Stable Blue Luminescent CsPbBr3 Perovskite Nanocrystals Confined in Mesoporous Thin Films

Article abstract of DOI:10.1002/anie.201802335

Creating CsPbBr₃ perovskite nanocrystals with bright blue emission is challenging because their optical properties depend sensitively on structure. Growing perovskites in mesoporous templates bypasses some of these purification issues because the size of the nanocrystal is governed by the dimensions of the pores. Mesoporous silica consisting of aligned channels with tunable diameter can be easily synthesized and used as a template. When the perovskite solution evaporates and retreats, some of the liquid remains trapped in the interconnecting pores by discontinuous dewetting. The precursor crystallizes, generating stable ca. 3.1 nm blue-emitting perovskite nanocrystals. The mesoporous template also serves as a protective barrier to preserve the optical properties of the CsPbBr₃ from atmospheric conditions. Compared to the bulk crystals and the powder composite, the strong blue-shift of the emission peak in the film is accompanied by a decrease in the longer lifetime component and an 8-fold increase in the external quantum efficiency.

Full text of DOI:10.1002/anie.201802335

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201802335
German Edition: DOI: 10.1002/ange.201802335
Perovskites
Stable Blue Luminescent CsPbBr Perovskite Nanocrystals Confined in
3
Mesoporous Thin Films
Victor Malgras, Joel Henzie,* Toshiaki Takei, and Yusuke Yamauchi*
Abstract: Creating CsPbBr3 perovskite nanocrystals with
bright blue emission is challenging because their optical
properties depend sensitively on structure. Growing perov-
skites in mesoporous templates bypasses some of these
purification issues because the size of the nanocrystal is
governed by the dimensions of the pores. Mesoporous silica
consisting of aligned channels with tunable diameter can be
easily synthesized and used as a template. When the perovskite
solution evaporates and retreats, some of the liquid remains
trapped in the interconnecting pores by discontinuous dewet-
ting. The precursor crystallizes, generating stable ca. 3.1 nm
blue-emitting perovskite nanocrystals. The mesoporous tem-
plate also serves as a protective barrier to preserve the optical
properties of the CsPbBr3 from atmospheric conditions.
Compared to the bulk crystals and the powder composite, the
strong blue-shift of the emission peak in the film is accom-
panied by a decrease in the longer lifetime component and an
been generated using various routes including hot injection
[
2]
[3]
synthesis, room temperature crystallization, and various
[
4]
microemulsion methods. Methylammonium lead bromide
(MAPbBr ) perovskites NCs are known for their bright green
3
emission, possessing photoluminescence quantum yields
[
5]
(PLQY) close to unity. Cesium lead bromide (CsPbBr3)
NCs are an all-inorganic perovskite counterpart that are more
stable than MAPbBr , and have already achieved PLQY
3
[
2]
values of more than 90%. To our knowledge, there are few
reports of blue-emitting cesium-based perovskite NCs
because band gaps in the blue region require the nanoparticle
to have one dimension that is less than 4 nm. This is
challenging owing to material stability issues, and to the
[6]
polydisperse nature of most synthetic colloidal methods.
Ultrathin two-dimensional (2D) CsPbBr nanoplates exhibit
3
blue emission, although their ionic nature makes them
[
7]
susceptible to degradation. Integrating these kinds of
colloids into devices is challenging because of their sensitivity
to water, which can require synthesis, purification, and
fabrication steps to be carried out in an inert atmosphere.
To realize a simple and reliable bench-top method to
generate stable blue-emitting all-inorganic perovskite nano-
particles, we used ordered mesoporous silica films as tem-
plates to direct and confine the growth ofNCs. Nanoporous
matrices offer numerous advantages in the synthesis of
nanostructured lead halide perovskite materials in particu-
8
-fold increase in the external quantum efficiency.
T
he optoelectronic properties of bulk lead halide perovskites
have enabled a whole new class of inexpensive, efficient,
solution processable light-emitting and light-harvesting devi-
ces. Doping and alloying approaches have long served as an
effective means of tuning the optical properties of semi-
conductors. But nanostructuring is a parallel and comple-
mentary strategy to control the excitonic band gap, as well as
[1]
[8]
relaxation, recombination, and energy transfer dynamics.
