CsPb2Br5 Single Crystals: Synthesis and Characterization

Article abstract of DOI:10.1002/cssc.201701131

CsPb₂Br₅ is a ternary halogen-plumbate material with close characteristics to the well-reported halide perovskites. Owing to its unconventional two-dimensional structure, CsPb₂Br₅ is being looked at broadly for potential applications in optoelectronics. CsPb₂Br₅ investigations are currently limited to nanostructures and powder forms of the material, which present unclear and conflicting optical properties. In this study, we present the synthesis and characterization of CsPb₂Br₅ bulk single crystals, which enabled us to finally clarify the material's optical features. Our CsPb₂Br₅ crystal has a two-dimensional structure with Pb₂Br₅^? layers spaced by Cs⁺ cations, and exhibits approximately 3.1 eV indirect band gap with no emission in the visible spectrum.

Full text of DOI:10.1002/cssc.201701131

Accepted Article
Title: CsPb2Br5 Single Crystals: Synthesis and Characterization
Authors: Ibrahim Dursun, Michele De Bastiani, Bekir Turedi, Badriah
Alamer, Aleksander Shkurenko, Jun Yin, Issam Gereige,
Ahmed Alsaggaf, Omar F. Mohammed, Mohamed Eddaoudi,
and Osman M. Bakr
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.201701131
Link to VoR: http://dx.doi.org/10.1002/cssc.201701131
A Journal of
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ChemSusChem
10.1002/cssc.201701131
COMMUNICATION
CsPb Br Single Crystals: Synthesis and Characterization
2
5
+
[a]
+ [a]
[a]
[a]
[b]
Ibrahim Dursun , Michele De Bastiani , Bekir Turedi, Badriah Alamer, Aleksander Shkurenko,
[
a]
[a]
[c]
[c]
[a]
Jun Yin, Ahmed M. El-Zohry, Issam Gereige, Ahmed AlSaggaf, Omar F. Mohammed,
[
b]
[a, d]
Mohamed Eddaoudi, and Osman M. Bakr*
[
6a]
Abstract: CsPb
close characteristics to well-reported halide perovskites. Due to its
unconventional two-dimensional structure, CsPb Br is being looked
at broadly for potential applications in optoelectronics. CsPb Br
2
Br
5
is a ternary halogen-plumbate material with
related structure, and/or as a converted product from CsPbBr
3
[
7a]
nanocrystals.
Pure powders of CsPb
2
Br
5
9]
were obtained and
[
2
5
characterized initially by Kuznetsova et al., ; and more recently,
[
10, 7b, 11]
2
5
nanostructures of CsPb
2
Br
5
were reported.
While these
Br , they
investigations are currently limited to nanostructures and powder
forms of the material, which present unclear and conflicting optical
properties. In this study, we present the synthesis and
works agree on structure and composition of CsPb
2
5
greatly diverge on the interpretation of its intrinsic optical
properties, which nowadays is a subject of controversy. For
characterization of CsPb
2
Br
5
bulk single crystals, which enabled us
Br crystal
layers spaced by Cs
example, reports on CsPb
2 5
Br nanoplatelets and microplates
to finally clarify the material’s optical features. Our CsPb
2
5
suggest a material with strong green emission and green-lasing
-
+
[10, 7b]
has a two-dimensional structure with Pb
2
Br
5
properties.
2 5
In stark contrast, studies on CsPb Br
cations, and exhibits a ~3.1 eV indirect bandgap with no emission in
the visible spectrum.
nanosheets suggest that the material is of an indirect bandgap
[
11]
nature and with no visible photoluminescence.
Thus, to
resolve the dispute arisen from the perceived optical properties
and to push the field towards controlling the key properties at
The development of solution processed halide perovskite-based
2 5
play in the material, we synthesized bulk CsPb Br single
[
1]
semiconductors has flourished in the last five years. This
emergent class of semiconductors, represented with the general
crystals as the purest state of the compound – free of any
possible ambiguities emanating from structural defects and
3
formula ABX (A is organic/inorganic cation, B is a lead cation,
dimensional properties. Bulk single crystals of CsPb
yet to be reported, since difficulties in the crystallization process
from melt precursors (CsBr and PbBr ) have only afforded so far
CsPbBr and Cs PbBr as stable crystals.
