A Room-Temperature Hybrid Lead Iodide Perovskite Ferroelectric

Article abstract of DOI:10.1021/jacs.8b08286

Organic-inorganic hybrid perovskite, [CH₃NH₃]PbI₃, holds a great potential for next-generation solar devices. However, whether the ferroelectricity exists in [CH₃NH₃]PbI₃ and results in the ultrahigh performance is not at present clear. Beyond that, no hybrid lead iodide perovskite ferroelectric has yet been found. Here, using precise molecular modifications, we successfully designed a room-temperature hybrid perovskite ferroelectric, [(CH₃)₃NCH₂I]PbI₃. Because of the high-symmetry and nearly spherical shape of [(CH₃)₄N]⁺ cation, [(CH₃)₄N]PbI₃ crystallizes in a centrosymmetric space group P6₃/m at room temperature and undergoes a structural phase transition at 184 K. Accompanied by the introduction of halogen atoms on the cation from F to I, the phase transition temperature gradually increases to 312 K and the space group transforms into a polar C2 at room temperature. The strongest halogen bond energy of [(CH₃)₃NCH₂I]-I and the largest volume of [(CH₃)₃NCH₂I]⁺ among these compounds might be possible reasons for the stabilization of ordered [(CH₃)₃NCH₂I]⁺ cation array and further reservation of its ferroelectricity at relatively high temperature. This work provides an efficient molecular design strategy toward the targeted harvest of room-temperature organic-inorganic perovskite ferroelectrics, and should inspire further exploration of the interplay between structure and ferroelectricity. The discovery of lead iodide perovskite ferroelectric also offers a foothold to the possibility for the existence of ferroelectricity in [CH₃NH₃]PbI₃.

Full text of DOI:10.1021/jacs.8b08286

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Article
A Room-Temperature Hybrid Lead Iodide Perovskite Ferroelectric
Xiu-Ni Hua, Wei-Qiang Liao, Yuan-Yuan Tang, Peng-Fei Li, Ping-Ping Shi, Dewei Zhao, and Ren-Gen Xiong
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08286 • Publication Date (Web): 05 Sep 2018
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A Room-Temperature Hybrid Lead Iodide Perovskite Ferroelectric
†
,‡
‡
‡
†
‡
Xiu-Ni Hua, Wei-Qiang Liao,* Yuan-Yuan Tang, Peng-Fei Li, Ping-Ping Shi, Dewei Zhao,
,
†,‡
and Ren-Gen Xiong*
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†Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing
11189, P. R. China.
2
‡Ordered Matter Science Research Center, Nanchang University, Nanchang 330031, P. R. China.
Supporting Information
ABSTRACT: Organic-inorganic hybrid perovskite, [CH
devices. However, whether the ferroelectricity exists in [CH
at present clear. Beyond that, no hybrid lead iodide perovskite ferroelectric has yet been found. Here, using precise mo-
lecular modifications, we successfully designed a room-temperature hybrid perovskite ferroelectric, [(CH NCH I]PbI
crystalizes in a centrosym-
/m at room temperature and undergoes a structural phase transition at 184 K. Accompanied by
3
NH
3
]PbI
3
, holds a great potential for next-generation solar
3
NH
3
]PbI
3
and results in the ultrahigh performance are not
3
)
3
2
3
.
+
Due to the high-symmetry and nearly spherical shape of [(CH
metric space group P6
3
)
4
N] cation, [(CH
3
)
4
N]PbI
3
3
the introduction of halogen atoms on the cation from F to I, the phase transition temperature gradually increases to 312
K and the space group transforms into a polar C2 at room temperature. The strongest halogen bond energy of
+
[(CH
3
)
3
NCH
2
I]-I and the largest volume of [(CH
3
)
3
NCH
2
I] among these compounds might be possible reasons for the
+
stabilization of ordered [(CH
3
)
3
NCH I] cation array and further reservation of its ferroelectricity at relatively high tem-
2
perature. This work provides an efficient molecular design strategy toward the targeted harvest of room-temperature
organic-inorganic perovskite ferroelectrics, and should inspire further exploration of the interplay between structure
and ferroelectricity. The discovery of lead iodide perovskite ferroelectric also offers a foothold to the possibility for the
existence of ferroelectricity in [CH
3
NH
3
]PbI .
