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Nano Res

1

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

© Tsinghua University Press 2016

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Nano Research

DOI 10.1007/s12274-016-1401-6

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

64 Nano Res.

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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.


65Nano Res.

Provide the authors’ webside if possible.

Song Jin, https://jin.chem.wisc.edu/



optoelectronics

Dewei Ma1,2,§

, Yongping Fu1,§

, Lianna Dang1, Jianyuan Zhai

1, Ilia A. Guzei

1, Song Jin

1 ()

1 Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States

2 Department of Applied Physics, Zhejiang University of Technology, Hangzhou 310014, China

§These authors contributed equally to this work.

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

ABSTRACT

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

materials community are a promising class of

semiconductor materials for high-performance

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

formula of (RNH3)2An-1MnX3n+1, in which R is a

long-chain alkyl or aromatic group, A is a small

Nano Research

DOI (automatically inserted by the publisher)

Address correspondence to Song Jin, [email protected]

Research Article


2Nano Res.

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 micro-

or nanostructures of 2D perovskites with


3 Nano 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

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

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

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


4Nano Res.

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 and morphological

characterizations

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

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 P 1 P 1

a (Å) 11.5219(6) 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)

γ (°) 89.9980(10) 90.573(11)

volume (Å3) 2169.1(2) 1211.7(6)

Z 2 2

ρcalc (g/cm3) 2.362 2.629

μ (mm-1) 15.147 12.059

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

independent reflections 7652 [Rint = 0.0630, Rsigma = 0.0655] 7416 [Rint = 0.0389, Rsigma = 0.0321]

goodness-of-fit on F2 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

largest diff. peak/hole (e.Å-3) 1.89/-1.41 1.50/-1.14

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:

2

2 4( ) 4 ( ) ( ) 2 ( )PbAc s Br sol PbBr sol Ac sol

(1)

24 2 4( ) 2 ( ) ( ) ( )PbBr sol PEA sol PEA PbBr 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, PbBr42-

ions need to diffuse to other areas (but close to the

PbAc2 source) where low supersaturation of PbBr42-

lingers 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


9 Nano 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].


10Nano Res.

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,

together with their nanocale thickness, make these

(PEA)2PbX4 microstructures interesting building

blocks for (nano)photonics and optoelectronics.

4 Conclusions

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.


11 Nano Res.

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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Appl. Phys. 2013, 113, 083523.

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Niesner, D.; Williams, K. W.; Jin, S.; Zhu, X. Y.

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1409–1413.

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E.; Jin, S.; Yu, D. Photocurrent mapping in single-crystal

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Electronic Supplementary Material



optoelectronics

64

64Nano Res.

Dewei Ma1,2,§

, Yongping Fu1,§

, Lianna Dang1, Jianyuan Zhai

1, Ilia A. Guzei

1, Song Jin

1 ()

1 Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States

2 Department of Applied Physics, Zhejiang University of Technology, Hangzhou 310014, China

§These authors contributed equally to this work.

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]

65

65 Nano Res.

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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66Nano Res.

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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67 Nano Res.

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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68Nano Res.

1 2 3 4 5 6 7

0

1

2

3

4

(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

x v

alu

e o

f (P

EA

) 2P

bI 4

-xB

r x

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.

400 450 500 550 600 650

on glass slide

PL

inte

nsi

ty (

a.u

.)

Wavelength (nm)

(PhEA)2PbBr

4

(PhEA)2PbBr

3.7I0.3

(PhEA)2PbBr

3.1I0.9

(PhEA)2PbBr

2.4I1.6

(PhEA)2PbBr

0.6I3.4

(PhEA)2PbI

4

on Si wafer

Figure S10 Photoluminescence spectra of various alloyed (PEA)2PbBrxI4-x microstructures on the precursor

glass substrate and Si substrate.

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69 Nano Res.

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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