From Precursor Powders to CsPbX3 Perovskite Nanowires: One-Pot Synthesis, Growth Mechanism, and Oriented Self-Assembly

Article abstract of DOI:10.1002/anie.201707224

The colloidal synthesis and assembly of semiconductor nanowires continues to attract a great deal of interest. Herein, we describe the single-step ligand-mediated synthesis of single-crystalline CsPbBr₃ perovskite nanowires (NWs) directly from the precursor powders. Studies of the reaction process and the morphological evolution revealed that the initially formed CsPbBr₃ nanocubes are transformed into NWs through an oriented-attachment mechanism. The optical properties of the NWs can be tuned across the entire visible range by varying the halide (Cl, Br, and I) composition through subsequent halide ion exchange. Single-particle studies showed that these NWs exhibit strongly polarized emission with a polarization anisotropy of 0.36. More importantly, the NWs can self-assemble in a quasi-oriented fashion at an air/liquid interface. This process should also be easily applicable to perovskite nanocrystals of different morphologies for their integration into nanoscale optoelectronic devices.

Full text of DOI:10.1002/anie.201707224

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Accepted Article
Title: Precursor Powders-to-CsPbX3 Perovksite Nanowires: One-pot
Synthesis, Growth Mechanism and Oriented Self-assemblies
Authors: Lakshminarayana Polavarapu, Yu Tong, Bernhard J Bohn,
Eva Bladt, Kun Wang, Peter Müller-Buschbaum, Sara Bals,
Alexander S Urban, and Jochen Feldmann
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707224
Angew. Chem. 10.1002/ange.201707224
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http://dx.doi.org/10.1002/ange.201707224

10.1002/anie.201707224
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Precursor Powders-to-CsPbX3 Perovksite Nanowires: One-pot
Synthesis, Growth Mechanism and Oriented Self-assemblies
Yu Tong, Bernhard J. Bohn, Eva Bladt, Kun Wang, Peter Müller-Buschbaum, Sara Bals,* Alexander S.
Urban,* Lakshminarayana Polavarapu,* and Jochen Feldmann
Dedication ((optional))
Abstract: Colloidal synthesis and assembly of semiconductor
nanowires continue to attract a great deal of research attention. Here,
we demonstrate a single-step ligand-mediated synthesis of single-
crystalline CsPbBr3 perovskite NWs directly from precursor powders.
Shedding light on the reaction process and morphological evolution
reveals that the initially formed CsPbBr3 nanocubes transform into
NWs through an oriented attachment mechanism. The optical
properties of the NWs can be tuned across the entire visible range by
varying the halide (Cl, Br and I) composition through a subsequent
halide ion exchange. Single particle studies show that these NWs
progress made in obtaining shape control of perovskite NCs, with
morphologies ranging from nanocubes[3b, 3c] to nanoplatelets[2e, 4]
5]
and NWs,[2d,
very few attempts have been made toward
understanding of the individual growth mechanisms.[2h, 6] Most of
the reported synthetic methods generally require inert conditions
and a presynthesis of the cesium precursor. However, we have
recently shown that precursor powders can be directly
transformed into CsPbX3 nanocubes of different halide
compositions and nanoplatelets by ultrasonication in the
presence of ligand molecules.[3b] In fact, the direct conversion of
exhibit strong polarized emission with a polarization anisotropy of 0.36. precursor powders into NCs of a desired morphology can
More importantly, we show that the NWs can self-assemble in a quasi-
oriented fashion at an air/liquid interface. This process should also be
easily applicable to perovskite nanocrystals of varying morphology for
their integration into future nanoscale optoelectronic devices.
