Green-Emitting Powders of Zero-Dimensional Cs4PbBr6: Delineating the Intricacies of the Synthesis and the Origin of Photoluminescence

Article abstract of DOI:10.1021/acs.chemmater.9b02944

A detailed investigation into the synthesis of green-emitting powders of Cs₄PbBr₆ and CsPbBr₃ materials by antisolvent precipitation from CsBr-PbBr₂ precursor solutions in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) is reported. Various solvated lead bromide and polybromide species (PbBr₂, [PbBr₃]^-, [PbBr₄]^2-, and possibly [PbBr₅]^3- or [PbBr₆]^4-) are detected in the precursor solutions by optical absorbance and emission spectroscopies. The solvodynamic size of the species in solution is strongly solvent-dependent: -1 nm species were detected in DMSO, while significantly larger species were observed in DMF by dynamic light scattering. The solvodynamic size of the lead bromide species plays a critical role in determining the Cs-Pb-Br composition of the precipitated powders: smaller species favor the precipitation of Cs₄PbBr₆, while larger species template the formation of CsPbBr₃ under identical experimental conditions. The powders have been characterized by ¹³³Cs and ²⁰⁷Pb solid-state nuclear magnetic resonance, and ¹³³Cs sensitivity toward the different Cs environments within Cs₄PbBr₆ is demonstrated. Finally, the possible origins of green emission in Cs₄PbBr₆ samples are discussed. It is proposed that a two-dimensional Cs₂PbBr₄ inclusion may be responsible for green emission at -520 nm in addition to the widely acknowledged CsPbBr₃ impurity, although we found no conclusive experimental evidence supporting such claims.

Full text of DOI:10.1021/acs.chemmater.9b02944

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Article
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Green-Emitting Powders of Zero-Dimensional CsPbBr: Delineating
the Intricacies of the Synthesis and the Origin of Photoluminescence
Aniruddha Ray, Daniela Maggioni, Dmitry Baranov, Zhiya Dang, Mirko Prato, Quinten
A. Akkerman, Luca Goldoni, Enrico Caneva, Liberato Manna, and Ahmed L. Abdelhady
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02944 • Publication Date (Web): 28 Aug 2019
Downloaded from pubs.acs.org on August 29, 2019
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Green-Emitting Powders of Zero-Dimensional
Cs4PbBr6: Delineating the Intricacies of the
Synthesis and the Origin of Photoluminescence
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Aniruddha Ray,†,‡ Daniela Maggioni,∥ Dmitry Baranov,† Zhiya Dang,† Mirko Prato,Δ Quinten A.
Akkerman,† Luca Goldoni,⊥ Enrico Caneva,§ Liberato Manna,*,† Ahmed L. Abdelhady*,†
†Nanochemistry Department, ΔMaterials Characterization Facility, and ⊥Analytical Chemistry
Lab., Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy
‡Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Via
Dodecaneso, 31, 16146, Genova, Italy
∥Dipartimento di Chimica, and §UNITECH COSPECT (COmprehensive Substances
characterization via advanced sPECTtrometry), Università degli Studi di Milano, Via Golgi 19,
20133 Milano, Italy
Corresponding Author
*liberato.manna@iit.it, ahmed.abdelhady@iit.it
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ABSTRACT. A detailed investigation into the synthesis of green-emitting powders of Cs4PbBr6
and CsPbBr3 materials by antisolvent precipitation from CsBr-PbBr2 precursor solutions in
dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) is reported. Various solvated lead
bromide and polybromide species (PbBr2, [PbBr3]-, [PbBr4]2-, and possibly [PbBr5]3- or [PbBr6]4-)
are detected in the precursor solutions by optical absorbance and emission spectroscopies. The
solvodynamic size of the species in solution is strongly solvent-dependent: ~1 nm species were
detected in DMSO, while significantly larger species were observed in DMF by dynamic light
scattering. The solvodynamic size of the lead bromide species plays a critical role in determining
the Cs-Pb-Br composition of the precipitated powders: smaller species favor the precipitation of
Cs4PbBr6, while larger species template the formation of CsPbBr3 under identical experimental
conditions. The powders have been characterized by 133Cs and 207Pb solid-state nuclear magnetic
resonance and 133Cs sensitivity toward the different Cs environments within Cs4PbBr6 is
demonstrated. Finally, the possible origins of green emission in Cs4PbBr6 samples are discussed.
It is proposed that a two-dimensional Cs2PbBr4 inclusion may be responsible for a green emission
at ~520 nm in addition to the widely acknowledged CsPbBr3 impurity, although we found no
conclusive experimental evidence supporting such claim.
