Polar Solvent Induced Lattice Distortion of Cubic CsPbI3 Nanocubes and Hierarchical Self-Assembly into Orthorhombic Single-Crystalline Nanowires

Article abstract of DOI:10.1021/jacs.8b05949

Despite the recent surge of interest in inorganic lead halide perovskite nanocrystals, there are still significant gaps in their stability disturbance and the understanding of their destabilization, assembly, and growth processes. Here, we discover that p

Full text of DOI:10.1021/jacs.8b05949

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Article
Polar solvent induced lattice distortion of cubic CsPbI3 nanocubes and
hierarchical self-assembly into orthorhombic single-crystalline nanowires
Jian-Kun Sun, Sheng Huang, Xiao-Zhi Liu, Quan Xu, Qing-Hua Zhang, Wen-
Jie Jiang, Ding-Jiang Xue, Jia-Chao Xu, Jing-Yuan Ma, Jie Ding, Qian-Qing
Ge, Lin Gu, Xiaohong Fang, Hai-Zheng Zhong, Jin-Song Hu, and Li-Jun Wan
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05949 • Publication Date (Web): 15 Aug 2018
Downloaded from http://pubs.acs.org on August 15, 2018
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Polar Solvent Induced Lattice Distortion of Cubic CsPbI Nanocubes
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and Hierarchical Self-assembly into Orthorhombic Single-crystalline
Nanowires
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†
,§,
‡,
∥
†,§
∥
†,§
∇
,§
∇
JianꢀKun Sun,
Sheng Huang , XiaoꢀZhi Liu, Quan Xu, QingꢀHua Zhang, WenꢀJie Jiang,
∥
†
†,§
†,§
†,§
†,§
DingꢀJiang Xue, JiaꢀChao Xu, JingꢀYuan Ma, Jie Ding, QianꢀQing Ge, Lin Gu, XiaoꢀHong
Fang, HaiꢀZheng Zhong, JinꢀSong Hu,* LiꢀJun Wan*
†
,§
‡
,†,§
,†,§
†
CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, Beijing National Research Center for Molecular
Sciences, CAS Research/Education Center for Excellence in Molecule Science, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China.
‡
Beijing Key Laboratory of Nanophotonics asnd Ultrafine Optoelectronic Systems, School of Materials Science &
Engineering, Beijing Institute of Technology, Zhongguancun South Street, Haidian District Beijing, 100081, China.
∥
Beijing National Laboratory for Condensed Matter Physics, Collaborative Innovation Center of Quantum Matter, Institute
of Physics, Chinese Academy of Sciences, Beijing 100190, China
§
School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
KEYWORDS: Perovskites, Polar solvent, CsPbI , Lattice distortion, Selfꢀassembly, Phase transition
3
ABSTRACT: Despite the recent surge of interest in inorganic lead halide perovskite nanocrystals, there are still significant gaps in
their stability disturbance and the understanding of their destabilization, assembly, and growth processes. Here, we discover that
polar solvent molecules can induce the lattice distortion of ligandꢀstabilized cubic CsPbI , leading to the phase transition into
3
orthorhombic phase which is unfavorable for photovoltaic applications. Such lattice distortion triggers the dipole moment on
CsPbI3 nanocubes, which subsequently initiates the hierarchical selfꢀassembly of CsPbI3 nanocubes into singleꢀcrystalline
nanowires. The systematic investigations and inꢀsitu monitoring on the kinetics of the selfꢀassembly process disclose that the more
amount or the stronger polarity of solvent can induce the more rapid selfꢀassembly and phase transition. These results not only
elucidate the destabilization mechanism of cubic CsPbI nanocrystals, but also open up opportunities to synthesize and store cubic
3
CsPbI for their practical applications in photovoltaics and optoelectronics.
3
1
.
Introduction
slightly distorted perovskite structure up cooling, and
eventually convert back to thermodynamically preferred nonꢀ
perovskite yellow δꢀphase.
Hybrid organicꢀinorganic halide perovskites have emerged
16
as intriguing photovoltaic materials and achieved a certified
power conversion efficiency (PCE) of > 22 %, on a par with1
the best singleꢀjunction chalcogenide and silicon devices.
However, the intrinsic structural instability leads to their
dissociation into PbX2 and volatile organic halide under
environmental moisture or heat, technically hurdling the
A couple of strategies have been explored to alleviate this
17ꢀ25
ꢀ
phaseꢀchange issue.
Alloying with Br is able to
significantly reduce the phaseꢀtransition temperature down to
o
18,19,26
~110 C although at a cost of increasing bandgap.
Protesescu et al. reported that CsPbI nanocrystals with oleic
3
2ꢀ4
commercialization at the current stage. Instead, allꢀinorganic
lead halide perovskites are explored to tackle this issue by
acid and oleylamine as ligands could keep cubic phase at room
20
temperature. Luther etc. demonstrated that the surface
replacing volatile organic components. CsPbX are promising
energyꢀleveraged cubic CsPbI3 quantum dots (QDs) could
3
+
21
candidates since Cs is the most feasible inorganic cation to
achieve over 10 % of PCE on a PV device. The surface
5ꢀ7
+
occupy ammonium sites with suitable tolerance factor.
Although cubic CsPbBr is relatively stable and shows great
modification
with
ethylenediamine cation
(EDA ),
+
+
formamidinium (FA ), or methylammonium (MA ) were
subsequently reported by Zhao and Luther etc. to stabilize
3
8
ꢀ13
potential as lightꢀemitting semiconductor,
the too wide
2
2,23
bandgap (E = 2.25 eV) makes it not suitable for singleꢀ
cubic phase, pushing the cell PCE up to 13.43 %.