Colloidal lead halide perovskite nanocrystals (NCs) have
lar. They enable a ligand-free approach to control the size of
the nanocrystal, and depending on the dielectric properties of
the template it can interconnect or isolate the NCs electroni-
cally and optically. For example, silica is ideal for studying the
properties of isolated nanoparticles, because its insulating
nature contributes minimal collateral electronic interactions.
Furthermore, encapsulation limits adventitious exposure to
water, promoting NC stability. Compared to their powdered
counterparts, mesoporous thin films yield larger periodic
domains (> 1 mm) with a fixed orientation relative to the
substrate. Well-ordered structures can even lead to mono-
disperse particles arranged according to the negative replica
of the mold. We envisioned film templates as a platform
towards functional composites that are ready-for-integration,
whereas mesoporous powders require further processing
steps that cause additional challenges. Finally, films produce
negligible scattering in the active region of the device since,
unlike the typical micrometer grain size in powders, their
features are far below the diffraction limit of light (! l).
Capillarity is the main driving force for infusing the
channels of the template with perovskite precursor solution.
As the diameter d of the channel decreases below some
threshold, nanofluidic effects begin to affect viscosity and
mass transport, thus hindering the effective filling of the
[
*] Dr. V. Malgras, Dr. J. Henzie, Dr. T. Takei
International Center for Young Scientists (ICYS) & International
Centre for Materials Nanoarchitectonics (MANA)
National Institute for Materials Science (NIMS)
1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan)
E-mail: Henzie.Joeladam@nims.go.jp
Prof. Y. Yamauchi
College of Chemistry and Molecular Engineering
Qingdao University of Science and Technology
Qingdao 266042 (China)
and
Department of Plant and Environmental New Resources
Kyung Hee University
1732 Deogyeong-daero, Giheung-gu, Yongin-si
Gyeonggi-do 446-701 (South Korea)
and
School of Chemical Engineering and Australian Institute for
Bioengineering and Nanotechnology
The University of Queensland, Brisbane (Australia)
E-mail: y.yamauchi@uq.edu.au
Supporting information and the ORCID identification number for
one of the authors of this article can be found under:
https://doi.org/10.1002/anie.201802335.
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[9]
template. To circumvent this nanofluidic limitation, we
sought to exploit a well-known property the mesoporous
SBA-15 family: its hierarchical porous composition. The main
mesochannels are the negative replica of the cylindrical
micelles arranged in a 2D hexagonal phase and separated by
the aqueous domain where the hydrolitic condensation of
SiO occurs. Two additional categories of micropores are also
2
[10]
present:
1) ultramicropores (< 1 nm): small pockets that
are the imprint of the hydrophobic polyethylene oxide (PEO)
subunits on the surface of the mesochannels; 2) interconnect-
ing pores (1.5–4 nm): likely form when adjacent micelles
come in contact or merge during the condensation of SiO2.
The perovskite precursor solution can access the entirety of
the mesoporous film because the main channels have less
resistance while the interconnecting pores prevent dead ends
(
Scheme 1). When the precursor solution begins to evaporate,
Figure 1. SEM images of a) the pristine mesoporous silica meso-
porous film with channels periodically aligned over >1 mm, parallel to
the surface; b) the CsPbBr perovskite-infused mesoporous silica film
3
with several particles (brighter contrast) enclosed inside the channels.
Inset: SAXS pattern of the nanocomposite film sample shown in (b),
attesting that the overall hexagonal periodicity is maintained despite
the perovskite inclusion.
Scheme 1. Illustration of the hierarchical filling (left), where the pre-
cursor solution infuses the network via the larger channels and smaller
interconnecting pores. As the solution evaporates and dewets the
channels, small amounts are trapped in the smaller pores, crystallizing
perovskite solution onto the mesoporous substrate, the
surface of the sample appears to be filled with nanoparticles,
evidenced by the bright contrasting regions inside the
channels (Figure 1b). The average channel width and the
to form CsPbBr NCs (right). Although some crystallization occurs in
3
the larger main channels to form green-emitting NCs, it mostly takes
place in the interconnecting pores, thus the primary emission comes
from the small blue-emitting NCs.
2
D hexagonal mesoporous periodicity remains unaffected, as
supported by the SAXS patterns in the inset (pore-to-pore
distance of 2 ꢀ d12
it retreats and dewets the surface of the channels. Surprisingly,
some of the solvent is trapped in the interchannel pores,
leaving behind 3.1 nm CsPbBr3 perovskite NCs. Some larger
NCs formed in the mesochannels, but they are a minority
product. This approach enables the fabrication of blue-
emitting nanocomposites consisting of stable all-inorganic
lead halide perovskite NCs in a pre-synthesized mesoporous
silica film.
