Employing Polar Solvent Controlled Ionization in Precursors for Synthesis of High-Quality Inorganic Perovskite Nanocrystals at Room Temperature

Article abstract of DOI:10.1002/adfm.201706000

All inorganic cesium lead halide (CsPbX₃, X = Cl, Br, I) perovskite nanocrystals (PeNCs) are synthesized by employing polar solvent controlled ionization (PCI) method in precursors. The new strategy can be easily carried out at room temperature and allow to employ smaller amount of weaker polarity and a broader range of low-boiling low-toxic solvents. The as prepared CsPbX₃ PeNCs reveal tunable emission spectra from 380 to 700 nm and high quantum yields over 80% with narrow full width at half maximum (FWHM). Meanwhile, larger “effective Stokes shifts” of PeNCs in PCI method, which enlarges 200% more than other PeNCs in regular methods, are observed. Most interestingly, the PeNCs growth process is coupling with some typical crystals formations. The main morphologies of CsPbI₃ PeNCs are hybrid of nanorods and nanoparticles. The primary morphologies of CsPbBr_xI_{3- x} and CsPbBr₃ PeNCs are nanowires, which are supposed to have great potentials for applying in laser arrays and highly sensitive photodetector applications. Furthermore, such superior optical is endowed to fabricate white light emitting diodes, which has wide color gamut covering up to 120% of the National Television Systems Committee color standard.

Full text of DOI:10.1002/adfm.201706000

FULL PAPER
Perovskite
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Employing Polar Solvent Controlled Ionization in
Precursors for Synthesis of High-Quality Inorganic
Perovskite Nanocrystals at Room Temperature
Fan Fang, Wei Chen, Yang Li, Haochen Liu, Ming Mei, Ruan Zhang, Junjie Hao,
Marus Mikita, Wanqiang Cao, Ruikun Pan, Kai Wang,* and Xiao Wei Sun
their spectra, but also offer tunable emis-
All inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals
(PeNCs) are synthesized by employing polar solvent controlled ionization
(PCI) method in precursors. The new strategy can be easily carried out at
room temperature and allow to employ smaller amount of weaker polarity
and a broader range of low-boiling low-toxic solvents. The as prepared
CsPbX3 PeNCs reveal tunable emission spectra from 380 to 700 nm and high
quantum yields over 80% with narrow full width at half maximum (FWHM).
Meanwhile, larger “effective Stokes shifts” of PeNCs in PCI method, which
enlarges 200% more than other PeNCs in regular methods, are observed.
Most interestingly, the PeNCs growth process is coupling with some typical
crystals formations. The main morphologies of CsPbI3 PeNCs are hybrid of
nanorods and nanoparticles. The primary morphologies of CsPbBrxI3-x and
CsPbBr3 PeNCs are nanowires, which are supposed to have great potentials
for applying in laser arrays and highly sensitive photodetector applications.
Furthermore, such superior optical is endowed to fabricate white light emit-
ting diodes, which has wide color gamut covering up to 120% of the National
Television Systems Committee color standard.
sion wavelength covering entire visible
light range by controlling the composite
ratio, which are supposed to be the two
important parameters for wide color
gamut display technique.[8]
The organic PeNCs were emerged
recently and possessed excellent photo-
electric performance.[6–14] Unfortunately,
organic perovskite nanocrystals are very
easily destructed. Many researchers tried
to improve their stability, such as Yuan
et al. and Li and co-workers have greatly
improved the stability of organic perovs-
kites by changing ligands and coating
inorganic silica.[15,16] One effective strategy
is introduction of all inorganic PeNCs
(e.g., CsPbX3, X = Cl, Br, or I). The replace-
ment of organic cation with all inorganic
counterparts could facilitate the perovskite
stability. Furthermore, inorganic PeNCs
possess more excellent performance than
organic PeNCs such as the photolumines-
cence (PL) has narrow emission line-width of 12−42 nm, high
quantum yields (QYs) of up to 95%, and radiative lifetimes in
the range of 1−29 ns.[17]
1. Introduction
Perovskite nanocrystals (PeNCs) have attracted intensive inves-
tigations in recent years due to their outstanding optoelectronic
and spectral properties and the great potentials for their
applying in photovoltaics, photodiodes, lasers, light-emitting
diodes (LEDs), and optical down-conversion devices.[1–7] Espe-
cially, for the applications in lighting and wide color gamut
