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Enhancing luminescence of intrinsic and Mn doped CsPbCl3 perovskite nanocrystals through Co2+ doping
Materials Research Bulletin 121 (2020) 110608
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Enhancing luminescence of intrinsic and Mn doped CsPbCl3 perovskite nanocrystals through Co2+ doping ⁎
Zhen Caoa, Ji Lia,b, , Li Wanga, Ke Xinga, Xi Yuana, Jialong Zhaoa, Xin Gaob, Haibo Lia, a b
T
⁎
Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping, 136000, China National Key Lab of High Power Semiconductor Lasers, Changchun University of Science and Technology, Changchun, 130022, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Luminescence Perovskite nanocrystals CsPbCl3 Mn doped perovskite quantum dots Co doping
In this work, the effects of Co2+ ions on the structural and luminescent properties of intrinsic and Mn doped CsPbCl3 NCs were studied by using photoluminescence (PL) spectroscopy, X-ray diffraction (XRD) spectroscopy, and transmission electron microscopy (TEM). The undoped and Mn doped CsPbCl3 NCs with various Co/Pb molar ratios were synthesized at 190 °C. It was found that the quantum yields of purple emissions in these CsPbCl3 NCs were greatly enhanced up to 30% after Co2+ ions were doped. The diffraction peaks in XRD patterns shifted to a large angle with the increase of Co/Pb molar ratio, indicating Co doping in Pb-site. Further both intrinsic and Mn doped CsPbCl3 NCs exhibited uniform cubes with average sizes of 8–10 nm and good crystallinity after Co doping. The enhancement of purple emissions from excitons in the CsPbCl3 NCs was related to the reduction of nonradiative defects in these NCs through Co2+ doping.
1. Introduction In recent years, lead halide perovskite materials (APbX3, A = Cs+, CH3NH3+; X = Cl, Br, I) have become very popular because of their excellent photoelectronic properties, low production cost and easy processing in solution [1–7]. Compared with organo-inorganic hybrid lead halide perovskite materials, which have poor stability due to air and humidity conditions, all inorganic lead halide perovskite materials CsPbX3 have good stability, high photoluminescence quantum yield (PL QY, > 90%), narrow emission line width (12–40 nm) and wide color range (150% NTSC) [6–10]. Therefore, all inorganic perovskite materials will have widely applications in the fields of light-emitting diodes (LEDs), displays, solar cells, photodetectors and lasers [11–13]. In the family of all inorganic lead perovskite materials, the PL QYs of green and red CsPbBr3 and CsPbI3 NCs have reached 90%, respectively [14,15]. However, the PL QY of purple CsPbCl3 NCs is relatively low due to many defects/traps in the NCs [16]. Up to now, the reasons for the low efficiency of these NCs have not been fully understood, which greatly limits the use of all-inorganic lead perovskite chloride materials, thus hindering their application in violet light-emitting devices (LEDs). Recently, the transition and alkaline earth metal ions such as Ni, Cu, Sn, Mn, Ca, and Mg have been doped into perovskite NCs to effectively suppress nonradiative defects and improve PL QYs [17–24]. The exciton
emission QY of CsPbCl3 NCs was enhanced up to 31% by the introduction of K and Rb ions [17,18]. The PL QYs of violet-emitting allinorganic CsPbCl3 NCs were significantly improved over 90% by Ni and Ca ion doping [19,20]. The nearly 100% PL efficiency of CsPbCl3 NCs was surprisingly realized with incorporation of Cd ions [21]. Similarly, the enhancement of emissions in Mn doped CsPbCl3 NCs was observed by addition of metal chlorides such as CuCl2 and NiCl2 [25,26]. The radius of cobalt (Co) ion (0.058 nm) is smaller than that of lead (Pb) ion and Co ions can be doped into CsPbCl3 NCs to replace Pb without destroying their structure at low doping level [27]. The doping of Co ions into CH3NH3Pb1-xCoxI3 solar cells for improved power conversion efficiencies has been reported [28,29]. Therefore, it is necessary to study the effects of Co ions on electronic and structural properties of CsPbCl3 NCs for better understanding the improvement mechanisms in optoelectronic performances of lead perovskite halides. In this work, we synthesized Co doped CsPbCl3 perovskite NCs with different Co/Pb feed ratios and studied their structural and luminescent properties by measuring UV–vis absorption and PL spectra, PL efficiencies, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The effects of Co ions on the structural and optical properties of Mn doped CsPbCl3 perovskite NCs with dual emission were also discussed by changing the Co ion concentration. The PL enhancement of excitons in the NCs was demonstrated through Co doping.
⁎ Corresponding authors at: Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping, 136000, China. E-mail addresses: [email protected] (J. Li), [email protected] (H. Li).