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TOC
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All-inorganic lead halide semiconductors have recently attracted renewed research interest due
to the various stoichiometries that can be crystallized from the same precursors. Lead halide
compounds demonstrate structural tunability based on the connectivity of the [PbX6]4- (X = Cl-,
Br-, or I-) octahedra that can form three-dimensional (3D) corner-sharing structures (perovskites),
layered 2D networks, chains of 1D, as well as 0D isolated octahedral clusters.1-5 In case of the
organic-inorganic lead halide compounds, the size of the organic cation has been found to be one
of the main factors determining the dimensionality of the octahedral framework. However, in the
all-inorganic Cs-Pb-Br compounds, all three dimensionalities can be formed with the same Cs
cation. Mixing CsBr and PbBr2, in specific ratios and in appropriate solvents, leads to the formation
of pure or mixed structures of CsPbBr3 (3D), CsPb2Br5 (2D), and Cs4PbBr6 (0D).6 PbBr2-rich
conditions lead to the 2D tetragonal CsPb2Br5 phase, characterized by a sandwich structure with
Cs+ ions in between layers of [Pb2Br5]-. On the other hand, under CsBr-rich conditions, a trigonal
0D Cs4PbBr6 structure is formed, which consists of disconnected [PbBr6]4− octahedra separated by
Cs+ ions. An equimolar amount or a slight excess of PbBr2 with respect to CsBr results in formation
of 3D CsPbBr3 with an orthorhombic structure of corner-sharing [PbBr6]4− octahedra, as in the
organic-inorganic perovskites (MAPbX3 and FAPbX3; MA = methylammonium and FA =
formamidinium) commonly used in optoelectronic applications.7
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Cs4PbBr6 based materials have recently gained increased interest due to the reports of strong and
stable green photoluminescence (PL) in their powder, thin-film, single crystal and nanocrystal
forms.8-15 However, it was reported that Cs4PbX6 compounds are colorless16 with bandgaps in the
UV region.12 Specifically, Cs4PbBr6 has a matching experimental and theoretical bandgap around
3.95 eV.17-19 Hence, the origin of the green PL has been proposed to be either due to intrinsic
13-15
defects within the wide bandgap Cs4PbBr6
or due to the presence of 3D CsPbBr3
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nanocrystals.19-24 Ling et al.25 suggested that the 3D CsPbBr3 phase is not the direct cause of the
intense PL; instead, they proposed trap states at the interface between the two phases (3D and 0D)
to be responsible for that intense green emission.
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Herein, we explore reaction parameters governing the formation of Cs4PbBr6 and CsPbBr3 phases
from CsBr-PbBr2 solutions and the origin of the green PL in the Cs4PbBr6 samples. The nature and
solvodynamic size of the lead bromide species formed in solution varies with the coordination
strength of the solvent and subsequently determines the composition of the formed Cs-Pb-Br
powders: species with a ~1 nm solvodynamic radius (presumably isolated octahedra) were
detected in dimethyl sulfoxide (DMSO) solutions of CsBr-PbBr2 leading to the formation of
Cs4PbBr6 phase upon addition of an antisolvent, while larger species were detected in
dimethylformamide (DMF) and resulted in the formation of CsPbBr3 phase. In addition to the
solvent, the solvent-antisolvent pair and the absolute concentrations of the precursors are key
parameters defining the structure of the precipitated material. For example, antisolvents such as
tetrahydrofuran (THF) and dichloromethane (DCM) precipitate mainly Cs4PbBr6 and CsPbBr3
phases, respectively, from the CsBr-PbBr2 solutions in DMSO when used in 1:6
(solvent:antisolvent by volume) ratios. Increasing the concentration of CsBr-PbBr2 while keeping
their molar ratio fixed (1:1) from 0.05 M to 0.5 M in DMSO favors formation of CsPbBr3 at higher
concentrations. In addition to the powder X-ray diffraction (XRD) technique commonly used for
a structural and compositional identification of the materials, we apply 133Cs and 207Pb solid-state
nuclear magnetic resonance (ssNMR) to the analysis of the various Cs-Pb-Br precipitates. We
found that 133Cs ssNMR is more sensitive toward detecting relatively low impurity content of the
different phases compared to its 207Pb counterpart. The 133Cs ssNMR results are in agreement with
XRD data in terms of observed phases, but 133Cs ssNMR could also distinguish between the two
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different Cs crystallographic positions in the 0D crystal. Finally, based on the observed absorbance
in the visible region, and the recurring green PL of Cs4PbBr6 samples, we hypothesize that a 2D
Cs2PbBr4 inclusion (not observed experimentally) may be responsible for a green PL in addition
to the widely acknowledged 3D impurity.
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EXPERIMENTAL SECTION
Materials. CsBr (99.9%, Sigma-Aldrich), PbBr2 (98%, Sigma-Aldrich), DMSO (99.9%, Sigma-
Aldrich), DMF (99.8%, Sigma-Aldrich), THF (99.9%, Sigma-Aldrich) and DCM (99.8%, amylene
stabilizer, Sigma-Aldrich).
