Benzoyl Halides as Alternative Precursors for the Colloidal Synthesis of Lead-Based Halide Perovskite Nanocrystals

Article abstract of DOI:10.1021/jacs.7b13477

We propose here a new colloidal approach for the synthesis of both all-inorganic and hybrid organic-inorganic lead halide perovskite nanocrystals (NCs). The main limitation of the protocols that are currently in use, such as the hot injection and the liga

Full text of DOI:10.1021/jacs.7b13477

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Article
Benzoyl Halides as Alternative Precursors for the Colloidal
Synthesis of Lead Based Halide Perovskite Nanocrystals
Muhammad Imran, Vincenzo Caligiuri, Menjiao Wang, Luca Goldoni,
Mirko Prato, Roman Krahne, Luca De Trizio, and Liberato Manna
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13477 • Publication Date (Web): 29 Jan 2018
Downloaded from http://pubs.acs.org on January 29, 2018
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Benzoyl Halides as Alternative Precursors for the Colloidal Synthesis
of Lead Based Halide Perovskite Nanocrystals
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Muhammad Imrana,b, Vincenzo Caligiuria, Menjiao Wanga,b, Luca Goldonic,d, Mirko Pratoe, Roman Krahnea,
Luca De Trizio*a and Liberato Manna*a
aNanochemistry Department, cD3 PharmaChemistry Line Department, d Analytical Chemistry Facility and eMaterials Characterization
Facility, Istituto Italiano di Tecnologia (IIT), via Morego 30, Genova, Italy
bDipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Via Dodecaneso 31, 16146 Genova, Italy
KEYWORDS. inorganic metal halides, organicꢀinorganic metal halides, perovskites, nanocrystals, acyl halide, colloidal synthesis.
ABSTRACT: We propose here a new colloidal approach for the synthesis of both allꢀinorganic and hybrid organicꢀinorganic lead halide
perovskite nanocrystals (NCs). The main limitation the protocols that are currently in use, such as the hotꢀinjection and the ligand assisted
reprecipitation routes, is that they employ PbX2 (X= Cl, Br, or I) salts as both lead and halide precursors. This imposes restrictions on being
able to precisely tune the amount of reaction species and, consequently, on being able to regulate the composition of the final NCs. In order
to overcome this issue, we show here that benzoyl halides can be efficiently used as halide sources to be injected in a solution of metal cations
+
(mainly in the form of metal carboxylates) for the synthesis of APbX3 NCs (in which A= Cs+, CH3NH3+ or CH(NH2)2 ). In this way, it is
possible to independently tune the amount of both cations and halide precursors in the synthesis. The APbX3 NCs that were prepared with
our protocol show excellent optical properties, such as high photoluminescence quantum yields, low amplified spontaneous emission threshꢀ
olds, and enhanced stability in air. It is noteworthy that CsPbI3 NCs, which crystallize in the cubic α phase, are stable in air for weeks without
any postꢀsynthesis treatment. The improved properties of our CsPbX3 perovskite NCs can be ascribed to the formation of lead halide termiꢀ
nated surfaces, in which Cs cations are replaced by alkylammonium ions.
INTRODUCTION
200°C) of the A cation (in the form of Csꢀoleate for Cs or methylꢀ
amine for MA) to a solution containing a metal halide salt (e.g.
Over the last few years, semiconductor metal halide nanocrystals
(NCs) with a perovskite crystal structure have emerged as one of
the most interesting materials for optoelectronic applications.1ꢀ11 In
particular, lead based halide perovskite NCs with formula APbX3,
in which A can be Cs+, CH3NH3+ (MA) or CH(NH2)2+ (FA) and X
is Cl, Br or I, have been recently shown to have outstanding optical
properties.12 Such compounds are characterized by a broad tunable
photoluminescence (PL) that ranges from the ultraviolet (UV) to
the near infrared region of the electromagnetic spectrum, a narrow
PbX2, X= Cl, Br, I) and surfactants (e.g. oleylamine and oleic acꢀ
28ꢀ33
id).19,
Immediately after the injection, a rapid salt metathesis
reaction occurs, forming ternary halide NC materials. Conversely,
the LARP strategy, which was originally proposed for the synthesis
of organicꢀinorganic MAPbX3 NCs but was later extended to the
synthesis of allꢀinorganic CsPbX3 systems, is performed at low
temperatures (typically from room temperature, RT, to 60°C).
