White light emission in Bi3+/Mn2+ ion co-doped CsPbCl3 perovskite nanocrystals

He Shao ab, Xue Bai *a, Haining Cui b, Gencai Pan a, Pengtao Jing c, Songnan Qu c, Jinyang Zhu b, Yue Zhai a, Biao Dong a and Hongwei Song *a
aState Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China. E-mail: baix@jlu.edu.cn; songhw@jlu.edu.cn
bCollege of Physics, Jilin University, Changchun 130012, China
cState Key Laboratory of Luminescence and Applications, Changchun Institute of Optics Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road, Changchun 130033, P. R. China

Received 1st November 2017 , Accepted 3rd December 2017

First published on 21st December 2017


Colloidal perovskite nanocrystals (NCs), especially the fully inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) NCs, have been considered as promising candidates for lighting and display applications due to their narrow band emission, tunable band gap and high photoluminescence quantum yields (QYs). However, owing to the anion exchange in the CsPbX3 NCs, stable multi-color and white light emissions are difficult to achieve, thus limiting their practical optoelectronic applications. In this work, dual ion Bi3+/Mn2+ codoped CsPbCl3 perovskite NCs were prepared through the hot injection method for the first time to the best of our knowledge. Through simply adjusting the doping ion concentrations, the codoped NCs exhibited tunable emissions spanning the wide range of correlated color temperature (CCT) from 19[thin space (1/6-em)]000 K to 4250 K under UV excitation. This interesting spectroscopic behaviour benefits from efficient energy transfer from the perovskite NC host to the intrinsic energy levels of Bi3+ or Mn2+ doping ions. Finally, taking advantage of the cooperation between the excitonic transition of the CsPbCl3 perovskite NC host and the intrinsic emissions from Bi3+ and Mn2+ ions, white light emission with the Commission Internationale de l'Eclairage (CIE) color coordinates of (0.33, 0.29) was developed in the codoped CsPbCl3 NCs.


Introduction

Currently, the fully inorganic cesium lead halide perovskite nanocrystals (NCs) have attracted considerable attention due to their excellent properties including narrow band emission, tunable band gap and high photoluminescence quantum yields (QYs).1–9 These merits render them with huge potential to be applied in the fields of solid-state lighting and displays.10–12 However, anion exchange always occurs between distinct halide ions;13–16 therefore it is difficult to achieve multi-color emission and white light emission directly in perovskite NCs.17,18 In addition, red light emission from the CsPbI3 or CsPb(Br/I)3 NCs was extremely unstable under ambient conditions, owing to the lattice phase transmission of CsPbI3 NCs and the phase separation of CsPb(Br/I)3 NCs.19,20 These drawbacks are urgent to be solved to meet the requirement of practical applications.

Doping of hetero-metal ions is a widely applied technique to modulate the physical and chemical properties of nanostructure materials. Extensive studies on metal ion dopants have been devoted to the transitional II–VI MX (M = Cd, Zn, X = S, Se) semiconductor NCs for improving their electrical, magnetic, and optical properties.21–23 Inspired by these achievements, some groups reported on utilizing the doping of metal ions into the perovskite NCs to adjust their optical performances and meanwhile maintain their crystalline structure and morphologies. For instance, Mn2+ ions successfully substituted Pb2+ ions in CsPbCl3 NCs in a recently published work by Yang's group,24 in which the NCs present additional red light emission at ca. 600 nm originated from the intrinsic transition of Mn2+ ions and also exhibit the enhanced photoluminescence QYs from 5% to 54%. Klimov's group reported Mn2+ ion doped CsPbX3 (X = Cl, Br, I) NCs and the band gap of the perovskite NC host was regularly varied.25 Furthermore, the doping effect on the charge transfer of perovskite NCs has also been studied in Bi3+ doped CsPbBr3 NCs through ultrafast spectroscopy investigation.26 In these studies, the obvious changes in optical performances have been observed; however, the emitting peaks were constrained to shift in few specific wavelength regions due to the limited energy levels of these doped ions.7 To the best of our knowledge, white light emission has still not been reported in doped perovskite NC structures.