displays, PeNCs could not only provide narrow emission from
Currently, there are two main methods for synthesis of
inorganic PeNCs: Hot injection (HI) method which has been
developed by Kovalenko and co-workers[17] and supersaturated
recrystallization (SR) method which has been invented by Zeng
et al.[18,19] HI method had been proved to be an efficient strategy
to synthesize inorganic PeNCs with both excellent photoelectric
performance and stability in previous work. However, there
are two factors during the synthesis process, high temperature
and inert atmosphere, which are required in HI method and
inevitably increased the cost and probably limited the output in
mass-production. To well overcome these two factors, the SR
method was developed to make it possible for obtaining high
quality inorganic PeNCs in regular atmosphere and at room
temperature. The method was progressive, however, the inor-
ganic PeNCs were observed to be very sensitive to polar sol-
vents which is summarized as the stronger the polarity of the
ambient solvent system is, the weaker stability the prepared
inorganic PeNCs against ambient will be. The normally used
polar solvent, dimethyl formamide (DMF), would therefore
bring inevitable damages to inorganic PeNCs, especially for
F. Fang, W. Chen, H. Liu, R. Zhang, J. Hao, M. Mikita, Prof. K. Wang,
Prof. X. W. Sun
Department of Electrical and Electronic Engineering
Southern University of Science and Technology
1088 Xueyuan Blvd, Xili, Nanshan, Shenzhen 518055, China
E-mail: wangk@sustc.edu.cn
F. Fang, Y. Li, M. Mei, Prof. W. Cao, Prof. R. Pan
School of Materials Science and Engineering
Hubei University
368 Youyi Road, Wuchang, Wuhan 430062, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201706000.
DOI: 10.1002/adfm.201706000
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CsPbI3 NCs.[20] Furthermore, inevitable DMF (or DMSO) has
a higher boiling point and is toxic, which are very disadvanta-
geous to large-scale production.
(T) remains constant, the activation energy (Ea) of the reaction
should be reduced or the concentration of reaction substance (C)
should be increased. Methods for increasing the rate of reaction
(r) can be used to design different synthetic strategy of CsPbX3
nanocrystals, one is HI method, and another is the PCI method.
The precursors of PeNCs, such as cesium oleate, lead oleate
and halogenated amine (specific molecular formulas are shown
in the Supporting Information), which are synthesized by inor-
ganic salts, organic carboxylic acid, and organic amine. These
methods for synthesis have been reported previously.[21–23] We
can here write the precursors formation reaction as below
Herein, we report a new strategy for synthesis of inorganic
PeNCs at room temperature via polar solvent controlled ioniza-
tion (PCI) of precursors. Novel mechanism is designed so that
synthesis of CsPbX3 PeNCs allows to employ smaller amount of
polar solvents, which has weaker polarity and broader range of
low-boiling low-toxic solvents. Due to the employment of weaker
polarity solvent, the CsPbX3 PeNCs via this strategy possess
better stability against ambient than those obtained from SR
method. Moreover, the as prepared CsPbX3 PeNCs also revealed
excellent optical performance that the PL QYs reached over
80%, and full width at half maximum (FWHM) of the emission
spectra remained at 35, 24, and 18 nm for red, green and blue
emissions, respectively. Most interestingly, we had also observed
some specified characteristics from the spectra and the mor-
phology for these PeNCs via the PCI method as following. First,
we obtained PeNCs that were observed to reveal larger “effec-
tive Stokes shifts,” as 42, 46 and 31 nm, for red, green and blue
emissions, respectively, which are enlarged 200% more than
those of traditional PeNCs. Second, PCI method-based PeNCs
revealed typical crystal formations from the transmission elec-
tronic microscope (TEM) images. The main morphologies of
CsPbI3 PeNCs are hybrid of nanorods and nanoparticles. The
primary morphologies of CsPbBrxI3−x and CsPbBr3 PeNCs are
nanowires. This new strategy thus provides a new approach to
the synthesis of perovskite nanowires, which probably be used
for laser arrays and highly sensitive photodetector devices.[4,5]
Finally, we also applied our obtained inorganic PeNCs in LEDs
for wide color gamut display backlight and realized 120% of
NTSC (National Television Systems Committee) standard.