3
only in one of the ten polar point groups: 1 (C
(C1h), mm2 (C2v), 4 (C ), 4mm (C4v), 3 (C ), 3m (C3v), 6
(C ) and 6mm (C6v). A molecular design approach is con-
1
), 2 (C
2
), m
INTRODUCTION
4
3
The remarkably versatile compositions and structures
of perovskites (A and B are different metal cations, X is
an anion) render a wide range of fascinating physical
properties, including ferroelectricity, piezoelectricity,
dielectricity, high-temperature superconductivity and
6
sequently proposed for addressing the issue of introduc-
ing ferroelectricity into OHPs, since subtle alternation of
a molecular structure will have direct influence on the
crystal structures and properties. In contrast to their
rigid inorganic counterparts, OHPs have high tunability
through the halides and organic and metal cations, so
their structural, electrical, and optical properties can be
easily tuned and modulated by facile chemical modifica-
tion, building up a promising platform for the design and
1
colossal magnetoresistance. Organic-inorganic hybrid
perovskites (OHPs), in which the A-site is occupied by
organic component, have been known as an important
branch of perovskite materials over more than 100 years.
The past decade has witnessed the arise of “perovskite
fever”, when such great enthusiasm to the study of OHPs
5
optimization of diverse functional materials. By select-
extended the applications ranging to varied electronic,
ing polar and dynamic organic cations, the possibility of
orientational disorder and polarization, or even bulk
ferroelectric ordering, can be smoothly induced. Moreo-
ver, especially noteworthy is that both organic compo-
nent and inorganic framework have templating influence
on each other, since the latter will affect the ordering,
orientation, and conformation of the former, and like-
wise, the former may distort the latter. It enables a cer-
tain degree of control over the overall structural charac-
teristics and properties of the material.
2
photovoltaic and optoelectronic devices. [CH
3
NH
3
]PbI ,
3
a prototype of OHPs, is attracting worldwide interest due
to the extraordinary light-harvesting property and high
3
efficiency above 20%. The polar nature and order-
+
disorder transition of [CH
3
NH ] cations raise the possi-
3
bility of ferroelectricity that is supposed to account for
the superior performance in solar cells. Nevertheless, so
far, experimental efforts to get the direct proof of the
4
ferroelectric property yield only contradictory results.
Fortunately, thanks to the unusual degree of structure-
Under precise molecular modifications or tailoring,
even those non-polar candidates can be endowed with
property flexibility, beyond the [CH
3
NH
3
]PbI are count-
3
less unexplored candidates for new ferroelectric OHPs.
What is urgently needed is to find the key to this treasure,
that is, developing useful constructing avenues.
ferroelectricity. [(CH
tris(μ -iodo)-lead, (TM)PbI
rived from [CH NH ]PbI , where the three-dimensional
(3D) symmetry is disrupted to 1D by replacing the cation
3
)
4
N]PbI
3
(tetramethylammonium
2
3
), is an example of OHP de-
3
3
3
According to the Neumann’s Principle, crystal structure
controls properties, including ferroelectricity that occurs
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Scheme 1. The space groups at room temperature and phase transition temperatures for compounds (TM)PbI ,
3
(
TMFM)PbI
3
, (TMCM)PbI
3
, (TMBM)PbI
3
and (TMIM)PbI , respectively.
3
with an excessively large one. The high-symmetric and
stantially exceptional and cost-effective for large-scale
ferroelectric applications.
+
nonpolar [(CH
space group P6
3
)
4
N] cation triggers a centrosymmetric
3
/m, disallowing the possibility of high-
temperature ferroelectricity, while its disorder-to-order
change is responsible for the structural phase transition
at 184 K. To change this, we proposed the idea of incor-
porating more electronegative species like halogen atoms
on the cation to induce molecular dipole and asymmetry.
RESULTS AND DISCUSSION
(
TM)PbI
and (TMIM)PbI
by slow evaporation of a clear HI solution containing
stoichiometric amounts of PbI and corresponding or-
3
, (TMFM)PbI
3
,
(TMCM)PbI
3
, (TMBM)PbI
3
3
were prepared as light yellow crystals
2
Three new 1D OHPs, [(CH
romethylammonium tris(μ
(CH NCH Cl]PbI (trimethylchloromethylammonium
tris(μ -iodo)-lead, (TMCM)PbI and
(CH NCH Br]PbI (trimethylbromomethylammonium
tris(μ -iodo)-lead, (TMBM)PbI ) were therefore obtained.
