Enhancing Luminescence and Photostability of CsPbBr3 Nanocrystals

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 3

Enhancing Luminescence and Photostability of CsPbBr Nanocrystals via Surface Passivation with Silver Complex Hongbo Li, Yang Qian, Xing Xing, Jianfeng Zhu, Xinyu Huang, Qiang Jing, Weihua Zhang, Chunfeng Zhang, and Zhenda Lu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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The Journal of Physical Chemistry

Enhancing Luminescence and Photostability of CsPbBr3 Nanocrystals via Surface Passivation with Silver Complex

Hongbo Li†, Yang Qian†, Xing Xing†, Jianfeng Zhu†, Xinyu Huang∥, Qiang Jing†, Weihua Zhang†, Chunfeng Zhang∥, Zhenda Lu*†‡

†

National Laboratory of Solid State Microstructures, College of Engineering and

Applied Sciences and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China ‡

Research Center for Environmental Nanotechnology (ReCENT), Nanjing University,

Nanjing 210023, China ∥National

Laboratory of Solid State Microstructures, School of Physics, and

Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China *

E-mail: [email protected]

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ABSTRACT We demonstrate Ag(I) complex can fix bromide on the nanocrystal surface, reduce surface trap density, and as a result efficiently passivate the surface of CsPbBr3 nanocrystals. This passivation makes the photoluminescence (PL) intensity increase several times. PL kinetics study clearly shows the decay lifetime increased after the passivation. TEM and XPS analysis demonstrated the existence of Ag on the nanocrystal surface. In addition, we utilize single-particle spectroscopy combined with in-situ light analysis to further confirm the PL enhancement effect. On the other hand, the passivation leads to an extraordinary photostability of CsPbBr3 nanocrystals, with PL intensity retained 80% after UV illumination for 5 days under ambient condition.

INTRODUCTION Lead halide perovskite thin films have emerged as the very promising materials for optoelectronic applications, particularly in photovoltaics where the power conversion efficiency has reached over 20%.1-5 Very recently, their colloidal counterparts, typically inorganic CsPbX3 (X = Cl, Br, I) nanocrystals (NCs), have also attracted considerable attention as a new class of nanosized emitters, offering bright prospects in light-emitting diodes (LEDs),6-15 lasers have

exceptional photoluminescence

16-21

and photodectors.22-24 CsPbX3 NCs

properties,

including

narrow emission

bandwidths, high photoluminescence quantum yields (PLQY) and broad emission peak tunability.25-27 Besides, they behave a large tolerance to surface defect, which is attributed to nonbonding interactions between the conduction bands and valence bands of perovskite materials.28-30 As a result, the directly synthesized CsPbBr3 NCs can achieve PLQY as high as 90% without special surface modification.8, 31 However, this tolerance is limited. Considering the ionic nature of perovskite, the 2


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halides in the materials tend to pair with surface amine ligand, forming ammonium halide ion pairs, which can be easily removed during the purification process. Sargent et al. reported that washing with antisolvents such as acetone could remove most of oleylamine ligands from the NC surface, likely some of these ligands lost as oleylammonium bromide form.32 This process certainly induces some surface defects, and then leads to an obvious PL intensity decay. Kovalenko et al. confirmed the existence of oleylammonium bromide ion pairs and their lability to the surface as well.33 Therefore, the surface state of perovskite NCs, to some extent, is even more fragile than that of traditional quantum dots considering their intrinsic highly ionic nature. Surface passivation is an efficient way to improve the optoelectronic properties and colloidal stability of the perovskite materials.34, 35 Halide ion pairs, such as di-dodecyl dimethyl ammonium bromide (DDAB)

36

and methylammonium bromide (MABr)

