Lawrence Berkeley National Laboratory
Recent Work
Title
Atomically thin two-dimensional organic-inorganic hybrid perovskites.
Permalink
https://escholarship.org/uc/item/2nv363g0
Journal
Science (New York, N.Y.), 349(6255)
ISSN
0036-8075
Authors
Dou, Letian
Wong, Andrew B
Yu, Yi
et al.
Publication Date
2015-09-01
DOI
10.1126/science.aac7660
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California
Matter MURI, NIST, and NSF (through the Physics Frontier Center
at the JQI). B.K.S. is a NIST–National Research Council
Postdoctoral Research Associate. L.M.A. was supported by the
NSF Graduate Research Fellowship Program. All authors except
I.B.S contributed to the data collection effort. B.K.S. and L.M.A.
configured the apparatus for this experiment. H.-I.L. led the
team on all aspects of the edge-current and skipping-orbit
measurements. H.-I.L., L.M.A., and B.K.S. analyzed data. B.K.S.,
H.-I.L., L.M.A, and I.B.S. performed numerical and analytical
calculations. All authors contributed to writing the manuscript.
I.B.S. proposed the initial experiment.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/349/6255/1514/suppl/DC1
Materials and Methods
Supplementary Text
Fig. S1
Reference (35)
Database S1
9 February 2015; accepted 28 July 2015
10.1126/science.aaa8515
NANOMATERIALS
Atomically thin two-dimensional
organic-inorganic hybrid perovskites
Letian Dou,
1,2
* Andrew B. Wong,
1,2
* Yi Yu,
1,2,3
* Minliang Lai,
1
Nikolay Kornienko,
1,2
Samuel W. Eaton,
1
Anthony Fu,
1,2
Connor G. Bischak,
1
Jie Ma,
2
Tina Ding,
1,2
Naomi S. Ginsberg,
1,2,4,5,6
Lin-Wa ng Wang,
2
A. Paul Alivisatos,
1,2,5,7
Peidong Yang
1,2,5,7
†
Organic-inorg anic hybrid perovskites, which have prov ed to be promising semiconductor
materials for photovoltaic applications, have been made into atomically thin two-dimensional
(2D) sheets. We report the solution-phase growth of single- and few-unit-cell-thick
single-crystalline 2D hybrid perovskites of (C
4
H
9
NH
3
)
2
PbBr
4
with well-defined square
shape and large size. In contrast to other 2D materials, the hybrid perovskite sheets
exhibit an unusual structural relaxation, and this structural change leads to a band gap
shift as compared to the bulk crystal.The high-quality 2D crystals exhibit efficient
photoluminescence, and color tuning could be achieved by changing sheet thickness as
well as composition via the synthesis of related materials.
T
he organic-inorganic hybrid perovskites,
especially CH
3
NH
3
PbI
3
, have recently been
used in solution-processable photovol-
taic devices that have reached 20% power
conversion efficiency (1–4). These layered
materials have a general formula of (RNH
3
)
2
(CH
3
NH
3
)
m-1
A
m
X
3m+1
, where R is an alkyl or
aromatic moiety, A is a metal cation, and X is
a halide. The variable m indicates the number
of the metal cation layers between the two lay-
ers of the organic chains (5–11). In the extreme
case where m = V, the structure becomes a
three-dimensionally bonded perovskite crystal
with a structure similar to BaTiO
3
. In the oppo-
site extreme where m = 1, the structure becomes
an ideal quantum well with only one atomic
layer of AX
4
2–
separated by organic chains, in
which the adjacent layers are held together by
weak van der Waals forces.
