Controlled synthesis of brightly fluorescent CH3NH3PbBr3 perovskite nanocrystals employing Pb(C17H33COO)2 as the sole lead source

Article abstract of DOI:10.1039/c7ra11832e

Organometal halide perovskite nanocrystals hold vast potential for application in photovoltaics, light emitting diodes, low-threshold lasers, and photodetectors due to their size-tunable bandgap energies and photoluminescence as well as excellent electron and hole mobilities. However, the synthesis of such nanocrystals typically suffers from poor structural stability in solution and the coexistence of lamellate nanocrystals (nanoplatelets) and spherical nanocrystals (nanoparticles). Here we show that the pure nanoparticle morphology of CH₃NH₃PbBr₃ nanocrystals can be realized by employing lead oleate (Pb(C₁₇H₃₃COO)₂) as the sole lead source and controlled using short- and long-chain mixed alkyl ammonium. These nanocrystals are monodispersed (2.2 ± 0.4 nm in diameter), highly fluorescent (with a quantum yield approaching 85%), and highly stable in the solution (for more than 30 days). Comparative studies reveal that the shape of CH₃NH₃PbBr₃ nanocrystals is strongly dependent on the lead source, PbBr₂ and Pb(C₁₇H₃₃COO)₂, and evolves as a function of the ratio of short- and long-chain alkyl ammoniums in the precursors. At an optimal short to long-chain alkyl ammonium ratio of 4:6, the growth of CH₃NH₃PbBr₃ nanoplatelets can be selectively suppressed with Pb(C₁₇H₃₃COO)₂ as the sole lead source, enhancing the overall photoluminescence quantum yield of the produced CH₃NH₃PbBr₃ nanocrystals. This work reveals important new insights for controlled synthesis of perovskite nanocrystals with pure crystal shape and significantly improved photoluminescence properties and stability.

Full text of DOI:10.1039/c7ra11832e

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PAPER
Controlled synthesis of brightly fluorescent
CH NH PbBr perovskite nanocrystals employing
3
3
3
Cite this: RSC Adv., 2018, 8, 1132
Pb(C H COO) as the sole lead source†
1
7 33
2
ac
b
a
a
a
Xiaoming Fu,
‡
Zhiwei Peng, ‡ Chi Zhang,
Yong Xia, Jianbing Zhang,
ae
e
d
b
ab
Wei Luo, L. Jay Guo, Honglang Li,* YuHuang Wang* and Daoli Zhang
*
Organometal halide perovskite nanocrystals hold vast potential for application in photovoltaics, light
emitting diodes, low-threshold lasers, and photodetectors due to their size-tunable bandgap energies
and photoluminescence as well as excellent electron and hole mobilities. However, the synthesis of such
nanocrystals typically suffers from poor structural stability in solution and the coexistence of lamellate
nanocrystals (nanoplatelets) and spherical nanocrystals (nanoparticles). Here we show that the pure
3 3 3
nanoparticle morphology of CH NH PbBr nanocrystals can be realized by employing lead oleate
(
Pb(C17 33COO) ) as the sole lead source and controlled using short- and long-chain mixed alkyl
H
2
ammonium. These nanocrystals are monodispersed (2.2 ꢀ 0.4 nm in diameter), highly fluorescent (with
a quantum yield approaching 85%), and highly stable in the solution (for more than 30 days).
Comparative studies reveal that the shape of CH
lead source, PbBr and Pb(C17 33COO) , and evolves as a function of the ratio of short- and long-chain
alkyl ammoniums in the precursors. At an optimal short to long-chain alkyl ammonium ratio of 4 : 6, the
growth of CH NH PbBr nanoplatelets can be selectively suppressed with Pb(C17 33COO) as the sole
lead source, enhancing the overall photoluminescence quantum yield of the produced CH NH PbBr
3 3 3
NH PbBr nanocrystals is strongly dependent on the
2
H
2
Received 27th October 2017
Accepted 19th December 2017
3
3
3
H
2
3
3
3
DOI: 10.1039/c7ra11832e
nanocrystals. This work reveals important new insights for controlled synthesis of perovskite nanocrystals
rsc.li/rsc-advances
with pure crystal shape and significantly improved photoluminescence properties and stability.
9
2
2.7% now. In addition to solar energy harvesting, the size-
Introduction
tunable bandgap energies and photoluminescence (PL) from
Organolead halide perovskites (OHP) are intensively researched perovskite materials also evoke the exploration of their poten-
for photovoltaics because of their high light conversion effi- tial uses as emission materials by decreasing the particle size to
1
,2
3,4
ciency, optimal bandgap energy and excellent electron and nanocrystals (NCs). With nanoscaled size, OHP NCs have sharp
5,6
hole mobilities. These compounds, possessing a general excitonic features, which provide a synthetically tunable system
formula of APbX
3
crystal structure (A ¼ organic ammonium for systematic investigations of the nature and dynamics of the
10
cation and X ¼ halide anion), have been applied to solar cells excited states of OHP. In the 1990s, Papavassiliou et al.