Several
g
1
4,15
junction photovoltaics.
Cubic (α) CsPbI3 has the most
examples also demonstrated that the organic surface ligands
appropriate bandgap of 1.7 eV for highꢀefficiency
photovoltaics, but not thermodynamically preferred. It will
transit spontaneously into the roomꢀtemperature stable yellow
δꢀphase (orthorhombic nonꢀperovskite double chain phase)
with an improper bandgap (Eg = 3.0 eV) over time. Although
the yellow nonꢀperovskite δꢀphase can be converted to the
black perovskite αꢀphase upon heating above 360 °C, it will
convert sequentially to the black βꢀphase and γꢀphase with
played an important role in controlling the size and shape of
27ꢀ30
CsPbX3 nanocrystals
. CsPbX3 nanocubes, nanodots,
nanoplates, nanorods, stacked column etc. were synthesized by
mediating organic ligands although the underline mechanism
still needs to be unveiled and phase control is still
31ꢀ33
challenging.
Therefore, understanding the role of surface
ligands on the structural evolution of CsPbI is essential to
3
explore the ambientꢀstable CsPbI for photovoltaic and lightꢀ
3
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emitting applications.
flask, 0.1734 g PbI and 10 mL ODE were degassed under
2
vacuum at 120°C for 1 h. Afterwards, 1 mL dried OAm and 1 mL
OA were injected into the flask under N2 flow. After PbI2
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Herein, we report for the first time that polar solvent
molecules can induce the lattice distortion of ligandꢀstabilized
cubic CsPbI3, triggering the dipole moment which
subsequently initiates the hierarchical selfꢀassembly of CsPbI3
nanocubes into singleꢀcrystalline nanowires via oriented
attachment (OA) process. Theoretical analyses indicate that
the adsorption of polar molecule such as ethanol causes the
completely dissolved, the reaction temperature was raised to
1
70 °C and 0.8 mL Csꢀoleate was swiftly injected. 5 s later, the
suspension was cooled down by iceꢀwater bath, then
centrifugalized to collect the solid products. The products were
then redispersed into hexane and centrifuged again. The
supernatant was filtered and store in glove box for further use in
the following experiments.
move of metal cation and the distortion of PbI octahedrons in
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cubic CsPbI , which is directly proved by atomicꢀresolution
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2
.3 Self-assembly of CsPbI nanocubes. The selfꢀassembly
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transmission electron microscopic observation. Such lattice
distortion initiates the dipole moment on CsPbI nanocubes
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and phase transition were induced by introducing polar solvents.
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In a typical procedure, 2 mL of CsPbI nanocrystal dispersion in
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where it should not intrinsically exist if the symmetry is not
broken. Driven by the dipoleꢀdipole force, the distorted
nanocubes selfꢀassemble and reꢀcrystallize into singleꢀ
crystalline nanowire in a same width as the size of a nanocube
under OA mechanism, accompanying with the phase transition
from cubic into orthorhombic phase. These nanowires can
further selfꢀassemble sideꢀbyꢀside into thick singleꢀcrystalline
nanowire with a diameter up to submicron to minimize the
surface energy. Unlike the OA growth in inorganic
semiconductor nanocrystal systems (such as CdS, CdTe and
PbSe etc.), the OA selfꢀassembly of CsPbI nanocubes into
nanowires/microwires is significantly accelerated due to the
easy migration of cations in halide perovskites.
findings not only shed light on the phase transition of organic
ligandsꢀstabilized cubic inorganic halide perovskites in polar
solvents, guiding the exploration of roomꢀtemperature stable
hexane containing ~2 mmol CsPbI3 were dispensed into glass
vials which cap has an injectable rubber in the middle. The polar
solvent mixing was carried out by injecting 0.1 mL solvent taken
from glove box under a constant stirring of 600 RPM via a
syringe pump at an rate of 0.5 mL/min.
2
.4
spectroscopy (FTIR) spectra were measured on TENSORꢀ27
Bruker). Transmission electron microscopy (TEM) images were
Characterizations.
Fourier
transform
infrared
(
recorded using a JEOL JEMꢀ2100F microscopy, working at an
accelerating voltage of 200 kV. The highꢀangle annular darkꢀfield
(HAADF) imaging experiments were performed on an ARMꢀ
200F (JEOL, Tokyo, Japan) STEM operated at 200 kV with a
CEOS Cs corrector (CEOS GmbH, Heidelberg, Germany). It
should be noted that the TEM sample should be prepared and
transferred as quickly as possible in dry condition since the
exposure to moisture will cause the change of cubic nanocrystals
over time in view of high polarity of water. Powder Xꢀray
diffraction (XRD) patterns were recorded on a Rigaku D/Maxꢀ
2500 diffractometer equipped with a Cu Kα1 radiation (λ ＝
3
34ꢀ37
These
cubic CsPbI for optoelectronic applications, but also reveal
3
how the perovskite nanocubes in a size of ~10 nm evolve into
singleꢀcrystalline nanowires in a length of several micrometers
and a diameter up to submicron.
ꢀ
1
1
.54056 Å) at a scan rate of 3° min . A homeꢀbuilt total internal
reflection fluorescence microscope (TIRFM) with a resolution of
singleꢀmolecule imaging was used to monitor the fluorescence
during the selfꢀassembly. The UVꢀvis absorption spectra were
collected on a UVꢀvis spectrophotometer (UH4150, HITACHI).