This arrangement is fundamentally different
from transition metal dichalcogenides, in which
one layer of the metal ions is sandwiched be-
tween two hexagonal layers of S or Se atoms,
affording a rigid backbone. In contrast, the lay-
ered hybrid perovskites normally have a tetrago-
nal or orthorhombic structure and are inherently
more flexible and deformable (5–11). By varying
the value of m, the thickness and the related
optoelectronic properties of the quantum well
can be tuned. To date, many organic amines,
metal cati ons (Cu
2+
,Mn
2+
,Cd
2+
,Ge
2+
,Sn
2+
,Pb
2+
,
Eu
2+
, etc.) and halides (Cl, Br, and I) have been
used to construct such layered materials (m =
1 ~ 3), and their corresponding optoelectronic
properties have been well studied (12–15). Pre-
vious reports have claimed that the organic layers
effectively isolate the two-dimensional (2D) quan-
tum wells in each layer from electronic coupling,
if the organic chain is longer than propyl amine
(16). This means that the properties of the atomi-
cally th in 2D quantum well should be the same
as those of the bulk layered material (microscopic
crystal, powder , or film). This hypothesis, as well
as the technical difficulty of separating individual
layers, has probably delayed investigation of free-
standing single layers of such 2D materials. Very
recently, at tempts to obtain ultrathin 2D perov-
skite samples by spin coating, chemical vapor de-
position, or mechanical exfoliation methods have
been made with limited success (17–19).
Here we report the direct growth of atomically
thin 2D hybrid perovskites [(C
4
H
9
NH
3
)
2
PbBr
4
]
and derivatives from solution. Uniform square-
shaped 2D crystals on a flat substrate with high
yield and excellent reproducibility were syn-
thesized by using a ternary co-solvent. We in-
vestigated the structure and composition of
individual 2D crystals using transmission elec-
tron microscopy (TEM), energy-dispersive spe c -
troscopy (EDS), grazing-incidence wide-angle
x-ray scattering (GIWAXS), and Raman spec-
troscopy. Unlike other 2D materials, a struc-
tural relaxation (or lattice constant expansion)
occurred in the hybrid perovskite 2D sheets
that could be responsible for emergent features.
We investigated the optical properties of the 2D
sheets using steady-state and time-resolved pho-
toluminescence (PL) spectroscopy and cathodolu-
minescence microscopy. The 2D hybrid perovskite
sheets have a slightly shifted band edge emission
that could be attributed to the structural relaxa-
tion. We further demonstrated that the as-grown
2D sheets exhibit high PL quantum efficiency as
well as wide composition and color tunability.
A structural illustration of a monolayer 2D
perovskite (Fig. 1A) shows the case with six Br
atoms surround ing each Pb atom, and the four
in-plane Br atoms are shared by two octahedrons,
forming a 2D sheet of PbBr
4
2–
.Thenegative
charges are compensated for by the butylam-
monium that caps the surfaces of the 2D sheet.
Thisstructureisamenabletofacilesolutionsyn-
thesis. The ionic character of such materials is
stronger than the transition metal disulfides
and diselenides, and the bulk solid is soluble in
polar organic solvents such as dimethylformamide
(DMF) (20). To grow 2D sheets, a very dilute pre-
cursor solution was dropped on the surface of a
Si/SiO
2
substrate an d dried under mild heating
[see the supplementary materials (21)]. A DMF
and chlorobenzene (CB) co-solvent was initially
investigated, because CB helps to reduce the sol-
ubility of (C
4
H
9
NH
3
)
2
PbBr
4
in DMF and pro-
mote crystallization. Because CB has a similar
boiling point and evaporation rate as DMF, the
drying of the solvents and the crystallization
process were uniform acros s the whole substrate.
We examined the products of this reaction by
optical microscopy and atomic force microscopy
(AFM), but instead of monolayers, thick par-
ticles with random shapes formed on the sub-
strate (fig. S1). Hybrid perovskites have limited
solubility in acetonitrile, and the solvent has
been used previously for making microscopic
hybrid perovskite single crystals (22). In this
case, acetonitrile evaporates more quickly and
helps induce the formation of the ultrathin 2D
hybrid perovskite sheets. When acetonitrile was
combined with DMF and CB, uniform square
sheets grew on the substrate (Fig. 1B). The edge
length of the square crystals ranged from 1 to
10 mm, with an average of 4.2 mm(thesizedis-
tribution statistics can be found in fig. S2). The
detailed synthetic procedure and discussion of
the role of each solvent can be found in the sup-
pl e mentary text (21).
1518 25 SEPT EMBER 2015 • VOL 349 ISSUE 6255 sciencemag.org SC IENCE
1
Department of Chemistry, University of California, Berkeley , CA
94720 , USA.
2
Materials Sciences Division, Lawrence Berkeley
National Laboratory , Berkeley , CA 94720 , USA.
