Room-Temperature Engineering of All-Inorganic Perovskite

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Cite This: Chem. Mater. 2017, 29, 8978-8982

Room-Temperature Engineering of All-Inorganic Perovskite Nanocrsytals with Different Dimensionalities Haoze Yang,†,# Yuhai Zhang,†,# Jun Pan,†,‡ Jun Yin,† Osman M. Bakr,*,†,‡ and Omar F. Mohammed*,† †

KAUST Solar Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia ‡ KAUST Catalysis Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia S Supporting Information *

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varied dimensionalities, including Cs4PbBr6, CsPb2Br5 and CsPbBr3 NCs. These perovskite NCs exhibit both a narrow size distribution and high phase purity. Importantly, such a general synthesis endows those NCs with an identical capping agent, i.e., oleic acid, as evidenced by Fourier transform infrared (FTIR) spectroscopy, allowing us to study their intrinsic properties without interference from surface ligands and surface defects. Interestingly, our temperature-dependent photoluminescence spectra reveal the distinct nature of excitons in those different dimensional perovskites. In this study, perovskite nanocrystals with different dimensionalities were synthesized through a modified reverse microemulsion method. Typically, a dimethylformamide (DMF) solution of lead bromide (PbBr2), hydrogen bromide (HBr), oleic acid (OA) and oleyamine (OAm) was thoroughly mixed and injected into a n-hexane solution of cesium oleate and OA under vigorous stirring. The immiscible nature of DMF and n-hexane leads to the formation of a reverse microemulsion system comprising “aqueous” DMF droplets suspended in the “oil” phase of hexane. A large amount of OA was added as a surfactant to decrease the size of those droplets, resulting in a minimized NC size. Importantly, HBr and cesium oleate were chosen as Br and Cs sources, respectively, due to the very limited solubility of CsBr in either DMF or hexane. Instead of using a singular CsPbBr3 precursor, such selection of reactants offers a large flexibility over the feeding ratio of Cs:Pb, which is essential to control the crystalline phase of the resulting NCs, as demonstrated in CsPb2Br5 and CsPbBr3 perovskite synthesis. Indeed, all-inorganic perovskite NCs were readily prepared by tuning the feeding ratio of Cs:Pb (see Figure 1). X-ray diffraction (XRD) patterns were used to determine the dimensionality of the perovskite NCs. Figure 2a shows the XRD patterns for the final product of all three different NCs. Major peaks of the three nanocrystals’ XRD patterns were labeled in the figure. When the molar ratio of Cs:Pb ratio is 4:1, the XRD pattern demonstrated that pure 0D Cs4PbBr6 NCs were obtained. After decreasing the ratio to 1:1 or 1:2, we found that 3D CsPbBr3 NCs are the final product. When the ratio is further decreased to 1:3, the product was a mixture of CsPb2Br5 and CsPbBr3 NCs. On the other hand, when the ratio decreases to 1:5, the XRD pattern indicated that pure CsPb2Br5

erovskite-based semiconductor nanocrystals have become a promising candidate as active-layer materials in optoelectronic applications due to their remarkable optical properties, including high photoluminescence quantum yield (>90%), narrow emission bands (full width at half-maximum, fwhm < 30 nm) and tunable bandgap energy (400−800 nm).1−11 Intrinsically, these unique physical properties originate from the lattice structures of basic functional units (i.e., PbX6 octahedra) existing in the crystalline perovskite solids. Based on the connection manner of the PbX6 octahedra, the dimensionality of Cs-based perovskite is classified into zerodimensional (0D), two-dimensional (2D) and three-dimensional (3D) crystal structures. For example, 0D perovskite Cs4PbBr6 displays the most significant quantum confinement effect (bandgap = 3.90 eV)12 among these perovskite analogues due to its isolated nature of PbX6 octahedra as shown in Figure 1. In contrast, the 3D perovskite CsPbBr3 shows the least confinement with a bandgap of 2.36 eV due to its couplednetwork nature of PbX6 octahedra.13 The understanding of the intrinsic correlation between dimensionality and optical property requires successful synthesis of various dimensional perovskites through a general method. The precise control over perovskite dimensionality has been realized due to the recent progress in synthetic technique. 3D perovskite CsPbBr3 NCs have commonly been synthesized by the hot-injection method,13−17 whereas perovskite CsPb2Br5 NCs were recently massively produced via a coprecipitation method.18−22 Very recently, our group successfully synthesized 0D perovskite NCs from a reverse microemulsion system at room temperature.23,24 Despite the success in synthesis, the large differences among synthetic conditions usually induce a large discrepancy in the surface property of as-obtained materials and an ensuing difference of intrinsic optical parameters derived from those obtained samples. For example, the exciton binding energy of 0D Cs4PbBr6 perovskite varies from 171 to 353 meV when samples synthesized using different methods are analyzed.23,25 Such dramatic discrepancy poses a severe challenge in understanding the intrinsic properties of perovskites with different dimensionalities. To quantitatively evaluate the dimensionality effect of perovskite, a general synthetic method simultaneously affording both identical surface properties and control over dimensionality is highly desired. Herein, we developed a single synthetic approach at room temperature to produce all-inorganic perovskite NCs with © 2017 American Chemical Society

Received: October 2, 2017 Revised: October 24, 2017 Published: October 25, 2017 8978

DOI: 10.1021/acs.chemmater.7b04161 Chem. Mater. 2017, 29, 8978−8982

Communication

Chemistry of Materials

Figure 1. Schematic micelle structure for the reverse microemulsion comprising an “oil” phase with n-hexane and an “aqueous” phase with DMF; with different Cs:Pb ratios, perovskite nanocrystals of different dimensionalities were formed.