This method is based on the reprecipitation of halide salts in the
presence of ligands: metal halide salts (or organic halide salts in the
case of hybrid perovskites) are solubilized in one or more polar
solvents, like DMF, and are subsequently added dropwise to a
solution of a nonꢀpolar medium, like toluene, in the presence of
ligands.2, 15, 18, 34ꢀ40 The low solubility of halide salts in the nonꢀpolar
solvent triggers their precipitation with the recrystallization of
halide NCs.
full width at halfꢀmaximum (FWHM), and high PL quantum yields
13ꢀ19
(PLQY).7ꢀ11,
Interestingly, the PL emission of such perovskite
NCs can be easily adjusted not only through size control, and
subsequently through quantum confinement, as is the case for
standard quantum dots, but also through compositional mixing, for
example via simple anion exchange reactions.20ꢀ27 Such properties
have inspired researchers to exploit this class of materials for use in
efficient solar cells, sensitive photodetectors, low threshold lasers,
and light emitting diodes (LEDs).3, 4, 7, 9ꢀ13
Both procedures have various limitations, the main one of which
is the use of inorganic salts (i.e. PbX2) as both cation and anion
precursors for the metathesis reaction. Indeed, since the ratio of
cations to anions employed in the synthesis is linked to that of the
chosen inorganic salts, it is not possible to precisely tune the comꢀ
position of the final NCs. Furthermore, a notable disadvantage is
that it is difficult to work with an excess of halide ions, which is an
experimental condition that has been shown to favor the formation
To date, various approaches have been proposed for the direct
synthesis of allꢀinorganic and organicꢀinorganic metal halide perovꢀ
skite colloidal NCs, with the hotꢀinjection and the ligandꢀassisted
reprecipitation (LARP) ones being the most used and most develꢀ
oped methods.7, 9ꢀ11 The former, which was initially devised for allꢀ
inorganic perovskite NCs, is based on the hotꢀinjection (up to
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of lead halide perovskite NCs with improved stability and optical
can be ascribed to the formation of lead halide terminated surfaces
in which Cs cations are replaced by alkylammonium ions. Indeed,
our XꢀRay photoelectron spectroscopy analysis revealed that all our
CsPbX3 NCs are Cs poor and they contain a considerable amount
of amines, most likely in the form of oleylammonium ions. Indeed,
the formation of these surfaces is favored under our synthetic conꢀ
ditions: the use of benzoyl halides provides a halide rich environꢀ
ment and, at the same time, an efficient protonation of the oleylaꢀ
mine.
properties.2, 29, 41
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The obstacles imposed by these two techniques were partially
overcome by Wei et al., who proposed an alternative colloidal route
for the preparation of CsPbX3 NCs, that was later called the “threeꢀ
precursors approach”.42 Such a synthesis is based on the dissolution
of Cs+ and Pb2+ cations in fatty acids followed by the injection of an
alkylammonium halide salt (as the halide precursor). This proceꢀ
dure was then adopted and revised by Yassitepe et al. for the prepaꢀ
ration of amineꢀfree CsPbX3 NCs for LEDs,43 and by Protesescu et
al. and Li et al. for the synthesis of FAPbX3 (X=Br, I) NCs.44ꢀ46 This
approach allows one to work with the desired stoichiometry of the
ions, since the halide ions and the metal cation sources are not
delivered together, i.e. they are not delivered with the same chemiꢀ
cal precursor. On the other hand, its potential versatility is limited
by the poor reactivity of the alkylammonium halide salts (i.e. CsPꢀ
bI3, CsPbCl3, MAPbX3 and FAPbCl3 NCs have not been reported
by using this strategy), which can also lead to the formation of
undesired secondary phases.
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EXPERIMENTAL DETAILS
Chemicals. Lead acetate trihydrate (Pb(CH3COO)2×3H2O,
99.99%), lead(II) oxide (PbO, 99.999%), sodium iodide (NaI,
99.99%) cesium carbonate (Cs2CO3, reagent Plus, 99%), cesium
acetate (CH3COOCs, 99.9%), methylamine (CH3NH2, 2M soluꢀ
tion in tetrahydrofuran, THF), formamidinium acetate salt
(HN=CHNH2ꢁCH3COOH, 99%), benzoyl bromide (C6H5COBr,
97%), benzoyl chloride (C6H5COCl, 98%), toluene (anhydrous,
99.5%), octadecene (ODE, technical grade, 90%), oleylamine
(OLAM, 70%) and oleic acid (OA, 90%) were purchased from
SigmaꢀAldrich. All chemicals were used without any further purifiꢀ
cation.