The structural modifications raise the potential energy
3
)
3
NCH
2
F]PbI
3
(trimethylfluo-
ganic iodide salts. Their phase purities were confirmed
by powder X-ray diffraction (PXRD) (Figure S1) and
infrared (IR) analysis (Figure S2). Infrared characteris-
tic peak of the C−F bond from TMFM cation, the C−Cl
bond from TMCM cation, the C−Br bond from TMBM
cation, and the C−I bond from TMIM cation was clearly
2
-iodo)-lead, (TMFM)PbI
3
),
[
3
)
3
2
3
2
3
)
[
3
)
3
2
3
2
3
-1
-1
-1
shown at around 1090 cm , 806 cm , 702 cm , and 645
+
barrier of the tumbling motion of [(CH
the phase transition temperatures of (TMFM)PbI
(TMCM)PbI and (TMBM)PbI are increased to 189, 269
3
)
4
N] cation, so
-1
cm , respectively.
3
,
We then directly checked the phase transition behav-
iors by differential scanning calorimetry (DSC) meas-
urements (Figure 1a). (TM)PbI was found to exhibit a
low-temperature phase transition at around 184 K with
an obvious endothermic peak at this temperature upon
3
3
and 291 K, respectively. However, because the F, Cl and
Br atoms are not large or heavy enough to make such
3
+
tumbling motion freezing, the [(CH
3
)
3
NCH
2
Cl] and
+
[
(CH
lar to the original [(CH
cancelled molecular dipoles, (TMFM)PbI
and (TMBM)PbI , disappointingly, adopt the centro-
symmetric space group P6 /m and P6 /mmc at room
3
)
3
NCH
2
Br] cations still show severe disorder simi-
heating. (TMFM)PbI shows a little higher phase transi-
3
+
3
)
4
N] cation. As a result of the
, (TMCM)PbI
tion temperature at around 189 K after one H atom of
TM cation substituted by a halogen atom F. When re-
placing one H atom of TM cation with a halogen atom Cl,
3
3
3
3
3
the phase transition temperature of (TMCM)PbI in-
3
temperature, respectively. By way of analogy, tying a
feather to the balloon is not enough to hinder its rise or
move, only tying a rock could work. Hence, upon pro-
ceeding down group VIIA, we noted the I atom that is
much heavier than F, Cl and Br, and successfully de-
creases significantly to 269 K, but it is still below room
temperature. When further substituting the H atom of
TM cation by one halogen atom Br, the phase transition
of (TMBM)PbI occurs near room temperature at 291 K.
3
The phase transition temperature was then further suc-
cessfully increased above room temperature in
signed a ferroelectric OHP, [(CH
3
)
3
NCH
2
I]PbI
3
(trime-
thyliodomethylammonium
tris(μ
2
-iodo)-lead,
(TMIM)PbI
another halogen atom I. As shown in the DSC curves of
TMIM)PbI , one small endothermic peak at around 312
3
by replacing an H atom of TM cation with
(
TMIM)PbI ). As expected, the TMIM cation is well or-
3
dered at room temperature, the alignment of molecular
dipoles does give rise to a polar space group C2, and the
ferroelectric phase transition occurs at 312 K with an
Aizu notation of 2/mF2. By exploring many possibilities
for systematically determining structure-property rela-
tionships, this work opened up a fine design path toward
the targeted harvest of ferroelectric OHPs. And, the low-
temperature solution processability will make them sub-
(
3
K was observed in the heating run, indicating a second-
order phase transition. The phase transitions of these
compounds were also verified by the temperature-
dependent real part (εʹ) of complex dielectric permittivi-
ty (Figure 1b). Apparent dielectric anomalies were pre-
sented near the phase transition temperatures in all the
compounds.
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Figure 2. (a) Packing diagram of (TMIM)PbI at 298 K
along b axis, containing three nonequivalent TMIM cations.
The pink dashed lines stand for the I···I edges interactions.
Hydrogen atoms were omitted for clarity. Comparison of
3
packing views of (TMIM)PbI along [101] direction at (b)
298 K and (c) 318 K. The TMIM cations occupy a general
symmetry position at 298 K while a special position of the m
symmetry plane (blue dashed lines) at 318 K.