3
School of
Physical Science and Technology, ShanghaiTech University,
Shanghai , 20121 0 , China.
4
Department of Physics, University of
California, Berkeley, CA 94720, USA.
5
Kavli Energy NanoScience
Institute, Berkeley, CA 94720, USA.
6
Physical Biosciences Division,
Lawrence Berkeley National Laboratory , Berkeley , CA 94720, USA.
7
Department of Materials Science and Engineering, University of
California, Berkeley, CA 94720, USA.
*These authors contributed equally to this work. †Corresponding
author. E-mail: p_yang@berkeley.edu
RESEARCH | REPORTS
on October 26, 2015www.sciencemag.orgDownloaded from on October 26, 2015www.sciencemag.orgDownloaded from on October 26, 2015www.sciencemag.orgDownloaded from on October 26, 2015www.sciencemag.orgDownloaded from
The thickness of the square sheets was quan-
tified with AFM. The thickness of the crystals
varied from a few to tens of nanometers; the
thinnest sheets were ~1.6 nm (±0.2 nm). The
AFM images of several monolayer and double-
layer sheets show thicknesses of 1.6 and 3.4 nm
(±0.2 nm) (Fig . 1, C and D), whereas the d
spacing in the bulk crystal was 1.4 nm (20).
AFM tapping mode (noncontact) was used to
avoid sample damage, which can lead to a minor
overestimation. For the separated monolayer, the
organic chain may also relax, and the apparent
thickness of the monolayer may increase slight-
ly. Additional AFM images of other similar 2D
sheets can be seen in fig. S3.
By combining AFM and optical microscopy,
we correlated the appearance of the sheets in
the optical image with their thickness as shown
in fig. S4. The thinnest sheet on Si/SiO
2
that we
could distinguish in an optical microscope was
a double layer. We also prepared large single
crystals of (C
4
H
9
NH
3
)
2
PbBr
4
and investigated
the conventional mechanical exfoliation method
using tape and the solvent exfoliation method
using h exane to disperse the thin sh eets (23).
Unfortunately,themajorityoftheproductsfrom
mechanical exfoliation were very thick flakes (fig.
S5A) and from solvent exfoliation they were ran-
domly shaped particles (fig. S5, B and C). The
monolayer-thick particles obtained were very
small (less than 1 mm), which suggests that these
hybrid perovskite layers are mechanically brittle.
To determine the crystal structure of the 2D
hybrid perovskites, x-ray diffraction (XRD) and
TEM were used. The XRD pattern revealed that
the (001) plane grew parallel to the substrate,
and the out-of-plane d spacing was 1.42 nm (fig.
S6), which is consistent with reported data for
this material (16).Thein-planestructuralinfor-
mation was revealed by selected-area electron
diffraction(SAED)inaTEM.Figure2Ashowsa
TEM image of a 2D sheet grown on a lacy car-
bon grid. After examining more than 20 indi-
vidual sheets by TEM, we found that they all
showed similar shape and identical diffraction
patterns (additional TEM images are shown fig.
S7); Fig. 2B shows the SAED pattern of an other
sheet. The calculated average in-plane lattice
constants are a =8.41Åandb = 8.60 Å from five
sheets,whichareslightlygreaterthanthelattice
constants in the bulk crystal measured by single-
crystal XRD [a =8.22Åandb =8.33Å,see(15)
and tables S1 and S2]. The electron diffra ct ion
patterns were consistent with structural simula-
tions, which further confirm the structure of the
atomically thin 2D sheets (see figs. S8 and S9 for
more discussion on the simulations/experiments
on the bulk and few-layer hybrid pe rovskites).
We observed rapid radiation damage of the
sheets under the strong electron beam. After
exposing the 2D sheets to the electron beam for
a few seconds, Pb was reduced and precipitated,
which caused the sample to be irreversibly dam-
aged (fig. S10). This phenomenon is similar to
that observed in alkali halides (24). More exam-
ples of SAED pat terns of indivi dual sheets and
their corresponding TEM images demonstrating
the degradation can be found in fig. S11. Figure 2,
C to F, shows the elemental distribution in the
thin sheets; lead, bromine, carbon, and nitrogen
are all present in the square.