To construct white light emission, the codoping of metal ions with distinct emitting energy levels would be a promising method that ensures multi-chromatic emitting colors. In this work, we constructed Bi3+ and Mn2+ ion codoped CsPbCl3 NCs by the hot injection approach.27 Through adjusting the doping concentrations of Bi3+ and Mn2+ ions, white light emission has been successfully achieved by the cooperation between the excitonic transition of the CsPbCl3 NC host and the intrinsic emissions of the doped metal ions, and an efficient energy transfer has also been observed between them.28 The results provide a good platform to develop white light emission, which is not only meaningful for white light-emitting device applications, but also provides more broad research space to study metal ion doped perovskite NCs.29–31

Materials

Cs2CO3 (99.9%), BiCl3 (90%), MnCl2 (90%), octadecene (ODE, technical grade, 90%), oleic acid (technical grade, 90%) and oleylamine (OLA, technical grade, 90%) were purchased from Macklin; toluene was purchased from Beijing Chemical Reagent Ltd, China. All the materials were used directly without further purification.

Preparation of CsPbCl3 NCs

Firstly, cesium-oleate was prepared: 0.8 g of Cs2CO3, 2.5 mL of OA, and 20 mL of ODE were loaded in a 100 mL three-neck flask and reacted at 120 °C for 1 h under a N2 atmosphere. Cesium-oleate was heated at 130 °C for the following reactions. Secondly, 0.105 g of PbCl2, 1 mL of OLA, 1 mL of OA and 10 mL of ODE were loaded in a 100 mL three-neck flask and degassed under vacuum at 120 °C for 1 h, and then the temperature was increased to 200 °C. Following that, 0.9 mL of Cs-oleate solution was quickly injected, and the resulting mixture was cooled using an ice-water bath.

Preparation of single Bi3+ ion/single Mn2+ ion doped CsPbCl3 NCs

For Bi3+ ion doped CsPbCl3 NCs, different amounts of BiCl3 (0.03 g, 0.06 g, 0.10 g and 0.15 g) were mixed with 10 mL of ODE, respectively, in a 100 mL three-neck flask and degassed under N2 gas at 160 °C for 20 min. And then 0.105 g of PbCl2, 1 mL of OLA and 1 mL of OA were injected into the mixtures under a N2 atmosphere for 1 h at 120 °C and then the temperature was elevated to 200 °C. Finally, 0.9 mL of hot 130 °C Cs-oleate solution that was mentioned above was quickly injected, and the reaction mixture was cooled down using an ice-water bath.

For the case of Mn2+ ion doped CsPbCl3 NCs, different amounts of MnCl2 (0.1 g, 0.2 g, 0.3 g, 0.47 g, and 0.6 g) were mixed with 10 mL of ODE, respectively. And then the same reaction procedure that was used for preparing Bi3+ ion doped CsPbCl3 NCs was followed.

Preparation of Mn2+/Bi3+ ion codoped CsPbCl3 NCs

0.1 g of BiCl3 was mixed with 10 mL of ODE in a 100 mL three-neck flask and degassed under vacuum at 160 °C for 20 min. And then 0.105 g of PbCl3, 1 ml of OLA and 1 ml of OA, and different amounts of MnCl2 (0.1 g, 0.2 g, 0.3 g, 0.47 g, and 0.6 g) were injected into the mixture under 120 °C for 1 h, respectively. After that the temperature was increased to 200 °C, and 0.9 mL of as-prepared Cs-oleate solution (the first experiment mentioned) was quickly injected, and then the reaction mixture was cooled down using an ice-water bath.

Purification

All the samples were extracted from the crude solution by centrifugation at 9800 rpm for 10 min to remove the foreign substances containing unreacted precursor and byproducts. After that, the precipitates were collected and washed thoroughly with toluene through centrifugation at 8500 rpm for 5 min. Finally, the precipitates were re-dispersed in toluene, forming a stable colloidal solution.

