Metal Halide Perovskite Supercrystals: Gold-Bromide Complex Triggered Assembly of CsPbBr3 Nanocubes

Article abstract of DOI:10.1021/acs.langmuir.7b03432

Using nanocrystals as "artificial atoms" to construct supercrystals is an interesting process to explore the stacking style of nanoscale building blocks and corresponding collective properties. Various types of semiconducting supercrystals have been constructed via the assembly of nanocrystals driven by the entropic, electrostatic, or van der Waals interactions. We report a new type of metal halide perovskite supercrystals via the gold-bromide complex triggered assembly of newly emerged attractive CsPbBr₃ nanocubes. Through introducing gold-bromide (Au-Br) complexes into CsPbBr₃ nanocubes suspension, the self-assembly process of CsPbBr₃ nanocubes to form supercrystals was investigated with the different amount of Au-Br complexes added to the suspensions, which indicates that the driven force of the formation of CsPbBr₃ supercrystals included the van der Waals interactions among carbon chains and electrostatic interactions between Au-Br complexes and surfactants. Accordingly, the optical properties change with the assembly of CsPbBr₃ nanocubes and the variation of mesoscale structures of supercrystals with heating treatment was revealed as well, demonstrating the ionic characteristics of CsPbBr₃ nanocrystals. The fabricated CsPbBr₃ supercrystal presents a novel type of semiconducting supercrystals that will open an avenue for the assembly of ionic nanocrystals.

Full text of DOI:10.1021/acs.langmuir.7b03432

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Article
Metal Halide Perovskite Supercrystals: Gold-Bromide
Complex Triggered Assembly of CsPbBr3 Nanocubes
Kun-Hua Wang, Jun-Nan Yang, Qian-Kun Ni, Hong-Bin Yao, and Shu-Hong Yu
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03432 • Publication Date (Web): 18 Dec 2017
Downloaded from http://pubs.acs.org on December 19, 2017
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Metal Halide Perovskite Supercrystals: Gold-Bromide Complex Triggered
Assembly of CsPbBr3 Nanocubes
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KunꢀHua Wang1, 2, JunꢀNan Yang2, QianꢀKun Ni2, HongꢀBin Yao1,2*, ShuꢀHong Yu1,2,3,4,5
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1Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at
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Microscale, Department of Chemistry, CAS Center for Excellence in Nanoscience, Hefei
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Science Center of CAS, Collaborative Innovation Center of Suzhou Nano Science and
Technology, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China
*Corresponding author. yhb@ustc.edu.cn, shyu@ustc.edu.cn
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ABSTRACT: Using nanocrystals as ‘artificial atoms’ to construct supercrystals is an interesting
process to explore the stacking style of nanoscale building blocks and corresponding collective
properties. Various types of semiconducting supercrystals have been constructed via the
assembly of nanocrystals driven by the entropic, electrostatic, or van der Waals interactions.
Here, we report a new type of metal halide perovskite supercrystals via the goldꢀbromide
complex triggered assembly of newly emerged attractive CsPbBr3 nanocubes. Through
introducing Goldꢀbromide (AuꢀBr) complexes into CsPbBr3 nanocubes suspension, the selfꢀ
assembly process of CsPbBr3 nanocubes to form supercrystals was investigated with the different
amount of AuꢀBr complexes added into the suspensions, which indicates that the driven force of
the formation of CsPbBr3 supercrystals included the van der Waals interactions among carbon
chains and electrostatic interactions between AuꢀBr complexes and surfactants. Accordingly, the
optical properties change with the assembly of CsPbBr3 nanocubes and the variation of
mesoscale structures of supercrystals with heating treatment were revealed as well demonstrating
the ionic characteristics of CsPbBr3 nanocrystals. The fabricated CsPbBr3 supercrystal presents a
novel type of semiconducting supercrystals which will open an avenue for the assembly of ionic
nanocrystals.
