Strong Blue Emission from Sb3+-Doped Super Small CsPbBr3

Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 1750−1756

pubs.acs.org/JPCL

Strong Blue Emission from Sb3+-Doped Super Small CsPbBr3 Nanocrystals Xiangtong Zhang,† Hua Wang,‡ Yue Hu,§ Yixian Pei,|| Shixun Wang,† Zhifeng Shi,⊥ Vicki L. Colvin,§ Shengnian Wang,|| Yu Zhang,*,† and William W. Yu*,†,‡

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†

State Key Laboratory of Integrated Optoelectronics and College of Electronic Science and Engineering, Jilin University, Changchun 130012, China ‡ Department of Chemistry and Physics, Louisiana State University, Shreveport, Louisiana 71115, United States § Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States || Institute for Micromanufacturing, Louisiana Tech University, Ruston, Louisiana 71270, United States ⊥ Key Laboratory of Materials Physics of Ministry of Education, Department of Physics and Engineering, Zhengzhou University, Zhengzhou 450052, China S Supporting Information *

ABSTRACT: Colloidal lead halide perovskite nanocrystals (NCs) have high tunability in the visible light region and high photoluminescence quantum yields (PL QYs) for green and red emissions, but bright blue emission is still a challenge. Super small CsPbBr3 perovskite NCs emit blue light around 460 nm with a narrow peak width, and they do not have the problem of phase separation like their Cl−Br counterparts. However, the blue emission from super small CsPbBr3 NCs easily becomes green over time, and their PL QY is still low. The doping of Sb3+ ions successfully reduced the surface energy, improved the lattice energy, passivated the defect states below the band gap, eventually boosted the PL QY of blue emission to 73.8%, and resulted in better spectral stability even at elevated temperatures in solution (40−100 °C). Its CIE coordinates were (0.14, 0.06), which are close to the primary blue color (0.155, 0.070) according to the NTSC TV color standard.

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quantum confinement effect,32,33 but their spectral stability is poor, and the blue emission easily turns to green. In this work, we synthesized Sb3+-doped super small CsPbBr3 NCs at room temperature. The doped CsPbBr3 NCs showed a higher PL QY (73.8%) with a blue emission of CIE coordinates (0.14, 0.06), very close to the primary blue color (0.155, 0.070) according to the NTSC TV color standard. In addition, the stability was also greatly enhanced. The crystal structure and optical performance of the doped NCs with the increased Sb3+ ion ratio are demonstrated here. We prepared five samples named as A−E (A is undoped; B−E are doped), which are outlined in the Experimental Section. After the injection of the precursor solution, the dark treatment of 12 h was an aging process.34,35 TEM images of the five samples are given in Figure 1a−e. The average sizes of the 5 samples were 2.6 nm (A), 2.4 nm (B), 2.2 nm (C), 2.9 nm (D), and 2.3 nm (E) (Figure 1f). High-resolution transmission electron microscopy (HRTEM) and fast Fourier transform (FFT) images highlighted that they had a well-defined crystalline structure with the same lattice fringe of 0.29 nm of (200) lattice planes. To further confirm the crystal structure, we offer more HRTEM images of samples A and C in Figure

ighly efficient and outstanding visual effect are always the goals of lighting and display.1 Because of the nonblinking,2−4 narrow emission bandwidth,5,6 high photoluminescence quantum yield (PL QY),7,8 facile synthesis, and tunable band gap over the whole visible range,9−11 lead halide perovskite NCs have been widely studied in many fields such as lasers,12 light-emitting diodes (LEDs),13−15 solar cells, and photodetectors.16 However, compared with the near-unit PL QYs in green and red regions, the insufficient PL QY of blue emission still needs improvement because of the requirements of lighting and display with high PL QY and low power consumption.17,18 To date there are many methods to boost the PL QY of blue emission from CsPbClxBr1−x NCs, such as tetrafluoroborate salt treatment, 19 Ni 2+ ion doping, and YCl 3 surface passivation.20,21 However, the phase segregation of mixedhalide lead halide perovskite NCs is inevitable.22−24 CsPbCl3 NCs are not attractive because of the very low PL QY (5%) and near violet emission (410 nm).6,25 Recently, doping engineering has drawn vast attention.26−28 The postaddition of Zn2+ and Cd2+ ions can shift the green emission of CsPbBr3 NCs to blue and maintain the original PL QY (∼60%).29 The doping of Al3+ ions makes CsPbBr3 NCs generate blue emission at 456 nm.30 For super small CsPbBr3 NCs, they themselves have blue emission around 460 nm and medium PL QY (the reported highest is value 54.6%),31 due to the strong © XXXX American Chemical Society

Received: March 20, 2019 Accepted: March 28, 2019

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DOI: 10.1021/acs.jpclett.9b00790 J. Phys. Chem. Lett. 2019, 10, 1750−1756

Letter

The Journal of Physical Chemistry Letters

Figure 1. (a−e) TEM images of samples A−E (scale bars represent 50 nm), respectively. Insets show HRTEM and FFT images of corresponding NCs. (f) Variation of average sizes of samples A−E. (g) XRD patterns and (h) absorption and emission spectra of the 5 samples. Photographs of the 5 samples under (i) sunlight and (j) UV light combined with the doping levels of Sb3+ ions and the variation of PL QYs, respectively.

