Gram-scale and solvent-free synthesis of Mn-doped lead halide perovskite nanocrystals

Article abstract of DOI:10.1016/j.jallcom.2019.152393

Although Mn-doped lead halide perovskites could alleviate the environmental burden of Pb, their synthesis process suffers from intricate operations, high operating temperature, low production, or toxic strong polar solvents. Developing a simple and green route for Mn-doped halide perovskites with mass production is highly desirable for perovskite-derived applications. In this work, we developed a facile and environmental-benign synthesis strategy for large-scale preparation of Mn-doped lead halide perovskite nanocrystals. The nanocrystals were prepared via a mechano-synthesis process without organic solvents. We investigated the influence of atomic composition on their morphology, crystal structure and optical performance. Our results show that the perovskite nanocrystals have dual-emissions from the ⁴T₁ to ⁶A₁ transition of Mn²⁺ and the excitonic recombination of perovskite hosts, respectively. The achieved Mn:CsPbX₃ nanocrystals show well-tailored color rendering when used as LED phosphors. This work presents a scalable and facile strategy for Mn-doped perovskite production without organic solvents, which promises low-cost and eco-friendly LEDs.

Full text of DOI:10.1016/j.jallcom.2019.152393

Journal of Alloys and Compounds 815 (2020) 152393
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Correspondence
Gram-scale and solvent-free synthesis of Mn-doped lead halide
perovskite nanocrystals
a b s t r a c t
Keywords:
Perovskite
Mechano-synthesis
Massive production
Color rendering
Although Mn-doped lead halide perovskites could alleviate the environmental burden of Pb, their syn-
thesis process suffers from intricate operations, high operating temperature, low production, or toxic
strong polar solvents. Developing a simple and green route for Mn-doped halide perovskites with mass
production is highly desirable for perovskite-derived applications. In this work, we developed a facile
and environmental-benign synthesis strategy for large-scale preparation of Mn-doped lead halide
perovskite nanocrystals. The nanocrystals were prepared via a mechano-synthesis process without
organic solvents. We investigated the influence of atomic composition on their morphology, crystal
structure and optical performance. Our results show that the perovskite nanocrystals have dual-
emissions from the 4T1 to 6A1 transition of Mn2þ and the excitonic recombination of perovskite hosts,
respectively. The achieved Mn:CsPbX3 nanocrystals show well-tailored color rendering when used as
LED phosphors. This work presents a scalable and facile strategy for Mn-doped perovskite production
without organic solvents, which promises low-cost and eco-friendly LEDs.
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
Generally, the preparation methods of Mn2þ-doped lead halide
perovskite NCs could be roughly divided into three categories: hot
All-inorganic lead halide perovskites (LHPs) are recognized as
promising materials for next-generation illumination and displays
due to their excellent optical performance [1e11]. Benefiting from
LHPs, significant progress has been achieved in solar cells