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Keywords: Supercrystals, CsPbBr3, Nanocubes, Assembly, GoldꢀBromide complex
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INTRODUCTION
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Threeꢀdimensional (3D) selfꢀassemblies of nanocrystals (NCs) to form superlattices or
supercrystals are of great interest for the development of advanced materials with potential
applications in fieldꢀeffect transistors,1 lightꢀemitting diodes,2 plasmonic waveguides,3 magnetic
recording media,4 and so on. Previous studies of the assembly of NCs have shown that the
assembled supercystals are highly related to the shapes of nanoscale building blocks and the
interactions among the building blocks.5ꢀ7 Generally, spherical NCs selfꢀassembled into
hexagonal close packed structures or faceꢀcentered cubic to perform the highest packing
efficiency, i.e., 74.04%.8ꢀ10 Nanocubes with flat faces and sharp edges prefer to assemble in
simple cubic superlattices.11ꢀ13 Diverse types of supercrystals have been obtained via the
assembly of nanocubes due to the high face to face interactions among nanocubes.14ꢀ17 These
reported 3D supercrystals consisting of singleꢀ or multicomponent NCs are routinely achieved by
the selfꢀassembly processes induced by entropic, electrostatic, or van der Waals interactions.18ꢀ20
Very recently, metal halide perovskite NCs have attracted intensive attention due to their
unique properties including high photoluminescence (PL), anion exchange tuning optical
bandgaps and narrow emissions.21ꢀ25 A series of metal halide perovskite NCs have been
synthesized and shown intriguing properties.26ꢀ31 In particular, monodisperse colloidal CsPbBr3
NCs with wellꢀdefined nanocube shapes have been synthesized and exhibited bright emission
with wide color gamut.28, 32 In addition, fast anion exchange performance has also been reported
in metal halide perovskite NCs.33ꢀ36 This new born nanomaterials will be interesting nanoscale
building blocks to study the assembly process of nanostructures due to its high PL, wellꢀdefined
cube shape, and ionic polar surfaces. However, the selfꢀassembly of colloidal CsPbBr3
nanocubes to form supercrystals and their collective properties are rarely reported.37ꢀ38
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Distinctive from traditional semiconductors, metal halide perovskites not only possess the
properties of classical semiconductors but also the complex defect physics of ionic crystals.
Compared to typical metal chalcogenides, such as ternary (CuInS2 and AgInS2) compounds,
ternary CsPbBr3 NCs own high ionic conductivities28 and the fast speed of anionꢀexchange in
perovskite NCs, which is rooted from the ionic properties of perovskite metal halide crystals.34
The highly ionic character of CsPbBr3 nanocubes would bring a new viewpoint for the 3D
supercrystals assemblies of ionic nanocrystals.
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Herein, we report a new type of 3D supercrystals based on the selfꢀassembly of CsPbBr3
nanocubes triggered by AuꢀBr complexes via the van der Waals interactions and electrostatic
forces, in which the electrostatic forces between AuꢀBr complexes and surfactants played an
important role on the formation of 3D supercrystals. The size and quantity of asꢀfabricated
supercrystals have been demonstrated to be tunable by varying the amount of AuꢀBr complexes
added into the CsPbBr3 nanocubes suspension. Extensive nanoscale structural characterizations
revealed that different number of faces on the CsPbBr3 nanocubes could interact with each other
via the van der Waals and electrostatic interactions. During the assembly process the quenching
of PL of CsPbBr3 nanocubes was observed, which is related to the the aggregation of nanocubes
and the introduction of trap states on the surfaces of CsPbBr3 nanocubes by AuꢀBr complexes.
More interestingly, the obtained CsPbBr3 supercrystals preferred to form mesocrystals rather
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than disassembling into individual nanocubes after a heat treatment at 80 C indicating the
inherently active ionic properties of CsPbBr3 nanocubes.
EXPERIMENTAL SECTION
Materials and Chemicals. Lead bromide (PbBr2, Afla Aesar, 98%), oleic acid (OA, Sinopharm
Chemical Reagent Co.Ltd (SCRC), AR), oleylamine (OAm, Aldrich, 70%), gold bromide
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(AuBr3, Aladdin, 99%), toluene (C7H8, SCRC, AR), 1ꢀbutanol (BuOH, SCRC, AR), octadecene
(ODE, Aldrich, 90%), cesium carbonate (Cs2CO3, 99.9%, J&K), didodecyldimethylammonium
bromide (DDAB, 98%, J&K), hexane (C6H14, SCRC, AR). All chemical were used as received
without further purification.