Photoluminescence (PL) spectra were measured in an Edinburgh
Instrument FLS 980 spectrophotometer.
2. Experimental Section
2
.1 Chemicals and Materials. Cesium carbonate (Cs CO ,
2 3
9
9
9.99 %), lead(II) iodide (PbI , 99.999 %), 1ꢀoctadecene (ODE,
0 %), oleic acid (OA, 90 %), and oleylamine (OAm, 90 %) were
2
purchased from SigmaꢀAldrich. All these chemicals were used as
received without any further purification. Methanol (Innochem,
2.5 Surface energy calculations. We use the equation (1) to
9
9.9%), ethanol (Acros, 99.8 %), acetone (Acros, 99.9 %),
calculate the surface free energy γ of a (abc) crystal face.
abc
isopropanol (Alfa, 99.9 %), methyl acetate (Acros, 99+%), and nꢀ
hexane (Innochem, 97.5%) were used for investigating the
influence of solvent polarity on the stability of CsPbI3
nanocrystals. All these solvents were dried with molecular sieves
and stored in glove box prior to use.
∆ꢀ
γ = ∆
(1)
ꢁ
where ∆E represents the free energy difference between the
material with new surface, and ∆A is the area of the surface. For a
3
8ꢀ40
slab the surface energy can be determined as,
ꢆ
(ꢀ
ꢇꢈꢀꢅꢉꢃꢊ
)
ꢂꢃꢄꢅ
γ =
(2)
2
.2 Synthesis of CsPbI nanocubes. CsPbI nanocubes were
3 3
ꢋꢁ
where N is the number of layers in the slab, ꢌꢈ is the total
20
synthesized according to modified previous procedures. In brief,
Csꢀoleate precursors were firstly prepared as follows. 0.0814 g
Cs CO , 0.5 mL OA, and 10 mL ODE were loaded in a 50 mL 3ꢀ
ꢍꢎꢏꢐ
energy of an Nꢀlayer slab, Ebulk represents the energy of a formula
unit of bulk material, which can be treated as a monolayer. A is
the area of the surface. The calculation is based on the density
functional theory (DFT), which is performed by the PWmat code.
A DFT Plane Wave materials
2
3
neck flask and degassed at 120 °C for 1 h under vacuum, then
heated up to 150 °C under N flow until all Cs CO were reacted
with OA. The Csꢀoleate precursor solution was kept at 100 °C to
prevent its precipitation out of ODE. In another 50 mL 3ꢀneck
2
2
3
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Figure 1. Pristine CsPbI nanocubes. (a) XRD patterns. (bꢀc) TEM images at lowꢀ (b) and highꢀ (c) magnification
.
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simulation code run on GPU clusters. The pseudo potential
nanocrystals via electrostatic interactions with surface iodide
ions and oleic acid can interacts with the ammonium head
group of OAMIs to form a bilayerꢀtype protection ligand
layer. FTIR spectrum confirms the existence of oleic acid
and alkylammonium cations as surface ligands on these
nanocubes (Figure S1). As shown in Figure 1b and c, CsPbI3
nanocubes are distributed equidistantly, which is the result of
multiple interactions. In general, the interactions between two
charged nanocrystals include steric and osmotic repulsion, van
der Waals attraction, and electrostatic interaction (such as
41,42
adopted the USPPꢀGBRV.
In all models, we set the value of
the vacuum regions is 10 Å and N equal to 5. K point grids of
2×2×2 are chosen for all systems. The structural parameters of
46
orthorhombic nonꢀperovskite CsPbI are based on the reference
3
(JCPDS No. 74ꢀ1970), i.e., a = 4.797 Å, b = 10.462 Å, c = 17.788
Å, and α = β = γ =90°.
2.5 Adsorption energy calculations.
The adsorption energy is calculated according to the following
equation 3,
ꢌꢏꢑꢍꢒꢓꢐ = ꢌꢔꢒꢕꢖꢎꢗꢘ ꢙ ꢌꢎꢚꢛꢏꢜꢑ ꢙ ꢌꢝꢍꢞꢐꢟꢠ
Where ꢌꢔꢒꢕꢖꢎꢗꢘ, ꢌꢎꢚꢛꢏꢜꢑ, ꢌꢝꢍꢞꢐꢟꢠ are the total energies of the
(3)
chargeꢀcharge, chargeꢀdipole, dipoleꢀdipole, chargeꢀinduced
47ꢀ49
dipole interactions).
Since the dipole moment of a highly
adsorbed complex, ligand and CsPbI perovskites, respectively.
symmetric nanocube is nearly zero, the electrostatic
interaction is chargeꢀcharge interaction. Therefore, the CsPbI3
nanocubes in equilibrium are under steric repulsion (supported
by ligand chains), van der Waals attraction, and chargeꢀcharge
3
2
.6 Dipole moment calculations. The dipole moment of single
43
relaxed primitive unit cell is carried out by Gaussian09 code.
We use the HF/Lanl2dz as the basis sets. The value of the dipole
moment is 3.5 Debye.
interaction. The distance among CsPbI nanocubes of ~2 nm
3
47
(Figure 1c) is the thickness of interdigitated capping ligands.