In order to overcome the restrictions associated with the aforeꢀ
mentioned synthetic procedures, we propose here a new colloidal
synthesis approach that can lead to either allꢀinorganic or organicꢀ
inorganic lead based halide perovskite NCs. This new approach
relies on the use of benzoyl halides as halide precursors, which can
be easily injected into a solution of metal cations (such as metal
carboxylates) and desired ligands (oleylamine and oleic acid). This
injection immediately triggers the nucleation and the growth of
metal halide NCs (see Scheme 1). By simply tuning the relative
amount of cation precursors, ligands, solvents, benzoyl halides and
the injection temperature, it is possible to synthesize either allꢀ
inorganic or organicꢀinorganic APbX3 (A= Cs, MA or FA and X=
Cl, Br or I) NCs with tight control over the size distribution and
with high phase purity. In addition, thanks to the strong reactivity
of benzoyl halides even at RT, they can be used for anion exchange
reactions using preꢀsynthesized CsPbX3 NCs.
Preparation of Benzoyl Iodide. The reaction was performed in a
N2ꢀfilled glovebox following the procedure reported by Theobald
and Smith.51 In short, sodium iodide (3g) was mixed with benzoyl
chloride (1.4mL) in a 20mL vial. The mixture was vigorously
stirred at 75°C on a hot plate for five hours. The reaction mixture
turned from colorless to an orange red color, indicating that the
transformation of the benzoyl chloride into the benzoyl iodide was
successful. Next, the reaction mixture was cooled down to RT and
diluted using 3mL of anhydrous ODE. Finally, the solution was
filtered, by using a polytetrafluorethylene membrane filter with a
0.45μm pore size, in order to collect the liquid precursor. The
unambiguous identification of all the organic species in the soluꢀ
tion, as well as the reaction yield (97.7%), was possible by means of
Nuclear Magnetic Resonance (NMR, see the Supporting Inforꢀ
mation, SI, for the complete assignment and quantification of the
species in the reaction mixture).
Scheme 1. Colloidal synthesis of lead based halide perovskite
nanocrystals using benzoyl halides as halide precursors.
Synthesis of CsPbX3 NCs. In a typical synthesis, cesium carꢀ
bonate (16mg), lead acetate trihydrate (76mg), 0.3ml of OA, 1ml
of OLAM and 5ml of ODE were loaded into a 25ml 3ꢀneck round
bottom flask and dried under vacuum for 1h at 130oC. Subsequentꢀ
ly, the temperature was increased to 165ꢀ200oC (See Table 1 for
details) under N2 and the desired amount of the benzoyl halide
precursor was swiftly injected (0.6mmol in the case of benzoyl
bromide and iodide, and 1.8mmol in the case of benzoyl chloride).
The reaction mixture was immediately cooled down in an iceꢀwater
bath for CsPbBr3 and CsPbI3 NCs, while it was quenched after
20sec for CsPbCl3 NCs. Finally, 5ml of toluene was added to the
crude NC solutions and the resulting mixture was centrifuged for
10 min at 4 krpm. The supernatant was discarded and the precipiꢀ
tate was redispersed in 5 mL of toluene for further use.
The APbBr3 NCs that were prepared using our protocol are
characterized by their PLQYs, which were as high as 92%, and by
their very low amplified spontaneous emission (ASE) thresholds.