Figure 2. (a) Absorption spectra in logarithmic scale and (b) PL decay curves and biexponential fitting curves of all five samples.

obvious change compared with the undoped CsPbBr3 NCs except for the sharper peak: the half width at half-maximum (hwhm) to the long wavelength side of the absorption peak reduced from 6 nm (A) to 5 nm (C), which may be attributed to the increase of the exciton binding energy.36−38 The full width at half-maximum (fwhm) of the emission peak also reduced from 17 to 14 nm. The emission peaks of all doped samples are located in the deep blue emission region at 461 nm. The doping levels of Sb3+ ions in the four doped samples were determined to be 3.4% (B), 3.6% (C), 3.9% (D), and

S1 that contain many particles with lattice fringes of 0.29 nm. The X-ray diffraction (XRD) patterns are given in Figure 1g. Obviously, all samples had a good agreement with the PDF card of CsPbBr3 (JCPDS No. PDF 75-0412). As distinguished by the red dashed line around 30°, the peak position of the doped NCs progressively shifted to higher degrees with the increased amount of Sb3+ ions, which was due to the shrinkage of crystal lattice because the ionic radius of Sb3+ (0.092 nm) is smaller than that of Pb2+ (0.119 nm). Figure 1h exhibits the absorption and emission spectra. With Sb3+ ion doping, the position of absorption peak had no 1751


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The charge of Sb3+ is higher than that of Pb2+, which may need some charge compensators to keep the charge balance. We used X-ray photoelectron spectroscopy (XPS) to analyze the charge balance brought by Sb3+ ion doping. Figure 3a shows the fitting results of XPS of sample C for Sb 3d. There are two peaks that locate at 530.9 (red line) and 540.3 eV (green line) belonging to Sb3+ ions and two fitting blue lines (the fitted curves of the Gaussian and Lorentz line-type) of the O 1s state. The XPS spectra of Cs, Pb, and Br in samples A and C are presented in Figure S3. The atomic percentages of samples A and C are exhibited in Figure 3b. For sample A, the percentages of each atom are 18.6% (Cs), 22.2% (Pb), and 59.0% (Br). For sample C, the percentages of each atom are 10.2% (Cs), 22.3% (Pb), 66.2% (Br), and 1.3% (Sb) (corresponding to a doping level of 5.5%). The charge compensator of sample C may be the negative Br− ions that increased from 59.0% to 66.2% after doping. Besides, the ratios of Br/Pb are 2.7 and 3.0 for samples A and C, which should not form a Br-rich environment to significantly boost the PL QYs as reported on green CsPbBr3 NCs.42 Finally, the stability of samples A and C was recorded in detail. In solution, for sample A, the position of the emission peak had 8 nm shift toward green in 10 days. Besides, a new emission peak around 485 nm gradually appeared (Figure S4a). For sample C, the position of emission peaks shifted only 2 nm (Figure S4b) in 10 days. Figure S4c shows the variation of the emission intensity. The quick descending sections were fitted by linear functions. For undoped sample A, the linear equation is y = −0.296x + 1.052; for sample C, the linear equation is y = −0.140x + 1.348. The initial PL enhancement of sample C can be attributed to the photoactivation.43−45 After Sb3+ doping, the stability of emission intensity had over one fold improvement, according to the rate of emission attenuation [(0.296 − 0.140)/0.140 = 1.11]. The stepwise TEM images exhibit that sample A gradually turned to large nanoplates (Figure S 4d) and sample B turned to thin nanosheets (the thickness was about 1.8 nm) (Figures S4e,f), which is consistent with the variation of PL peaks from different morphologies.31 Additionally, the thermostability of doped sample C also gains a great improvement. As exhibited in Figure 4a, the undoped sample A lost its blue emission at 120 °C in about 5 min, but about 25 min was required for the doped sample C to lose most of its emission (Figure 4b). These results clearly demonstrated that Sb3+ ion doping effectively slowed the growth of super small CsPbBr3 NCs. These obvious changes caused by Sb3+ ion doping may be attributed to the enhanced lattice energy. The bond

