity (ON) periods reached ∼100 counts/bin (=5.0 × 105 cps). The count rate gradually decreased with time under continuous illumination, and the PL peak is shifted to a higher frequency as shown in Figure 3d, which are typical signatures of photochemical degradation of QDs.15,42−44 These photodegradation features become more dramatic at higher excitation intensities (not shown). The PL intensity trajectory is analyzed using a threshold method. In brief, the threshold intensity is selected, and intensities above (below) the threshold are considered as ON (OFF). We calculate the probability distributions P(τON/OFF) for duration times of ON/OFF periods (τON/OFF) as P(τON/OFF) =

Article

(1)

where the exponential component appears as a truncation cutoff of the power-law behavior. From the fitting, the ON duration cutoff time constant TON C is 10.5 ms and the powerlaw exponent nON is 0.85. The OFF duration time distribution of the QD occurs over a wide range of time scales (∼105) over many decades for a probability density (over 107). As can be seen in Figure 3f, this distribution shows a power law with a truncation cutoff at shorter duration times ( 1). This result also agrees with recent studies by Park et al. on CsPbI3 all-inorganic perovskite QDs, where ν = 1.3,15 and with the results of Gibson et al. on CsPbBr3, where the ON truncation time decreases with the excitation intensity and saturates at ⟨N⟩ ≈ 1.50 Those features were explained by mutual contributions of DCET and Auger ionization for the ON to OFF trapping process.15,50 Figure 5a shows the OFF duration time distributions of the same QD in Figure 4a with the fitting curves given by eq 2.

Under this circumstance, we suggest that the OFF to ON transition for type-A blinking of FAPbBr3 QDs can be partially explained by DCET. To better understand the blinking mechanism of FAPbBr3 QDs, more systematic studies are required. Our efforts to improve the photon detection capability and to identify a more suitable analysis scheme are ongoing.

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CONCLUSIONS In conclusion, we studied the optical properties of single FAPbBr3 perovskite QDs. The strong photon antibunching property of these QDs at room temperature suggests that they can serve as a high-quality single-photon source. This property is due to a fast nonradiative Auger recombination process for multiexciton creation. However, they exhibited photochemical degradation that was more noticeable at much higher excitation intensities and/or longer illumination time periods. We also observed a strong fluctuation of the PL intensity, known as blinking. The truncation cutoff features observed for ON- and OFF-probability distributions confirm that “type-A blinking” is associated with ON/OFF states as a consequence of the neutral/charged states of the system. Further, the tailing power-law statistics at longer duration time of the OFFprobability distribution suggest the contribution of another mechanism, i.e., “type-B blinking”, where the ON/OFF times can be explained by activated/deactivated fast nonradiative recombination centers. Our analyses of the ON(OFF) duration cutoff time distribution as a function of the excitation intensity suggest that the blinking behavior is driven by both DCET and Auger ionization in type-A blinking of FAPbBr3 QDs. Our results are in support of DCET as a possible mechanism for the OFF to ON transition, but more systematic studies are required to elucidate the underlying mechanism. Other experimental techniques and theoretical models should be tested for a comprehensive understanding of the blinking mechanisms involved in FAPbBr3 perovskite QDs. For example, fluorescence-lifetime-intensity-distribution measurements can further examine the validity of type-B blinking in FAPbBr3 perovskite QDs. Finally, our results show that FAPbX3 QDs can be a good test bench for studies related to the optical properties of single organic−inorganic perovskite QDs, including the OFF time behaviors.