¯
ꢀ 10.4 nm). The semi-elliptical shape of the
SAXS pattern is the direct consequence of the distortion
induced on the mesoporous structure. It is known that after
calcination of the templated film, the periodicity along the
[11]
vertical axis shrinks and the pores adopt an elliptical shape.
The STEM image in Figure 2a shows a typical region of
the mesoporous film containing the perovskite nanoparticles.
Large crystals arranged along the direction of the channels
can be observed, suggesting an effective penetration of the
precursor solution during the synthesis. The particles had
a broad size distribution (5 to 8 nm) caused by the elliptical
shape of the mesopores and random viewing angle in TEM.
The complementary high-resolution (HRTEM) images (Fig-
ure 2b,c) confirm the crystallinity of the perovskite particles,
with lattice fringes aligned in the direction of the encapsulat-
ing silica channels. FFT analysis of the image (Figure 2d)
indicates the d-spacing is 2.94 ꢁ, which matches the (004)
interplanar distance of cubic CsPbBr3 perovskite (Pbnm,
ICSD card 97852). This observation suggests that the crystals
The mesoporous films are prepared by spin-coating glass
or silicon substrates with a sol–gel solution of silica precursor
(
tetraethylorthosilicate) containing a precise concentration of
triblock copolymer (EO -PO -EO , Pluronic P123) that
2
0
70
20
yields a 2D hexagonal mesophase of in-plane cylindrical
micelles. The polymer and residual solvent are then removed
via thermal treatment (see the Supporting Information for the
detailed procedure). An SEM image of the top surface of the
pristine mesoporous silica film (Figure 1a) highlights the
curved mesochannels domains that are uniformly distributed
throughout the sample and appear continuous over several
micrometers. After casting and annealing the CsPbBr3
2
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Figure S3). As shown in Figure 1, the curved mesochannel
domains span the surface of the film. Thus infiltration and
transport of precursor solution can only pass through the top
during the spincoating process. It is possible that the
centrifugal force generated by the spin-coating process
helps fill the channels. But as mentioned previously, the
mesochannels in SBA-15 are interconnected by micro-
[
13]
pores. A network of large channels and small pores offers
innumerable paths for the perovskite precursor solution to
permeate the entirety of the film template. When the solution
evaporates and retreats, it dewets the surface of the template.
The interchannel pores have a lower surface free energy than
the solution, so they likely remain filled via a discontinuous
dewetting mechanism that is well explained in 2D systems and
[
14]
has been used to grow small NCs. Therefore, the inter-
channel pores in the films offer two advantages: 1) they offer
more paths for the precursor solution to fill the film, and
2
) they act as a local discontinuity to trap the solution and
direct the growth of perovskites NCs that are small enough to
experience quantum confinement and enable blue emission.
Figure 3a shows a photograph of CsPbBr perovskite NCs
3
in mesoporous powder and mesporous film. The blue
emission from the film was further confirmed by the clear
displacement of the photoluminescence (PL) peak (Fig-
ure 3b) compared to the mesoporous powder and bulk
perovskite samples. The bulk perovskite has a peak at
5
25 nm, which is shifted to 507 nm in the mesoporous
powder because the quasi-spherical particles experience
moderate 3D quantum confinement, expanding the band
gap from 2.36 to 2.45 eV. The PL peak from the mesoporous
film further shifts to 487 nm, which indicates the particles are
much smaller and experience additional confinement (the
reproducibility over nine different samples can be assessed in
the Supporting Information, Figure S4). Although a band gap
of 2.55 eV for crystals of ca 3.1 nm is supported by previous
Figure 2. a) STEM image of the CsPbBr -infused silica mesoporous
film nanocomposite. Inset: histogram highlighting the bimodal particle
size distribution, counted over 400 particles. b) Low- and c) high-
3
magnification HRTEM image emphasizing the CsPbBr NCs encapsu-
3
[8b,15]
studies,
other publications also reported different val-
Compared to the bulk, the emission linewidth of the
lated in the mesoporous silica film. d) FFT of the NCs shown in (c).