We compute the distribution of dipolar potential field with the
44
following formula,
3.2 Polar solvent induced lattice distortion of CsPbI
3
nanocubes. As discussed above, the asꢀsynthesized CsPbI3
ꢖ
ꢓꢠ
ꢩ
ꢖ
ꢪ
ꢫ( ꢜꢬ ꢖ
ꢩ
)( ꢜꢬ ꢖ
ꢪ
)
ꢜ,ꢕ
ꢣ
ꢢ
ꢡ =
∑
ꢑꢚꢖꢒꢎꢗ,ꢑꢚꢖꢒꢎꢗ
(
ꢙ
(4)
ꢓꢠ
ꢤꢥꢦ
ꢧ
ꢦ
ꢨ
nanocubes can be stabilized in cubic phase by OAMIs and
where p represents the dipole moment; ꢭꢬ is the unit vector
20
i
oleic acid. However, these ligands are not favorable for
along the vector r between the dipoles; ε is the dielectric constant
0
carrier transportation and need to be removed before the
of vacuum; and ε is the relative dielectric constant.
50
r
fabrication of optoelectronic devices. In our experiments, we
found that the polarity of washing solvent significantly
influenced the stability of CsPbI3 nanocubes. The polar
molecules could induce the lattice distortion and phase
3
.
Results and Discussion
3
.1 Characterization of CsPbI3 nanocubes. CsPbI3
nanocubes were first characterized by XRD technique. All
XRD peaks in Figure 1a matches with those simulated from
transition of cubic CsPbI . To get clear insight into the
3
influence of polar solvent on the lattice distortion, we use
aberration corrected scanning transmission electron
microscope (ACꢀSTEM) to reveal the structural evolution in
atomic resolution before and after the introduction of ethanol
singleꢀcrystalline cubic CsPbI (ICSD #161481), indicating
that the product is cubic CsPbI . Transmission electron
microscope (TEM) image (Figure 1b) shows that the product
is composed of uniform and nearly perfect nanocubes in a size
of around 14 ± 2 nm. Highꢀresolution TEM image in Figure 1c
clearly displays the lattice fringes with a distance of 0.63 nm
from the (100) planes of cubic CsPbI . It is known that bulk
CsPbI tends to crystallize in orthorhombic phase at room
temperature. The cubic phase of CsPbI nanocubes here can be
attributed to the stabilization of surface ligands.
3
3
(an example of polar molecules). Without ethanol, high angle
annular darkꢀfield (HAADF) STEM image of a pristine CsPbI3
nanocube (Figure 2a) clearly shows the atomic structure of
cubicꢀphase perovskite. Figure 2b and Figure 2c are the zoomꢀ
in view of the inner and outer parts marked by red and yellow
squares in Figure 2a, respectively. PbꢀI columns, Cs atom
column, and I column are labeled as red, cyan and purple dots,
respectively. It is obvious that
3
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28,45
It is
reported that oleylamine ions (OAMIs) can attach to CsPbI3
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Figure 2. HAADF images of the pristine and ethanolꢀadsorbed CsPbI nanocrystal. (a) HAADF image of the pristine CsPbI nanocrystal.
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(bꢀc) Zoomꢀin view of the outer and the inner area in (a), marked with a red and yellow square, respectively. Red, cyan and purple dots,
represent PbꢀI column, Cs atoms and I atoms column. PbꢀI octahedra along [001] projection are overlaid on the image. (dꢀi) HADDF
6
images of ethanolꢀadsorbed CsPbI (etꢀCsPbI ) nanocrystal. (d) HADDF images with zone axis perpendicular to the outer planes of etꢀ
3
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CsPbI . (eꢀf) Zoomꢀin image corresponding to the outer red square and center yellow square in (d). (g) HADDF images with zone axis
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perpendicular to the center planes of etꢀCsPbI . (hꢀi) Zoomꢀin image of the outer and the inner area of etꢀCsPbI in red and yellow square
3
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in (g)
.
both inner and outer parts of CsPbI nanocube exhibit the
Pb configuration deviates from its original linear states (as
shown in Figure 2b), resulting in a “zigꢀzag” structure.
Concomitantly, the overlapped Cs atom column projected
along [001] direction evolves from dot shape into oval shape,
as marked by blue ellipses, indicating the movement of Cs
column in octahedral cavities. Moreover, the inner part of
CsPbI3 nanocube (yellowꢀsquare area) displays complex
pattern due to the misalignment between the electron beam
and the zone axis of the inner nanocube (Figure 2f),
3
perfect distortionꢀfree cubic perovskite structure in the [001]
projection where PbꢀI column is in a higher contrast compared
with Cs atoms or I atoms column. No structural distortion is
observed on the whole nanocube. While HAADF STEM
images (Figure 2dꢀi) displays the clear lattice distortion after
introducing ethanol into the dispersion of asꢀsynthesized
CsPbI nanocubes. By aligning the incident electron beam to
3
the zone axis of the outer nanocube, the zoomꢀin view (Figure
2
e) of redꢀsquare area in Figure 2d clearly shows that the PbꢀIꢀ
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Figure 3. (aꢀe) Polyhedral and ballꢀandꢀstick model of CsPbI . Aquamarine blue, gray and purple spheres represent Cs, Pb, I atoms,
3
respectively. (a) Cubic CsPbI . (b) Differential charge density of ethanol molecule adsorbed CsPbI (cyan represents the area of losing
3
3
electrons, while yellow is gaining electron. Blue is the cross profile). (cꢀe) Results of ethanol molecules adsorbed on CsPbI structure with
3
different layers. (fꢀg) Distribution of dipolar potential field around one or two adjacent CsPbI nanoparticles induced by ethanol molecules.
