CsPbBr3 QD/AlOx Inorganic Nanocomposites with Exceptional Stability in Water, Light, and Heat

Article abstract of DOI:10.1002/anie.201703703

Herein, the assembly of CsPbBr₃ QD/AlO_x inorganic nanocomposites, by using atomic layer deposition (ALD) for the growth of the amorphous alumina matrix (AlO_x), is described as a novel protection scheme for such QDs. The nucleation and growth of AlO_x on the QD surface was thoroughly investigated by miscellaneous techniques, which highlighted the importance of the interaction between the ALD precursors and the QD surface to uniformly coat the QDs while preserving the optoelectronic properties. These nanocomposites show exceptional stability towards exposure to air (for at least 45 days), irradiation under simulated solar spectrum conditions (for at least 8 h), and heat (up to 200 °C in air), and finally upon immersion in water. This method was extended to the assembly of CsPbBr_xI_3?x QD/AlO_x and CsPbI₃ QD/AlO_x nanocomposites, which were more stable than the pristine QD films.

Full text of DOI:10.1002/anie.201703703

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Accepted Article
Title: CsPbBr3 QD/AlOx inorganic nanocomposites with exceptional
stability in water, light and heat
Authors: Anna Loiudice, Seryio Saris, Emad Oveisi, Duncan T. L.
Alexander, and Raffaella Buonsanti
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201703703
Angew. Chem. 10.1002/ange.201703703
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CsPbBr3 QD/AlOx inorganic nanocomposites with exceptional
stability in water, light and heat
A. Loiudice,+[a] S. Saris,+[a] E. Oveisi,[b] D. T. L. Alexander,[b] and R. Buonsanti*[a]
Abstract: Herein, the assembly of CsPbBr3 QD/AlOx inorganic
nanocomposites, using atomic layer deposition (ALD) for the growth
of the amorphous alumina matrix (AlOx), is proven as a novel
protection scheme for this new class of QDs. The nucleation and
growth process of AlOx on the QD surface was thoroughly
investigated by a miscellanea of techniques which highlighed the
importance of the interaction between the ALD precursors and the
QD surface to uniformly coat the QDs while preserving the
optoelectronic properties. These nanocomposites show an
exceptional stability against exposure to air (for at least 45 days),
irradiation under simulated solar spectrum (for at least 8h), to
thermal treatment (at least up to 200ºC in air), and finally against
immersion in water. The method was extended to assembly
CsPbBrxI3-x QD/AlOx and CsPbI3 QD/AlOx nanocomposites which
were more stable compared to the pristine QD films.
study, the as-obtained films were not uniformly stabilized and
loss of luminescence in the central region and at the edges of
the films was evident. Recently, Li et al. have utilized
trimethylaluminum cross-linking on the surface of thin film
perovskite QDs with the only purpose to prevent their dissolution
in organic solvents.[11] Silica has also been used as a matrix to
protect the inorganic perovskite QDs.[12,13] While increased
stability in solution was demonstrated with this approach, long-
term stability in thin films has not been studied. The bottom line
is that, while for organic-inorganic perovskites different
stabilizing approaches have been successfully implemented,
based mostly on polymers, no reliable approach exists for
inorganic perovskite QDs.[14,15]
Herein, the encapsulation with an amorphous alumina
(AlOx) matrix deposited by atomic layer deposition (ALD) is
proposed as an effective strategy to stabilize CsPbX3 QDs.
Compared to the protection schemes mentioned above, the
advantages of the metal oxide encapsulation by ALD include
conformity, uniformity and low solid-state ion diffusion
coefficients.[16–19] Previous works have proven metal oxide
matrices (Al2O3 and ZnO) to improve the oxidative and
photothermal stability of other QDs (PbSe, PbS, CdSe,
CdSe@ZnS and CdTe).[16–18,20–25] However, no application to the
perovskite QDs has been reported so far. The high sensitivity to
moisture, temperature and light of this class of QDs makes the
development of an optimal ALD process not trivial. For example,
low temperature and short water pulses, are mandatory to avoid
QD degradation. In this work, a low temperature ALD process
for the deposition of AlOx on a CsPbX3 QD thin film has been
successfully developed. The AlOx matrix protects the perovskite
QDs from oxygen and moisture in air, confers them stability in
water and prevents sintering, thus improving their stability at
high temperature and under light exposure for hours. The
process is of general applicability and has been successfully
exploited for different composition, specifically for CsPbBr3,
CsPbI3 and CsPbBr3-xIx.