https://doi.org/10.1016/j.materresbull.2019.110608 Received 12 August 2019; Received in revised form 28 August 2019; Accepted 30 August 2019 Available online 31 August 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 121 (2020) 110608
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2. Experimental
compared with the undoped ones. The obviously gentle rise in the absorption spectra of Co2+ doped CsPbCl3 NCs from 600 nm to absorption edge may be attributed to the scattering of the large-size particles from residual chemicals. The exciton absorption band of CsPbCl3 NCs is at 402 nm. It slightly decreases with increasing Co2+ doping concentration. Similarly, the exciton PL peak is at 406 nm with full width at half maximum (FWHM) of 9.51 nm (0.071 meV) for undoped CsPbCl3 NCs. It slightly shifts to the higher energy side from 406 to 404 nm after Co doping. The shifts of the exciton absorption and PL bands may be related to the change in the size of NCs. The reduction of size may cause a small shift in the exciton absorption and emission bands of the NCs, which is consistent with the quantum size effect of CsPbX3 NCs [34,35]. The change in FWHM of PL band in the doped NCs may be related to the size distribution of the NCs. The size distribution of the NCs with low doping level is relatively uniform, resulting in a small FWHM. After the molar ratio exceeds 3/1, the FWHM increases slightly. As seen in Fig. 1c, it is interestingly found that Co doped CsPbCl3 NCs show a significant increase in band edge emission efficiency with increasing the Co doping concentration. The PL QY of the undoped CsPbCl3 NCs is 6%, and it is enhanced to 30% as Co/Pb ratio increases to 2/1. Table 1 summarizes the actual Co doping concentrations of Co doped CsPbCl3 NCs with various Co/Pb ratios of 0/1, 0.5/1, 1/1, 2/1, and 3/1. On the other hand, as displayed in Fig. 1d, Co2+ doped CsPbCl3 NCs exhibit a clear magnetic hysteresis loop, indicating room-temperature ferromagnetism. It is also found that ferromagnetism increases with the increase of Co2+ content. The observed ferromagnetism of doped NCs may be attributed to the presence of Co2+ in the lattice as a substituent for carrier-induced ferromagnetism, which is usually reported in Co doped ZnO diluted magnetic semiconductors [36,37]. The XRD patterns of Co doped CsPbCl3 NCs with different Co doping concentrations are shown in Fig. 2. The Co/Pb feed molar ratios of these NCs are 0/1, 0.5/1, 1/1, 2/1, and 3/1, respectively. It is obviously observed that undoped CsPbCl3 NCs have two strong diffraction peaks at 15.8° and 31.9°, corresponding to (100) and (200) directions, respectively. They have typical cubic structure, which is consistent with a reference from a standard card (JCPDS: 75-0411). No new diffraction peak appears after the NCs are doped with different Co2+ ion concentrations. This shows that the structure of doped NCs is still the same as that of undoped ones. Fig. 2b shows the enlarged patterns of (200) diffraction peaks for Co doped CsPbCl3 NCs with different Co ion concentrations. It can be clearly seen that the diffraction peaks shift to a large angle with the increase of Co doping concentration, which is in agreement with Mn doped CsPbCl3 NCs [38,39]. We believe that this is due to the doping of Co ions into the lattice of NCs for the substitution of Pb. Because the radius of Co ions is less than that of Pb ions, replacing Pb ions in the lattice of NCs by Co ions can reduce the lattice parameter and the average size of NCs. Based on the fitted results of XRD by Debye-Scherrer formula, the average sizes were estimated to be 12.69, 11.35, 11.16, 10.36, 9.59 nm, respectively, for Co doped CsPbCl3 NCs with various Co/Pb molar ratios of 0/1, 0.5/1, 1/1, 2/1, and 3/1. As a result, the maximum diffraction peak (200) of perovskite NCs doped with different Co concentrations shifts slightly to a larger angle. Fig. 3a-d shows the TEM images of Co doped CsPbCl3 NCs with various Co/Pb feed ratios 0/1, 1/1, 2/1, and 3/1, respectively. From these photos, it can be clearly seen that Co doped CsPbCl3 NCs with different Co concentrations are uniformly distributed and monodispersed cubes. The average size of undoped NCs is about 10.5 nm. After doping of Co ions, the size distribution of perovskite NCs become more uniform. The average size of Co doped NCs decreases from 10.5 nm to 8.1 nm with increasing Co/Pb ratio to 3/1. The reason is that the radius of Co ion is smaller than that of Pb ion, and Co ion is doped into the lattice of NCs to replace Pb ion, resulting in the slight reduction of the average size of NCs, which is also consistent with the results of XRD as shown in Fig. 2. Moreover, the chloride of Co in the reaction may reduce the number of chlorine vacancies, thus inhibiting the
2.1. Materials Lead chloride (PbCl2, 99.99%), Cesium carbonate (Cs2CO3, 99.99%) and trioctylphosphine (TOP, 90%) were purchased from Aladdin; 1octadecene (ODE, 90%,) was purchased from Alfa Aesar; Oleic acid (OA, 90%) and oleylamine (OLA, 70%) were purchased from Aldrich. All chemicals were used without further purification. 2.2. Synthesis of Co doped CsPbCl3 NCs The Cs-oleate precursors, CsPbCl3 and Mn doped CsPbCl3 NCs were synthesized, which followed our previous reports, respectively [30–33]. For the synthesis of Co doped CsPbCl3 NCs in a typical procedure, the PbCl2 (0.2 mmol) and CoCl2 (x mmol) were mixed with OLA (1.6 mL), OA (1.6 mL), TOP (1 mL), and ODE (5 mL) in a 50 mL three-neck roundbottomed flask. The reaction mixture was degassed at 110 °C for 30 min, and then heated up to 190 °C under argon flow. A 1 mL portion of Cs-oleate precursor was quickly injected and 60 s later the reaction mixture was cooled by an ice-water bath. The solution was centrifuged for 5 min at 5000 rpm after the reaction, the supernatant was discarded and then the particles were redispersed in 2 mL of hexane and centrifuged again for 5 min at 5000 rpm to remove the residual reaction mixture. The Co concentration was varied by changing the ratio of PbCl2 and CoCl2 from 1:0 to 1:3. 2.3. Synthesis of Co and Mn codoped CsPbCl3 NCs For the synthesis of Co and Mn codoped CsPbCl3 NCs in a typical procedure, the PbCl2 (0.2 mmol), MnCl2 (0.2 mmol) and CoCl2 (x mmol) were mixed with OLA (1.6 mL), OA (1.6 mL), TOP (1 mL), and ODE (5 mL) in a 50 mL three-neck round-bottomed flask. The reaction mixture was degassed at 110 °C for 30 min, and then heated up to 190 °C under argon flow. A 1 mL portion of Cs-oleate precursor was quickly injected and 60 s later the reaction mixture was cooled by an ice-water bath. The solution was centrifuged for 5 min at 5000 rpm after the reaction, the supernatant was discarded and then the particles were redispersed in 2 mL of hexane and centrifuged again for 5 min at 5000 rpm to remove the residual reaction mixture. The Co concentration was varied by changing the ratio of PbCl2 and CoCl2 from 1:0 to 1:3. 2.4. Characterization UV-visible absorption spectra were recorded by Shimadzu UV-2700 spectrophotometer. The steady-state and time-resolved fluorescence spectra were collected by a Horiba Jobin Yvon fluorologo-3 fluorescence spectrometer 450 W Xenon lamp and N-305 nano-LED were used as excitation source. The absolute photoluminescence quantum yield (PL QY) was recorded by Otsuka QE-2000. Transmission electron microscopy was performed using JEOL-JEM-2100 microscope. X-ray diffraction (XRD) was characterized by incident radiation from Rigaku D/ Max-2500 and copper Kα radiation diffractometer (λ = 1.54 A). The actual compositions of NCs were analyzed with an inductively coupled plasma mass spectrometer (ICP-MS, PerkinElmer Nexion 350-X). Magnetic measurements were carried out with vibrating sample magnetometer (VSM, Lake Shore M-7407). 3. Results and discussion Fig. 1 shows the UV–vis absorption and steady-state PL spectra, PL QYs, and magnetization M (H) curves of Co doped CsPbCl3 NCs synthesized with different Co/Pb molar ratios (0/1, 0.5/1, 1/1, 2/1, and 3/ 1). As shown in Fig. 1a-b, the exciton absorption band and PL peak of the Co doped CsPbCl3 NCs with Co2+ ions have almost no clear change, 2
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Fig. 1. Absorption (a), PL spectra (b), PL QYs (c), and magnetization M (H) curves (d) of Co doped CsPbCl3 NCs with various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/ 1, 2/1 and 3/1, respectively.