Preparation of Cs-Pb-Br Powders. Generally, CsBr and PbBr2 were dissolved in DMSO or
DMF at the required ratio and at the required concentration by sonication for 1 hour. Then the
solutions were filtered using PTFE filter with 0.2-mm pore size and the precipitation was carried
out by quick injection of THF or DCM to the filtrate.
Characterization. The optical absorbance of the precursor solutions was recorded on a Varian
Cary Eclipse UV−vis spectrophotometer. PLE and PL of precursor solutions were performed on a
Varian Cary Eclipse fluorescence spectrophotometer. DLS was performed using a Zeta Sizer
(Malvern Instruments) equipped with a 4.0 mW He-Ne laser operating at 633 nm and an avalanche
photodiode detector, in order to determine the average size of the precursors in the mother solution.
X-ray Diffraction analysis was performed using PAanalytical Empyrean X-ray diffractometer
equipped with a 1.8 kW Cu Kα ceramic X-ray tube and a PIXcel3D 2 × 2 area detector, operating
at 45 kV and 40 mA. Cs-Pb-Br powders were studied under ambient conditions using a parallel
beam bragg Brentano geometry. XRD data was analysed using the HighScore 4.1 software from
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PANanalytical. Step sizes of 0.026° and 0.013° and scan speeds of 0.102 and 0.013°/s were used
for the standard and slow scans, respectively.
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PL and PLQY measurements on dried powders were conducted on an Edinburgh Instruments
FLS920 fluorescence spectrometer equipped with a calibrated integrating sphere exciting at 405
nm. Absorbance measurements for the powder samples were carried out in a Cary 5000
spectrometer using the diffuse reflectance accessory. Optical microscopy of the 4:1_DMSO
powder was performed on a Nikon 80i microscope equipped with a grayscale digital camera and
a high-pressure mercury lamp (Nikon) and optical filters. The sample was prepared by mixing a
small amount of the finely ground powder with a drop (~5 μl) of octadecene-1. The mixture was
prepared on top of a rectangular microscope slide and covered with a thin glass coverslip. Confocal
PL microscopy on the same sample was performed using a Nikon A1 confocal laser microscope
with 488 nm excitation (Oxxius LBX-488 diode laser). The laser power setting was kept at 0.5%,
and the resolution of a 32-channel spectral detector set at 2.5 nm during the experiments. The
spectrally-resolved imaging data and the z-series used for a volume view reconstruction (Figure
S8) were acquired (as *.nd2 files) using Nikon NIS-Elements High Content Software Analysis
ver. 4.30.02 and viewed and processed in either Nikon NIS-Elements Viewer ver. 4.20.00 or
ImageJ 1.51j8 as previously described.26
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XPS measurements were carried out on a Kratos Axis UltraDLD spectrometer, using a
monochromatic Al Kα source (15 kV, 20 mA). The spectra were taken on a 300 × 700 μm2 area.
Wide scans were collected with a constant pass energy of 160 eV and an energy step of 1 eV.
High-resolution spectra were acquired at a constant pass energy of 10 eV and energy step of 0.1
eV. The binding energy scale was referenced to the C 1s peak at 284.8 eV. The spectra were
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analyzed using the CasaXPS software (version 2.3.17). Samples were prepared by covering an
indium substrate with the Cs-Pb-Br powder.
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High-resolution transmission electron microscopy (HRTEM) analysis was carried out on a 200
kV TEM (JEOL JEM-2200FS) and a Bruker Quantax 400 system with an XFlash 5060 detector
was used for EDS analysis. The sample was prepared by dispersing the powders in THF before
depositing on an ultrathin carbon film supported on a holey carbon film coated 400 mesh TEM
copper grids.
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Solid-state NMR (ssNMR) measurements were collected on a 500 MHz Bruker BioSpin NMR
Spectrometer AvanceTM 500, operating at a static field of 11.7 Tesla and equipped with a 4 mm
CPMAS probe, by spinning the sample at the magic angle (MAS), with speed rates from 10 to 14
kHz. All samples were packed into Zirconia (ZrO2) rotors and closed with Kel-F caps (80 μL
internal volume). The working frequency of the 133Cs nucleus was 65.598 MHz and spectra were
acquired by using a single pulse sequence (zg), with a spectral width of 104166 Hz, wide enough
to ensure a homogeneous irradiation of the whole spectrum. The optimal spinning speed (MAS)
was achieved at 10 KHz; however, comparative spectra were also acquired at different spinning
rates (12 KHz and 14 KHz), to assign all sidebands in the spectrum and to exclude any possible
signal impurity overlapping to the manifold SSB. The NMR parameters used for 133Cs experiments
were: 90° pulse length (p1) = 2.5 μs, repetition time (d1) = 3-60 s, number of scans (ns) = 200-
800. The working frequency of 207Pb nucleus was 104.63 MHz and spectra were acquired by using
a single pulse sequence (solidecho, with an echo time of 7 μs), with a spectral width of 1600 ppm
(corresponding to 167.000 Hz), wide enough to allow an homogeneous irradiation of the whole
spectrum, but not enough to cover the wide anisotropic chemical shift range, typical of such a
nucleus. Different experiments were needed (with the same spectral width but changing the offset)
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to cover the whole possible range of detectable signals. The spinning speed (MAS) was optimized
at 10 kHz, a value that returned resolved spectra. The NMR parameters used for 207Pb experiments
were the following: 90° pulse length (p1) = 2 μs, repetition time (d1) = 0.5 s, ns = 28.000-105.000.
