Microwave-Enhanced Chemistry at Solid-Liquid Interfaces: Synthesis of All-Inorganic CsPbX3 Nanocrystals and Unveiling the Anion-Induced Evolution of Structural and Optical Properties

Article abstract of DOI:10.1021/acs.inorgchem.0c00294

We demonstrate how microwaves could enhance the chemistry at interfaces of heterogeneous reactions involved in the microwave-solvothermal (MW-ST) synthesis of all-inorganic CsPbX₃ (X = Cl, Br, I) perovskite nanocrystals (PNCs) within 6 min, unlike a conventional hot-injection method that requires 3 h. The enhanced MW-ST reaction rate was quantitatively analyzed by the Eyring equation, and it has been observed that the decreased activation free energy (ΔG^?) and increased activation entropy (ΔS^?) are caused by changes in the relative energies of reactants at their solid-liquid interfaces, leading to the formation of hot spots , where microwave energy absorption is at its maximum. This rapid and homogeneous microwave heating could facilitate the self-assembly of uniformly distributed CsPbX₃ nanocubes with precise control over the stoichiometric ratio, as confirmed by high-resolution transmission electron microscopy and energy-dispersive X-ray analyses. X-ray diffraction and Raman results indicate that lattice contraction and expansion in CsPbBr_3-yX_y have occurred because of an increase in the metal-halide bond length upon moving down the groups Cl → Br → I, as further ascertained by the Rietveld refinement studies. These anion-induced structural variations accordingly affected the electronic properties of MW-ST-synthesized CsPbX₃ PNCs, which is apparent from the shifts in their conduction-band (CB) and valence-band (VB) positions. Consequently, the optical properties were also altered, resulting in a color-tuned emission from blue to red, with excellent photoluminescence quantum yields (up to 92%) and narrow emission line widths, as is evident from UV-vis and photoluminescence spectroscopy. The MW-ST-synthesized CsPbX₃ PNCs were used as color-conversion layers for the fabrication of light-emitting diodes (LEDs) with commercial 456 nm UV-LED chips.

Full text of DOI:10.1021/acs.inorgchem.0c00294

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Article
Microwave-Enhanced Chemistry at Solid−Liquid Interfaces:
Synthesis of All-Inorganic CsPbX Nanocrystals and Unveiling the
3
Anion-Induced Evolution of Structural and Optical Properties
Kabalaraj Thesika and Arumugam Vadivel Murugan*
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sı Supporting Information
ABSTRACT: We demonstrate how microwaves could enhance the
chemistry at interfaces of heterogeneous reactions involved in the
microwave-solvothermal (MW-ST) synthesis of all-inorganic CsPbX3
(
X = Cl, Br, I) perovskite nanocrystals (PNCs) within 6 min, unlike a
conventional hot-injection method that requires 3 h. The enhanced
MW-ST reaction rate was quantitatively analyzed by the Eyring
equation, and it has been observed that the decreased activation free
energy (ΔG ) and increased activation entropy (ΔS ) are caused by
changes in the relative energies of reactants at their solid−liquid
interfaces, leading to the formation of “hot spots”, where microwave
energy absorption is at its maximum. This rapid and homogeneous
microwave heating could facilitate the self-assembly of uniformly
distributed CsPbX3 nanocubes with precise control over the
⧧
⧧
stoichiometric ratio, as confirmed by high-resolution transmission electron microscopy and energy-dispersive X-ray analyses. X-
ray diffraction and Raman results indicate that lattice contraction and expansion in CsPbBr3−yXy have occurred because of an
increase in the metal−halide bond length upon moving down the groups Cl → Br → I, as further ascertained by the Rietveld
refinement studies. These anion-induced structural variations accordingly affected the electronic properties of MW-ST-synthesized
CsPbX PNCs, which is apparent from the shifts in their conduction-band (CB) and valence-band (VB) positions. Consequently,
3
the optical properties were also altered, resulting in a color-tuned emission from blue to red, with excellent photoluminescence
quantum yields (up to 92%) and narrow emission line widths, as is evident from UV−vis and photoluminescence spectroscopy. The
MW-ST-synthesized CsPbX PNCs were used as color-conversion layers for the fabrication of light-emitting diodes (LEDs) with
3
commercial 456 nm UV-LED chips.
1
0−19
1
. INTRODUCTION
variety of solution chemical methodologies.
These
methods yield solution-dispersible CsPbX PNCs, which can
be compositionally tuned via systematic anion-exchange
The HOT-I synthesis temperatures normally
fall in the range of 90−240 °C, followed by a swift injection
process that could effectually dissociate, followed by the
nucleation process, resulting in uniform homogeneous
colloidal CsPbX PNCs. Protesescu et al. reported the
successful synthesis of all-inorganic CsPbX PNCs by the
HOT-I method for the first time, in which an organic cesium
precursor was quickly hot-injected into the reaction mixture
containing lead halides and organic capping ligands in a
noncoordinating solvent octadecene (ODE) at elevated
3
Semiconducting lead halide perovskite nanocrystals (PNCs)
have attracted considerable attention because of their
enormous potential for solar cells and optoelectronic
15−18
procedures.
1
−4
applications.
In particular, all-inorganic CsPbX (X = Cl,
3
Br, I) PNCs have established higher chemical and temperature
stability, as well as exceptional photoluminescence (PL)
quantum yields (PLQYs).
of the CsPbX PNCs over the whole visible region can be
achieved by varying the stoichiometric ratio of the anions,
which makes them potential substitutes for conventional metal
chalcogenide quantum dots (QDs).
CsPbX PNCs have been conventionally prepared by hot-
injection (HOT-I) methods reported previously.
then, several efforts have been chosen to optimize the synthesis
20
2
−5
3
The tunable PL emission spectra
3
3
6
,7
These all-inorganic
3
1
−9
Since
Received: January 29, 2020
10−13
of high-quality CsPbX PNCs including anion-exchange,
ultrasonication, coprecipitation, solvothermal, chemical
vapor deposition, and room-temperature methods. Mor-
3
1
2
14
15
16
17
phological tuning of CsPbX PNCs with shapes including QDs,
3
nanowires, and nanoplatelets has also been achieved by a
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.inorgchem.0c00294
A
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Table 1. HOT-I and MW-ST Syntheses of CsPbX PNCs in Previous Literature for Comparison with Our Work
3
sample no.
NC size (nm)
QY (%)
lifetime (ns)
favorable phase
cubic
cubic
cubic
cubic
cubic
synthesis method
ref
7
1
2
3
4
5
6
7
8
9
8.0−8.4
4−15
8−10
8−10
16−20
4−15
9
14−15
15
11−16.9
10−11.4
9.8−20.7
5−8
78
90
10−80
84
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
HOT-I
solvothermal
microwave
microwave
microwave
Anton-Paar MW-ST
Akkerman et al.
Protesescu et al.
Nedelcu et al.
9
1−29
1
1
1
8
Bekenstein et al.
33
12.3
Liu et al.
Gomez et al.
3
4
85
60
35
13.9
orthorhombic
Palazon et al.
36
Li et al.
Zhang et al.
37
1.34−7.78
9.6−11.6
cubic
orthorhombic, cubic
38
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
Lee et al.
Uddin et al.
39
80−99
4
0
cubic
cubic, orthorhombic
Wu et al.
4
1
95
60−80
38−50
Li et al.
42
Parobek et al.
43
3.2−10.5
19
10
10−12
8
9
7−10
5−10
10
8.2
10.8
cubic, orthorhombic
cubic
Li et al.
Li et al.
44
45
Lao et al.
Mondala et al.
4
6
51−62
60−90
7−17
47
Jizhong et al.
Wang et al.
Emre et al.
Bertolotti et al.