37

were naturally added to compensate the bromide loss on the NC surface and subsequently improve the optoelectronic properties. Corresponding metal bromide demonstrated the same effect, whenever they are introduced by in-situ synthesis 38 or post-treatment 39. Another kind of efficient surface passivation reagent is Lewis base, including thiophene,40 pyridine,40, 41 oxygen,42 S2-,43 and SCN-.44 These Lewis bases are believed to provide electronic passivation to the lead-rich surface of perovskite NCs and then enhance the PL intensity. In addition, surface passivation by S2- made the perovskite NC an excellent long-term photostability at continuous UV irradiation under ambient conditions.43 This surface effect study should be very important for the perovskite materials, but thus far, it has been relatively original. To the best of our knowledge, besides the halide salts and Lewis bases, other reagent has been rarely reported with the similar passivation functions. Here, we report a facile but effective surface passivation on CsPbBr3 NCs by silver-trioctylphosphine complex (denoted as Ag-TOP) treatment. Small amount of Ag-TOP reagent reacts with surface bromide to form AgBr on the nanocrystals, 3



effectively reducing the surface defects that originated from the loss of labile oleylammonium bromide. This passivation significantly improves the PL intensity of NCs. The study of PL decay kinetics clearly shows the decay lifetime increased after the passivation, which is originated from reduction of surface charge traps. TEM and XPS characterization confirms the existence of Ag element on the NC surface, and most of Ag stays at the oxidation state. To obtain more information beyond ensemble average, we also investigated PL properties of individual NC by single-particle spectroscopy combined with atomic force microscopy (AFM). The statistical result demonstrates PL intensity of single NC has increased by two to three times after Ag treatment. The in-situ measurement recorded the PL brightness trajectory during dropping process of Ag-TOP, which further confirms the enhancing effect of Ag on PL intensity. Furthermore, the Ag treatment significantly improves the photostability of CsPbBr3 NCs. After Ag passivation, the PL intensity of the CsPbBr3 NCs was retained about 80% after UV illumination (365 nm, 35 mW/cm2, 12 cm away from the sample) for 5 days. All these results demonstrate Ag passivation can effectively fix bromide on the NC surface, reduce surface trap density and subsequently enhance the PL intensity and photostability of perovskite NCs.

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Figure 1. (a) Absorption spectra of CsPbBr3 NCs and silver treated samples dispersed in cyclohexane (The volume number is the adding amount of Ag-TOP reagent). The inset is the Tauc plot of the corresponding samples. (b) PL spectra (λex=365 nm) and (c) Time-resolved PL decay 5



curves for the pristine and Ag treated CsPbBr3 NCs, the emission wavelength is 514 nm.

RESULTS AND DISCUSSION To ensure the controllable surface passivation of CsPbBr3 NCs with Ag precursors, our reaction was carried out in a nonpolar solvent, in which the concentration of CsPbBr3 NCs and Ag precursor can be easily tuned. CsPbBr3 NCs were first synthesized through the typical procedure reported by Kovalenko et al.45 with minor modifications. The CsPbBr3 NCs have cubic shapes with edge sizes from 8.9 nm to 12.3 nm (Figure S1a), dispersing in cyclohexane. Ag passivation reagent was prepared by dissolving AgNO3 in tri-n-octylphosphine (TOP) and then diluted in cyclohexane, forming Ag-TOP precursor. In a typical surface passivation experiment, different amount of Ag-TOP precursor (0.06 M based on Ag) was directly added to the NC dispersion (2 mL, 0.05 M based on Br) in a cuvette. The absorption and PL spectra were recorded two minutes after adding Ag because most optical changes occurred within the first minute. The absorbance of CsPbBr3 NCs decreased a little after Ag treatment, but with no obvious change in the bandgap, as demonstrated in the Tauc plot (Figure 1a). The size of NCs was mostly preserved after passivated with small amount of Ag precursor. On the contrary, the change of PL intensity was very pronounced, with up to two times increase when 30 µL of Ag-TOP precursor was added (Figure 1b). An excess of Ag precursor led to a quick decreasing of PL intensity because they further reacted with Br to form sliver bromide, and decomposed the CsPbBr3 NCs. As shown in Figure 1a, the absorbance was markedly reduced when 200 µL of Ag-TOP precursor was introduced, and a prominent peak at 370 nm appeared, which could be ascribed to the peak of sliver bromide. To exclude the influence of TOP reagent, TOP instead of Ag-TOP reagent was used as a control test. The result showed that only TOP reagent had little influence on PL intensity (Figure S2), which confirms that Ag plays a key role in the passivation process. The 6


