Nano Research
1
DOI 10.1007/s12274-016-1401-6
Nano Res
Singe-crystal
microplates
of
two-dimensional
organic-inorganic lead halide layered perovskites for
optoelectronics
Dewei Ma1,2,§, Yongping Fu1,§, Lianna Dang1, Jianyuan Zhai1, Ilia A. Guzei1, and Song Jin1 ()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-016-1401-6
http://www.thenanoresearch.com on Dec. 2, 2016
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Singe-crystal microplates of two-dimensional
organic-inorganic
lead
halide
layered
perovskites for optoelectronics
Dewei Ma, Yongping Fu, Lianna Dang, Jianyuan
Zhai, Ilia A. Guzei, Song Jin*
University of Wisconsin-Madison, United States
Zhejiang University of Technology, China
We report a facile solution growth of single-crystal microplates of
layered perovskites (C6H5CH2CH2NH3)2PbX4 (X = Br, I) with
well-defined rectangular geometry and nanoscale thickness through
a solution-phase transport growth process and study the growth
mechanism. Through halide alloying, the photoluminescence
emission with narrow peak bandwidth can be readily tuned from
violet to green color.
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Nano Res.
65
Provide the authors’ webside if possible.
Song Jin, https://jin.chem.wisc.edu/
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Nano Research
DOI (automatically inserted by the publisher)
Research Article
Singe-crystal
microplates
of
two-dimensional
organic-inorganic lead halide layered perovskites for
optoelectronics
Dewei Ma
1,2,§
1,§
1
1
1
1
, Yongping Fu , Lianna Dang , Jianyuan Zhai , Ilia A. Guzei , Song Jin ()
1
Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
Department of Applied Physics, Zhejiang University of Technology, Hangzhou 310014, China
§
These authors contributed equally to this work.
2
ABSTRACT
Received: day month year
Revised: day month year
Accepted:
day
month
year
(automatically inserted by the
publisher)
© Tsinghua University Press
and
Springer-Verlag
Berlin
Heidelberg 2014
KEYWORDS
layered
lead
halide
perovskite,
phenylethylammonium lead
halide
perovskites,
microplate,
nanoplate,
dissolution-recrystallization,
photoluminescence
Organic-inorganic hybrid perovskites are now the focus of attention due to
their applications in high efficiency solar cells and light emission. Compared
with the three-dimensional (3D) perovskites, two-dimensional (2D) layered
hybrid perovskites possess higher exciton binding energy and promise
potentially more efficient light emission. Growth of high-quality crystalline
2D perovskites with well-defined nanoscale morphology is desirable because
they could be suitable building blocks for integrated optoelectronics and
(nano)photonics. Herein, we report a facile solution growth of single-crystal
microplates of 2D perovskites based on 2-phenylethylammonium
(C6H5CH2CH2NH3+, PEA) cation, (PEA)2PbX4 (X = Br, I), with a well-defined
rectangular
geometry
and
nanoscale
thickness
through
a
dissolution-recrystallization process. The crystal structures of (PEA)2PbX4 are
first confirmed using single-crystal X-ray diffraction. A solution-phase
transport growth process is further developed to grow microplates with
typical size of tens of micrometers and thickness of hundreds of nanometers
on another clean substrate from the substrate coated with lead acetate
precursor film. Surface topography study suggests that the formation of the
2D microplates is likely driven by the wedding cake growth mechanism.
Through halide alloying, the photoluminescence emission of (PEA)2Pb(Br,I)4
perovskites with narrow peak bandwidth can be readily tuned from violet
(~410 nm) to green (~530 nm) color.
1 Introduction
Organic-inorganic hybrid metal perovskites that
recently have attracted significant interest in the
solution-processed photovoltaics [1-3], light
emitting diodes [4], lasers [5-8], field effect
transistors [9] and photo-/X-ray detectors [10, 11].
These hybrid perovskites generally adopt the
Address correspondence to Song Jin, [email protected]
materials community are a promising class of
semiconductor materials for high-performance
formula of (RNH3)2An-1MnX3n+1, in which R is a
long-chain alkyl or aromatic group, A is a small
Nano Res.
2
cation
such
as
methylammonium
(MA),
formamidinium (FA) or Cs+, M is a metal ion, such
as Pb2+, Sn2+, and X is a halide anion (Cl-, Br-, and I-).
When n is infinite, the resulting materials,
methylammonium lead triiodide (MAPbI3) or
formamidinium lead iodide (FAPbI3) and their
alloys [12, 13] adopt a three-dimensional (3D)
perovskite structure, which are the focus of
attention for high efficiency solar cells and efficient
light emitting diodes (LEDs). The 3D perovskites
behave as free-carrier semiconductors, exhibiting
long carrier lifetime and carrier diffusion length,
and strong photoluminescence (PL) [14, 15]. In the
case of n = 1, these hybrid perovskites become
two-dimensional (2D) layered structures, in which
each layer consists of an extended network of
corner-sharing metal halide octahedra (MX6) and
two layers of organic cations capping both sides to
balance the charge. Due to the lower dielectric
constants of organic species, the layered
perovskites are essentially natural multi-quantum
wells (MQWs) with charge carriers mainly
confined in the 2D crystal planes and possess little
interlayer electronic interactions [16, 17]. As a result,
previous report has shown that the exciton binding
energy of 2D perovskites is up to a few hundred
meV (significantly higher than that of 3D
perovskites) [18], which can potentially lead to
more efficient light emission than the 3D
perovskites.