Wang et al.
4
8
3.1
38
4
9
70
cubic
cubic, orthorhombic
50
5
1
87
80
1
5
1.3−6.7
cubic
cubic
monoclinic
cubic
orthorhombic
Chen et al.
3
1
Pan et al.
32
16.8−20.8
6−10
12−15
90
33−56
88−92
1.9−135.7
67−119
10−29
Liu et al.
5
2
Li et al.
this work
9
temperatures (140−200 °C). The HOT-I method also
microwaves would provide more homogeneous heating by
reducing the thermal gradient effects of the whole volume, thus
favoring identical nucleations, which is crucial for the
provides a domain for facile alteration of the halide
composition and reaction temperature of the CsPbX PNCs,
resulting in a fine-tuned absorption and PL over the entire
visible region. However, the HOT-I method requires harsh
3
2
5,26
formation of monodispersed nanoparticles.
Colloidal
9
CsPbX PNCs possessing exceptionally fast and irrepressible
3
conditions such as vacuum and inert environments as well as
longer reaction times, and it is also tedious in terms of scale-up
production, complicated experimental setup, and high-energy
input. A conventional solvothermal method was first adopted
crystallization rates are well-suited for the rapid and
homogeneous temperature rise in microwave synthesis
methods. This method promotes the nucleation as well as
development of high-quality PNCs and also reduces the
synthesis time. Recently, sustainable, environmentally benign,
and energy-efficient processes have drawn much attention for
the synthesis of various nanostructured materials. In this
regard, recently our group reported the microwave-assisted
solvo/hydrothermal (MW-ST/HT) methodology is a con-
sistent technique for synthesizing high-quality, morphology-
controlled, and phase-tuned nanostructured energy materials
by Chen et al. to prepare CsPbX PNCs, where cesium acetate,
3
lead halide, oleic acid (OA), oleylamine (OLA), and ODE
were mixed in an autoclave, with trioctylphosphine being
employed to improve the solubility of the lead halide
1
5
precursors. The solvothermal synthesis is usually conducted
in a pressurized vessel at high temperatures, in an isochoric
atmosphere, and hence it is considered to be a simple,
ascendable, and flexible synthesis methodology for the
synthesis of nanomaterials. It is also considered to gain
pronounced control over the morphology, stoichiometry,
monodispersity, and crystallinity of the product; however, it
consumed more reaction time (≥1 h).
2
8,29
for photovoltaic and photocatalytic applications.
The
synthesis of optical materials via a simple, low-cost domestic
microwave-refluxing method is also desired for its elevated
30
quantum yields and long lifetimes. The microwave-assisted
synthesis of CsPbX PNCs with trioctylphosphine treatment
On the other hand, microwave-assisted synthesis method-
ologies that employ microwave irradiation are becoming more
prevalent in synthetic chemistry because it is a comparatively
prompt input energy source to drive reactions. The heating of
the reactants generates dipole interactions of the molecules
with high-frequency electromagnetic radiation, which makes
the microwave-assisted technique a very favorable approach to
3
was reported for the first time by Pan et al., thus providing a
novel synthetic route that is rapid and efficient for the
31
preparation of high-quality PNCs. Subsequently, Liu et al.
reported a one-step synthesis of CsPbX PNCs with a domestic
3
microwave oven by replacing the conventionally used capping
ligand OA by a long-chain bis(2,4,4-trimethylpentyl)-
2
1−27
32
obtaining good-quality semiconductor nanocrystals.
phosphinic acid. Motivated by the rapid advancement in
Compared to a conventional heating strategy that involves
the synthesis of CsPbX PNCs, we have attempted to combine
3
conductive and/or convective heat energy transfer, the
the effects of microwave and solvothermal synthetic strategies
B
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Figure 1. Schematic illustration of the MW-ST and HOT-I syntheses followed by their structural and optical studies.
for the preparation of CsPbX PNCs employed from the
naturally available resources as capping ligands.
Here, we have demonstrated an innovative, high-throughput,
facile microwave-assisted solvothermal (MW-ST) method to
compared to the HOT-I method. Thus, we have successfully
demonstrated the MW-ST synthesis of self-assembled CsPbX3
PNCs by systematically varying the anion ratio, which could
provide a comparative understanding of anion-induced
variations in lead halide perovskites. The significance of the
MW-ST methodology has been compared with the previously
3
synthesize high-quality all-inorganic CsPbX PNCs using the
3
commercial Anton-Paar microwave reaction system SOLV.
Our MW-ST approach is widely desired because of its rapidity,
which does not require any predissolving or pretreating as in
the case of a conventional HOT-I method. The novelty of the
MW-ST process is the drastic reduction in the reaction time
from many hours to few minutes without requiring any
vacuum and inert gas atmosphere. Another significance of the
MW-ST method is that the temperature and pressure of the
reaction mixture can be controlled automatically in a closed
chamber with high safety compared to a domestic microwave
oven. However, to the best of our knowledge, there is no
quantitative report of the MW-ST-assisted synthesis of CsPbX3
PNCs to suggest changes on the reaction kinetics and
thermodynamic parameters. Hence, the enhanced overall
reaction rate and thermodynamic parameters such as activation
free energy, activation enthalpy, and activation entropy of the
MW-ST-assisted synthesis of CsPbX3 PNCs have been
quantitatively investigated by using the Eyring equation. In
addition, it has been emphasized that localized superheating of
reported synthesis of CsPbX PNCs by conventional HOT-I
3
and other microwave methods, as given in Table 1.
2. EXPERIMENTAL SECTION
2
.1. Materials. Cesium carbonate (CsCO ; 99%), lead(II)
3
bromide (PbBr ; 99.99%), lead(II) iodide (PbI ; 99%), lead(II)
2
2
chloride (PbCl ; 98%), 1-octadecene (ODE; technical grade, 90%),
2
oleylamine (OLA; technical grade, 90%), and oleic acid (OA;
technical grade, 90%) were purchased from Sigma-Aldrich. Hexane
was purchased from Loba Chemie. All of the purchased chemicals
were used as received without any additional purification. The
commercial 456 nm UV light-emitting diodes (LEDs) were purchased
from DNA Solutions, Nashik, India.
2
.2. Preparation and Purification of CsPbX PNCs. CsPbX
3 3
PNCs were synthesized via HOT-I methodology as previously
8
reported. Briefly, 10 mL of ODE, 0.2 mmol of PbX (X = Cl, Br,
2
I), 1 mL of OLA, 1 mL of OA, and 0.8 mL of Cs-oleate (see the
Supporting Information) were mixed and poured into a quartz vessel,
which was positioned on a turntable stage for uniform heating in the
Anton Paar microwave reactor. The microwave irradiation time was
set at a ramp for 5 min and at hold for 1 min, the temperature was set
to 180 °C, as programmed with the Multiwave PRO software, and the
pressure was around 12 bar. After completion of the process, the
reaction vessels were cooled to room temperature by a built-in fast-
the solid-phase (PbX ) and liquid-phase [cesium oleate (Cs-
2
Oleate), OA, and OLA] precursors at their interface leads to
direct conversion of the reaction mixture into products. The
MW-ST method also facilitates a domain for the anion-
induced tuning of the structural and optical characteristics and
is well-suited for precise control over the stoichiometry of
cooling fan system. The CsPbX PNCs were deposited at the bottom
3
of the quartz vessel and separated by centrifuging the obtained
solution for further purification. To obtain high-quality CsPbX3
PNCs, the obtained solution was centrifuged at 8000 rpm for 5
min to remove the supernatant consisting of unreacted precursors
and/or byproducts. Then, 10 mL of hexane was added and
centrifuged at 1000 rpm for 5 min to remove large PNCs. Finally,
the supernatant containing CsPbX3 PNCs was stored for further
characterization.