Cs2CO3 + 2ROOH = 2CsOOR + H2O ↑ + CO2 ↑
PbX +2ROOH+2RNH = Pb OOR +2RNH3X
(3)
(4)
(
)
2
2
2
Equations of ionization equilibrium of precursors are also
given as following
CsOOR = Cs+ + ROO−
(5)
Pb OOR = Pb2+ + 2ROO−
(6)
(
)
2
RNH3X = RNH3+ + X−
(7)
There is an ionization balance between free and combined
states in this synthesis system, described with Equations (5),
(6), and (7). It is well known that the ionization equilibrium
(5), (6), and (7) will shift to the right direction by increasing
system temperature, and the concentration of Pb2+, X− and Cs+
increases. At the same time, the reaction rate constant (k) will
increase and the reaction rate (r) will also be accelerated. This
mechanism has been applied in HI method. The formation of
CsPbX3 PeNCs by ions of precursors was following the prin-
ciple. The reaction principle could be written as below
2. Results and Discussion
Cs+ +Pb2+ +3X− = CsPbX3
(8)
2.1. Mechanism for Synthesis of Inorganic PeNCs by High
Temperature Hot Injection
The crystallization of CsPbX3 PeNCs will consume large
amounts of Cs+, Pb2+ and X−. According to Le Chatelier’s prin-
ciple, the ionization equilibrium (5), (6), and (7) will shift again
to the right direction. When the precursors cannot continue to
ionize and release Pb2+, X−, and Cs+, the Equation (8) would
turn into a new balance.
Before describing the reaction process, it is necessary to intro-
duce two main equations. First one is the Arrhenius equation,
which is written as
−E
a
k = Aexp RT
(1)
And the other one is reaction rate equation, which is as
following
2.2. Synthesis of CsPbX3 Nanocrystals by PCI Method
at Room Temperature
a
b
c
r = k C
C
C
3
(2)
[
1 ] [ 2 ] [
]
Polar solvent controlled ionization method is based on the
similar compatibility principle. The precursors for forming
the perovskites are getting soluble in the polar solvents by
controlling the ionization degree and therefore increasing
the ions concentration of cesium, lead as well as the halides.
Equations (1) and (2) show that high temperature is not a neces-
sary condition, increasing the concentration (C) can also obtain
perovskite nanocrystals. Controlling the concentration of ions
in precursors is therefore a crucial factor for perovskite syn-
thesis at room temperature. In our experiment, 0.5 mL cesium
where k is the rate constant, A is the pre-exponential factor, Ea
is the activation energy for the reaction (in Joules mol−1), R is
the universal gas constant, T is the absolute temperature (in
Kelvins), r is the reaction rate, C is the concentration of reac-
tion substance, and a, b, c stands for Stoichiometric numbers,
respectively. As can be seen from Equations (1) and (2), when
the temperature (T) increases, the reaction rate constant (k)
increases as well, followed by acceleration of the reaction rate
(r). To obtain the desired reaction rate (r) when the temperature
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Figure 1. a,b) The absorption and PL spectra (the excitation wavelength is 365 nm) of composition-tunable CsPbX3 PeNCs. c) The rainbow emission
spectra of the CsPbX3 PeNCs under UV light (365 nm).
precursor consists of cesium oleate, 1.5 mL lead and halogen
precursor consists of lead oleate and halogenated amine were
mixed in 10 mL hexane solution. 0.5 mL isopropanol as polar
solvent was also added with stirring for a few seconds. Bright
fluorescence from the solution under the irradiation ultraviolet
lamp was observed afterward.