formation of structural defects in NCs and making lattice in NCs more orderly. The large area TEM image of Co doped CsPbCl3 NCs with Co/ Pb molar ratio of 2/1 is shown in Fig. 3e, and their size distribution is seen in the inset of Fig. 3e. The Co doped CsPbCl3 NCs with a feed ratio of 2/1 have quite uniform distribution. As seen in Fig. 3f, it is obviously observed from this photograph that the crystallinity of NCs becomes better. The lattice spacings of NCs in (100) and (110) directions were estimated to be about 0.56 and 0.38 nm, respectively, which is in
Table 1 The actual Co doping concentrations of Co doped CsPbCl3 NCs with various Co/ Pb ratios of 0/1, 0.5/1, 1/1, 2/1, and 3/1, measured by ICP-MS. Feed ratio
0/1
0.5/1
1/1
2/1
3/1
Concentration
0
1.8%
2.7%
4.9%
7.5%
Fig. 2. (a) XRD patterns of Co doped CsPbCl3 NCs with various Co/Pb molar ratios of 0/1, 0.5/1, 1/1, 2/1, and 3/1. (b) The enlarged XRD patterns at about 31.9° are also shown. 3
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Fig. 3. (a–d)TEM images of Co doped CsPbCl3 NCs with various Co/Pb feed ratios of 0/1, 1/1, 2/1, and 3/1, (e) the large area TEM image of Co doped CsPbCl3 NCs with Co/Pb molar ratio of 2/1 (their size distribution is shown in the inset of this figure) and (f) the high resolution TEM image of the NCs with a Co/Pb ratio of 2/1.
Fig. 4. Absorption (a) and PL spectra (b), PL QYs (c), and PL decay curves (d) of Co and Mn codoped CsPbCl3 NCs with a Mn/Pb ratio of 1/1 and various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1.
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The feed ratio of Mn/Pb is fixed to 1/1 and the feed ratios of Co/Mn are varied from 0/1 to 3/1, respectively. It is obviously observed from Fig. 5a that there are two strong diffraction peaks at 15.8 and 31.9 degrees for undoped, Mn doped (feed ratio is 1/1 Mn/Pb) and Co and Mn codoped CsPbCl3 NCs, which correspond to (100) and (200) directions, respectively, which are the same as those for Co doped CsPbCl3 NCs in Fig. 2a. Compared with the reference standard card (JCPDS: 75-0411), Mn doped CsPbCl3 NCs exhibit the same structure as that of undoped CsPbCl3 ones, having a typical cubic structure. After codoping of Co and Mn ions, no new diffraction peak appears, indicating that the cubic structure of CsPbCl3 NCs was not changed. Fig. 5b displays an enlargement of (200) diffraction peaks for undoped, Mn doped, and Co and Mn codoped CsPbCl3 NCs. It can be clearly seen that after doping Mn ions, the diffraction peaks move slightly towards a large angle, which is consistent with previous reports [38,39]. This indicates that Mn ions are doped in CsPbCl3 NCs to replace Pb ions in the lattice of NCs. Then, the diffraction peak shifts to a larger angle with the increase of Co doping concentration. This trend is the same as that of Co ion doped CsPbCl3 NCs as shown in Fig. 2. As shown in XRD patterns, Co and Mn ions enter the lattice of CsPbCl3 NCs to replace the Pb ions, respectively. Based on the fitted results of XRD by DebyeScherrer formula, the average sizes were estimated to be 11.29, 9.94, 9.31, 9.03, 8.95 nm, respectively, Co and Mn codoped CsPbCl3 NCs with a Mn/Pb ratio of 1/1 and various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1. This is due to the fact that both Co and Mn ions are smaller than Pb ions, which is similar to Ni and Mn ion codoped CsPbCl3 NCs [26]. The TEM images of undoped, Mn doped (feed ratio Mn/Pb is 1/1), and Co and Mn codoped CsPbCl3 NCs are shown in Fig. 6. The feed ratio of Mn/Pb in Co and Mn codoped CsPbCl3 NCs is fixed to 1/1. The feed ratios of Co/Pb in Co and Mn codoped NCs are 0/1, 0.5/1, 1/1, 2/1 and 3/1, respectively. From these photos, it can be clearly seen that undoped, Mn doped, and Co and Mn codoped CsPbCl3 NCs are uniformly distributed and monodispersed cubes, which is consistent with the results of XRD. The average size of undoped CsPbCl3 NCs is about 10.5 nm. The average size of Mn doped CsPbCl3 NCs (feed ratio Mn/Pb is 1/1) is about 9.7 nm. After doping with Co ions, the size distribution of Co and Mn codoped CsPbCl3 NCs becomes more uniform, and the average size of undoped CsPbCl3 NCs decreases from 9.7 nm to 7.7 nm. Until the feed ratio reaches 3/1, the size distribution of Co and Mn codoped CsPbCl3 NCs becomes uneven and the average size becomes larger, about 10.9 nm. This may be due to the excessive chloride concentration of Co in the reaction, or the effect of high Co ion concentration on the growth process of NCs in the reaction solution. As shown in Fig. 4c-d, Co and Mn codoped CsPbCl3 NCs with a Co/Pb feed ratio of 3/1 have the lowest Mn PL QY and the shortest Mn PL lifetimes among samples, which is similar to the previous results reported in Ni and Mn codoped CsPbCl3 NCs [26,30].