significantly reduce the fabrication time, laborious effort and thus
costs of production, all key factors for commercialization. In
addition to shape-controlled synthesis, self-assembly of NCs is a
crucial step for their integration into devices.[7] Herein, we
describe a facile method for the ligand-assisted conversion of
precursor powders into single crystalline CsPbBr3 perovskite NWs
by ultrasonication (Figure S1). The emission colour of NWs can
Over the past two decades, semiconductor nanowires (NWs)
have played an indispensable role in the evolution of nanodevice
technologies for a wide range of applications such as sensing,
optoelectronics, nanolasers, waveguides, light-emitting diodes
and photovoltaics.[1] Despite great progress in the controlled
synthesis of high quality semiconductor NWs, full colour tunability
has been challenging. However, the rapid emergence of metal
halide perovskite nanocrystals (NCs) as a new potential class of
semiconductor materials has opened the doors for NWs with wide
emission and wavelength tunability through halide composition.[2]
Particularly, all inorganic CsPbX3 (X=Cl, Br and I) perovskite NCs
have gained special attention owing to their high
photoluminescence quantum yields, enhanced stability and the
ability to control their morphologies.[3] In spite of the rapid
be tuned across the full visible spectrum by applying
a
subsequent halide ion (Cl- or I-) exchange reaction on the CsPbBr3
NWs. Mechanistic studies reveal that NWs are formed through
the oriented attachment of the initially formed nanocubes. These
NWs show strong polarization-dependent optical properties.
Furthermore, we found that these perovskite NWs can self-
assemble into quasi-oriented closely packed arrays by an
air/liquid interface technique.
The synthesis of colloidal CsPbBr3 perovskite NWs was
carried out by adapting the ultrasonication-assisted synthesis of
perovskite nanocubes and nanoplatelets developed by our
group.[3b] We have found that prolonged ultrasonication under
similar reaction conditions leads to the evolution of NWs together
with only a few residual nanocubes. A subsequent centrifugation
step yields dispersions with NWs of uniform thickness with over
90% purity (Figure S1). The prepared colloidal solution of
CsPbBr3 NWs emits pure green emission under UV light
illumination as shown in Figure 1a. The corresponding TEM
images clearly show the high purity of NWs (width ~12 nm) with a
very small percentage of nanocubes (Figures 1c & S1). The as
prepared NWs tend to form bundles in the colloidal solution due
to strong hydrophobic interactions between ligand molecules, and
thus tend to settle down at the bottom of the sample vial due to
their larger mass. However, unlike the synthesis of CsPbX3
nanocubes of different halide compositions, CsPbCl3 and CsPbI3
NWs cannot be produced directly.[3b] Instead, the reaction leads
to a non-fluorescent yellow phase in the case of CsPbI3 NCs. It
is nevertheless possible to tune the emission colour of perovskite
NWs across the visible range by applying the well-known halide
ion exchange reaction on CsPbBr3 NW templates (Figure 1a).[2f, 8]
More importantly, the halide ion
[*] Y. Tong, B. Bohn, Dr. A.S. Urban, Dr. L. Polavarapu, Prof. J. Feldmann
Chair for Photonics and Optoelectronics,
Department of Physics and Center for NanoScience (CeNS)
Ludwig-Maximilians-Universität München,
Amalienstr. 54, 80799 Munich, Germany
E-mail: (urban@lmu.de; l.polavarapu@lmu.de)
Y. Tong, B. Bohn, Dr. A.S. Urban, Dr. L. Polavarapu, Prof. J. Feldmann
Nanosystems Initiative Munich (NIM) Schellingstraße 4 80799 , Munich ,
Germany
B. Eva, Prof. Dr. S. Bals
EMAT, University of Antwerp,
Groenenborgerlaan 171, B-2020 Antwerp, Belgium
E-mail: sara.bals@uantwerpen.be
K Wang, Prof. Dr. Peter Müller-Buschbaum
Department of Physics, Chair for Functional Materials,
Technische Universität München
James-Franck-Straße 1, 85747 Munich, Germany
Supporting information
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Figure 1: (a) Photograph of colloidal dispersions of CsPbX3 perovskite NWs with different halide (X = Cl, Br and I) compositions under UV light illumination (λ = 367
nm). CsPbBr3 NWs were obtained by ultrasonication of Cs2CO3 & PbBr2 in octadecene in the presence of oleylamine and oleic acid. The NWs with mixed halide
compositions were obtained by halide ion exchange reactions (CsPbBr3 + PbCl2 (or) PbI2). (b-d & e-g) Bright field-TEM & atomically resolved high resolution HAAD-
STEM images of CsPbCl3, CsPbBr3 and CsPbI3 NWs, respectively. (h) Corresponding UV/Vis absorption (dotted lines) and photoluminescence (solid lines) spectra
of colloidal CsPbX3 perovskite NWs with different halide (X = Cl, Br and I) compositions.