In the last year, all-inorganic perovskite quantum dots (CsPbX3
QDs with X = Br, I, Cl) have emerged as a new class of
semiconductor nanocrystals with outstanding optical properties
and with huge promise in QD-based photovoltaics, light emitting
diodes and lasing applications.[1–5] One of the main issues that
still remains is to achieve stability against air, temperature, light
irradiation and water. In addition to their implementation in
optoelectronic devices, developing strategies to stabilize the
perovskite QDs is crucial to explore their intrinsic optoelectronic
properties, which may require relatively long measurements in
air and under light.[6,7] Currently, a few attempts have been made
to stabilize CsPbX3 QDs.[8-15] As one example, Pan et al. have
demonstrated stability in air at high optical fluence for more than
one day by passivating their surface with didodecyl
dimethylammonium
sulphide.[8]
The
introduction
of
poly(maleicanhydride-alt-1-octadecene) during the synthesis of
CsPbX3 QDs has been suggested to improve their stability
during irradiation under UV light.[9] At the same time this
treatment was not enough to prevent structural degradation, as
sintering and phase transition were still observed. Palazon et al.
have discovered increased stability in air and water upon
exposure of CsPbX3 QD films to a low flux of X-rays for 5 hours
as a result of the formation of intramolecular C=C bonding
between adjacent ligands.[10] While this represents an interesting
The CsPbBr3 QDs, synthesized according to the procedure
by Protesescu et al., were spin-coated on silicon or glass
substrates before the ALD process (Figure S1).[1] The latter was
carried at 50 ºC with trimethylaluminum (TMA) and H2O as the
precursor and the co-reactant, respectively. Low temperature
was essential to avoid sintering and structural changes of the
QDs (Figure S2).[26]
[a]
[b]
[a]
Dr. A. Loiudice, S. Saris
Department of Chemical Sciences and Engineering, EPFL Valais
Wallis, CH-1951 Sion, Switzerland.
Dr. E. Oveisi, Dr. D. T. L. Alexander
Interdisciplinary Centre for Electron Microscopy (CIME), EPFL, CH-
1015 Lausanne, Switzerland.
Prof. Buonsanti*
Department of Chemical Sciences and Engineering, EPFL Valais
Wallis, CH-1951 Sion, Switzerland.
Each deposition cycle consisted of one TMA pulse (t1), purging
(t2), one H2O pulse (t3), purging (t4). The ALD parameters were
varied to identify the optimal conditions for a uniform protective
coating without degrading the QDs. Optimal times were found to
be t1 = 0.015 s, t2 = 10 s, t3 = 0.010 s and t4 = 180 s. Figure 1
gives an overview of the structural analysis conducted by
different electron microscopy techniques to fully characterize a
typical CsPbBr3 QD/AlOx inorganic nanocomposite film.
e-mail: raffaella.buonsanti@epfl.ch
+
A.L. and S.S. contributed equally to this work.
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Figure 1. A) Schematic representation of the CsPbBr3/AlOx nanocomposite and cross-sectional STEM-HAADF image of a typical nanocomposite obtained
by performing 100 ALD alumina cycles on a 60nm thick QD film. B) In-plane STEM-HAADF image of CsPbBr3 QD film infilled with AlOx. C) Cross-
sectional STEM-HAADF image and corresponding EDX elemental colored maps of Al, O, Pb, Br and Br+Al and D) EELS color-coded elemental intensity
maps of Al and Br.
Figure 1A shows
a
cross-sectional scanning transmission
in EDX, could be mapped, confirming not only the pure Al-
containing overcoating layer, but also identifying the ALD-
deposited AlOx bottom substrate layer. Finally, the infilling of the
alumina throughout the CsPbBr3 QD films and the absence of
any noticeable change in chemical composition and oxidation
states in the CsPbBr3 QDs after the ALD process was
corroborated by X-ray photoelectron spectroscopy (XPS)
(Figures S5 and S6). The XPS data gave also some insights into
the stoichiometry of the amorphous alumina matrix. While the
Al:O ratio was 2:3 in the overcoat layer, a deviation from
stoichiometry with increasing oxygen content was measured in
the infilling layer (Figure S6). Oxygen-rich alumina is quite
common in low temperature ALD deposition due to uncompleted
reaction of the TMA precursor with the Al-OH surface species
which results in a higher hydroxyl concentration in the film.[29]
The nucleation and the growth steps of the alumina deposition
process were studied in depth by combining miscellanea of
techniques (Figure 2 and Figure S9).