133Cs and 207Pb shifts were referenced to solid CsBr (δiso = 268.0 ppm vs. CsCl: δiso = 223.2 ppm)
and Pb(NO3)2 (δ = -3490 ppm), respectively, which were used as external calibration standards.
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RESULTS AND DISCUSSION
CsBr-PbBr2 solutions in DMSO and DMF. PbBr2-containing solutions in DMF and DMSO are
precursors for the Cs4PbBr6 and CsPbBr3 samples, thus it is anticipated that the nature of the
solvated species has an effect on the resulting material obtained from them. Steady-state optical
spectroscopy is a simple method for investigation of the lead bromide speciation. Absorbance,
photoluminescence (PL), and photoluminescence excitation (PLE) spectra of 0.05 M solutions of
CsBr:PbBr2 mixtures (1:1 molar ratio) in DMSO and DMF were collected and are reported in
Figure 1. The DMSO solution showed an absorbance peak at 285 nm that is attributed to PbBr2,
while the DMF solution had a clear absorbance peak at 310 nm corresponding to [PbBr3]- (Figure
1a), consistent with previous studies.27-28 Both solutions showed an absorbance tail in the longer
wavelength region (340 - 360 nm, inset in Figure 1a), suggesting the possible formation of
[PbBr4]2- species.27 Both [PbBr3]- and [PbBr4]2- species are emissive (at ~600 nm and 560 nm,
respectively)27-28 and their presence can be confirmed through selective wavelength excitation
(Figure 1b). The DMSO solution emitted at 560 nm regardless of the excitation wavelength,
indicating the presence of mainly [PbBr4]2-. On the other hand, the DMF solution exhibited
emissions at 560 and 600 nm, confirming the presence of detectable amounts of both [PbBr3]- and
[PbBr4]2- species. A relatively higher PL intensity of the DMF solution compared to DMSO under
360 nm excitation (Figure 1b) suggests relatively higher concentration of [PbBr4]2- species in DMF
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with respect to DMSO (consistent with a higher absorbance at around ~360 nm). The presence of
an extra PLE peak at 374 nm in the DMF solution (Figure 1b) that is red shifted relative to that of
[PbBr4]2- in the DMSO solution suggests that higher-order lead polybromide complexes, such as
[PbBr5]3- or [PbBr6]4- might be present in DMF (by analogy with their iodide counterparts).29
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We performed DLS measurements on both solutions to further investigate the discussed
differences in lead bromide speciation between two solvents. It was found that the DMSO solution
contained roughly 1 nm-sized lead bromide species while the DMF solution was composed of
significantly larger clusters (Figure 1c). The large-sized clusters in the DMF solution could be due
to an aggregation or polymerization of the lead bromide species, because of the different solvent-
solute interaction between PbBr2 and DMF compared to DMSO (similar to PbI229-31). Our findings
indicate that a change of solvent significantly alters the speciation and the solvodynamic size of
the lead bromide species, as schematically summarized in Figure 1d. The results hold in the
absence of CsBr, as summarized in the SI (see Figure S1 and an accompanying discussion).
Previously, the changes in the solvodynamic size of the lead polyhalide species have been
attributed to the relative ratio of salts (e.g. MAI:PbI2; MA = methylammonium)32 or to the effect
of additives such as hydrohalic acids.33 This is of high significance to the perovskite photovoltaic
community as the solvodynamic size of the species present in the precursor solution influences the
composition of the resulting material (see below) and subsequent optoelectronic properties of the
devices.32-33
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Figure 1. (a) Normalized Absorbance, (b) PLE (solid lines) and PL (dotted and dashed lines)
spectra of 0.05 M 1:1 CsBr:PbBr2 precursor solutions in DMF and DMSO. PLE is reported at the
highest intensity emission (560 nm) while PL is recorded at different excitation wavelengths. (c)
Corresponding DLS measurements. (d) Scheme illustrating the photoactive species formed upon
dissolving 0.05 M 1:1 CsBr:PbBr2 in DMSO and DMF.
Cs4PbBr6 and CsPbBr3 formation. Next we investigated the differences in the composition of
the powders precipitated from the abovementioned CsBr-PbBr2 solutions in DMSO and DMF.