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Preparation of Cs-oleate. Cs2CO3 (0.814 g) was loaded into a 100 ml threeꢀneck flask along
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with ODE (40 ml) and OA (2.5 ml), dried for 1 h at 120 C, and then heated under N2 to 150 oC
until all Cs2CO3 reacted with OA. Since Csꢀoleate precipitates out from ODE at room
temperature, it must be preheated to 100 oC before the following injection process.
Synthesis and purification of CsPbBr3. PbBr2 (0.035g) and ODE (5 ml) were loaded into a 25
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ml 3ꢀneck flask and dried under vacuum at 120 C for 30 min. Then OA (0.5 ml) and OAm (0.5
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ml) were injected into the 3ꢀneck flask at 120 C under N2 flow. After complete dissolution of
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PbBr2 in ODE, the temperature was raised to 180 C and Csꢀoleate ODE solution (0.2 ml) was
swiftly injected and 1 min later the reaction mixture was cooled down by iceꢀwater bath. The
crude solution was precipitated by BuOH before centrifugation and then purified with toluene at
12000 rpm for 3 min. The final precipitate was reꢀdispersed in 5 ml of hexane for further use.
Assembly of CsPbBr3 into supercrystals. The AuꢀBr complex solution was prepared containing
0.01g of AuBr3 (0.024mmol) and 0.01g of DDAB (0.024mmol) in 4 ml of toluene and vibrated
for 10 min at room temperature. AuBr3 was easily dissolved in the toluene/DDAB mixture
forming a clear dark orange solution. 50 ꢁl of OA and 100 ꢁl of DDAB toluene solution (0.01
M) were added into 2.5 ml of asꢀprepared hexane suspension of CsPbBr3 solution under vigorous
stirring, respectively, then a desired amount of DDABꢀAuBr3 solution was added into the
CsPbBr3 suspension to trigger the assembly of CsPbBr3 nanocrystals into supercrystals. The
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assembly process was carried out under N2 atmosphere. After 20 min stirring for assembly, the
assembled sample was collected by centrifugation at 12000 rpm.
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Heating treatment of the assembled CsPbBr3 supercrystals. The obtained assembled CsPbBr3
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supercrystals were reꢀdispersed in 2.5 ml of toluene, heated at 80 C for 30 min under N2
atmosphere, and then collected by centrifugation at 12000 rpm.
Characterizations
Transmission electron microscope (TEM) images were acquired on a Hitachi HTꢀ7700
transmission electron microscope with an accelerating voltage of 120 kV. Highꢀangle annular
darkꢀfield scanning transmission electron microscopy (HAADFꢀSTEM), High resolution TEM
(HRTEM) images and the energy dispersive spectra (EDS) were acquired on a Jeol JEMꢀ2100f
transmission electron microscope an accelerating voltage of 200 kV equipped with an energy
dispersive detector.
Optical properties characterization. UVꢀVisible absorption spectra were collected by using a
PekinElmer in transmission mode. The PL spectra and PLQYs were taken by a Hamamatsu
absolute PL yield spectrometer C11347.
Raman Spectroscopy. Raman spectroscopy was performed on a Raman microscope (inVia,
Renishaw, UK) using a 532 nm laser.
X-ray diffraction (XRD) measurements. Powder Xꢀray diffraction was measured using a
Philips X’Pert PRO SUPER Xꢀray diffractometer equipped with graphite monochromatized Cu
Kα radiation.
Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Molar ratio of Cs, Br, Pb and Au
were measured through Inductively Coupled PlasmaꢀMass Spectrometry on Optima 7300 DV.
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Proton Nuclear Magnetic Resonance (1H-NMR) measurements. HꢀNMR measurements
were recorded on a Bruker Avance III HD Spectrometer operating at a 1H frequency of 400
MHz and equipped with a BBFOꢀZ probe.
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Thermal Gravimetric Analysis (TGA). Thermal gravimetric analysis (TGA) was performed
under N2 atmosphere on a SDT Q600 TGA instrument, using a heating rate of 10 °C/min up to a
temperature of 900 °C.