CsPbX PNCs. The microwave synthetic strategies employed
3
previously have reported CsPbX nanocrystals (NCs) in their
3
kinetically stable phases, and their time-resolved photo-
luminescence (TRPL) measurements exhibited multiexponen-
tial decay traces. However, the MW-ST method produces
CsPbX NCs in their thermodynamically stable orthorhombic
3
phase, and it also reveals many similar optical properties
C
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2
.3. Characterization. Crystal structures of the synthesized
revealed that the nonradiative recombination of CsPbBr3
decreased as the reaction temperature increased during the
MW-ST synthesis, as shown in Figure S2. The lifetimes of the
samples synthesized at different temperatures ranging from 100
samples were analyzed by X-ray diffraction (XRD) studies using a
Rigaku Ultima IV X-ray diffractometer with Cu Ka radiation (λ = 1.54
Å) at 40 kV and 30 mA to scan the diffraction angles from 10 to 60°
−
1
at a step size of 0.02° s . Phase identification was carried out using
the standard patterns reported in the International Centre for
Diffraction Data database. GSAS software and CIF files were used for
Rietveld analyses of both the MW-ST and HOT-I samples. The
tolerance and octahedral tilting factors were calculated by the
Goldshcmidt equation. Room-temperature confocal Raman spectra
were recorded using a Renishaw in-Via system with a laser excitation
energy of 785 nm. The absorbance and diffuse-reflectance spectros-
copy (DRS) of the synthesized MW-ST and HOT-I samples were
recorded using a UV−vis spectrophotometer (UV-2550, Shimadzu,
Japan). PL emission and TRPL spectra were recorded using a
spectrofluorometer (Fluorolog FL3-11) with a 488 nm femtosecond
laser. High-resolution transmission electron microscopy (HR-TEM)
images were acquired using a transmission electron microscope (FEI-
to 220 °C are tabulated in Table S1. The CsPbBr PNCs
3
synthesized at 180 °C exhibited longer lifetimes compared to
the samples synthesized at temperatures lower and higher than
180 °C. Consequently, the average particle sizes of the
CsPbBr PNCs synthesized at 200 and 220 °C were found to
3
increase drastically from 46.03 nm at 180 °C to 416.5 and
6
24.2 nm at 200 and 220 °C, respectively, as shown in Figure
S2. This suggested the agglomeration of CsPbBr PNCs when
3
the synthesis temperature increases, resulting in poor optical
properties. Hence, these results confirm that the MW-ST
synthesis requires an optimal temperature of 180 °C for the
effective formation of CsPbX3 PNCs with better optical
properties.
2
Technai G ) at 200 kV, equipped with an energy-dispersive X-ray
(
EDX) spectrometer. Particle-size analysis was carried out using a
3.2. MW-ST Reaction Kinetics and Thermodynamics.
The overall reaction rate kinetics and thermodynamic
Zeta Sizer (Malvern Zeta Sizer - Nano S). The Commission
Internationale de L’Eclairage (CIE, 1931) calorimetry system was
used to identify the color gamut.
parameters of both HOT-I and MW-ST can be discussed
53,54
according to the Eyring equation.
same can be calculated by eq 3.4.
The reaction rates of the
3
. RESULTS AND DISCUSSION
.1. Influence of the Reaction Parameters on the
TkB
−ΔG /RT
⧧
3
k =
e
h
(3.4)
MW-ST Synthesis of CsPbX PNCs. The all-inorganic PNCs
3
were synthesized by both conventional HOT-I and MW-ST
methods for deducing the microwave induced over all reaction
kinetics and deviation in the photophysical properties of the
Here k stands for the rate constant, T stands for the absolute
temperature, k stands for the Boltzmann constant, h is the
B
Planck constant, R denotes the universal gas constant, and
⧧
CsPbX PNCs, as shown in a schematic illustration (Figure 1).
3
ΔG denotes the free energy difference between the reactants
The MW-ST method using ODE as the solvent, combined
with microwave irradiation, rapidly initiates the nucleation
and transition state, i.e., the activation free energy. The
⧧
⧧
activation enthalpy (ΔH ), activation entropy (ΔS ), and
⧧
process and growth rate of CsPbX PNCs at 180 °C with a
3
activation free energy (ΔG ) were calculated with the Eyring
equation, shown as eq 3.5.
pressure of 12 bar. The conventional HOT-I method involves
the rapid injection of precursors, followed by quenching of the
reaction mixture immediately in an ice bath, which enormously
slows the nucleation and growth dynamics of all-inorganic
PNCs. For a typical HOT-I synthesis of PNCs, the generalized
conversion of reactants into their reactive intermediates is
given in eq 3.1, and the final product reaction is given in eq 3.2.
TkB
−ΔS /R −ΔH /RT
⧧
⧧
k =
e
e
h
(3.5)
(3.6)
i k y ΔH⧧
1
T
ik y ΔS⧧
j z
j
B z
lnj z =
+ lnj z +
j z
j
j
z
z
k {
T
h
k
h
{
R
PbX + 2OA + 2OLA → Pb(OA) + 2OLAX
(3.1)
The rate constants of the MW-ST and HOT-I syntheses were
plotted between 1/T and ln(k/T) following eq 3.6, ΔH and
ΔS were derived from the slope and intercept, respectively,
2
2
⧧
⧧
CsOA + Pb(OA) + 3OLAX → CsPbX + 3OA + OLA
2
3
⧧
(3.2)
and ΔG was obtained using eq 3.7, as tabulated in Table 2.
⧧
⧧
⧧
However, during the MW-ST synthesis of PNCs, all of the
precursors were mixed together, leading to the conversion of
reactants into their respective products, as given in eq 3.3.
(3.7)
ΔG = ΔH + TΔS
Generally, the activation energy for the conversion of
precursors of the solid phase (PbX ) and liquid phase (Cs-
2
Oleate, OA, and OLA) to their unstable transition states
significantly contributes to an increase or a decrease in the
overall rate of reaction. It is observed that the activation free
PbX + 3OLA + 3OA + CsOA
2
→
CsPbX + 3OA + 3OLA
(3.3)
3
⧧
energy, ΔG , of the HOT-I synthesis through conventional
In order to comprehend the influence of the temperature in
the MW-ST synthesis of CsPbX3 PNCs, we synthesized
Table 2. Thermodynamic Parameters Obtained Using the
Eyring Equation
pristine CsPbBr in different temperatures ranging from 100 to
3
2
20 °C with varied powers ranging from 400 to 800 W at
similar reaction times with a ramp time of 5 min and hold time
of 1 min. The samples were analyzed using PL spectroscopy, as
given in Figure S1. The PL spectra showed a distinct increase
in the emission intensity with an increase in the synthesis
temperature from 100 to 180 °C, and from 200 to 220 °C, the
PL intensity drastically reduces. Thus, 180 °C was found to be
the optimum temperature for the MW-ST synthesis of CsPbX3
PNCs using the Anton-Paar system. The TRPL spectra
reaction parameter
MW-ST
180
HOT-I
180
150
9.26 × 10
147.498
−3409.95
304.811
temperature (°C)
reaction time (min)
reaction rate (s−1)
6
−3
−5
2.78 × 10
134.67
−3409.95
−333.13
ΔG⧧ (kJ mol−1
)
ΔH (J mol−1)
⧧
ΔS (J mol K−1
⧧
−1
)
D
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Figure 2. (a and b) Schematic representations of conventional and microwave heating of the reaction mixture, respectively. Comparison of the
⧧
⧧
⧧
differences in the (c) activation free energy (ΔG ), (d) enthalpy (ΔH ), and (e) entropy (ΔS ) of the HOT-I and MW-ST processes.
heating is approximately 13 kJ higher than that of the MW-ST
synthesis involving microwave heating, as represented in
Figure 2c. It is mainly due to the decrease in the activation
greater than the temperature of the individual solid (PbX ) or
2
liquid (Cs-Oleate, OA, and OLA) phase. This delivers a rapid
direct interaction with the reactive species at the interfaces,
leading to the creation of “hot spots” in the synthesis mixture
significantly varying their capacity to absorb microwaves, as
depicted in Figure 2b. Thus, microwave exposure leads to a
relative enhancement in the effective temperatures of the
reactive intermediates, thus enhancing the relative energies of
the reactive intermediates in the reaction sequence. These
energy variations of the reaction intermediates certainly
enhance the overall rate kinetics, as observed in the case of
catalyst-mediated reactions.