Similar to 3D MAPbI3, thin films of 2D layered
lead halide perovskites can be easily accessed by a
large variety of methods [19-22], including spin
coating, vacuum vapor deposition and two-step
dipping
conversion.
Layered
lead
halide
perovskites have been used in electroluminescence
(EL) devices [23, 24], scintillation detectors for
X-ray radiation [25], optical microcavities with
strong exciton-photon coupling [26, 27], and
exciton or bi-exciton lasing [28]. Recently, the 2D
layered lead halide perovskites have also attracted
attention for solar applications due to their better
moisture stability [29-32]. Due to the strong
quantum confinement effect intrinsic to the 2D
crystal structures, highly luminescent thin films
[33-36] and powders of 2D layered perovskites [37,
38] have been demonstrated to have high quantum
yield, narrow emission peak and easily accessible
wavelength tunability, making them particularly
interesting for light emitting applications [39-41].
However, the device performance and stability of
2D perovskites remains to be improved before
practical usage. Early studies showed EL or lasing
using polycrystalline thin films of 2D perovskites
as active materials was only observed under
cryogenic temperature [23, 28], and improving the
crystal quality of 2D perovskites enhanced the
quantum efficiency and enabled room temperature
EL [39]. Recent studies have shown that the grain
boundaries in thin films of MAPbI3 are less PL
active and exhibit faster non-radiative decay [42].
In contrast, bulk single crystals or single-crystal
MAPbI3 perovskite micro- or nanostructures with
fewer boundaries demonstrate exceptional low trap
density, leading to much longer charge carrier
diffusion length and near unity quantum efficiency
[6, 14, 43]. Therefore, improving the crystal quality
of 2D perovskites would also benefit both
fundamental studies and the development of
optoelectronic devices using these materials, yet
little effort has been made to understand and
control the crystal growth of 2D perovskites.
Due to superior optoelectronic properties
compared
with
their
bulk
counterparts,
one-dimensional (1D) micro/nano wires and 2D
micro/nano plates of single-crystal semiconductors
have been intensely investigated for the
applications of nanoelectronics and nanophotonics
in the past two decades [44-47]. These micro/nano
scale building blocks are also used as a model
system to study the physical properties of
semiconductors [48]. It is interesting to note that
the past few years have witnessed a remarkable
progress in atomically thin 2D semiconductor
layers, such as MoS2, from the fundamental studies
of new physics to the development of transistors
and optoelectronic devices [49]. From the point of
view of a single layer of PbX4 consisting of
corner-sharing metal halide octahedra, these
layered perovskites are a new family of 2D
materials due to their intrinsic 2D crystal structures,
which could open new opportunity for the
development of solution-processed optoelectronic
devices. However, there have been few reports on
the syntheses of high-quality single-crystal microor nanostructures of 2D perovskites with
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3Nano Res.
well-defined morphology [50] that are suitable
building blocks for the applications in integrated
(nano)photonic circuits and networks [51].
In this work, we first re-examine the crystal
structures of 2D lead halide layered perovskites
using phenethylammonium (C6H5CH2CH2NH3+,
PEA) as the organic cation, (PEA)2PbX4 (X = Br, I).
This specific organic cation of PEA was known to
lead to 2D perovskites that display narrow PL peak
with high quantum efficiency [52] and has enabled
room temperature LED devices [39-41]. Then we
report a facile solution synthesis of single-crystal
microplates of (PEA)2PbX4 with well-defined
rectangular geometry and nanoscale thickness
through a dissolution-recrystallization process.
Moreover, we improve the synthesis using a
solution-phase transport growth process from the
precursor to product substrate and reveal that the
formation of these 2D microplates is likely driven
by the wedding cake growth mechanism. Through
halide alloying, we readily tune the emission with
narrow peak bandwidth from violet (~410 nm) for
(PEA)2PbBr4 to green color (~530 nm) for
(PEA)2PbI4. Our successful synthesis and
understanding on the crystal growth of layered
PEA perovskite microplates not only provide a new
material system to explore the fundamental
photophysics, such as nonlinear optical properties
[53, 54], quantum confinement and carrier
dynamics [55], but also offer guidelines to
synthesize other 2D perovskites with different
organic
cations
in
the
micro/nano-scale
morphology.
2 Experimental
precipitated from the solution. After the solution
was cooled, the powder product was filtered and
rinsed with diethyl ether several times before it was
dried at 80 °C in a vacuum oven for ∼24 h to
remove the residual water.
