Chemical Cutting of Perovskite Nanowires into Single-Photon Emissive Low-Aspect-Ratio CsPbX3 (X=Cl, Br, I) Nanorods

Article abstract of DOI:10.1002/anie.201810110

Post-synthetic shape-transformation processes provide access to colloidal nanocrystal morphologies that are unattainable by direct synthetic routes. Herein, we report our finding about the ligand-induced fragmentation of CsPbBr₃ perovskite nanowires (NWs) into low aspect-ratio CsPbX₃ (X=Cl, Br and I) nanorods (NRs) during halide ion exchange reaction with PbX₂–ligand solution. The shape transformation of NWs-to-NRs resulted in an increase of photoluminescence efficiency owing to a decrease of nonradiative decay rates. Importantly, we found that the perovskite NRs exhibit single photon emission as revealed by photon antibunching measurements, while it is not detected in parent NWs. This work not only reports on the quantum light emission of low aspect ratio perovskite NRs, but also expands our current understanding of shape-dependent optical properties of perovskite nanocrystals.

Full text of DOI:10.1002/anie.201810110

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Accepted Article
Title: Chemical Cutting of Perovskite Nanowires into Single-Photon
Emissive Low-aspect Ratio CsPbX3 (X= Cl, Br & I) Nanorods
Authors: Lakshminarayana Polavarapu, Yu Tong, Ming Fu, Eva Bladt,
He Huang, Alexander F. Richter, Kun Wang, Peter Müller-
Buschbaum, Sara Bals, Philippe Tamarat, Brahim Lounis, and
Jochen Feldmann
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201810110
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Chemical Cutting of Perovskite Nanowires into Single-Photon
Emissive Low-aspect Ratio CsPbX (X= Cl, Br & I) Nanorods
3
Yu Tong, Ming Fu, Eva Bladt, He Huang, Alexander F. Richter, Kun Wang, Peter Müller-Buschbaum,
Sara Bals, Philippe Tamarat, Brahim Lounis, Jochen Feldmann, Lakshminarayana Polavarapu*
Dedication ((optional))
Abstract: Post-synthetic shape-transformation processes provide
access to colloidal nanocrystal morphologies that are unattainable by
direct synthetic routes. Herein, we report our finding about the ligand-
nanosheets and NWs.[3-4] Whereas in the case of metal and
conventional semiconductors, myriad of nanostructure
a
[
5]
morphologies have been achieved. Among all, colloidal NRs
played an indispensable role in various applications of
induced fragmentation of CsPbBr
low aspect-ratio CsPbX (X=Cl, Br and I) nanorods (NRs) during
halide ion exchange reaction with PbX -ligand solution. The shape
3
perovskite nanowires (NWs) into
[
5]
3
nanotechnology. However, this striking morphology has been
rarely achieved in the field of colloidal perovskite NCs.[ Previous
studies clearly suggest that it is challenging to stop the growth of
all-inorganic perovskite NWs at the early stages of their growth in
order to obtain NRs,[3, 7] which makes it difficult to prepare low-
aspect ratio NRs by direct synthetic approach. Essentially, post-
synthetic chemical and shape transformation processes offer
accessibility to morphologies and chemical compositions that are
difficult to synthesize by direct methods.[5, 8] These processes are
also gaining attention in the field of perovskite NCs due to the fact
that perovskites are soft in nature.[9] For instance, we have
previously shown the ligand-induced transition from bulk hybrid
perovskites to colloidal nanoplatelets of tunable thicknesses.[
6]
2
transformation of NWs-to-NRs resulted in an increase of
photoluminescence efficiency owing to a decrease of nonradiative
decay rates. Importantly, we found that the perovskite NRs exhibit
single photon emission as revealed by photon antibunching
measurements, while it is not detected in parent NWs. This work not
only reports on the quantum light emission of low aspect ratio
perovskite NRs, but also expands our current understanding of shape-
dependent optical properties of perovskite nanocrystals.