The lattice expansion in the 2D sheets was fur-
ther confirmed through macroscopic GIWAXS
measurements. Figure 2G shows the GIWAXS
image and Fig. 2H shows the integrated pattern.
In addition, to the (200) and (020) peaks ob-
served in TEM diffraction, many other peaks ca n
be assigned. The d spacing of the (200), (020), (111),
and (113) planes is 4.19 (lattice constant a =8.38Å),
4.25 (lattice constant b = 8.50 Å), 5.81, and 5.00 Å;
respectively. These values are all greater than
those of the bulk crystals and are consistent
with our TEM measurements of single 2D sheets.
In addition, we examined the Raman spectra
of the bulk crystal and the thin sheet as shown
in fig. S12. The peaks found at 57.7 and 43.6 cm
−1
for the bulk crystal shifted to 55.2 and 41.3 cm
−1
for the 2D sheet, respectively. These peaks can be
attributed to Pb-Br stretching and librational
motions of both inorganic and organic ions (25),
indicating that relaxation of the crystal lattice
occurs in the thin sheets. Meanwhile, the peak
at 122.2 cm
−1
(from -CH
3
group torsional mo-
tion and insensitive to lattice distortion) did
not change. The peaks for the 2D sheet became
narrower, suggesting better-defined vibrational
states in the thin sheet. Furthermore, our density
functional theory (DFT) calculation indicat es a
small lattice expansion of around 0.1 Å for the mono-
layer as compared to the bulk (C
4
H
9
NH
3
)
2
PbBr
4
crystal (see the supplementary material s for de-
tails about DFT simulation).
Strong crystal lattice distortions are common
for hybrid perovskites (5–10). Structural distortion–
induced optical and electronic changes have
been reported in bulk hybrid perovskites (26–29).
Single-crystal XRD data of the bulk crystal of
(C
4
H
9
NH
3
)
2
PbBr
4
revealed that the PbBr
4
2–
layer
in the bulk crystal is highly distorted, with a
SCIENCE sciencemag.org 25 SEPTEMBER 2015 • VOL 349 ISSUE 6255 1519
Fig. 1. Synthesis of atomically thin 2D (C
4
H
9
NH
3
)
2
PbBr
4
crystals. (A) Structural illustration of a
single layer (C
4
H
9
NH
3
)
2
PbBr
4
(blue balls, lead atoms; large orange balls, bromine atoms; red balls,
nitrogen atoms; small orange balls, carbon atoms; H atoms were removed for clarity). (B) Optical
image of the 2D square sheets. Scale bar, 10 mm. (C) AFM image and height profile of several single
layers. The thickness is around 1.6 nm (T0.2 nm). (D) AFM image and height profile of a double layer.
The thickness is around 3.4 nm (T0.2 nm).
RESEARCH | REPORTS
Pb-Br-Pb bond angle of 152.94° (see fig. S13 and
tables S3 and S4 for more details), which may
provide driving forces for lattice relaxation in
the 2D thin sheet. In other reported 2D mate-
rials, the in-plane crystal structure does not
change from the bulk crystal to isolated sheets;
and the optical and electronic properties change
because of electronic decoupling between adja-
cent layers .
We investigated the PL properties of indi-
vidual 2D crystals under 325-nm laser excitation.
Figure 3A shows the PL spectra of the bulk
(C
4
H
9
NH
3
)
2
PbBr
4
crystal and 2D sheets with
different thickness (22, 8, and 3 layers thick, see
fig. S14 for AFM images), and Fig. 3, B to E, shows
the corresponding PL image of each sheet. Both
the bulk materials and the sheets exhibit similar
strong purple-blue light emission. The bulk crystal
hasanemissionpeakat411nm(2.97eV),andthe
2D sheets have slightly blue-shifted peaks at
~406 nm (3.01 eV). The slightly increased optical
band gap for the ultrathin 2D sheets is probably
induced by the lattice expansion. Our DFT sim-
ulation also suggests a 20-meV blue shift in PL
for the single-layer 2D sheets, which is a trend
consistent with the experimental observation.
The 2D sheets with different thickness (from 22
to 3 layer s) have similar PL spectra, the peak po-
sition shift is within 1 nm, and any lattice con-
stant difference is within the experimental error
from SAED observed between these samples.