From another point of view, the synthesis process of PCI
method could be understood as the opposite process of SR
method. Figure 2 shows the relationship between solvent
polarity and ionization of precursors. It simply describes the
synthesis process of these two methods. From state A to state B
to state C, in the system, the solvent polarity is getting increased
with adding more polar solvent. The degree of ionization of the
precursors thus increases, and more and more precursors are
presented as ions. When the concentrations of Pb2+, X−, and
Cs+ increase to a critical value, the reaction will turn to form
the CsPbX3 PeNCs. This process is state B to state D. Growth
and dissolution rates of the CsPbX3 PeNCs reach an equilib-
rium in state D. At this time, if a small amount of polar solvent
continues to be added, balance in state D will be broken, the
growth rate of the CsPbX3 PeNCs is greater than the dissolu-
tion rate, the particle size of nanocrystals will increase, even
precipitation will occur. If a large amount of polar solvent con-
tinues to be added, the dissolution rate of the CsPbX3 PeNCs is
larger than the growth rate, formed CsPbX3 nanocrystals will
be re-dissolved into ions. There is no doubt that polar solvents
can promote ionization of ions. Contrarily, the nonpolar solvent
can inhibit the ionization of ions. Supersaturated recrystalliza-
tion method is a good example, state C to state B to state D,
corresponding to the CsPbX3 PeNCs synthesis process by SR
method. In state D, Cs+, Pb2+, and X− are dissolved in polar sol-
vent (e.g., DMF or DMSO), and the polar solvent transferred
The emission spectra of as prepared CsPbX3 PeNCs are
mainly adjusted by tuning the composition ratio of the hal-
ogen and the emission wavelength was observed being tunable
from 380 to 700 nm. Figure 1a,b shows the absorption and
PL spectra of composition-tunable CsPbX3 PeNCs. Compared
with previous reported works,[17,18] the PL spectrum of CsPbCl3
PeNCs reveals significantly blueshift about 20 nm owing to the
quantum size effect. The reaction conditions of the PCI method
are more moderate than in case of the HI method and the SR
method result in smaller size and consequent broader emis-
sion band gap. Figure 1c shows the rainbow emission spectra
of the CsPbX3 PeNCs under UV light. The kinetics of CsPbX3
PeNCs synthesis method is similar to hot injection method, but
the difference is polar solvent is used to promote the precursors
ionization, and the concentration of Cs+, Pb2+ and X− increased
with polar solvent adding. As can be seen from the Equation
(2), the reaction rate will increase with the ion concentration
increasing, so the increased concentrations for Cs+, Pb2+,
and X− could thus induce the fast crystallization according to
Equation (8) under the activation of rapid stirring.
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2.3. Characterization and Analysis of CsPbX3
Perovskite Nanocrystals Synthesized via Polar
Solvent Controlled Ionization
Figure 3a,b demonstrates the PL spectra of
the CsPbX3 PeNCs synthesized by using
polar solvents with different ratio, wherein,
Figure 3a reveals the PL spectra for CsPbBr3
PeNCs which was prepared with different
amount of isopropanol adding, Figure 3b
demonstrates the spectra CsPbI3 PeNCs
prepared with different ratio of n-butanol
adding. With the proportion of the polar sol-
vent in the system increases, the emission
peak of the CsPbX3 PeNCs was observed
moving toward larger wavelength. This
could be explained by the increased size and
growth rate of as prepared CsPbX3 PeNCs
which were determined by the volume ratio
of the polar solvent adding. With the polarity
of the system increasing, the growth of
CsPbX3 PeNCs is getting faster due to higher
degree of ionization in precursor solvent.
Simultaneously, higher volume ratio also
caused the reaction moving toward to state D
shift and obtaining larger size crystals, hence
the PL spectrum of crystals revealed obvious
red-shift. As shown in Figure 3a,b, the PL
Figure 2. Schematic illustration of the relationship between solvent polarity and ionization of
precursors and the synthesis process of CsPbX3 PeNCs.
to a large number of nonpolar solvent, ionization of Cs+, Pb2+,
and X− are inhibited, inducing crystallization rapidly according
to Equation (8).
spectra of CsPbBr3 and CsPbI3 PeNCs respectively shift from
450 to 520 nm and 600 to 700 nm due to the crystals automati-
cally grew in polarity solvent.
Figure 3. a,b) The PL spectra of CsPbX3 PeNCs synthesized by different ratio of the polar solvent; c,d) The PL spectra of the CsPbX3 PeNCs synthesized
by different polar solvents; a,c) CsPbBr3 PeNCs; b,d) CsPbI3 PeNCs.
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Figure 4. Crystallographic results. a) HR-TEM images of the CsPbBr3 PeNCs prepared via PCI method; b) XRD patterns of CsPbX3 PeNCs, the top three
prepared via PCI method, the bottom one prepared via HI method.
The optimal volume ratio of polar solvent is 4–8%. Figure 3c,d
exhibits the PL spectra of the CsPbX3 PeNCs synthesized by
adding different polar solvents, and all molecular formulas of
these polar solvent are shown in the Supporting Information.
Although all these polar solvents could induce the formation of
perovskite nanocrystals, the obtained PL spectra would be quite
different. As mentioned above, the solvent system with stronger
polarity could induce the faster growth and larger size of the
CsPbX3 PeNCs with obvious red-shift from their emission
spectra, however, the emission position of as obtained PeNCs
synthesized by using acetone or acetidin is smaller than using
n-butanol in same synthesis conditions, indicated that using
n-butanol with weaker polarity generated bigger crystals size.