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initial quantum yield of the CsPbBr3 is about 52.0%. Upon adding the Ag-TOP reagent into the CsPbBr3, the quantum yield increased from 52.0% to 90.1% (Figure S3). The improved quantum yield of PL emission can be ascribed to the suppression of nonradiative recombination induced by silver treatment.46, 47 Such assessment is consistent with different PL dynamics in CsPbBr3 NCs before and after Ag treatment as characterized by time-resolved PL (TRPL) spectroscopy (Figure 1c). The TRPL curves can be well reproduced by a bi-exponential function (SI for details). In the pristine sample, the lifetime parameters (amplitude ratios) of the two decay components are τ1 ~ 2.7 ns (A1 ~ 58%) and τ2 ~ 9.6 ns (A2 ~ 42%), respectively; In the treated sample, the lifetime parameters (amplitude ratios) of the two decay components are τ1 ~ 3.9 ns (A1 ~ 64%) and τ2 ~ 11.7 ns (A2 ~ 36%), respectively. Since PL decay is contributed by both radiative and non-radiative recombination processes, the longer lifetime in the treated sample with higher PL yield can be explained if the non-radiative recombination is suppressed. This is reasonable since some trap centers at the surface can be passivated by silver treatment. In addition, PL decay curves at shorter (504 nm) and longer (524 nm) emission wavelength show similar changes after silver treatment (Figure S4), suggesting the effect of surface passivation is independent of nanocrystal size.

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Figure 2. (a) TEM and (b) HRTEM images of the Ag treated CsPbBr3 NCs. (c) HAADF-STEM

and (d) corresponding EDS elemental mapping of Ag treated samples (The scale bar is 30 nm). (e) The survey XPS spectra of CsPbBr3 before and after Ag treatment; Insets show the magnified Ag 3d spectra of the samples. (f) The high-resolution Ag 3d spectrum of Ag-treated samples.

While the sliver treatment is very effective in impelling the optical performance, it is of interest to examine what happens on the surface of the CsPbBr3 NCs. The silver treated NCs present regular cubic shape and size distributions, with little change compared to the untreated ones. As shown in Figure 2a, the measured lattice 5.8 Å in the high-resolution STEM-HAADF image (Figure 2b) is in good agreement with the cubic phase of CsPbBr3 NCs. The EDS mapping result (Figure 2c and 2d) clearly shows silver is evenly distributed on the NC surface, although the passivation layer is too thin to be identified under the regular TEM. The X-ray photoelectron spectroscopy (XPS) characterizations were carried out to further verify the existence of Ag on the surface. XPS survey spectra exhibited strong peaks from lead (Pb), carbon (C), Nitrogen (N) and Oxygen (O), with relatively weak but clear peaks from Ag for Ag-treated NCs (Figure 2e). The two peaks at 374.2 and 368.2 eV (Figure 2f) 8


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can be ascribed to Ag 3d3/2 and Ag 3d5/2 binding energies, indicating the Ag is in the oxidation state.48 From these results, it can be suggested that AgBr was successfully deposited on the nanocrystal surface after Ag treatment, and provided an efficient passivation layer to enhance the PL intensity. The PL of individual CsPbBr3 NC was investigated to have a further understanding the Ag passivation. The NCs in nonpolar solvent were dispersed on the glass substrate through spin-coating method. The glass surface was modified by hydrophobic alkyl chain first to prevent the aggregation during the dispersion. Atomic Force Microscope (AFM) results (Figure 3a-d) show the NCs can be well separated on the glass substrate; and the height of the individual particle (~10 nm) verifies that there is no aggregation at least in the vertical direction. To further verify there is only one single NC at each bright spot, second-order photon correlation measurement was carried out on tens of these bright spots under a pulsed excitation at 405 nm with a repetition rate of 40 MHz. A typical antibunching effect (Figure 3c) of these spots shows that we are investigating really single NC. Micrographs of these NCs and statistics on their light intensity are shown in Figure 3d-g. It is clear that NCs after Ag treatment are much brighter than that of untreated ones, particularly more than doubled in the average intensity, which is highly consistent with the assembled result from conventional spectroscopic method.