and led to a breakthrough performance in photovoltaic power systematically investigated the structure, optical and other
11,12
conversion efficiencies from 3.1% (ref. 8) in 2009 to a certied related properties of OHP NCs,
in which the high photo-
luminescence quantum yield (PLQY) and large exciton binding
energy offered interesting opportunities for application in eld-
effect transistors (FET) and light emitting diodes (LEDs).
a
13,14
School of Optical and Electronic Information, Huazhong University of Science and
Technology, 1037 Luoyu Road, Hongshan District, Wuhan City, Hubei Province,
In 2014, OHP NCs captured the attention of researchers
4
30074, P. R. China. E-mail: zhang_daoli@hust.edu.cn
15
again when Schmidt et al. synthesized 6 nm-sized CH NH -
b
3
3
Department of Chemistry and Biochemistry, University of Maryland, 8051 Regent
Drive, College Park, MD 20742, USA. E-mail: yhw@umd.edu
PbBr
experiments, lead bromide (PbBr
(MABr) and octylammonium bromide (OABr) were added to
3
perovskite NCs via a simple solution process. In their
c
), methylammonium bromide
School of Physics, Communication and Electronics, Jiangxi Normal University, 99
2
Ziyang Avenues, Nanchang City, Jiangxi Province, 330022, P. R. China
d
Institute of Acoustics, Chinese Academy of Sciences, 21 North 4th Ring Road, Haidian a stirring solution of oleic acid (OLA) and 1-octadecene (ODE) at
District, Beijing 100190, P. R. China. E-mail: lhl@mail.ioa.ac.cn
ꢁ
80
C and the reaction was quenched by injecting acetone
e
Department of Electrical Engineering and Computer Science, University of Michigan,
quickly. It is believed that the long alkyl chain cations acted as
the capping ligands of the NCs to restrict the growth of the
Ann Arbor, MI 48109, USA
†
1
‡
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra11832e
CH NH PbBr array extending in three dimensions. Subsequent
3
3
3
research found that two-dimensional nanoplatelets of
These authors contribute equally to this work.
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organolead bromide perovskites coexisted with nanoparticles in hydrobromic acid (5 mL, 44 mmol) was added to a solution of
16
the synthesis. Later on, a large number of experiments based excess methylamine (12 mL, 96 mmol) in ethanol (50 mL) or
ꢁ
on the solution process were carried out and these OHP NCs slight less n-octylamine (7 mL, 42 mmol) at 0 C. The mixture
1
7–20
ꢁ
with higher PLQY were applied to novel optical devices.
However, these OHP NCs are unstable in solution and the alkylammonium bromide (MABr or OABr) was achieved using
was continuously stirred for 4 h at 0 C. Crystallization of
21
structures are heterogeneous, with different crystal shapes a rotary evaporator and washed three times with diethyl ether.
including nanoparticles and nanoplatelets coexisting as The resulting powder was dried under vacuum (60 C, 12 h) for
ꢁ
mixtures. Light emitting devices require phosphors with high future use.
22
PLQY and narrow-band emissions. The coexistence of CH
3
-
NH PbBr nanoplatelets and nanoparticles hampers applica-
tions in practical optical devices because they behave different
3
3
Synthesis of Pb-oleate
The reaction was performed on a Schlenk line using standard
air-free techniques. In a typical synthesis, 3 mmol PbO,
PL emission peaks and PLQY. From a synthetic point of view,
when using inorganic PbBr
PbBr
moieties between the Pb–Br–Pb layers of the crystalline PbBr
2
as the lead precursor, CH
nanoplatelets will form by intercalation of guest organic
3
NH
3
-
7
.5 mmol OLA and 30 mL ODE were mixed in a 150 mL ask,
3
ꢁ
and then heated to 120 C for 1 h under vacuum to obtain a clear
solution, followed by natural cooling to the ambient tempera-
2
23
host, implying that the formation of CH
3
NH
3
PbBr
3
nano-
2
ture under N blow protection. The solution was then quenched
platelets would be suppressed if PbBr
precursors as lead sources.
2
is replaced by other
using acetone and the precipitates were redispersed in toluene
ꢂ1
to a concentration of 0.4 mol L
.
In the present work, we developed a facile method to synthesize
monodispersive and stable CH NH PbBr perovskite NCs using
3
3
3
lead oleate (Pb(C17
and ligand to replace PbBr
resulting CH NH PbBr NCs showed pure and monodispersive
nanoparticle structure with an average size of 2.2 nm, improved
PLQY of 85%, and higher stability than those employing PbBr as
H
33COO)
2
, Pb-oleate) as the sole lead source
Synthesis of hybrid CH NH PbBr NCs using PbBr and Pb-
oleate as lead sources
3
3
3
2
2
and OLA in the precursors. The
3
3
3
Stock solutions of PbBr and MABr, with the same concentra-
2
ꢂ1
tion 0.5 mol L , were prepared in DMF. The solution of lead
2
source was prepared by mixing the same mole of PbBr –DMF
2
lead source and OLA or Pb-oleate as ligand. Our investigation
further revealed that the mole ratios of long- and short-chain alkyl
and Pb-oleate–toluene solutions. The alkyl ammonium halides
were prepared in DMF, and the molar ratios of OABr and MABr
were tuned as 9 : 1, 7 : 3, 6 : 4, 5 : 5, 4 : 6, 3 : 7, and 1 : 9. Then
the reagents were mixed to obtain a series of precursor solutions
with xed total mole mass of PbBr (4 mmol), alkyl ammonium
2
bromides (4 mmol) and Pb-oleate (4 mmol). To obtain the
desired perovskite crystallization, the precursor solution was
+
+
ammonium halides (OA : MA ) in the precursors can signicantly
affect the crystalline structures of CH NH PbBr NCs toward
nanoparticles and nanoplatelets with different layers via the
3
3
3
+
+
quantum connement effect. At an optimal OA : MA ratio of
: 6 with Pb-oleate as the sole lead source, the growth of nano-
3
platelets was essentially suppressed, generating pure CH NH -
4
3
added dropwise into 10 mL toluene under vigorous stirring at
PbBr3 nanoparticles with enhanced optical properties.