2.3 Growth of single crystals and microplates of
(PEA)2PbX4 (X = Br and I)
First, Fluoride-doped tin oxide coated (FTO) glass
substrate was partially coated with a thin film of
lead acetate (PbAc2) by drop-casting an aqueous
solution of PbAc2•3H2O (100 mg/mL) and then
dried in an oven for 30 min at 60 °C. The
microplates of (PEA)2PbBr4 were synthesized by
placing the PbAc2 film into PEABr solution in
isopropanol (IPA) with various concentrations from
1 mg/mL to 10 mg/mL at room temperature, with
the lead precursor-coated side facing down in the
glass vial. After a specified reaction time, typically
from 1 min to 20 h, the FTO substrate was taken
out and dipped into IPA to remove any leftover
solution on the substrate, and then dried under a
stream of nitrogen flow. Large single crystals that
occasionally formed were picked up for X-ray
structure analyses. For the solution-phase transport
growth of microplates of (PEA)2PbX4 and their
alloys, a clean Si wafer or CaF2 substrate was first
placed on the bottom of a glass vial containing a 10
mg/mL PEABr solution, or a 15 mg/mL PEAI
solution, or a mixed PEABr and PEAI solution in
IPA, then the glass substrate coated with PbAc2
film were placed over the clean substrate with the
lead precursor-coated side facing the clean
substrate. The reaction time was ~ 20 h.
2.4 Single-crystal X-ray structure data collection
and determination
2.1 Materials
All chemicals and reagents were purchased from
Sigma-Aldrich and used as received unless
specified otherwise.
2.2 Syntheses of PEA halides (PEAX, X = Br, I)
The PEAXs were synthesized by a similar method
reported previously [6, 13]. Briefly, solution of HBr
(48 wt.% in water) or HI (57 wt.% in water) was
added slowly to phenylethylamine with an equal
molar ratio of 1:1 in a flask at 0 °C. Then the water
was evaporated in a hood at an elevated
temperature (~100 °C) until PEAX crystals
A single crystal with dimensions ~0.1 × 0.1 × 0.05
mm3 was selected under oil under ambient
conditions and attached to the tip of a MiTeGen
MicroMount©. The crystal was then mounted in a
stream of cold nitrogen at 100(1) K and centered in
the X-ray beam using a video camera. The structure
of the single crystal was resolved on a Bruker
Quazar SMART APEXII diffractometer with Mo Kα
(λ = 0.71073 Å) radiation and the diffractometer to
crystal distance of 4.96 cm. The initial cell constants
were obtained from three series of ω scans at
different starting angles. The reflections were
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Nano Res.
4
successfully indexed by an automated indexing
routine built in the APEXII program suite. The final
cell constants were calculated from a set of strong
reflections from the actual data collection. A
successful structure solution by the direct methods
provided most non-hydrogen atoms from the
E-map. The remaining non-hydrogen atoms were
located in an alternating series of least-squares
cycles and difference Fourier maps. All
non-hydrogen atoms were refined with anisotropic
displacement coefficients. All hydrogen atoms were
included in the structure factor calculation at
idealized positions and were allowed to ride on the
neighboring atoms with relative isotropic
displacement coefficients. More details of the data
collection and structure solution are tabulated in
Table 1.
2.5
Structural
characterizations
and
morphological
Optical images were obtained with an Olympus
BX51M optical microscope. The PL of single
microplates was collected with an Aramis Confocal
Raman/PL microscope excited by a 442 nm laser.
The sample was transferred to a Si substrate by a
dry-transfer method prior to the PL measurement.
Scanning electron microscopy (SEM) images were
acquired on a LEO SUPRA 55 VP field-emission
SEM operated at 3.0 kV. Energy-dispersive X-ray
spectroscopy (EDX) was performed on sample
transferred onto a Si wafer using a LEO 1530
field-emission SEM equipped with an EDS detector
operating at 15.0 kV. Atomic force microscopy
(AFM) was performed with an Agilent 5500 AFM
instrument in contact mode (sharp silicon tip on
nitride lever with reflective gold back coating,
SNL-10 from Bruker AFM Probes, k: 0.12 N/m).
Powder X-ray diffraction (PXRD) data were
collected using Cu Kα radiation on a Siemens
STOE diffractometer (40 kV, 40 mA).
3 Results and Discussion
We first synthesized large plate-like single crystals
of (PEA)2PbX4 (X = Br, I) with dimensions ~
0.1×0.1×0.05 mm3 by reacting lead acetate film
coated on glass slide with PEAX (X = Br, I) solution
in IPA. Single-crystal X-ray diffraction data were
collected on these crystals to allow the
determination of their crystal structures (CCDC
number 1515121-1515122), which are slightly
different from what have been previously reported
for these compounds [35, 56, 57]. Both (PEA)2PbI4
and (PEA)2PbBr4 crystallize in a triclinic crystal
system with the space group of P 1 (see Table 1 for
detailed lattice parameters). As shown in Fig. 1, the
basic structural unit of these compounds consists of
a layer of corner-sharing PbX6 octahedra, with a
layer of PEA cations capping on both sides of the
lead halide layer through hydrogen bonds between
the ammonium groups and halogen atoms
(N-H•••X). There are some minor differences
between the structures of the PbI4 and PbBr4 layers
and there is some positional disorder in the
bridging X atoms in the PbX4 layers. The crystals of
these 2D perovskites are then formed via stacking
the neutral (PEA)2PbX4 layers along the c axis via
weak van der Waals interactions. Interestingly,
even though the radius of iodide is larger than that
of bromide, (PEA)2PbI4 has a smaller c lattice
parameter, which is likely due to the different
conformation of PEA cations affected by the
specific hydrogen bonding interactions (N-H•••X)
and the disorder of lead halide planes, which
influence the details of the layer stacking [21].