accordance with previous reports [22]. The UV–vis absorption and PL spectra, PL QYs, and Mn PL decay curves of Mn doped CsPbCl3 NCs doped with different Co ion concentrations are shown in Fig. 4. There is no new absorption band in the absorption spectra after Co doping in Mn doped CsPbCl3 NCs as seen in Fig. 4a. The width of the exciton absorption band is expressed by the half width at half-maximum (HWHM) to the side of the long wavelength. [12] With the increase of Co doping, the exciton absorption band shifts to the blue and becomes broad and its HWHM to the long wavelength side is estimated to be 6, 8, 10, 13, 15 nm, respectively, for Co and Mn codoped CsPbCl3 NCs with various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1. The blue shift and broadening in exciton absorption band reflects the change in electronic structure and size of CsPbCl3 NCs due to doping of Mn and Co ions. It is known that Mn doped CsPbCl3 NCs exhibit a narrow exciton PL emission band at about 400 nm and a wide Mn ion PL emission band at about 600 nm [38,39]. As shown in Fig. 4b, when the feed ratio of Mn/Pb is fixed to 1/1, the feed ratio of Co ion can be changed to 0/1, 0.5/1, 1/1, 2/1, and 3/1, respectively. It is found that the relative PL emission intensity of Mn ions increases significantly with the increase of Co/Pb feed ratio, compared the exciton PL. The exciton, Mn ion luminescence and total QYs of Co and Mn codoped CsPbCl3 NCs with various Co doping concentrations were tested, as shown in Fig. 4c. The exciton emission QY of Co and Mn codoped CsPbCl3 NCs increases from 7% to 14% with increasing Co/Pb ratio to 0.5/1 and then decreases to 3% with further increasing Co/Pb ratio to 3/1. It becomes low due to the efficient energy transfer from exciton to Mn ion, compared with only Co doped CsPbCl3 NCs. It is surprisingly observed that the Mn ion emission QY significantly increases from 30% to 50% and then decreases to 24%. The total QY of Co and Mn codoped CsPbCl3 NCs increases from 38% to 60% and then decreases to 27%. The increase in exciton, Mn ion luminescence, and total PL QYs is related to the reduction of nonradiative recombination centers such as defects/traps in the NCs, which is similar to enhancement of exciton PL in CsPbCl3 NC due to incorporation of CoCl2 as shown in Fig. 1. On the other hand, it is this noted that the clear decrease in exciton and Mn ion PL QYs occurs as the Co doping concentration increases to 2/1 and 3/1, respectively. The increase and decrease in exciton and Mn PL QYs can be understood by measuring the change in PL spectra and decay curves. As seen in Fig. 4b, Mn PL peak has a clear red shift from 594 nm to 603 nm as Co doping concentration increases, meaning the formation of Mn-Mn pairs due to the high level Mn ion doping [30,40]. Further, we tested the time resolved PL spectra of Mn ions as shown in Fig. 4d. It is found that the PL decay of Mn ions gradually becomes multi-exponential from mono- exponential as the Co doping concentration increases. The Mn PL lifetimes of Mn doped CsPbCl3 NCs with various Co/Pb ratios are 1.78, 1.71, 1.70, 1.55, and 1.46 ms, respectively. The reduction in PL lifetimes of Mn ions for NCs with higher Co/Pb ratios is perhaps related to formation of Mn-Mn pairs [30,40]. This also indicates that Mn doping concentration is improved by increasing addition of CoCl2. Table 2 gives the actual doping concentrations of Co and Mn ions in Co and Mn codoped CsPbCl3 NCs with a Mn/Pb ratio of 1/1 and various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1, respectively. Therefore, the experimental results indicate that the luminescence properties of CsPbCl3 NCs have been significantly improved by Co ion doping. Fig. 5 shows the XRD patterns of Co and Mn codoped CsPbCl3 NCs.
4. Conclusions In summary, we have studied the structural and luminescent properties of intrinsic CsPbCl3 and Mn codoped CsPbCl3 NCs doped with various Co/Pb molar ratios. The PL QY of excitons in intrinsic CsPbCl3 NCs was enhanced from 6% to 30% by changing Co doping concentration. The diffraction peak shifts toward a large angle in XRD patterns demonstrated the successful doping of Co into CsPbCl3 NCs. The TEM images showed the average NC sizes of 8–10 nm for Co doped CsPbCl3 NCs with increasing Co doping concentration. For Co doping in Mn doped CsPbCl3 NCs, it was surprisingly found that the exciton and Mn PL QYs of Mn doped CsPbCl3 NCs were enhanced from 6% to 14% and from 30% to 50%, respectively, with the increase of Co doping. The similar shift in XRD patterns and size change were also observed in the NCs due to Co and Mn codoping. The PL enhancement of excitons in CsPbCl3 NCs was attributed to the reduction of nonradiative defects due to passivation of NC surface through Co ion incorporation. Therefore,
Table 2 The actual doping concentrations of Co and Mn ions in Co and Mn codoped CsPbCl3 NCs with a Mn/Pb ratio of 1/1 and various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1, measured by ICP-MS. Feed ratio
0/1
0.5/1
1/1
2/1
3/1
Mn Co
1.5% 0
2.0% 2.2%
2.3% 5.1%
2.5% 7.7%
3.1% 9.3%
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Fig. 5. XRD patterns of Co and Mn codoped CsPbCl3 NCs with a Mn/Pb ratio of 1/1 and various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1.
Fig. 6. TEM images of Co and Mn codoped CsPbCl3 NCs with a Mn/Pb ratio of 1/1 and various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1.
the highly luminescent CsPbX3 NCs with doping of metal ions will exhibit important practical application in solid state lighting and displays.