Figure 1. (a) DSC curves in a heating−cooling cycle and (b)
temperature dependence of the real part of complex dielec-
tric permittivity for polycrystalline samples of compounds
(TM)PbI
3
3
,
(TMCM)PbI
3
, (TMBM)PbI
3
and
Fortunately, compared with the high-symmetry cen-
(
TMIM)PbI
trosymmetric hexagonal space groups in (TM)PbI
(TMFM)PbI (TMCM)PbI and (TMBM)PbI
TMIM)PbI
ric monoclinic space group C2 at 298 K (Table S1),
which belongs to one of the 10 polar point groups, C
The crystal structure is comprised of three independent
3
,
3
,
3
3
,
Structurally, (TM)PbI
(TMBM)PbI
hexagonal ABX
3
, (TMFM)PbI
3
, (TMCM)PbI
3
,
(
3
adopts a low-symmetry noncentrosymmet-
3
are isostructural, having a
2
.
ganic cations occupy the cavities between adjacent
-
[
PbI
a and S3). Above phase transition temperature,
TM)PbI and (TMFM)PbI crystalize in the same hexag-
onal space group P6 /m and point group C6h at 298 K
Table S1). The TM and TMFM cations, lying on a spe-
cial position with 6/m symmetry, show highly orienta-
tional disorder (Figure S3). Resembling (TM)PbI and
(TMFM)PbI , (TMCM)PbI and (TMBM)PbI also crys-
talize in the hexagonal crystal system at 298 K, with the
different space group P6 /mmc and point group D6h
Table S1). The TMCM and TMBM cations situated on
the 6/mmm symmetry positions are severely disordered
Figure S3). Both the C6h and D6h point groups are cen-
trosymmetric, meaning that it is impossible for (TM)PbI
TMFM)PbI , (TMCM)PbI and (TMBM)PbI to have
3
]
n
chains of face-sharing PbI
6
octahedra (Figures
-
TMIM cations and infinite [PbI
quivalent face-sharing PbI octahedra running along [101]
direction (Figure 2a). The hexagonal ABX perovskite
structure in (TMIM)PbI is distinct from the cubic ABX
perovskite structure in [CH NH ]PbI , which contains
] cation and three-dimensional [PbI
work of corner-sharing PbI octahedra. Different from
the 6/m symmetry position of TM cation in (TM)PbI
and TMFM cations in (TMFM)PbI , as well as the
/mmm symmetry positions of TMCM cation in
TMCM)PbI and TMBM cations in (TMBM)PbI , each
3
]
n
chains of four none-
2
(
6
3
3
3
3
(
3
3
3
+
-
[CH
3
NH
3
3
] frame-
3
6
3
3
3
3
3
6
(
(
3
3
of three TMIM cations is located on a general position.
Consequently, the TMIM cation is ordered with one ori-
entation in the ferroelectric phase. As shown in Figure
2b, all the C−I bonds of TMIM cations point bottom
right or left. Such an alignment should result in a spon-
taneous polarization along b axis from the viewpoint of
polarization. Based on a point charge model, the sponta-
(
3
,
(
3
3
ferroelectricity above room temperature. We tried to
determine their structures below phase transition tem-
perature to check the space groups of low-temperature
phase. But we failed because of the very poor diffraction
data, induced by the common multiple twining in hybrid
neous polarization in the direction of b axis is estimated
2
to be about 0.67 μC/cm (Table S2). The PbI
6
octahedra
perovskites.
show weak distortion with the average trans I–Pb–I an-
gles of 171.83°, 175.25°, 172.73°, and 177.03° for Pb1, Pb2,
Pb3, and Pb4 center, respectively. The TMIM cation is
-
situated in the free space between adjacent [PbI
3 n
]
chains. Interestingly, the organic cation links the inor-
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Page 4 of 8
the utilization of the intermolecular interactions for the
development of new molecular crystals with desired
physical properties. A halogen bond occurs when there is
evidence of a net attractive interaction between an elec-
trophilic region associated with a halogen atom in a mo-
lecular entity and a nucleophilic region in another, or the
same, molecular entity. Due to the unique chemical na-
ture of halogen bonding, this intermolecular interaction
serves as an indispensable tool for the development of
organic-inorganic hybrid crystal systems. In the TMXM-
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gen···halogen interaction instead of hydrogen-bonding
interaction found in most of the organic-inorganic metal
halides, such as N−H···I hydrogen bonds in
[
CH
3
NH
3
]PbI . That is, the I atom of TMIM cation forms
3
-
C−I···I−Pb bond with the I atom of [PbI
3
] chain, with
n
average I···I distance of 3.624 Å.
