Controlled Synthesis of Organic/Inorganic van der Waals Solid for Tunable Light-Matter Interactions

Article abstract of DOI:10.1002/adma.201503367

High-quality organic and inorganic van der Waals (vdW) solids are realized using methylammonium lead halide (CH₃NH₃PbI₃) as the organic part (organic perovskite) and 2D inorganic monolayers as counterparts. By stacking on

Full text of DOI:10.1002/adma.201503367

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Controlled Synthesis of Organic/Inorganic van der Waals
Solid for Tunable Light–Matter Interactions
Lin Niu, Xinfeng Liu, Chunxiao Cong, Chunyang Wu, Di Wu, Tay Rong Chang,
Hong Wang, Qingsheng Zeng, Jiadong Zhou, Xingli Wang, Wei Fu, Peng Yu,
Qundong Fu, Sina Najmaei, Zhuhua Zhang, Boris I. Yakobson, Beng Kang Tay,
Wu Zhou, Horng Tay Jeng, Hsin Lin, Tze Chien Sum, Chuanhong Jin,
Haiyong He,* Ting Yu,* and Zheng Liu*
Van der Waals (vdW) solids are vertically stacked heterostruc-
tures that consist of different layered components. These
materials have opened a window into a new landscape of next
generation electronics and optoelectronics such as transis-
tors,[1–5] sensors,[6–8] photodetectors,[9–14] and spin-valleytronic
devices.[15–17] In light of the discoveries in inorganic 2D crystals,
such as graphene,[5,7,18] hexagonal boron nitride (h-BN),[5,19,20]
and transition metal dichalcogenides (TMDs),[21,22] one could
engineer composition, thickness, and stacking sequences of
layered components in vdW solids for design of new material
architectures and properties suited for specific applications.
Light–matter interaction at the interfaces of vdW solids is
the key to build high-performance optoelectronic devices. Such
interfaces have been realized in inorganic WS2/graphene vdW
solids.[10] In order to engineer an excellent inorganic/organic
interface for the study of light–matter interaction, a suitable
photoactive material needs to be identified. Recently, among
plenteous photoactive materials, inorganic–organic perov-
skites have attracted great attention due to their remarkable
performance and relatively low cost as good lighter absorbers
in solid-state photovoltaic devices.[23–26] Perovskites have shown
highly stable excitons,[27] strong exciton absorption, sharp
exciton emission even at room temperature,[28,29] high optical
absorption coefficient, optimal band gap, and long electron/
hole diffusion lengths,[23] which grant their high efficiencies
for energy conversion. The highest efficiency of organometal
perovskite is ≈20%.[30] It turns out that the performance of the
inorganic–organic perovskite devices highly relies on the mate-
rial quality and the surfaces. Various 2D layers could be excel-
lent substrates for such materials and one should examine their
interfacial impact, in greater details, on the optical properties
of perovskite.[26] These expected complementary properties of
such hybridizations suggest that marrying inorganic 2D mate-
rials with inorganic–organic perovskites may provide oppor-
tunities to understand the light–matter interactions at their
hetero-interfaces. Although this organic perovskite/2D hetero-
structure has shown promising light-based potentials in the vis-
ible regime, the difficulty of fabrication remains.