Using the equimolar (0.05 M of each precursor) solutions in DMSO and DMF, the precipitation
of the powders was done by a quick injection of 6 mL of tetrahydrofuran (THF) to 1 mL of the
solution. Powders were then dried overnight in a vacuum oven at 40 °C. X-ray diffraction (XRD)
patterns of the dried powders are plotted in Figure 2a along with the simulated patterns of Cs4PbBr6
and CsPbBr3. It is observed that mainly the 0D trigonal phase was obtained from the DMSO
solution, with only low intensity peaks corresponding to a 3D impurity, while 3D CsPbBr3 was
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the main precipitate from the DMF solution, with an extremely low contamination from the 0D
phase (inset in Figure 2a). The 0D impurity was only detected upon performing a slow XRD scan
(see experimental section). We could eliminate the 0D impurity by using a CsBr:PbBr2 ratio of
1:1.2 in DMF. This is relevant for perovskite-based light-emitting devices, as we observed that
even this extremely small amount of 0D impurity in the 3D sample could significantly enhance
the PL peak (Figure S2). The differences in the compositions obtained from DMSO (mainly
Cs4PbBr6) and DMF (mainly CsPbBr3) under identical precipitation conditions can be rationalized
on the basis of the DLS data of the precursor solutions (Figure 1c). The formation of clusters of
the lead bromide species in the DMF solution possibly templates the growth of the 3D
orthorhombic phase upon the addition of the antisolvent. On the other hand, the approx. one nm-
sized particles in DMSO, which is in agreement with a previous report,34 suggests that most of the
complexes in solution are isolated octahedra, making the crystallization of the 3D phase less
favorable. Coordination strength of the solvent is likely a main factor responsible for these
differences. DMSO is known to have a stronger coordination with Pb2+ compared to DMF;30 hence,
the Cs-rich phase Cs4PbBr6 can be easily precipitated from DMSO solution while the composition
of precipitates from DMF solution is closer to the stoichiometry of the feed solution (1:1
CsBr:PbBr2). Nonetheless, 3D CsPbBr3 could be precipitated out from a DMSO solution by
changing the antisolvent or the absolute concentration of the precursors, as discussed next.
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Figure 2. (a) XRD patterns of Cs-Pb-Br powders precipitated from 0.05 M 1:1 CsBr:PbBr2 DMSO
and DMF solutions along with simulated patterns for trigonal Cs4PbBr6 (ICSD-162158) and
orthorhombic CsPbBr3 (ICSD-97851) phases. (b) XRD of Cs-Pb-Br powders precipitated from
different DMSO:antisolvent volume ratio. (c) Scheme illustrating the effect of the solvodynamic
size and solvent-antisolvent pair on the formed phases.
We studied the effect of two different antisolvents on an equimolar (0.05 M) mixture of CsBr and
PbBr2 dissolved in DMSO. THF and dichloromethane (DCM) were chosen as antisolvents. XRD
patterns of the powders are presented in Figure 2b. Simply by changing the antisolvent from THF
(1:6 THF) to DCM (1:6 DCM), despite the absence of any large-sized clusters, 3D perovskite is
the main precipitate (see sketches in Figure 2c). Hence, it is not only the solvodynamic size of the
different lead polybromide in the precursor solution that controls the phase of the obtained
precipitate but also the solvent-antisolvent pair. In a 1:1 CsBr:PbBr2 DMSO solution, addition of
THF favors the precipitation of the Cs-rich phase (Cs4PbBr6) possibly due to the higher solubility
of PbBr2, compared to CsBr, in this solvent-antisolvent pair. On the other hand, using DCM as an
antisolvent resulted in the precipitation of the 3D phase as both precursors are relatively insoluble
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in the DMSO-DCM pair. Nevertheless, when an excess amount of THF is added (1:30 THF), the
3D phase is the primary formed phase (see Figure 2b). It could be possible that excess THF forces
full precipitation of the PbBr2 present in the precursor solution, hence the stoichiometric 3D phase
is formed. The effect of the solvodynamic size of the different lead polybromide species and
solvent-antisolvent pair on the resulting Cs-Pb-Br phase is demonstrated in Figure 2c. We also
explored a mixed DMSO/DMF solution and found that both THF and DCM leads to the
precipitation of mainly the 3D phase with minor 0D impurity (Figure S3). Another parameter that
we varied was the absolute concentration of the solution (0.05 M to 0.5 M) using DMSO-THF as
the solvent-antisolvent pair, and interestingly discovered that the lower concentration favors the
precipitation of majority 0D while the higher concentration favors a majority 3D perovskite
(Figure S4). The 3D precipitated from the 0.5 M DMSO solution is likely due to the high
concentration that results in significant interaction between adjacent octahedra after the addition
of the antisolvent.35 In fact, upon direct addition of the antisolvent, we observed a white precipitate,
most probably 0D phase or ultra-small 3D clusters36 that in few seconds turned into orange colored
3D perovskite.