At 318 K, (TMIM)PbI also crystalizes in a monoclinic
3
but centrosymmetric space group C2/m with the point
group of C2h (Table S1), corresponding to the paraelec-
PbI (X = F, Cl, Br and I) crystal system, all the halogen
3
tric phase. From the Aizu rule, (TMIM)PbI
3
is classified
as the 2/mF2 type ferroelectric. In the structure, the
PbI octahedra are still distorted, with a little increase in
0
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bonds tend to form at approximately 180° angles. The
strength of halogen bonds are found to be increased from
6
-
4
-3
8
.3*10 to 2.89*10 Hartree with increased atom num-
the average trans I–Pb–I angle of 177.72°. However, the
symmetry site of three nonequivalent TMIM cations
changes to a special one of m symmetry plane perpen-
dicular to b axis. Two of them are orientationally disor-
dered over two equivalent positions related by m sym-
metry plane (Figure 2c). Their C−I bonds point to the
bottom right and upper right of this plane, while the C−I
bond of other TMIM cation lies on the symmetry plane.
Thus, the dipole moment along b axis cancels each other
out, leading to the disappearing of spontaneous polariza-
tion.
ber (Figure 3). The strongest halogen bond energy
-
3
(
2.89*10 Hartree) in TMIM-I may be responsible for
the formation of ordered TMIM cation array in one-
dimensional PbI chains at room temperature phase. On
the other hand, compared with the molecular volume
3
3
3
(
Table 1) of TMFM (134.70 Å ), TMCM (141.27 Å ) and
3
TMBM (159.70 Å ), the much larger volume of TMIM
(182.01 Å ) could also be another possible reason for the
3
stabilization of ordered TMIM cation array and further
reservation of its ferroelectricity in such organic-
inorganic hybrid crystalline systems.
Figure 3. Calculated halogen bond energy in the system of
TMXM-I (X = F, Cl, Br and I).
Figure 4. Temperature-dependent SHG intensity of
Table 1. Structural parameters for TM cation series. (The
(
TMIM)PbI
3
,
revealing
a
noncentrosymmetric-to-
computed molecular volume is defined as the volume inside
3
centrosymmetric phase transition at around 312 K.
a contour of 0.001 electrons/Bohr density.)
The symmetry change accompanying the phase transi-
Mole-
cule
Bond
length (Å)
Dipole
(D)
Volume
(Å )
Sym-
metry
3
tion in (TMIM)PbI was further investigated by the
3
measurement of second harmonic generation (SHG) re-
sponse as a function of temperature, which is a reliable
method to detect the change of inversion symmetry. As
1
.08864 (C
TM
0
134.18
134.70
141.27
159.70
182.01
T
d
-
H)
1
.35551 (C
shown in Figure 4, (TMIM)PbI is SHG active with de-
3
TMFM
TMCM
TMBM
TMIM
3.1617
4.6531
6.7510
8.3246
C
s
C
s
C
s
C
s
-
F)
tectable intensities below 312 K, which indicates a non-
centrosymmetric phase, consistent with the ferroelectric
1
1
2
.77049 (C
C
2
point group having symmetry elements of E and C
The SHG intensity at 298 K is approximately 0.65 times
that of KH PO (KDP) (Figure S4). No SHG signal is
2
.