Dr. L. Niu, Dr. H. Wang, Dr. Q. Zeng, J. Zhou, Dr. W. Fu,
Dr. P. Yu, Q. Fu, Dr. H. He, Prof. Z. Liu
T. R. Chang, Prof. H. T. Jeng
Department of Physics
School of Materials Science and Engineering
Nanyang Technological University
National Tsing Hua University
Hsinchu 30013, Taiwan
Singapore, Singapore 639798, Singapore
E-mail: HYHe@ntu.edu.sg; Z.Liu@ntu.edu.sg
X. Wang, Prof. B. K. Tay, Prof. Z. Liu
NOVITAS
Dr. X. Liu, Dr. C. Cong, Prof. T. C. Sum, Prof. T. Yu
School of Physical and Mathematical Sciences
Nanyang Technological University
Singapore, Singapore 637371, Singapore
E-mail: YuTing@ntu.edu.sg
Nanoelectronics Centre of Excellence
School of Electrical and Electronic Engineering
Nanyang Technological University
Singapore 639798, Singapore
Dr. S. Najmaei
Dr. X. Liu
United States Army Research Laboratories
Sensors and Electron Devices Directorate
2800 Powder Mill Road, Adelphi
MD 20783, USA
National Center for Nanoscience and Technology
Beijing 100190, China
Dr. C. Wu, Prof. C. Jin
State Key Laboratory of Silicon Materials and
School of Materials Science and Engineering
Zhejiang University
Dr. Z. Zhang, Prof. B. I. Yakobson
Department of Materials Science and Nanoengineering
Rice University
Hangzhou 310027, P. R. China
Houston, TX 77005, USA
D. Wu, Prof. H. Lin
Dr. W. Zhou
Centre for Advanced 2D Materials and Department of Physics
National University of Singapore
Materials Science and Technology Division
Oak Ridge National Lab
Singapore, Singapore 117542, Singapore
Oak Ridge, TN 37831, USA
DOI: 10.1002/adma.201503367
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To date, most inorganic vdW solids such as graphene/h-
BN, graphene/MoS2, MoS2/WS2, graphene/h-BN superlattice,
and graphene/h-BN/MoS2 stacks were engineered via layer-by-
layer transfer of 2D films. By using the similar method, inor-
ganic–organic perovskite films will suffer from the organic
solutions used during the transfer process.[7,19,31–34] Moreover,
it generally requires a few cycles of dry transfer, alignment,
e-beam lithography, plasma etching, and metal deposition to
engineer vdW devices,[1] which may have detrimental effects
on perovskite quality. Recently, deposition technologies such
as chemical vapor deposition (CVD) have been applied to pre-
pare vdW solids such as BN/graphene and TMD vdW hetero-
structures.[2,35–39] In order to address these problems, we pro-
pose a three-step method for controlled synthesis of organic/
inorganic vdW solids. This protocol enables production of
large-size and highly crystalline inorganic/organic vdW solid
families. The inorganic monolayers could be substituted by
many available 2D materials. Surprisingly, in such organic/
inorganic interfaces, the subnanometer monolayer beneath the
organic component can dramatically tune the optical proper-
ties of the vdW solids, by coupling with the electron-hole gen-
erated in the organic perovskites. Further studies reveal that
h-BN monolayer is an excellent complement to organic per-
ovskites for preserving their original optical properties, as also
supported by theoretical calculation and the patterned organic
perovskite/h-BN vdW solids as red light emitters. This work
shines light on ways for designing vdW solids with great poten-
tial for a wide variety of applications in optoelectronics.
The feasibility of the strategy and method developed have
been demonstrated by using the most popular carbon-based
2D material, graphene, and noncarbon 2D materials, h-BN
and MoS2, as the substrates for growing the organic/inorganic
vdW solids. First, large-size 2D monolayers are prepared as
inorganic components (Figures S1–S4, Supporting Informa-
tion). Next, epitaxial growth of highly crystalline PbI2 nano-
platelets on the prefabricated 2D substrates is performed via
a physical vapor deposition (PVD) process. The PbI2 crystals
are then converted into perovskite by reacting with CH3NH3I
under vacuum, forming organic perovskite (methylammonium
lead halide)/2D vdW solids, denoted as perovskite/2D vdW
solids. The morphologies of the organic perovskite/2D vdW
solids were examined by optical microscopy, scanning electron
microcopy (SEM), and atomic force microscopy (AFM) (see
Figure 1 and Figures S5–S7 in the Supporting Information).
The CH3NH3PbI3 nanoplatelets display different colors in the
optical images, responding to different thickness. Their sur-
face roughness is measured to be ≈2.0 nm by AFM in Figures
S6–S7 (Supporting Information), indicating the high quality
of the perovskite/2D vdW solids. Organic perovskites sitting
on polycrystalline graphene, h-BN films and single crystal
MoS2 domains are illustrated in Figure 1g–i, respectively. The
yellow, gray, and blue balls correspond to organic molecule
Figure 1. Overall morphologies of the perovskite/2D vdW solids. a–f) False-colored SEM images of perovskite 2D vdW solids on a,d) graphene, b,e)
MoS2, and c,f) h-BN, respectively. Their corresponding structure models are illustrated in panels (g, h, and i), respectively. a,d) Graphene and c,f) h-BN
films cover the whole SiO2/Si substrates while b,e) triangular MoS2 single-crystal domains are directly grown on SiO2/Si substrates. a–f) Hexagonal
+
and triangular organic perovskites are all in light blue. In the structure models (g–i), the yellow, gray, and blue balls are organic molecule CH3NH3
ions, Pb atoms, and I atoms, respectively. Pb atoms locate at the center of the halide octahedrons.