[
2,8d,16]
ues.
grow in the < 001 > direction along the channels. The XRD
pattern of the composite film reveals several weaker peaks
that support this assignment (Supporting Information, Fig-
ure S1). There is currently an ongoing debate about the phase
of perovskite colloidal NCs and whether they adopt the cubic
or orthogonal phase. Unfortunately the subtle difference in
the lattice parameters of these two phases could not be
resolved in our surfactant-free system with our existing
instrumentation.
In addition to these NCs, we noticed that the template is
riddled with much smaller particles in the 2.5 to 3.6 nm size
range. This bimodal particle size distribution is highlighted by
the histogram in the inset of Figure 2a. The smaller particles
were not observed in the powder sample (Supporting
Information, Figure S2), although the same precursor con-
centration was used. This indicates that the smaller particles
may originate from how the precursor solution infiltrates and
dewets the interior spaces of the template. Furthermore, the
adsorption/desorption isotherms revealed that although the
specific surface area of the powder sample is considerably
larger, the overall contribution of the interconnecting pores in
the film sample is more important (Supporting Information,
film sample remains narrow (96 and 108 meV, respectively),
suggesting good crystal quality, while the peak for the larger
particles in the powder is much broader (144 meV). Owing to
the location of the emission feature, we can conclude that
most of the PL signal emitted from the film sample originates
from the smaller crystals formed in the interchannel pores
observed by TEM. The peak is asymmetrically shaped, and
a double-gaussian fit (white curves) reveals the presence of
a weak contribution located at 510 nm, which most likely
corresponds to the larger 7 nm particles. This is in good
agreement with the bimodal distribution of particle sizes
discussed earlier. It is also possible that variations in the
surface stoichiometry can be partly responsible for the shift in
wavelength, as Br-rich compositions tend to blue-shift the PL
features. The overall mechanism describing the formation of
NCs in confined spaces is not fully understood, but we assume
that the inner walls of the interconnecting pores could impart
considerable residual pressure on the surface of the NCs to
quench their growth. According to previous study on
methylammonium lead halide, it was evidenced that external
pressure tends to induce a blue-shift in the PL emission owing
[
12]
[
17]
to amorphization. We ruled out this effect, however, since
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Supporting Information, Table S1. The fast component of the
lifetime t is slightly larger in the film than in the powder. This
1
result seems counterintuitive at first because the radiative
recombination rate typically increases inversely with crystal
size because of the larger oscillator strength. The excitonic
spontaneous recombination lifetime of surfactant-stabilized
CsPbBr perovskite NCs, however, has been reported with
3
[
12b,18b]
values ranging between 2 and 8 ns,
suggesting a strong
dependence on factors such as the synthetic conditions,
morphology, and surface passivation. On the other hand,
the mesoporous film has a faster t (slow component) and
2
larger PLQY than the bulk and powder composite, which
results in a strong reduction in the calculated non-radiative
lifetime. Presently there is still vigorous debates about the
photophysical events that are responsible for the tail of the PL
time-response in this family of perovskite materials. If t can
2
be attributed to trap-assisted mechanisms involving charge
capture, thermal release and delayed radiative/non-radiative
[
20]
decays, then our results indicate the NCs generated in the
interconnecting pores are well-passivated single crystals that
contain minimal defects, hence they possess only a limited
amount of surface recombination centers. Others have
suggested that photon recycling might lead to longer apparent
recombination lifetimes, accompanied by a drastic drop of the
[21]
external yield compared to the internal quantum efficiency.
Our results also support this mechanism because the 3 nm
crystals in the film are scattered and monodisperse, which
reduces the probability of photons being readsorbed by
neighbouring NCs.
The PL emission of the CsPbBr mesoporous films should
3
be very stable to various environmental conditions because
the NCs are embedded in a composite film. To test stability,
one of the film samples was kept under atmospheric
conditions (relative humidity 60–70%) in the dark for five
months, while another was placed under solar simulated light
Figure 3. a) Photograph comparing the powder and film samples
under UV light. b) PL spectra comparing the position of the main
emission peak for the three samples. The solid white line represents
a double-Gaussian fitting of the emission peak from the film sample.
(l > 300 nm) for 6 h (Supporting Information, Figure S5). In
both samples, the PL emission spectra and PL lifetimes and
PLQY remained essentially unchanged, showcasing its strong
resistance to degradation due to adventitious exposure.