Characterization

Characterization was performed using inductively coupled plasma optical emission spectrometry (ICP-OES) on a Varian 720-ES ICP-optical emission spectrometer. UV-Vis absorption spectra were recorded by using a Shimadzu UV-3101PC UV-Vis scanning spectrophotometer in the range 300–900 nm. Photoluminescence spectra were recorded under ambient conditions by using an FLS365 spectrometer. Transmission electron microscopy (TEM) imaging and X-ray diffraction (XRD) studies were carried out on an X-ray diffractometer (Rigaku) using Cu K radiation (λ = 1.5418 Å). The luminescence decay curves were measured on an FLS920 spectrofluorometer. The color of the emitting light was identified by using the CIE (Commission Internationale de L'Eclairage 1931) calorimeter system. Transient absorption was measured by a pump–probe setup monitoring the bleach recovery at the peak of exciton absorption after band gap excitation.

Results and discussion

The structure, morphology and optical performances of Bi3+ ion doped CsPbCl3 NCs

Crystalline structure and morphology of the as-prepared undoped and Bi3+ ion doped CsPbCl3 NCs were firstly revealed by XRD and TEM measurements. The XRD patterns (Fig. 1a) identified that all the as-prepared NCs exhibited a tetragonal perovskite structure (PDF# 18-0366),32 while with the increase of the Bi3+ ion doping concentration, a slight shift and broadening of the (101) diffraction peak were observed. The crystalline lattice constants for the (100), (101), and (200) diffraction planes were calculated as shown in Table 1, which showed that the lattice constant clearly decreased with the increase of the Bi3+ ion doping concentration. The morphology change of undoped and doped CsPbCl3 NCs was tracked using TEM images. As shown in Fig. 1b–f, the shape of CsPbCl3 NCs underwent no clear variations with the doping of Bi3+ ions, but the NC size decreased gradually with the increase of the doping concentration, which was consistent with the XRD result. The statistical charts of particle size distribution are shown in the insets of the TEM images, and the average sizes were calculated to be 7.0 ± 0.1 nm, 6.9 ± 0.3 nm, 6.7 ± 0.1 nm and 6.6 ± 0.2 nm for the samples with the Bi3+ ion doping concentrations of 2.2%, 5.1%, 8.7%, and 12.0%, respectively, which are smaller than 7.2 ± 0.2 nm of the undoped CsPbCl3 NCs (Fig. 2a). The decease of particle size with the doping concentration can be attributed to the smaller radius of Bi3+ ions (1.17 Å) compared with that of Pb2+ ions (1.33 Å).33 This result readily suggested that the Pb2+ ions were partly substituted by Bi3+ ions during the synthesis process.
image file: c7nr08136g-f1.tif
Fig. 1 (a) XRD patterns of the undoped CsPbCl3 NCs and Bi3+ ion doped NCs with different doping concentrations. (b–f) TEM images for undoped CsPbCl3 NCs and Bi3+ ion doped NCs with different doping concentrations (b, undoped; c, 2.2%; d, 5.1%; e, 8.7% and f, 12.0%).

image file: c7nr08136g-f2.tif
Fig. 2 (a) UV-Vis absorption spectra and (b) emission spectra of undoped CsPbCl3 NCs and Bi3+ ion doped NCs with different doping concentrations in toluene solution. (c, d) Excitation spectra of CsPbCl3 NCs doped with different Bi3+ ion concentrations (2.2%, 5.1%, 8.7%, and 12.0%) monitored at 410 nm (c) and at 463 nm (d), respectively. (e) Time-resolved photoluminescence decays monitored at 410 nm for the undoped CsPbCl3 NCs and Bi3+ ion doped NCs with different doping concentrations. (f) The fs-transient absorptions of undoped and Bi3+ ion (0.6% and 8.7%) doped CsPbCl3 NCs monitored at the respective GSB maxima.
Table 1 The crystalline lattice constant of undoped CsPbCl3 NCs and Bi3+ ion doped NCs with different doping concentrations
  Undoped 2.2% 5.1% 8.7% 12.0%
100 (Å) 5.661 5.608 5.542 5.511 5.493
101 (Å) 3.968 3.966 3.935 3.917 3.902
200 (Å) 2.796 2.799 2.787 2.773 2.772


We further investigated the optical properties of the undoped and Bi3+ ion doped CsPbCl3 NCs. The undoped CsPbCl3 NCs presented an intense absorption band centred at ca. 408 nm that was related to the excitonic absorptions of the perovskite NC host. The Bi3+ ion doped NCs exhibit a clear blue shift of the excitonic absorption position with the increase of the doping concentration because the lattice constant decreased with the doping concentration of NCs34 (see Table 1).

