Surface engineering for improved stability of CH3NH3PbBr3 perovskite nanocrystals

Artavazd Kirakosyan , Seokjin Yun , Soon-Gil Yoon and Jihoon Choi *
Department of Materials Science and Engineering, Chungnam National University, Daejeon, South Korea. E-mail: jihoonc@cnu.ac.kr; Fax: +82 42 821 5850; Tel: +82 42 821 6632

Received 3rd September 2017 , Accepted 14th December 2017

First published on 14th December 2017


Organohalide perovskite nanocrystals (NCs) with a variety of nano-scale structures and morphologies have shown promising potential owing to their size- and composition-dependent optoelectronic properties. Despite extensive studies on their size-dependent optical properties, a lack of understanding on their morphological transformation and the relevant stability issues limits a wide range of applications. Herein, we hypothesize a mechanism for the morphological transformation of perovskite NCs, which leads to dissolving NCs and forming microscale rectangular grains, resulting in a reduction of photoluminescence. We found that the morphological transformation from nanocrystal solids to microscale rectangular solids occurs via Ostwald ripening. A surface treatment with a surfactant suppresses the transformation, resulting in nearly monodisperse NCs with a square shape (∼20 nm edge size), and thus improves the stability of NC solution, as well as their photoluminescence performance and quantum yield (PLQY = 82%). Furthermore, we employed similar amine derivatives to investigate the effect of a molecular architecture (i.e. steric hindrance) on perovskite NC stability, which exhibited much enhanced PLQY (93%). These experimental results provide new insights into the fundamental relationship between the physical properties and the structure of perovskite nanocrystals required to understand their diverse optoelectronic properties.


Introduction

Organometal halide perovskites with a basic formula of ABX3, where A is CH3NH3 (MA) or NH2CH[double bond, length as m-dash]NH2 (FA), B is Pb or Sn, and X is Cl, Br, or I,1–4 have attracted considerable attention in optoelectronic fields such as photovoltaic energy harvesting,5,6 light emitting devices,7–9 and photodetectors10,11 since the variation of the constituent ions and their mixtures is shown to afford more opportunities for controlling their energy band structures and the corresponding absorption and photoluminescence (PL). Furthermore, a recent observation on well-defined size-dependent excitonic behaviors in colloidal organohalide perovskite nanocrystals (NCs) motivated significant research efforts on the nanostructured forms of perovskites to provide additional functionalities via the quantum confinement effect for a wide range of applications including piezoelectric energy generators,12,13 sensors for gas14,15 and chemical substances,16,17 as well as polarity chemosensors.18 Because these optical and electrical characteristics mostly depend on the dimension and length scale of the perovskite NCs, recent efforts have been taken to control their size and shape, involving optimization of their morphology.19–22

Although several synthetic approaches such as ligand-assisted reprecipitation (LARP) and ligand-assisted exfoliation are widely used to prepare organohalide metal perovskite NCs with a desired chemical composition,19–28 the control of the NC size and shape remains a challenge at present. For example, the LARP method produces coarse crystals as well as nanocrystals with a highly polydisperse size distribution, which requires an additional process to separate the nanocrystals by centrifugation, as reported by the prior studies.24,26 In our previous work, we modified the LARP method to control the particle size and prevent the formation of micro-scale particles during the synthetic process by limiting the amount of precursors.24 The amount of precursors is not enough to feed the further growth of nanocrystals whose size was successfully controlled within 3 to 8 nm. In the case of the exfoliation method, a gradual size separation is also required to achieve the desired particle size since the exfoliation method also produces nanocrystals with an uncontrolled size distribution.25,27 More detailed control of the morphology could be achieved by employing oleylamine (C18H35NH2) to tune the particle shape from spherical nanocrystals to nano-sized plate-shaped particles.20 However, only a few studies highlight the issues associated with the performance and morphological stability of perovskite NCs over aging.7,26,27 A lack of understanding on the key parameters and the associated processes in the morphological transformation and the relevant stability issues limits their practical applications. To the best of our knowledge, there is no report on post morphological transformation of perovskite NCs in the solution phase. Thus, more experimental, as well as theoretical studies are required to identify and understand the role of the fundamental factors in order to achieve a desired NC morphology.