RESULTS AND DISSCUSION
The inoic CsPbBr3 nanocubes present a different assembly style comparing to traditional
semiconductor nanocrystals at the atomic scale. As shown in Figure 1a, the cubic phase of
CsPbBr3 behaves a threeꢀdimensional connected structure, in which the octahedral coordination
of Pb2+ with six Brꢀ extends to three dimensions via sharing the vertex and Cs+ ions localize in
the octahedral voids. In this 3D ionic crystal structure, bromide anions can be easily extracted
and replaced by other halides, due to their single ionic charge, the rigid nature of the cationic
sublattice, as well as an efficient vacancyꢀassisted diffusion mechanism.34 In addition, from a
structural standpoint, the exposure of rich Brꢀ on the surface of CsPbBr3 NCs makes it feasible to
give rise to strong affinity with positive surfactants.39 Figure 1b illustrates the proposed driven
force for the assembly of CsPbBr3 nanocrystals triggered by AuꢀBr complexes, e.g. van der
Waals interactions among carbon chains and electrostatic forces between AuꢀBr complexes and
surfactants. In particular, the electrostatic forces between AuꢀBr complexes and positively
charged surfactants would play the key role on the assembly of CsPbBr3 nanocrystals. The
interactions between these CsPbBr3 nanocubes results in the face to face contaction, which
further induces the formation of 3D supercrystals.
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Figure 1. (a) crystal structural model of cubic CsPbBr3. (b) illustration of the formation
mechanism of supercrystals. (c) TEM image of asꢀsynthesized CsPbBr3 nanocubes (Inset: the
photo of asꢀsynthesized CsPbBr3 nanocube suspension). (d) TEM image of ligand exchanged
CsPbBr3 nanocubes (Inset: the photo of ligand exchanged CsPbBr3 nanocube suspension). (e)
TEM image of assembled CsPbBr3 nanocubes (Inset: the photo of assembled CsPbBr3 nanocube
suspension).
To trigger the assembly of CsPbBr3 nanocubes into supercrystals, the ligand exchange and the
addition of AuꢀBr complexes into the CsPbBr3 nanocube suspension were carried out in a
sequent process as shown in Figure 1cꢀe. Typically, the CsPbBr3 nanocubes were firstly
synthesized according to the literature34 and dispersed into the hexane forming the suspension
(1.6 mg/ml) (Inset in Figure 1c). In this suspension, the CsPbBr3 nanocubes are monodisperse
and only form the small monolayer aggregations on the TEM grid after the evaporation of
hexane (Figure 1c). Then, 50 ꢁl of oleic acid (OA) and 100 ꢁl of didodecyldimethylammonium
bromide (DDAB, 0.01 M) solution in toluene were added into 2.5 ml of asꢀprepared hexane
suspension of CsPbBr3 nanocubes under stirring to further improve the dispersion of nanocubes
in the solvent via the ligand exchange role of DDAB.40 After the ligandꢀexchange, the color of
suspension still maintained bright green (Inset in Figure 2d) and the transparency and PL
intensity were enhanced (Supporting Information Figure S1) indicating the improved dispersion
of CsPbBr3 nanocubes in the solvent induced by the ligand exchange. Similarly, the ligand
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exchanged CsPbBr3 nanocubes still only formed the monolayer aggregations on the TEM grid
after the evaporation of solvent (Figure 1d). Finally, 1 ml of DDABꢀAuBr3 solution (0.006 M) in
toluene was injected into the ligand exchanged CsPbBr3 suspension to induce the assembly of
monodisperse CsPbBr3 nanocubes, with the color change of suspension from yellow to orange
(Inset in Figure 1e). TEM image of the assembled CsPbBr3 nanocubes (Figure 1e) clearly shows
that the nanocubes stacked together into cubic superstructures with the size of about 300 nm after
the addition of 1 ml of DDABꢀAuBr3 solution into the CsPbBr3 suspension. Furthermore, the
assembled CsPbBr3 nanocubes still maintain cubic phase without any phase change (Figure S2).
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Figure 2. (a)ꢀ(f) TEM images of assembled CsPbBr3 nanocubes with addition of different
amount of DDABꢀAuBr3 solution, 0.1 ml, 0.2 ml, 0.3 ml, 0.4 ml, 1 ml and 1.5 ml, respectively.