Figure 2. (a) XRD patterns of Cs4PbBr6, CsPb2Br5 and CsPbBr3 perovskite nanocrystals synthesized by the microemulsion method; (b-d) TEM images of Cs4PbBr6, CsPb2Br5 and CsPbBr3 perovskite nanocrystals, CsPb2Br5 nanocrystals are close to cubic shape; and the size distributions of histogram of these NCs can be found in Figures S5−S7 of the Supporting Information); (e−g) absorption (blue) and PL (green) spectra of Cs4PbBr6, CsPb2Br5 and CsPbBr3 perovskite NCs in toluene solution, respectively. Note that those perovskite NCs are stable in solvents, such as hexane and toluene.

peak at 513 nm with excitation at 375 nm, and the full width at half-maximum (fwhm) is 18 nm. The PL quantum yield (PLQY) was measured as 64%. The measurement of PLQY was conducted in an integrated sphere with an excitation wavelength of 375 nm. The absorption peak of the colloidal solution is approximately 315 nm, which is due to the 1S0→3P1 transition of Pb2+ centers, as was reported by Nikl.6 This peak is also observed in the absorption spectra of CsPb2Br5 and CsPbBr3 nanocrystals. In addition to this peak, there is a long tail from 350 to 500 nm. The TEM image shown in Figure 2b indicates that the obtained sample has a hexagonal shape, confirming its zero-dimensional phase. For CsPb2Br5 nanocrystals on the other hand, the PL spectrum exhibits a peak position at 508 nm with 22 nm fwhm at excitation wavelength of 375 nm, agrees well with the previous report on the strong green-emissive CsPb2Br5 nanoplatelets.21 However, Jiang’s group’s experimental results and DFT simulation results indicated that CsPb2Br5 nanosheets have an indirect bandgap of 2.98 eV and they should be PL-inactive.26 Meanwhile, Zhou’s

NCs were synthesized. In addition, energy-dispersive X-ray (EDX) spectroscopy was used to analyze the element ratio of Cs:Pb:Br to confirm the final product of our NCs (see Figures S1−S3 of the Supporting Information). The element ratios of Cs4PbBr6, CsPb2Br5 and CsPbBr3 NCs are 5.28:1:7.4, 1:1.92:5.73 and 1.3:1:3.45, respectively. It should be noted that the Cs and Br content in Cs4PbBr6 is slightly higher than expected due to the excess CsBr added during the synthesis of NCs. A similar result has recently been reported by Zhang et al.23 To analyze the optical properties of the NCs, UV/vis absorption and photoluminescence emission spectra of the colloidal solutions of the three nanocrystals are presented in Figure 2e−g. The absorption spectra of the three nanocrystals are dominated by sharp exciton peaks, which are similar to the optical features reported in the literature.18,21,23 For the PL spectrum, with excitation at 375 nm, the three perovskite nanocrystals show similar PL peaks at approximately 510 to 520 nm. For 0D Cs4PbBr6, the PL emission spectrum exhibits a 8979

DOI: 10.1021/acs.chemmater.7b04161 Chem. Mater. 2017, 29, 8978−8982

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Chemistry of Materials

CsPbBr3, OA density is 2.19/nm2. The similar surface ligand density proves that different dimensionality perovskite NCs have same surface. This is lower than the previously reported density of 3.9−6.7/nm2.32 The reason might be that oleic acid was lost during the washing and drying process before TGA measurement. In addition, through TGA measurement, the reaction yield was measured to be 85% for Cs4PbBr6, 48.5% for CsPb2Br5 and 26.5% for CsPbBr3 NCs, respectively. To study the exciton nature of these different kinds of NCs, we conducted temperature-dependent PL measurement for the thin film sample (see Figure 4).33 The PL integrated intensity was plotted against temperature and fitted with the following Arrhenius equation:

group indicated that they calculated an indirect band gap of 2.44 eV with a larger direct band gap of 2.52 eV.27 In our case, active PL was measured as 507 nm. The PLQY of CsPb2Br5 nanocrystal was measured to be 84%, coinciding well with Zhou’s group results. Because CsPb2Br5 is an indirect bandgap material,26,28 one possible explanation for the high PLQY21,27 is structural defects, such as Br vacancy as recently reported in 0D Cs4PbBr6.29 The absorption peaks of the CsPb2Br5 colloidal solution are approximately 390 and 510 nm, also with a long tail. For 3D CsPbBr3, the PL position is observed at 520 nm with 17 nm fwhm. The PL quantum yield was measured to be 70%. The absorption peak is located at 505 nm. The TEM image shown in Figure 2d indicates that 3D CsPbBr 3 nanocrystals have a cubic shape, which is consistent with recent reports.30,31 To analyze the capping ligand in these NCs with different dimensionalities, Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) experiments were conducted. In the process of synthesis, oleic acid (OA) and oleyamine (OAM) were used as ligands. Figure 3b shows