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XRD analysis of the precipitate from the lower absolute concentration (0.05 M) detected around
15% 3D CsPbBr3 (see Note S1), hence, we used a 4:1 CsBr:PbBr2 ratio in order to precipitate 3D-
free 0D Cs4PbBr6 (Figure 3, blue pattern). We observed that the (121) peak at 21.5° (2theta)
corresponding to the 3D orthorhombic phase completely disappeared. However, at this precursor
ratio, peaks, especially the (110) peak at 29.5° (2theta), corresponding to cubic CsBr are detected.
Importantly, powders precipitated from a 3.75:1 of CsBr:PbBr2 contained detectable 3D impurity
(Figure 3, green pattern). These results suggest that the formation of CsBr- and CsPbBr3-free
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Cs4PbBr6 from DMSO and DMF solutions could be challenging. It is worth mentioning that the
dried powders precipitated from all the different ratios were green emitting under the UV-lamp.
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Figure 3. XRD patterns of powders precipitated from a 3.75:1 (green pattern) and a 4:1 (blue
pattern) CsBr:PbBr2 precursor solutions in DMSO.
ssNMR on Cs-Pb-Br powders. We carried out 207Pb and 133Cs solid-state nuclear magnetic
resonance (ssNMR) analysis to explore the composition of our precipitates. ssNMR is a powerful
characterization tool that can distinguish between different crystal structures of the same
material,37 and it can also identify the formation of amorphous and hydrated hybrid perovskite
phases.38-40 Since it is well known that the main drawback for NMR is the intrinsic poor sensitivity,
we first checked the sensitivity of ssNMR in detecting perovskite impurities in the Cs-based
perovskites. For this purpose, two samples were used, specifically, a 3D CsPbBr3 with very low
0D Cs4PbBr6 impurity (1:1_DMF) and 0D Cs4PbBr6 with very low 3D CsPbBr3 impurity
(3.75:1_DMSO) as suggested by their XRD patterns shown in Figure 2a (black pattern) and Figure
3 (green pattern), respectively. All ssNMR data are summarized in Table 1. The detection of the
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impurity phases was successful by 133Cs ssNMR (Figure 4a) but not by 207Pb ssNMR (Figure 4b),
despite the first nuclide being quadrupolar (I=7/2, natural abundance = 100%) while 207Pb is not
(I=1/2, natural abundance = 22.6%), suggesting higher sensitivity of 133Cs ssNMR compared to its
207Pb counterpart in these high crystalline materials. It is worthwhile to notice that the Pb signals
show a fine structure with well-defined components of the multiplet originating from the coupling
between 207Pb and the six 79/81Br atoms (I= 3/2 natural abundance 79Br 50.54% and 81Br 49.46%),
surrounding the metal ions in an octahedral geometry, as already reported for similar Cl
complexes.41 We found that 133Cs ssNMR was also able to distinguish between two kinds of Cs
atoms, lying in different environments in the 0D crystal structure42 with a ratio between 2:1 to 3:1,
as expected from the 0D unit cell. Moving to the “3D-free” 0D sample (4:1_DMSO), the XRD
pattern of which we report in Figure 3 (blue pattern), 133Cs ssNMR identified the CsBr (δiso = 268
ppm), which was also detected by XRD. Importantly, 133Cs ssNMR did not detect any 3D impurity
in agreement with powder XRD.
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133Cs
207Pb
3D
3D
(a)
(b)
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*
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0D
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0D
#
#
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0D
#
§
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100
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0
-200
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-600
Figure 4. (a) 133Cs and (b) 207Pb ssNMR spectra (10 kHz MAS, 11.75 T, 312 K) of the samples
1:1_DMF (red trace), 3.75:1_DMSO (green trace), and 4:1_DMSO (blue trace). The asterisks (*)
mark the spinning sidebands (SSB) of 3D phase signal, the section signs (§) mark the SSB of the
less intense 0D peak while the hash signs (#) mark the SSB of the more intense 0D peak.
Table 1. Summary of powder XRD, and 133Cs and 207Pb ssNMR analysis on 3 different samples.
In brackets the δiso (ppm) of the different phases is reported. For 207Pb signal the scalar coupling
constant 1JPb-Br typical for these systems is also included.43
Sample
Powder XRD
133Cs ssNMR
207Pb ssNMR
CsBr:PbBr2_solvent
1:1_DMF
3D (120 ppm) + 0D
impurity
3D only (246 ppm,
3D + 0D impurity
1JPb-Br=2425 Hz)
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0D (354 ppm and
233.6 ppm ) + 3D
impurity
0D only (-373 ppm,
3.75:1_DMSO
4:1_DMSO
0D + 3D impurity
0D + CsBr
1JPb-Br=2050 Hz)
0D + CsBr (268 ppm)
0D only
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Optical and compositional analysis of green-emitting Cs4PbBr6 powder. We focused on the
“3D-free” 0D powder (4:1_DMSO). Although XRD (Figure 3 blue pattern) and ssNMR (Figure 4
blue trace) did not detect any 3D impurity in this sample, diffuse reflectance measurements, as
plotted in Figure 5a, showed absorption throughout the green region with a matching PLE (Figure
S5) and a single PL emission peak at 520 nm (Figure 5a). It is worth mentioning that in the case
of our 0D powders with (3.75:1_DMSO) or without (4:1_DMSO) detectable 3D impurity, the as-
precipitated wet powders are non-emissive under the UV-lamp. However, they turn green emitting
after some time, whether the precipitate is left in solution, dried in a vacuum oven at 40 °C, or
under N2 at room temperature.