More discussion about the PL can be found in
the supplementary text (21).
Cathodoluminescence microscopy, a technique
that provides a map of the light emitted from a
sample when excited by a focused electron beam
with excellent lateral resolution, was used to de-
termine the spatial distribution of emissive sites
on the 2D sheets. As shown in Fi g. 3G, the cat h -
odoluminescence mapping from 395 to 435 nm
shows a square shape identical to the correspond-
ing scanning electron microscopy (SEM) images
as shown in Fig. 3F, indicating that the emission is
fr o m the who l e squ are. The PL internal quantum
efficiency (QE) of the 2D sheets was estimated by
comparing the integrated PL inten sity (from 390
to 450 nm) of the band edge emission at room
temperature (298 K) and helium cryogenic tem-
perature (6 K), a nd the results are shown in Fig.
3H (30). There is a small red shift of the main
peak from 406 to 412 nm as the temperature
decreases. The emission at 421 nm (2.91 eV) at
6 K is known from the G
1
–
state, which cannot be
distinguished from the G
2
–
state at room temper-
ature (26). The PL QE for the 2D sheet is calculated
to be ~2 6 %, which is much higher than the QE of
the bulk crystal (<1%), indicating the high quality
of the single-crystalline 2D sheets. The PL inten-
sity increased linearly as the excitation power in-
creased (fig. S15), suggesting that the PL QE was
constant within our measurement range. The PL
lifetime of the 2D sheets was measured by time-
resolved PL. As shown in Fig. 3I, the decay curve
showed a bi-exponential feature with lifetimes of
0.78 ns (67%) and 3.3 ns (33%), which are near
the reported data for the bulk crystals (15).
The chemistry of synthesizing these ultrathin
2D sheets was extended to other hybrid perovskites
(8). We prepared (C
4
H
9
NH
3
)
2
PbCl
4
,(C
4
H
9
NH
3
)
2
PbI
4
,
(C
4
H
9
NH
3
)
2
PbCl
2
Br
2
,(C
4
H
9
NH
3
)
2
PbBr
2
I
2
,and
(C
4
H
9
NH
3
)
2
(CH
3
NH
3
)Pb
2
Br
7
ultrathin 2D sheets
using similar methods, and their PL spectra and
optical images are shown in Fig. 4 (see fig. S16
forXRDandfig.S17forAFMimages).Forthe
(C
4
H
9
NH
3
)
2
PbCl
4
sheet (i), the band edge emis-
sion was in the ultraviolet at ~340 nm, which was
beyond our detection range for the single-sheet
measurement. Three states in the visible region
were observed at 486, 568, and 747 nm, which
made the sheets appear nearly white. This emis-
sion is attributed to the transient formation of
self-trapped excitons (31). For the (C
4
H
9
NH
3
)
2
PbI
4
sheet (iii), the band edge emission was at 514 nm,
which is blue-shifted by 9 nm as compared to
the bul k (5). This blue shift is consistent with the
bromidecase(ii)discussedabove.Forthechloride-
bromidealloycrystal,(C
4
H
9
NH
3
)
2
PbCl
2
Br
2
(iv),
thebandedgeemissionpeakwasat385nm,
and a broad self-trapped exciton emission appeared
at longer wavelength. However , the bromide-iodide
a lloy (v) showed only one peak at 505 nm. In
thecaseof(C
4
H
9
NH
3
)
2
(CH
3
NH
3
)Pb
2
Br
7
, no well-
defined squares were observed, and the thick-
ness of the plates was ~10 nm. Preliminary PL
study indicates a band edge emission at 453 nm,
which is red-shifted slightly as compared to
the bulk. These results indicate that the 2D hy-
br id per o v s kites have excellent composition and
color tunability.
The direct growth of atomically thin sheets
overcomes the limitations of the conventional
exfoliation and chemical vapor deposition meth-
ods, which normally produce relatively thick
perovskite plates (17–19, 32, 33). In contrast to
other 2D materials, the structural framework of
1520 25 SEPTEMBER 2015 • VOL 349 ISSUE 6255 sciencemag.org SC IENCE
Fig. 2. TEM, EDS, and GIWAXS
studies. (A) TEM image of a
thin (C
4
H
9
NH
3
)
2
PbBr
4
sheet.