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Enhancing luminescence of intrinsic and Mn doped CsPbCl3 perovskite nanocrystals through Co2+ doping ⁎
Zhen Caoa, Ji Lia,b, , Li Wanga, Ke Xinga, Xi Yuana, Jialong Zhaoa, Xin Gaob, Haibo Lia, a b
T
⁎
Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping, 136000, China National Key Lab of High Power Semiconductor Lasers, Changchun University of Science and Technology, Changchun, 130022, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Luminescence Perovskite nanocrystals CsPbCl3 Mn doped perovskite quantum dots Co doping
In this work, the effects of Co2+ ions on the structural and luminescent properties of intrinsic and Mn doped CsPbCl3 NCs were studied by using photoluminescence (PL) spectroscopy, X-ray diffraction (XRD) spectroscopy, and transmission electron microscopy (TEM). The undoped and Mn doped CsPbCl3 NCs with various Co/Pb molar ratios were synthesized at 190 °C. It was found that the quantum yields of purple emissions in these CsPbCl3 NCs were greatly enhanced up to 30% after Co2+ ions were doped. The diffraction peaks in XRD patterns shifted to a large angle with the increase of Co/Pb molar ratio, indicating Co doping in Pb-site. Further both intrinsic and Mn doped CsPbCl3 NCs exhibited uniform cubes with average sizes of 8–10 nm and good crystallinity after Co doping. The enhancement of purple emissions from excitons in the CsPbCl3 NCs was related to the reduction of nonradiative defects in these NCs through Co2+ doping.
1. Introduction In recent years, lead halide perovskite materials (APbX3, A = Cs+, CH3NH3+; X = Cl, Br, I) have become very popular because of their excellent photoelectronic properties, low production cost and easy processing in solution [1–7]. Compared with organo-inorganic hybrid lead halide perovskite materials, which have poor stability due to air and humidity conditions, all inorganic lead halide perovskite materials CsPbX3 have good stability, high photoluminescence quantum yield (PL QY, > 90%), narrow emission line width (12–40 nm) and wide color range (150% NTSC) [6–10]. Therefore, all inorganic perovskite materials will have widely applications in the fields of light-emitting diodes (LEDs), displays, solar cells, photodetectors and lasers [11–13]. In the family of all inorganic lead perovskite materials, the PL QYs of green and red CsPbBr3 and CsPbI3 NCs have reached 90%, respectively [14,15]. However, the PL QY of purple CsPbCl3 NCs is relatively low due to many defects/traps in the NCs [16]. Up to now, the reasons for the low efficiency of these NCs have not been fully understood, which greatly limits the use of all-inorganic lead perovskite chloride materials, thus hindering their application in violet light-emitting devices (LEDs). Recently, the transition and alkaline earth metal ions such as Ni, Cu, Sn, Mn, Ca, and Mg have been doped into perovskite NCs to effectively suppress nonradiative defects and improve PL QYs [17–24]. The exciton
emission QY of CsPbCl3 NCs was enhanced up to 31% by the introduction of K and Rb ions [17,18]. The PL QYs of violet-emitting allinorganic CsPbCl3 NCs were significantly improved over 90% by Ni and Ca ion doping [19,20]. The nearly 100% PL efficiency of CsPbCl3 NCs was surprisingly realized with incorporation of Cd ions [21]. Similarly, the enhancement of emissions in Mn doped CsPbCl3 NCs was observed by addition of metal chlorides such as CuCl2 and NiCl2 [25,26]. The radius of cobalt (Co) ion (0.058 nm) is smaller than that of lead (Pb) ion and Co ions can be doped into CsPbCl3 NCs to replace Pb without destroying their structure at low doping level [27]. The doping of Co ions into CH3NH3Pb1-xCoxI3 solar cells for improved power conversion efficiencies has been reported [28,29]. Therefore, it is necessary to study the effects of Co ions on electronic and structural properties of CsPbCl3 NCs for better understanding the improvement mechanisms in optoelectronic performances of lead perovskite halides. In this work, we synthesized Co doped CsPbCl3 perovskite NCs with different Co/Pb feed ratios and studied their structural and luminescent properties by measuring UV–vis absorption and PL spectra, PL efficiencies, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The effects of Co ions on the structural and optical properties of Mn doped CsPbCl3 perovskite NCs with dual emission were also discussed by changing the Co ion concentration. The PL enhancement of excitons in the NCs was demonstrated through Co doping.
⁎ Corresponding authors at: Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping, 136000, China. E-mail addresses: [email protected] (J. Li), [email protected] (H. Li).