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Figure 3. (a) AFM image (30 × 30 µm2) of CsPbBr3 NCs dispersed on glass surface and (b)

corresponding height profiles of the line indicated in the AFM image. (c) Typical second-order photon correlation measurement for single bright spot at the polarization angle of 45º. (d) PL microscopy image (50×50 µm2) of CsPbBr3 NCs and (f) silver treated samples obtained with wide-field excitation. (e, g) Corresponding brightness statistics of single CsPbBr3 NC and silver treated samples.

We performed an in situ PL study on single CsPbBr3 NC to confirm the enhancement 10


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effect of Ag surface passivation. A small drop of Ag-TOP cyclohexane solution (5 µL, 0.06 mM) was put on the NC-deposited cover glass under optical microscopy, while the PL image was continuously recorded (Supporting Information, Movie 1). Figure 4a demonstrates images taken before and after dropping Ag-TOP solution, showing that PL intensity is significantly enhanced after Ag treatment. The intensity trajectory in Figure 4b recorded the detailed PL change during the Ag passivation process. At the point of adding Ag-TOP solution (100 s), the PL intensity decreases because of the defocus effect. After the solution is evaporated in several second, the focus is recovered automatically. PL intensity quickly recovers and reaches much higher level than before. Again, only TOP solution without Ag performs negligible effect on PL intensity of single NC. These in situ characterizations prove the PL enhancement of Ag passivation on CsPbBr3 NCs.

Figure 4. (a) Single-particle PL images of CsPbBr3 NCs before and after dropping Ag-TOP

reagent. The scale bar is 5 µm. (b) Typical fluorescence traces observed during 405 nm laser 11



irradiation in air (the point indicated by the blue circle in Figure 4a). The integration time per frame was 100 ms. The Ag-TOP solution was added at ∼100 s, as indicated by the red arrow.

The photostability is still a remaining issue for perovskite NCs, not only for the optoelectronic characterizations, but also for their practical applications in solid-state devices.49-51 To test the photostability of CsPbBr3 NCs before and after Ag treatment, we monitored the PL intensity decay of NCs in both solution and solid film under ambient conditions (70 ± 10% humidity), with a constant UV-light irradiation (365 nm, 35 mW/cm2, 12 cm away from the sample). Figure 5a-c shows the result in the solution. The intensity stability of CsPbBr3 NCs is significantly enhanced after Ag passivation, about 80% retained in five operation days, which is much superior to the pristine ones. The same phenomenon was observed in solid film, as shown in Figure 5d. The PL of pristine CsPbBr3 NCs became weak as time went by, lost most brightness in 2 days, and totally vanished after 5 days. As a comparison, the silver treated NCs were still strongly ﬂuorescent after 5 days, remaining most brightness as the beginning. Therefore, it is very clear that Ag surface passivation on perovskite NCs not only achieves improved PL intensity, but also realizes long-term photostability under ambient conditions.

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Figure 5. (a, b) Temporal evolution of PL spectra of CsPbBr3 NCs with and without Ag treatment.

(c) Normalized PL intensity as a function of running time. (d) The photographs of CsPbBr3 NC films under 5 days of UV irradiation.

CONCLUSIONS In conclusion, we have demonstrated a facile but efficient silver passivation method for highly stable CsPbBr3 NCs. It is able to efficiently decrease the surface traps by stabilizing the surface bromide. As a result, the passivation increases PL intensity, slows PL decay kinetics, and remarkably improves the photostability. Compared to the reported surface passivation reagents, including halide-ammonium ion pairs, metal halides and Lewis bases, Ag-based one is very unique and effective with great tenability. This facile passivation strategy should be highly encouraging to the perovskite materials and corresponding optoelectronic devices.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxx. Details of the experimental section; additional experimental results, including the TEM, absorbance and PL spectra, lifetime and the fitting parameters. (PDF) AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] ORCID Zhenda Lu: 0000-0002-9616-8814 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This

work

was

supported

by

National

Key

R&D

Program

of

China

(2016YFA0201100), Thousand Talents Program for Young Researchers, National Natural Science Foundation of China (Grant No. 21601083), the Fundamental Research Funds for the Central Universities, Jiangsu Innovative and Entrepreneurial Talent Award and Natural Science Foundation of Jiangsu Province (Grant No. BK20160614).

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Enhancing Luminescence and Photostability of CsPbBr3 Nanocrystals

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