Comparative studies also veried that the nanoplatelet structure
lowered the overall quantum yields of CH NH PbBr NCs. To the
ꢁ
8
0 C. Aer centrifugation at 7000 rpm for 10 min to discard the
precipitates, a bright yellow-green suspension or blue suspen-
sion was obtained because of different proportions of nano-
platelets and nanoparticles in solution. The solution of hybrid
perovskite NCs was ltered using a polytetrauoroethylene
3
3
3
best of our knowledge, this is the rst report obtaining pure
monodispersive CH NH PbBr nanoparticles with favourable
3
3
3
stability and good dispensability using Pb-oleate as the sole lead
source in the reactants, and the results shown here may pave the
way for further application of OHP NCs in optoelectronic devices.
(PTFF) syringe lter (pore size 0.45 mm).
Synthesis of hybrid CH NH PbBr NCs using PbBr and OLA
3
3
3
2
Except for using OLA to replace Pb-oleate in the precursors, the
synthesis scheme is similar to the synthesis of hybrid CH3-
NH PbBr NCs using PbBr and Pb-oleate as lead sources.
3 3 2
Experimental section
Chemicals
Lead(II) oxide (PbO, 99.9%, Alfa), lead(II) bromide (PbBr
2
, 99%,
Aladdin), methylamine (CH NH , 33 wt% in absolute ethanol,
Aladdin), hydrobromic acid (HBr, 48 wt% in water, Aladdin), N,
N-dimethylformamide (DMF, anhydrous, 99.8%, Alfa), oleic acid
3
2
3 3 3
Synthesis of pure CH NH PbBr nanoparticles
The precursor solution was prepared by mixing the Pb-oleate–
toluene solution (4 mmol) and the alkyl ammonium bromides
(OLA, $99%, Sigma-Aldrich), 1-octadecene (ODE, >90%, Sigma-
+
+
(
4 mmol) with an OA : MA ratio of 4 : 6. To obtain the desired
Aldrich) and n-octylamine (OA, C , 99%, Aladdin). All
8 2
H17NH
pure perovskite nanoparticles, the precursor solution was
the chemicals were used as received without further purication.
added into 10 mL toluene quickly under vigorous stirring at
ꢁ
8
0 C. Aer centrifugation at 7000 rpm for 10 min to discard the
Synthesis of alkylammonium bromide
precipitates, a bright green suspension was obtained, which was
Alkylammonium bromide was synthesized by reaction between ltered using a polytetrauoroethylene (PTFF) syringe lter
alkyl amine (CH
3
NH
2
or
C
8
H17NH
2
)
and HBr. Briey, (pore size 0.45 mm). Finally, there was nearly 100% chemical
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3 3 3
yield of the small CH NH PbBr perovskite nanoparticles using
+
+
Pb-oleate as sole lead source and an OA : MA ratio of 4 : 6.
It is noted that the above-mentioned synthesis of CH NH -
3
3
PbBr nanoparticles can be scaled up. In addition, if only the two
3
2
DMF solutions of PbBr and MABr in 1 : 1 ratio were employed in
the absence of any ligands in the synthesis system and then
dropped into 10 mL of anhydrous toluene under vigorous stir-
3 3 3
ring, CH NH PbBr would precipitate out as bulk crystals.
3 3 3
Characterization of CH NH PbBr NCs
The UV-vis absorption was recorded using UV-3600 plus of Shi-
madzu corporation and PL emission was collected in air using F-
2
80 of Tianjin Gangdong Science & Technology Co. Ltd. Relative
ꢂ1
quantum yield measurements were taken using a 0.5 mol L
quinine sulphate as a reference and exciting at 336 nm. CH3-
NH PbBr NCs samples in toluene were deposited on amorphous Fig. 1 Morphology of CH
3
3
NH PbBr
3
NCs synthesized from different
3
3
+
+
lead sources at OA : MA ratio of 4 : 6. (a) The TEM image of mon-
odispersed CH NH PbBr nanoparticles synthesized from Pb-oleate
as sole Pb source. (b) The HRTEM image of an individual, crystalline
CH NH PbBr nanoparticle. (c) Corresponding FFT pattern of the
HRTEM image in (b). (d) Size distribution of monodispersed CH NH
nanoparticles. (e) TEM image of CH NH PbBr NCs synthesized
from Pb-oleate + PbBr as Pb source. It shows a mixture of nano-
platelets and nanoparticles. (f) TEM image of CH NH PbBr NCs
synthesized from OLA + PbBr as Pb source.