https://doi.org/10.1016/j.materresbull.2019.110608 Received 12 August 2019; Received in revised form 28 August 2019; Accepted 30 August 2019 Available online 31 August 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
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2. Experimental
compared with the undoped ones. The obviously gentle rise in the absorption spectra of Co2+ doped CsPbCl3 NCs from 600 nm to absorption edge may be attributed to the scattering of the large-size particles from residual chemicals. The exciton absorption band of CsPbCl3 NCs is at 402 nm. It slightly decreases with increasing Co2+ doping concentration. Similarly, the exciton PL peak is at 406 nm with full width at half maximum (FWHM) of 9.51 nm (0.071 meV) for undoped CsPbCl3 NCs. It slightly shifts to the higher energy side from 406 to 404 nm after Co doping. The shifts of the exciton absorption and PL bands may be related to the change in the size of NCs. The reduction of size may cause a small shift in the exciton absorption and emission bands of the NCs, which is consistent with the quantum size effect of CsPbX3 NCs [34,35]. The change in FWHM of PL band in the doped NCs may be related to the size distribution of the NCs. The size distribution of the NCs with low doping level is relatively uniform, resulting in a small FWHM. After the molar ratio exceeds 3/1, the FWHM increases slightly. As seen in Fig. 1c, it is interestingly found that Co doped CsPbCl3 NCs show a significant increase in band edge emission efficiency with increasing the Co doping concentration. The PL QY of the undoped CsPbCl3 NCs is 6%, and it is enhanced to 30% as Co/Pb ratio increases to 2/1. Table 1 summarizes the actual Co doping concentrations of Co doped CsPbCl3 NCs with various Co/Pb ratios of 0/1, 0.5/1, 1/1, 2/1, and 3/1. On the other hand, as displayed in Fig. 1d, Co2+ doped CsPbCl3 NCs exhibit a clear magnetic hysteresis loop, indicating room-temperature ferromagnetism. It is also found that ferromagnetism increases with the increase of Co2+ content. The observed ferromagnetism of doped NCs may be attributed to the presence of Co2+ in the lattice as a substituent for carrier-induced ferromagnetism, which is usually reported in Co doped ZnO diluted magnetic semiconductors [36,37]. The XRD patterns of Co doped CsPbCl3 NCs with different Co doping concentrations are shown in Fig. 2. The Co/Pb feed molar ratios of these NCs are 0/1, 0.5/1, 1/1, 2/1, and 3/1, respectively. It is obviously observed that undoped CsPbCl3 NCs have two strong diffraction peaks at 15.8° and 31.9°, corresponding to (100) and (200) directions, respectively. They have typical cubic structure, which is consistent with a reference from a standard card (JCPDS: 75-0411). No new diffraction peak appears after the NCs are doped with different Co2+ ion concentrations. This shows that the structure of doped NCs is still the same as that of undoped ones. Fig. 2b shows the enlarged patterns of (200) diffraction peaks for Co doped CsPbCl3 NCs with different Co ion concentrations. It can be clearly seen that the diffraction peaks shift to a large angle with the increase of Co doping concentration, which is in agreement with Mn doped CsPbCl3 NCs [38,39]. We believe that this is due to the doping of Co ions into the lattice of NCs for the substitution of Pb. Because the radius of Co ions is less than that of Pb ions, replacing Pb ions in the lattice of NCs by Co ions can reduce the lattice parameter and the average size of NCs. Based on the fitted results of XRD by Debye-Scherrer formula, the average sizes were estimated to be 12.69, 11.35, 11.16, 10.36, 9.59 nm, respectively, for Co doped CsPbCl3 NCs with various Co/Pb molar ratios of 0/1, 0.5/1, 1/1, 2/1, and 3/1. As a result, the maximum diffraction peak (200) of perovskite NCs doped with different Co concentrations shifts slightly to a larger angle. Fig. 3a-d shows the TEM images of Co doped CsPbCl3 NCs with various Co/Pb feed ratios 0/1, 1/1, 2/1, and 3/1, respectively. From these photos, it can be clearly seen that Co doped CsPbCl3 NCs with different Co concentrations are uniformly distributed and monodispersed cubes. The average size of undoped NCs is about 10.5 nm. After doping of Co ions, the size distribution of perovskite NCs become more uniform. The average size of Co doped NCs decreases from 10.5 nm to 8.1 nm with increasing Co/Pb ratio to 3/1. The reason is that the radius of Co ion is smaller than that of Pb ion, and Co ion is doped into the lattice of NCs to replace Pb ion, resulting in the slight reduction of the average size of NCs, which is also consistent with the results of XRD as shown in Fig. 2. Moreover, the chloride of Co in the reaction may reduce the number of chlorine vacancies, thus inhibiting the
2.1. Materials Lead chloride (PbCl2, 99.99%), Cesium carbonate (Cs2CO3, 99.99%) and trioctylphosphine (TOP, 90%) were purchased from Aladdin; 1octadecene (ODE, 90%,) was purchased from Alfa Aesar; Oleic acid (OA, 90%) and oleylamine (OLA, 70%) were purchased from Aldrich. All chemicals were used without further purification. 2.2. Synthesis of Co doped CsPbCl3 NCs The Cs-oleate precursors, CsPbCl3 and Mn doped CsPbCl3 NCs were synthesized, which followed our previous reports, respectively [30–33]. For the synthesis of Co doped CsPbCl3 NCs in a typical procedure, the PbCl2 (0.2 mmol) and CoCl2 (x mmol) were mixed with OLA (1.6 mL), OA (1.6 mL), TOP (1 mL), and ODE (5 mL) in a 50 mL three-neck roundbottomed flask. The reaction mixture was degassed at 110 °C for 30 min, and then heated up to 190 °C under argon flow. A 1 mL portion of Cs-oleate precursor was quickly injected and 60 s later the reaction mixture was cooled by an ice-water bath. The solution was centrifuged for 5 min at 5000 rpm after the reaction, the supernatant was discarded and then the particles were redispersed in 2 mL of hexane and centrifuged again for 5 min at 5000 rpm to remove the residual reaction mixture. The Co concentration was varied by changing the ratio of PbCl2 and CoCl2 from 1:0 to 1:3. 2.3. Synthesis of Co and Mn codoped CsPbCl3 NCs For the synthesis of Co and Mn codoped CsPbCl3 NCs in a typical procedure, the PbCl2 (0.2 mmol), MnCl2 (0.2 mmol) and CoCl2 (x mmol) were mixed with OLA (1.6 mL), OA (1.6 mL), TOP (1 mL), and ODE (5 mL) in a 50 mL three-neck round-bottomed flask. The reaction mixture was degassed at 110 °C for 30 min, and then heated up to 190 °C under argon flow. A 1 mL portion of Cs-oleate precursor was quickly injected and 60 s later the reaction mixture was cooled by an ice-water bath. The solution was centrifuged for 5 min at 5000 rpm after the reaction, the supernatant was discarded and then the particles were redispersed in 2 mL of hexane and centrifuged again for 5 min at 5000 rpm to remove the residual reaction mixture. The Co concentration was varied by changing the ratio of PbCl2 and CoCl2 from 1:0 to 1:3. 2.4. Characterization UV-visible absorption spectra were recorded by Shimadzu UV-2700 spectrophotometer. The steady-state and time-resolved fluorescence spectra were collected by a Horiba Jobin Yvon fluorologo-3 fluorescence spectrometer 450 W Xenon lamp and N-305 nano-LED were used as excitation source. The absolute photoluminescence quantum yield (PL QY) was recorded by Otsuka QE-2000. Transmission electron microscopy was performed using JEOL-JEM-2100 microscope. X-ray diffraction (XRD) was characterized by incident radiation from Rigaku D/ Max-2500 and copper Kα radiation diffractometer (λ = 1.54 A). The actual compositions of NCs were analyzed with an inductively coupled plasma mass spectrometer (ICP-MS, PerkinElmer Nexion 350-X). Magnetic measurements were carried out with vibrating sample magnetometer (VSM, Lake Shore M-7407). 3. Results and discussion Fig. 1 shows the UV–vis absorption and steady-state PL spectra, PL QYs, and magnetization M (H) curves of Co doped CsPbCl3 NCs synthesized with different Co/Pb molar ratios (0/1, 0.5/1, 1/1, 2/1, and 3/ 1). As shown in Fig. 1a-b, the exciton absorption band and PL peak of the Co doped CsPbCl3 NCs with Co2+ ions have almost no clear change, 2
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Fig. 1. Absorption (a), PL spectra (b), PL QYs (c), and magnetization M (H) curves (d) of Co doped CsPbCl3 NCs with various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/ 1, 2/1 and 3/1, respectively.