⧧
entropy, ΔS , of the HOT-I synthesis compared to that of the
MW-ST reaction, as given in Figure 2d. Here, we observe that
microwave exposure influences the activation energy, and it
can also be deduced that the microwaves could afford a more
favorable reaction coordinate, which leads to less organized
reactive intermediates having high entropy. This causes a
substantial rise in the overall reaction rate of the MW-ST
reaction by 2 orders of magnitude compared to the HOT-I
⧧
reaction. However, the activation enthalpy, ΔH , is the same
3.4. Anion-Induced Structural Evolution and Mor-
for both the HOT-I and MW-ST processes because the
reaction temperature for both synthesis methodologies was
constant, as represented in Figure 2e. Thus, the effect of
microwave exposure can be considered athermal, as previously
phological Studies. The mixed halide CsPbX PNCs were
3
synthesized by using both the MW-ST and HOT-I methods.
The samples with different halide compositions have been
abbreviated, and the sample codes have been mentioned in
5
4
reported. Conversely, when related to the conventional
HOT-I method, the reaction profile does not exhibit any
significant change except a periodic variation due to localized
heating during the microwave process.
Table 3. The purified CsPbX PNCs were dispersed in hexane,
3
Table 3. Composition with the Corresponding Sample
Codes of Both the MW-ST- and HOT-I-Synthesized
Samples
3
.3. MW-ST-Influenced Chemistry at the Interfaces.
The synthesis of CsPbX3 PNCs involves heterogeneous
reactions between solid and liquid phases, where nucleation
and growth occurs in the interfacial surface. In the case of the
composition
MW-ST sample code
HOT-I sample code
CsPbBr1.5Cl1.5
MC-3
MC-2
MC-1
MB-0
MI-1
MI-2
MI-3
HC-3
HC-2
HC-1
HB-0
HI-1
HOT-I synthesis of CsPbX PNCs, the solid−liquid interface
3
CsPbBr Cl1
2
between the solid-phase reactants (metal halide salts PbBr ,
2
CsPbBr2.5Cl0.5
CsPbBr3
PbCl , and PbI ) and the liquid-phase organic reactants (OA
2
2
and OLA) undergoes a conventional nonuniform heating, as
depicted in Figure 2a. Consequently, there is no difference in
temperature of the solid−liquid interfaces compared to that of
the reaction mixture. However, the MW-ST conditions
influence interfacial reactions that occur in a heterogeneous
system where the reactants reorganize or adsorb at the
interfaces. This could be attributed to the Maxwell−Wagner
interfacial microwave polarizations that commonly arise in a
CsPbBr2
I
.5 0.5
CsPbBr2I1
CsPbBr1
HI-2
HI-3
I
.5 1.5
and the morphologies along with the elemental compositions
have been analyzed by HR-TEM and EDX analyses. The HR-
TEM images of HOT-I-synthesized HB-0 and MW-ST-
synthesized MB-0, MC-3, and MI-3 have shown the distinct
cube and rectangular-edged, self-assembled highly monodis-
persed PNCs (Figure 3a−d).
55
heterogeneous system.
Because of microwave exposure, the solid−liquid interfaces
exhibit different effective temperatures according to their
different permittivities, where the temperature is effectively
E
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Figure 3. (a−d) HR-TEM and (e−h) magnified HR-TEM images showing lattice fringes. (i−l) SAED patterns, (m−p) particle-size distribution
graphs, and (q−t) EDX spectra of the HB-0, MB-0, MC-3, and MI-3 samples, respectively.
The pristine HOT-I-synthesized CsPbBr PNCs (HB-0)
smaller-anion, chlorine-substituted MC-3 sample shows a
lattice spacing of 0.398 nm, which corresponds to the (121)
plane of CsPbBr Cl PNCs (shown in Figure 3g,k), whereas
3
with a lattice fringe of 0.29 nm corresponding to the (202)
plane (also observed in the SAED pattern) clearly revealed
high crystalline nature with an average particle size of 14.64 nm
1
.5 1.5
the bigger-anion, iodine-substituted MI-3 sample shows an
enhanced lattice spacing of 0.412 nm for the same
corresponding (121) plane in CsPbBr I PNCs, which can
(
(
Figure 3e,i). The pristine MW-ST-synthesized CsPbBr PNCs
3
MB-0) are also highly monodispersed, having an average
1
.5 1.5
particle size of 15.58 nm with a well-defined lattice fringe of
.269 nm corresponding to the (202) plane, and hence it
confirmed the uniform crystallinity of the self-assembled
also be clearly attributed to the crystalline nature of PNCs in
Figure 3h,l. The average particle size of MC-3 is 15.29 nm
larger than that of MI-3, which is only 12.21 nm, as presented
in Figure 3o,p. The reciprocal lattice vector of length 3.81
0
PNCs, as shown in Figure 3f,j. Similarly, MW-ST-synthesized
F
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Figure 4. (a and b) XRD patterns of the MW-ST and HOT-I samples, respectively, and (c and d) (121) plane peak shift of the MW-ST and HOT-
I samples respectively. Rietveld refinement graphs of the (e) MB-0 and (f) HB-0 samples. (g) Orthorhombic lattice parameters of the MW-ST
samples. (h) Packing coefficient of the MW-ST and HOT-I samples. (i) Octahedral tilting angles of the pristine CsPbBr sample according to a
3
change in the halide composition.
nm− (0.262 nm in the direct lattice) indicating the (222)
1
4a−d, the XRD peaks are located at 2θ = 15.3°, 21.1°, and
31.1°, which correspond to the (101), (121), and (202)
planes, respectively, and they remain unaltered irrespective of
plane can be observed for all of the samples, which is unique
for the orthorhombic phase (PDF 98-007-7608) of CsPbBr3.
The EDX spectroscopy of MB-0 (Cs:Pb:Br ∼ 1:0.9:2.9)
proved to be in good agreement with the elemental atomic
ratio of CsPbBr compared to that of HB-0 (Cs:Pb:Br ∼
:0.7:2.9), as given in Figure 3q,r. This shows that the
stoichiometry of CsPbX PNCs is preserved more efficiently
because of rapid and homogeneous heating during the MW-ST
synthesis rather than the HOT-I synthesis. The similar trend of
the elemental atomic ratio is also observed in the MW-ST-
56
−
−
variations in the concentration of I or Cl anions. The
presence of the (101), (121), and (202) planes in pristine as
well as anion-substituted samples can also be confirmed by the
SAED patterns. The peak at 28.54° corresponding to the (222)
plane is unique for the orthorhombic phase, which was also
observed in the HR-TEM results. The XRD peaks gradually
shift to lower angles for the HI-1, HI-2, and HI-3 samples
because of the lattice dilation caused by the replacement of a
3
1
3
−
−
−
synthesized Cl -substituted MC-3 (Cs:Pb:Br:Cl ∼ 1:1:1.4:1.4)
larger I ion for the smaller Br ion. On the contrary, the peaks
−
and I -substituted MI-3 (Cs:Pb:Br:Cl ∼ 1:1:1.4:1.4) PNCs,
shift to higher angles for the HC-1, HC-2, and HC-3 samples
−
which also suggested the effective conservation of stoichiom-
according to an increase in the Cl concentration, which can
etry in the MW-ST synthesis of CsPbX PNCs.