The emission spectra of undoped and Bi3+ ion doped NCs were recorded at the excitation of 365 nm (see Fig. 2b). The emission spectrum of undoped NCs presented an obvious excitonic emission band at ca. 410 nm, which was consistent with the previous reports.27 The excitonic emission band also existed in all the doped samples, while the position shifted to higher energy with the increase of the doping concentration due to the lattice contraction.34 Along with the excitonic component, another broad component spanning from 420 nm to 520 nm was observed in the doped NCs, which originated from the intrinsic transitions (3P11S0) of Bi3+ ions.35 It is necessary to note that the decrease of the overall intensity of emissions may be attributed to the strong perturbation of state density resulting from the high doping degree of Bi3+ ions into the CsPbCl3 NC lattice.26 Intriguingly, the relative intensity between the intrinsic transitions of Bi3+ ions to the excitonic component of the NC host increase obviously with the increase of the Bi3+ ion doping concentration, which indicated that an efficient energy transfer occurred from the CsPbCl3 NC host to the energy level of Bi3+ ions. The energy transfer can be further identified by the excitation spectra (Fig. 2c and d). When monitoring the excitonic emission of 410 nm (Fig. 2c), the excitation spectra for undoped NCs presented a broad band spanning from 275 nm to 375 nm that was associated with the electronic transitions from the valence band to the conduction band of the CsPbCl3 NC host. This excitonic component was also observed in all the doped samples when monitoring the intrinsic transition (463 nm) of Bi3+ ions (Fig. 2d), suggesting that the main excitation path of Bi3+ ions was from the energy transfer of the CsPbCl3 NC host.24

The CsPbCl3 NCs and Bi3+ ion doped NCs all exhibited biexponential photoluminescence decay traces when monitored at 410 nm (see Fig. 2e) and the calculated lifetime values are shown in Table 2. On increasing the Bi3+ ion doping concentration, the average lifetime gradually reduced from 11.0 ns of undoped NCs to 3.2 ns of 12.0% Bi3+ ion doped NCs. The decrease of the time constant of excitonic transition would result from two factors. On the one hand, the energy transfer from excitons of the CsPbCl3 NC host to the Bi3+ ions accelerates the depletion of the excitonic population, thus inducing a decrease in the lifetime of excitonic transitions.28 In addition, more defects would be introduced by the doping of Bi3+ ions that is responsible for the non-radiative recombination of electrons and holes.36 To evaluate these two effects, comparative analysis of the exciton relaxation dynamics in the undoped and Bi3+ ion doped NCs was performed through femtosecond transient absorption (fs)-TA spectroscopy. From Fig. 2f and Table S1, the recovery of the ground state bleach (GSB) for the undoped NCs showed two distinct time components, where the fast time constant (τ1) of 4.32 ± 0.05 ps resulted from the depletion of the population of carriers in the CsPbCl3 host through electron trapping to the defects, and the slow time constant (τ2) of 8.56 ± 0.02 ns was related to the combination of radiative and other slower nonradiative recombination of excitons without having rapid electron trapping.37 Comparing the recovery of GSB in the Bi3+ ion doped NCs with that in undoped NCs, the fast time constant (τ1) associated with electron trapping to the defect state decreased from 4.32 ± 0.05 ps in undoped NCs to 2.15 ± 0.02 ps in the NCs doped with 8.7% Bi3+ ions, which indicated that more defects were induced by the doping of Bi3+ ions. In addition, one new recovery component with a time constant of 663 ± 1 ps associated with the exciton-doping ion energy transfer appears in the doped NCs, readily confirming the occurrence of energy transfer from excitons to the Bi3+ ions.37