exchange reaction preserves the CsPbBr3 NW template
morphology as evidenced by TEM measurements (Figures 1b &
d. See the large area TEM and SEM images in Figure S2) and
from high resolution high-angle annular dark field scanning TEM
(HAADF-STEM) images recorded along the [100] direction
(Figures 1e-g). A difference in the lattice parameters of the NWs
is measured, which corresponds to approximately 5.5-5.6, 5.8-5.9
and 6.1-6.2 Å, for CsPbCl3, CsPBr3 and CsPbI3, respectively. X-
ray diffraction (XRD) patterns of the NWs (Figure S4) and grazing-
incidence wide-angle X-ray scattering (GIWAXS) measurements
(Figure S5) reveal that the prepared NWs exhibit either cubic or
orthorhombic crystal structures, but they are difficult to distinguish
as they exhibit nearly similar diffraction peaks. The crystal
structures of the NWs were further studied by high resolution
HAADF-STEM and it was found that they exhibit an orthorhombic
crystal structure (Figure S3).
The optical properties of the CsPbBr3 NWs before and after
the halide ion exchange were characterized by UV/Vis absorption
and photoluminescence (PL) spectroscopy. The absorption onset
at 524 nm for the Br NWs clearly red-shifts with increasing iodide
content and blue-shifts as the chloride content is increased
(Figure 1h, dashed lines). Thus, the optical bandgap and PL of
the NWs is tunable across the full visible range of 400-700 nm.
Additionally, a pronounced excitonic absorption peak can be seen
for the Br/Cl NWs indicative of a larger exciton binding energy for
these compositions. As depicted in Figure 1h, all colloidal NWs
exhibit a single narrow PL emission peak with a small Stokes shift.
The emission wavelengths of CsPbCl3 & CsPbI3 NWs are quite
similar to the PL emission of nanocubes of the same composition
prepared by direct synthesis, thus suggesting the nearly complete
replacement of Br with Cl or I.[3b] However, energy dispersive X-
ray spectroscopy (EDX) on the NWs indicates that trace amounts
of Br are still present after the completion of the halide ion
exchange (Figure S6). The shape of the PL emission indicates
that the halide ion exchange produces NWs with a single, uniform
halide composition rather than NWs comprising different halide
ion contents. Time-resolved PL measurements of NWs show that
the average PL decay lifetime decreases with decreasing halide
ion size (Figure S7), similar to the trend observed for nanocubes
in our previous study.[3b] The PL quantum yield (PLQY) of well
purified CsPbX3 NWs was found to be significantly lower (0.5%,
10.8%, & 10.3% for Cl, Br & I, respectively) than that of the
corresponding nanocubes.[3b] Time-resolved PL measurements
further revealed that NWs have a much shorter PL lifetime
compared to nanocubes (Figure S8). There are two main possible
explanations for the low PLQYs and faster decay of NWs. First,
the removal of surface ligands during the purification likely leads
to an increase of surface traps and concomitant nonradiative
decay resulting from them. The second reason is due to the
elongated nature of the wires, which enables a greater exciton
delocalization. Through this larger delocalization length, the
exciton is far more likely to encounter a trap, leading to
nonradiative decay than if the exciton was confined to a single NC.
Especially if some of the nanocubes initially contained trap states,
greatly reducing their PLQY, then incorporating these into NWs,
would greatly enhance the probability that excitons from
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Figure 2: (a) UV-Visible extinction spectra of colloidal CsPbBr3 perovskite NCs obtained after different ultrasonication times during the transformation of precursor
powders into NWs and their corresponding TEM images (b-g). (h) HAAD-STEM images of CsPbBr3 NCs obtained after 20 min. The arrows show the oriented
attachment of nanocubes to NWs. See figure S11 for additional HRSTEM image and corresponding atom-counting results to visualize the oriented attachment (i)
Schematic illustration of the transformation of precursor powders into NWs by ultrasonication, in which initially formed CsPbBr3 nanocubes are transformed into
NWs.