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Figure 2. Electron microscopy characterization of the perovskite/2D vdW solids. a) Low-resolution STEM image of a perovskite platelet. b–f) Element
mapping obtained by energy-dispersive X-ray spectroscopy show the uniformity of the perovskite nanoplatelets. g) High resolution TEM (HRTEM)
image showing the structure of the perovskite nanoplatelets. Inset is the corresponding fast Fourier transform pattern from this HRTEM image along
the [−120] zone axis (ZA). h) Filtered HRTEM image of the area highlighted in (g).
+
CH3NH3 ions, Pb atoms, and I atoms, respectively. In the
octahedral structure of the PbI2, Pb atoms locate at the center
of the halide octahedrons. During the conversion, CH3NH3I
(Figure 2; Figures S11–S13, Supporting Information). HRTEM
imaging indicates a highly crystalline PbI2 with sixfold sym-
metric diffraction patterns from selected area electron diffrac-
tion (details of the TEM results of PbI2 nanoplatelets can be
found in Figures S11 and S12 in the Supporting Information).
Then, we investigate the crystal structure transformations with
a complete conversion process from PbI2 into CH3NH3PbI3.
Figure 2a shows a typical perovskite flake and the corre-
sponding elemental mapping of carbon (C), nitrogen (N), lead
(Pb), and iodine (I) are presented in Figure 2b–e, followed by an
overlapped image in Figure 2f. This mapping confirms the ele-
mental uniformity of the perovskite over the whole flake after
conversion. In addition to XRD spectra, the crystal structure of
the converted perovskite is evaluated by HRTEM imaging in
Figure 2g,h. The lattice of perovskite is quite clear in Figure 2g.
Inset is the corresponding fast Fourier transform (FFT) pat-
tern from this HRTEM image along the [−120] zone axis (ZA).
During the conversion process from PbI2 into perovskite, with
+
reacts with PbI2, and the CH3NH3 ions (orange balls in the
schematic) insert into the octahedral structure of PbI2 and stay
at the center of eight lead halide octahedrons. XRD patterns of
perovskite nanoplatelets are examined to evaluate the quality of
perovskite/2D vdW solids. For converted perovskite vdW solids,
peaks at 14.06°, 28.38°, 3l.74°, and 43.14° are located, assigned
to (110), (220), (310), and (330), respectively, for CH3NH3PbI3
perovskite with a tetragonal crystal structure (Figures S8–S10,
Supporting Information).[40] It is worth noting that all the 2D
monolayers we used in our experiments remain intact during
the conversion.
The quality of as-grown organic perovskites has been fur-
ther examined by selected area electron diffraction (SAED),
high-resolution transmission electron microscopy (HRTEM)
imaging, and energy dispersive X-ray spectroscopy (EDX)
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Figure 3. Raman characterization of the perovskite/2D vdW solids. a,d,g) Optical images of perovskite grown on graphene, MoS2 and h-BN, respec-
tively. Scale bars are 2, 5, and 2 µm, respectively. b,e,h) Corresponding Raman intensity mapping of phonon modes of A11 and A12 of the perovskite/2D
vdW solids. c,f,i ) Corresponding Raman spectra of the perovskite/2D vdW solids. The presence of graphene, MoS2 and h-BN layers underneath the
perovskite can be confirmed by the peaks at 1590 cm−1 (G mode of graphene), 382 and 408 cm−1 (E2g and A1g modes of MoS2), and 1369 cm−1(E2g mode
of h-BN), respectively. The excitation wavelength used for Raman measurements is 532 nm.
the chemical reaction of CH3NH3I and PbI2, the space group
is transferred from P3m1 for PbI2 into I4 cm for perovskite.
Figure 2h shows a close-up of the area from the yellow dashed
rectangle in Figure 2g. The interplanar distances of ≈0.38 and
≈0.32 nm can be determined, which are attributed to the (21−1)
planes and (004) planes, respectively, with the tetragonal lattice
of perovskite. The corresponding structural schematic of the
perovskite are be found in Figure S13c (Supporting Informa-
tion). Apparently, along the [001] direction, there are alternative
lines, similar to the HRTEM results in Figure 2g. One con-
sists of I and Pb atoms, while another consists of I atoms and
CH3NH3+ ions.