be attributed to lattice contraction, which is clearly evident
from the Rietveld refinement results. The shifts of peaks
corresponding to the (121) plane of MW-ST- and HOT-I-
synthesized samples are presented in Figure 4c,d. The presence
of some extra peaks in both the MW-ST- and HOT-I-
synthesized samples is due to the presence of impurities such
as CsBr, PbI , and PbCl . This can be evidenced by the
3
The CsPbX PNCs are known to form at least three different
3
̅
phases, cubic (space group Pm3m), tetragonal (space group
P4/mbm), and orthorhombic (space group Pnma), of which
the orthorhombic phase is thermodynamically stable at room
temperature. The XRD patterns shown in Figure 4a,b further
indicate that the as-synthesized CsPbX PNCs crystallize in the
3
2
2
thermodynamically stable orthorhombic phase (PDF 98-007-
enlarged XRD patterns given in Figure S4. In the MW-ST-
synthesized MI-3 sample, the intensity of the peaks
representing the orthorhombic phase is greatly reduced,
whereas the appearance of the high-intensity peaks corre-
sponding to the (221), (312), and (224) planes of a secondary
phase is observed, which could be attributed to the presence of
7
608) with space group Pnma, adopting a distorted perovskite
structure. The MW-ST-synthesized CsPbX PNCs in their
3
orthorhombic phase is unique compared to the previously
reported microwave-synthesized CsPbX NCs in their kineti-
3
cally stable monoclinic and cubic phases. As shown in Figure
G
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Table 4. Composition with the Corresponding Sample Codes and Their Tolerance Factors, Octahedral Factors, and Packing
Coefficients
packing coefficient
composition
MW-ST sample code
HOT-I sample code
tolerance factor
octahedral factor
MW-ST
HOT-I
CsPbBr1.5Cl1.5
MC-3
MC-2
MC-1
MB-0
MI-1
MI-2
MI-3
HC-3
HC-2
HC-1
HB-0
HI-1
0.8171
0.8164
0.8156
0.8149
0.8136
0.812
0.626
0.619
0.613
0.607
0.595
0.578
0.5721
0.14636
0.14697
0.14894
0.15439
0.1576
0.14799
0.14884
0.15147
0.14986
0.15743
0.16824
0.17476
CsPbBr Cl1
2
CsPbBr2.5Cl0.5
CsPbBr3
CsPbBr2
I
.5 0.5
CsPbBr2I1
CsPbBr1
HI-2
HI-3
0.16779
0.17404
I
0.811
.5 1.5
rA + r
X
cubic CsPbI (PDF 98-011-4280), as given in Figure 4a. The
3
t =
MW-ST-synthesized samples have found to be more crystalline
compared to the HOT-I-synthesized PNCs, and this is clearly
evident from the sharp peaks, which can be identified by the
Rietveld refinement studies. To gain more insight into the
structure-related modifications of the photophysical properties
2 (r + r )
(
3.8)
B
X
where r is the Pauling radius of the A cation, r is the Pauling
radius of the B cation, and r is the Pauling radius of the anion.
In the case of ABBr3 X (A is the Cs cation, B is the Pb
cation, and X is the Cl or I anion), an effective anionic radius
is required for determination of the tolerance factor as
proposed by Shannon. Thus, the Goldschmidt tolerance factor
A
B
X
+
2+
−y
y
−
−
and crystal structure stability of CsPbX with different anion
3
compositions, Rietveld refinement analysis was performed with
the XRD data obtained from all of the anion-substituted
samples synthesized by both the MW-ST and HOT-I methods.
Parts e and f of Figure 4 show the Rietveld graphs of the MB-0
and HB-0 samples, respectively. The calculated structural
refinement parameters for all of the samples have been given in
Table S2. This provides valuable information regarding the
lattice parameters, unit cell volume, packing coefficient, phase
percentage of a phase, etc., which are tabulated in Table S3.
The cell parameters of both the MW-ST- and HOT-I-
synthesized samples have been affected as a result of anion
58
was calculated using the modified form of eq 3.9.
rA + reffective(X)
t =
2
[rB + reffective(X)]
(3.9)
where reffective is the radius
yr + (3 − y)r
Br
X
reffective(X) =
3
(3.10)
The tolerance factor of the CsPbBr3−yXy materials was
calculated, and it is observed that the tolerance factor increases
as the concentration of Cl increases and decreases as the
−
substitution because of the increase in the ionic radii of Cl ,
−
−
−
Br , and I accordingly. The variation in the lattice parameters
of the orthorhombic phase obtained by Reitveld refinement
analysis of the MW-ST samples is plotted, as given in Figure
−
concentration of I increases. The experimentally calculated
packing coefficient is found to be inversely proportional to the
theoretically calculated tolerance factor. This implies that the
higher the packing density, the lower the tolerance factor,
which could be attributed to the ionic size of the anion
substitution. The interatomic distances between the halides in
the perovskite structure immensely affect the rotation of the
4
g. The experimentally calculated packing coefficients of the
MW-ST and HOT-I samples have been plotted, as shown in
Figure 4h. Increasing the chlorine concentration in
CsPbBr3−yCl leads to a decrease in the unit cell volume due
y
to lattice contraction, which further could be attributed to a
decrease in the packing coefficient compared to the pristine
MB-0 and HB-0 samples synthesized by the MW-ST and
HOT-I methods, respectively. Similarly, increasing the iodine
concentration in CsPbBr3−yIy leads to an increase in the unit
cell volume due to lattice expansion, which could be attributed
to an increase in the packing coefficient compared to the
pristine MB-0and HB-0 samples. The MW-ST-synthesized
samples exhibit a distinct increase in the packing coefficient
compared to the HOT- I-synthesized samples, which could be
due to the effective preservation of the stoichiometry in the
MW-ST samples. Here, we also successfully related the packing
coefficient with the Goldschmidt tolerance factor (t), which is
a reliable empirical index to predict the stability and distortion
4−
2+
[
PbX6] octahedra. Moreover, the Pb ion is placed in a
59
slightly off-centered position because of the 6s lone pair. This
causes a distortion in the perovskite, which deviates from the
orthogonal geometry of the ideal perovskite crystal structure.
This leads to the exceptional photophysical properties of the
CsPbX PNCs resulting from the different orientations of the
PbX6] octahedra in the PNCs. The formability of [PbX6]
3
4−
4−
[
octahedra can be explained using the octahedral tilting factor
μ) determined by eq 3.11.
(
μ = r
B
rX
(3.11)
If μ < 0.41, it implies that the B cation is very small and its
effective coordination would involve an overlap between the
halide anions, and thus such a compound would not exist. The
octahedral tilting factor of the CsPbBr3−yXy materials was
calculated, and it is observed that the octahedral tilting factor
also increases as the concentration of Cl increases and
decreases as the concentration of I increases, as given in Table
57
of perovskite crystal structures. Generally, materials with a
tolerance factor in the range of 0.9−1.0 have an ideal cubic
structure, whereas corner-sharing perovskites can be formed in
57
−
the range of 0.8 < t < 1.0. A tolerance factor ranging from
.71 to 0.9 leads to a distorted perovskite structure having
−
0
tilted octahedra. For the ABX -type perovskites, the Gold-
4.