Herein, we first studied the stability of MAPbBr3 perovskite NCs in the solution phase and the associated factors that can influence their optoelectronic properties. These NCs have been shown to undergo size- and morphological deterioration via Ostwald ripening due to the insufficient coverage of the NC surfaces by surfactants (ligands). To understand the role of the surfactant in the stability and growth of NCs and the relevant physical properties, we extended our previous study to stabilize the as-synthesized NCs in solution by surface engineering, which prohibits further growth of the perovskite NCs. In particular, the addition of extra surfactant after NC synthesis improves the stability threefold over two days and enhances the quantum yield. Precise control of the precursor and surfactant contents terminates the particle growth process leading to uniform and monodisperse CH3NH3PbBr3 perovskite NCs. Furthermore, we studied the effect of the molecular architecture of surfactants on the NC stability and PL performance by employing various amine derivatives. Our findings will help to understand and extend our knowledge in the relationship of perovskite NC's size, morphology, transformation, and optical properties, and are important in developing perovskite materials with desired properties and morphologies for optoelectronic application.

Results and discussion

The perovskite NC solution was prepared according to the LARP method.23 Briefly, precursors including lead bromide (PbBr2), methyl ammonium bromide (CH3NH3Br), n-octylamine, and oleic acid dissolved in N,N-dimethylformamide (DMF) were precipitated in a non-solvent (i.e. toluene). See the ESI for the detailed experimental procedure. X-Ray diffraction patterns (Fig. S1) obtained from the NCs confirm that MAPbBr3 NCs belong to the space group of Pm[3 with combining macron]m with a lattice spacing of a = 5.9896 Å.

Fig. 1 demonstrates the photoluminescence (PL) emission and UV-vis absorbance of perovskite solution (a), and the scanning and transmission electron microscopy (SEM and TEM) micrographs (b–d) of the sample obtained at different stages of aging. The initial sample exhibits PL emission centered at 498 nm and an absorption edge at 483 nm, consistent with the literature.24 The emission peak at 498 nm (2.48 eV) is relatively sharp with a full width at half maximum (FWHM) of 35 nm and blue-shifted by ∼35 nm compared to the bulk counterpart.5,24,29 The absorption edge at 483 nm (2.56 eV) is also blue-shifted by ∼45 nm (230 meV) compared to the bulk counterpart.5,6,24,29 The observed Stokes shift (80–100 meV) indicates that the PL emission originates from excitonic recombination for these perovskite NCs.28,29 SEM and TEM images (Fig. 1b) revealed that the initial sample mainly consists of NCs with 3–5 nm size with a narrow size distribution (Fig. S2) along with a very little amount of submicron-sized crystals. However, the number density of micro-scale crystals abruptly increases with aging while nano-crystallites disappear rapidly (Fig. 1c and d). Fig. 2 shows the normalized integral PL emission and the maximum emission wavelength (λmax) of the whole spectrum depending on the aging duration. As the aging progresses, the wavelength of the PL emission maxima (λmax) and optical absorption edge were red-shifted, approaching those of the bulk counterpart (λbulk) (Fig. S3), and their intensity was gradually reduced. In the meantime, PL emission showed a multimodal spectrum.


image file: c7nr06547g-f1.tif
Fig. 1 (a) Time-dependent UV-vis absorbance (thick solid line) and PL emission (thin solid line) spectra. Scanning electron micrographs for (b) the initial, (c) 1 day, and (d) 5 day-aged samples, respectively. The insets depict their transmission electron micrographs. Excitation wavelength (λExc) was fixed at 365 nm.

image file: c7nr06547g-f2.tif
Fig. 2 Normalized integral PL emission intensity (squares) and maximum emission wavelength (λmax) (circles) depending on the aging duration. PL intensity significantly decreases with aging, and the emission spectra shift to longer wavelengths towards that of bulk CH3NH3PbBr3 perovskite materials (Fig. S3).