Confocal micro-Raman mapping and spectroscopy are com-
monly used powerful tools to characterize materials thanks to
their unique advantages of relatively high spatial resolution
(a few hundreds of nanometer), no requirement for special
sample preparation, high efficiency, and non-destructiveness.
Figure 3 shows the micro-Raman mapping and spectroscopy
of the perovskite/2D vdW solids. Optical images of hexagonal
perovskite nanoplatelets sitting on CVD graphene, MoS2, and
h-BN are shown in Figure 3a,d,g. The thickness of the per-
ovskite nanoplatelets on CVD graphene, MoS2, and h-BN are
around 300, 290, and 270 nm, respectively (see Figure S9 in
the Supporting Information). Figure 3b,c, e,f, and h,i are the
corresponding Raman mapping and spectra of the perovskite/
graphene, perovskite/MoS2, and perovskite/h-BN, respectively.
It can be seen from the Raman images of the sum intensity of
A11
and A12 modes that the perovskite nanoplatelets grown on
CVD graphene, MoS2, and h-BN are homogenous. In the meas-
ured spectroscopy range of 10–160 cm−1, the Raman spectra
reveal four distinct bands at ≈14, 70, 94, and 110 cm−1 as shown
in Figure 3c,f,i, which indicate that the perovskite nanoplate-
lets maintain the 4H polytype of PbI2 (see Figure S14 in the
Supporting Information).[41] The phonon vibration located at
≈14 cm−1 was assigned to E23, the shear-motion rigid-layer mode
of PbI2 with 4H polytype,[41] while the Raman peaks at ≈70, 94,
and 110 cm−1 were assigned to E1 , A1 , and A2 , respectively.[42]
2
1
1
The presence of graphene, MoS2, and h-BN layers underneath
the perovskite are confirmed by the Raman peaks at 1590 cm−1
(G mode of graphene), 382 and 408 cm−1 (E2g and A1g modes
of MoS2), and 1369 cm−1(E2g mode of h-BN) as shown in
Figure 3c,f,i, respectively.
We also explore the UV–vis absorption and PL excitation
spectra of the perovskite/2D vdW solids on quartz. The absorp-
tion peak (760 nm) of perovskite/2D vdW solids is attributed to
the direct gap transition from the valence band maximum to
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Figure 4. Photoluminescence of the perovskite/2D vdW solids. a–c) PL intensity mapping of perovskite on graphene, MoS2 and h-BN, respectively.
The mapping is collected at 760 nm under the same conditions. Considerable difference in the PL intensity of the perovskites is found on graphene
(purple in (a), ≈104), MoS2 (blue in (b), ≈106), and h-BN (light blue in (c), ≈107), respectively. The profile of the MoS2 layer is also found in dark
blue in (b). d) Comparison of the PL spectra extracted from the PL mapping. e) Time-resolved PL decay transients measured at 760 10 nm for
perovskites on h-BN (light blue, lifetime of 5.8 0.5 ns), MoS2 (blue, lifetime of 1.3 0.25 ns), and graphene (purple, lifetime of 0.42 0.01 ns).
The excitation wavelength for PL measurements is 532 nm. The solid lines in (e) are the single-exponential fits of the PL decay transients. f–h) Sche-
matic band alignment and illustrative transfer of photo-excited carriers at the f) perovskite/h-BN, g) perovskite/MoS2, and h) perovskite/graphene
hetero-interfaces.