In this manuscript, we present for the first time the
synthesis and characterization of CsPb Br millimetre-sized
single crystal. Our crystals are synthesized from solutions of
CsBr and PbBr precursors by means of antisolvent vapour
crystallization (AVC). Transparent CsPb Br crystals were grown
2 5
Br have
and X is a halogen anion), exhibits remarkable light absorption
and long carriers diffusion length together with tunable
2
[
2]
bandgaps obtained by tailoring the halogen composition.
Hybrid organic-inorganic lead halogen perovskites of the
CH NH PbX -type (X = Cl, Br, and I) represent the most suitable
candidates for photovoltaic applications, while full-inorganic
CsPbX perovskites are prime contenders for light emitting
3
4
6
3
3
3
2
5
[
3]
3
2
[
4]
[5]
devices
halogen perovskites, the recent comeback of low-dimensional
perovskite-related structures, with general formula A
and laser applications.
Among inorganic lead
2
5
with an absorption edge close to 370 nm and without noticeable
photoluminescence (PL). DFT calculations agree well with the
experimental evidence, by confirming an indirect bandgap of
n
[
BX2+n, is
6]
due to their attractive photoluminescence properties. Among
these compounds, CsPb Br represents an inspiring candidate
for both light-emitting devices and lasing applications.
CsPb Br is often obtained as a secondary product during the
syntheses of CsPbBr
2
5
3
.09 eV. From Single-Crystal X-Ray Diffraction (SC-XRD) we
[
7]
resolved the full CsPb
2
Br
5
structure, which is composed of two-
2
5
-
+
dimensional layers of Pb
2
Br
5
spaced by Cs cations, in
[
8]
3
perovskites,
Cs
4
PbBr
6
perovskite-
[7, 10, 11]
agreement with previously predicted CsPb
CsPb Br
established AVC technique
2 5
Br structures.
2
5
single crystals were obtained from the well-
[
12]
using CsBr and PbBr
2
as
[a]
I. Dursun, Dr. M. De Bastiani, B. Turedi, B. Alamer, Dr. J. Yin, Dr. A.
El Zohry, Prof. O. F. Mohammed, Prof. O. M. Bakr
KAUST Solar Center (KSC)
precursors together with dimethyl sulfoxide (DMSO) and
methanol (MeOH) as solvent and antisolvent (see details in
Division of Physical Sciences and Engineering (PSE)
King Abdullah University of Science and Technology (KAUST),
Thuwal 23955-6900, Saudi Arabia
Experimental
Section).
Interestingly,
the
2 5
CsPb Br
crystallization can be divided into two steps (Figure 1a). In the
first 48 hours, the diffusion of MeOH in DMSO initiates the
nucleation and precipitation of orange crystallites with proper
E-mail: osman.bakr@kaust.edu.sa
[b]
Dr. A. Shkurenko, Prof. M. Eddaoudi
Functional Materials Design, Discovery and Development research
characteristics of CsPbBr
during the following five weeks, the orange crystallites grow and
slowly convert to transparent crystals of CsPb Br . This process
is reminiscent of processes already observed for CsPb Br
nanocrystals. Indeed, the co-existence of a nanometric dual-
3
(see Figure S1). Subsequently,
3
group (FMD ), Advanced Membranes & Porous Materials Canter,
Division of Physical Sciences and Engineering
King Abdullah University of Science and Technology (KAUST),
Thuwal 23955-6900, Saudi Arabia
2
5
2
5
[
c]
d]
Dr. I. Gereige, Dr. A. Al Saggaf
Saudi Aramco Research & Development Center, Dhahran 31311,
Kingdom of Saudi Arabia
[7a]
phase of CsPbBr
and then supported by Wang et al.
CsPbBr nanocubes to CsPb Br nanosheets in the presence of
an excess of PbBr . In our work, the excess of PbBr is
3 2 5
-CsPb Br was first reported by Zhang et al.