Moreover, the APbI3 NCs had PLQYs of around 55% and, finally,
the CsPbCl3 NCs exhibited a record PLQY value of 65%. Also, they
exhibited a much higher phase stability than that which has been
previously reported for NCs prepared with other synthesis methꢀ
ods. It is noteworthy that cubic CsPbI3 NCs are stable in air for
weeks without any postꢀsynthetic treatment, which is different from
those prepared using the classic hotꢀinjection or LARP approachꢀ
es.28, 44, 47ꢀ50 The optimal properties of our CsPbX3 perovskite NCs
Synthesis of mixed CsPb(Br,Cl)3 and CsPb(Br,I)3 NCs. Cesium
carbonate (16mg), lead acetate trihydrate (76mg), 0.3ml of OA,
1ml of OLAM and 5ml of ODE were loaded into a 25ml 3ꢀneck
round bottom flask and dried under vacuum for 1h at 130°C. Subꢀ
sequently, the temperature was increased to 170°C under N2 and
the 0.6mmol mixture of benzoyl chloride/bromide (a precursor
ratio of 1:1 led to NCs emitting at 448nm) or benzoyl broꢀ
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mide/iodide (precursor ratios of 5:1 and 1:1 led to NCs emitting at
544nm and 594nm, respectively) was swiftly injected. The reaction
mixture was immediately cooled down in an iceꢀwater bath. The
NCs were collected by adding 5ml of toluene to the crude solution
followed by centrifugation at 4 krpm for 10 min. The supernatant
was discarded and the precipitate was redispersed in 5 mL of toluꢀ
ene.
P
L
Q
u
a
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m
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i
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a
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r
e
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o
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m
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a
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r
e
ꢀ
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ments. The samples were measured with an Edinburgh FLS900
fluorescence spectrometer equipped with a Xenon lamp, a monoꢀ
chromator for steadyꢀstate PL excitation, and a timeꢀcorrelated
single photon counting unit coupled with a pulsed laser diode (λex
375 & 405 nm, pulse width = 50 ps) for timeꢀresolved PL. The
PLQY was measured using a calibrated integrating sphere (λex
350nm for all the chloride and the bromide samples, and λex
450nm for all the iodide samples). All solutions were diluted to an
optical density of 0.1 or lower (at the corresponding excitation
wavelength) in order to minimize the reꢀabsorbance of the fluoroꢀ
phore.
=
=
=
Synthesis of MAPbX3 NCs. Lead oxide (44mg), 2.5mL of OA,
0.025mL of OLAM and 5mL of ODE were mixed in a 25mL 3ꢀneck
round bottom flask and dried under vacuum for 1h at 125oC. Subꢀ
sequently, the temperature was lowered to 65oC under N2. Methylꢀ
amine (0.170mL) was injected, followed by the injection of
0.6mmol of the benzoyl halide precursor (see Table 1 for details).
The reaction was quenched by the addition of 5mL of toluene after
30 seconds in the case of MAPbBr3 and MAPbI3 NCs, and after 10
seconds in the case of MAPbCl3 NCs. The NCs were collected by
centrifuging the crude solution at 4 krpm for 10 min.
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Amplified Spontaneous Emission (ASE) measurements. All the
APbBr3 (A=Cs+, MA+, or FA+) NC samples that were used for ASE
dynamics were cleaned twice by precipitation using ethyl acetate
(the volume ratio of toluene to ethyl acetate was 5:1) and reꢀ
dispersion in toluene. Eventually, thick NC films were produced on
glass substrates by dropꢀcasting the colloidal solutions. The NC
films were optically excited by a pulsed laser source at λ = 405nm,
with a pulse width of 50fs, and a repetition rate of 1kHz at normal
incidence. The pump beam was focused on the sample by a cylinꢀ
drical lens, producing an excitation stripe of about 0.6cm in length.
The emission spectra were recorded using a collection lens and a
fiber coupled Ocean Optics HR4000 spectrometer at an angle close
to 90° with respect to that of the excitation beam.
Synthesis of FAPbX3 NCs. Lead acetate trihydrate (76mg),
formamidinium acetate (40mg), 2.5mL of OA, 0.025mL of OLAM
and 5mL of ODE were mixed in a 25mL 3ꢀneck round bottom flask
and dried under vacuum for 1h at 125oC. Subsequently, the temꢀ
perature was lowered to 75ꢀ95oC (See table S1) under N2 and
0.6mmol of the benzoyl halide precursor was rapidly injected. After
30 seconds, the reaction mixture was cooled down in an iceꢀwater
bath. 5ml of toluene was added to the crude solution and the resultꢀ
ing mixture was centrifuged for 10 min at 4 krpm. FAPbI3 NCs
were washed once with ethyl acetate (using a toluene/ethyl acetate
ratio of 5/1) and were eventually redispersed in toluene.
Transmission Electron Microscopy (TEM) characterization.
The samples were prepared by dropping dilute solutions of NCs
onto carbon coated copper grids. Lowꢀresolution TEM measureꢀ
ments were performed on a JEOLꢀ1100 transmission electron
microscope operating at an acceleration voltage of 100 kV.