4.2% (E) by using inductively coupled plasma-optical emission spectrometry (ICP-OES) (Figure 1i). The doping had a great impact on the PL QY. As shown in Figure 1i, the toluene solution of Sb3+-doped CsPbBr3 NCs (B and C) exhibit obvious blue emission under sunshine, implying high PL QYs. The variation of emission capability of all samples can be easily seen and compared under UV light. The CIE coordinates of (0.14, 0.06) were found by the emission spectrum of sample C, which are very close to the primary blue CIE coordinates of (0.155, 0.070) according to the NTSC TV color standard shown in Figure S2. The PL QY results of all samples are shown in Figure 1j. The PL QY of undoped CsPbBr3 NCs is 50.0%, which is comparable to the highest reported value, 54.6%.31 With an appropriate Sb3+ doping level, the PL QY reaches 73.8% for sample C. The variation of the PL QY for different CsPbBr3−Sb3+ NCs may be understood by the Urbach tail39,40 and PL decay lifetime. In Figure 2a, the Urbach tail gradually decreases then increases with the inflection point located at sample C. This means that some defect states near the band gap are passivated at low doping level but generated again at high doping level. Meanwhile, the PL decay curves further explained the fluctuation of PL QYs. In Figure 2b, all curves are fitted by biexponential functions. Table 1 lists the fitting results. Table 1. Biexponential Fitting Results of PL Decays for Samples A−E sample

τ1 (10−9 s)

τ2 (10−9 s)

A1 (%)

A2 (%)

τavg (10−9 s)

A B C D E

3.3 3.7 3.8 3.9 3.4

12.9 9.1 9.6 9.1 10.4

74.0 85.3 88.1 84.7 41.8

26.0 14.7 11.9 15.3 58.2

5.8 4.5 4.5 4.7 7.5

There are two parts in the PL decay curves. τ1 and A1 represent the decay time and percentage of intrinsic radiative recombination, respectively;31,41 τ2 and A2 represent the decay time and percentage of nonradiative recombination, respectively.31,41 τavg represents the weighted average PL decay time. Obviously, sample C has the largest percentage of radiative recombination. Then, from the relationship between the PL QY and doping level it is clear that the doping of the Sb3+ ion passivates some defect states below the band gap and increases the percentage of radiative recombination at the optimal doping level that sample C has.

Figure 3. (a) XPS spectra of sample C. (b) Atomic percent of inorganic elements in samples A and C acquired by XPS. 1752


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Figure 4. Emission stability of (a) undoped sample A and (b) doped sample C in toluene at 120 °C. Emission changes of (c) undoped sample A and (d) doped sample C in toluene with PS at different temperatures. The samples were held at the indicated temperatures for 3 min before PL measurements, except the specified time at 100 °C in panel d.

Figure 5. (a) PL spectra of fresh films (left) and aged films of samples A and C (right). Inset shows photographs of two samples under UV light. (b) XRD patterns of aged films after storage in air for 20 days. The black (bottom) and red (top) vertical lines are from CsPbBr3 (JCPDS No. PDF 75-0412) and Cs4PbBr6 (JCPDS No. PDF 73-2478), respectively. Contact angles of samples (c) A and (d) C with water droplets.

for more details), PS could not prevent the undoped sample A from changing to green emission (Figure 4c), but the doped sample C showed a much greater thermostability and its PL peak shifted very little (to 471 nm) upon 15 min at 100 °C (Figure 4d). This indicates it is good for the NCs to be mixed with a polymer when making photoluminescent light-emitting diodes.

dissociation energies of Pb−Br and Sb−Br are 248.5 and 314.0 kJ/mol, respectively.46 The increased bond dissociation energy means higher lattice energy of super small CsPbBr3 NCs with Sb3+ ions. We also found that polymers can further improve the stability of CsPbBr3−Sb3+ against heating. When the mixtures of the dissolved polystyrene (PS) in the toluene solution of samples A and C were heated (see the Experimental Section 1753