Furthermore,
microplates
of
(PEA)2PbX4
perovskites with well-defined morphology can be
grown
by
similar
methods
after
some
modifications to the PbAc2 film deposition
procedure, precursor concentration and reaction
time (see Experimental Section for details). It is
important to note that we immersed the glass
substrate partially coated with PbAc2 film into
PEAX/IPA solution with the PbAc2 film facing
down. We found that well-defined microplates
usually formed on the clean area of the substrate
near the PbAc2 film, while the product grown on
PbAc2 film often exhibited irregular rectangular
shape with crystal defects. Here we use
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Figure 1 Extended crystal structures of (a) (PEA)2PbBr4, viewed approximately along the b axis, and (b) (PEA)2PbI4, viewed
approximately along the a axis. The unit cell outline is shown by the dashed lines. The hydrogen atoms were omitted for clarity.
(PEA)2PbBr4 as an example to illustrate the growth
behaviors of free standing microplates. First, we
investigated the effect of PEABr concentration on
the crystal growth with the reaction time fixed at 1
h. Figures 2(a-h) display the SEM images of
(PEA)2PbBr4 microplates grown using different
concentrations of PEABr/IPA solution ranging from
1 mg/mL to 8 mg/mL, and the corresponding PXRD
patterns are shown in Fig. 2(i). At a low
concentration of 1 mg/mL, only a few rectangular
platelets were formed and sparsely distributed on
the substrate. The yield of microplates increased
with the concentration of PEABr. The PXRD
patterns of the products grown using the PEABr
concentration of ≥4 mg/mL showed a group of
strong diffraction peaks with regular spacings at
5.27 °, 10.57 °, 15.90 °, and 21.26 °, that could be
assigned to the (001), (002), (003) and (004) lattice
planes of the (PEA)2PbBr4 layered structure.
However, peaks associated with PbAc2 (the bottom
trace) clearly showed that a significant amount of
PbAc2 was unreacted at the low concentration of
1-2 mg/mL (Fig. 2(i)), suggesting much slower
reaction kinetics at a lower concentration.
Therefore, an optimized concentration to
synthesize well-defined (PEA)2PbBr4 microplates
with proper dimensions for photonics and
electronics is ≥ 4 mg/mL. However, we should
note there is large dimensional variation among
as-grown microplates. Figure 2(j) shows an optical
image of a typical region of (PEA)2PbBr4
microplates grown using a 4 mg/mL PEABr
solution for 2 h. The size of as-grown microplates
varies from several micrometers to tens of
micrometers.
The effect of reaction time on the crystal growth
of (PEA)2PbBr4 was then investigated, while the
concentration of PEABr was fixed at 4 mg/mL.
Figures S1(a-h) in the Electronic Supplementary
Material (ESM) show the SEM images of
(PEA)2PbBr4 microplates synthesized at a reaction
time of 1 min, 5 min, 10 min, 45 min, 2 h, 5 h and
18.5 h, respectively. After a short reaction of 1-5 min,
the strong (001) diffraction peak at 5.27 ° confirms
the formation of (PEA)2PbBr4 phase (see Fig. S1(i)
in the ESM). The corresponding SEM images (Figs.
S1(b) and S1(c) in the ESM) show small plate-like
products with size of ~ 1 μm on the substrate;
however, their edges are not well-defined. After
extending the reaction time to 10 min, the products
(Fig. S1(d) in the ESM) start to display well-defined
geometry and smooth surfaces. In general, the size
and thickness of these platelets continue to increase
with the reaction time (Figs. S1(d-h) in the SEM).