formation of structural defects in NCs and making lattice in NCs more orderly. The large area TEM image of Co doped CsPbCl3 NCs with Co/ Pb molar ratio of 2/1 is shown in Fig. 3e, and their size distribution is seen in the inset of Fig. 3e. The Co doped CsPbCl3 NCs with a feed ratio of 2/1 have quite uniform distribution. As seen in Fig. 3f, it is obviously observed from this photograph that the crystallinity of NCs becomes better. The lattice spacings of NCs in (100) and (110) directions were estimated to be about 0.56 and 0.38 nm, respectively, which is in
Table 1 The actual Co doping concentrations of Co doped CsPbCl3 NCs with various Co/ Pb ratios of 0/1, 0.5/1, 1/1, 2/1, and 3/1, measured by ICP-MS. Feed ratio
0/1
0.5/1
1/1
2/1
3/1
Concentration
0
1.8%
2.7%
4.9%
7.5%
Fig. 2. (a) XRD patterns of Co doped CsPbCl3 NCs with various Co/Pb molar ratios of 0/1, 0.5/1, 1/1, 2/1, and 3/1. (b) The enlarged XRD patterns at about 31.9° are also shown. 3
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Fig. 3. (a–d)TEM images of Co doped CsPbCl3 NCs with various Co/Pb feed ratios of 0/1, 1/1, 2/1, and 3/1, (e) the large area TEM image of Co doped CsPbCl3 NCs with Co/Pb molar ratio of 2/1 (their size distribution is shown in the inset of this figure) and (f) the high resolution TEM image of the NCs with a Co/Pb ratio of 2/1.
Fig. 4. Absorption (a) and PL spectra (b), PL QYs (c), and PL decay curves (d) of Co and Mn codoped CsPbCl3 NCs with a Mn/Pb ratio of 1/1 and various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1.
4
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The feed ratio of Mn/Pb is fixed to 1/1 and the feed ratios of Co/Mn are varied from 0/1 to 3/1, respectively. It is obviously observed from Fig. 5a that there are two strong diffraction peaks at 15.8 and 31.9 degrees for undoped, Mn doped (feed ratio is 1/1 Mn/Pb) and Co and Mn codoped CsPbCl3 NCs, which correspond to (100) and (200) directions, respectively, which are the same as those for Co doped CsPbCl3 NCs in Fig. 2a. Compared with the reference standard card (JCPDS: 75-0411), Mn doped CsPbCl3 NCs exhibit the same structure as that of undoped CsPbCl3 ones, having a typical cubic structure. After codoping of Co and Mn ions, no new diffraction peak appears, indicating that the cubic structure of CsPbCl3 NCs was not changed. Fig. 5b displays an enlargement of (200) diffraction peaks for undoped, Mn doped, and Co and Mn codoped CsPbCl3 NCs. It can be clearly seen that after doping Mn ions, the diffraction peaks move slightly towards a large angle, which is consistent with previous reports [38,39]. This indicates that Mn ions are doped in CsPbCl3 NCs to replace Pb ions in the lattice of NCs. Then, the diffraction peak shifts to a larger angle with the increase of Co doping concentration. This trend is the same as that of Co ion doped CsPbCl3 NCs as shown in Fig. 2. As shown in XRD patterns, Co and Mn ions enter the lattice of CsPbCl3 NCs to replace the Pb ions, respectively. Based on the fitted results of XRD by DebyeScherrer formula, the average sizes were estimated to be 11.29, 9.94, 9.31, 9.03, 8.95 nm, respectively, Co and Mn codoped CsPbCl3 NCs with a Mn/Pb ratio of 1/1 and various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1. This is due to the fact that both Co and Mn ions are smaller than Pb ions, which is similar to Ni and Mn ion codoped CsPbCl3 NCs [26]. The TEM images of undoped, Mn doped (feed ratio Mn/Pb is 1/1), and Co and Mn codoped CsPbCl3 NCs are shown in Fig. 6. The feed ratio of Mn/Pb in Co and Mn codoped CsPbCl3 NCs is fixed to 1/1. The feed ratios of Co/Pb in Co and Mn codoped NCs are 0/1, 0.5/1, 1/1, 2/1 and 3/1, respectively. From these photos, it can be clearly seen that undoped, Mn doped, and Co and Mn codoped CsPbCl3 NCs are uniformly distributed and monodispersed cubes, which is consistent with the results of XRD. The average size of undoped CsPbCl3 NCs is about 10.5 nm. The average size of Mn doped CsPbCl3 NCs (feed ratio Mn/Pb is 1/1) is about 9.7 nm. After doping with Co ions, the size distribution of Co and Mn codoped CsPbCl3 NCs becomes more uniform, and the average size of undoped CsPbCl3 NCs decreases from 9.7 nm to 7.7 nm. Until the feed ratio reaches 3/1, the size distribution of Co and Mn codoped CsPbCl3 NCs becomes uneven and the average size becomes larger, about 10.9 nm. This may be due to the excessive chloride concentration of Co in the reaction, or the effect of high Co ion concentration on the growth process of NCs in the reaction solution. As shown in Fig. 4c-d, Co and Mn codoped CsPbCl3 NCs with a Co/Pb feed ratio of 3/1 have the lowest Mn PL QY and the shortest Mn PL lifetimes among samples, which is similar to the previous results reported in Ni and Mn codoped CsPbCl3 NCs [26,30].