carbon-coated copper grids, and the morphology of the samples
was investigated using an FEI Tecnai G2 20 or a JEM 2100 TEM
operating at 200 kV for images of transmission electron micros-
3
3
3
3
3
3
copy (TEM). The solid colloidal NCs were obtained by fast evap-
3
3
-
ꢁ
orating the toluene on a hot plate at 80 C. The X-ray diffraction PbBr
3
3
3
3
(XRD) patterns were obtained on X'Pert PROX-ray diffractometer
2
3
3
3
from PANalytical B.V. equipped with Cu K radiation. The
a
ꢁ
ꢁ
2
samples were scanned from 5 < 2q < 50 in 5 min.
PLQY was calculated by the following formula:
S
P
3 3 3
produced CH NH PbBr NCs presented a mixture of small-sized
hP ¼ S ꢃ hQ
and large-sized cubic nanoparticles in addition with a thick
organic cluster layers surrounding CH NH PbBr NCs. Addi-
Q
3
3
3
where h
P
and h
Q
3 3 3
represent PLQY of CH NH PbBr NCs and
and S represent the photo-
P Q
luminescence integral area of CH NH PbBr NCs and quinine
tionally, the CH NH PbBr NCs synthesized from Pb-oleate
3
3
3
quinine sulfate, while S
were stable in toluene solution under ambient air conditions
up to 30 days, but the ones from PbBr + OLA or PbBr + Pb-
3
3
3
2
2
sulfate, respectively.
oleate gradually aggregated and precipitated out from the
solution during the same period. The observed differences on
the morphology and stability of the produced CH
NCs suggests that (1) OLA as a ligand cannot effectively inte-
To investigate the impact of lead sources and ligand on the grate with the CH NH PbBr NCs, and produces undesired
3 3 3
NH PbBr
Result and discussion
3
3
3
structure of produced CH NH PbBr NCs, we used Pb-oleate, organic impurities that embed the CH NH PbBr NCs; (2) the
3
3
3
3
3
3
PbBr + Pb-oleate, and PbBr + OLA in the precursors with an use of inorganic PbBr2 as lead source results in the co-
2
2
+
+

xed OA : MA ratio of 4 : 6, and the morphology of the production of both nanoplatelets and nanoparticles; and (3)
synthesized CH NH PbBr NCs were rst examined by TEM the use of Pb-oleate as both sole lead source and ligand can
with an average diameter of 2.2 nm and a deviation of ꢀ0.4 nm effectively restrain the growth of CH NH PbBr nanoplatelets,
Fig. 1d). High-resolution TEM image in Fig. 1b revealed that producing pure CH NH PbBr nanoparticles with ultra-small
3
3
3
3
3
3
(
3
3
3
these nanoparticles are crystalline with a measured d-spacing of sizes, narrow size distribution and higher stability. It is
˚
2
.6 A. Together with the corresponding FFT pattern as shown in possible that during the formation of CH NH PbBr NCs when
3
3
3
ꢂ
Fig. 1c, the exposing crystal facet was determined as (210). On using Pb-oleate as sole lead source, C H COO ion from Pb-
1
7
33
the contrary, if PbBr together with Pb-oleate was used as lead oleate was effectively coordinate at the surface of CH NH -
2
3
3
sources, the synthesized perovskite NCs showed a mixture of PbBr
nanoparticles and nanoplatelets (Fig. 1e). When Pb-oleate was stability of CH
3
NCs, conning the nanocrystal size and enhancing the
NH PbBr NCs. When PbBr was added into the
used as sole lead source and ligand in the reaction, the resulting reaction, guest organic moieties intercalated between the Pb–
perovskite NCs showed a monodispersed nanoparticle structure Br–Pb layers of the crystalline PbBr host forming CH NH
Fig. 1a). Obviously, the use of inorganic lead source (PbBr PbBr nanoplatelets.
promoted the growth of CH NH PbBr NCs in the nanoplatelet The structure and crystallinity of CH NH PbBr NCs
3
3
3
2
2
3
3
-
(
2
)
3
3
3
3
3
3
3
formation with larger size than nanoparticles, possibly due to synthesized from three different precursors were further
less amount of size-conning organo attachments at the crystal investigated with XRD (Fig. 2). Here, bulk CH NH PbBr was
3
3
3
edges. As an additional control, PbBr
the lead source and ligand in the precursors, however the reference. For the CH
2
+ OLA was also utilized as also synthesized without the help of ligand and compared as
NH PbBr NCs synthesized from Pb-
3
3
3
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Fig. 3 Optical properties of CH
3
3
NH PbBr
3
NCs synthesized from
+
+
different lead sources at OA : MA ratio of 4 : 6. (a) Absorption curves
of CH NH PbBr NCs synthesized from OLA + PbBr (black), Pb-oleate
PbBr (blue) and Pb-oleate (red) as lead source. The curves were
offset for better comparison. (b) PL emission of CH NH PbBr NCs
synthesized from OLA + PbBr (black), Pb-oleate + PbBr (blue) and
Pb-oleate (red) as lead source. The excitation wavelength was set at
50 nm. The curves were offset for better comparison. (c) Pictures of
three CH NH PbBr NCs in toluene solutions taken under 365 nm
laser illumination. (d) Quantum yield comparison of three CH NH
PbBr NCs in toluene solutions.