The corresponding PXRD
Table 1 Crystallographic data for (PEA)2PbBr4 and (PEA)2PbI4 single crystals.a
Empirical formula
C32H48Br8N4Pb2
C16H24I4N2Pb
formula weight
1542.40
959.16
crystal system
Triclinic
triclinic
space group
a (Å)
P1
11.5219(6)
P1
8.679(2)
b (Å)
11.5236(6)
8.684(2)
c (Å)
17.2667(10)
16.410(4)
α (°)
80.3860(10)
94.453(14)
β (°)
73.9000(10)
100.588(13)
89.9980(10)
90.573(11)
volume (Å )
2169.1(2)
1211.7(6)
Z
2
2
2.362
2.629
15.147
12.059
γ (°)
3
3
ρcalc (g/cm )
-1
μ (mm )
F(000)
1424.0
856.0
crystal size (mm3)
0.12 × 0.10 × 0.05
0.10 × 0.08 × 0.05
color
colorless
yellow
index ranges
-13 ≤ h ≤ 13, -13 ≤ k ≤ 13, -20 ≤ l ≤ 20
-12 ≤ h ≤ 12, -12 ≤ k ≤ 12, -23 ≤ l ≤ 23
7652 [Rint = 0.0630, Rsigma = 0.0655]
7416 [Rint = 0.0389, Rsigma = 0.0321]
1.065
1.143
final R indexes [I>=2σ (I)]
R1 = 0.0388, wR2 = 0.0927
R1 = 0.0331, wR2 = 0.0605
final R indexes [all data]
R1 = 0.0579, wR2 = 0.0977
R1 = 0.0404, wR2 = 0.0626
1.89/-1.41
1.50/-1.14
independent reflections
goodness-of-fit on F
2
-3
largest diff. peak/hole (e.Å )
a
With Mo Kα radiation (λ = 0.71073 Å) at 100 K.
patterns (Fig. S1(i) in the ESM) also show the
dramatic increase of the (001) diffraction peak of
(PEA)2PbBr4 as the reaction time increases,
indicating significant crystallinity enhancement.
The crystal growth behaviors observed above
can be explained by a dissolution-recrystallization
mechanism [58], that is, the PbAc2 precursor is first
dissolved to form the PbBr42- complex ions in the
solution and then recrystallize with the PEA
organic cations to form (PEA)2PbBr4 crystals and
nanostructures on a different region of the
substrate. The chemical reactions can be described
as following:
PbAc2 ( s )  4 Br  ( sol )  PbBr42  ( sol )  2 Ac  ( sol )
(1)
PbBr42 ( sol )  2 PEA ( sol )  ( PEA)2 PbBr4 ( s)
(2)
We believe that the local supersaturation of the
PbBr42- complex can strongly affect the growth rate,
crystal quality, and morphology. As noted above,
the free-standing microplates with well-defined
geometry and flat facets were usually found in the
clean (uncoated with PbAc2) regions of the
substrate neighboring the PbAc2 film; on the other
hand, the product grown at the PbAc2 film often
exhibited complex over-growth with much
disorder. We attribute these distinct growth
behaviors to the difference of local supersaturation
on
the
substrate.
The
relatively
high
supersaturation of PbBr42- precursor over the PbAc2
film could lead to fast crystal growth kinetics,
resulting in uncontrollable
Figure 2 Structural characterizations of (PEA)2PbBr4 micoplates. (a−h) SEM images of (PEA)2PbBr4 microplates grown using
PEABr/IPA solutions with concentration ranging from 1 mg/mL to 8 mg/mL, respectively. (i) The corresponding PXRD patterns of
as-grown (PEA)2PbBr4, together with that of drop-casted PbAc2 on FTO substrate (bottom trace). (j) A representative optical image
of (PEA)2PbBr4 microplates grown using a PEABr concentration of 4 mg/mL for 2 h. The inset shows a uniform rectangular
microplate.
overgrowth. However, for the growth of
well-defined nano- and microstructures, PbBr42ions need to diffuse to other areas (but close to the
PbAc2 source) where low supersaturation of PbBr42lingers to recrystallize with PEA cations to form
(PEA)2PbBr4, which might enable the crystal
growth in a more controllable way [58, 59].
To verify this hypothesis and further improve the
control of crystal growth, we further designed a
solution-phase transport growth process to directly
grow these microplates on another clean substrate
(i.e. not coated with PbAc2 precursor film) by
placing the glass slide coated with PbAc2 film over
a silicon wafer or CaF2 substrate, as illustrated in
Fig. 3(a). In this process, the perovskite products
formed on the product substrate must have
transported through the solution between the two
substrates via the dissolution-recrystallization
process, hence the name of “solution-phase
transport growth”. The PXRD patterns of the
products on both the precursor substrate and the Si
substrate grown at 10 mg/mL PEABr for 19 h show
identical diffraction peaks associated with
(PEA)2PbBr4 (Fig. 3(b)). Figures 3(c) and 3(d) show
optical images of (PEA)2PbBr4 microplates with
rectangular shapes and smooth surfaces grown on
both the clean areas of the precursor substrate and
the Si substrate (Fig. S2 in the ESM for SEM
images), respectively. The success of microplate
growth on a clean substrate through the solution
clearly confirms our hypothesis above and the
dissolution-recrystallization process. Figures 3(e)
and 3(f) highlight individual microplates with
well-defined geometry grown on the Si substrate.
(PEA)2PbBr4 microstructures can still be observed
on the precursor substrate in the region originally
coated with PbAc2 precursor, but they display
much more disorder and poorly controlled
morphology and size (Fig. S3 in the ESM for SEM
images). The microstructures can also be grown on
other substrates, for example, CaF2 substrate (Fig.