[12e16], lasers [17e20], photodetectors [21e26] and light-
emitting diodes [27e34]. LHPs have long charge carrier diffusion
length, tunable bandgap and high light-absorption coefficient
[23,35e41], especially in nanocrystal (NC) states. Perovskite NCs
exhibit high brightness, well-tailored emission, and narrowed
line-widths [42]. The phosphor-converted light emitting diodes
(LEDs) based on perovskite NCs render a color gamut covering
over 140% of the National Television System Committee (NTSC)
standard, which is superior to commercial organic LEDs and quan-
tum dot (QD) LEDs [43]. However, perovskite-based lighting de-
vices encounter two major obstacles, i.e., the notorious Pb
component, and the unstable red fluorescence [29,44,45]. There-
fore, a rational strategy has been developed to alleviate their envi-
ronmental hazards by substitution of Pb with non-toxic or less toxic
multivalent cations [32,46e48]. Similar to the well-studied II-VI
injection, ligand-assisted re-precipitation (LARP) and cation ex-
change. For the hot injection strategy, MnX2 and PbX2 are foremost
dissolved in a mixture of solvent and surfactants in vacuum at
120 ꢀC. Subsequently, the pre-heated cesium-oleate precursor was
rapidly injected into the PbX2 solution at specific temperature
and the reaction solution is kept at high temperature to maintain
NC growth [46,49]. Whereas, the hot injection process is complex
and the injection temperature, reaction time and precursor concen-
tration need to be precisely controlled. In comparison, the LARP
method seems quite simple and versatile by transferring inorganic
ions from rich solvent into poor one [45]. Although, LARP was a
one-step synthetic strategy without high temperature and inert at-
mosphere requirements, this process involves large amount of high
polar solvents and toxic solvents. It needs complex sewage dis-
posals, which may induce serious environmental issues. Cation ex-
change, as another one-step synthesis strategy, avoids intricate
operations, high reaction temperature and high polar solvents
[51]. However, it is still challenging to realize gram-scale produc-
tion. Therefore, developing a simple and green route for mass pro-
duction of luminescent Mn2þ-doped perovskite NCs will pave a
pathway to the perovskite phosphors based LEDs.
semiconductor QDs, transition metal ions such as Mn2þ, Co2þ
,
Cu2þ and Ag2þ were frequently incorporated into perovskite NCs
to tune their optical, electronic and magnetic properties. It was re-
ported that the addition of Mn2þ are capable of introducing a new
emission channel in red regions, due to an internal 4T1 to 6A1 tran-
sition of Mn2þ, and the replacement of Pb2þ with Mn2þ could
leverage the potential environmental burden caused by Pb, in favor
of the practical application of perovskite-based LEDs [32,46,49,50].
In this work, we developed a simple, environmental-benign and
mass productive strategy to prepare Mn2þ-doped lead halide
perovskite NCs. Up to 50% of the Pb composition have been
substituted by the incorporation of Mn ions. Perovskite NCs with
various Mn substitution ratio and halide compositions have been
synthesized and investigated. The perovskite NCs exhibit dual-
https://doi.org/10.1016/j.jallcom.2019.152393
0925-8388/© 2019 Elsevier B.V. All rights reserved.