Table 1. Synthetic parameters used for the synthesis of APbX3 NCs
Sample
OLAM (mL)
OA (mL)
0.3
Temperature (oC)
Xꢀray diffraction (XRD) characterization. The XRD analysis was
performed on a PANanalytical Empyrean Xꢀray diffractometer,
equipped with a 1.8 kW CuKα ceramic Xꢀray tube and a PIXcel3D
2x2 area detector, operating at 45 kV and 40 mA. Specimens for the
XRD measurements were prepared by dropping a concentrated
NC solution onto a quartz zeroꢀdiffraction single crystal substrate.
The diffraction patterns were collected under ambient conditions
using parallel beam geometry and symmetric reflection mode. XRD
data analysis was conducted using the HighScore 4.1 software from
PANalytical.
CsPbCl3
CsPbBr3
CsPbI3
1
200
170
165
65
1
0.3
1
0.3
MAPbCl3
MAPbBr3
MAPbI3
FAPbCl3
FAPbBr3
FAPbI3
0.1
2.5
0.025
0.150
0.025
0.025
0.200
2.5
65
2.5
65
2.5
95
Xꢀray Photoelectron Spectroscopy (XPS) characterization.
Measurements were performed on a Kratos Axix Ultra DLD specꢀ
trometer, using a monochromatic Al Kα source (15kV, 20mA). The
photoelectrons were detected at a takeꢀoff angle of ϕ = 0° with
respect to the surface normal. The pressure in the analysis chamber
was maintained below 7×10ꢀ9 Torr for data acquisition. The data
was converted to VAMAS format and processed using CasaXPS
software, version 2.3.17. The binding energy (BE) scale was interꢀ
nally referenced to C1s peak (BE for CꢀC = 284.8 eV).
2.5
75
2.5
95
Anion Exchange Reactions. In short, 0.500 mL of the CsPbX3
NC dispersion was diluted with 2 mL of anhydrous toluene, and
different amounts of a 0.12M solution of benzoyl halide in toluene
(ranging from 30ꢂl to 500ꢂl) were swiftly injected under vigorous
stirring at RT. Finally, the NCs were collected by centrifugation at
4krpm for 10 min.
UVꢀVIS absorption and PL measurements. The UVꢀVisible abꢀ
sorption spectra were recorded using a Varian Cary 300 UVꢀVIS
absorption spectrophotometer. The PL spectra were measured on a
Varian Cary Eclipse spectrophotometer using an excitation waveꢀ
length (λex) of 350nm for all the chloride and the bromide samples,
and 450nm for all the iodide compounds. Samples were prepared
by diluting NC solutions in toluene, in quartz cuvettes with a path
length of 1 cm.
RESULTS AND DISCUSSION
The synthetic strategy we propose here was inspired by the one
employed for the nonꢀaqueous synthesis of metal oxide NCs.52 In a
typical synthesis of metal oxide NCs, the release of water, i.e. the
oxygen precursor, can be achieved by reacting carboxylic acids with
either amines or alcohols at relatively high temperatures (above
200°C), as is illustrated by Equations 1 and 2.