S4 in the ESM), by the solution-phase transport
growth process. Typically, the size of these
microplates on Si substrates varies from a few
micrometers to a hundred micrometers, and the
thickness ranges from tens of nanometers to up to a
few micrometers (see AFM images in Fig. S5 in the
ESM). The large variation in dimensions among
these microplates might be explained by the
different timing of the nucleation that triggers the
growth of each micorplate and the local
supersaturation of the PbBr42- could be spatially
dependent. EDS analysis on individual microplates
grown on Si substrate yields a Br/Pb ratio ~4, in
good agreement with the stoichiometry of
(PEA)2PbBr4. Further EDS mapping shows Br and
Pb elements are uniformly distributed within the
whole microplate (Fig. 3(g)). This
Figure 3 (a) Schematic illustration of the solution-phase transport growth process of (PEA)2PbX4 microplates. (b) PXRD patterns
of the products on precursor substrate and Si substrate. Optical image of (PEA)2PbBr4 microplates grown on (c) the precursor
substrate, and (d) the Si substrate. (e-f) Optical images of individual well-defined microplates with different dimensions. (g) EDS
mapping of a representative microplate showing uniform distribution of Pb and Br elements.
improved solution-phase transfer growth method
not only leads to more well-defined microplates
with sharp and smooth facets, but also can enable
convenient growth of perovskite nanostructures
onto arbitrary substrates for further property
studies and potential device fabrication.
Furthermore, we used AFM images to reveal that
step terrace morphology was clearly noticeable on
the surface of most (PEA)2PbBr4 nanoplatelets, as
shown in Fig. 4(a). An average step height
generated from the topographic image is ~1.7 nm,
in good agreement with the thickness of single
layer (PEA)2PbBr4 (1.69 nm). The absence of spiral
center suggests that the growth is likely dominated
by the wedding cake growth [60], where the new
nucleation forms on the top layer generating
terrace feature akin to screw-dislocation growth
patterns. Interestingly, we also occasionally found a
few plates possessing both a dislocation core and
spiral growth (Fig. S6 in the ESM), suggesting the
coexistence and competition of both growth
mechanisms. Nevertheless, these two growth
mechanisms both require low supersaturation
conditions, highlighting the importance of proper
low supersaturation on the formation of
well-defined microplate morphology [59, 61].
We observed very similar crystal morphology
and growth behavior for (PEA)2PbI4 using the
s
o
l
u
t
i
o
n
Figure 4 (a) AFM image of a representative (PEA)2PbBr4
microplate showing growth terrace on the surface. (b) The
corresponding step height profile from the line 1, yielding an
average step height of 1.7 nm.
transport growth method by replacing PEABr with
PEAI. Figure S7(a) in the ESM shows a
representative
SEM
image
of
(PEA)2PbI4
microstructures grown using a PEAI/IPA solution
with a proper concentration of 15 mg/mL for ~ 20 h.
Figure S7(b) in the ESM highlights some magnified
optical images of individual microplates and
9Nano Res.
microrods with well-defined morphology. The
lateral dimensions are 10-100 μm with thickness
varying from hundreds of nanometers to a few
micrometers, depending on the reaction time,
precursor concentration, and the growth area on
the substrate. EDS analysis on individual
microstructures was further carried out to confirm
the expected stoichiometry of Pb/I, which is ~1:4.
Moreover, by simply mixing the PEA halides
with various molar ratios, we can further
synthesize microstructures of a series of halide
alloys of the (PEA)2Pb(Br,I)4 perovskites. A
representative SEM image of the microstructures of
(PEA)2Pb(Br,I)4 alloys on the original precursor
substrate grown using a mixed precursor solution
of PEABr at 6 mg/mL and PEAI at 9 mg/mL (Fig.
5(a)) shows the as-grown microstructures with
irregular shapes. Figure 5(b) shows the SEM and
optical (Fig. 5(b) inset) images of the
microstructures with more defined shape sparsely
grown on a clean Si substrate after the solution
transport growth. Interestingly, unlike pure
(PEA)2PbI4 or (PEA)2PbBr4, we found that the use of
mixed precursors tends to promote the growth of
other geometric shapes beyond rectangular shape,
such as hexagonal, rhomboic and octagonal shapes
(see Fig. S8 in the ESM for more examples). EDS
analysis of an individual hexagonal microplate was
performed to confirm the successful alloying of Br
and I (Fig. 5(c)), yielding an average estimated
stoichiometry
of
(PEA)2PbBr2.4I1.6
through
quantitative analysis on several microplates (Fig. S9
in the ESM). Figure 5(d) shows the PXRD patterns
of the alloys grown using different ratios of Br/I in
the precursor solutions. Interestingly, the (00l)
peaks (corresponding to c lattice parameter) change
discontinuously with increasing Br/I ratio. A
sudden shift of the (00l) peaks was observed
around the alloy composition of (PEA)2PbBr0.6I3.4
(the ratio was determined by EDS analysis), but
then remained unchanged as the Br content further
increased. The unusual trend has been observed in
the thin films of
Figure 5 Structural characterization of microstructures of
(PEA)2Pb(Br,I)4 alloys. SEM images of representative
microstructures of (PEA)2Pb(Br,I)4 alloys grown using a mixed
solution of 6 mg/mL PEABr and 9 mg/mL PEAI (a) on the
precursor substrate and (b) on the Si substrate. The inset of (b)
shows an optical image of (PEA)2Pb(Br,I)4 microstructures on
Si substrate. (c) EDS mapping of an individual hexagonal
microplate showing the successful alloying of Br and I. (d)
PXRD patterns of various (PEA)2Pb(Br,I)4 alloys grown using
mixed solutions with different Br/I ratios in the precursor
solution.