the conduction band minimum (Figure S15, Supporting Infor-
mation). It is found that all the absorption peaks are similar to
those previously reported, where PbI2 and perovskites are pre-
pared by spinning coating and vapor deposition. These results
indicate that the 2D monolayers do not change the excitation
absorption of organic perovskites.[23,25] In order to examine
the importance of the hetero-interface in perovskite/2D vdW
solids, we characterize the samples using photoluminescence
(PL) spectroscopic techniques, including PL spectrum, PL
mapping, and time-resolved PL measurement, as presented
in Figure 4. Figure 4a–c shows the PL mapping of perovskite
(mapping wavelength at ≈760 nm) on graphene, MoS2, and
h-BN, respectively. All the maps are collected under the same
conditions. Their optical images are shown in Figure 3a,d,g,
respectively. For comparison, the three PL mapping images
are displayed using the same scale bar with the PL intensity
ranging from 1 to 107. It turns out that the PL emission in per-
ovskite is significantly modulated by the inorganic 2D compo-
nents. For perovskite/graphene vdW solid, little contrast can be
found. The PL emission is completely quenched (only a little
intensity difference between perovskite and graphene). How-
ever, for perovskite/MoS2, a sharp contrast can be observed
from SiO2 substrate (purple), monolayer MoS2 (dark blue), and
perovskite/MoS2 (blue). In perovskite/h-BN, a much higher
PL intensity from perovskite is recorded. The intensity is ≈103
stronger than that on perovskite/graphene vdW solids. Their
representative PL spectra are shown in Figure 4d. In addition,
the PL intensity mapping in Figure 4c also shows distinctly
different from the edge to the core region, as indicated by
the white arrow, and the shape of outer layer has a consistent
morphology layout as that observed in the optical microscopy
image. It is ascribed to light scattering effect at edges that
caused the difference of emission intensity at side and core
regions. In order to understand the carrier dynamics at these
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distinct hetero-interfaces, time-resolved PL measurements are
conducted as shown in Figure 4e. From the single decay fitting,
the lifetimes for perovskite/h-BN (green), perovskite/MoS2
(pink), and perovskite/graphene (navy) vdW solids are 5.8, 1.3,
and 0.42 ns, respectively. The lifetime of the perovskite/h-BN
is comparable to previous reported value of ≈6.8 ns for per-
ovskite on mica, suggesting almost no charge transfer at the
interface,[43] and thus the combining rate of the electron-hole
pairs remains intact on h-BN due to the very large optical band
gap of h-BN (≈6.0 eV). It is a fascinating venue to preserve the
intrinsic optical properties of perovskite by using this uniform,
smooth, and nonconductive h-BN monolayer (see Figure 4f).
In contrast, the exciton lifetime in the perovskite/MoS2 inter-
face drops remarkably, indicating that considerable charge
transfer occurs at the interface. Our calculations estimate that
such interface has a type II band alignment with band offset
of ≈0.3 eV (Figure 4g), which provides a strong driving force to
separate the electron-hole pairs excited by photons. Moreover,
as the electron-hole pairs in the perovskite have a small binding
energy,[44] they are particularly easy to split under a built-in
electric field across the interface. Such effect dominates the car-
rier dynamics at perovskite/graphene interface, as evidenced
by the entirely quenching behavior. In this situation, graphene
has linear band dispersion with a zero band gap, which ren-
ders graphene as a collector for both electrons[45] and holes[46]
transferred from the perovskite (Figure 4h). As such, carriers
can hardly survive in the perovskites and thereby no PL peak
can be measured. This efficient process for charge transfer has
been demonstrated and served as electron collection layer and
high performance hybrid photodetectors.[45] Therefore, tailoring
the interfaces provides us an efficient way to tune and design
the light–matter interactions in 2D material based heterostruc-
tures, which will benefit the fabrication of layered material
based optoelectronic devices such as photodetectors, photovol-
taics, light emission diodes, and on-chip lasers.
Understanding the growing mechanism is the key to
improving the quality of perovskite/2D vdW solids and
engineering the organic/inorganic interfaces. Ex situ time
dependent experiments are carried out to clarify this. The sam-
ples with different reaction time (tR) are synthesized and char-
acterized by optical microscopy to monitor the evolution of PbI2
nanoplatelets. As shown in Figure S21 (Supporting Informa-
tion), only small red dots can be observed at the early stage of
the reaction (tR = 1 min). Our density functional theory (DFT)-
based calculations estimate that the diffusion of a PbI2 nano-
flake (2 nm in size) on the h-BN substrates is around 50 meV,
thus being highly mobile (Figure S22a, Supporting Informa-
tion). When the reaction time is increased to 5 min, plenty of
PbI2 nanoplatelets with irregular shape were formed and some
of these PbI2 nanoplatelets tend to coalesce with each other.