The order of solvent polarity is given as following: ethanol >
isopropanol > n-butanol, acetone > acetidin > n-butanol. It indi-
cated that the crystal formation of CsPbX3 PeNCs was not only
dependent on the polarity of the solvent system, but also related
to the interaction between polar solvent and precursors. In the
system, compared with carbonyl and esterfunction, hydroxyl is
supposed to be easier to interact with the precursors, therefore,
the crystalline of PeNCs by using n-butanol is better than using
acetone or acetidin.
The morphology evolution of PCI synthesized CsPbX3 PeNCs
is shown in Figure 5, which exhibits novel crystals formations
which are different from crystals in other methods, HI or SR
method. For the CsPbI3 PeNCs samples, we can observe two
distinguished morphologies, nanorods and nanoparticles, as
shown in Figure 5a,b. The average size of the nanorods was
measured and calculated as 45 × 500 nm, and the average size
of the nanoparticles is about 40 nm, which is slightly smaller
than the nanorods. For CsPbBr1.5I1.5 and CsPbBr3, we can also
observe the nanowires from the TEM images. The CsPbBr1.5I1.5
nanowires have a width of about 5–6 nm, and the CsPbBr3
nanowires have the width of about 4–5 nm. The nanowires
are supposed to be used for lasers arrays and photodetectors
devices,[4,5] and the PCI method provides a potential approach
to fabricate perovskite nanowires.
Besides, it is worth noting that many nanoparticles can be
easily observed in the direction along the nanowires. And the
diameter of nanoparticles is about 1 nm smaller than width
of nanowires. Numerous smaller nanoparticles are adsorbed
on the nanowires, and this particular structure will endow the
CsPbX3 PeNCs special optical properties, such as large effective
Stokes shifts.
Figure 4a shows the high-resolution transmission electron
microscopy (HR-TEM) image of the CsPbBr3 PeNCs, which use
5% isopropanol as polar solvent. The average space between
the lattice fringes is measured and calculated as 0.3 nm, cor-
responding to the (200) plane. The X-ray diffraction (XRD) pat-
terns of CsPbX3 PeNCs are also shown in Figure 4b. Overall,
the main diffraction peaks (110), (200), (210), (211), and (202)
are all well matching the typical perovskite scattering pattern.
Peaks shifted toward to small angle direction had been observed
when the halide element in PeNCs shifted from Cl− to Br− to
I−, due to different halide ion radius as R(Cl−) < R(Br−) < R(I−),
which could be well explained by Bragg’s theory.[2] About the
crystal structures of CsPbX3 PeNCs, all CsPbX3 PeNCs present
good crystallinity, and most of them have cubic phases. But
the CsPbBr3 PeNCs, which prepared via PCI method, exhibit
obvious monoclinic phase. At 31° we can clearly observe a split
peak different from the cubic phase, which corresponds to the
monochromatic CsPbBr3 PDF card # 18–0364. The similar
phenomenon also appeared in the synthesis of PeNCs with SR
method before.[16]
Figure 6a–c illustrates emission peaks of CsPbBr3,
CsPbBr1.5I1.5, and CsPbI3 PeNCs via PCI method at 492, 538,
and 703 nm, respectively. The first exciton absorption peaks from
their absorption spectra were observed at 450, 492, and 672 nm,
and thus the obtained “effective Stokes shifts” were calculated
as 42, 46, 31 nm, correspondently. Compared with Figure 6d–f,
we can clearly see that CsPbBr3 PeNCs which synthesized via
our PCI method revealed larger “effective Stokes shifts” than
those PeNCs with SR method (16 nm) and HI method (15 nm).
Large Stokes shifts will reduce the self-absorption of PeNCs.
The CsPbX3 PeNCs possess the large Stokes shifts is related to
the specific morphology which many smaller nanoparticles are
adsorbed on the nanowires very intuitive are shown by TEM
images. It is well known that the energy bandgap is related to
the size of nanocrystals due to the quantum confinment effect.
According to Equation (9), we can calculate the effective bandgap
of nanoparticles and nanowires. Equation (9) can be written as
h2π 2
1
1
1.786e2
εR




E(R)=Eg +
+
−
−0.248ER*y
(9)
2R2 me mh 
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Figure 5. TEM images of CsPbX3 PeNCs prepared via PCI method (polar solvent is isopropanol). TEM image of a,b) CsPbI3 PeNCs; c) CsPbBr1.5 1.5
I
PeNCs; d) CsPbBr3 PeNCs.