To show the role of AuꢀBr complexes on the selfꢀassembly of CsPbBr3 nanocubes, we further
investigated the influence of the amount of DDABꢀAuBr3 solution on the selfꢀassembly
behaviors of CsPbBr3 nanocubes. We first confirmed that the 1:1 molar ratio of DDAB:AuBr3 is
key for triggering the selfꢀassembly and other ratios including 2:1 and 1:2 are unable to enable
the assembly of CsPbBr3 nanocubes (Figure S3). Then, a series amount of DDABꢀAuBr3 (1:1)
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toluene solution were added into the CsPbBr3 nanocubes suspensions, respectively, to investigate
the effect of AuꢀBr complexes on the assembly of CsPbBr3 nanocubes. With increasing the
amount of DDABꢀAuBr3 solution added into the CsPbBr3 nanocubes suspension, the assemblies
of nanocubes changed from 2D square arrays to 3D simple cubic superlattices. Figure 2a and
Figure 2b show that when 0.1 ml and 0.2 ml of DDABꢀAuBr3 solution were added into the
CsPbBr3 nanocubes suspension, respectively, each of these nanocubes is surrounded by four
others, forming 2D square arrays. As shown in Figure 2c, with the addition of 0.3 ml of DDABꢀ
AuBr3 solution into the CsPbBr3 nanocube suspension, more nanocubes were gradually
aggregated and all of the six faces of each nanocubes interacted with other nanocubes, forming
3D superlattices. Figure 2dꢀe demonstrate that the size of assemblies increased with increasing
the amount of DDABꢀAuBr3 solution from 0.4 ml to 1 ml. However, as shown in Figure 2f, the
size would not further increase even if the amount of DDABꢀAuBr3 solution increased to 1.5 ml.
The size variation of assembled CsPbBr3 supercrystals with the increase of amount of DDABꢀ
AuBr3 solution added into the suspension is plot in Figure S4, which indicates that the size of
supercrystals reached to a saturated value when the amount of DDABꢀAuBr3 solution increased
to 1 ml due to the balance of assembly and disassembly of CsPbBr3 nanocubes around the
supercrystals. Furthermore, we compared the size of asꢀsynthesized single CsPbBr3 nanocube
and the CsPbBr3 nanocube in the assembled supercrystal. As shown in Figure S5, the size
distribution of initially synthesized single CsPbBr3 nanocrystal before assembly indicated that
the size of most CsPbBr3 nanocubes (75%) was in the range of 13ꢀ17 nm. Figure S5bꢀg show that
the sizes of most of single CsPbBr3 particles (75%) in the assembled supercrystals were in the
range of 16ꢀ24 nm, which is possibly caused by the size variation of CsPbBr3 nanocrystals
synthesized at different batches. Moreover, we also synthesized CsPbBr3 nanocubes with smaller
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size (75%, 10ꢀ13 nm) as initial building blocks to conduct the assembly. As shown in Figure S6,
the assembled CsPbBr3 supercrystals with addition of different amount of DDABꢀAuBr3 solution
can be obtained as well. The results indicate that the assembly behavior of CsPbBr3 nanocrystals
could be achieved at relatively smaller sizes as well. In addition, the size distribution statistics
(insets in Figure S6) indicate that the sizes of most (75%) of single CsPbBr3 nanocubes after
assembly are in the range of 10ꢀ15 nm.
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To reveal the dominate role of AuꢀBr complexes for the formation of CsPbBr3 supercrystals, we
further studied other interactions possibly existed in the selfꢀassembly process of CsPbBr3
nanocubes. In order to illustrate that only dipoleꢀdipole interactions of DDAB could not be
enough to intrigue the formation of 3D CsPbBr3 supercrystals, the same amount of DDAB
solution without AuBr3 was added to the CsPbBr3 nanocubes suspension and the other conditions
kept the same. Figure S7 shows that the CsPbBr3 nanocubes failed to assemble into 3D
supercrystals indicating that the assembly force of CsPbBr3 nanocubes could not only be
attributed to the entropic force of DDAB indicating that the electrostatic interactions between
AuꢀBr complexes and surfactants is indispensable. To reveal the possible electrostatic force
among the CsPbBr3 nanocubes for the formation of superlattices, we also measured the zeta
potentials of asꢀsynthesized CsPbBr3 nanocubes, ligand exchanged CsPbBr3 nanocubes, and
assembled CsPbBr3 supercrystals, respectively. The results were summarized in Table S1.