Fig. 2 XRD patterns of CH
3
NH
3
PbBr
3
bulk powders (black) and NCs
(red) and OLA +
(green) as lead source (C bulk unit cell, A nanoplatelets stacks).
3
3
3
2
synthesized from Pb-oleate (blue), Pb-oleate + PbBr
PbBr
2
+
2
2
3
3
3
2
2
ꢁ ꢁ ꢁ
2
oleate + PbBr , a series of diffraction peaks at 8.2 , 12.4 , 16.5 ,
3
ꢁ
ꢁ
ꢁ
ꢁ
3
3
3
2
0.8 , 25.0 , 29.3 , and 33.7 were detected representing the
3
3
-
layered diffractions of CH NH PbBr nanoplatelets stacking in
3
3
3
3
24
the sample. These diffraction peaks were at a regular interval
ꢁ
of ꢄ4.2 , which corresponds to an average spacing of ꢄ2.1 nm
24,25
between the layers in these nanoplatelets.
The observation
520 nm as indicated by the orange arrow in Fig. 3a. Here, the
of higher order peaks also testied that CH NH PbBr nano-
3
3
3
higher absorption energy of nanoplatelets than nanoparticles is
11,19,26
platelets are well crystallized. Due to much larger size of
nanoplatelet than nanoparticles as revealed previous in the
TEM image (Fig. 1e), the diffraction peaks from nanoplatelets in
this sample were so dominating that any peaks from nano-
because of larger exciton binding energy.
As for Pb-oleate,
the absorption peaks from nanoplatelets were disappeared, but
the peak from nanoparticles remained, demonstrating again the
pure nanoparticle morphology in this sample. PL emissions
particles were almost negligible. With PbBr
source and ligand, the series layered diffraction peaks of CH
NH PbBr nanoplatelets were absent while the diffraction peaks
2
+ OLA as lead
provide further evidence as shown in Fig. 3b. CH
from OLA + PbBr presented only peaks at 442 and 476 nm rep-
resenting bi- and four-layer CH NH PbBr nanoplatelets, while
3 3 3
NH PbBr NCs
3
-
2
11
3
3
3
3
3
of bulk unit cell became preeminent, indicating that large-sized
and crystalline CH NH PbBr nanoparticles were formed.
When Pb-oleate was used both as sole lead source and ligand,
the diffraction peak positions from the nanoparticles showed
3 3 3
up and correlated well with the bulk CH NH PbBr with a much
broader peak width, verifying the nanoparticle structure with
ultra-small size in the sample. Also, the absence of series
layered diffraction peaks from nanoplatelets indicates the high
CH NH PbBr NCs from Pb-oleate + PbBr showed both nano-
3
3
3
2
3
3
3
platelet peak at 434 nm and nanoparticle peak at 529 nm. As
a comparison, the use of Pb-oleate as sole lead source resulted in
only nanoparticle emission at 520 nm with no emissions from
nanoplatelets. A picture of three solutions under UV 365 nm laser
illumination was provided in Fig. 3c. The green colour in the
samples synthesized from Pb-oleate and PbBr
2
+ Pb-oleate clearly
3
nanoparticles, in
indicated the existence of CH NH PbBr
3
3
purity of CH NH PbBr NCs in the nanoparticle structure. A
3
3
3
contrast with the one from PbBr + OLA showing the blue colour.
2
summary of XRD peaks and calculated d-space of the as-
It is also worth noting that PLQY of CH NH PbBr NCs from Pb-
3
3
3
synthesized CH
comparison to bulk CH
3
NH
3
PbBr
3
NCs from Pb-oleate and its
PbBr is shown in Table S1.†
oleate as sole lead source were enhanced a factor of 2 and 4.3,
respectively, compared to those from PbBr + Pb-oleate and PbBr
OLA, leading to an improved overall PLQY to 85% (Fig. 3d).
3
NH
3
3
2
2
With their distinct structure and morphology, these CH
NH PbBr NCs synthesized from different precursors also pre-
sented different optical properties (Fig. 3). The CH NH PbBr
NCs from OLA + PbBr showed a dominated absorption peak at
3
-
+
3
3
Other than the type of lead sources, the amount of them also
played an important role in determining the optical properties of
CH NH PbBr NCs. When changing the amount of Pb-oleate
3
3
3
2
3
3
3
3
96 nm and a small shoulder peak at 431 nm, corresponding to
from 4 mmol, which is equivalent to the total amount of alkyl
11
+
+
mono- and bi-layer CH NH PbBr nanoplatelets (Fig. 3a). When
changing to Pb-oleate + PbBr
3
3
3
ammonium halides (OA plus MA ), to less (2 mmol) or more (7
mmol), the absorption as well as PL emission from CH NH -
2
, the peak at 431 nm for bi-layer
3
3
CH NH PbBr nanoplatelet became stronger, along with the
3
3
3
3
PbBr nanoparticles were signicantly weakened (Fig. S1†),
appearance of an absorption peak from nanoparticles at around
indicating the lower quality of the produced perovskite NCs.