3
schmidt tolerance factor can be calculated by the ionic radii of
the atoms from eq 3.8:
The packing coefficient calculated experimentally also agrees
with the octahedral factor obtained theoretically. The
H
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Figure 5. Raman spectra of (a) MW-ST-synthesized and (b) HOT-I-synthesized CsPbX PNCs. (c) Influence of the lead−halogen bond length in
3
Raman frequencies.
octahedral tilting angle (Br−Pb−Br bond angle) of the MW-
of TO phonon modes (TO , TO , and TO ) positioned at (46,
1 2 3
−
1
−1
−1
ST-synthesized pristine CsPbBr sample, as calculated from the
68) cm , (69, 88) cm , and (123, 140) cm for the MB-0
3
−
−1
−1
−1
Rietveld refinement results, is 7.264°, whereas the Cl -
substituted MC-3 sample shows an increased octahedral tilt
of 7.385° and the I -substituted MI-3 sample shows a
decreased octahedral tilt of 7.21°, as represented in Figure
sample and (45, 65) cm , (67, 92) cm , and (121, 138) cm
for the HB-0 sample. The assignments of the Raman bands for
the other samples synthesized by both the MW-ST and HOT-I
−
methods are also listed in Table 5. LO , LO , and LO are
1 2 3
4
i. This shows that the ionic size of the halide ions substituted
in the pristine CsPbBr PNCs plays a vital role in tuning the
structural properties of all-inorganic PNCs.
Table 5. Raman TO Phonon Modes of Both the MW-ST-
and HOT-I-Synthesized Samples
3
Room-temperature Raman spectroscopy analysis was
performed for both the MW-ST- and HOT-I-synthesized
sample
TO (cm−1)
TO (cm−1)
TO (cm−1)
1
2
3
MC-3
MC-2
MC-1
MB-0
MI-1
MI-2
MI-3
HC-3
HC-2
HC-1
HB-0
HI-1
(54, 73)
(51, 72)
(47, 69)
(46, 68)
(45, 67)
(44, 66)
(43, 64)
(52, 69)
(48, 68)
(47, 67)
(45, 65)
(44, 65)
(43, 64)
(42, 63)
(75, 96)
(74, 93)
(75, 92)
(69, 88)
(67, 86)
(67, 83)
(67, 86)
(79, 99)
(76, 97)
(72, 95)
(67, 92)
(66, 90)
(66, 85)
(65, 82)
(130, 156)
(135, 159)
(133, 164)
(123, 140)
(118, 139)
(104, 124)
(101, 123)
(130, 154)
(129, 159)
(127, 150)
(121, 138)
(119, 142)
(109, 131)
(106, 124)
samples. The obtained Raman studies of the CsPbX PNCs
3
−
1
reveal that the vibrational peaks ν (60−90 cm ) and ν
1
2
−
1
(
[
109−164 cm ) are related with the vibrational mode of the
PbX ] octahedron and motion of cesium cations, respec-
6
6
0
tively. Parts a−c of Figure 5 show that, upon an increase in
−
the concentration of Cl in the CsPbX PNCs, a continuous
3
blue shift of the Pb−Cl vibrational peaks occurs because of the
decrease in the bond length of the Pb−Cl bond, leading to a
crystal structure contraction. Contrastingly, increasing the
−
concentration of I exhibits a continuous red shift of the Pb−I
vibrational peaks due to an increase in the bond length of the
Pb−I bond, resulting in a crystal structure expansion, whereas
the profile of Raman spectra stays in the original state for both
cases. These lattice contractions and expansions are well-
corroborated with the XRD results, as observed by a similar
peak shift depending on the concentration of the halide ions in
HI-2
HI-3
absent for all CsPbX PNCs because of overlap of the LO
modes with their strong TO modes. Only the TO mode could
be attributed to the [PbX ] octahedral vibrations, whereas the
motion of cesium cations can be represented by the TO and
3
the CsPbX PNCs synthesized by both the MW-ST and HOT-
3
2
I methods.
6
The halide-dependent structural variations may perhaps
reduce the space group symmetry caused by condensation of
the zone boundary phonons that correspond to the [PbX6]
1
63
TO modes.
3
3.5. Anion-Induced Optical Studies. The optical and
electronic properties were observed by the UV−vis absorption
and DRS measurements. The absorbance spectra of the MW-
ST- and the HOT-I-synthesized samples show a visible blue
shift of the absorption peak with an increase in the
concentration of Cl but a red shift with an increase in the
concentration of I , as represented in Figure 6a,b. This
behavior could be corroborated with lattice contraction of the
perovskite due to an increase in the Cl concentration and
lattice expansion due to an increase in the I concentration.
octahedral rotations about the high-temperature cubic axes and
61
alter the number of CsPbX formulas per unit cell. As
3
observed by the XRD studies, the CsPbX PNCs predom-
3
inantly crystallize in the orthorhombic phase, which is
described by the D2h point group. The degeneracy of the
unit cell is totally removed owing to the one-dimensionality of
all of the D2h representations, resulting in a phonon spectrum
consisting of three nondegenerate longitudinal-optical (LO)
phonon modes and three doubly degenerate transverse-optical
−
−
−
−
(
TO) phonon modes, derived from the Lyddane−Sachs−
These lattice variations might contribute to the band-gap
deviation, which could be understood by DRS studies. The
60,62
Teller relationship.
Thus, we are able to identify three sets
I
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Figure 6. (a and b) UV−vis spectra of the HOT-I- and MW-ST-synthesized CsPbX PNCs, respectively. (c−f) Tauc plots of the HOT-I- and MW-
3
ST-synthesized samples. (g) Comparison of the band gaps of the CsPbX PNCs synthesized by the MW-ST and HOT-I methods. (h) Molecular
3
orbital diagram of CsPbX showing the CB and VB energy levels. (i) Schematic illustration of the energy transfer of excitons to recombination
3
centers, providing tunable emission.
6
4
compression deviates the lattice structure of CsPbX PNCs,
(Figure 6h). The conduction band (CB) is obviously more
dispersive compared to the valence band (VB) because of the
highly delocalized 6p orbitals, and the CBM is composed of a
3
consequently redefining the boundary conditions for the
electronic wave functions, and the optoelectronic properties
are certainly impacted, as reinforced by band-gap narrowing.
+
dominant Pb 6p and a minor X np character. The Cs cations
65
The band gaps of CsPbX PNCs were obtained by extending
do not directly contribute to the electronic band edges, but
they influence the crystal structure and vary the band gap
indirectly, signifying that the photophysical properties of
CsPbX are predominantly determined by PbX octahedra. As
the concentration of Cl is increased, the shorter bond length
of Pb−Cl improves the coupling between halide 3p and Pb 6s
orbitals, which intensifies the band dispersion and uplifts the
VBM to higher energy levels. On the other hand, upon
increasing the concentration of I , the longer bond length of
the Pb−I bond leads to an ineffective coupling between halide
5p and Pb 6s orbitals, which decreases the band dispersion and
downshifts the VBM to lower energy levels. Contrastively, the
3
2
the linear portion of the (αhν) versus hν curve in direct-band-
gap Tauc plots (Figure 6c−f), where α denotes the absorption
coefficient, d denotes the thickness of the sample, and hν is the
photon energy. Generally, in semiconductors, the valence-band
maximum (VBM) and conduction-band minimum (CBM)
emerges from the bonding and antibonding orbital pairs.
Therefore, the energy gap varies between the VBM and CBM
according to deviations in the size of the perovskite lattice.