[
11]
with the conversion of
[
Prof. O. M. Bakr
KAUST Catalysis Center, King Abdullah University of Science and
Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
3
2
5
2
2
+
generated by the low solubility of CsBr in DMSO, which
continuously decreases during the MeOH antisolvent diffusion.
The chemical reactions involved in the crystallization process
are shown in Scheme 1.
[
] These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
1
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ChemSusChem
10.1002/cssc.201701131
COMMUNICATION
grinded CsPb
2
Br
5
crystals (Figure 2c). The reflections of the two
patterns are identical; thus we can exclude the presence of any
CsPbBr impurity.
3
Furthermore, to test the chemical composition and
homogeneity of our crystals, we performed elemental mapping
by means of Energy Dispersive X-Ray Spectroscopy (EDS).
Scheme 1. The reaction steps involved in the crystallization process that leads
CsPb
S2) following the final stoichiometry of the CsPb
The optical properties of CsPb Br represent an intriguing
topic of conflicting interpretations. Strong and narrow green PL
stemming from CsPb Br nanostructures was described in refs
in contrast Jiang et al. reported CsPb Br nanosheets with
large indirect bandgap and a PL inactive behavior.
2
Br
5
crystals exhibit the atomic ratio of 1:2:5 (see Figure
2 5
to the formation of CsPb Br .
2
5
Br crystals.
2
5
In our study, it is necessary to highlight the important role
of antisolvent choice. Indeed it was already demonstrated that
2
5
Cs
4
PbBr
6
crystallites could be obtained under similar conditions
[7b, 10]
;
2
5
from AVC using DMSO/diethyl ether (DE) as a solvent and
[11]
We
[
6b]
antisolvent pair.
We believe that the ultimate crystallization
investigated the PL properties of our crystals by exciting all over
the visible and near UV spectrum. We found no detectable
products are defined by the miscibility of solvent-antisolvent.
Thus, high miscibility (like MeOH in DMSO) produces first
[11]
emission, which is in agreement with Wang et al. The
CsPbBr
like DE in DMSO) directly yields Cs
After five weeks of crystallization, all the crystals in the vial
appeared transparent, and no orange CsPbBr crystallites are
visible. Scanning Electron Microscopy (SEM) images confirmed
the high quality of the CsPb Br crystals with a smooth surface
and well-defined tetragonal shape (Figure 1b). SC-XRD analysis
confirmed that the transparent crystals are CsPb Br with
3
and then subsequently CsPb
2 5
Br ; while low miscibility
absorption spectrum of our crystals exhibits only one edge at
(
4
PbBr
6
.
3
70 nm, from which we deduced a 3.35 eV bandgap (Figure 3a).
3
2
5
2
5
tetragonal symmetry and I 4/mcm space group (see Figure 2).
The unit cell dimensions a = b = 8.490 Å, and c = 15.197 Å are
in agreement with what was previously reported for powders
(
crystal data and structure refinement details can be found in
[
9]
Table S1). As shown in Figure 2a, CsPb
2 5
Br crystals present a
+
two-dimensional structure composed by Cs cation sandwiched
-
-
2+
between Pb
coordinate with eight Br , forming an elongated polyhedron
2 5 2 5
Br layers. The Pb Br layers consist of Pb atoms
-
[
11]
(
Figure 2b).
To further validate the absence of any CsPbBr
3
2 5
Figure 2. a) Crystal structure of the CsPb Br single crystal obtained by
+
SC-XRD and detail of the Cs coordination. b) Pb-Br schematic
coordination. c) Powder XRD of the grinded crystal along with the
2 5
calculated spectrum of tetragonal CsPb Br .
To gain a better insight into the optical properties of
Br crystals, we calculated the electronic band struture
CsPb
2
5
and projected density of states with the generalized gradient
approximation (GGA)/Perdew-Burke-Ernzerhof (PBE) theory
(
see the computational details in Supporting Information). As
shown in Figure 3b, we obtained an indirect bandgap between
X-point (top valence band) and Γ-point (bottom conduction band)
of 3.09 eV and the lowest direct band gap (3.19 eV) at the Γ-
point. The projected densities of states (PDOS) show that the
dominant contributions to the valence bands around the Fermi
level are from Br-4p and the conduction bands are provided by
+
Pb-6p; the Cs cations have no direct contributions to the
2 5
Figure 1. a) Schematic diagram of the CsPb Br single crystal growing by
electronic states in this energy region. In Figure 3c, we also
show the charge density distributions of valence band maximum
2 5
AVC method. b) SEM image and c) optical image of the CsPb Br single
crystal. White and red scale bar: 200 µm.
