Article pubs.acs.org/cm
Room-Temperature Synthesis of Widely Tunable Formamidinium Lead Halide Perovskite Nanocrystals Duong Nguyen Minh, Juwon Kim, Jinho Hyon, Jae Hyun Sim, Haneen H. Sowlih, Chunhee Seo, Jihye Nam, Sangwon Eom, Soyeon Suk, Sangheon Lee, Eunjoo Kim, and Youngjong Kang* Department of Chemistry, Research Institute for Natural Sciences, Institute of Nano Science and Technology, Hanyang University, 222 Wangsimni-Ro, Seongdong-Gu, Seoul 04763, Korea S Supporting Information *
ABSTRACT: Colloidal perovskite nanocrystals based on formamidinium lead halide (FAPbX3) have been synthesized by the ligand-assisted reprecipitation method using PbX2− dimethyl sulfoxide complexes as precursors at room temperature. Well-defined cubic-shaped FAPbX3 nanocrystals have been obtained with a size d of ∼10 nm. The synthesized FAPbX3 nanocrystals show bright photoluminescence with a high photoluminescence quantum yield (75% for FAPbBr3). The lifetimes of FAPbBr3 nanocrystals were measured for the samples isolated at several different centrifugal speeds. The photoluminescence can be tuned from the blue to nearinfrared region (λpeak = 408−784 nm) by changing either the amount of oleylamine or the composition of X. The color expression range is 135% of the NTSC standard. The bandwidth of the photoluminescence spectra of FAPbX3 nanocrystals is narrow (full width at half-maximum of 18−48 nm). FAPbX3 nanocrystals show thermal stability that is better than that of MAPbBr3 nanocrystals.
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INTRODUCTION Metal halide perovskites have attracted a growing level of research interest because of their remarkable optical and electronic properties, which include high carrier mobilities, long charge diffusion lengths, small exciton binding energies, broadly tunable photoluminescence, high photoluminescence quantum yields (PLQYs), and narrow bandwidths.1−6 After their successful demonstration as sensitizers in solar cells,7 metal halide perovskites have quickly emerged as popular optoelectronic materials and have been extensively tested for potential applications in solar cells, light-emitting devices, photodetectors, and lasers.8−17 In particular, lead halide perovskite nanocrystals of the composition APbX3 (where A is a cation and X is a halogen anion) have been recently utilized in lightemitting diodes because of their photo- and electroluminescence efficiencies are much better than those of their bulk polycrystalline counterparts.18,19 Over the past several years, therefore, much effort has been spent on the synthesis of colloidal perovskite nanocrystals with controlled size and geometry.20−22 In this case, the compositions satisfying the tolerance factor are limited to methylammonium (CH3NH3+, MA), formamidinium [CH(NH2)2+, FA], and cesium (Cs+) for cations and chloride (Cl−), bromide (Br−), and iodide (I−) for anions.23 Among them, MAPbX3 and CsPbX3 nanocrystals have been successfully synthesized.24−29 For example, CsPbX3 nanoparticles were synthesized by using the so-called hotinjection method, which requires a high reaction temperature.25 © 2017 American Chemical Society
MAPbX3 nanoparticles were also synthesized by the ligandassisted reprecipitation method.26 This method utilized roomtemperature synthesis by changing the quality of the solvent from good [dimethylformamide (DMF)] to bad (toluene). Formamidinium lead halide (FAPbX3) was recently introduced as an alternative to MAPbX3 because of its high thermal stability and low band gap energy.30 Because of the large size of FA, however, the synthesis of FAPbX3 is much more challenging than that of MAPbX3. While some synthetic methods for FAPbX3 nanocrystals have been investigated,31−36 a simple route for synthesizing FAPbX3 nanocrystals in a controlled manner is still highly desirable. In this work, we report a synthetic route for well-defined FAPbX3 (X = Cl, Br, I, or mixed halide) nanocrystals at room temperature. The synthesized FAPbX3 nanocrystals show high thermal stability, very broad color tunability (408 nm ≤ λpeak ≤ 784 nm), a high PLQY (75%), and a narrow full width at half-maximum (fwhm) (18−48 nm). Our approach utilizes the PbX2−dimethyl sulfoxide (DMSO) complexes (PbX2−DMSO) as precursors instead of the conventional PbX2 precursors. It is known that DMSO forms stable complexes with many metal compounds, including lead halide.37 Once the PbX2−DMSO complex is formed, the ligand DMSO can be easily replaced with Received: April 26, 2017 Revised: June 12, 2017 Published: June 13, 2017 5713
DOI: 10.1021/acs.chemmater.7b01705 Chem. Mater. 2017, 29, 5713−5719
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Chemistry of Materials Scheme 1. Synthesis of FAPbX3 Nanocrystals Using the PbX2−DMSO Precursor
Figure 1. FT-IR spectra of DMSO and PbX2−DMSO precursors. The right panel shows magnified spectra showing more clearly the SO stretch and C−S stretch regions.