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O
O
O
O
benzoyl halides since they are lowꢀcost and have a sufficiently high
1
2
3
4
5
6
7
8
H2O
+
+
R2
NH2
+
(1)
(2)
boiling point (~200°C) which allows for the synthesis of metal
halide NCs even at relatively high temperatures. Furthermore,
considering the strong reactivity of acyl halides toward nucleophilic
species, another important aspect of benzoyl halides is that they are
more stable than aliphatic acyl halides as a result of their stabilizaꢀ
tion by the πꢀoverlap in the ground state.54 Indeed, the more stable
the anion precursor is, the more controlled the release of the halide
ions should be, which, in turn, could lead to the fine tuning of the
size distribution of the NC.55
R2
R1
N
H
R1
OH
OH
H2O
R2
OH
+
R2
R1
O
R1
Similarly, acyl halides are well known for their strong reactivity
toward nucleophilic compounds (i.e. amines, alcohols, carboxylic
acids) which form carboxylic acid derivatives even at room temperꢀ
ature (i.e. amides, esters, anhydrides) and simultaneously release
hydrohalic acids (see Equations 3ꢀ5).53
9
In order to understand the efficacy of our synthetic protocol, we
tested the synthesis of leadꢀbased halide perovskites NCs, paying
particular attention to the optimization of their phase purity, size
distribution and optical properties. To this end, we chose to synꢀ
thesize the benzoyl iodide precursor due to the commercial availaꢀ
bility of only the benzoyl chloride and bromide compounds. This
precursor can be easily prepared by reacting benzoyl chloride with
anhydrous sodium iodide at 75°C in an inert atmosphere (see
Experimental Details and Figure S1 of the SI).51, 56
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O
O
HX
+
R2
NH2
+
(3)
R2
R1
N
H
R1
X
O
O
O
First, we will illustrate the results obtained for CsPbX3 NCs. In a
typical synthesis, cesium carbonate and lead acetate were dissolved
and degassed in oleylamine, oleic acid and octadecene at 130°C in a
three neck flask. Subsequently, the solution was heated up to the
desired temperature (170ꢀ200°C) and the benzoyl halide precursor
was swiftly injected into the reaction flask, triggering the immediate
nucleation and growth of the NCs (see Experimental Details and
Table 1). Bright field TEM images of CsPbX3 NCs show that the
crystals had a cubic shape and a narrow size distribution (see Figure
1aꢀc and Figure S2 of the SI). The average size of the CsPbCl3 NCs
was 8.6±1.0nm, while it was 7.8±1.3nm for the CsPbBr3 ones and
10.1±1.6nm for the CsPbI3 ones.
HX
HX
+
R2
OH
+
(4)
R2
R1
O
R1
X
X
O
O
O
+
+
(5)
R
O
R
2
R
OH
R
1
1
2
Thus, the idea behind our colloidal approach is to inject acyl
halide molecules into a solution of metal cations that have been
dissolved in nucleophilic molecules, namely amines and carboxylic
acids, at a desired temperature, in order to trigger the release of
halide ions and, consequently, the nucleation and growth of metal
halide NCs. Among the possible acyl halide molecules, we selected
Figure 1. Bright field TEM images of (a) CsPbCl3, (b) CsPbBr3, and (c) CsPbI3 NCs. Scale bars are 100 nm in all images. XRD patterns of
(d) CsPbCl3, (e) CsPbBr3, and (f) CsPbI3 NCs along with the corresponding bulk cubic reference patterns. Absorption and PL spectra of (g)
CsPbCl3, (h) CsPbBr3, and (i) CsPbI3 NCs dispersed in toluene.
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The XRD patterns of CsPbX3 NCs nicely match the cubic perovꢀ
to the orthorhombic CsPbI3 bandꢀedge absoption,59 and the NCs
retained their PL emission (see Figure 2b).
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skite structure (CsPbBr3 ICSD code 29073, CsPbCl3 ICSD code
23108, CsPbI3 ICSD code 181288) in all three cases, and no secꢀ
ondary phases were present (see Figure 1dꢀf). Remarkably, the
CsPbX3 NCs exhibited a narrow PL emission linewidth, ranging
from 11nm (CsPbCl3), to 18nm (CsPbBr3) to 32nm (CsPbI3), and
had PLQYs as high as 92% (see Figure 1gꢀi). CsPbCl3 NCs were of
particular interest: while cesium lead chloride perovskite NCs are
typically characterized by a significant nonꢀradiative decay, the
PLQY of CsPbCl3 NCs was measured to be as high as 65%, which is
In order to understand the reason behind the phase stability of
CsPbI3 NCs, and possibly also the PL properties of our CsPbX3
NCs, we performed XPS characterization to study their surface and
composition. The surface chemistry of lead halide based perovskite
NCs has been shown to play a fundamental role in determining not
only their stability in air or under annealing, but also their optical
performances.17, 28, 60ꢀ64 The analysis of the Cs 3d, Pb 4f and X peaks
(Cl 2p, Br 3d and I 3d) revealed that our CsPbX3 NCs were subꢀ
stoichiometric in Cs, while the Pb:X ratio was always close to 3 (see
Figure S4 of the SI). In more detail, the Cs:Pb:X ratios found in our
NCs were: 0.9:1:3.1 in the case of chlorides, 0.8:1:2.8 in the case of
bromides and 0.8:1:2.7 in the case of iodides. We also roughly
estimated the amount of oleylammonium ions bound to CsPbX3
NCs by analyzing the N 1s peak (see Figure S4 of the SI). While,
the chloride and bromide NCs had a ratio of Cs:N close to 1:0.5,
the iodide ones had a ratio of 1:1.2. These results suggest that the
surface of our CsPbX3 NCs is lead halide terminated, i.e. the surface
Cs ions are replaced by oleylammonium ions. Lead halide perovꢀ
skite NCs with this type of surface have been reported to have
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a record value.16, It is important to highlight that such a high
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PLQY was observed only when employing a large excess of the Cl
precursor, i.e. 1.8mmol of benzoyl chloride and 0.2mmol of the Pb
precursor (see the Experimental Details). On the other hand, CsPꢀ
bCl3 NCs were characterized by a weak PL emission when they
were prepared using a lower amount of benzoyl chloride. For exꢀ
ample, using 0.6mmol of benzoyl chloride and 0.2 mmol of the Pb
precursor led to NCs with a PLQY of just a few percentage points.