Figure 6. PL and absorption spectra. a) CsPbBr3 PeNCs, b) CsPbBr1.5 1.5
I
, c) CsPbI3 PeNCs prepared via PCI method; CsPbBr3 PeNCs prepared with
d) PCI, e) SR, f) HI method, respectively.
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Figure 7. a) TEM images of CsPbBr3 PeNCs synthesized by oleic acid as polar solvents. b) Time-resolved PL decay curves of different CsPbBr3 PeNCs
sample. c) Scheme of the synthetic procedure of CsPbX3 nanoparticles and nanowires.
where E is the bandgap energy, R is the particle radius, Eg is the
bulk bandgap, h is the reduced Planck constant, me and mh are
the effective masses of the electron and the hole, respectively, e
is the elementary charge, ε is the bulk dielectric constant, and
E*Ry is the effective Rydberg energy, which is equal to e4/(2ε2h2
(me−1 + mh−1)).[24] Comparing with nanowires, the nanoparti-
cles possess stronger quantum confinement effect due to the
smaller size, so absorption spectra will present blue shifts and
have a more noticeable first exciton absorption peak. Therefore,
the first exciton absorption peaks of the CsPbX3 PeNCs are
close to the absorption peaks of nanoparticles. Moreover, these
nanoparticles with large bandgap are adsorbed on the sur-
face of nanowires with small bandgap to form a similar type I
structure, so the CsPbX3 PeNCs emission peaks are mainly
determined by small bandgap of nanowires. Consequently, the
nanoparticles. Figure 7a shows the TEM images of CsPbBr3
PeNCs synthesized by oleic acid as polar solvents, the average
size of CsPbX3 PeNCs is around 4 nm. Figure 7b shows PL
decay curves of different CsPbBr3 PeNCs samples. We had
compared the CsPbBr3 PeNCs prepared via HI method (blue
line) with those PeNCs prepared via PCI method in which
IPA was used as polar solvent (black line), they possess sim-
ilar radiative recombination lifetime τ1 (5 and 4.48 ns), but
CsPbBr3 prepared via PCI method have an obvious nonradia-
tive recombination lifetime τ2 (12.96 ns), which may be caused
by the presence of an electron trap in type I heterojunction
structure.[23] The CsPbBr3 prepared via PCI method using OA
as polar solvent (red line) possess longer nonradiative recom-
bination lifetime τ2 (60.48 ns) than IPA as polar solvent. This
indicates that the CsPbBr3 prepared via OA as polar solvent
possess more traps state, and the lower QYs also proved the
phenomenon.[25] The difference of PeNCs morphology is
important evidence which speculates the reasons for the for-
mation of nanowires.
CsPbBr3 PeNCs and the CsPbBr1.5 1.5 PeNCs synthesized by the
I
PCI method possess a larger “effective Stokes shifts,” which is
over 200% more than traditional PeNCs. However, the CsPbI3
PeNCs have no such typeI structure, so the emission peaks are
located between nanoparticles and nanorods emission peaks.
Therefore, the Stokes shifts of CsPbI3 PeNCs are less than
When the polar solvent is OA, CsPbX3 PeNCs grow to form
nanoparticles, but when the solvent is isopropanol, CsPbX3
PeNCs grow to form nanowires. We believe that the mecha-
nism for the formation of CsPbX3 nanowires includes two main
aspects, the mild temperature for reaction and crystal growth,
and the function of ligands induction. Ligands with different
CsPbBr3 PeNCs and CsPbBr1.5 1.5 PeNCs.
I
To replace isopropanol (IPA), using oleic acid (OA) as a
polar solvent to synthesize the CsPbX3 PeNCs, we found that
the nanowires disappeared and all the CsPbX3 PeNCs are
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Figure 8. Demonstration of CsPbBr3 films application in down conversion white LEDs. a) Structural diagram of white LEDs. b) Spectrum of the three
devices (red LEDs, green LEDs, white LEDs). c) CIE color coordinate corresponding to above device and NTSC standard, color coordinate (black dot)
of the white LEDs device is (0.31, 0.34).
activities (e.g., short chain ligands with high activity and
long chain ligands with low activity) can make the nanocrys-
tals orientation grow into nanowires at room temperature.