wavenumber. Simultaneously, the formation of the coordination bond strengthens the C−S bond, which shifts the peak to the higher-wavenumber region.37,40,42 As shown in Figure 1, the original SO peak at 1059 cm−1 shifts to 992, 970, and 978 cm−1 and the C−S peak at 700 cm−1 shifts to 709, 714, and 724 cm−1 for the PbI2−DMSO, PbBr2−DMSO, and PbCl2−DMSO complexes, respectively. These results suggest the formation of PbX2−DMSO complexes. The synthesized PbX2−DMSO complexes are highly soluble in polar solvents such as DMF. It is notable that the PbCl2− DMSO complex is well dissolved in DMF while PbCl2 is almost insoluble in DMF. For the synthesis of FAPbX3 nanocrystals, the PbX2−DMSO complex (50 μmol), FAX (50 μmol), and oleylamine (OLA, 5−25 μL, 10.6−53 μmol) were dissolved together in DMF (3.5 mL), and then the solutions were transferred in a dropwise manner into a copious amount of a toluene (175 mL)/oleic acid (OLAc, 787 μL) mixture while being vigorously stirred (Scheme 1). In this case, OLAc was added to increase the stability of FAPbX3 nanocrystals. Upon being mixed with toluene, the solution immediately became pale green and showed PL under ultraviolet (UV) exposure. The reaction was very fast and occurred at room temperature. Finally, the solution was centrifuged at 5300 RCF to remove weakly fluorescent large particles. MAPbX3 nanocrystals also can be synthesized by the same procedures (Figure S2). It is worth mentioning that the PbX2−DMSO complex and FAX react even in the solid state without solvent. As shown in Figure
alkylammonium halide at room temperature by the intramolecular exchange reaction.38−40 For example, perovskite thin films of FAPbI3 were successfully prepared by using PbI2− DMSO precursors.38−41 We report that PbX2 −DMSO complexes can also be utilized in the synthesis of colloidal perovskite nanocrystals as well as a thin film.
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RESULTS AND DISCUSSION The synthesis of FAPbX3 nanocrystals was performed by the ligand-assisted reprecipitation method using PbX2−DMSO complexes as precursors (Scheme 1). Briefly, PbX2−DMSO complexes were first prepared by dissolving PbX2 salts in DMSO. For example, 1 g of PbBr2 was dissolved in 7 mL of DMSO for the synthesis of the PbBr2−DMSO complex. The reaction proceeded for 12 h at 70 °C. The PbX2−DMSO complex can be easily isolated by precipitation in toluene. The slow addition of 20 mL of toluene to the PbBr2−DMSO solution yielded a white crystal powder. The powder was dried in a vacuum oven for 24 h before being used. PbCl2−DMSO and PbI2−DMSO complexes were also prepared by the same procedures (Figure S1). The synthesized PbX2−DMSO complexes were characterized by Fourier transform infrared spectroscopy (FT-IR) (Figure 1). For neat DMSO, the stretching vibrations of SO and C−S appear at 1059 and 700 cm−1, respectively.37 The formation of a coordination bond between the lead of PbX2 and the oxygen of DMSO weakens the SO bond, which causes the peak shift to the lower 5714
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Chemistry of Materials S3, when the PbX2−DMSO complex and FAX powder were just mixed together, they form a pale green powder and show PL like that collected from the solution. The PL of FAPbBr3 nanocrystals was tuned by controlling the amount of OLA (Figure 2). The sample prepared by using
well with the size calculated via X-ray diffraction (XRD) using the Scherrer equation.34 Repeated experiments show that our results are quite reproducible and several parameters, including the precursor concentration and the speed of solution mix, slightly affect the size and distribution. For the 5 μL OLA sample, small particulates (d = 3 ± 1 nm) as well as cubic particles (d = 11 ± 3 nm) were observed in the TEM micrographs (Figure 3a). In this case, the dominant species is still large cubic particles. The amount of small particulates decreased as the amount of OLA increased, and they were barely observed in the 10 μL OLA sample. When the amount of OLA increased more than 10 μL, the PL and absorbance bands shifted to the shorter wavelength (Figure 2). When the amount of OLA