Table 2 Lifetime values of undoped CsPbCl3 NCs and Bi3+ ion doped NCs with different doping concentrations (2.2%, 5.1%, 8.7%, and 12.0%) monitored at 410 nm
  Undoped NCs 2.2% 5.1% 8.7% 17.0%
τ 1 (ns) 1.6 1.6 1.5 1.5 1.5
Percent (%) 36.3 44.6 52.5 60.2 72.2
τ 2 (ns) 17.2 16.0 6.4 8.2 6.7
Percent (%) 63.7 55.3 47.4 39.7 27.7
Average 11.0 9.5 5.4 4.5 3.2


It is also interesting to observe that both the intensity of the emission spectrum (Fig. S1) and the recovery time constant (τ1) (Table S1) of GSB increased in the NCs doped with a lower Bi3+ ion concentration (0.6%) compared with those in the undoped and higher Bi3+ ion doping concentration NCs. This result indicated that some defects were modified when a small amount of Bi3+ ions was introduced. As reported in the literature, plenty of defects such as Cl vacancies exist in CsPbCl3 perovskite NCs.37 Upon doping of Bi3+ ions with relatively low concentrations, the Cl vacancies have been partly removed because of the excess Bi chlorides that are added as the precursor during the synthesis procedure.37

Bi3+/Mn2+ codoped CsPbCl3 NCs with white light emission

To realize white light emission in the CsPbCl3 NCs, we further introduced Mn2+ ions into 8.7% Bi3+ ion doped CsPbCl3 NCs. The Mn2+ ion doping concentrations were identified at 0.8%, 1.7%, 2.5%, 3.2%, and 4.6% respectively, by the ICP-OES elemental analyses. First of all, the doping effect of Mn2+ ions on the crystalline structures was investigated through XRD measurement. The crystal structure of the codoped NCs presented a tetragonal perovskite structure (PDF# 18-0366) similar to the undoped and Bi3+ ion doped NCs, while the (101) diffraction peak shifted obviously to the larger diffraction angle, and the lattice constants of the (100), (101), and (200) diffraction planes decreased with the Mn2+ ion doping concentrations (see Table 3). These results suggested that the crystalline size of the codoped NCs was further contracted by the introduction of Mn2+ ions.
Table 3 The crystalline lattice constants of undoped CsPbCl3 NCs, 8.7% Bi3+ ion doped NCs, and codoped NCs with 8.7% Bi3+ ions and different Mn2+ ion doping concentrations: 0.8%, 1.7%, 2.5%, 3.2%, and 4.6%, respectively
Undoped NCs (Å) 8.7% Bi3+ ion doped NCs (Å) Codoped NCs with 8.7% Bi3+ ion doping concentration and different Mn2+ ion doping concentrations (Å)
0.8% 1.7% 2.5% 3.2% 4.6%
100 5.661 5.511 5.533 5.521 5.518 5.502 5.483
101 3.968 3.917 3.923 3.879 3.865 3.845 3.817
200 2.796 2.773 2.782 2.795 2.778 2.772 2.734


The doping effect of Mn2+ ions on the NC morphology was examined using TEM images. All codoped NCs preserve a similar cubic shape to the undoped and Bi3+ ion doped NCs (Fig. 3b–f). The NC size gradually decreased with the Mn2+ doping concentrations due to the smaller radius of Mn2+ ions compared with that of Pb2+ or Bi3+ ions.33 We also calculated the average size of the codoped NCs from distribution statistics to be 6.3 ± 0.2 nm, 6.2 ± 0.3 nm, 6.0 ± 0.1 nm, 5.9 ± 0.4 nm, and 5.8 ± 0.2 nm for the NCs with Mn2+ ion doping concentrations of 0.8%, 1.7%, 2.5%, 3.2%, and 4.6%, respectively.


image file: c7nr08136g-f3.tif
Fig. 3 (a) XRD patterns of codoped CsPbCl3 NCs with 8.7% Bi3+ ions and different Mn2+ ion doping concentrations. (b–f) TEM images of codoped CsPbCl3 NCs with 8.7% Bi3+ ions and different Mn2+ ion doping concentrations: b, 0.8%; c, 1.7%; d, 2.5%; e, 3.2%; and f, 4.6%, respectively.