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Correspondence / Journal of Alloys and Compounds 815 (2020) 152393
emissions from Mn2þ-related radiative relaxation and excitonic
recombination of perovskite hosts. The performance of the
Mn:CsPbX3 NCs as LED phosphors with tailored color rendering
have been demonstrated on commercial UV LED chips. This paper
presents a universal strategy for large-scale and solvent-free pro-
duction of Mn2þ-doped perovskite NCs for potential applications
in LEDs.
with a high energy ball milling. The as-synthesized Mn-doped
CsPbX3 NCs exhibit bright multi-color fluorescence.
2.3. Fabrication of LED devices
The as-prepared perovskite NCs were thoroughly mixed with
PDMS precursor at a weight ratio of 6:10. The mixture of perovskite
NCs and PDMS was de-bubbled and casted onto a 395 nm UV-chip,
and then cured in an oven at 70 ꢀC for 1 hour.
2. Experimental
2.1. Materials
2.4. Measurements and characterizations
All chemicals were used as purchased without any further puri-
fication. The regents are cesium chloride (CsCl; >99.9%, Aladdin),
cesium bromide (CsBr; >99.9%, Aladdin), cesium iodide (CsI;
>99.9%, Xi’an Polymer Light Technology Corp.), lead chloride
(PbCl2; >99.99%, Aladdin), lead bromide (PbBr2; >98%, Aladdin),
lead iodide (PbI2; >99.99%, Aladdin), manganese chloride (MnCl2;
>99.0%, Macklin), octadecylamine (ODA; >97.0%, Aladdin), manga-
nese carbonate (MnCO3; >99.0%, Macklin), hydrobromic acid (HBr;
48% by weight in H2O). The silicone elastomer base and curing
agent of PDMS (SYLGARDTM 184) were purchased from Dow
Chemical Company.
Manganese bromide (MnBr2) was synthesized in the lab via the
following process: certain amount of MnCO3 was slowly added into
excess aqueous HBr solution at 120 ꢀC under continuously stirring
until the powder completely dissolved. The MnBr2 solution was
then transferred into a rotatory evaporator to remove excessive sol-
vent. The resultant powders were grounded in an agate mortar and
dried in vacuum for 24 h.
X-ray diffraction (XRD) patterns were collected by an X’Pert Pro
diffractometer (Malvern Panalytical) using Cu
Ka radiation
(l
¼ 1.5406 Å). The step size and the integration time are set as
0.01ꢀ and 0.6 sec/step, respectively. Transmission electron micro-
scopy (TEM) and field-emission scanning electron microscopy
(FESEM) images were captured on JEM-2010 and JSM6700F respec-
tively. Energy dispersive X-ray (EDX) analysis was also conducted
on a JSM6700F scanning electron microscopy. Fourier transform
infrared (FTIR) spectra were obtained on a Thermo Scientific Nicolet
iS10 FTIR spectrometer over the range 4000e400 cmꢁ1. Photolumi-
nescence (PL) spectra were measured by Hitachi F-7000 spectro-
photometer and ultraviolet visible absorption (UVeVis) spectra
were characterized by UH4150 spectrophotometer. Photolumines-
cence quantum yield (PLQY) was measured by Edinburgh fluores-
cence spectrometer FS 5.
3. Results and discussion
The schematic solvent-free mechano-synthesis process for
large-scale production of Mn-doped lead halide perovskite NCs is
illustrated in Fig. 1. After grinding for a few minutes, the color of
the initial raw materials turns yellow, indicating the formation of
luminescent perovskite NCs during the mechano-synthesis process.
Small amount of ODA was introduced as surfactant to facilitate the
formation of highly luminescent perovskite NCs. The resultant
perovskite NCs exhibit bright green fluorescence much stronger
than the perovskites without ODA, as previously reported in other
ligands [52]. Meanwhile, MnX2 was added to partly replace the pre-
cursor of Pb2þ. The fluorescence of the perovskite NCs changes to
orange color gradually as the doping concentration increases. It
should be noted that the sample displays dual emission bands:
the peak centered at about 480 nm belongs to the band-edge of
the perovskite host, and the emission at around 610 nm arises
from Mn2þ d-d transition [32,46].
2.2. Mechano-synthesis of CsPbxMn1-xCl1.5Br1.5 NCs
Typically, equivalent molar amount of CsCl (0.4 mmol, 0.067 g),
PbCl2 (0.4 mmol, 0.111 g), CsBr (0.4 mmol, 0.085 g), and PbBr2 (0.4
mmol, 0.148 g) were mixed and grounded in an agate mortar.
Within a few minutes, the color of the mixture changed from white
to yellow, and a faint blue fluorescence could be observed under UV
light irradiation (365 nm). The grounding operation lasted for 1 h
and then 0.02 g ODA was added to the agate mortar. A few minutes
later, bright blue fluorescence appears from the resultant powders.
Mn-doping was realized by adopting appropriate amount of MnCl2
instead of PbBr2. In addition, multi-halide Mn-doped CsPbX3
(X ¼ Cl, Br, and I) NCs were prepared via the similar mechano-
synthesis process with corresponding precursor compositions. In
this work, well-cleaned agate mortar and pestle were adopted
while no grinding media was involved, so as to avoid the contami-
nation of the resultant NCs.
Perovskite NCs with nominal molar ratio of Mn/Pb at 12.5%, 25.0%,
37.5%, and 50.0% were synthesized to investigate the influence of
dopant on the microscopic morphology, structure, and fluorescence
of the resultant NCs. Fig. 2 presents the TEM images of the
mechano-synthesized perovskite NCs with different doping
For large-scale synthesis of the CsPbX3 (X ¼ Cl, Br, and I) NCs
phosphors, the amount of raw materials were multiplied propor-
tionally, and the mechano-synthesis procedure was performed
Fig. 1. The schematic mecano-synthesis process of Mn:CsPbMnX3 NCs.