(
VBM) and conduction band minimum (CBM) within GGA/PBE,
impurity, we compared the calculated powder spectrum from
SC-XRD results with the powder spectrum obtained from
the charge densities of VBM at both Γ- and X-points are highly
localized in the Br atoms, while charge density of CBM at Γ-point
2
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ChemSusChem
10.1002/cssc.201701131
COMMUNICATION
localizes at the Pb sites. From the calculated optical absorption
spectrum by taking into account of electron-hole interaction
PbBr
2
(≥98%), CsBr (99.9% trace metals basis), MeOH and DMSO were
purchased from Sigma-Aldrich and used without further purifications.
CsPb Br single crystals were obtained from a 0.25 M solution of PbBr
and CsBr in DMSO. For AVC, MeOH was used as antisolvent with a 4-to-
ratio respect to precursors’ solution. After one month of undisturbed
crystallization, CsPb Br crystals were collected and washed with MeOH,
2
5
(
using BSE method) as shown in Figure 3a-bottom panel, we
observe a sharp increase in the calculated absorption coefficient
around 390 nm, which corresponds to the direct optical
transitions (Br-4p → Pb-6p) at Γ-point of the band structure.
1
2
5
prior to stock in a nitrogen atmosphere.
2 5
In light of these results, we conclude that intrinsic CsPb Br
is an indirect bandgap material, which does not display
appreciable intrinsic PL. Our findings suggest that observations
Characterizations
[
7b, 10]
of emission in this material could be either due to the
Single-crystal X-ray diffraction data were collected using a Bruker X8
potential formation of emissive sub-bandgap defects (due the
peculiarities of the synthesis procedure) or secondary phases.
Indeed, it is very likely that under particular conditions, like those
manifested during the synthesis of nanostructures, residual
phases or defects induced by the surface strain may give rise to
PL. In this sense, a very similar scenario was recently reported
PROSPECTOR APEX2 CCD diffractometer (Cu Kα, λ = 1.54178 Å).
Powder X-ray diffraction was performed on
a Bruker AXS D8
diffractometer using Cu Kα radiation. SEM images and EDS
characterization were performed with an FEI Quanta 600 microscope.
Steady-state absorption spectra were collected from an Edinburg F900
4
Spectrometer with sample reflectance correction by using BaSO as a
reflectance standard. A FluoroMax-4 spectrofluorometer from Horiba
Scientific was used to investigate photoluminescence properties. For
DFT calculations and SC-XRD details, see the Supporting Information.
for Cs
4 6
PbBr perovskite-related structure, where emissive and
non-emissive nanocrystals with the same composition were
[
6c, 6d]
observed.
Acknowledgements
The authors acknowledge the financial support of King Abdullah
University of Science and Technology (KAUST) and Saudi Aramco.
Keywords: halogen perovskites
•
CsPb
2
Br
5
•
single crystal
•
photoluminescence • indirect bandgap
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COMMUNICATION
COMMUNICATION
Ternary halogen plumbate compounds,
Ibrahim Dursun, Michele De Bastiani,
Bekir Turedi, Badriah Alamer, Aleksander
Shkurenko, Jun Yin, Ahmed M. El-Zohry,
Issam Gereige, Ahmed AlSaggaf, Omar F.
Mohammed Mohamed Eddaoudi and
Osman M. Bakr*
like CsPb
attention
2
Br
of
5
,
are capturing the
the
However,
optoelectronic
fundamental
community.
knowledge of their intrinsic properties is
still missing. In this work, we present the
synthesis and characterization of
CsPb
2
Br
5
single crystal. Our crystals
Page No. – Page No.
exhibit ~3.1 eV indirect bandgap and
photo-luminescence inactive behavior.
CsPb
2
Br
5
Single Crystals: Synthesis
and Characterization
5
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