We carried out our experiments according to the inspiration
from the previous works by Yang and co-workers[26] and Deng
and co-workers[19] Yang and co-workers believed that the key
parameters controlling nanowires formation are the reaction
temperature, the concentration of the reactants, the concentra-
tion and the composition of the stabilizing agents (ligands). If
the reaction temperature is too high, the stable phase during
reaction would be a highly symmetric cubic phase, which lacks
the inherent anisotropy of crystal structure, and thus could
only yield nanocubes and large crystals. Since the PCI method
reaction just happened at room temperature in this work, we
thus mainly concerned about the influences from the ligands.
Moreover, Deng and co-workers also synthesized perovskite
nanowires (nanorods) with acetate acid (short chain ligands)
and dodecylamine (long chain ligands) as ligands at room tem-
perature. Due to the different activities between short and long
chain ligands, growth on orientation was therefore produced
and resulted in the formation of nanowires or nanorods. This
has indicated that the ligands with different activities could
also induce CsPbX3 PeNCs orientation growth. Meanwhile,
we found that isopropanol itself could produce C3H7O− and
shows scheme of the synthetic procedure of CsPbX3 nanoparti-
cles and nanowires. And the isopropanol ionization itself could
be written as
+
2C3H7OH = C3H7O− +C3H7OH2
(10)
+
The C3H7O− (or C3H7OH2 ) can replace OA− (or OAm+) as
the ligands of CsPbX3 PeNCs. C3H7O− ligands are shorter than
OA− and therefore have higher activity. Thereby, the orienta-
tion of growth is changed to form nanowires. This strategy of
using different kinds of ligands with different activities also has
been proved to be feasible in the synthesis of cadmium selenite
nanorods.[27,28]
The all inorganic CsPbX3 nanocrystals have been considered
as a promising candidate material for applying in lighting and
display because of their excellent optical properties. Herein,
three simple devices (encapsulated with red PeNCs-based
LEDs, green PeNCs-based LEDs, R-G-B white LEDs) are fab-
ricated by combining the green emissive films and red emis-
sive films of CsPbX3 with blue emissive GaN LEDs chips.
Figure 8a shows structural diagram of white LEDs. Spectra of
the three devices are shown in Figure 8b, the inset shows the
photographs of the fabricated red LEDs, green LEDs, and white
LEDs, respectively, which is under an applied current of 20 mA.
It is noteworthy that to obtain more stable LEDs devices, red
PeNCs are prepared by HI method. The color coordinate of the
obtained white emission is (0.31, 0.34), which is almost equal
to the standard white color (0.33, 0.33). Besides, these CsPbX3
PeNCs possess a wide gamut of pure colors (Figure 8c), which
cover a much larger area over 120% than the color space of the
NTSC TV color standard.
+
C3H7OH2 (specific molecular formulas showed in the Sup-
porting Information) via ionization process. Therefore, when
the polar solvent is OA, CsPbX3 PeNCs growth direction is the
same on all orientations resulting in the nanoparticles as final
products, but when the solvent is isopropanol, CsPbX3 PeNCs
growth direction on different orientations is getting different,
and thus resulting in wire-shape crystals finally. Figure 7c
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Measurement and Characterization: The TEM images were done on
a FEI Tecnai G2 F30 transmission electron microscope operating at an
acceleration voltage of 300 kV. XRD measurements were carried out
on an X-ray diffractometer (Bruker Advance D8 Ew, Germany) with Cu
Kα radiation (λ = 1.54178 Å). The operation voltage and current was
40 kV and 25 mA, respectively. The 2θ range was from 10° to 80° in
steps of 0.02°. UV–vis absorption spectra were recorded on a PERSEE
TU-1901 spectrophotometer. Florescence spectra and QY were carried
out using a Ocean Optics QE65pro. The lifetimes τ were measured by
the Edinburgh Instrument FLS980.
3. Conclusion
In summary, in this work, we report a facile PCI method for
the synthesis of highly emissive CsPbX3 PeNCs, and the syn-
thesis dynamics is clearly described. Emissions of CsPbX3
PeNCs covering the entire visible region can be obtained via
fine controlling of composition. CsPbX3 PeNCs have high QYs,
narrow FWHM, large Stokes shifts, and particular morphology.
At the same time, we explore the relationship between the large
Stokes shifts and the particular structure of CsPbX3 PeNCs.