Contrastively, the band-edge states of the all-inorganic lead
halide perovskites are reversed compared to classical semi-
conductors, where the CBM and VBM are formed by the
cationic p and s orbitals, respectively. In the case of the
3
6
−
66
−
rotation and tilting of [PbX ] octahedra could affect the orbital
6
CsPbX PNCs, the VBM is formed majorly by the p orbital of
coupling and change the position of the VBM, consequently
altering the band gap. The CBM is in a typical nonbonding
localized state, and so it is almost insensitive to contraction or
3
the halide with minor antibonding contributions from the Pb
2
6
s orbital and the CBM is made up of empty Pb 6p orbitals
J
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Figure 7. (a and b) PL spectra of the MW-ST- and HOT-I-synthesized CsPbX PNCs, respectively, (c and d) TRPL spectra of the MW-ST and
3
HOT-I samples, respectively. (e and f) Chromaticity diagrams showing the color gamuts of the MW-ST and HOT-I samples, respectively. (g) MW-
ST samples under UV illumination. (h) Schematic of the LEDs fabricated with the MW-ST samples. (i) Blue, green, and red LEDs fabricated in our
laboratory.
expansion of the Pb−X bond. Pb−X bond contraction or
Table 6. Optical Properties of CsPbX PNCs synthesized by
both MW-ST and HOT-I methods
3
expansion is instigated by the twists and tilts of PbX octahedra
6
due to the halide concentration. These variations consequently
contribute to the shift in the band gap, which provides a more
substantial influence on the electronic structure in this regime,
which can be observed in the Tauc plots, and it can also be
well-correlated to the peak shifts observed in the XRD patterns
upon moving from Cl to Br to I . Hence, the VB position is
comparatively sensitive to the halide composition, where the
valence atomic orbitals change from 3p → 4p → 5p moving
down the periodic table. Consequently, lattice dilation
downshifts the VBM potential energy; meanwhile, the CBM
potential energy is slightly upshifted and, thus, the band gap of
the lead halide perovskite is decreased according to alteration
of the halogens from Cl to Br to I, which could be observed in
both the MW-ST- and HOT-I-synthesized PNCs, as given in
Figure 6g.
PL peak
absorbance peak
wavelength
(nm)
fwhm
(nm)
QY
lifetime
(ns)
sample wavelength (nm)
(%)
MC-3
MC-2
MC-1
MB-0
MI-1
MI-2
MI-3
HC-3
HC-2
HC-1
HB-0
HI-1
440
460
476
498
509
520
533
443
472
494
501
506
521
534
449
474
487
502
517
528
547
453
485
495
512
524
534
545
30.99
40.17
37.66
26.57
30.81
27.12
36.36
17.07
22.74
22.67
22.15
32.52
29.77
26.56
43.44
83.91
74.21
88.2
59.83
21.1
10.20
12.29
13.48
19.61
11.85
19.58
26.03
17.84
14.19
25.89
16.14
22.51
27.00
25.51
−
−
−
17.9
29.57
33.92
32.71
91.02
65.25
45.28
22.75
The optical characteristics of CsPbX PNCs obtained via the
HI-2
HI-3
3
MW-ST and HOT-I methods were examined by PL measure-
ments at an excitation of 358 nm, which showed tunable PL
emission wavelength from 449 to 547 nm for the MW-ST
samples and from 453 to 545 nm for the HOT-I samples by
adjusting the halide composition, as given in Figure 7a,b. As
mentioned in Table 6, the MW- ST-synthesized CsPbBr3
shows an absorbance at 502 nm, which is red-shifted to 517,
528, and 547 nm with an increase in the I concentration,
whereas the absorbance is blue-shifted to 487, 474, and 449
−
K
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−
nm with an increase in the Cl concentration. This shift shows
a relationship with the XRD patterns, which exhibit a steady
shift of the (121) and (202) planes toward higher angles and
color gamut of both the MW-ST- and HOT-I-synthesized
CsPbX PNCs. The as-synthesized CsPbX PNCs by the MW-
ST method also exhibited high PLQYs and narrow emission
line widths (fwhm), and therefore the PNCs were used as
color-conversion materials for the fabrication of LEDs. We
3
3
−
−
lower angles, implying the substitution of Cl with Br and the
substitution of I with Br , respectively. As the concentration
of Cl increases, there is a shrinking of the [PbX ] octahedra of
the CsPbX PNCs and their band gap is broadened, thus
−
−
−
6
fabricated LEDs by spin coating the MW-ST CsPbX colloidal
3
3
solutions into precleaned glass substrates, as shown in the
schematic representation of the LEDs fabricated in our
laboratory (Figure 7h). This coating served as phosphors for
obtaining blue, green, and red emissions. The coated substrates
were mounted on the commercial 456 nm UV LEDs, and then
the films were pumped by the LEDs below, to achieve efficient
blue, green, and red emissions, as presented in Figure 7i,
through a direct-current process. This would provide the
possibility for large-scale practical applications of all-inorganic
PNCs in optoelectronics and photovoltaics, which could be
achieved by the energy-efficient, high-throughput MW-ST
resulting in blue-shifted PL. In the same way, as the
concentration of I− increases, there is a dilation of the
[
PbX ] octahedra of the CsPbX PNCs and their band gap is
6
3
narrowed, thus resulting in red-shifted PL. PL measurement of
the HOT-I-synthesized samples also revealed similar results.
These chemical and structural variations of the CsPbX PNCs
act as trap-facilitated recombination spots for photoexcited
charge carriers.
These trap states are energy levels within the band gap that
can be deep or shallow depending their position relative to the
3
6
7
band edges. The deep trap states that are away from the band
process for the synthesis of CsPbX PNCs.
3
68
edges contribute to the nonradiative recombination. The
dominant trap states that are situated in the shallow energy
region close to the CBM and VBM inhibit the recombining of
the electron−hole pair, leading to a radiative recombination.
The PLQYs of the pristine CsPbBr PNCs prepared by both
the MW-ST and HOT-I methods are higher compared to the
corresponding mixed halide CsPbX PNCs, which can be
attributed to the overall decrease in the rate of nonradiative
4. CONCLUSION
In summary, CsPbX PNCs have been synthesized by a rapid,
3
68
single-step, high-throughput MW-ST approach at a temper-
ature of 180 °C in 6 min, without requiring any vacuum/inert
gas atmosphere compared to a conventional HOT-I method
requiring a longer reaction time (3 h). The effect of the
3
3
recombination within the CsPbBr PNCs. The full-widths at
reaction conditions in the MW-ST synthesis of CsPbX PNCs
3
3
half-maximum (fwhm) of the MW-ST- and HOT-I-synthe-
sized samples were found to be between 26 and 40 and
between 17 and 32 nm, respectively. The optical properties are
slightly enhanced in the HOT-I-synthesized samples because
of their smaller size distribution, which is evident from the
narrower fwhm compared to that of the MW-ST samples,
which reaches up to 40 nm. This can also be due to the
formation of slightly bigger NCs with high crystallinity, caused
by rapid heating during the MW-ST reaction, compared to the
HOT-I samples, revealing smaller NCs with lesser crystallinity,
as is evident from the XRD results. The PLQYs of both the
MW-ST and HOT-I samples are high, up to 90%.
was optimized by varying the synthesis temperature from 100
to 220 °C, with a power ranging from 400 to 800 W, and
subsequently high-quality PNCs with outstanding photo-
physical properties were also systematically investigated. The
influence of microwave exposure on the chemistry at the
interfaces was quantitatively analyzed by the kinetics and
thermodynamics of the MW-ST reaction, and it was observed
that the activation free energy is approximately 13 kJ less than⧧
that of a conventional HOT-I synthesis. The decreased ΔG
⧧
and increased ΔS of the MW-ST synthesis were ascribed to
changes in the relative energies of individual reactive species of
the solid-phase (PbX ) and liquid-phase (Cs-Oleate, OA, and
2
The TRPL spectra of both the MW-ST- and HOT-I-
synthesized samples were measured to define the average
OLA) reactants, at their interfaces. We have also systematically
tuned the ratio of anions, which provided a major contribution
for deviations in the crystal lattice parameters of the CsPbX3
PNCs due to the PbX6 octahedral tilting and twisting.