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To further investigate the growth mechanism of CH
3
NH
3
-
nanoparticles was the highest, again proving that the growth of
PbBr3 NCs and control the morphology (nanoparticles or CH NH PbBr nanoplatelets needs to be suppressed in order to
3
3
3
nanoplatelets), the ratios of long- and short-chain alkyl obtain the high PLQY. These PLQY values are consistent with
+
+
27
ammonium halides (OA : MA ) were varied (1 : 9, 3 : 7, 4 : 6, previous reported on two-dimensional layered OHP NCs and
28
5
: 5, 6 : 4, 7 : 3 and 9 : 1) in the precursors with Pb-oleate as monodisperse OHP nanoparticles obtained through different
+
+
sole lead source. At the starting OA : MA ratio of 1 : 9, a strong synthesis methods.
absorption peak at 425 nm representing bi-layer CH NH PbBr The growth mechanism of CH
nanoplatelets appeared while the absorption peak from CH revealed by the schematic overview in Scheme 1. Generally
NH PbBr NH PbBr NCs with both
nanoparticles at ꢄ520 nm was weak (Fig. 4a), indi- speaking, during the growth of CH
cating that the formation of nanoplatelets was dominated. With short-chain methylammonium (MA ) and long-chain octy-
3
3
3
3 3 3
NH PbBr NCs can be
3
-
3
3
3
3
3
+
+
+
+
+
+
increasing percentage of OA to 50% (OA : MA ratio of 5 : 5), lammonium (OA ), the small MA ts in the centre of eight
the absorption peak of nanoplatelets quickly decreased to be PbBr corner-shared octahedral, while the large OA ts only the
+
6
negligible, and that of nanoparticles became noticeable, periphery of a set of four PbBr
6
octahedral in the formation of
24,29,30
demonstrating the more favorable formation of nanoparticle CH NH PbBr NCs because of steric effects.
3
3
3
An interesting
+
structure. At increasing OA , the absorption peak of nano- phenomenon aroused when a mixture of short and long organic
+
platelets reappears along with the decrease of nanoparticle cations was employed. While the small MA entering the centre
absorption. Similar trends were also observed in PL emission as of PbBr
6
corner-shared octahedral tends to comfort the three-
shown in Fig. 4b, where the nanoplatelet PL peak at around dimensional (3D) bulk perovskite structure, the long-chain
+
+
+
4
40 nm followed by a U-shape with varying OA : MA ratios cations (for example OA ) occupying the position of the small
+
from 1 : 9 to 9 : 1. On the contrary, the nanoparticle PL (around MA makes the 3D perovskite structure unsuitable and thus it
25 nm) peaks evolved in the opposite tendency, with lower breaks up into smaller sized perovskite NCs. When using
intensity at larger OA : MA amount differences and higher inorganic PbBr
intensity at even amount of OA : MA . Interestingly, the overall (including nanoparticles and nanoplatelets) will form by inter-
PLQY of the solution followed the same tendency with nano- calation of the small MA and large OA between the Pb–Br–Pb
particle PL intensity (Fig. 4d). Highest PLQY of 85% was ach- layers of the crystalline PbBr host (Scheme 1a). However, using
5
+
+
2
as the lead precursor, CH
3 3 3
NH PbBr NCs
+
+
+
+
2
+
+
ieved at an OA : MA ratio of 4 : 6 when PL peak intensity from Pb-oleate as sole lead source, PbBr corner-shared octahedral,
6
nanoplatelets was the lowest and PL peak intensity from provided bromide source by alkylammonium bromide, are
formed rstly, and then are assembled to OHP NCs which are
capped with the ligands of fatty acid (Scheme 1b). If the ratios of
long- and short-chain alkyl ammonium halides are suitable, the
growth of nanoplatelets would be essentially suppressed,
3 3 3
generating pure CH NH PbBr nanoparticles with enhanced
optical properties.
+
+
Fig. 4 Investigation of the impact of OA : MA ratios on the optical
properties of the resulting CH NH PbBr NCs solutions using Pb-
oleate as sole lead source. (a) UV-vis absorption curves of as-made
3
3
3
+
+
3 3
CH NH PbBr
3
NCs solutions with varying OA : MA ratios. The curves
were offset for better comparison. (b) PL emissions of as-made Scheme 1 Schematic overview of the grow mechanism of CH
3
NH
3
-
+
+
CH
3
NH
3
PbBr
3
NCs solutions with varying OA : MA ratios. The PbBr
NH PbBr nanoplatelets) are both formed by intercalation of the small MA and
NCs solutions synthesized from varying OA : MA ratios taken under large OA between the Pb–Br–Pb layers of the crystalline PbBr
UV light. The samples labeled from S1 to S7 represent different using inorganic PbBr as the lead precursor. (b) Pure CH NH
OA : MA ratios from 1 : 9 to 9 : 1. (d) The overall PLQY of as-made nanoparticles are obtained at an optimal OA : MA ratio of 4 : 6 with
CH NH PbBr NCs solutions and respective PL peak intensity of Pb-oleate as the sole lead source, suppressing the growth of
nanoplatelets and nanoparticles with varying OA : MA ratios.