Prior studies on MAPbBr3 perovskite NCs revealed a layered crystalline nature with a different number (n) of layers.29,30 Interestingly, the multi-modal emission spectra could be deconvoluted into several emission peaks corresponding to the layered nanoplatelets with n = 1, 2, 3, etc. For example, the PL spectrum of the aged sample (2 days) was deconvoluted into five different PL emission peaks centered at 444, 467, 494, 507, and 526 nm (Fig. S4).29,30 Both multimodal emission and contrast difference in TEM images (Fig. 1c and d) reveal that the perovskite NCs experience a morphological transition to form polydisperse particles with various morphologies including platelets during the aging process. Two-dimensional (2D) platelet perovskites are highly emissive when their thickness is lower than Bohr radii (<2 nm).1,3,29,30 The aged sample (7 days) exhibits a significantly reduced PL intensity, and mainly consists of micro-scale crystals. Such multimodal emission behavior negatively influences their practical application, which motivates the development of the surface passivation to prevent such a transformation. We monitored such a behavior by tracking the maximum wavelength as an indicator to characterize the overall transformation. The detailed study on the deconvoluted spectra and their contribution (i.e. PL emission relevant to the different sizes of nanocrystals) would provide more information on the morphological transformation, however, that is beyond the scope of the current study.

At the initial stage, large numbers of NCs are immediately formed by rapid precipitation owing to the low solubility of precursors in toluene (Fig. 3a). Despite the successful synthetic methods of the perovskite NCs based on the precipitation of precursors in a poor solvent, a certain solubility of precursors is still observed. For instance, PbBr2 dissolved in neat toluene and DMF containing toluene (2 mL of DMF in 10 mL of toluene) has intense absorbance (Fig. S5). Moreover, the presence of n-octylamine, oleic acid, and methyl ammonium bromide may increase the Pb solubility that accelerates the mass transfer process. Therefore, the as-synthesized NC solution is in the quasi-equilibrium state between two simultaneously occurring processes (i.e. dissolving and re-precipitation of the perovskite materials), in which the dissolving of nano-sized particles provides additional precursors required for the growth of large-sized crystallites in solution, as suggested in the scheme (Fig. 3c). Many nanocrystal solids will be reconstructed into a few micro-scale solids with a highly crystalline structure mediated by the liquid phase (i.e. solid–liquid–solid), which is confirmed by TEM images (Fig. 3a and b), via processes such as Ostwald ripening.31 Alternatively, the particle growth could occur by the aggregation of nanocrystals into aggregates, however, no aggregates of NCs were observed in the aged samples (Fig. 3b). The SEM of NC aggregates obtained by intense centrifuging is demonstrated in Fig. S6 for comparison.


image file: c7nr06547g-f3.tif
Fig. 3 TEM and SEM images of (a) initial and (b) 7 day-aged samples. (c) Scheme illustrating the morphological transformation of the perovskite NCs via Ostwald ripening.31

In practical applications, the organic–inorganic perovskite NCs can undergo undesired growth and morphological changes that affect the associated electronic and optical properties. Although an excess amount of surfactants could be introduced during the synthesis process, such an approach results in a wide range of variations in their size and morphology.10,26 In addition, the surfactant (n-octylamine) treatment can lead to uncontrolled exfoliation of layered nanoplatelets.30,32 To circumvent these issues, we adopted a post surface engineering of the perovskite NCs (synthesized by our previous study24) to suppress their transformation, and thus improve their stability by adding an identical surfactant solution (i.e. oleic acid and n-octylamine dissolved in DMF).

UV-vis spectra of the initial sample (Fig. 4a) exhibit a significant time-dependent variation in the absorption edge from ∼2.58 eV to ∼2.29 eV, arising from the reduction in the quantum confinement effect along with the growth of the perovskite NCs. Furthermore, PL emission shows clearly defined double peaks centered at about 460 and 525 nm after 2 days (Fig. 4c), corresponding to the particles with two distinct sizes (i.e. n = 1–2 and n = ∞). This observation is consistent with the above-mentioned transformation process of NCs. In contrast, no shift of the absorption edge is observed over 2 days when the surfactant solution is added (Fig. 4b) and even more intense PL emissions with a narrow spectral distribution (FWHM ∼25 nm at λmax = 500 nm) are shown in Fig. 4d, indicating a considerably improved stability of the perovskite nanocrystals. Fig. 4e depicts how the addition of extra amounts of surfactants (0–12 drops) affects the shift of λmax with duration. At very low amounts of surfactants (0–2 drops) a significant shift is observed, but at higher amounts of surfactants λmax does not show any noticeable shift with aging, suggesting that the excess surfactants could delay or terminate these growth processes. Fig. 4f shows the PLQY for samples depending on the surfactant content and aging time. The PLQY of nanocrystals increases with the amount of the additional surfactants from 54% to 82%. The surface engineered nanocrystal solution maintains a relatively high (e.g. PLQY = 70%) over two days of aging while the PLQY value for the reference sample drops to ∼22%. In a comparison with the values reported in the literature, MAPbBr3 NCs synthesized by the LARP and emulsion-based method show 50–70% and 80–90% of PLQYs, respectively.23,33 In both cases, NCs experience a significant change in the surfactant concentration during post-processing (e.g. centrifuging, washing). However, in our case, the surface treatment has shown to significantly enhance the PLQY as a consequence of the reduced non-radiative pathways through the defect states on the NC surfaces by passivating the surface defects with the additional surfactants.