The coalescence process is energetically favorable because it
reduces the edge energy of PbI2 nanoplatelets by decreasing
their total edge length. Moreover, the rotation barrier of the
nanoplatelets is also rather small, being only 45 meV for a 2 nm
diameter nanoflake on a monolayer h-BN sheet (Figure S22b,
Supporting Information). Thus, the nanoplatelets can readily
adjust their crystal orientations during the coalescence process
so that the misoriented interfaces (or grain boundaries) can be
largely avoided. This fact agrees with our TEM characterization
that the PbI2 nanoplatelets can be in a single crystal over tens
of micrometer size. Finally, by extending the reaction time to
20 min, the density of nuclei is decreased dramatically and hex-
agonal PbI2 appears. On the basis of gradual morphology evolu-
tion, we propose that PbI2 is formed through a van der Waals
epitaxy mechanism. According to the time dependent experi-
ments, there are two key points in the growth of perovskite/2D
vdW solids: nucleation site and van der Waals epitaxy. This
assumption is further supported by our perovskite/2D vdW
patterns (Figure 5), where the 2D materials are patterned into
arrays to separate nucleation sites (the dangling bonds within
the 2D materials) from a defect-free surface. In a brief sum-
mary, initially, PbI2 nanoparticles are nucleated around the
edges and defects of 2D materials. These nanoparticles then
grew along surface of 2D materials and began to merge to form
highly crystalline nanoplatelets. Next, the PbI2 nanoplatelets
extended on 2D material surface and covered all the area shown
in Figure S23a,d,g (Supporting Information). Finally PbI2 will
be converted into organic perovskite.
Inspired by the discussion above, based on our expertise on
the controlled growth of perovskite/2D vdW and understanding
of the charge transfer dynamic at the interfaces, patterned
perovskite/h-BN vdW solids have been demonstrated and show
excellent light emission performance as shown in Figure 5.
Considering that it is energetically favorable for the PbI2 to
nucleate at the defects, one could control the growth of PbI2 and
perovskite by predesigning the 2D films. First, periodic h-BN
patterns are patterned by lithography (Figure S24, Supporting
Information). Then PbI2 is deposited on h-BN arrays via PVD
methods. Finally, patterned perovskite/h-BN vdW solids are
obtained after conversion. It is worth noting that the patterning
of 2D films is compatible with traditional lithography pro-
cessing. Perovskite/2D vdW solids with arbitrary geometries
could be fabricated by this approach. Figure 5a presents the SEM
image of perovskite/h-BN disk array in a hexagonal densely
packed arrangement with patch diameter of d = 20 µm and
space distance of D = 50 µm. All the h-BN patches are covered
by perovskites without missing pixel, and the space between
patches is very clean, indicating that our method can effectively
control the growth of perovskite/2D vdW solid on targeted area.
Figure 5c shows the “NTU” array derived from Figure S18b
(Supporting Information), where one can see clearly how the
“NTU” array of perovskite/2D vdW solid formed. The diam-
eter of the perovskite/h-BN hexagonal patches in the “NTU”
array is around 10 µm and the gap distance between all adja-
cent patches is 10 µm. The gap distance between all adjacent
“NTU” units is around 30 µm. Fluorescent microscopy images
were taken under the excitation of irradiation (365 nm) from a
high pressure mercury lamp (Figure 5b,d). Bright red lumines-
cent patterns can be seen unambiguously without defects. The
dark regions in Figure 5b,c correspond to the spaces without
perovskite.
In summary, organic perovskite/2D vdW solids are pre-
pared via a three-step process. Such vdW solids can be grown
in a scalable fashion and designed into arbitrary shapes by
prepatterning the inorganic 2D monolayer using lithography.
Highly crystalline organic perovskite crystals are confirmed
by geometric, spectroscopic, and chemical analysis. Distinct
organic/inorganic interfaces are formed by using various 2D
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Figure 5. Patterned perovskite/h-BN vdW solids for red light emitting array. a) SEM image of a perovskite/h-BN dotted array. The diameter of the dots
is ≈20 µm and the spacing is ≈50 µm. b) Corresponding fluorescent image showing a high intensity emission of red light. c) SEM and d) fluorescent
image of perovskite/h-BN “NTU” arrays. The diameter of perovskite/h-BN hexagonal patches in the NTU array is ≈10 µm and the distance between
adjacent patches is ≈10 µm. The gap distance between adjacent NTU units is around 30 µm. The center panel is a schematic of a perovskite/h-BN film.