-
Cl)
.93731 (C
2
4
-
Br)
.16621 (C
I)
observable above 312 K due to the appearing of inversion
symmetry in the centrosymmetric C2/m (symmetry ele-
-
ments: E, C
2
, i, and σ ) space group. Thus, the SHG re-
h
sults combined with the structure analyses clearly reveal
that the phase transition around 312 K is from a noncen-
trosymmetric ferroelectric phase to a centrosymmetric
Halogen bonding effect is being exploited for an im-
portant element in crystal engineering, which involves
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paraelectric one. In addition, the continuous variation of
ages demonstrates the polarizations in the two adjacent
domains should be anti-parallel. Meanwhile, the ampli-
tude contrasts for the adjacent domains separated by
domain walls are very close, which provides a solid proof
for the existence of 180° domain structure, supporting its
crystal structure determination (2/mF2). When the ver-
tical and lateral PFM mappings were overlaid on three-
dimensional (3D) topography, it is clear that the piezore-
sponse has no obvious correlation with the local topology
of the sample surface, which excludes topographic cross-
talk and further demonstrates the presence of ferroelec-
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SHG intensity in the vicinity of 312 K suggests a second-
order phase transition feature, matching well with the
DSC results. Similar to that observed in the DSC cures of
(
TMIM)PbI , there is a small thermal hysteresis between
3
the heating and cooling runs, which is related to the
temperature changing rates of 10 K/min in the testing
process. We also examined the SHG response of
(
TM)PbI
3
, (TMFM)PbI
3
, (TMCM)PbI
3
and (TMBM)PbI
3
at room temperature. All of them are SHG inactive, con-
firming the centrosymmetric structure (Figure S4).
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tric polarization in the (TMIM)PbI thin film (Figure
3
S6).
Figure 6. Topographic images (top) and vertical PFM am-
plitude (middle) and phase (bottom) images of the
(
3
TMIM)PbI film surface. (a) Initial state. (b) After the first
Figure 5. Vertical and lateral PFM phase (a, c) and ampli-
tude images (b, d) for the (TMIM)PbI thin film. (e, f) Verti-
3
cal PFM phase and amplitude signals as functions of the tip
voltage for a selected point, showing local PFM hysteresis
loops.
switching with positive bias at +90 V. (c) After the succeed-
ing back-switching with negative bias at -90 V. The yellow
and blue regions in phase images indicate the polarizations
oriented upward and downward, respectively.
To explore the polarization switching behavior, local
PFM-based hysteresis loop measurement was performed
The most intrinsic property of ferroelectrics is that
they possess a spontaneous, stable and electrically
switchable polarization. Piezoresponse force microscopy
on the (TMIM)PbI thin film. As shown in Figure 5e,f,
3
the obvious 180° reversal of phase signal and the charac-
teristic butterfly loop of amplitude signal can be ob-
served on the film surface, which is a typical demonstra-
tion for the switching of ferroelectric polarization. Sub-
sequently, we carried out local poling test to visualize the
domain switching process (Figure 6). In the initial state,
the vertical PFM images of as-grown film show a distinct
domain pattern, just like a flying elephant (Figure 6a).
When a tip bias of +90 V was used to scan the whole re-
gion, the polarization directions of domains were com-
pletely switched downward, as the color contrast in the
phase image changed from yellow to blue and the do-
main walls disappeared in the amplitude image (Figure
6b). After a poling with opposite bias of -90 V on the tip,
the polarization direction of central region can also be
switched upward, demonstrating a reproducible polari-
zation switching process (Figure 6c). All the above evi-
(
PFM) is a powerful tool for addressing the statics and
dynamics of ferroelectric polarization, which can nonde-
structively visualize a microscale domain structure with
8
very high spatial resolution. Each PFM image contains
the amplitude and phase parameters that provide infor-
mation on the relative strength of the piezoelectric coef-
ficient and the polarization direction in each individual
domain, respectively. Figure S5 presents the vertical
PFM phase and amplitude images and the topographic
images for the (TMIM)PbI
3
thin film. The top half
demonstrates the classical lamellar domain structure
and the bottom half shows the stripe one, like that in
9
triglycine sulfate (TGS). For a more unambiguous visu-
alization of the domain structures, the vertical and lat-
eral PFM images have been simultaneously acquired and
analyzed in the left bottom area (Figure 6). Note that it
is almost the same for the domain patterns in the two
components. The 180° phase contrast in both phase im-
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dence unambiguously establish the presence of stable
and switchable polarization in the (TMIM)PbI thin film.
foothold to the possibilities for enriching the family of
room-temperature organic-inorganic perovskite ferroe-
lectrics.