Figure 5. (a) OFF duration time distribution probability of QD1 for different excitation intensities (or average exciton ⟨N⟩). (b) OFF as a function of ⟨N⟩ for three QDs. Solid duration cutoff time TOFF C ≈ I−1.0 for QD1 (black), ∼I−0.85 for lines are the fittings with TOFF C P P for QD3 (red). (c) Marcus parabolas in the QD2 (blue), and ∼I−0.97 P DCET model.28,29

While the tailing power-law exponent at a long duration time is quite constant (mOFF ≈ 1.6 ± 0.2), the OFF truncation time TOFF decreases with a dependency on the pump intensity given C by TOFF ≈ I−ν C P (ν ≈ 1) (Figure 5b). This slight sublinear dependency can be distinguished from the ON truncation time case (1 < ν < 2) where a quadratic contribution by the biexciton is observed before the saturation point of ⟨N⟩ ≈ 1. This is possibly because bi- or multiexciton formation is hardly possible during the OFF times due to a much faster decay of an exciton through Auger recombination. In addition, unlike the case of the ON time, not as much clear saturation near ⟨N⟩ ≈ 1 is observed for the OFF time, especially for QD1, as shown in Figure 5b. This is possibly because in the ON to OFF transition in the DCET model as depicted in Figure 5c, an exciton needs to be created so that the ON time cutoff is a function of the exciton population. However, for the OFF to ON transition, the pre-existing trapped charge is involved and possibly yields different dependencies of the cutoff based on the pump intensity. Nonetheless, we do not have a conclusive explanation for the cutoff behavior of the OFF time. We also emphasize the smaller values of the cutoff times for the OFF time compared to the ON time, which suggests a higher probability of artifacts in the data analysis of the OFF time.49

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METHODS

Materials. PbBr2 (98%, Sigma-Aldrich), formamidine acetate (≥98%, TCI), hydrobromic acid (HBr, 48%, SigmaAldrich), dimethyl sulfoxide (≥99.5%, Daejung), anhydrous Ndimethylformamide (DMF, 99.8%, Sigma-Aldrich), anhydrous toluene (99.8%, Sigma-Aldrich), oleylamine technical grade (OLA, 70%, Sigma-Aldrich), oleic acid (OLAc, ≥99%, SigmaAldrich), and poly(methyl methacrylate) (Mw ∼350 000, Sigma-Aldrich) were used as-received without further purification. Synthesis of FAPbBr 3 QDs. FAPbBr 3 QDs were synthesized by following the procedure highlighted in a previous report.31 Briefly, precursor solution was first prepared by mixing an equal molarity of 0.1 mmol (0.0445 g) of PbBr2DMSO and 0.1 mmol (0.0125 g) of FABr and 20 μL of oleylamine in 7 mL of anhydrous DMF. The precursor solution was then transferred dropwise into a copious amount of toluene (175 mL)−OLAc (787 μL) while stirring E