An inspection of the 4:1_DMSO sample with optical microscope under white light (Figure S6a)
and UV light (Figure S6b) revealed that the green emission is localized in bright spots scattered
across the sample. This observation was further confirmed by confocal PL microscope using 488
nm excitation (Figures 5b). The PL spectrum collected from a local area containing several
emission centers peaked at ~520 nm (Figure S7), in agreement with the PL spectrum in Figure 5a.
A spatial distribution of the emitting centers in three dimensions was obtained through the series
of z slices of ~40×40×5 micron volume of the sample (Figure S8). It further confirms that the
emission originates from the localized spots in the sample, which are unevenly distributed across
the sample. The size of the emissive domains is sub-micron and resolving their shape is beyond
the spatial resolution of the microscope used.
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Figure 5. (a) Optical absorption (solid line) of 4:1_DMSO powder sample obtained by
converting reflectance (R) to absorption using the Kubelka-Munk equation6: F(R) = (1-R)2/2R and
PL emission (dotted line) at excitation wavelength of 405 nm. (b) Confocal PL microscopy image
recorded with 488 nm excitation, the PL spectrum from an area highlighted by the white square is
shown in Figure S7.
Elemental analysis of the powder was done using X-ray photoelectron spectroscopy (XPS) and
Energy-dispersive X-ray spectroscopy (EDS) and the compositional data is listed in Table S1. Both
XPS and EDS analyses reveal that the sample is Br-deficient. This Br-deficiency contradicts most
reports on green-emitting 0D material where samples were reported to be Br-rich with Pb:Br ratios
up to 1:7.4.44 The bromine deficiency in our 0D sample could be due to incorporation of other
phases with lower bromine content. Next, we measured the separation between the onset of valence
band maxima (VBM) and Fermi level shown in Figure 6a. We observed only a small shift when
compared to reported VBM onsets of 3D CsPbBr3, suggesting a possible 3D or 3D-like impurity
in our 0D sample.45 Similarly, high-resolution transmission electron microscopy (HRTEM)
analysis on the 0D sample detected a domain with lattice spacing of 5.8 Å that is indicative of 3D-
like nature (Figure 6b,c). Based on elemental analysis of different particles in the 4:1_DMSO
sample, Cs4PbBr6 and the impurity CsBr particles were observed as individual entities and not as
a composite (Figure S9).
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Figure 6. (a) XPS results on the 4:1_DMSO sample showing the separation between the onset of
valence band maxima (VBM) and Fermi level. (b) HRTEM image and (c) corresponding Fast
Fourier transform (FFT) of the white square area showing lattice spacing of 5.8 Å.
Origin of the green emission. While bromine vacancy (VBr) transition level energy in the
Cs4PbBr6 was calculated by Yin et al.13 to be 2.3 eV above the VBM, matching the green emission,
recently, VBr was instead suggested to form a shallow defect level.46 In addition, as we have
previously reported,12 the intense and sharp emission and the absence of Stokes shift does not
match with a deep-trap emission. Therefore, impurity emission would be more reasonable as the
origin of the green emission. Impurity emission is in line with our confocal PL microscopy, VBM
analysis, and HRTEM findings. 3D CsPbBr3 could be the impurity source of the green PL
emission, as non-quantum confined nanocrystals (> 8 nm) do indeed emit at around 2.39 – 2.41
eV.47 However, in agreement with Wang et al.48 it is expected that the 3D CsPbBr3 impurity might
not be the only origin of the green emission. For instance, a previous report detected quantum
confined ~3 nm CsPbBr3 nanocrystals embedded within larger-sized Cs4PbBr6 using HRTEM,
however, the sample was green emitting at 520 nm,22 defying reported PL values for such small
CsPbBr3 nanocrystals.47 In fact, PLQYs of our 0D samples with detectable 3D impurity
(3.75:1_DMSO) and 3D-free (4:1_DMSO) were 25% and 50%, respectively.