3 3 3 3
NCs. (a) CH NH PbBr NCs (including nanoparticles and
+
excitation wavelength was set as 350 nm. (c) Picture of CH
3
3
3
+
+
+
2
host
PbBr
2
3
3
3
+
+
+
+
3
3
3
+
+
nanoplatelets.
1136 | RSC Adv., 2018, 8, 1132–1139
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+
+
To further analyse the impact of OA : MA ratio on PL Table 1 Comparison of the absorption and PL peaks of 2D CH
NH PbBr nanoplatelets reported in ref. 12 and this work using Pb-
oleate + PbBr
3
-
3
3
properties of CH NH PbBr nanoparticles, the position and full
width at half maximum (FWHM) of PL peaks of CH NH PbBr
3
3
3
3
+
+
2
as lead source with OA : MA of 9 : 1
3
3
nanoparticles were summarized in Fig. S2.† We observed a blue
shi of PL peak of CH NH PbBr nanoparticles from 527 nm to
16 nm with increasing the ratio of OA : MA , indicating
Abs (Ref)
(nm)
Abs (Exp)
(nm)
PL (Ref)
(nm)
PL (Exp)
(nm)
a
3
3
3
n
+
+
5
1
2
3
4
5
396
434
450
472
490
532
396
431
448
470
485
—
405
442
456
482
492
534
403
435
—
474
489
508
a decreased nanoparticle size. This observation also proves the
+
concept that the longer alkyl chain (OA ) restricts the 3D growth
3 3 3
of CH NH PbBr nanoparticles and thus strengthens the nano-
+
+
size quantum connement effect. At OA : MA ratios ranging
from 3 : 7 to 7 : 3, FWHM was below 24 nm, showing narrow
N
size distribution of CH NH PbBr nanoparticles and in agree-
a
3
3
3
n represents the number of layers of nanoplatelets. When n
ment with previous TEM imaging results (Fig. 1). The emission approaches to N, the corresponding structure changes to
nanoparticle formation.
colours of different species under 365 nm laser illumination
were illustrated in Fig. 4c. Considering all factors, the optimal
growth conditions for monodispersed CH
3 3 3
NH PbBr NCs were
determined using Pb-oleate as sole lead source and an
OA : MA ratio of 4 : 6, from which highly uorescent CH3-
+
+
indicated that an even OA /MA amount would promote the
growth of perovskite NCs in nanoparticle formation even with
+
+
NH PbBr nanoparticles could be synthesized as the potential
3
3
PbBr as lead source. Although the exact position of absorption
2
phosphors in LEDs.
We further studied the impact of alkyl ammonium halides
3 3 3 2
on the formation of CH NH PbBr perovskite NCs using PbBr
peak from nanoparticles was hard to be recognized, the blue-
shiing effect was clearly observed as indicated by the red
dashed arrow in Fig. 5a. Similarly, PL emission position of
nanoparticles was blue-shied from 528 nm at the OA : MA
ratio of 1 : 9 to 507 nm at the OA : MA ratio of 9 : 1 (Fig. 5d),
again demonstrating the enhanced quantum connement
effect due to the increasing amount of long-chain OA . As for
the absorption from nanoplatelets, while the intensity of the
peak at around 430 nm from bi-layer nanoplatelets followed
a decreasing trend, a new peak at 396 nm originated from
and Pb-oleate as lead sources. As shown in Fig. 5a and b, in
general the absorption and emission peaks from nanoparticles
at around 520 nm followed similar trends when using Pb-oleate
as sole lead source in which the intensity increased rst and
+
+
+
+
+
+
+
then decreased along with increasing OA : MA ratios. This
+
+
monolayer nanoplatelets started to appear at OA : MA ratio of
: 6 and continued to increase with increasing OA amount
+
4
(
Fig. 5c). Apparently, when PbBr was used as lead source,
2
+
+
nanoplatelets were always formed with varying OA : MA ratios,
and more OA tended to promote the growth of nanoplatelets
+
with fewer layers. This is in agreement with the assumption that
long-chain ammonium halide will cause the size reduction of
perovskite NCs. For mono- and bi-layer nanoplatelets, their
absorption peak positions kept generally constant in each
sample (Fig. 5c), which indicates the thickness of these nano-
platelets was essentially unchanged. Interestingly, even though
the absorption intensity from monolayer nanoplatelets was in
+
+
dominance at higher OA : MA ratios, its corresponding PL
emission at 403 nm was almost absent in these samples. It
reveals that the monolayer nanoplatelets contribute much less
to the overall PLQY of perovskite NCs. Tyagi et al. observed
similar phenomenon that the absorbed energy downhill trans-
ferred from the blue-emitting monolayer nanoplatelets to the
+
+
Fig. 5 Investigation of the impact of OA : MA ratios on the optical
properties of the resulting CH NH PbBr NCs solutions when using
PbBr + Pb-oleate as lead source. (a) UV-vis absorption curves of as- that the dominant relaxation pathway is recombination of free
made CH
curves were offset for better comparison. (b) PL emissions of as-made
16
red-emitting thicker nanoplatelets. Manser et al. suggested
3
3
3
2
+
+
3
NH
3 3
PbBr NCs solutions with varying OA : MA ratios. The
electrons and holes via femtosecond transient absorption
31
spectroscopy measurements. The recombination of thinner
+
+
CH
3
NH
3
PbBr
3
NCs solutions with varying OA : MA ratios. The
excitation wavelength was set as 350 nm. (c) The absorption peak
intensity and position of mono- and bi-layer nanoplatelets from as-
nanoplatelets with its inherently high surface to volume ratio
will be lower because of exceptional sensitivity to surface
+
+
31
+
+
made CH
3 3 3
NH PbBr NCs solutions with varying OA : MA ratios. (d) defects. Moreover, at an OA : MA ratio of 9 : 1, multiple
The PL peak intensity and position of nanoplatelets and nanoparticles
absorption and PL emission peaks can be resolved and assigned
to different number of nanoplatelet layers from 1 to 5 (Fig. S3†),
+
+
from as-made CH
ratios.