used as a source and placed in the center of the quartz tube while CVD
graphene or MoS2 or BN substrate having as-grown lead halide platelets
were placed around 5–6 cm away from the center in the downstream
region. The quartz tube was first evacuated to a base pressure of 3 mTorr,
layers as inorganic components, providing an excellent scaffold
for exploring and tuning the carrier dynamic at the interfaces,
which is supported by our time-resolved PL experiments and
DFT calculations. Based on the novel optical behaviors, com-
followed by a 50 sccm flow of high-purity Ar gas. The pressure was then
plex pattern-designed perovskite/h-BN vdW solids based light
emission arrays are further fabricated. This work will shed light
on organic perovskite based optoelectronic devices.
stabilized to 30 Torr and the temperature was elevated to 120 °C and
kept there for 1 h after which the furnace was allowed to cool down
naturally to ambient temperature.
Measurement: X-ray diffraction pattern (2θ scans) were obtained
from perovskite/2D vdW solids supported on the SiO2/Si substrates
using an X-ray diffractometer (XRD Shimadzu Thin Film), using Cu
Kα radiation (λ = 1.54050 Å) within a diffraction angle (2θ) from 5° to
60°. The SEM images of perovskite flakes were obtained using JEOL
JEM7600F operated at an accelerating voltage of 10 kV. AFM experiments
were performed in tapping mode under ambient conditions (Dimension
ICON SPM system, Bruker, USA). Commercial silicon tips with a
nominal spring constant of 40 N m−1 and resonant frequency of 300 kHz
were used in all the experiments. The PbI2 samples for transmission
electron microscope (TEM) were flaked off from the 2D substrates by
using toluene (99.85%, Acros Organics) and then transferred onto the
TEM grids (Quantifoil Mo grids). The CH3NH3PbI3 perovskite samples
for TEM measurement were converted from the PbI2 onto the TEM grids,
with the similar method we introduced in the materials synthesis part
above. The high resolution transmission electron microscopy (HRTEM)
and the selected area electron diffraction (SAED) pattern were done
with a FEI Tecnai F20 operated with an acceleration voltage of 200 kV.
The chemical composition of lead iodide and CH3NH3PbI3 perovskite
was determined by means of energy dispersive X-ray spectroscopy
(EDX attached to FEI TecnaiF20). A WITec alpha300 RAS Raman system
with a piezocrystal controlled scanning stage, an objective lens of
100× magnification (numerical aperture, NA = 0.95), and an Electron
Multiplying CCD was used for recording photoluminescence (PL) and
Raman spectra/images. For PL spectra/images, a 600 lines mm−1 grating
was used. For Raman spectra/images, a low-wavenumber coupler
and a 2400 lines mm−1 grating were used for observing low-frequency
Raman modes and achieving a good spectral resolution. All the PL
Experimental Section
Synthesis of Lead Halide Platelets: PbI2 powder (99.999%, Aldrich) was
used as a single source and put into a quartz tube mounted on a single-
zone furnace (Lindberg/Blue MTF55035C-1). The freshly CVD graphene
or MoS2 or BN substrate (1 cm × 3 cm) was placed in the downstream
region inside the quartz tube. The quartz tube was first evacuated to a
base pressure of 3 mTorr, followed by a 50 sccm flow of high-purity Ar
gas. The temperature and pressure inside the quartz tube were set and
stabilized to desired values for each halide (390 °C and 30 Torr). In all
cases, the synthesis was carried out within 30 min and the furnace was
allowed to cool down naturally to ambient temperature.
Synthesis of CH3NH3I : The synthesis of CH3NH3I: 18.0 mL of
methylamine (40 wt% in water, Sigma) was dissolved in 100 mL of
absolute ethanol (absolute for analysis, Merck Millipore) in a 250 mL
round bottom flask. 20.0 mL of hydroiodic acid (57 wt% in water, Alfa
Aesar) was added into the solution dropwise. After that, the solution was
stirred at 0 °C for 2 h. The raw product was obtained by removing the
solvents using a rotary evaporator. A recrystallization process of the raw
product, including the redissolution in 80 mL absolute ethanol and the
precipitation after the addition of 300 mL diethyl ether, was carried out
twice to get a much purer product. Finally, the white colored powders
were collected and dried at 60 °C for 24 h in a vacuum oven.