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Other than the known perovskite or perovskite-related phases of 3D (CsPbBr3), 2D (CsPb2Br5)
and 0D (Cs4PbBr6), Jizhong et al.49 reported the synthesis of a Cs2PbBr4 material, which they
described as a 2D sheet of corner sharing [PbBr6]4- octahedra passivated by Cs+ ions, exhibiting
an absorbance and emission in the green region of the visible spectrum. This phase has not been
previously reported in the phase diagram of this ternary Cs-Pb-Br system and the synthesis of a
bulk sample of Cs2PbBr4 appears to be challenging, as the phase was suggested to be
thermodynamically unstable.50 However, this 2D sheet of corner-shared [PbBr6]4- octahedra is in
essence a monolayer of 3D CsPbBr3. Furthermore, the 2D Ruddlesden-Popper (RP) phase of
Csn+1PbnBr3n+1 with n = 1 or 2 has also been detected as an impurity within 3D CsPbBr3
nanosheets.51 Theoretically, at n = 1, the Cs2PbBr4 layer is predicted to be a direct bandgap
material, and upon introducing spin orbital coupling (SOC) combined with hybrid function (HSE),
Yang et al.52 calculated its bandgap to be 2.29 eV.
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In pure 2D perovskites such as 2D MA2PbI4, the difference between the bandgaps of a monolayer
and bulk material is theoretically calculated to be of only 0.01 eV.53 Experimentally, unlike the
quasi-2D e.g. (BA)2(MA)n-1PbnI3n+1 (BA = butylammonium) that possess considerable blue shift
in the PL peak position as number of layers (n) decreased,54 pure 2D perovskites showed
insignificant differences.55-56 Hence, a 2D perovskite structure with small Cs cation as an interlayer
spacer could explain the narrow range of PL (515 nm to 524 nm)12, 57 reported for the emissive 0D
single crystals, powders and nanocrystals. The proposed Cs2PbBr4 is different from 3-5 layered
CsPbBr3 nanosheets or nanoplatelets that usually emit between 440 nm to 460 nm.58-59 This
discrepancy in the PL peak position could be attributed to the fact that, in case of nanosheets and
nanoplatelets, their top and bottom surfaces are covered with large organic cations such as
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oleylammonium or octylammonium, hence, they could be considered as quasi-2D perovskites,
leading to the pronounced blue shift in their emissions.
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This hypothesis of 2D Cs2PbBr4 inclusions being responsible for the absorbance and emission in
the green region of the visible spectrum is in agreement with Liu et al.60 where they suggested the
formation of [PbBr4]2- intermediates (in the form of sheet structures observed in TEM) during the
transformation of CsPbBr3 nanocrystals to Cs4PbBr6 nanocrystals. It is possible that the 0D matrix
stabilizes this 2D Csn+1PbnBr3n+1 with n = 1 (Cs2PbBr4) phase similar to what was detected in 3D
nanosheets.51
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CONCLUSION
To summarize, a mixture of CsBr-PbBr2 salts dissolved in DMSO and DMF results in the
formation of different lead bromide species with a solvent-dependent solvodynamic radius of the
species. Solutions in DMSO feature smaller species (~1 nm solvodynamic size) compared to the
solutions in DMF, likely due to the stronger coordination of DMSO to Pb2+ compared to DMF.
The size of lead bromide species influences the composition of the Cs-Pb-Br precipitates upon
addition of the antisolvent: smaller species favor formation of the Cs4PbBr6 phase, while larger
species template the precipitation of CsPbBr3. Overall, 0D Cs4PbBr6 has a higher tendency to be
precipitated out from solutions with stronger coordinating solvents to Pb2+, lower absolute
concentration of the precursors and higher CsBr:PbBr2 ratios, compared to its 3D CsPbBr3
counterpart. We have discussed a possible origin of the green emission in the wide bandgap
Cs4PbBr6 and concluded that 3D impurities might not be the only source of the emission and high
PLQY. Alternatively, an impurity of 2D Cs2PbBr4 has been proposed but no conclusive
experimental evidence of its existence was obtained. Our work highlights the importance of the
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less investigated small cation spacers (e.g. Cs or MA) in 2D perovskites. Future work would target
understanding the formation mechanism of Cs2PbBr4 as an impurity within the Cs4PbBr6.
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ASSOCIATED CONTENT
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Supporting Information. The Supporting Information is available free of charge on the ACS
Publications website. PLE, PL, and DLS of PbBr2-only solutions. Optical images, confocal PL
microscopy volume view, and high-angle annular dark-field (HAADF) and elemental analysis of
the 4:1_DMSO sample.
AUTHOR INFORMATION
Corresponding Author
*Liberato Manna: liberato.manna@iit.it
*Ahmed L. Abdelhady: ahmed.abdelhady@iit.it
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The research leading to these results has received funding from the European Union 7th Framework
Programme under Grant Agreement No. 614897 (ERC Consolidator Grant “TRANS-NANO”).
The work of D.B. was supported by the European Union’s Horizon 2020 research and innovation
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programme under the Marie Sklodowska-Curie grant agreement No. 794560 (RETAIN). We thank
Dr. Federico Locardi and Dr. Guilherme Almeida for helpful discussions and suggestions and Dr.
Francesca Benevelli (BrukerBiospin) and Pasquale Illiano (University of Milan) for NMR
technical assistance.
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