image file: c7nr06547g-f4.tif
Fig. 4 (a, b) UV-Vis absorbance and (c, d) PL emission spectra depending on the aging duration of (a, c) reference (5 drops of precursor) and (b, d) 12 drops of surfactant added samples, respectively. The insets show the Tauc plots. Insets in (c) and (d) demonstrate the photoimages under visible and UV light, respectively. (e) PL emission maximum wavelength (λmax) depending on the aging duration with different amounts of surfactant. (f) Time-dependent PLQY for samples containing 5 drops of precursors and different amounts of surfactant solution (1 to 12 drops). Excitation wavelength (λExc) was fixed at 365 nm.

Further experimental studies (Fig. 5a) on the stabilization of the perovskite NCs depending on the molecular architecture of the surfactants were performed using stearylamine (CH3(CH2)17NH2) and trimethylamine ((CH3CH2)3N), as illustrated in Fig. 5b. While the PLQY of the initial sample drops to 22% with time duration, the samples with excess amounts of linear molecules (i.e. n-octylamine or stearylamine) show over 70% of PLQY after 2 days (Fig. S7). In particular, a relatively longer stearylamine (i.e. two times longer carbon backbones) was more effective to stabilize the perovskite NCs compared to n-octylamine in terms of morphology control and PLQY enhancement that reached about 93%. In contrast, the triethylamine with a bulkier structure (i.e. three ethyl groups that are spatially oriented to the other side) has shown a poor performance, which could be associated with the surface coverage of these amine derivatives as illustrated in Fig. 5b. Longer aliphatic hydrocarbon molecules localized on the NC surface provide a thicker protecting layer, leading to extra stability. However, the localization of triethylamines on the NC surface could be less preferable due to steric repulsion, and the side-by-side neighbors can screen out, resulting in bare surfaces of NCs. This process was also confirmed by the size- and morphological changes in the perovskite NCs (Fig. 5c–f). The reference sample consists of submicron-sized 2D nanoplatelets with an edge in the range of 20–500 nm. The addition of n-octylamine leads to the formation of irregular shapes with a nearly monodisperse size distribution and negligible amounts of microscale crystals. The stearylamine-stabilized sample consists of monodisperse square-shaped NCs with 15–20 nm edge size (Fig. S8). However, mainly larger sized crystals and their aggregates are observed for the trimethylamine case, indicating their poor stabilizing ability. A more detailed theoretical and experimental investigation of the influence of the surfactant type on their optical properties and the corresponding stability will be the subject of a future publication.


image file: c7nr06547g-f5.tif
Fig. 5 (a) Time dependent PLQY of NC solution, (b) schematic representation of perovskite NCs coordinated by different amine molecules and (c–f) TEM images of the reference and various amine treated samples after two days of aging. The insets in TEM images (c–f) show diffraction patterns.

Conclusions

We have systematically investigated the stability of CH3NH3PbBr3 NCs and further surface engineering to prevent the undesired growth and morphological changes in solution. It has been found that the as-synthesized perovskite NCs suffer severe degradation in their size and morphology via Ostwald ripening, resulting in significantly reduced optical characteristics. Post treatment of the perovskite NCs using the amine-derivative surfactant could terminate their growth resulting in monodisperse and uniform NCs and maintains the stability of the 70% level for two days of aging. Post treatment by the surfactants improves the PLQY to 93% and enhances PL emission. These results are important in understanding the relationship between the organohalide lead perovskite NC structures and their optoelectronic properties, and provide a fundamental background for the development of organohalide lead perovskite NCs with enhanced performance and improved stability. The post treatment approach has potential to be extended to other NC systems as well.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work is supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (no. NRF-2015R1C1A1A01052865, NRF-2016R1D1A1B03933212, and NRF-2013R1A4A1069528).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nr06547g

This journal is © The Royal Society of Chemistry 2018