Correspondence / Journal of Alloys and Compounds 815 (2020) 152393
3
Fig. 2. TEM images of the CsPb1-xMnxCl1.5Br1.5 NCs prepared with different Pb/Mn nominal molar ratios. a) x: 12.5%, b) x: 25.0%, c) x: 37.5%, d) x: 50.0%. Insets: digital images of the
corresponding samples in ambient light and under UV light, and the histogram of the size distribution.
Fig. 3. a) XRD patterns of CsPbCl1.5Br1.5 NCs with different Mn2þ doping concentration. b) Enlarged features of (a) with continuous diffraction peak shift. c) Absorption and d) PL
spectra of Mn: CsPbCl1.5Br1.5 NCs of varied dopant concentration.

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Correspondence / Journal of Alloys and Compounds 815 (2020) 152393
concentration. All samples are cubic shaped, whose size increases
slightly with the increase of Mn2þ concentration. Large area observa-
tion of the NCs size distribution is also presented in the inset histo-
gram. Energy dispersive X-ray (EDX) analysis was carried out to
identify the composition of the perovskite NCs, and the as-
compared with that of the undoped one, which is consistent with
the lattice contraction caused by the substitution of larger sized
Pb2þ by smaller sized Mn2þ [46]. Fig. 3c and d presents the absorp-
tion and PL spectra of Mn:CsPbCl1.5Br1.5 NCs with different dopant
concentration. Both spectra experienced an obvious batho-shift
with the increase of Mn-concentration, while the corresponding
PL peaks could be tuned in the range of 451e496 nm (Fig. 3d).
The continuous batho-shift in PL spectra is assumed to result
from the increase in the perovskite NCs size.
We also prepared perovskite NCs with various halide composi-
tions via this mechano-synthesized process. Fig. 4 shows the char-
acterizations of a series of representative perovskite NCs with
tunable halide composition and emission peaks covering a broad
range. It is worth noting that several interesting trends could be
prepared NCs has
CsPb0.78Mn0.22Cl1.5Br1.5
CsPb0.51Mn0.49Cl1.5Br1.5, respectively.
a
,
composition of CsPb0.90Mn0.10Cl1.5Br1.5
,
CsPb0.58Mn0.42Cl1.5Br1.5
,
Fig. 3a and b shows the XRD patterns of CsPbCl1.5Br1.5 NCs with
different Mn2þ doping concentrations. The NCs are highly crystal-
lized, and all the diffraction peaks can be assigned to the ortho-
rhombic perovskite (ICSD No. 97851). As shown in the enlarged
features in Fig. 3b, with the increase of dopant concentration, the
diffraction peaks slightly shift to higher angle by about 0.02ꢀ
Fig. 4. a) Absorption and b) PL spectra of a series of mechano-synthesized CsPb0.75Mn0.25X3 NCs. c) the energy level diagram of Mn: CsPbX3 NCs, and △ is the energy difference
between the band edge and Mn-related transitions. d) Photographs of the CsPb0.75Mn0.25X3 NCs under indoor light and UV illumination.

Correspondence / Journal of Alloys and Compounds 815 (2020) 152393
5
Fig. 5. a) XRD patterns and b) PL spectra of gram-scale synthesized CsPb0.75Mn0.25Cl1.5Br1.5 NCs. EL spectra and CIE chromaticity coordinate of the UV-pumped multi-color LED
packaged with gram-scale synthesized perovskite NCs of c) CsPb0.75Mn0.25Cl3, d) CsPb0.75Mn0.25Cl2Br1, e) CsPb0.75Mn0.25Cl1.5Br1.5, f) CsPb0.75Mn0.25Br3, g) CsPb0.75Mn0.25Br2I1, and h)
CsPb0.75Mn0.25Br1.5I1.5. The inset shows the corresponding photographs. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version
of this article.)