was 12.5 μL, the green PL turned to blue with a λpeak of 488 nm. Furthermore, the absorbance peak shifted to 455 nm with an additional peak at 425 nm. As the amount of OLA increased to 25 μL, the PL peak and absorbance peak gradually shifted to λpeak values of 438 and 425 nm, respectively. The absorbance peaks at a wavelength shorter than the bulk peak (λpeak = 530 nm) are generally attributed to the formation of two-dimensional (2D) platelets with a certain thickness.20 The sample prepared with 15 μL of OLA exhibited two PL peaks at 470 and 486 nm that are generally considered to be the formation of bimodal species of perovskite nanocrystals.44,45 This is well supported by TEM. As shown in Figure 3c, large thin 2D particles were also found along with cubic particles (d = 8 ± 2 nm) in TEM micrographs of 15 μL of the OLA sample. The formation of 2D platelets is also well supported by the absorption spectrum profile. The band edge-like absorption (5−10 μL of OLA) turned into the distinctive molecular-like absorption (12.5−25 μL of OLA) with an increase in the amount of OLA. Those sharp absorption peaks are generally considered as proof of the strong Coulombic hole−electron interaction caused by dielectric confinement in thin 2D materials.46−48 From these data, we conclude that the addition of OLA above a certain critical amount (≥10 μL) induces the formation of quantumconfined nanoplatelets with a discrete thickness, which results in the blue-shift of PL and absorption.20 The time-resolved PL spectra of FAPbBr3 nanocrystals are depicted in Figure 4b. For the comparison of PL lifetimes, MAPbBr3 nanocrystals were also prepared following the procedure reported elsewhere.26,44 The details of three nonexponential decay lifetime values are listed in Table S1, and the PL and UV−visible (UV−vis) spectra of their samples are presented in Figure S5. The lifetime of the FAPbBr3 nanocrystal (τavg = 80.17 ns) was much longer than that of the MAPbBr3 nanocrystal (τavg = 12.78 ns), which is consistent with other previous results.49,50 The precipitates collected by
Figure 2. PL emission (solid lines) and UV−visible absorbance (dashed lines) spectra of FAPbBr3 nanocrystals synthesized by varying the amount of OLA from 5 to 25 μL. The PL excitation wavelength (λex) was 365 nm. The photographs at the right present corresponding FAPbBr3 solutions under UV irradiation (λpeak = 365 nm).
5 μL of OLA exhibited green PL with a λpeak of 531.5 nm. With an increase in the amount of OLA to 7.5 and 10 μL, there was no significant change in the band position. Meanwhile, the PL intensity gradually increased as the amount of OLA increased from 5 to 10 μL. PL showed the highest intensity when the amount of OLA was 10 μL. Under this optimized condition, the OLA:FABr mole ratio in the precursor solution was 3:7 (21.2 μmol:50 μmol). In this case, the PL showed a narrow spectrum with a 22 nm fwhm (Figure 4a), and the PLQY was measured to be 75%. The details of the PLQY calculation are described in the legend of Figure S4. In the UV spectra, the samples prepared by using 5−10 μL of OLA showed a single absorption at 525 nm, which is consistent with that of bulk FAPbBr3 (Figure 2).43 This result indicates that the synthesized FAPbBr3 nanocrystals are three-dimensional particles.44 This is well supported by transmission electron microscopy (TEM) data. As shown in Figure 3b, FAPbBr3 nanocrystals were uniform cubic particles with a size d of 10 ± 2 nm, which agrees
Figure 3. TEM micrographs of FAPbBr3 nanocrystals prepared using (a) 5, (b) 10, and (c) 15 μL of OLA. 5715
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Figure 4. (a) PL emission (solid line) and UV−visible absorbance (dashed line) of the FAPbBr3 nanocrystal solution prepared using 10 μL of OLA. The PL spectrum shows an emission peak at 530 nm with a narrow fwhm of 22 nm. The photoluminescence quantum yield was 75%. The inset photograph shows the image of 100 mL of as-synthesized FAPbBr3 nanocrystals. (b) Time-resolved PL spectra of FAPbBr3 nanocrystals, the precipitate of the FAPbBr3 solution collected by centrifugation at 5300 RCF, and MAPbBr3 nanocrystals.