Over the last few years, colloidal halide perovskite nanocrystals
9a, 9e]
(
NCs) have received significant interest owing to their
extraordinary optical properties such as near-unity
In this work, we report the shape-transformation of CsPbBr
perovskite NWs into a nearly-monodisperse low aspect ratio
CsPbX NRs during the ion exchange reaction (Figure 1). The
colloidal dispersion of CsPbBr perovskite NWs were prepared by
the polar solvent-free, one-pot ultrasonication approach reported
by our group.[3] The length of the as-prepared CsPbBr
NWs
ranges from 1-2 m with a uniform width of ~12nm (Figure 2a)
and they exhibit green emission under UV light illumination. The
3
CsPbBr NWs exhibit relatively low PLQY (<10%), which was
previously attributed to the presence of defects. To tune the
emission color of NWs, we implemented well known halide ion
3
photoluminescence quantum yields (PLQYs), strong quantum
confinement effects and quantum light emission.[1] These unique
properties make them stand out in the forefront of many potential
semiconductor materials for light emitting and photovoltaic
applications.[ The optical properties of perovskite NCs are
tunable either by halide composition or through morphology
control.[2a, 3] Despite recent success in the shape-control of
perovskite NCs, currently, unlike for conventional semiconductor
and metal NCs, we have access only to a limited number of
perovskite nanostructures such as nanocubes, Nanoplatelets,
3
3
2]
3
3
exchange reaction between colloidal solution of CsPbBr NWs
-ligand complex in toluene.[
3, 10]
The complete halide ion
and PbI
2
exchange of Br with I in the NWs resulted in a well dispersed
[
a]
Y. Tong, Dr. H. Huang, A. F. Richter, Prof. Dr. J. Feldmann and
Dr. L. Polavarapu
Chair for Photonics and Optoelectronics
Department of Physics and Center for NanoScience (CeNS),
Ludwig-Maximilians-Universität München
Amalienstr.54, 80799, Munich, Germany
Email: l.polavarapu@lmu.de
[b]
[c]
[d]
Dr. M. Fu, Prof. Dr. Philippe Tamarat, Prof. Dr. B. Lounis
LP2N, Université de Bordeaux, Talence F-33405, France
LP2N, Institut d’Optique and CNRS, Talence F-33405, France
Dr. E. Bladt, Prof. Dr. S. Bals
EMAT, University of Antwerp
Groenenborgerlaan 171, B-2020 Antwerp, Belgium
K. Wang, Prof. Dr. P. Müller-Buschbaum
Figure 1. a) Schematic illustration showing the transformation of CsPbBr
3
Technische Universität München, Physik-Department, Lehrstuhl für
Funktionelle Materialien, James-Franck-Str. 1, 85748 Garching,
Germany
perovskite NWs into CsPbX
Photograph of the colloidal dispersions of CsPbX
X=Cl, Br, and I) compositions under UV light (=367 nm) illumination.
3
2
NRs by the addition of PbX -ligand solution. b)
3
NRs with different halide
(
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
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Figure 2. Bright-field TEM (a, b) and HAADF-STEM (c, d) images of parent CsPbBr
3
NWs and the CsPbI
3
NRs obtained after the halide ion exchange reaction with
PbI
2
-ligand solution. The diffractograms shown in the insets of figures c & d indicate that the CsPbBr
3
NWs and CsPbI
3
NRs have an orthorhombic crystal structure.
NWs and NRs.
(e) UV-vis extinction and PL spectra of CsPbBr
3
NWs, CsPbI NWs and CsPbBr NRs. (f) PL decay dynamics of CsPbI
3
3
3
colloidal solution with bright red luminescence under UV lamp. In
addition, weekly luminescent precipitate can be seen at the
bottom of the glass bottle (Figure S1). The change in the PL
emission color of NCs from green to red with a PL peak of the
resultant NCs at ~680 nm indicates the nearly-complete
exchange of Br with I (Figure 2e, S2). Such halide ion exchange
generally preserves the morphology of the parent perovskite
NCs.[6, 10] To our surprise, TEM images of the resultant red
addition, the PLQY of the CsPbI
significantly enhanced from ~10% (CsPbBr
(CsPbI NRs) after the shape transformation. There are two main
possible reasons for the enhanced PLQY after halide ion