3
corroborating the apparent distortion generated from outside
and penetrated to inside of nanocube. By aligning the incident
electron beam to the zone axis of the inner nanocube, the
similar distorted cubic pattern could be obtained as shown in
Figure 2g. As the distortion directions of adjacent atom
columns are apparently distinct from each other, the PbꢀIꢀPb
distortion. DFT calculations were carried out to get further
insight into the polar molecule induced lattice distortion of
cubic CsPbI . As shown in Figure 3a, cubic CsPbI belongs to
3
3
a space group Pm 3ꢮ m with high symmetry. I atoms occupy the
vertices of regular cornerꢀsharing PbI6 octahedra, while
divalent Pb atoms sit at the octahedra centers. The monovalent
Cs atoms are hosted by a cavity enclosed by neighboring
octahedra. We first simulated the adsorption process of
“
zigꢀzag” configuration can be seen in both Figure 2h and 2i,
confirming the lattice distortion occurs in the whole nanocube
in the presence of ethanol molecules. These results reveal at
atomic scale that the ethanol molecules can induce the rotation
and distortion of the cornerꢀsharing PbI6 octahedrons and
offset of Cs atoms in octahedrons cavities throughout whole
ethanol molecule on CsPbI with alkylamine surface ligands.
3
Results show that ethanol molecule prefers to impact onto
CsPbI instead of attacking alkylamine ligand. As shown in
3
Figure S2, the adsorption of ethanol on CsPbI crystal is
3
CsPbI nanocube.
spontaneous and the attachment of ethanol molecule will
release energy. In contrast, breaking iodide ion and alkylamine
3
3.3 Theoretic analysis of polar solvent induced lattice
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ligand is an endothermic process. Therefore, when ethanol ethanol molecules via breaking the symmetry. Figure 3fꢀg
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molecules approachCsPbI nanocubes, they prefer to attach
simulate the distribution of the dipolar potential field of one or
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onto CsPbI rather than attack surface ligands. Based on this
two CsPbI nanocubes at the existence of ethanol molecules,
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3
result and considering the computation consume, we use twoꢀ
respectively. In the dipolar potential field of twoꢀpolarized
CsPbI nanoparticles, when another similarly polarizedꢀCsPbI
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layer cubic CsPbI structural units without surface ligands for
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3
further modelling the impact of ethanol molecules. The
differential charge density profile (Figure 3b) of ethanolꢀ
nanocrystal approached, the gradient of the dipole filed would
be the smallest along the rectilinear direction from the side of
the opposite charged end of the dipole; and the energy state of
the approached nanoparticle is the lowest and most stable.
adsorbed CsPbI (where cyan represents the area of losing
3
electrons and yellow is gaining electron) corroborates Cs
atoms in cubic structure are independently filled in the cavity
without polarity, while polar molecule can induce the shift of
electron cloud of Cs and thus polarity. From Figure 3b, it can
This explains that the polarizedꢀCsPbI nanoparticles tends to
3
arrange in a line.
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.4 Polar solvent induced hierarchal self-assembly of
+
be seen that Cs loses electron when the ethanol molecules are
CsPbI nanocubes. As mentioned above, although the cubic
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3
adsorbed on CsPbI , resulting in symmetry breaking of PbI6
CsPbI nanocube itself does not show dipole moment, the
3
octahedra nearby and the polarization of CsPbI . This result
3
attachment of polar molecules induces its lattice distortion,
generating the dipole moment which can initiate the selfꢀ
assembly of these nanocubes. After the introduction of
ethanol, the structure deformation and crystal polarization
cause the decrease of surface energy and the fallꢀoff of
alkylamine ligands, leading to the decline of steric repulsion
among CsPbI3 nanocubes. Meanwhile, the dipoleꢀdipole
attraction, i.e., orientation force, will attract them closer. As
evidenced in Figure 4aꢀd and Figure S4, the interꢀdistances of
nanocubes clearly shrink over time. For example, the statistic
results on a basis of TEM observations show that the interꢀ
distance decreases from 1.92 nm at initial stage to 1.36, 1.07,
then 0.76 nm after 1, 2.5, and 5 min since the addition of
ethanol, respectively (Figure S4). Due to the lowest field
gradient and energy distribution in the aligning direction of
agrees with the energy calculation. The mulliken charge data
(
Table S1) gives the specific charge value of original cubic
and polarized CsPbI . The increased charge values of Cs
3
atoms justify the increased electropositivity and electron loss,
verifying the polarization effect of ethanol molecule on cubic
CsPbI . This means that the adsorption of ethanol molecule
3
leads to the migration of Cs cation, following by distortion of
PbI octahedra and octahedral cavities (Figure 3cꢀe) in view of
6
the shifting in PbꢀI lengths and PbꢀIꢀPb angles. As listed in
Table S2, when one ethanol molecule absorbed on a twoꢀlayer
cubic CsPbI unit, PbꢀI bond length on the first and second
3
layer deviate from its original length of 3.174 Å by 0.0897 Å
and 0.0580 Å, respectively; and PbꢀIꢀPb bond angle deviates
by 6.5340° from its original value of 180°. Further calculation
was carried out by using two ethanol molecules. As shown in
Figure 3cꢀd, the movement of metal cations and the distortion
nanocubes (Figure 3g), the polarized CsPbI nanocubes tends
3
to align in a line under multiple interactions (Figure 4c, d). At
the same time, the ethanolꢀinduced polarization of CsPbI3
nanocube increases over time, resulting in its further distortion
and peelꢀoff of amine ligands. Once the nanocubes get
attached, they fuse together into a singleꢀcrystalline elongated
nanocrystals to reduce the surface energy (Figure 4e, f), like
what happens in the oriented attachment growth of
of PbI octahedrons are intensified compared with the oneꢀ
6
molecule case. Specifically, When two ethanol molecule
attacks, PbꢀI bond length on the first and second layer shifts
0.1228 Å and 0.1384 Å, respectively; and PbꢀIꢀPb bond angle
shifts by 14.5560° (Table S2). These results indicates that
more ethanol molecules will aggravate the polarization
51
deformation of CsPbI . Moreover, even if expanding the
3
nanocrystals.