(300) at 2.0 Å (Figure 5b). The XRD pattern of the precipitate collected by centrifugation of the FAPbBr3 nanocrystal solution at 5300 RCF (Figure 5c) is quite consistent with other previous reports of bulk FAPbBr3.49,50 FAPbBr3 nanocrystals showed a broad 2θ = 19° peak. This broad peak can be removed by another high-speed centrifugation at 24400 RCF. As shown in Figure 5d, the intensity of the 2θ = 19° peak significantly decreased after the centrifugation. Meanwhile, the supernatant collected by centrifugation at 24400 RCF showed a highintensity 2θ = 19° peak (Figure 5e). This result suggests that the broad 2θ = 19° peak originated from the small size of the particulates. Interestingly, the supernatant exhibited several new strong peaks at 7.85 Å (2θ = 11.27°), 6.73 Å (2θ = 13.14°), 5.21 Å (2θ = 17.01°), and 4.60 Å (2θ = 19.30°). None of these peaks come from the PbBr2−DMSO precursor (Figure 5a). We think that these peaks originate from the 2D nanoplatelets with very few layers.44,47 A detailed analysis of XRD patterns, including 2D nanoplatelets contained in the supernatant, is presented in Table S4. The optical properties of the supernatant were slightly different from those of the purified FAPbBr 3 nanocrystals. The PL emission and UV−vis absorption of the supernatant shifted to the shorter wavelength region (Figure S6). XRD analysis of the supernatant is quite consistent with the very short PL lifetime (τavg = 13.23 ns). Our synthetic method can be applied to the synthesis of other FAPbX3 nanocrystals, including FAPb(Cl1−α/Brα)3 and FAPb(Br1−β/Iβ)3, where 0 ≤ α ≤ 1 and 0 ≤ β ≤ 1. Following the same procedures, a variety of FAPbX3 nanocrystals were synthesized. In this case, an optimized OLA:FAX ratio of 3:7 was used. As shown in panels a and b of Figure 6 and Table S5, the PL of FAPbX3 nanocrystals can be broadly tuned from 408 to 784 nm by varying the α:β molar ratio. For the FAPb(Cl1−α/ Brα)3 nanocrystals, PL can be tuned from 408 to 530 nm with an increase in α from 0 to 1. Similarly, PL could also be tuned from 530 to 774 nm with an increase in β from 0 to 1 for FAPb(Br1−β/Iβ)3 nanocrystals. In contrast to the case of tuning via the amount of OLA, the absorption profile retained the band edge-like structure with a change in halide composition. This implies that PL is tuned by the energy gap change by the halide composition rather than dimensional confinement. FAPb(Cl1−α/Brα)3 nanocrystals showed quite narrow spectral bands (18 nm ≤ fwhm ≤ 26 nm) regardless of the composition. However, the PL bandwidths of FAPb(Br1−β/Iβ)3 nanocrystals (25 nm ≤ fwhm ≤ 66 nm) were much broader than those of FAPb(Cl1−α/Brα)3 nanocrystals. In particular, the bandwidth
centrifugation at 5300 RCF showed a much longer lifetime (τavg = 146.16 ns). Such a long lifetime originates from a long charge diffusion length in large perovskite particles.50 Because of the long lifetime, the precipitates show very weak fluorescence.41 The lifetime of the FAPbBr3 nanocrystal is highly dependent on the centrifugal speed of purification. With an increase in the g force from 5300 to 6000 RCF, the average lifetime (τavg) decreased from 80.17 to 20.87 ns (Table S2). When the highspeed centrifugation (g force = 24400 RCF) was further performed using the 6000 RCF sample, a majority of the FAPbBr3 nanocrystals were precipitated and a small amount was left in the supernatant. The lifetime of the supernatant decreased to 13.23 ns, while the precipitate showed a larger value, 32.50 ns (Table S3). To further study the crystal structure of FAPbBr3 nanocrystals, XRD analysis was performed (Figure 5). FAPbBr 3 nanocrystals showed the characteristic (100) peak at 6.0 Å and its corresponding higher-order peaks (200) at 3.0 Å and
Figure 5. XRD patterns of (a) the PbBr2−DMSO precursor, (b) FAPbBr3 nanocrystals, (c) the precipitate collected by centrifugation of the FAPbBr3 nanocrystal solution at 5300 RCF, (d) purified FAPbBr3 nanocrystals collected by centrifugation at 24400 RCF, and (e) the supernatant of the FAPbBr3 solution collected at 24400 RCF. 5716
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Figure 6. Color tunability of FAPbX3 nanocrystals. (a) FAPbX3 nanocrystals dispersed in toluene under UV irradiation (λpeak = 365 nm). (b) PL emission spectra of FAPbX3 nanocrystals and (c) corresponding color gamut of FAPbX3 nanocrystals displayed on the CIE diagram.