exchange. First, the ligands present in the PbI precursor solution
can passivate the surface trap states of NC, thus reducing
nonradiative recombination channels. Second, the breakdown of
NWs into NRs shortens the extent of exciton diffusion, thereby
reducing the probability to find a nonradiative trap. Among these
two reasons, it is likely that the observed increase of PLQY is a
consequence of change of morphology due to that fact that the
3
NRs were found to be
3
NWs) to ~47%
3
2
luminescent colloidal dispersion revealed that the CsPbBr
perovskite NWs fragment into low aspect-ratio CsPbI NRs during
3
3
the halide ion exchange process (Figure 2a, b). The
corresponding bright-field TEM images of the supernatant show
CsPbI
similar to the templated CsPbBr
studies revealed that the CsPbI
3
NWs present in the sediment exhibit only 10% PLQY
NWs. Time resolved PL decay
NRs exhibit longer exciton
the presence of uniform CsPbI
ranging from 2.5-3.5 (Figure 2b, S1), whereas the sediment
consists of CsPbI NWs with preserved morphology of CsPbBr
3
NRs with an aspect-ratio mainly
3
3
3
3
lifetime than the NWs due to reduced nonradiative decay rates
(Figure 2f). These results suggest that the part of a NW having
traps gets separated from the non-defective parts when the NW
breaks into NRs, otherwise the excitons are likely to find a trap
owing to large exciton diffusion lengths of perovskites. These
features are exactly the opposite effects observed in the
NW templates (Figure S1). It was staggering to see such low
aspect ratio perovskite NRs due to the fact that it is difficult to
obtain them by direct colloidal synthesis. The high-resolution
high-angle annular dark field scanning TEM (HAADF-STEM)
images of single NCs and corresponding diffractograms revealed
that the orthorhombic crystal structure of the CsPbBr
retains after the shape transformation into CsPbI NRs (Figures
c, d). The HAADF-STEM images of multiple CsPbI NRs further
confirm the high quality single crystalline nature of CsPbI NRs
Figure S3). These perovskite NRs tend to self- assemble into
3
NWs
transformation of nanocubes to NWs through oriented
attachment.[ However, the PLQY of CsPbI
3]
NRs is not as high
3
3
4c]
2
3
as it was reported for corresponding nanocubes,[ which confirm
that some of these NRs must still possess defect sites that can
act as nonradiative channels. Temperature-dependent PL
3
(
ordered domains on silicon substrate as revealed by SEM
characterization (Figure S4).
The CsPbI NRs exhibit slightly blue-shifted extinction and
3
photoluminescence compared to that of NWs due to quantum
confinement caused by their decrease in length (Figure 2e). In
3
measurements revealed that the PLQY of CsPbI NRs increases
up to 90% at cryogenic temperatures (Figure S5). We attribute
this to a decrease of the nonradiative decay rate, but we can also
expect that there is an increase in the radiative decay of free
excitons.[
11]
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Figure 3. PL (a), UV-vis extinction (b) of colloidal CsPbBr
3
NCs obtained by the reaction of CsPbBr
3
NWs with oleylamine for different periods of time. (c-e)
Corresponding TEM images. (f) Scheme showing the ligand-induced breakdown of NWs into NRs. (g) Corresponding PL decay dynamics and (h) plot depicting the
PLQY, radiative and nonradiative decay of the obtained CsPbBr3 NCs as a function of reaction time.
Interestingly, similar shape transformations were observed
by the reaction of CsPbBr NWs with PbCl and PbBr precursor
solutions, as illustrated in figure 1. The composition of the
resultant CsPbX NRs and thus their emission color is tunable by
varying the amount of PbX (X= Cl, Br and I)-ligand solution used
for the ion exchange reaction (Figure S2). The optical bandgap of
CsPbX perovskite NRs is also tunable across the full visible
wavelength range of 400-700 nm and the PL decay becomes
slower while going from chloride to iodide via bromide (Figure S6).