neighboring NCs would diffuse to the trap and recombine there
via radiative decay. Temperature-dependent PL measurements
of CsPbBr3 NWs show that the PL intensity increases strongly with
decreasing temperature, in contrast to CsPbBr3 nanocubes, which
show nearly no increase (Figure S9). This also suggests that the
low PLQY of NWs at room-temperature originates not from a
reduced radiative decay, but instead from a higher non-radiative
decay, which can be significantly suppressed at lower
temperatures. However, the PLQYs of NWs can be improved by
excess ligand passivation.[9]
report (Figure 2c).[3b] As the synthesis further progresses, NWs
begin to appear with lengths on the order of 1-3 µm and lateral
dimensions comparable to
a single nanocube, while the
nanocubes progressively vanish (Figures 2d-f). The percentage
of nanocubes significantly decreased after 60 minutes, leaving
mainly NWs in the dispersions (Figure 2g). A HAADF-STEM
image of NWs obtained after 20 min of reaction clearly showed
the oriented attachment of a nanocube to an intermediate
nanowire (Figures 2h
&
S11). Through these optical
measurements and the HAADF-STEM analysis we postulate the
NWs are obtained through the oriented attachment of nanocubes
In general, inorganic semiconductor NWs are formed either
[10]
through the oriented attachment of NCs
or by surfactant-
assisted, seed-mediated anisotropic growth.[11] Both of these
growth mechanisms have been proposed for the formation of
3g, 6]
perovskite NWs, on the basis of TEM analysis.[2d,
To
elucidate the formation process of the CsPbBr3 NWs in our
synthesis, the reaction was monitored by absorption and PL
spectroscopy and complimentary TEM characterization (Figures
2 & S10). The emergence of the steep absorption onset at 520
nm happens quickly, within 10 minutes, and then slightly redshifts
over the next 50 minutes (Figure 2a). Additionally, the signal for
larger wavelengths increases with synthesis time, indicative of an
increased scattering due to larger crystals present in the
dispersions. Similarly, the typical single PL peak emerges after 10
minutes. This peak then progressively decreases in intensity with
increasing synthesis time, while also slightly red-shifting (see
Figure S10 for normalized absorption and PL). TEM analysis of
the reaction product at each of these stages shows a significant
change in the morphology with synthesis time. At five minutes
(Figure 2b) there are only a few small NCs (5-8 nm) visible, with
the rest mainly being amorphous structures. After 10 minutes, the
reaction product consists of homogeneous nanocubes of 12 nm
in size with a small percentage of nanorods, as in our previous
Figure 3: (a) Dark field PL image of a CsPbBr3 NW obtained by UV light
excitation. (b) Photoluminescence (PL) intensity of CsPbBr3 NWs as a function
of emission angle upon excitation with blue light. The PL could be reproduced
with a cos² function and the polarization anisotropy was found to be ~0.36. The
inset in (b) shows the dark field microscope image of a NW bundle under blue
light excitation
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Figure 4: (a) Schematic illustration of the formation of perovskite NW self-assemblies by the air/liquid interface technique. A colloidal perovskite NW dispersion in
hexane is dropped onto a ethylene glycol solution in a petri dish. The evaporation of hexane leads to the formation of densely packed quasi-oriented self-assemblies
of perovskite NWs at the air/liquid interface. The assemblies are transferrable to any solid substrate applying the Langmuir–Schaefer deposition technique. (b-d)
SEM images of quasi-oriented self-assemblies of perovskite NWs prepared at the air/liquid interface. (e) SEM image of a perovskite NW film prepared by a simple
drop-casting approach.
(Figure 2i). It is worth mentioning that oriented attachment of
nanowire formation has been previously reported for silver
nanowires.[10] We rule out a seed-mediated growth mechanism,
as this process leads to a huge increase in the absorption when
a nanocube is transformed into a nanowire,[11] which was not
observed in our measurements (Figure S10).
CsPbBr3 NW assemblies at the air/liquid interface.[7] This
technique relies on the assembly of hydrophobic nanoparticles by
spontaneous evaporation of volatile organic solvents on a
hydrophilic liquid surface (Figure 4a), similar to the Langmuir-
Blodgett technique.[13] A slight excess amount of oleylamine is
added to the colloidal dispersion in order to protect the particles
from degradation (see experimental section). The excess
oleylamine molecules likely form bilayers on the NW surfaces,
preventing the ethylene glycol from reaching the NW surface and
degrading it. Furthermore, the hexane, or a similarly volatile
organic solvent seems to be necessary, as the NWs degrade,
when a less volatile solvent, such as toluene is used instead. The
rapid evaporation of the hexane leads to the formation of a NW
film floating on the surface of the ethylene glycol, which was then
transferred to a silicon substrate for SEM characterization. The
NW films obtained at the air/liquid interface consist of densely
packed quasi oriented self-assembled NW domains (Figures 4b-
d, & S12), in contrast to non-uniform films with randomly oriented
NWs obtained through direct drop-casting of the NW dispersions
(Figure 4e). The oriented self-assembly process is driven by the
minimization of Gibbs free energy of the system through
maximum Van der Waals interactions between the ligands on the
surface of neighboring NWs. However the self-assembly process
failed in the case of CsPbI3 NWs, which were degraded
immediately upon coming into contact with the ethylene glycol.