The proposed nucleation mechanism is sketched in Figure 2A.
The nucleation of alumina in the ALD process requires the
presence of polar groups on the solid surface, commonly
hydroxyl groups (-OH).[25] Thus, it is suggested that, in the initial
stage of nucleation, the TMA reacts with the partially oxidized
NC surface. Upon saturation of the oxidized surface sites, the
TMA molecule intercalates at the hydrophilic ligand/QD interface
as recently proposed by Li et al.[11]
electron microscopy high angle annular dark field (STEM-
HAADF) image of the CsPbBr3 QD/AlOx nanocomposite along
with a sketch of the final structure. This analysis beautifully
reveals that the QDs are embedded in the alumina matrix
without any change of size and shape compared with the
pristine QDs (Figure S1). In this sample an AlOx overcoating
layer of around 10 nm was measured in the STEM-HAADF
image (Figure 1A). High resolution STEM-HAADF was also
performed on single QDs covered with 5 cycles of AlOx (Figure
1B). The alumina coating conferred stability to the QDs under
the electron beam, and enabled high resolution imaging without
special low dose techniques.[27] No change in the lattice
parameters (5.7 Å) was observed compared with the reported
values for CsPbBr3 QDs.[28] The XRD patterns of the QD film
before and after the AlOx deposition were also identical (Figure
S3). STEM-HAADF imaging and area selective energy-
dispersive X-ray spectroscopy (EDX) maps are shown in Figure
1C. The Cs:Pb:Br ratio was constant at 1:1:3 in the EDX
spectrum, indicating that the stoichiometry is preserved in the
nanocomposites (Figure S4, Table 1). The higher Al atomic
percentage on the top layer suggests that an alumina
overcoating layer ultimately seals the QD film. To confirm the
infilling of AlOx in the interstices between the CsPbBr3 QDs,
STEM electron energy-loss spectroscopy (EELS), which allows
to distinguish the Al K edge from that of the Br L edge, was
applied (Figure 1D). In this way, the two signals, which overlap
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Partial surface oxidation was assessed by XPS. Figure 2B
shows representative XPS spectra corresponding to the Pb 4f
core levels for the pristine QDs (black), the QDs after one TMA
pulse (blue) and the final QD/AlOx nanocomposite (red). In the
peak deconvolution, the component at 137.6 eV in the pristine
QDs and in the QD film after one TMA pulse corresponds to
oxygen bonded lead (Pb-O), in agreement with O1s core level
energy in Figure S5 (The metallic lead may form under the XPS
beam).[30] Fourier Transform Infrared Spectroscopy (FTIR) was
utilized to investigate the fate of the ligands and the eventual
intercalation of TMA in the ligand shell (Figure 2C). The results
evidence that the ligands do not decompose during the ALD
process, which is reasonable considering the low temperature
used in the process. The spectrum of the pristine QDs is
consistent with the presence of long-chain aliphatic ligands
(oleylamine and oleic acid).[31] After the TMA pulse, the main
change in the spectrum is the appearance of two strong and
sharp peaks at 1591 cm−1 and 1496 cm−1, which are
characteristic of the COO- stretching modes. The FTIR spectrum
corresponding to the nanocomposites shows a strong -OH peak
centered around 3450 cm−1. Such a signal is expected for an
alumina grown by an ALD process at low temperature, which
should contain many hydroxyl impurities.[18,29] The N1s and C1s
signals present in the XPS depth profiling throughout the film
thickness are in agreement with the FTIR results (Figure S7).