(PEA)2Pb(Br,I)4 alloys [52], which may be explained
by the varying c lattice constants in the two
(PEA)2PbX4 crystal structures (see Table 1 and Fig. 1)
due to a sudden conformation change of the PEA
cations in these alloys.
Due to quantum confinement within the lead
halide layers, these layered perovskites exhibit
several features, such as high quantum efficiency
[40], high color purity (narrow emission
bandwidth), and controllable color tunability, that
could be attractive for light emitting applications.
Our preliminary optical studies reveal these 2D
perovskite microstructures have strong room
temperature PL with a small full width at half
maximum (FWHM). PL spectra collected on the
as-grown (PEA)2PbBr4 and (PEA)2PbI4 on Si
substrates show band edge emissions centered at
406 nm and 519 nm, with FWHMs of ~11 nm and
~17 nm, respectively (Fig. 6(a) purple and green
curves, respectively). The PL spectra collected on
individual (PEA)2Pb(Br,I)4 nano- or microstructures
with different stoichiometry show a continuous
blue shift from green to violet color with increasing
Br content (Fig. 6(a)). This is in agreement with the
increasing bandgap expected due to the alloying of
the Br into the (PEA)2PbI4, which have been also
observed on thin films previously [33].
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano
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Nano Res.
10
together with their nanocale thickness, make these
(PEA)2PbX4 microstructures interesting building
blocks for (nano)photonics and optoelectronics.
4 Conclusions
Figure 6 PL spectra of various alloyed (PEA)2PbX4 perovskite
microstructures. (a) Confocal microscopy PL spectra of
individual (PEA)2Pb(Br,I)4 microstructures excited by a 442
nm laser source at room temperature. Note the PL spectrum of
(PEA)2PbBr4 was excited by a 365 nm laser. (b-f) Optical
images of a series of individual microstructures of
(PEA)2Pb(Br,I)4 alloys showing strong waveguiding effect, (b)
elongated hexagonal (PEA)2PbBr3.1I0.9 microplate, (c)
rectangular (PEA)2PbBr3.1I0.9 microoplate, (d) hexagonal
(PEA)2PbBr2.4I1.6 microplate, (e) rectangular (PEA)2PbBr0.6I3.4
microplate, (f) (PEA)2PbI4 microwire. All scale bars are 10
µm.
We also noticed an increase of FWHM of the PL
peaks with the increase of the Br content in these
alloys, which might be due to the increased
inhomogeneity arising from structural and
chemical disorder. Interestingly, we noticed the PL
spectra of the (PEA)2PbX4 samples grown on
precursor substrates were characterized by more
asymmetric shapes with clear PL tails than those
grown on a clean Si substrate through the solution
transport growth (Fig. S10 in the ESM). The
broadening of PL has been previously attributed to
the formation of self-trapping due to the
exciton-phonon interactions [62, 63]. Earlier studies
also suggested that the imperfections of layer
stacking in the single crystal could introduce
additional red-side band emission [64]. We further
note that the PL peak for the (PEA)2PbBr3.1I0.9
microplate sample has the most asymmetric shape
and likely contains shoulder, which could be due to
light-induced halide segregation of the perovskite.
Furthermore, a series of optical images of these
alloys excited by a 442 nm CW laser (Figs. 6(b-f))
clearly demonstrate tunable emission and strong
waveguiding effect among these microstructures
(see Fig. S11 in the ESM for more examples). Such
high efficient and tunable light emitting properites,
In summary, we synthesized single-crystal
microplates of 2D perovskite (PEA)2PbX4 (X = Br, I)
with well-defined rectangular geometry and
nanoscale thickness via a facile solution route.
Single-crystal X-ray diffraction study confirmed
that both (PEA)2PbBr4 and (PEA)2PbI4 have layered
crystal structures. Under optimal precursor
concentrations to form microplate structures, the
typical size of these microplates varies from a few
micrometers to a hundred micrometers, and the
typical thickness ranges from tens of nanometers to
up to a few micrometers. The crystal growth
proceeds through a dissolution-recrystallization
mechanism and can be further improved by using a
solution-phase transport growth method. AFM
measurement suggests the formation of these
microplates is likely driven by the wedding cake
growth. By using mixed PEA halide precursor
solutions, microstructures of a series of
(PEA)2Pb(Br,I)4 alloy perovskites can be readily
grown to tune the PL emission from violet (~410
nm) to green (~530 nm) color. Our successful
synthesis and understanding on the crystal growth
of these micro/nanoscale 2D perovskites not only
provide a new material system to explore their
fundamental photophysics [65-67], but also offer
guidelines to synthesize other 2D perovskites with
different organic cations in the micro/nano scale
morphology.