Similar to a negative value of ꢀ3.98 mV for the zeta potential of pristine CsPbBr3 nanocubes,
ligand exchanged CsPbBr3 nanocubes also shows a negative zeta potential of ꢀ4.92 mV, which is
consistent with previous report.40 This negative zeta potential faciliated the adsorption of
positive charged ligands, DDA+, on the surface of CsPbBr3 nanocubes, which acted as the
bridges to connect the CsPbBr3 nanocubes together under the help of AuꢀBr complexes. In
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addition, the zeta potential of finally assembled CsPbBr3 supercrystals is still negative (ꢀ6.48
mV), implying that the Brꢀ always kept rich on the surface of CsPbBr3 nanocubes confirming the
ionic characteristics of CsPbBr3.
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Figure 3. (a) TEM image of assembled CsPbBr3 nanocubes by adding 1 ml of DDABꢀAuBr3
solution into the CsPbBr3 suspension. (b) Corresponding HAADFꢀSTEM image of assembled
CsPbBr3 nanocubes. (c)ꢀ(f) EDS mappings of assembled CsPbBr3 nanocubes. (g)ꢀ(j) XPS spectra
of assembled CsPbBr3 nanocubes for the Cs (3d), Pb (4f), Br (3d) and Au (4f) as marked in
corresponding curves, respectively.
To verify the existence of AuꢀBr complexes among the assembled CsPbBr3 nanocubes to
induce the formation of supercrystals and specifically describe the AuꢀBr complexes, we
comprehensively characterized the CsPbBr3 supercrystals by highꢀangle annular darkꢀfield
scanning transmission electron microscopy (HAADFꢀSTEM), energy dispersive Xꢀray
spectroscopy (EDS) and Xꢀray photoelectron spectroscopy (XPS). Figure 3a shows the TEM
image of wellꢀdefined CsPbBr3 supercrystals obtained by adding 1 ml of DDABꢀAuBr3 solution
into CsPbBr3 suspension, which indicates that CsPbBr3 nanocrystals were tightly connected by
face to face with each other forming the wellꢀdefined supercrystals. Figure 3b displays the
corresponding HAADFꢀSTEM image, also showing the uniform supercrystals assmebled by the
CsPbBr3 nanocrystals and maitianing the consistence with the EDS mappings. As shown in
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Figure 3cꢀf, the EDS mappings of Pb, Au and Br overlapped very well, which indicates that Au
element is homogeneously dispersed in the whole supercrytals. In addition, the signal intensity of
Br is much stronger than Pb indicating that the AuꢀBr complexes acted the main forms to
connect the CsPbBr3 nanocrystals. Meanwhile, the low content of Au in the supercrystals is
consistent with the little amount of AuꢀBr complex added into the CsPbBr3 nanocube suspension.
For comparision, we also used XPS to characterize the CsPbBr3 nanocubes before addition of
AuꢀBr complexes, the results of which are shown in Figure S8. As we can see that all elements
(Cs, Pb, and Br) can be detected due to the pure phase of asꢀsynthesized CsPbBr3 nanocubes. In
particular, the Br 3d binding energy curve for CsPbBr3 nanocubes before addition of AuꢀBr
complex can be fitted into only two peaks at 69.15 eV and 68.1 eV. After addition of AuꢀBr
complexes to trigger the assembly of CsPbBr3 nanocubes, besides the Cs, Pb, and Br, the Au
element can be detected as shown in Figure 3gꢀj. The binding energy curve of Au 4f can be fitted
into two peaks at 88.5 eV and 84.8 eV, indicating the existence of AuꢀBr complex ([Au2Br6]2ꢀ) in
the assembled supercrystals. More interestingly, the Br 3d core level of CsPbBr3 nanocubes after
addition of AuꢀBr complexes can be fitted into three peaks at 70 eV, 68.8 eV and 67.8 eV,
indicating the existence of BrꢀBr bridge bonding in the assembled samples.