accordance with previous reports [22]. The UV–vis absorption and PL spectra, PL QYs, and Mn PL decay curves of Mn doped CsPbCl3 NCs doped with different Co ion concentrations are shown in Fig. 4. There is no new absorption band in the absorption spectra after Co doping in Mn doped CsPbCl3 NCs as seen in Fig. 4a. The width of the exciton absorption band is expressed by the half width at half-maximum (HWHM) to the side of the long wavelength. [12] With the increase of Co doping, the exciton absorption band shifts to the blue and becomes broad and its HWHM to the long wavelength side is estimated to be 6, 8, 10, 13, 15 nm, respectively, for Co and Mn codoped CsPbCl3 NCs with various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1. The blue shift and broadening in exciton absorption band reflects the change in electronic structure and size of CsPbCl3 NCs due to doping of Mn and Co ions. It is known that Mn doped CsPbCl3 NCs exhibit a narrow exciton PL emission band at about 400 nm and a wide Mn ion PL emission band at about 600 nm [38,39]. As shown in Fig. 4b, when the feed ratio of Mn/Pb is fixed to 1/1, the feed ratio of Co ion can be changed to 0/1, 0.5/1, 1/1, 2/1, and 3/1, respectively. It is found that the relative PL emission intensity of Mn ions increases significantly with the increase of Co/Pb feed ratio, compared the exciton PL. The exciton, Mn ion luminescence and total QYs of Co and Mn codoped CsPbCl3 NCs with various Co doping concentrations were tested, as shown in Fig. 4c. The exciton emission QY of Co and Mn codoped CsPbCl3 NCs increases from 7% to 14% with increasing Co/Pb ratio to 0.5/1 and then decreases to 3% with further increasing Co/Pb ratio to 3/1. It becomes low due to the efficient energy transfer from exciton to Mn ion, compared with only Co doped CsPbCl3 NCs. It is surprisingly observed that the Mn ion emission QY significantly increases from 30% to 50% and then decreases to 24%. The total QY of Co and Mn codoped CsPbCl3 NCs increases from 38% to 60% and then decreases to 27%. The increase in exciton, Mn ion luminescence, and total PL QYs is related to the reduction of nonradiative recombination centers such as defects/traps in the NCs, which is similar to enhancement of exciton PL in CsPbCl3 NC due to incorporation of CoCl2 as shown in Fig. 1. On the other hand, it is this noted that the clear decrease in exciton and Mn ion PL QYs occurs as the Co doping concentration increases to 2/1 and 3/1, respectively. The increase and decrease in exciton and Mn PL QYs can be understood by measuring the change in PL spectra and decay curves. As seen in Fig. 4b, Mn PL peak has a clear red shift from 594 nm to 603 nm as Co doping concentration increases, meaning the formation of Mn-Mn pairs due to the high level Mn ion doping [30,40]. Further, we tested the time resolved PL spectra of Mn ions as shown in Fig. 4d. It is found that the PL decay of Mn ions gradually becomes multi-exponential from mono- exponential as the Co doping concentration increases. The Mn PL lifetimes of Mn doped CsPbCl3 NCs with various Co/Pb ratios are 1.78, 1.71, 1.70, 1.55, and 1.46 ms, respectively. The reduction in PL lifetimes of Mn ions for NCs with higher Co/Pb ratios is perhaps related to formation of Mn-Mn pairs [30,40]. This also indicates that Mn doping concentration is improved by increasing addition of CoCl2. Table 2 gives the actual doping concentrations of Co and Mn ions in Co and Mn codoped CsPbCl3 NCs with a Mn/Pb ratio of 1/1 and various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1, respectively. Therefore, the experimental results indicate that the luminescence properties of CsPbCl3 NCs have been significantly improved by Co ion doping. Fig. 5 shows the XRD patterns of Co and Mn codoped CsPbCl3 NCs.
4. Conclusions In summary, we have studied the structural and luminescent properties of intrinsic CsPbCl3 and Mn codoped CsPbCl3 NCs doped with various Co/Pb molar ratios. The PL QY of excitons in intrinsic CsPbCl3 NCs was enhanced from 6% to 30% by changing Co doping concentration. The diffraction peak shifts toward a large angle in XRD patterns demonstrated the successful doping of Co into CsPbCl3 NCs. The TEM images showed the average NC sizes of 8–10 nm for Co doped CsPbCl3 NCs with increasing Co doping concentration. For Co doping in Mn doped CsPbCl3 NCs, it was surprisingly found that the exciton and Mn PL QYs of Mn doped CsPbCl3 NCs were enhanced from 6% to 14% and from 30% to 50%, respectively, with the increase of Co doping. The similar shift in XRD patterns and size change were also observed in the NCs due to Co and Mn codoping. The PL enhancement of excitons in CsPbCl3 NCs was attributed to the reduction of nonradiative defects due to passivation of NC surface through Co ion incorporation. Therefore,
Table 2 The actual doping concentrations of Co and Mn ions in Co and Mn codoped CsPbCl3 NCs with a Mn/Pb ratio of 1/1 and various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1, measured by ICP-MS. Feed ratio
0/1
0.5/1
1/1
2/1
3/1
Mn Co
1.5% 0
2.0% 2.2%
2.3% 5.1%
2.5% 7.7%
3.1% 9.3%
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Fig. 5. XRD patterns of Co and Mn codoped CsPbCl3 NCs with a Mn/Pb ratio of 1/1 and various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1.
Fig. 6. TEM images of Co and Mn codoped CsPbCl3 NCs with a Mn/Pb ratio of 1/1 and various Co/Pb feed molar ratios of 0/1, 0.5/1, 1/1, 2/1 and 3/1.
the highly luminescent CsPbX3 NCs with doping of metal ions will exhibit important practical application in solid state lighting and displays.
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