The optical performances of the codoped NCs were firstly revealed by UV-Vis absorption spectra. Fig. 4a shows the evolution of excitonic absorption for the codoped NCs. A gradual blue shift was observed with the increase of the Mn2+ ion doping concentration owing to the lattice contraction.38 The codoped NCs exhibited interesting emission features as shown in Fig. 4b. Accompanied by the blue emitting component originating from the excitonic transition of the NC host and the green emitting component associated with the intrinsic transition of Bi3+ ions, the codoped samples revealed the presence of an additional intense red emitting band centered at ca. 600 nm, which can be related to the intrinsic transition of Mn2+ ions. Therefore, benefiting from the dual Bi3+ and Mn2+ ion dopants, emission with tunable correlated color temperatures from 19[thin space (1/6-em)]000 K to 4250 K was achieved by adjusting only the ion doping concentrations (Fig. 4c). Under excitation of 365 nm, white light emission with a quantum yield (QY) of 4.2%, Commission Internationale de l'Eclairage (CIE) color coordinates of (0.33, 0.29), was achieved in the codoped NCs (8.7% doping concentration of Bi3+ ions and 2.5% doping concentration of Mn2+ ions).


image file: c7nr08136g-f4.tif
Fig. 4 (a) UV-Vis absorption and (b) emission spectra of Bi3+ ion doped and codoped CsPbCl3 NCs with 8.7% Bi3+ ions and different Mn2+ ion doping concentrations under 365 nm excitation. (c) CIE chromaticity for the codoped NCs with 8.7% Bi3+ ions and different Mn2+ ion doping concentrations under 365 nm excitation.

From Fig. 4b, it is found that both the relative intensity of intrinsic transition of Mn2+ ions to the excitonic emission and to the intrinsic transition of Bi3+ ions increased obviously with the Mn2+ ion doping concentration, indicating that the energy transfer occurred from the CsPbCl3 NC host and Bi3+ ions to the Mn2+ ions.28 This differed from Mn2+ doped II–VI NCs,39,40 which possessed a very weak transition dipole relying on the exchange coupling between the donor and acceptor.28 Furthermore, it was also interesting to find that both the excitonic emission and the intrinsic transition of Bi3+ ions increased in the codoped NCs when the Mn2+ ion doping concentration increased from 0.8 to 3.2%, because the Cl vacancy was further modified by the introduction of MnCl2 during the synthesis procedure as discussed above. With the further increase in the Mn2+ ion doping concentration to 4.6%, both the excitonic emission and the intrinsic transition of Bi3+ ions decreased due to energy transfer from the NC host and Bi3+ ions to Mn2+ ions evolved predominantly.

Energy transfer from excitons to Mn2+ ions can be proved by the excitation spectra (Fig. 5a and b). Comparing the excitation spectra monitored at the excitonic emitting position (410 nm) with that monitored at the Mn2+ ion intrinsic emitting position (600 nm), the excitation spectra showed almost identical excitation components and only presented variations in the intensity. This result confirmed that the emission of Mn2+ ions resulted from the energy transfer of the CsPbCl3 NC host. Moreover, photoluminescence as well as the time-resolved spectra for the single Mn2+ ion doped NCs can also identify the energy transfer from the CsPbCl3 NC host to Mn2+ ions (Fig. S2 and S3). The relative intensity of the intrinsic transition of Mn2+ ions to excitonic transition of the NC host increased obviously (Fig. S2) and the average lifetime values of the excitonic transition decreased dramatically when the doping concentration increased from 2.5 to 3.2% as shown in Fig. S3 and Table S2 (the initial increase in the time constant at the lower doping concentration from 0.8 to 1.7% resulted from the modification of Cl vacancies). These results confirmed again the occurrence of energy transfer from the CsPbCl3 NC host to Mn2+ ions.


image file: c7nr08136g-f5.tif
Fig. 5 Excitation spectra of Bi3+ ion doped CsPbCl3 NCs and codoped NCs with 8.7% Bi3+ ions and different Mn2+ ion doping concentrations monitored at (a) 410 nm and (b) 600 nm. Emission decays of Bi3+ ion doped CsPbCl3 NCs and codoped NCs monitored at (c) 410 nm and (d) 463 nm.