6
Correspondence / Journal of Alloys and Compounds 815 (2020) 152393
extracted from these data. CsPb0.75Mn0.25Br3 will be token as the
boundary in the following discussion to differentiate samples
with substitution of Brꢁ by Clꢁ and Iꢁ. The first excitonic absorption
peak of the perovskite NCs exhibits hypso-shift when the Cl con-
centration increases, and batho-shift when Iꢁ increases. The short-
est excitonic peak at 401 nm is observed for CsPb0.75Mn0.25Cl3,
whereas the longest absorbance edge wavelength at 516 nm is real-
exhibit a strong Mn-related emission, which is attributed to the en-
ergy transfer process from the perovskite host to the Mn2þ dopants,
resulting in multi-colored emission. The as-prepared Mn-doped
NCs are further employed as the phosphors on commercially avail-
able 395 nm GaN LED chip for fabricating multi-color LED. The re-
sults present a universal strategy for large-scale and solvent-free
production of Mn2þ-doped perovskite NCs as the phosphors, which
promises lead-free perovskite NCs as low-cost and eco-friendly
phosphors.
ized by CsPb0.75Mn0.25Br1.5 1.5 (Fig. 4a). Similar trends can also be
I
found in their PL spectra. The PL peak of host perovskite shifts
from 404 nm to 617 nm when Cl concentration decreases and I con-
centration increases. While the wavelength center of Mn-related
emission retains nearly constant, as shown in Fig. 4b. And the emis-
sion of host perovskite vanished when Iꢁ is incorporated in the
perovskite. This phenomenon might stem from the energy transfer
between the Mn2þ and the perovskite host [49]. For NCs with the
concentration of Mn:CsPbCl3, the conductive band of perovskite
host is higher than the 4T1 state of Mn2þ dopant, energy transfer
from the host to Mn2þ is allowed. Therefore, as shown in Fig. 4c,
the perovskite host absorbs incident light and forms exciton, which
then excites Mn2þ d-electron to 4T1 state via an energy transfer pro-
cess, followed by 4T1 to 6A1 de-excitation process. Dual-color light
emission will then be realized by the excitonic recombination of
perovskite and Mn2þ-mediated radiative relaxation. The band-
edge PL peak monotonically shifts to longer wavelength with the
substitution of Clꢁ by Brꢁ and Iꢁ until the band-edge energy even-
tually becomes smaller than that of the Mn2þ transitions. Beyond
that point, there is no longer any trance of Mn2þ-mediated emis-
sion [47]. Fig. 4d shows the corresponding photographs of the
multi-color emission Mn:CsPbX3 NCs.
Notably, this method is quite facile to realize massive production
of perovskite NCs with tunable emission wavelength by simply
enlarging the precursor dosage proportionally and utilizing high
energy ball milling. A batch production of 6.45 g has been achieved
for CsPb0.75Mn0.25Cl1.5Br1.5 phosphors via this method. Fig. 5a
shows the XRD patterns of the resultant NCs with gram-scale pro-
duction in comparison with that of the experimental-synthesized
NCs. All the patterns exhibit similar full width at half maximum
(FWHM), and can be assigned to orthorhombic perovskite (ICSD
No. 97851), indicating that the gram-scale synthesized perovskite
NCs are of identical structure and in high crystallinity. As shown
in Fig. 5b, the PL spectra and emission intensity of the gram-scale
synthesized samples are nearly the same as that of the
experimental-scale synthesized NCs. Therefore, the gram-scale sol-
vent-free synthesis process can be employed for massive produc-
tion of promising perovskite phosphors for LEDs. The mechano-
Acknowledgements
This work is supported by the National Natural Science Founda-
tion of China (11674290, U1704138 and 11974317), and the Zhengz-
hou University Physics Discipline Improvement Program.
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Henan Key Laboratory of Diamond Optoelectronic Materials &
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School of Materials Science & Engineering, Zhengzhou University,
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** Corresponding author. Henan Key Laboratory of Diamond
Optoelectronic Materials & Devices, School of Physics &
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* Corresponding author.
E-mail address: liukaikai@ciomp.ac.cn (K. Liu).
E-mail address: ldong@zzu.edu.cn (L. Dong).
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15 May 2019
5 September 2019
21 September 2019
Available online 25 September 2019