Finally, to closely follow the interaction of TMA with the QDs
surface during the initial phases of nucleation, 1H Nuclear
Magnetic Resonance (NMR) spectra were recorded on QDs
reacted with different amount of TMA. To make these
experiments possible, a solution-based process was developed
to mimic the reaction of the TMA with the QD surface during the
ALD (see SI for experimental details). The alkene and the TMA
spectral regions are included in Figure 2D and the full spectra
are reported in Figure S8. The alkene resonances around 5.6
ppm correspond to the oleyl species, deriving from oleylamine
and oleic acid, which are the native ligands for the CsPbBr3
QDs.[31] As the TMA/NC ratio increases, changes in the alkene
region indicate modification of the ligand shell while the broad
and shifted resonance (compared to the pure compound, Figure
S8) around -0.4 ppm strongly suggests that the TMA binds the
NC surface.[31,32,33]
To estimate the alumina growth per cycle (GPC), Inductively
Coupled Plasma Mass Spectrometry (ICP-MS) analysis was
used as the main tool to calculate the amount of alumina
deposited per each ALD cycle and complementary techniques
supported the obtained trends (Figures S9 and S10). Two
different growth regime were identified: Step 1 (number of cycles
<75, GPC =1.06 ng cm-2 cycle-1) and Step 2 (number of cycles
>75, GPC = 0.21 ng cm-2 cycle-1), which are associated with the
infilling and the overcoating processes, respectively. The
Scanning Electron Microscopy (SEM) cross-section analysis on
QD films with different thickness and the same number of ALD
cycles (Figure S10) reveals a consistent full infilling of the QD
films up to a thickness of around 250 nm, when the pore
clogging effect during ALD becomes evident.[34]
The comparison between the optical properties of the pristine
QDs and the nanocomposites was then carried out (Figure S11).
No significant change of the absorption features and no peak
shift were observed, consistent with the absence of
compositional and morphological changes from TEM, SEM,
XRD and XPS analyses. However, a photoluminescence (PL)
intensity decrease of around 50% with respect to the initial value
was recorded (Figure S11). Such a change was too big to be
solely attributed to the change of total reflectance due to the
alumina matrix.[23] Previous works on alumina encapsulated QDs
have observed similar PL decrease and attributed it to ligand
desorption in the vacuum step inducing surface-trap states.[18]
Several experiments were carried out to isolate the effects of
different parameters on the PL decrease and are described in
the Supporting Information. While the effects of temperature,
time, reactants are interrelated and difficult to disentangle, the
interaction of the TMA with the NC surface was determined to be
the key factor in the PL quenching (Figure S12).
The stability of the CsPbBr3 QD/AlOx nanocomposites was then
tested in conditions which are expected to degrade the
perovskite QDs: long-term storage in air, immersion in water,
light irradiation, and annealing at relatively high temperatures
(Figure 3).[9,35–37] Figure 3A shows a comparison of the PL
properties of the pristine QDs and of the nanocomposites as a
function of the storage time in air. Over the course of 45 days,
no apparent change was observed in the PL properties for the
CsPbBr3 QD/AlOx nanocomposites. On the contrary, the pristine
samples showed a sharp PL quenching and a red-shift after only
a few days of storage. Absorption measurements revealed
similar behavior, indicative of rapid decomposition and sintering
for untreated samples compared to long-term stability for the
nanocomposites (Figure S13).
Figure 2. A) Scheme of the possible nucleation mechanism of alumina on
the QD surface during the ALD process. B) Pb 4f XPS spectra and C)
FTIR spectra and of the pristine QD film (black), the QDs after one pulse
of TMA (blue) and the final QD/AlOx nanocomposite (red); D) 1H NMR in
the region of the alkene protons (top) and TMA protons (bottom) while
varying the concentration of TMA.
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Figure 3: Stability studies on CsPbBr3 QD/AlOx nanocomposites obtained by performing 100 ALD cycles on 60 nm thick QD films. A) PL properties over
45 days of storage in ambient conditions. Red color shows changes in PL intensity and blue color shows changes in the PL peak position. B) Photographs
taken under UV illumination (λ=365 nm) after 1 h of soaking in water. C) PL properties after 8 h photosoaking under solar spectrum irradiation at
10mW/cm2. D) XRD spectra after annealing in air at 200 ºC. For the graphs showing PL data typical error range is Ierr= ±5 % and λerr = ±1 nm.