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To reveal the ratio of components in the assembled supercrystals, we used the Inductively
Coupled PlasmaꢀMass Spectrometry (ICPꢀMS), Proton Nuclear Magnetic Resonance (1HꢀNMR)
(Figure S9ꢀ12) and Thermal Gravimetric Analysis (TGA) (Figure S13) to analyze the
compositions including inorganic and organic parts in the supercrystals. For inorganic part, we
performed ICPꢀMS to determine the molar ratio of Cs, Br, Pb and Au. The results show that the
molar ratio of Cs: Br: Pb: Au is 1.1:3.2:1:0.03 in assembled CsPbBr3 supercrystals with the
addition of 0.5 ml of DDABꢀAuBr3 solution. When assembling CsPbBr3 nanocrystals with the
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addition of 1 ml of DDABꢀAuBr3 solution, the molar ratio of Cs: Br: Pb: Au in the obtained
supercrystals is 1.01: 3.1: 1: 0.06. The ICPꢀMS results show that the ratio of Cs: Br: Pb varied
little in the assembled samples obtained by adding different amount of DDABꢀAuBr3 solution
but the ratio of Pb: Au increased with the amount of DDABꢀAuBr3 added into the assembly
solution. This means that CsPbBr3 nanocubes were stable during the assembly and higher
concentration of DDABꢀAuBr3 solution induced more Au complexes embedded in the obtained
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supercrystals. For the organic part, we used HꢀNMR to study the variation of ligands on the
surface of CsPbBr3 nanocubes before and after assembly. Compared with the reference spectrum
of 1ꢀoctadecene (ODE), oleic acid (OA), oleylamine (OAm) and DDA+ (Figure S9), Figure S10
clearly shows the characteristic resonances of ODE, OA and OAm in CsPbBr3 nanocubes and
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their ratio was determined as 5.3:2.7:2.0 according to integral area of specific HꢀNMR peaks.
After adding 0.5 ml of DDABꢀAuBr3 solution to trigger the assembly of CsPbBr3 nanocubes,
only OA and DDA+ can be detected as shown in Figure S11 and their ratio is 8.9:1.1. Similarly,
after adding 1 ml of DDABꢀAuBr3 solution to trigger the assembly of CsPbBr3 nanocubes, the
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amount of OA declined, and the DDA+ raised, and their ratio is 8.5:1.5 (Figure S12). The Hꢀ
NMR results indicate that with the assembly processing DDA+ gradually to replace OA and
OAm to conduct the assembly of CsPbBr3 nanocubes due to the stronger affinity of DDA+ with
the Brꢀ on the surface of nanocubes. In accordance with integrally weak peaks in 1HꢀNMR, TGA
(Figure S13) also indicates that the content of organic component is only 8.9 wt.% in the
assembled CsPbBr3 supercrystals with the addition of 1 ml of DDABꢀAuBr3 solution. In
combination of all above analysis, the formula of assembled supercrystals with the addition of 1
ml of DDABꢀAuBr3 solution was determined as (OA)0.159(DDA+)0.028Au0.06CsPbBr3.
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Figure 4. (a) UVꢀvis absorption spectra of asꢀsynthesized CsPbBr3 nanocubes, assembled
CsPbBr3 nanocubes with the addition of different amount of DDABꢀAuBr3 solution and the pure
DDABꢀAuBr3 solution. (b) PL emission spectra of asꢀsynthesized CsPbBr3 nanocubes and
assembled CsPbBr3 nanocubes with the addition of different amount of DDABꢀAuBr3 solution.
The collective optical properties of CsPbBr3 supercrystals were also studied with the variation
of amount of DDABꢀAuBr3 added into the suspension. Figure 4a shows the UVꢀvis absorption
spectra of CsPbBr3 nanocubes before and after the addition of different amount of DDABꢀAuBr3
solution. It is obvious that the optical absorption edge around 512 nm changed from sharp to flat
and the extended tail absorption appeared, which indicate the variation of the electronic density
of states of CsPbBr3 nanocubes probably caused by AuꢀBr complexes or aggregation of
nanocubes. In addition, when the amount of DDABꢀAuBr3 solution increased to 0.5 ml and 1 ml,
a broad absorption peak around 420 nm appeared, which is consistent with the absorption peak
of DDABꢀAuBr3 indicating the existence of AuꢀBr complexes in the supercrystals. The PL
spectra of CsPbBr3 nanocubes at different status were also collected and plotted in Figure 4b.