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3
EXPERIMENTAL SECTION
Materials. Tetramethylammonium iodide (TM·I) was
commercially available. Trimethylfluoromethylammoni-
um iodide (TMFM·I) was synthesized from trime-
thylfluoromethylammonium chloride (TMFM·Cl), which
was first prepared by reacting equimolar amounts of
trimethylamine (30 wt % in water) with chlorofluoro-
methane in ethanol at room temperature. Then, excess
0
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Ag
2
CO was added to a TMFM·Cl aqueous solution with
3
stirring, resulting in the formation of AgCl precipitates.
After filtering, the clear solution was acidized by excess
HI solution. TMFM·I was obtained as colorless solid af-
ter removing the solvent under reduced pressure. For
trimethylchloromethylammonium iodide (TMCM·I) and
trimethylbromomethylammonium iodide (TMBM·I), the
synthetic procedure was similar to that of TMFM·I, with
the chlorofluoromethane replaced by dichloromethane
and dibromomethane, respectively. Trimethyliodome-
thylammonium iodide (TMIM·I) was directly synthe-
sized by the reaction of equimolar amounts of trimethyl-
amine (30 wt % in water) and diiodomethane in ethanol.
3
Figure 7. UV−vis absorption spectrum of (TMIM)PbI . The
inset shows the Tauc plot. The estimated band gap is 2.82
eV regarded as a direct band gap semiconductor.
The optical property of (TMIM)PbI was investigated
3
by the solid state UV-vis diffuse reflectance spectrum at
room temperature. As shown in Figure 7, the UV−vis
absorbance spectrum displays an intense absorption at
the band edge onset 470 nm (~2.64 eV), corresponding
well with its yellow appearance of bulk crystal. Consider-
ing the power law for the variation of absorption coeffi-
cient versus photo energy (Figure 7, inset), the feature
of direct band gap semiconductor can be concluded.
From the Tauc plot, the measured optical band gap for
the direct transition is 2.82 eV, slightly larger than initial
absorption photon energy of the band edge. Another sig-
nificant character of direct band gap is that a strong PL
Light yellow crystals of (TM)PbI
TMCM)PbI , (TMBM)PbI , and (TMIM)PbI
tained by slowly evaporating clear HI solutions contain-
ing equal molar amounts of PbI and TM·I, TMFM·I,
TMCM·I, TMBM·I, and TMIM·I, respectively.
TMIM)PbI shows excellent thermal stability (Figure S8)
and air stability (Figure S9). PXRD (Figure S1) and ele-
3
,
(TMFM)PbI
3
,
(
3
3
3
were ob-
2
(
3
mental analyses confirm the phase purity of (TMIM)PbI
(Table S3).
3
Measurements. PXRD, IR, DSC, dielectric, SHG and
10
PFM measurements were described elsewhere. For
(
photoluminescence) emission can be found with photon
PFM measurements, the samples of thin film crystals of
energy centered at the value of energy gap owing to the
high quantum efficiency of the radiative recombination
(TMIM)PbI
3
were prepared by spreading a drop (20 µL)
of the precursor solution containing 10 mg (TMIM)PbI
3
in direct band gap materials. In the case of (TMIM)PbI ,
3
in 1 mL N,N-dimethylmethanamide on a clean ITO-
coated glass substrate. Isolated thin film crystals were
grown by slowly evaporating the solvent.
an obvious PL emission was observed with photon ener-
gy centered at 2.33 eV (Figure S7), whose emission
maximum does not shift with different excitation wave-
lengths, indicates that the band gap type of (TMIM)PbI
is more likely to be direct.
3
UV-vis diffuse-reflectance spectra measurements were
performed at room temperature using a Shimadzu UV-
2
450 spectrophotometer mounted with ISR-2200 inte-
grating sphere operating from 200 to 900 nm. BaSO
was used as a 100% reflectance reference. Powdered
crystal of (TMIM)PbI was prepared for measurement.
CONCLUSION
4
In conclusion, we report a room-temperature organic-
inorganic hybrid perovskite ferroelectric,
(CH NCH I]PbI . The spherical [(CH
tends to exhibit dynamical disorder at room temperature,
allowing [(CH N]PbI crystalize in a centrosymmetric
space group P6 /m and undergo a structural phase tran-
3
+
The generated reflectance-versus-wavelength data were
used to estimate the band gap of the material by convert-
[
3
)
3
2
3
3
) N] cation
4
ing reflectance data to absorbance according to the Ku-
3
)
4
3
2
belka−Munk equation: F(R
∞
) = (1− R
∞
) /2R
∞
. Therefore,
3
the optical band gap can be determined by the variant of
the Tauc equation:
sition at relatively low temperature (184 K). Through
precise molecular modification by replacing a H atom
with a halogen atom from F to I, the phase transition
temperature gradually increases to 312 K and the space
group transforms into a polar C2 at room temperature.