The corresponding X-ray diffraction patterns (Figure S7) as well
as HAADF-STEM images (Figure 2d, S8) indicate that the
continuous blue shift over the next 60 min (Figure 3a). Similarly,
the UV-vis extinction spectra show a slight gradual blue shift with
an increase in reaction time (Figure 3b). The corresponding TEM
images acquired from the samples collected at different reaction
time indicate that the blue shift of absorption and PL is caused by
the change of morphology from NWs to NRs (Figure 3c-e). While
3
2
2
3
2
3
the CsPbBr
aspect ratio of the CsPbBr
3
samples exhibit high polydispersity, the average
NRs appears to be relatively
3
decreased with increase of reaction time, thus resulting in a
gradual blue shift of absorption and PL. While the PLQY increases
significantly from 8-50%, the blue shift is small due to the fact that
change of morphology is not leading to the size where the NCs
exhibit strong quantum confinement. Time resolved PL
measurements revealed that the PL lifetime gradually increases
with decreasing average aspect ratio of NRs due to a significant
decrease of nonradiative decay rate (Figure 3g). As discussed
earlier, the separation of defect sites from the NWs during shape
CsPbX
3
NRs exhibit either cubic or orthorhombic crystal
structures, while the HAADF-STEM analysis of CsPbBr
3
NWs
suggest that they exhibit orthorhombic phase only (Figure 2c).[
Despite interesting shape transformation from NW to NR, the
mechanism of this process is still unclear. One possible reason
could be the reduction of crystal lattice induced by the breakage
of NWs into NRs. However, a quantitative analysis of the high
resolution HAADF-STEM measurements images revealed that
there is no significant trend in the interatomic distances of the
nanocrystal lattices of before and after shape transformation
3]
transformation can result in
a significantly decrease of
nonradiative channels, while the radiative decay rate increases
slightly due to the decrease of size (Figure 3h). These results
demonstrate that the shape transformation from NWs to NRs are
(
Figure S8), which rules out our hypothesis.
2
most likely due to the presence of ligand molecules in the PbX -
Such break down of NWs into NRs has some striking similarities
with our previous findings on ligand-assisted fragmentation of
bulk perovskites into perovskite nanoplatelets. [9a, 9e] Herein, we
ligand solution as schematically shown in Figure 3f. Now, the
question is whether the ligands induce melting or fragmentation
of NWs in to NRs. The melting process can be ruled out based on
the fact that there is no significant difference in the absorption
intensity before and after the shape transformation. Therefore, the
ligand molecule induces the fragmentation of NWs into NRs in a
process similar to the fragmentation of 3D perovskites into 2D
nanoplatelets due to their soft nature.[
used the ligands oleylamine and oleic acid to dissolve the PbX
precursors in organic solvents. The excess ligands present in
PbX precursor solution can cause the fragmentation of NWs.
2
2
Although the exact mechanism of this cutting process is unclear,
it is likely that the ligands reacts with NWs randomly at different
portions of the NWs, thus break them into NRs by particle
dissolution at reaction sites. To test our assumption, a small
volume of pure oleylamine (100 L) was added to the colloidal
9a]
Recently, there has been a growing interest in exploring the
perovskite NCs as single-photon source (quantum emitter), [
which may have implications in quantum information technologies.
To this end, we explored the quantum emitting properties of the
12]
3
solution of CsPbBr NWs under continuous stirring and then the
reaction was monitored by UV-vis extinction, PL, PL lifetime and
correlative TEM characterization over a period of time (Figure 3).
After 10 min, there is a steep increase in intensity and blue shift
of PL peak and is then gradually increased in intensity with a
3
prepared CsPbX NRs in comparison with their NW counterparts.
For single particle PL measurements, the colloidal NR solutions
were first diluted in 1wt% poly(methyl methacrylate)/toluene
solution and then spin-coated onto clean glass slides, as
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blinking characteristics (Figure S11). Although bundling of NWs
cannot be completely ruled out, these results suggest that under
intense laser excitation, multiple excitons can coexist at emission
sites distributed all along a same NW and that their simultaneous
radiative recombination is not efficiently quenched by Auger
processes, and thus it is difficult to find only a single emitting event
at any given time. It is thus a remarkable property that these NWs
can be efficiently broken into low-aspect ratio NRs that exhibit
quantum signatures in their emitted light. Furthermore, similar
results were found from the single particle measurements of
3
CsPbBr NWs and NRs (Figure S12). It is worth mentioning that
unlike nanocubes the anisotropic morphology of NRs may have
additional benefits as single photon source.
In summary, we have presented the fragmentation of
CsPbBr
CsPbX
3
NWs into nearly-monodisperse and low aspect ratio
NRs of different halide compositions by halide ion
3
exchange reaction. Through control experiments, we revealed
that the ligands present in the precursor solution drive this
process. This shape transformation leads to an increase of PLQY
while retaining anisotropic morphology. The increase of PL
efficiency is likely due to the removal of nonradiative trap sites
from NWs by their fragmentation into NRs. Importantly, we have
shown that these single NRs can serve as quantum light sources
at room temperature. This work not only opens a door to access
the morphologies such as NRs through post-synthetic shape
transformations but also expands our current understanding of
shape-dependent optical properties.