Since
the
cubicꢀphase
CsPbI3
is
CsPbI models, the movement and distortion can be observed
3
thermodynamically unstable at room temperature, the
nanocubes were stabilized by the surface energy of organic
amine ligands. Accompanying with ligand peelꢀoff and lattice
fusion, the cubic CsPbI3 nanocrystals undergo the phase
transformation into thermodynamically stable orthorhombic
phase, which has a lower internal energy than cubic phase as
calculated in Figure S5. The lattice fringes and corresponding
Fourierꢀtransformed diffractograms in HRTEM image (Figure
throughout the whole model, as shown in Figure 3e. To further
understand the adsorption process of polar molecules, we
simulate the adsorption of ethanol molecules from different
sides (Figure S3). Calculated results indicate that the
attachment energy for ethanol molecules from opposite side
(△Q1 = ꢀ1.7235 eV) is just slighter lower than that for
ipsilateral adsorption (△Q2 = ꢀ1.9819 eV) and the lattice
distortion occurs in both cases. This result suggests that the
4
f) confirms that the fused nanocrystal is becoming
ethanol molecules can adsorb on CsPbI crystals from either
3
orthorhombic structure (red square region), while the
approaching nanocube still remains in distorted cubic phase
(white square region).
ipsilateral side or opposite sides, inducing the lattice distortion
of cubic CsPbI to nonꢀsymmetric configuration.
3
Although cubic CsPbI nanocube is a highly symmetric
structure with no dipole moment in original state, the dipole
moment can be created after the lattice distortion induced by
3
Concomitant with such nanocrystal fusion, the anisotropic
interactions, such as the electric dipole force, will be enhanced
in the aligning direction, further driving
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Figure 4. Lowꢀ and highꢀmagnification TEM images displaying the timeꢀdependent selfꢀassembly of CsPbI nanocubes into nanowires. (a,
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b) Original CsPbI nanocubes. (cꢀf) Selfꢀassembly of CsPbI nanocubes at 1 min (c, d) and 5 min (e, f) after the addition of ethanol,
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respectively. The insets in f are the corresponding Fourierꢀtransform diffractograms on the marked region. The red square region shows
the evolved orthorhombic structure while the white square keeps distorted cubic structure. (gꢀl) Selfꢀassembled CsPbI nanowires in
3
different diameters collected at 10 min (g, h), 20min (i, j), and 30 min (k, l) after the addition of ethanol, respectively. The insets in h is the
corresponding Fourierꢀtransform diffractogram.
more nanocubes to fuse into its end. The more nanocubes fuse
into nanorod/nanowire the larger the dipole force is. Over
time, the linearly selfꢀassembled nanocubes evolves into
singleꢀcrystalline nanowires in a width same as the nanocube,
i.e. ~15 nm. The length of nanowires can reach over 5 µm
(Figure S6), meaning over 300 nanocubes can selfꢀassemble
and fuse together in the same direction. Distinct from other
oriented attachment and selfꢀassembly in inorganic systems,
phase transition. It is believed that the phase transition process
accompanies with lattice match and atomic reconstruction,
resulting in the nanowires with nearly perfectly smooth
surfaces although the building blocks (nanocubes) are not in
perfectly cubic shape. After 10 min, all nanocubes evolve into
CsPbI nanowires with a uniform width corresponding to the
3
size of single nanocube (Figure 4g). HRTEM image and
diffraction pattern (Figure 4h) evidence that the nanowires are
in orthorhombic phase and oriented in [100]
the selfꢀassembly and fusion of CsPbI nanocubes take place
3
very fast, probably due to the concomitant energyꢀfavorable
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Figure 5. Schematic diagram (top image) and TIRFM micrographs of selfꢀassembly process of CsPbI3 nanocubes into nanowires. TIRFM
micrographs of (AꢀF) and (aꢀf) are captured through a D679/41 and D460/60 bandꢀpass filter, respectively.
direction, which agrees with the calculated growth direction
with lowest surface energy and doubleꢀchain nature of
selfꢀassembly of nanowires into thicker ones would take place
simultaneously. The preꢀformed nanowire parts have a chance
to attach and merge with others while each nanowire can still
attract nanocubes and keep growing. Therefore, it is usually
seen that the final product contains broomꢀlike nanowire
bundles with merged end or middle part as well as unmerged
nanowire end, as shown in Figure S6. It should be noted that
the nanowire thickness and morphology in the final product
can be controlled to some extent by adjusting the
concentration of nanocubes in the solution, polar solvent
amount, and assembling time. The less amount of nanocubes,
polar solvent, or short time will deliver thinner final nanowires
and less nanowire bundles.
orthorhombic CsPbI (Table S3).