Furthermore, the nanocomposites exhibited
a
striking
XRD measurements showed no particular change after
photosoaking, indicating that neither degradation nor sintering
took place (Figure S18). Quenching was still observed in
additional UV-filtered and air-free photosoaking experiments
(Figure S19), which allows one to exclude the contribution of
photochemical degradation of the ligands from the alumina
band-gap (>3.4 eV) excitation and also of UV-induced
coarsening.[17,38] An alternative explanation for this quenching
effect at higher irradiation densities is the photo-induced
desorption of surface ligands and the subsequent formation of
carrier trapping defects, as previously reported for chalcogenide
QD films.[6,39,40] The effect of annealing on the CsPbBr3 QD/AlOx
nanocomposites was investigated by heating the samples at
200ºC in air. Figure 3D shows the XRD spectra for the pristine
QDs and the nanocomposites before and after annealing. Upon
annealing, the pristine QD film shows clear sintering and
additional peaks which are consistent with the presence of the
thermodynamically stable orthorhombic phase (Figures S20-
S21).[2,25,43–46] On the contrary, there are no additional XRD
peaks in the nanocomposites. However, a decrease of 20% in
the full width at half maximum (FWHM) of the (200) peak
suggests that necking of those QDs not fully covered by the
improvement in stability when immersed in water as they were
stable for at least 1 hour compared to immediate degradation
observed for the pristine QDs (Figures 3B and S14, video in the
Supporting Information). The crucial role of the overcoat layer as
diffusion barrier to confer water stability to the nanocomposites
was evidenced by testing QD films of the same thickness at
varying ALD cycles (Figure S15). Interestingly, while only subtle
structural differences were found when reducing t4 from 180 s to
20 s, the nanocomposites with lower purging time after the water
pulse were less stable (Figure S16). A lower density of the
alumina matrix and decreased conformity of the alumina layer
on the QD surface might explain this observation.[17,29] The effect
of the ALD treatment on the photostability of CsPbBr3 QD films
was studied. Photo-soaking experiments were conducted under
simulated solar spectrum irradiation in air. Figure 3C evidences
the dramatic PL intensity decay and energy red-shift of the
pristine QDs that result from the combination of chemical and
photo-degradation.[17,38] Instead, the nanocomposites have
minimal PL quenching after 8 h of irradiation with no apparent
energy shifting. However, when the irradiation power density
was increased from 10 mW/cm2 to 100 mW/cm2, an irreversible
PL intensity quenching of more than 50% after 8 h was observed, alumina might occur (Figures S20-S21). All together, the stability
which stabilizes afterwards (Figure S17). Yet, absorption and tests point at the importance of combining the alumina infilling
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layer with the overcoat layer to build an effective protection
scheme where the main function of the former is to prevent ion
diffusion and thus sintering during annealing, while the latter is
crucial to confer stability in water by acting as a diffusion barrier
to oxygen and water.
Finally, the same ALD process was applied to obtain CsPbI3
QD/AlOx and CsPb(I/Br)3 QD/AlOx nanocomposites which
possessed increased stability compared to the bare QD film
counterparts as evidenced in Figures S22-S25. It should be
noted that the CsPbI3 QD/AlOx, while more stable that the
unprotected films, do not show exceptional stability, most likely
as a result of the intrinsic instability of this particular composition.
Keywords: perovskites • quantum dots • nanocomposites •
atomic layer deposition • stability
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Experimental Section
Experimental details are reported in the Supporting Information, which
contain: details of material synthesis, characterization methods, TEM and
SEM images, XRD patterns, PL, UV-Vis absorption, XPS data, videos.
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Acknowledgements
A.L acknowledges the H2020-Marie Curie Individual Fellowship
with grant agreement number 701745 for financial support. R.B.
thanks the Swiss SNF (AP Energy Grant, project number
PYAPP2_166897/1). We thank Dr. Adam Shwartzberg for useful
discussion on the ALD process, Daniele Laub for cross section
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in water
CsPbBr3/AlOx
Stabilizing the unstable: CsPbBr3
QDs are stabilized against water,
light and heat by embedding them in
an alumina matrix (AlOx) deposited
by atomic layer deposition (ALD).
The interactions between the ALD
precursor and the surface of the
QDs are key to uniformly coat the
A. Loiudice+, S. Saris+, E. Oveisi, D. T.
L. Alexander, and R. Buonsanti*
CsPbBr3/AlOx
Page No. – Page No.
CsPbBr3 QD/AlOx inorganic
nanocomposites with exceptional
stability in water, light and heat
20 nm
QDs
while
preserving
their
optoelectronic properties.
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