Time-resolved spectra provided the arguments for the modification of defects through the doping of Mn2+ ions (Fig. 5c and d). The lifetime distribution statistics monitored at the excitonic emission of the NC host and intrinsic transition of Bi3+ ions are shown in Tables 4 and 5. The emission decays monitored at the excitonic emission of the NC host (410 nm) prolonged from 4.5 ns of Bi3+ ion doped NCs to 5.5 ns of codoped NCs (8.7% Bi3+ ion and 0.8% Mn2+ ion doping concentration) and then decrease to 4.7 ns with further increase in the Mn2+ ion doping concentrations (Fig. 5c). This suggested that the defect states were removed by adding Mn2+ ions initially, and subsequently the efficient energy transfer occurred at the higher Mn2+ ion doping concentrations, leading to a decrease in the lifetime. Moreover, the decrease of the average lifetime of Bi3+ ions from 24.4 ns of NCs doped with 3.2% Mn2+ ions to 20.5 ns of NCs doped with 4.6% Mn2+ ions confirmed that the energy transfer occurred from Bi3+ to Mn2+ ions at the higher Mn2+ ion doping concentrations. Finally, according to the aforementioned results, the possible photoluminescence mechanism for the Bi3+/Mn2+ ion codoped CsPbCl3 perovskite NCs is shown in Fig. S4.

Table 4 Lifetime values of codoped CsPbCl3 NCs with 8.7% Bi3+ ions and different Mn2+ ion doping concentrations (0.8%, 1.7%, 2.5%, 3.2%, and 4.6%) monitored at 410 nm
  8.7% Bi3+ ion doped NCs Codoped NCs with 8.7% Bi3+ ions and different Mn2+ ion doping concentrations
0.8% 1.7% 2.5% 3.2% 4.6%
τ 1 (ns) 1.5 1.8 1.7 1.9 1.9 1.2
Percent (%) 60.2 51.6 49.9 51.0 54.4 60.2
τ 2 (ns) 8.2 8.9 8.3 7.8 8.0 49.3
Percent (%) 39.7 48.3 50.0 48.9 45.5 39.8
Average (ns) 4.5 5.5 5.0 4.8 4.7 3.6


Table 5 Lifetime values of codoped CsPbCl3 NCs with 8.7% Bi3+ ion doping concentration and different Mn2+ ion doping concentrations (0.8%, 1.7%, 2.5%, 3.2% and 4.6%) monitored at 463 nm
  8.7% Bi3+ ion doped NCs Codoped NCs with 8.7% Bi3+ ions and different Mn2+ ion doping concentrations
0.8% 1.7% 2.5% 3.2% 4.6%
τ 1 (ns) 4.4 5.2 5.4 6.2 6.7 5.8
Percent (%) 53.4 49.2 46.3 43.8 41.5 44.4
τ 2 (ns) 26.4 28.7 30.6 34.5 37.4 32.3
Percent (%) 46.4 50.8 53.7 56.2 58.5 55.6
Average (ns) 14.5 16.9 19.0 22.0 24.4 20.5


Conclusions

In summary, Bi3+/Mn2+ ion codoped CsPbCl3 perovskite NCs were successfully synthesized by a hot injection approach. The structure and morphology of the as-prepared NCs were investigated by XRD and TEM measurements, and the doping concentrations for the Bi3+ and Mn2+ ions were carefully controlled and measured. When the Bi3+ and Mn2+ ion doping concentrations were set at 8.7% and 2.5%, respectively, white light emission was achieved. Furthermore, the correlated color temperature of emission was facilely changed from 19[thin space (1/6-em)]000 K to 2750 K by adjusting only the ion doping concentrations. To our knowledge, single-component white light emission was achieved for the first time in perovskite NCs, revealing promising potential in lighting and displays.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 11674127, 11674126, and 21403084), the Jilin Province Science Fund for Excellent Young Scholars (20170520129JH and 20170101170JC), the Major State Basic Research Development Program of China (973 Program) (no. 2014CB643506 and 2014CB921302), and the National Key Research and Development Program (2016YFC0207101).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nr08136g

This journal is © The Royal Society of Chemistry 2018