3 3 3
NH PbBr NCs solutions with varying OA : MA
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which is in agreement with reported values from ref. 12 (Table
4 V. D'Innocenzo, G. Grancini, M. J. Alcocer, A. R. S. Kandada,
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H. J. Snaith, Science, 2013, 342, 341–344.
1). The only absent emission peak of the tri-layer nanoplatelets
may be due to interference by the adjacent strong peaks. The
+
+
appearance of multiple PL emission peaks at higher OA : MA
ratios indicates a weaker structure control of the grown perov-
skite NCs.
6
7
8
G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Gr ¨a tzel,
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Conclusions
We developed a new strategy for controllable synthesis of
monodispersed and stable CH NH PbBr perovskite NCs using
3 3 3
Pb-oleate as sole lead source. A series of control experiments
were designed and revealed that the mole ratios of long- and
short-chain alkyl ammonium halides (OA : MA ) in the
NREL, Best Research-Cell Efficiencies, https://www.nrel.gov/
pv/assets/images/efficiency-chart.png,
accessed
26
November 2017.
Y. Hassan, Y. Song, R. D. Pensack, A. I. Abdelrahman,
Y. Kobayashi, M. A. Winnik and G. D. Scholes, Adv. Mater.,
9
+
+
precursors have a signicant impact on the crystal shape of
2
016, 28, 566–573.
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+
+
CH NH PbBr NCs. At an optimal OA : MA ratio of 4 : 6, the
3
3
3
1
1
1
1
1
growth of CH
3
NH
3
PbBr
3
NCs in the nanoplatelet shape is
NH PbBr perov-
1
essentially suppressed, resulting in pure CH
3
3
3
skite nanoparticles with an average size of 2.2 nm and an
improved photoluminescence quantum yield up to 85%. The
result shown here provides important new insights for
controlled synthesis of perovskite NCs with pure crystal shape
and high quantum yield, paving the way for further applications
of OHP NCs in optoelectronic devices.
2
2
3
4 L. C. Schmidt, A. Perteg ´a s, S. Gonz ´a lez-Carrero,
O. Malinkiewicz, S. Agouram, G. Minguez Espallargas,
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Conflicts of interest
There are no conicts of interest to declare.
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015, 6, 1911–1916.
2
1
6 Y. Ling, Z. Yuan, Y. Tian, X. Wang, J. C. Wang, Y. Xin,
K. Hanson, B. Ma and H. Gao, Adv. Mater., 2016, 28, 305–311.
7 J. A. Sichert, Y. Tong, N. Mutz, M. Vollmer, S. Fischer,
K. Z. Milowska, R. Garc ´ı a Cortadella, B. Nickel,
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Acknowledgements
1
Financial support from National Natural Science Foundation of
China (Grant No. 51302096), China Scholarship Council (Grant
No. 201606165006), the Fundamental Research Funds for the
Central Universities (Grant No. 2015TS051), the Fundamental
Research Funds of Wuhan City (Grant No. 2016060101010075),
and the Innovation Foundation of Shenzhen Government
6521–6527.
1
8 S. Pathak, N. Sakai, F. Wisnivesky Rocca Rivarola,
S. D. Stranks, J. Liu, G. E. Eperon, C. Ducati,
K. Wojciechowski, J. T. Griffiths and A. A. Haghighirad,
Chem. Mater., 2015, 27, 8066–8075.
(Grant No. JCYJ20160429182959405) are acknowledged. HLL
acknowledges partial support from the National Key Research
and Development Program of China (Grant No.
1
2
2
2
9 F. Zhang, H. Zhong, C. Chen, X.-g. Wu, X. Hu, H. Huang,
J. Han, B. Zou and Y. Dong, ACS Nano, 2015, 9, 4533–4542.
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1 Y. Shirasaki, G. J. Supran, M. G. Bawendi and V. Bulovi ´c , Nat.
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AFOSR (Grant No. MURI FA9550-16-1-0150). The authors thank
the Analytical and Testing Center of Huazhong University of
Science and Technology and Maryland NanoCenter for making
available the shared experimental facilities.
2 S. Ahmad, P. K. Kanaujia, W. Niu, J. J. Baumberg and
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