(hν·F(R ))1/n = A(hν – E ), Where:
∞
g
h: Planck's constant, ν: frequency of vibration, F(R
∞
):
Kubelka−Munk equation, E : band gap, A: proportional
g
The strongest halogen bond energy of [(CH
3
)
3
NCH
I] among these
compounds might be possible reasons for the stabiliza-
2
I]-I
constant. The value of the exponent n denotes the nature
of the sample transition. For direct allowed transition, n
+
and the largest volume of [(CH
3
)
3
NCH
2
=
1/2; for indirect allowed transition, n = 2. Hence, the
+
tion of ordered [(CH
3
)
3
NCH I] cation array and further
2
optical band gap E can be obtained from a Tauc plot by
g
reservation of its ferroelectricity at relatively high tem-
perature. This fine molecular design strategy offers a
1/n
plotting (hν·F(R
∞
)) against the energy in eV and ex-
trapolation of the linear region to the X-axis intercept.
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Calculation condition. All the geometric optimiza-
Rong, Y.; Xu, M.; Hu, M.; Chen, J.; Yang, Y.; Gratzel, M.; Han, H.
Science 2014, 345, 295.
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tion, harmonic vibration analysis and rigid potential en-
ergy scan are performed at b3lyp/6-311G** (Lanl2DZ for
iodine) level using Gaussian 09 package. The initial
structures before geometric optimization are obtained
from single crystal X-ray diffraction data.
(
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K.; Liu, J.-M.; Li, J.; Yan, Y.; Xiong, R.-G. Science 2017, 357, 306;
ASSOCIATED CONTENT
Supporting Information.
Figures S1–S9, Tables S1–S3, and discussion. This material
is available free of charge via the Internet at
http://pubs.acs.org.
(
b) Tang, Y.-Y.; Li, P.-F.; Liao, W.-Q.; Shi, P.-P.; You, Y.-M.; Xiong,
R.-G. J. Am. Chem. Soc. 2018, 140, 8051; (c) Shi, P.-P.; Tang, Y.-Y.;
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Ryu, S.; Seok, S. I. Nat. Mater. 2014, 13, 897.
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AUTHOR INFORMATION
Corresponding Author
(
6) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Inorg.
*
liaowq@ncu.edu.cn; *xiongrg@seu.edu.cn
Chem. 2013, 52, 9019.
(7) Aizu, K. J. Phys. Soc. Jpn. 1969, 27, 387.
Notes
(
8) (a) Garcia, V.; Fusil, S.; Bouzehouane, K.; Enouz-Vedrenne, S.;
The authors declare no competing financial interests.
Mathur, N. D.; Barthelemy, A.; Bibes, M. Nature 2009, 460, 81; (b)
Tang, Y.-Y.; Li, P.-F.; Shi, P.-P.; Zhang, W.-Y.; Wang, Z.-X.; You, Y.-
M.; Ye, H.-Y.; Nakamura, T.; Xiong, R.-G. Phys. Rev. Lett. 2017,
ACKNOWLEDGMENT
1
19, 207602; (c) Kalinin, S. V.; Rodriguez, B. J.; Jesse, S.;
This work was supported by the National Natural Science
Foundation of China (21427801, 21703033) and Natural
Science Foundation of Jiangsu Province (BK20170658).
Karapetian, E.; Mirman, B.; Eliseev, E. A.; Morozovska, A. N. Annu.
Rev. Mater. Res. 2007, 37, 189; (d) Ye, H.-Y.; Tang, Y.-Y.; Li, P.-F.;
Liao, W.-Q.; Gao, J.-X.; Hua, X.-N.; Cai, H.; Shi, P.-P.; You, Y.-M.;
Xiong, R.-G. Science 2018, 361, 151; (e) Tang, Y.-Y.; Li, P.-F.;
Zhang, W.-Y.; Ye, H.-Y.; You, Y.-M.; Xiong, R.-G. J. Am. Chem. Soc.
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