Synthesis of Perovskites via the Reaction with CH3NH3I and PbI2: The
conversions were done using a similar CVD system. CH3NH3I were
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2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, 27, 7800–7808

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and Raman spectra/images were recorded under an excitation laser of
532 nm (Elaser = 2.33 eV). To avoid the laser-induced heating, laser power
was kept below 0.1 mW. The laser spot was of ≈500 nm in diameter.
UV–vis absorption spectra of perovskite/2D vdW solids prepared on
quartz were recorded on SHIMADZU UV-3101PC UV–vis–NIR scanning
spectrophotometer. The excitation pulse (400 nm) was generated by
frequency doubling the 800 nm output (with a BBO crystal) from the
Coherent Oscillator Mira 900F (120 fs, 76 MHz, 800 nm). The pump laser
source was introduced into a microscope (Nikon LV100) and focused
onto samples via a 20× objective (Nikon, numerical aperture: 0.4). The
PL emission signal was collected in a standard backscattering geometry
and dispersed by a 0.25 m DK240 spectrometer with 150 g mm−1 grating.
The emission signal was time-resolved using an Optoscope Streak
Camera system that has an ultimate temporal resolution of ≈10 ps.
The fluorescence images were obtained by an Olympus fluorescence
microscope. A mercury lamp was used as the excitation light source. All
FL images are as taken without any artificial image processing.
h-BN Patterns Fabrication: As-transferred h-BN film could be used to
prepare various patterns for inducing the growth of PdI2 patterns. The
fabrication process consisted of the following steps: (1) The h-BN film
was covered by a photoresist layer spin-coated (AZ5214, 4000 rpm) on
top of the h-BN surface; (2) Standard photolithography was performed to
pattern the photoresist layer as a mask; (3) Argon-based plasma etching
(power is 50 W, pressure is 200 mTorr and time is 30 s) was performed
to transfer the photoresist mask pattern onto underlying h-BN; and
(4) Photoresist mask was completely removed in acetone, and a h-BN
pattern was created, such as the hexagon and “NTU” patterns shown in
Figure S21 in the Supporting Information.
DFT Calculations: The results are based on first principle calculations
within density functional theory (DFT) as implemented in the VASP[47]
(Vienna ab initio simulation package) code. The ion–electron interaction
was modeled by projector augmented plane wave (PAW)[48] and the
electron exchange correlation was treated by Perdew–Burke–Ernzerhof
(PBE) parameterization of the generalized gradient approximation
(GGA).[49] The plane wave basis set with kinetic energy cutoff of 500 eV
was used. The Brillouin zone was sampled by an 8 × 8 × 1 Γ-centered
k-point mesh. A vacuum layer of 15 Å was adopted to avoid interactions
between the neighbor surfaces. All structures were fully relaxed until the
Hellmann–Feynman force on each atom was smaller than 0.01 eV Å−1.
Potential Energy Calculations: The energies were calculated by
first-principles calculations as implemented in VASP code as well.
Ultrasoft pseudopotentials were employed for the core region and
spin-unpolarized density functional theory based on local density
approximation, which could give a reasonable interlayer distance
between the PbI2 fl ake and h-BN sheet. A kinetic energy cutoff of 400 eV
was chosen for the plane-wave expansion.
the financial support from the National Science Foundation of China
(Grant Nos. 51222202 and 51472215), the National Basic Research
Program of China (Grant Nos. 2014CB932500 and 2015CB921000), and
the Fundamental Research Funds for the Central Universities (Grant
No. 2014XZZX003-07).
Received: July 13, 2015
Revised: August 29, 2015
Published online: October 27, 2015
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Acknowledgements
L.N., X.L., C.C., and C.W. contributed equally to this work. This work
was supported by the Singapore National Research Foundation (NRF)
under RF Award No. NRF-RF2013-08, the start-up funding from
Nanyang Technological University (M4081137.070 and M4080514),
and the Ministry of Education AcRF Tier 2 Grants MOE2013-T2-1-081
and MOE2014-T2-1-044. X.L and T.C.S. also acknowledge the financial
support by the Singapore NRF through the Singapore-Berkeley Research
Initiative for Sustainable Energy (SinBerRISE) CREATE Programme.
C.C and T.Y thank the support of Ministry of Education, Singapore
(MOE2012-T2-2-049). C.W. and C.J. thank the Center for Electron
Microscopy of Zhejiang University for the access to TEM facilities, and
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wileyonlinelibrary.com
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2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Adv. Mater. 2015, 27, 7800–7808