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The Journal of Physical Chemistry Letters The film stability of samples A and C was tested. In Figure 5a, both of them have strong blue emission under UV light, but sample A has a small green emission peak at 520 nm. After storage in air for 6 h, sample C generated a weak green emission peak. Figure 5b reveals that after 20 days sample A generated many other XRD diffraction peaks that contained mainly Cs4PbBr6 (JCPDS No. PDF 73-2478) and some PbBr2 (JCPDS No. PDF 31-0679; not shown for clarity), but sample C still matched well with the standard PDF card of CsPbBr3 (JCPDS No. PDF 75-0412). Panels c and d of Figure 5 show that the contact angle of sample C increased from 54.6°/57.2° to 75.9°/75.9°. These revealed that the surface energy decreased with Sb3+ doping.47 The doping of Sb3+ ions enhanced the crystal stability and slowed the growth of super small CsPbBr3 particles by the strengthened lattice energy and the reduced surface energy. We reported a facile method to acquire highly efficient deep blue emission from super small CsPbBr3 NCs with a PL QY of 73.8% by Sb3+ doping. The doping of Sb3+ ions greatly slowed the growth of super small CsPbBr3 NCs and improved their crystal stability. The Sb3+-doped CsPbBr3 perovskite NCs indicate that it is possible to inhibit the further growth of super small CsPbBr3 NCs and may have applications in the fabrication of blue lasers, light-emitting diodes, and photodetectors.

Absorbance spectra were obtained with a Shimadzu UV-2550 spectrophotometer. The morphologies of the NCs were observed by a Technai F20 transmission electron microscope. XPS spectra were analyzed on an ESCALAB250 spectrometer. The doping amounts were found by a Varian720-ES ICP-OES instrument. XRD patterns of NCs were acquired using a Bruker D8 Advance X diffractometer. The visible absolute PL QYs and time-resolved PL lifetime were obtained by an integrating sphere and an FLS920P fluorescence spectrometer. The contact angle measurements were conducted using a Dataphysics OCA 15 Pro video-based optical contact angle measuring system.

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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.jpclett.9b00790.

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CIE coordinates of sample C, HRTEM images, XPS spectra, and solution stability of samples A and C (PDF)

AUTHOR INFORMATION

Corresponding Authors

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*E-mail: [email protected] (Y.Z.). *E-mail: [email protected] (W.W.Y.).

EXPERIMENTAL SECTION Materials. Dimethylformamide (DMF, 99.8%, Sigma-Aldrich), cesium bromide (CsBr, 99.9%, Aladdin), lead(II) bromide (PbBr2, 99.0%, Aladdin), antimony(III) bromide (SbBr3, 99.999% metals basis, Alfa Aesar), oleic acid (OA, technical grade 90%, Sigma-Aldrich), oleylamine (OAm, 80−90%, Aladdin), toluene (GR, Beijing Chemical Reagent Ltd.), molecular sieves (3 Å, Aladdin), methyl acetate (GR, Beijing Chemical Reagent Ltd.), and polystyrene beads (PS, average Mw 35000, Sigma-Aldrich) were used as received. Solvent Drying. Molecular sieves were activated under vacuum at 200 °C for 2 h then cooled and added into the solvent bottles to dry DMF and toluene overnight. Synthesis of CsPbBr3 NCs. At room temperature, CsBr (0.4 mmol) and PbBr2 (0.4 mmol) were dissolved in DMF (10 mL) with OA (1 mL) and OAm (0.5 mL) to make a precursor solution. Then, the precursor solution (1 mL) was injected into cold toluene (10 mL, 0 °C) under vigorous stirring. Synthesis of Sb3+-Doped CsPbBr3 NCs. All steps followed the synthesis of CsPbBr3 NCs except that a special amount of SbBr3 was dissolved in cold toluene (10 mL, 0 °C) before the injection of the precursor solution. The amount of SbBr3 varied (0, 0.01, 0.02, 0.03, and 0.04 mmol). The corresponding doped NCs were named A, B, C, D, and E, respectively. After the injection of the precursor solution, the products were stored in darkness for about 12 h before purification. Preparation of PS−CsPbBr3 and PS−CsPbBr3−Sb3+ Mixtures. PS beads (1.5 g) were added into the synthesized products (5 mL), and the mixture was placed on a shaker for 1 h to become an evenly distributed mixture. Purif ication. First, the products were centrifuged at 5000 rpm for 10 min to discard the large precipitates. Second, the supernatants were added 2-fold volume of methyl acetate and then centrifuged at 10000 rpm for 10 min. Lastly, the collected powders were dispersed in toluene (3 mL). Characterizations. Fluorescence emission spectra were recorded using an Ocean Optics QE Pro spectrometer.

ORCID

Zhifeng Shi: 0000-0002-9416-3948 Vicki L. Colvin: 0000-0002-8526-515X Yu Zhang: 0000-0003-2100-621X William W. Yu: 0000-0001-5354-6718 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the following financial support: National Key Research and Development Program of China (2017YFB0403601), National Natural Science Foundation of China (61675086, 61722504, and 51772123), Science and Technology Development Program of Jilin Province (20190101016), and State Scholarship Fund of China Scholarship Council.

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Strong Blue Emission from Sb3+-Doped Super Small CsPbBr3

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