3
Figure 4. (a) Wide-field fluorescence image of CsPbI rods dispersed in PMMA.
(b) Time trace of the PL intensity of a single CsPbI
3
rod, recorded with a bin time
2
of 20 ms under a CW excitation intensity of 100 W/cm . (c) Histogram of time
delays between consecutive photon pairs detected from the fluorescence of a
single CsPbI
3
rod, measured under a weak CW excitation intensity of 100 W/cm2.
The dip which approaches zero at zero time delay (with 푔 (2)(0) ~ 0.1), is a clear
signature of photon antibunching. The single exponential fit (red solid line)
corresponds to an exciton lifetime of 16 ns. (d) Distribution of 푔 (2)(0) values.
Acknowledgements
described in the previous reports.[12d, 13] Figure 4a shows a wide-
This work was supported by the Bavarian State Ministry of Science,
field fluorescence image of CsPbI
from each other for single NR PL measurements. A representative
time trace of PL intensity of single CsPbI NR under continuous
wave excitation at 488 nm is shown in figure 4b (see figure S9 for
other examples). The PL of individual CsPbI rods exhibits a
3
NRs which are well-separated
Research, and Arts through the grant “Solar Technologies go Hybrid
(SolTech)”, by the China Scholarship Council (Y.T. and K.W.), by the
3
Horizon 2020 research and innovation program under the Marie
Skłodowska-Curie Grant Agreement COMPASS No. 691185 and by
LMU Munich’s Institutional Strategy LMU excellent (L.P., J.F.). M.F.,
P. T. and B. L. acknowledge the financial support from the French
National Agency for Research, the French Excellence Initiative (Idex
Bordeaux, LAPHIA Program) and the Institut Universitaire de France.
E.B. and S.B. acknowledge the financial support from the European
Research Council Starting Grant #335078-COLOURATOMS. L.P
thank the EU Infrastructure Project EUSMI (European Union’s
Horizon 2020, grant No 731019).
3
characteristic blinking behavior, which is possibly due to the
photoionization driven charging and discharging process, with a
dominant Auger recombination in the decay of the charged
excitons.[12a, 12b] While negligible in bulk semiconductors, the
Auger effect is indeed strongly increased in quantum confined
(
QC) nanocrystals due to the enhanced Coulomb interaction
between charge carriers and the reduced kinematic restriction on
momentum conservation.[14] As the CsPbI
NRs are weekly QC, it
3
is likely that the Auger processes can provide an efficient channel
for exciton-exciton annihilation under intense laser excitation.
This process drastically reduces radiative recombination of
multiple excitons simultaneously created in the single NRs.
3
Keywords: CsPbX nanocrystals • perovskite nanowires •
perovskite nanorods • chemical cutting • shape transformation•
single-photon emission
(2)
Indeed, the PL intensity autocorrelation function 푔 () measured
with a Hanbury Brown and Twiss setup under CW excitation
reveals strong photon antibunching, which manifests as a dip of
[
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(2)
푔 () at zero delay time (=0), as exemplified in Figure 4C (see
(2)
other examples in Figure S9). We find 푔 (0) ~0.1 (see Fig. 4d),
close to the ideal signature 푔(2)(0) = 0 of a pure single photon
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biexciton radiative recombinations. Fitting 푔 (2)() with a single
exponential rise function yields an exciton lifetime of 16 ns for this
rod. Figure S10 displays a histogram of the lifetimes measured on
[
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3
corresponding CsPbI NWs neither show photon antibunching nor
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Angewandte Chemie International Edition
10.1002/anie.201810110
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Single photon emissive perovskite
Y. Tong, M. Fu, E. Bladt, H. Huang, A.
F. Richter, K. Wang, P. Müller-
Buschbaum, S. Bals, P. Tamarat, B.
Lounis, J. Feldmann, L. Polavarapu
nanorods (NRs): CsPbBr
nanowires (NWs) break into low
aspect-ratio CsPbX (X=Cl, Br and I)
NRs of different halide compositions by
reaction with PbX -ligand solution.
3
perovskite
3
Page No. – Page No.
2
These perovskite NRs can serve as
quantum light sources at room
temperature, while the corresponding
NWs did not show photon antibunching
characteristics.
Chemical Cutting of Perovskite
Nanowires into Single-Photon
Emissive Low-aspect Ratio CsPbX
3
(
X= Cl, Br & I) Nanorods
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