I (T ) =

I0 1 + A e−E b /(kBT )

where I(T) and I0 are the integrated PL intensities at temperature T and 0 K, respectively. Eb is the exciton binding energy, and kB is the Boltzmann constant. It is found that the exciton binding energy decreases as the dimensionality increases. For 0D Cs4PbBr6, the exciton binding energy is 239.0 ± 34.8 meV obtained from the average of three different experiments (see Figure S8 of the Supporting Information), which is slightly higher than our previous reported value (171 ± 22 meV).23 This change in the exciton binding energy may arise from different fitting parameters and quality. This is 1 order of magnitude higher than that of CsPb2Br5 and CsPbBr3, as shown in Figure 4. One possible explanation is that the generated exciton in 0D perovskite is confined within the isolated octahedral structure. With a tightly bound exciton, it is unlikely to dissociate and diffuse in the crystal lattice. This might be the reason that the PLQY of Cs4PbBr6 is not sensitive to the environment such as solvent in the colloidal suspension and even in the thin film.23 Even though some group has reported that 0D NCs have no green emission,22 recently, it has been reported that the active PL is due to the recombination from the midband gap states arising from the crystal defects.29,34 For CsPb2Br5 and CsPbBr3 NCs, the exciton binding energy was measured to be 31.0 ± 7.8 and 18.4 ± 3.5 meV, respectively. It should be noted that this value for CsPbBr3 NCs is 2 times lower than those for NCs synthesized by another method (40 meV),16 the ligand and synthesis method might influence the binding energy. In CsPb2Br5 and CsPbBr3 perovskite, due to the low binding energies, they are more likely Wannier−Mott excitons, whereas Cs4PbBr6 has a much higher binding energy, and the excitons are more likely a kind of Frenkel excitons rather than Wannier−Mott excitons.35,36 In conclusion, we reported a reverse microemulsion method at room temperature as an approach to synthesize Cs4PbBr6, CsPb2Br5 and CsPbBr3 NCs by adjusting only the cesium-tolead ratio. These NCs exhibited high phase purity and high photoluminescence quantum yield. More importantly, this single-synthesis approach enabled us to synthesize NCs under identical experimental conditions, including the same capping agent, but with different dimensionalities. This method gives possibility for the first time to investigate and optimize the optical and electronic properties simultaneously for potential applications including LED and solar cells without the interference of surface discrepancy and defect states.

Figure 3. (a) TGA analysis of Cs4PbBr6, CsPb2Br5 and CsPbBr3 perovskite nanocrystals; (b) FTIR analysis of Cs4PbBr6, CsPb2Br5 and CsPbBr3 perovskite nanocrystals, oleic acid (OA) and oleyamine (OAm).

the FTIR spectra of the three kinds of nanocrystals, OA and OAm. For OA, there are peaks at 2920 and 1710 cm−1, which can be assigned to CH and CO stretching vibrations. For OAm, the peaks are located at 2920 and 1480 cm−1, and all peaks appear in the three nanocrystals. In addition, TGA was measured, and the results are displayed in Figure 3a and Figure S4 of the Supporting Information, indicating that approximately 5−10 wt % is lost from 250 to 360 °C, which matches the boiling point of oleic acid at 360 °C. Because the amount of OAm during synthesis is only 1% comparing to OA, OAm can be ignored in TGA curve. The hydrophobic ligands on the nanocrystal surface enable a dispersion of colloids in nonpolar solvents such as n-hexane and toluene. The three NCs with different dimensionalities have the same ligand surface, i.e., oleic acid, which proves that by this method, we are able to obtain perovskite nanocrystals with the same capping ligand. The oleic acid ligand density was calculated by TGA measurement. For Cs4PbBr6, OA density is 1.89/nm2; for CsPb2Br5, OA density is 0.96/nm2; for 8980

DOI: 10.1021/acs.chemmater.7b04161 Chem. Mater. 2017, 29, 8978−8982

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Figure 4. (a−c) Temperature-dependent PL spectra of Cs4PbBr6, CsPb2Br5 and CsPbBr3 nanocrystals, respectively; (d−f) integrated PL emission intensity as a function of temperature of Cs4PbBr6, CsPb2Br5 and CsPbBr3 nanocrystals, respectively.

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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.7b04161. Materials, synthesis methods, EDX, TGA measurement results, size distribution and Arrhenius plots of integrated PL intensity (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*O.M.B., email: [email protected]. *O.F.M., email: [email protected]. ORCID

Osman M. Bakr: 0000-0002-3428-1002 Omar F. Mohammed: 0000-0001-8500-1130 Author Contributions #

H.Y. and Y.Z. contributed equally to this work.

Funding

The authors gratefully acknowledge funding support from KAUST. Notes

The authors declare no competing financial interest.

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REFERENCES

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