Overall, these results suggest that interfacial self-assembled
perovskite NCs can be a promising method, but that significant
challenges to be addressed in the future remain.
It is has been known that semiconductor NWs generally
exhibit anisotropic absorption and PL.[12] We investigated the
emission from perovskite NWs in a dark-field microscopy setup
(Figure 3a & see the supporting information for more details). The
samples were prepared by spin coating a diluted solution of
colloidal CsPbBr3 NWs on a cover slide. Individual elongated
structures resembling NWs were clearly distinguishable under UV
light excitation (inset of Figure 3b). The studied nanostructures
are likely not single NWs, but bundles of parallel oriented NWs,
as the observed intensity seems to be too bright for a single NW
and due to the fact that the NWs tend to form aggregates as seen
in SEM images. The PL intensity was then acquired as a function
of the angle of the polarization filter (Figure 3b). The NWs exhibit
a strong polarized emission, which can be well fitted with a cosine-
squared function of the polarizer angle (θ). The polarization
anisotropy P = (I∥ − I⊥)/( I∥ + I⊥) was calculated to be ~ 0.36 (where
I∥ and I⊥ are emission of NWs along the parallel and perpendicular
axes of the NW, respectively). This value is comparable to the
reported polarization anisotropy of conventional semiconductor
NWs such as CdTe NWs and can be attributed to the anisotropic
shape of NWs and their parallel alignment.[12]
Besides the synthesis of high-quality colloidal nano-building
blocks with excellent control over morphology and composition,
their self-assembly into well-defined architectures is essential for
device applications. We demonstrate that it is possible to obtain
In summary, a single-step ligand-assisted synthesis of
precursor powders into single-crystalline colloidal CsPbBr3 NWs
of uniform thickness has been demonstrated. The optical
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bandgap and emission wavelength of the NWs are tunable across
the visible light spectrum by varying the halide composition
through a subsequent ion exchange reaction. The prepared NWs
exhibit strongly polarized emission comparable to some
conventional semiconductors. The morphological evolution and
corresponding optical properties revealed that the NWs are likely
formed through the oriented attachment of nanocubes.
Importantly, we have demonstrated the fabrication of quasi-
oriented NW self-assemblies at an air/liquid interface, which could
be potentially applied to other morphologies. This work not only
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perovskite NWs, but is also expected to open new avenues for
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Acknowledgements
This work was supported by the Bavarian State Ministry of
Science, Research, and Arts through the grant “Solar
Technologies go Hybrid (SolTech)”, by the China Scholarship
Council (Y.T and K.W.), by the Alexander von Humboldt-Stiftung
(L.P.), by Flemish Fund for Scientific Research (FWO
Vlaanderen) (E.B). S.B. acknowledges the financial support from
European Research Council (ERC Starting Grant #335078-
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Keywords: CsPbX3 perovskite nanowires • ultrasonication •
oriented attachment • self-assemblies • polarized emission
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Entry for the Table of Contents
COMMUNICATION
Perovskite nanowires and their
self-assemblies: Single crystalline
perovskite nanowires can be prepared
directly from precursor powders in a
single-step ligand-assisted
Y. Tong, B. J. Bohn, E. Bladt, K. Wang,
P. M. Buschbaum, S. Bals, A. S. Urban,
L. Polavarapu, J. Feldmann
Page No. – Page No.
ultrasonication process. Nanowires
are obtained likely through the
oriented attachment of nanocubes.
Quasi-oriented self-assemblies of
perovskite NWs can be fabricated at
the air/liquid interface.
Precursor Powders-to-CsPbX3
Perovksite Nanowires: One-Pot
Synthesis, Growth Mechanism and
Oriented Self-assemblies
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