Scale bar, 1 mm. (B) Electron
diffraction pattern of a thin
sheet of (C
4
H
9
NH
3
)
2
PbBr
4
.
Scale bar, 2 nm
–1
.(C to F)
EDS analysis showing the
elemental distribution of
lead, bromide, carbon, and
nitrogen, respectively. Scale
bars, 1 mm. (G) GIWAXS
image of the 2D thin sheets.
Q, wave vector in reciprocal
space. (H) Integrated
GIWAXS spectrum of the
2D thin sheets. a.u.,
arbitrary units.
RESEARCH | REPORTS
hybrid per ovskites is flexible and deformable;
and unlike the bulk crystal, the thin 2D sheets
exhibit new features such as structural relaxa-
tion and PL shift. This study opens up opportu-
nities for fundamental research on the synthesis
and characterization of atomically thin 2D hy-
brid perovskites and introduces a new family of
2D solution-processed semiconductors for nano-
scale optoelectronic devices.
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AC KN OW LE D GM E NT S
This work was supported by the U.S. Department of Energy under
contract no. DE-AC02-05CH11231 (PChem KC3103). TEM and CL
characterization were carried out at the National Center for Electron
Microscopy and Molecular Foundry, supported by the U.S. Department
of Energy. GIWAXS measurements were carried out at beamline 7.3.3
at the Advanced Light Sour ce, supported by the U.S. Department of
Energy. X-ray crystallography was supported by NIH Shared
Instrumentation Grant S10-RR027172; data collected and analyzed by
A. DiPasquale. Theoretical calculation was supported by the BES/SC,
U.S. Department of Energy, under contract no. DE-AC02-05CH11231
through the Material Theory program. CL characterization was
supported by a David and Lucile Packard Fellowship for Science and
Engineering to N.S.G. C.G.B. acknowledges an NSF Graduate Research
Fellowship (DGE 1106400), and N.S.G. acknowledges an Alfred P. Sloan
Research Fellowship. S.W.E. thanks the Camille and Henry Dreyfus
Foundation for funding, award no. EP-14-151. M. L. thanks the fellowship
support from Suzhou Industrial Park. We thank C. Zhu, C. Liu, and
D. Zhang for the help with GIWAXS, AFM, and XRD measurements and
H. Peng and F. Cui for fruitful discussions.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/349/6255/1518/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S17
Tables S1 to S4
References (34, 35)
10 June 2015; accepted 25 August 2015
10.1126/science.aac7660
SCIENCE sciencemag.org 25 SEPTEMBER 2015 • VOL 349 ISSUE 6255 1521
Fig. 3. PL properties of the 2D (C
4
H
9
NH
3
)
2
PbBr
4
sheets. (A) Steady-state PL spectrum of a piece of
bulk crystal and several 2D sheets. (B) The corr esponding optical image of the bulk crystal under
ex citation. Scale bar , 20 mm. (C to E) Optical images of the 2D sheets with 22 layers, 8 layers, and 3 layers.
Scale bars, 2 mm. (F) SEM image of a 2D sheet. Scale bar, 2 mm. (G) The corres ponding catho dolu-
minescence image showing the emission (with a 40-nm bandpass filter centered at 415 nm). (H)PLspectra
of a 2D sheet at 298 and 6 K. (I) Time-resolved PL measurements showing a bi-exponential decay .
Fig. 4. Photoluminescence of different 2D hybrid perovskites. (C
4
H
9
NH
3
)
2
PbCl
4
(i), (C
4
H
9
NH
3
)
2
PbBr
4
(ii),
(C
4
H
9
NH
3
)
2
PbI
4
(iii), (C
4
H
9
NH
3
)
2
PbCl
2
Br
2
(iv), (C
4
H
9
NH
3
)
2
PbBr
2
I
2
(v), and (C
4
H
9
NH
3
)
2
(CH
3
NH
3
)Pb
2
Br
7
(vi)
2D sheets demonstrate that the solution-phase direct growth method is generalizable. The correspond-
ing optical PL images are shown in the inset. Scale bars, 2 mmfor(i)to(v)and10mmfor(vi).
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