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Quantum Dot Vertical Cavity Lasers with Low Threshold and High Stability. ACS Photonics 2017, 4, 2281−2289. (8) Papagiorgis, P.; Manoli, A.; Protesescu, L.; Achilleos, C.; Violaris, M.; Nicolaides, K.; Trypiniotis, T.; Bodnarchuk, M.; Kovalenko, M. V.; Othonos, A.; Itskos, G. Efficient Optical Amplification in the Nanosecond Regime from Formamidinium Lead Iodide Nanocrystals. ACS Photonics 2018, 5, 907−917. (9) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (10) Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; et al. Iodide Management in Formamidinium-Lead-Halide−based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376−1379. (11) Levchuk, L.; Osvet, A.; Tang, X.; Brandl, M.; Perea, J. D.; Hoegl, F.; Matt, G. J.; Hock, R.; Batentschuk, M.; Brabec, C. J. Bright Luminescent and Color-Tunable Formamidinium Lead Halide Perovskite FAPbX3 (X= Cl, Br, I) Colloidal Nanocrystal. Nano Lett. 2017, 17, 2765−2770. (12) Huang, H.; Polavarapu, L.; Sichert, J. A.; Susha, A. S.; Urban, A. S.; Rogach, A. L. Colloidal Lead Halide Perovskite Nanocrystals: Synthesis, Optical Properties and Applications. NPG Asia Mater. 2016, 8, e328. (13) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341− 344. (14) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Gratzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344−347. (15) Park, Y.-S.; Guo, S.; Makarov, N. S.; Klimov, V. I. Room Temperature Single-Photon Emission from Individual Perovskite Quantum Dots. ACS Nano 2015, 9, 10386−10393. (16) Hu, F.; Zhang, H.; Sun, C.; Yin, C.; Lv, B.; Zhang, C.; Yu, W. W.; Wang, X.; Zhang, Y.; Xiao, M. Superior Optical Properties of Perovskite Nanocrystals as Single Photon Emitters. ACS Nano 2015, 9, 12410−12416. (17) Rainò, G.; Nedelcu, G.; Protesescu, L.; Bodnarchuk, M. I.; Kovalenko, M. V.; Mahrt, R. F.; Stoferle, T. Single Cesium Lead Halide Perovskite Nanocrystals at Low Temperature: Fast Single Photon Emission, Reduced Blinking, and Exciton Fine Structure. ACS Nano 2016, 10, 2485−2490. (18) Eperon, G. E.; Stranks, S. D.; Menelaou, C.; Johnston, M. B.; Herz, L. M.; Snaith, H. Formamidinium Lead Trihalide: A Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar Cells. Energy Environ. Sci. 2014, 7, 982−988. (19) Arora, N.; Dar, M. I.; Abdi Jalebi, M.; Giordano, F.; Pellet, N.; Jacopin, G.; Friend, R. H.; Zakeeruddin, S. M.; Gratzel, M. Intrinsic and Extrinsic Stability of Formamidinium Lead Bromide Perovskite Solar Cells Yielding High Photovoltage. Nano Lett. 2016, 16, 7155− 7162. (20) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Bertolotti, F.; Masciocchi, N.; Guagliardi, A.; Kovalenko, M. V. Monodisperse Formamidinium Lead Bromide Nanocrystals with Bright and Stable Green Photoluminescence. J. Am. Chem. Soc. 2016, 138, 14202− 14205. (21) Galland, C.; Ghosh, Y.; Steinbruck, A.; Sykora, M.; Hollingsworth, J. A.; Klimov, V. I.; Htoon, H. Two Types of Luminescence Blinking Revealed by Spectroelectrochemistry of Single Quantum Dots. Nature 2011, 479, 203−207. (22) Cordones, A. A.; Leone, S. R. Mechanism for Charge Trapping in Single Semiconductor Nanocrystals Probed by Fluorescence Blinking. Chem. Soc. Rev. 2013, 42, 3209−3221. (23) Efros, A. L.; Nesbitt, D. J. Origin and control of blinking in quantum dots. Nat. Nanotechnol. 2016, 11, 661−671.

vigorously. Upon mixing with toluene, the solution immediately turned pale green. Finally, some large particles were removed from the FAPbBr 3 nanocrystal solution by centrifugation at 5300 RCF for 10 min. Preparation of Thin Film. The FAPbBr3 QDs solution was diluted by the addition of a sufficient amount of 3% PMMA (Mw = 350 000 g/mol) in a toluene solution to isolate the particles. A thin film was formed by spin coating the diluted solution at a speed of 2000 rpm for 45 s. Photon Detection and Analysis. In measuring g(2)(τ) and the PL intensity time traces, photons are detected by single photon counting APDs (PerkinElmer, SPCM-AQ4C). The g(2)(τ) curves in Figure 2a are fitted using the function g(2)(τ ) = A + (1 − A) × (1 − e− τ / τx), which considers the background noise and the convolution effect caused by the limited time resolution of the detection system.35 The temporal resolution of our photon detection system is estimated to be approximately 0.65 ns by measuring temporally short laser pulses (∼3 ps) and deconvoluting the measured curves.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Youngjong Kang: 0000-0001-5298-9189 Kwang-Geol Lee: 0000-0002-9795-4403 Author Contributions ‡

C. T. Trinh and D. N. Minh contributed equally to this work.

Author Contributions

C.T.T. performed the experiments and analyzed the data. D.N.M. suggested the first experiment and prepared samples. K.A., Y.G., and K.L. supervised the project. C.T.T. and K.L. wrote the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea and funded by the Ministry of Science and ICT (Grant Nos. 2016R1A2B4014370, 2017R1A2B2007618, 2012R1A6A1029029, and 2018R1A2B6001449). The authors thank Y.-S. Park for fruitful discussions.

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