3
Along with the evolution of CsPbI3 nanocubes into
nanowires with a large aspect ratio, it was found that when
nanowires attached together sideꢀbyꢀside they could also fused
into a thicker singleꢀcrystalline nanowire by lattice matching
(
Figure 4i). HRTEM image in Figure 4j shows a clear lattice
matching process of two nanowires merging into one singleꢀ
crystalline nanowire. A couple of initial nanowires can merge
into thicker ones, which can further merge with other
nanowires, leading to even thicker ones (Figure 4i). Through
this hierarchical selfꢀassembly and fusion process, the width of
final nanowires can reach up to hundreds of nanometers
building from a bunch of nanowires as shown in Figure 4k.
While the zoomꢀin view TEM image clearly shows that its tail
is still composed of a bunch of initial nanowires (in a width of
nanocube size) (Figure 4l). This result further supports the
proposed mechanism that the thick singleꢀcrystalline wire is
formed by the oriented attachment and lattice fusion of thinner
nanowires. The hierarchical selfꢀassembly process of CsPbI3
nanocubes into nanowires is schematically illustrated in the
top of Figure 5. If the concentration of nanocubes is high, the
selfꢀassembly of nanocubes into nanowire and the secondary
It should be noted here that during the lattice distortion and
selfꢀassembly the polar solvent molecules did not go into the
lattice CsPbI . We carefully checked the XRD patterns of
3
orthorhombic CsPbI nanowires after the selfꢀassembly. As
3
shown in Figure S7 There are no shifts of diffraction peaks
compared with the standard reference (JCPDS No. 74ꢀ1970).
The lattice fringes in HRTEM images (Figure 4h and Figure
S8) also exactly match with the interplanar distances of the
reference. Therefore, the polar solvent should induce the
lattice distortion and initiate the selfꢀassembly through the
surface interaction instead of integrating into the lattice.
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Figure 6. Timeꢀdependent UVꢀvis absorption (a) and PL (b) spectra of CsPbI at the different time after addition of 0.1 mL ethanol. The
3
inset is the zoomꢀin view of Figure 6a in 600ꢀ750 nm. (c) XRD patterns of the products during ethanolꢀinduced selfꢀassembly. (d) Timeꢀ
dependent absorbance (at about 430 nm) for different amounts of ethanol. (eꢀf) Timeꢀdependent UVꢀvis absorption spectra (e) and the
absorbance change (f) for polar solvents with different dielectric constants. 0.1 mL solvents are added.
3
.5 In-situ monitoring hierarchal self-assembly of
investigated by UVꢀvis and PL spectroscopy. The optical
photos show that the solution color gradually changes from
CsPbI nanocubes. The polar molecule induced selfꢀassembly
3
and phase transition process of cubic CsPbI nanocubes into
red for initial CsPbI nanocubes solution to yellow for selfꢀ
3
3
orthorhombic nanowires were further monitored by
fluorescence evolution. Fluorescence spectra (PL) indicate that
pristine nanocubes show a very strong red emission at 690 nm,
assembled CsPbI nanowire solution (Figure S10) in 30 min
after the addition of 0.1 mL ethanol into 2 mL hexane
3
containing ~1 mmol/mL CsPbI nanocubes. In UVꢀvis spectra
3
which is typical for cubic CsPbI , while assembled nanowires
(Figure 6a), the absorption edges at ~700 nm from the cubic
3
presents much weak blue emission centered at 445 nm
CsPbI gradually fade away and distinct absorption peaks rise
3
corresponding to orthorhombic CsPbI (Figure S9). A total
up at 430 nm, which can be assigned to the first absorption
band of orthorhombic nanowires. Meanwhile, the optical
photos taken under UV light show the solution color changes
from red to blue. The photoluminescence (PL) spectra indicate
that PL emission intensity at 690 nm gradually decrease,
suggesting the selfꢀassembly (Figure 6b). Xꢀray diffraction
patterns were also recorded during the selfꢀassembly process
to reveal the phase transition process. As shown in Figure 6c,
3
internal reflection fluorescence microscope (TIRFM) with a
resolution of singleꢀmolecule imaging was used to inꢀsitu
image the fluorescence evolution during the selfꢀassembly
process on a basis of the different emission from nanocubes
and nanowires. Luminescence is divided to red and blue
channel through D679/41 and D460/60 bandꢀpass filter. For
the red channel, the fluorescence was attenuated by OD4
dimmer to get clear images in view of too strong emission
the diffraction peaks from orthorhombic CsPbI clearly show
3
from cubic CsPbI . As shown in Figure 5AꢀF and Movie S1,
the gradual fading of red fluorescence indicates the cubic
up 30 min after the addition of ethanol and the peaks from
cubic phase decrease. After 1 h, all diffractions can be indexed
to orthorhombic CsPbI3 and no cubic phase is detected,
suggesting the completion of phase transition.
3
CsPbI disappears bit by bit over time. After 30 min, almost no
3
red emission was observed in the red channel. Concomitantly,
nanowire emission shows up and progressively increases in
the blue channel (Figure 5aꢀf), indicating the formation of
orthorhombic nanowires accompanying with the dissipation of
nanocubes.
Furthermore, it was found that the kinetics of selfꢀassembly
is closely dependent on the amount and the dielectric constant
of polar solvent. The absorptions at 430 nm are plotted as a
function of time under the different adding amounts of
ethanol. According to Figure 6d,
The kinetics of polar molecule induced selfꢀassembly was
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Figure 7. TEM images taken at 5 min after adding 0.1 mL of methanol (a), ethanol (b), isopropanol (c) and methyl acetate (d) into 1
mmol/mL CsPbI3 solution, respectively.
increasing ethanol addition can accelerate the selfꢀassembly
process. For example, 5 min after addition of 0.150 mL
ethanol the absorption intensity reaches 95 % of maximum,
similar to that 30 min after addition of 0.100 mL ethanol.