Acknowledgements
This work is supported by the Department of
Energy, Office of Basic Energy Sciences, Division of
Materials Sciences and Engineering, under Award
DE-FG02-09ER46664. D.M. also acknowledges
financial support from the China Scholarship
Council and the Natural Science Foundation of
Zhejiang Province of China (No. LY13F040002). L.
D. also thanks the UW-Madison Advanced
Opportunity Fellowship (AOF) and NSF Graduate
Research Fellowship for support.
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano
Research
Nano Res.
11
Electronic
Supplementary
Material:
Supplementary
material
(Structural
characterizations on (PEA)2PbBr4 microplates
synthesized at different reaction times; SEM,
optical and AFM images of
(PEA)2PbBr4
microplates grown by a solution-phase transport
process; SEM and optical images of (PEA)2PbI4
microstructures; Optical images, EDS analyses and
PL
spectra
of
alloyed
(PEA)2Pb(Br,I)4
microstructures) is available in the online version
of
this
article
at
http://dx.doi.org/10.1007/s12274-***-****-* .
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Electronic Supplementary Material
Singe-crystal
microplates
of
two-dimensional
organic-inorganic lead halide layered perovskites for
optoelectronics
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano
Research
Nano Res.
Dewei Ma
1,2,§
64
1,§
1
1
1
1
, Yongping Fu , Lianna Dang , Jianyuan Zhai , Ilia A. Guzei , Song Jin ()
1
Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
Department of Applied Physics, Zhejiang University of Technology, Hangzhou 310014, China
§
These authors contributed equally to this work.
2
Supporting information to DOI 10.1007/s12274-****-****-*
Figure S1 Effect of reaction time on the crystal growth of (PEA)2PbBr4 microplates. SEM images of (a)
drop-casted PbAc2 film on FTO substrate and (PEA)2PbBr4 microstructures grown with a reaction time of (b)
1 min, (c) 5 min, (d) 10 min, (e) 45 min, (f) 2 h, (g) 5 h, and (h) 18.5 h, respectively, while the PEABr
precursor concentration was fixed at 4 mg/mL. (i) The corresponding PXRD patterns of as-grown
(PEA)2PbBr4.
Address correspondence to Song Jin, [email protected]
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Nano Res.
65
Figure S2 (a) SEM image of (PEA)2PbBr4 microplates grown on Si substrate by a solution-phase transport
process. (b) Magnified SEM images highlight several individual microplates with well-defined rectangular
shape. All scale bars are 10 μm.
Figure S3 SEM images of (PEA)2PbBr4 microstructures grown on the precursor substrate in the region
originally coated with PbAc2 precursor.
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66
Figure S4 Optical images of (PEA)2PbBr4 microstructures grown on CaF2 substrate by a solution-phase
transport process.
Figure S5 AFM image of (PEA)2PbBr4 microplates with various thickness, and the corresponding step
height profile from the line in the AFM image.
Figure S6 AFM images of (PEA)2PbBr4 microplate (a) with a dislocation core, and (b) without a dislocation
core.
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Figure S7 (a) SEM image of (PEA)2PbI4 microstructures synthesized using a 15 mg/mL PEAI/IPA solution
with a reaction time of
~20 h. The inset SEM image shows a rectangular microplate with flat facet. (b)
Optical images of individual microplate and microrod of (PEA)2PbI4 single crystals.
Figure S8 Optical images of mixed halide alloys of (PEA)2Pb(Br,I)4 with various geometry beyond the most
common rectangular shape.
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x value of (PEA)2PbI4-xBrx
4
3
2
(PEA)I_12 mg/mL, (PEA)Br_3 mg/mL
(PEA)I_9 mg/mL, (PEA)Br_6 mg/mL
(PEA)I_6 mg/mL, (PEA)Br_9 mg/mL
(PEA)I_3 mg/mL, (PEA)Br_12 mg/mL
1
0
1
2
3
4
5
6
7
Sample No.
Figure S9 Determination of Br/I ratio in the alloyed microstructures grown using mixed halide precursor
solution with different concertation ratios through quantitative EDS analysis.
(PhEA)2PbBr4
(PhEA)2PbBr3.7I0.3
(PhEA)2PbBr3.1I0.9
(PhEA)2PbBr2.4I1.6
PL intensity (a.u.)
(PhEA)2PbBr0.6I3.4
(PhEA)2PbI4
on Si wafer
on glass slide
400
450
500
550
600
650
Wavelength (nm)
Figure S10 Photoluminescence spectra of various alloyed (PEA)2PbBrxI4-x microstructures on the precursor
glass substrate and Si substrate.
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Figure S11 Additional optical images of individual microstructures of (PEA)2Pb(Br,I)4 alloys with different
geometries showing strong waveguiding effect.
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