Besides, through the influence of different polar solvents on
the CsPbX3 PeNCs morphology, we explain the reasons for the
generation of 1D nanowires CsPbX3 PeNCs. Finally, white LED
devices are fabricated based on blue emissive GaN LEDs chips.
There is no doubt that this efficient approach for the synthesis
of the outstanding perovskite nanocrystals will be in favor of
a range of applications including display, lighting, laser arrays,
and highly sensitive photodetector devices.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was supported by National Natural Science Foundation
of China (No. 51402148, No. 61674074), National Key Research
Project administrated by the Ministry of Science and Technology of
China (No. 2016YFB0401702), Guangdong High Tech Project (No.
2014A010105005, No. 2014TQ01C494), Shenzhen Peacock Team Project
(No. KQTD2016030111203005), Shenzhen Innovation Project (No.
JCYJ20150630145302223, No. JCYJ20160301113537474) Doctoral Fund
of Ministry of Education of China (No. 2017M610484, 2017M612497)
and Foshan Innovation Project (No. 2014IT100072).
4. Experimental Section
Chemicals: Cs2CO3 (99.99%), PbCl2 (99.9%), PbBr2 (99.9%), PbI2
(99.9%), CsCl (99.99%), CsBr (99.99%), CsI (99.99%), octadecene (ODE,
90%), oleic acid (OA, AR), oleylamine (OAm, 70%) were purchased
from Aladdin. n-Hexane (>97.0%), toluene (>99.5%), DMF (>99.5%),
dimethyl sulfoxide (DMSO, >99%), acetone (>99.5%), ethanol (>99.7%),
isopropanol (>99.7%), n-butanol (>99.5%), and acetidin (>99.5%) were
purchased from Shanghai Ling Feng Chemical Reagent Co., Ltd.
Preparation of Precursors: Cs precursors: Cs2CO3 (0.0814 g), OA
(0.25 mL), and ODE (4.75 mL) were added into a 25 mL three-neck
flask, dried for 1 h at 120 °C and then heated under N2 to 150 °C until all
Cs2CO3 reacted with OA. The concentration of Cs precursors was 0.1 m.
Pb and Halogen Precursors: 0.5 mmol PbX2, ODE (9 mL), OA (0.5 mL),
and OAm (0.5 mL) were loaded into a 25 mL three-neck flask and dried
under vacuum at 120 °C for 1 h, the optically transparent liquid could be
obtained. The concentration of Pb precursors was 0.05 m, and halogen
precursors was 0.1 m.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
inorganic perovskite nanocrystals, ionization, nanowires, polar solvent
control
Synthesis of CsPbX3 PeNCs via PCI Method: 0.5 mL Cs precursors,
1.5 mL Pb and halogen precursors, and 10 mL n-hexane were mixed in a
25 mL beaker with vigorously stirring at room temperature. Then 0.5 mL
isopropanol was swiftly added into the beaker, after a few seconds, the
bright CsPbX3 PeNCs could be obtained. All precursors synthesized
via high temperature in ODE or directly purchased could be used to
synthesize CsPbX3 PeNCs.
Received: October 16, 2017
Revised: November 25, 2017
Published online:
Synthesis of CsPbX3 PeNCs via HI Method: ODE (5 mL) and
0.188 mmol of PbX2 were loaded into a 25 mL three-neck flask and
dried under vacuum at 120 °C for 1 h. Dried OA (0.5 mL) and OAm
(0.5 mL) were injected at 120 °C under N2 flow. After PbX2 salt
completely dissolved, the temperature was raised to 150–180 °C and
0.5 mL Cs precursors solution (prepared as described above) were
swiftly injected. The reaction mixture was cooled down by a water bath
after 5 s. For obtaining CsPbCl3 PeNCs, 1 mL of n-Trioctylphosphine was
necessary to solubilize PbCl2.
Synthesis of CsPbX3 PeNCs via SR Method: In a typical synthesis
process, PbBr2 (0.4 mmol) and CsBr (0.4 mmol) were dissolved in
DMF or DMSO (10 mL), OA (1 mL), and OAm (0.5 mL) were added to
stabilize the precursor solution. Then, 1 mL of the precursor solution
was quickly added into toluene (10 mL) under vigorous stirring. Strong
green emission was observed immediately after the injection. Other
samples with different colors were fabricated with the mixture of PbX2
and CsX (X = Cl, Br, I).
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