was broadened at high I contents. We found that the bandwidth of FAPb(Br1−β/Iβ)3 can be improved by changing the feed molar ratio of γ = (moles of FAX)/(moles of PbBr2−DMSO complex + moles of PbI2−DMSO complex). Originally, FAPb(Br1−β/Iβ)3 nanocrystals were synthesized under stoichiometric conditions (γ = 1.0). Under the slightly hypostoichiometric condition (γ = 0.8), the PL bandwidth of FAPb(Br1−β/Iβ)3 nanocrystals decreased (19 nm ≤ fwhm ≤ 48 nm) (Figures S7 and S8), and PL peaks at the same β were slightly shifted to a wavelength longer than that under the stoichiometric conditions; for example, the PL peak of FAPbI3 changed from 774 nm (γ = 1.0) to 784 nm (γ = 0.8) (Figure S7). The additional images of nanocrystal solutions synthesized at a γ ratio of 0.8 are shown in Figure S9. The hyperstoichiometric condition (γ = 1.2), however, destabilized FAPb(Br1−β/Iβ)3 nanocrystals and caused large amounts of precipitation. The PL colors of FAPbX3 nanocrystals are displayed on a CIE diagram (Figure 6c). The color gamut of FAPbX3 nanocrystals was 135% wider than that of the 1953 NTSC color standard. The high thermal stability of perovskite nanocrystals is in strong demand for their engineering applications, including solar cells and LED devices. It was originally reported that FAPbX3 thin films showed thermal stability better than that of MAPbX3.30 We have compared the thermal stability of FAPbBr3 and MAPbBr3 nanocrystal solutions. For the experiments, MAPbBr3 nanocrystals were synthesized by using the conventional reprecipitation method reported selsewhere.26,44 Both FAPbBr3 and MAPbBr3 nanocrystals were placed in a 100 °C oil bath, and PL spectra were obtained every 5 min for 1 h. Both samples showed green PL, but the peak positions were slightly different each other (Figure S5). The integrated PL intensity changes of MAPbBr3 and FAPbBr3 nanocrystals are shown in Figure 7. The PL intensity of MAPbBr3 nanocrystals quickly decreased by almost 50% over the first 10 min and gradually decreased to 17% for 1 h. For FAPbBr3 nanocrystals, interestingly, the PL intensity increased to 120% after a 20 min annealing and gradually decreased to 38% after a 1 h annealing. It is not yet clear why the PL increases during the first stage of annealing, but similar behavior was reported elsewhere.51 However, FAPbBr3 nanocrystals are less stable than MAPbBr3 nanocrystals under high-humidity conditions (Figure S10). We
Figure 7. Thermal stability comparison between FAPbBr3 and MAPbBr3 at 100 °C.
think the stability of perovskite nanocrystals can be further increased either by changing ligands or by removing the residual DMF by dialysis, which is detrimental for the longterm stability of perovskites.52,53
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CONCLUSION
In conclusion, we have successfully demonstrated a synthetic approach to FAPbX3 colloidal nanocrystals using the PbX2− DMSO complex as a precursor. Our approach allows the synthesis of well-defined FAPbX3 nanocrystals at room temperature. The synthesized FAPbX3 nanocrystals showed bright PL with a PLQY of 75%. In this case, the PL of FAPbX3 nanocrystals can be broadly tunable from the blue to nearinfrared region (λpeak = 408−784 nm) by changing either the amount of OLA or the composition of X. Under the optimized condition, the bandwidth of the PL spectra of FAPbX3 nanocrystals was quite narrow (fwhm = 18−48 nm). Our approach is versatile and scalable. Because of their fascinating thermal and optical properties presented in this work, FAPbX3 nanocrystals are promising for their applications in LEDs. 5717
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01705. Experimental details, PbX2−DMSO precursors, solid state reaction of the PbBr2−DMSO complex and FABr, PLQY of FAPbBr3, PL and UV−vis spectra of MAPbBr3 and FAPbBr3 nanocrystals, PL spectra of purified FAPbBr3 nanocrystals, PL and UV−vis spectra of FAPb(Cl1−α/Brα)3 and FAPb(Br1−β/Iβ)3, effect of the stoichiometric factor (γ) on PL, humidity stability of MAPbBr3 and FAPbBr3 nanocrystals, time-resolved PL data, lifetimes of FAPbBr3, and analysis of XRD patterns (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Youngjong Kang: 0000-0001-5298-9189 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2012R1A6A1029029) and partially by the Ministry of Science, ICT & Future Planning (2017R1A2B2007618).
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REFERENCES
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Chemistry of Materials
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DOI: 10.1021/acs.chemmater.7b01705 Chem. Mater. 2017, 29, 5713−5719