The emission peak at 517 nm of CsPbBr3 nanocubes and CsPbBr3 supercrystals exhibit no
evident shift following the addition of DDABꢀAuBr3 solution, which indicates that the assembly
process would not change the bandꢀgap of CsPbBr3 nanocubes. However, an obvious decrease in
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the absolute PL quantum yield (PLQY) was observed, namely from 37% to 5% (Figure S14)
with the increase of amount of DDABꢀAuBr3 solution from 0 to 0.5 ml, due to the quenching
effect of the assembly of CsPbBr3 nanocubes on their PL. The variation of collective optical
properties of assembled CsPbBr3 supercrystals is interesting to be related with the electron
tranfer from conducting band of CsPbBr3 nanoparticles to the surface trap states induced by the
AuꢀBr complex and explore their potentials for photocatalysis.
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Figure 5. (a) TEM image (b) HRTEM image and (c) HAADFꢀSTEM image of the assembled
CsPbBr3 nanocubes after the heating treatment. (d) Overlapped EDS mapping image. (eꢀh)
Corresponding elementary mapping of Cs, Pb, Br and Au, respectively.
It is well known that the 3D supercrystals is relatively easier to be disassembled if it is
generated by entropic, or van der Waals interactions. However, our obtained CsPbBr3
supercrystals induced by AuꢀBr complexes is stable and could maintain the mesostructure after a
o
heating treatment at 80 C for 30 minutes (Figure 5a). High resolution TEM image (Figure 5b)
further shows that the CsPbBr3 nanocubes fused together via the oriented attachment of (100)
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crystal planes, indicating the inherently reactive ionic surfaces of CsPbBr3 nanocubes. The
absence of Au in the heating treated CsPbBr3 mesocrystals was further confirmed by the EDS
mapping analysis (Figure 5cꢀh) implying that AuꢀBr complexes were excluded by the fusion of
CsPbBr3 nanocubes under the oriented attachment effect. Moreover, the Au element was also
hard to be detected by XPS after the heat treatment (Figure S15) and the Br 3d core level for heat
treated CsPbBr3 supercrystals can be only fitted into two peaks at 69.0 eV and 68.0 eV due to the
exclusion out of Au by the transformation of supercrystals to mesocrystals. The UVꢀvis
absorption and PL emission intensity of CsPbBr3 supercrystals (Figure S16) were further
suppressed due to the growth of CsPbBr3 nanocubes into mesocrystals.
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CONCLUSIONS
In summary, we reported a novel CsPbBr3 supercrystals generated by the van der Waals
interactions among carbon chains and electrostatic forces between AuꢀBr complexes and
surfactants to induce the anisotropic assembly of CsPbBr3 nanocubes. Through tuning the
amount of AuꢀBr complexes during the assembly process, 2D squareꢀarrays and 3D cubic
supercrystals of CsPbBr3 nanocubes are obtained, indicating the important role of AuꢀBr
complexes on the assembly. Moreover, the obtained CsPbBr3 supercrystal is stable and tends to
form the mesocrystal via the oriented attachment growth. With the formation of supercrystal or
mesocrystal, the optical properties of CsPbBr3 nanocubes are intensively suppressed. This new
type of CsPbBr3 supercrystal will offer an alternative insight into the exploration of attractive
metal halide perovskite nanomaterials.
ASSOCIATED CONTENT
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Supporting Information. The Supporting Information is available free of charge on the ACS
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Publications website.
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AUTHOR INFORMATION
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Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We acknowledge the funding support from the National Natural Science Foundation of China
(Grants 51571184, 21431006, 21501165, 21573203, 91645202), the Foundation for Innovative
Research Groups of the National Natural Science Foundation of China (Grant 21521001), Key
Research Program of Frontier Sciences, CAS (Grant QYZDJꢀSSWꢀSLH036), the National Basic
Research Program of China (Grant 2014CB931800), and the Users with Excellence and
Scientific Research Grant of Hefei Science Center of CAS (2015HSCꢀUE007).
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Table of Contents Graphic
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Metal Halide Perovskite Supercrystals:
A new type of 3D supercrystals based on the assembly of CsPbBr3 nanocubes triggered by the
AuꢀBr complexes has been reported.
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