In summary, we demonstrated a simple route to obtain
c) Time-resolved PL decay (l =365 nm) of the three samples, along
ex
with their respective bi-exponential fitting. Fitting parameters are
reported in the Supporting Information, Table S1.
stable blue-emitting CsPbBr NCs confined in the intercon-
3
necting pores present in adjoining walls of a mesoporous SiO2
film. These pores are very small and can trap the equivalent of
the sharp and intense emission peak does not indicate
a disordered structure.
ꢁ
24
hundreds of yoctoliters (10 liters) of precursor solution that
eventually evaporates and forms ultra-small blue-emitting
CsPbBr3 NCs. We provided sufficient evidence, through
electron microscopy and optical measurements, to demon-
strate that the blue-shift of the NC film compared to the bulk
(DE ꢀ 0.2 eV) is mainly due to strong quantum confinement,
although some stress-induced crystallographic distortions
may play a small role. Compared to the bulk and mesoporous
powder samples, the film possesses a shorter PL lifetime and
a much higher PLQY. This decrease in non-radiative recom-
bination is combined with a strong resistance to photochem-
ical and moisture-based degradation. This work illustrates the
simplicity of the templating approach in generating mono-
disperse nanocrystals with properties that are governed by
strong quantum confinement effects. The template also helps
to maintain the stability of the perovskite NCs under
The time-resolved PL curves in Figure 3c were fit numeri-
cally for each sample with a bi-exponential decay (Supporting
Information, Table S1). Biexponential fits are frequently used
[12b,18]
to describe the dynamics of this family of NCs.
In our
case, the mixed perovskite/air and perovskite/SiO interfaces
2
have dissimilar optical-mode densities and refractive indices
and are therefore likely to provide different transition
[
19]
dynamics. The average luminescent lifetime ht i is assessed
L
to be 24.0 ns, thus much shorter than that of the bulk
perovskite and mesoporous powders (equal to 41.5 and
5
3.7 ns, respectively). Confining the growth of CsPbBr in
3
a such small volume also led to a substantial increase of the
PLQY up to 31.8% for the films versus 1.6% and 4.1% for
the bulk and mesoporous powder sample, respectively. All
optical parameters for the three samples are listed in the
4
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atmospheric conditions, thereby providing an effective strat-
egy to harness the potential of this material in light-emitting
and light-harvesting devices.
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Experimental Section
The starting precursor solution consisted of a stoichiometric
mixture of CsBr and PbBr dissolved in DMF (0.03m) stirred at room
2
temperature overnight. From this solution, a bulk reference sample,
a powder composite and a film composite were all prepared. The bulk
sample was obtained by drop casting the precursor solution on a glass
slide. The powder sample was prepared via the previously reported
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8a]
method: in brief, the perovskite solution was added dropwise to
SBA-15 silica powder (synthesized according to Ref. [22]) before
ꢁ1
being vortex-mixed vigorously (1 mLmg ). For the composite film,
first a sol–gel solution consisting of a mixture of TEOS, P123, H O,
HCl and ethanol (molar ratio 18.5:1.5:100:0.02:110) was prepared
and aged for 3 h, according to a previously published method. The
solution was then spin-coated on various substrates (glass, silicon) at
2
[
23]
3
000 rpm for 15 s, aged overnight and heat-treated at 4008C for 5 h,
ꢁ
1
with a ramping of 18Cmin . The perovskite solution is then spin-
casted at 3000 rpm for 15 s on the mesoporous film. Finally, all three
samples (bulk, powder, and film) are heat treated on a hotplate at
8
3
08C, under mild vacuum, for 30 min, followed by 1508C, in air, for
0 min.
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Manuscript received: February 22, 2018
Revised manuscript received: April 15, 2018
Version of record online: && &&, &&&&
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Angew. Chem. Int. Ed. 2018, 57, 1 – 6
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Perovskites
Perovskite nanocrystals were grown in
mesoporous templates. The hierarchical
nature of the porous structure allowed
the interconnecting pores to be easily
filled with perovskite precursor and gen-
erate stable blue-emitting perovskite
nanocrystals (ca. 3.1 nm) that are well-
distributed throughout the film. The
mesoporous template also serves as
a protective barrier to preserve their
optical properties from atmospheric
conditions.
V. Malgras, J. Henzie,* T. Takei,
Y. Yamauchi*
&&&& — &&&&
Stable Blue Luminescent CsPbBr3
Perovskite Nanocrystals Confined in
Mesoporous Thin Films
6
www.angewandte.org
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2018, 57, 1 – 6
These are not the final page numbers!