While the absorption after 20 min in the case of 0.050 mL
ethanol only reach 85 % of maximum, similar to that 1 min
after addition of 0.125 mL ethanol.
nanocubes and still consists of nanocubes (Figure 7d). No
assembled nanowire or even larger nanocrystals are observed.
These TEM results are well consistent with the above
analyses.
Additionally, we are aware that the dielectric constants of
the liquid solvents we used here increases as their molecular
size decreases, but it does not mean the molecular size is the
measure of the interaction between solvent and cubic CsPbI3
nanocubes. To demonstrate this point, a control experiment
was carried out. As mentioned, the asꢀsynthesized cubicꢀphase
The dielectric constant (ε) is a measure of solvent polarity.
Higher dielectric constant means higher polarity and greater
ability to induce the lattice distortion of CsPbI nanocubes.
3
The UVꢀvis spectra in Figure 6e clearly show the different
kinetics of selfꢀassembly in solvents with different dielectric
constants. The solvent with higher dielectric constant can
induce the selfꢀassembly and phase transition much faster. For
example, methanol (ε = 33.0) can induce the production of
nanowires in 1 min. While the absorbance change in the case
of isopropanol (ε = 18.3) and acetone (ε = 20.7) takes place
much slower (Figure 6f). Moreover, methyl acetate was used
CsPbI nanocrystals are very stable in hexane due to its nonꢀ
3
polarity. However, when we use 1ꢀoctyl alcohol with a larger
size and dielectric constant as well than hexane (10.914 Å vs.
8.095Å for size and 3.4 vs. 2.0 for dielectric constant) as the
polar solvent to disperse cubic CsPbI nanocrystals, the phase
3
transition and selfꢀassembly will take place in tens of minutes,
as implied by the optical image in Figure S11. Since the polar
solvent acts through surface interaction instead of going into
the lattice, it is more reasonable that the solvent polarity
to replace amine ligands on CsPbI nanocubes while basically
3
keeping their cubic phase, achieving CsPbI quantum dotꢀ
matters its interaction with CsPbI rather than its size.
3
3
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1
based solar cells with a high efficiency of 10.77 %. It can be
seen in Figure 6eꢀf that methyl acetate (ε = 6.7) barely cause
the change of absorption, rationalizing its application in the
4
.
Conclusion
In conclusion, we found for the first time here that polar
molecules can induce the lattice distortion of ligandꢀstabilized
fabrication of highꢀefficiency CsPbI solar cells. As a direct
3
cubic CsPbI , triggering the dipole moments. The theoretical
3
demonstration of the influence of solvent polarity on the selfꢀ
assembly, TEM images were taken 5 min after adding
different solvents (0.1 mL). As shown in Figure 7, the sample
with methanol shows plenty of nanowires in several
micrometer long and some nanowires are much thicker due to
sideꢀbyꢀside selfꢀassembly of primary nanowires (Figure 7a).
The sample with ethanol (ε = 24.3) shows nanowires in a
length of hundreds of nanometer and a width similar to
nanocube size (Figure 7b). While the sample with isopropanol
analyses indicate that the attachment of polar molecules causes
the move of metal ions in perovskite and the distortion of PbI6
octahedra, which is directly visualized and evidenced by
atomic resolution transmission electron microscopic
observation. The induced dipole moment drives the selfꢀ
assembly of CsPbI3 nanocubes into singleꢀcrystalline
nanowires under OA mechanism, accompanying with the
phase transition from cubic into orthorhombic phase. The
formed CsPbI nanowires can further hierarchically sideꢀbyꢀ
3
(ε = 18.3) is composed of plenty of nanocubes with a few
side selfꢀassembled into much thicker nanowires up to
submicrometer in width. Importantly, the systematic
nanowires (Figure 7c). For the methyl acetate (ε = 6.7), the
sample does not exhibit detectable change from the pristine
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investigation on the kinetics of selfꢀassembly reveals that the
(11) Cho, H.; Jeong, S. H.; Park, M. H.; Kim, Y. H.; Wolf, C.; Lee,
C. L.; Heo, J. H.; Sadhanala, A.; Myoung, N.; Yoo, S.; Im, S. H.;
Friend, R. H.; Lee, T. W. Science 2015, 350, 1222.
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more amount or the stronger polarity of polar solvent can
induce the more rapid selfꢀassembly and phase transition.
These results not only provide the new insight into the
(
12) Zhou, Q.; Bai, Z.; Lu, W. G.; Wang, Y.; Zou, B.; Zhong, H.
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Supporting Information.
FTIR spectra, PL spectra, additional XRD patterns and TEM
images, timeꢀdependent interꢀdistances statistics during selfꢀ
assembly process, and supplementary theoretical calculation
analyses are provided as Supporting Information which are
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(20) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.;
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(24) Liu, C.; Li, W.; Zhang, C.; Ma, Y.; Fan, J.; Mai, Y. J. Am.
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Author Contributions
∇These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
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25) Lin, J.; Lai, M.; Dou, L.; Kley, C. S.; Chen, H.; Peng, F.; Sun,
This work was supported by the National Key Project on Basic
Research (2015CB932302), National Natural Science Foundation
of China (21573249, and 91645123) and the Strategic Priority
Research Program of the Chinese Academy of Sciences
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D. T.; Yang, P. Nat. Mater. 2018, 17, 261.
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