Deviations in the metal−halide bond length resulted in a
systematic crystal structure disparity caused by the anion
substitution was also confirmed by the Raman studies. It was
also corroborated by Tauc plots that the lattice dilation
downshifts the VBM potential energy, while the CBM
potential energy is slightly upshifted, consequently modifying
lifetime (τ ) of electron−hole pairs, which is given in Figure
avg
7
c,d. The PL decay curves were triexponentially fitted and the
average lifetime (τ ) of the samples were calculated using eq
avg
3
.12
τavg
where B , B , and B are the relative amplitudes of the decay
2
2
2
B τ + B τ + B τ
1
1
2 2
3 3
=
B τ + B τ + B τ
(3.12)
1
1
2 2
3 3
the band gap of the CsPbX PNCs with alteration of the
1
2
3
3
−
−
−
factors, whereas τ , τ , and τ are the respective decay times.
halides from Cl to Br to I . Hence, these studies proved the
excellent correlation of the structural parameters with the
electronic as well as optical properties. Moreover, this facile,
high-throughput, cost-effective, energy-efficient approach paves
1
2
3
The average lifetimes were found to range from 10 to 26 ns for
the MW-ST samples and from 14 to 27 ns for the HOT-I
samples. The TRPL spectra of the MW-ST sample measure-
ments exhibit triexponential decays similar to those of the
HOT-I method, which is different from the multiexponential
decay traces observed in the conventional (domestic oven)
a new avenue to the synthesis of anion-tuned CsPbX PNCs
3
with excellent optical properties for scale-up manufacturing of
next-generation electronic displays.
microwave-irradiated synthesis of CsPbX NCs reported
3
3
1,52
previously.
ASSOCIATED CONTENT
■
The color gamut of both the MW-ST- and HOT-I-
synthesized CsPbX3 PNCs was identified using a CIE
calorimetry system, as shown in Figure 7e,f. The chromaticity
diagram revealed that there is no notable difference in the
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00294.
L
https://dx.doi.org/10.1021/acs.inorgchem.0c00294
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Synthesis procedure for cesium oleate, details of the
pubs.acs.org/IC
Article
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Temperature and Gram-Scale Synthesis of CsPbX (X = Cl, Br, I)
3
XRD refinement parameters, lattice parameter plot for
the HOT-I samples, table containing Rietveld refine-
ment parameters, table containing crystallographic
parameters, Rietveld graphs of the all of the other
samples, table containing orthorhombic peaks and d-
spacing of all of the samples, band-gap calculation of
each samples, and CBM and VBM calculation of all of
the samples (PDF)
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AUTHOR INFORMATION
■
Corresponding Author
(8) Shamsi, J.; Dang, Z.; Bianchini, P.; Canale, C.; Di Stasio, F.;
Brescia, R.; Prato, M.; Manna, L. Colloidal Synthesis of Quantum
Confined Single Crystal CsPbBr3 Nanosheets with Lateral Size
Control up to the Micrometer Range. J. Am. Chem. Soc. 2016, 138,
Arumugam Vadivel Murugan − Advanced Functional
Nanostructured Materials Research Laboratory (AFNM Lab),
Centre for Nanoscience and Technology (CNST), Madanjeet
School of Green Energy Technologies, Pondicherry University (A
Central University), Kalapet, Puducherry 605014, India;
orcid.org/0000-0003-4601-9615; Phone: 91-413-
7
(
240−7243.
9) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.;
Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V.
Nanocrystals of Cesium Lead Halide Perovskites (CsPbX , X = Cl, Br,
and I): Novel Optoelectronic Materials Showing Bright Emission with
3
2
654975; Email: avmrajeshwar@gmail.com,
avmurugan.nst@pondiuni.edu.in
Wide Color Gamut. Nano Lett. 2015, 15, 3692−3696.
(10) Guhrenz, C.; Benad, A.; Ziegler, C.; Haubold, D.; Gaponik, N.;
Author
Eychmu
̈
ller, A. Solid-State Anion Exchange Reactions for Color
Kabalaraj Thesika − Advanced Functional Nanostructured
Materials Research Laboratory (AFNM Lab), Centre for
Nanoscience and Technology (CNST), Madanjeet School of
Green Energy Technologies, Pondicherry University (A Central
University), Kalapet, Puducherry 605014, India
Tuning of CsPbX Perovskite Nanocrystals. Chem. Mater. 2016, 28,
3
9
(
033−9040.
11) Nedelcu, G.; Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.;
Grotevent, M. J.; Kovalenko, M. V. Fast Anion-Exchange in Highly
Luminescent Nanocrystals of Cesium Lead Halide Perovskites
(
CsPbX , X = Cl, Br, I). Nano Lett. 2015, 15, 5635−5640.
3
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00294
(
12) Tong, Y.; Bladt, E.; Ayguler, M.; Manzi, A.; Milowska, K.;
̈
Hintermayr, V.; Docampo, P.; Bals, S.; Urban, A.; Polavarapu, L.;
Feldmann, J. Highly Luminescent Cesium Lead Halide Perovskite
Nanocrystals with Tunable Composition and Thickness by Ultra-
sonication. Angew. Chem., Int. Ed. 2016, 55, 13887−13892.
(13) Lignos, I.; Stavrakis, S.; Nedelcu, G.; Protesescu, L.; deMello,
A. J.; Kovalenko, M. V. Synthesis of Cesium Lead Halide Perovskite
Nanocrystals in a Droplet-Based Microfluidic Platform: Fast Para-
metric Space Mapping. Nano Lett. 2016, 16, 1869−1877.
Author Contributions
K.T. designed the concept and carried out the experiments,
analyzed the results, and discussed the manuscript preparation.
A.V.M. supervised the work.
Notes
The authors declare no competing financial interest.
(14) Zhang, J.; Fan, L.; Li, J.; Liu, X.; Wang, R.; Wang, L.; Tu, G.
Growth mechanism of CsPbBr Perovskite Nanocrystals by a Co-
3
ACKNOWLEDGMENTS
■
Precipitation Method in a CSTR System. Nano Res. 2019, 12, 121−
The authors express their gratitude to the funding agency:
Department of Science and Technology (DST), Government
of India, New Delhi, India, and Clean Energy Research
Initiative (CERI) Program under Project DST/TM/CERI/C-
127.
(15) Chen, M.; Zou, Y.; Wu, L.; Pan, Q.; Yang, D.; Hu, H.; Tan, Y.;
Zhong, Q.; Xu, Y.; Liu, H.; Sun, B.; Zhang, Q. Solvothermal Synthesis
of High-Quality All-Inorganic Cesium Lead Halide Perovskite
Nanocrystals: From Nanocube to Ultrathin Nanowire. Adv. Funct.
Mater. 2017, 27, 1701121.
2
64(G) for financial support. K.T. particularly thanks the
Central Instrumentation Facility, Pondicherry University, for
material characterizations and Tulsi Satyavir Dabodiya (Ph.D.
Research Scholar, AFNM Lab, CNST, Pondicherry University
for helping Rietveld refinement and Raman studies).
(16) Wang, Y.; Sun, X.; Shivanna, R.; Yang, Y.; Chen, Z.; Guo, Y.;
Wang, G.; Wertz, E.; Deschler, F.; Cai, Z.; Zhou, H.; Lu, T.; Shi, J.
Photon Transport in One-Dimensional Incommensurately Epitaxial
CsPbX Arrays. Nano Lett. 2016, 16, 7974−7981.
3
(17) Liu, Y.; Guo, M.; Dong, S.; Jiao, X.; Wang, T.; Chen, D. Room
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