Time correlated single photon counting (TCSPC) measureꢀ
ments that were conducted on APbX3 NCs revealed, as expected,
that systems with higher bandgaps had faster PL decay rates (see
Figure S3a and Table S1 of the SI).19 In particular, the calculated
average lifetimes were 7.7ns for CsPbCl3 NCs, 12.5ns for CsPbBr3
NCs and 21ns for CsPbI3 NCs. The average radiative and nonꢀ
radiative decay rates that were estimated from the PLQYs and the
average PL decay times are reported in Table S1 of the SI.
63
improved stability and enhanced optical properties.29, Indeed,
Woo et al. ascribed the improved stability of their CsPbBr3 NCs
(which were obtained with the hotꢀinjection method, adding ZnBr2
as an extra bromide source) to their lead bromide rich surfaces.29 In
addition, Ravi et al. demonstrated that alkylmmonium ions have
the ability to substitute Cs ions on the surface of CsPbX3 NCs,
which consequently improves their stability and optical
properties.63 We believe that our new synthetic procedure is favorꢀ
able with regards to the formation of oleylammoniumꢀleadꢀhalide
surfaces thanks to the halide rich conditions that are used (see the
Experimental Details). Moreover, the release of Xꢀ ions from the
acyl halides is accompanied by the concomitant release of H+ ions
which can drive the protonation of the oleylamine in solution (see
Equations 3ꢀ5).
While, in general, leadꢀhalideꢀbased perovskite NCs exhibit exꢀ
cellent optical properties, some of these materials are known for
their poor structural stability. In particular, red emitting CsPbI3
NCs, the most interesting materials for photovoltaics applications,
suffer from a delayed phase transformation from the metastable
cubic (α) phase into the nonꢀluminescent orthorhombic (δ) phase,
also known as the “yellow phase”.58 For this reason, different apꢀ
proaches have been reported for the stabilization of the cubic
phase, such as: the use of alkyl phosphonic acids or phosphines in
48
the synthesis of CsPbI3 NCs;47, washing procedures employing
ethyl acetate;28 replacing part of Cs+ cations with bigger cations;44
50
and replacing Pb2+ ions with Mn2+ cations.49, In this regard, we
observed that the cubic CsPbI3 NCs that were synthesized with our
procedure had a high phase stability, without any postꢀsynthesis
treatment. The XRD patterns of CsPbI3 NC films exposed to air
indicated that no phase transition occurred after 20 days (see Figꢀ
ure 2a).
Figure 3. (a) Evolution of the PL spectra of CsPbBr3 NCs by the
addition of benzoyl chloride or benzoyl iodide. (b) Picture of the
different CsPbX3 NC solutions obtained by anion exchange under a
UV lamp.
As benzoyl halides can be easily mixed together, we also tested
our procedure to synthesize mixed halide NCs, namely
CsPb(Cl/Br)3 and CsPb(Br/I)3. These compounds, as well as the
starting CsPbX3 NCs, could be successfully synthetized with a
narrow size distribution, phase purity and good optical properties,
simply by injecting mixtures of benzoyl halides in appropriate ratios
(see Experimental Details and Figure S5 of the SI). Also, given the
strong reactivity of benzoyl halides even at room temperature, we
tested the CsPbX3 NCs for postꢀsynthesis transformations, namely
for anionꢀexchange reactions. So far, the most commonly used
precursors for anion exchange reactions have been metal halide
Figure 2. (a) XRD patterns, (b) UVꢀVIS and PL curves of CsPbI3
NCs exposed to air up to 20 days.
Furthermore, our optical characterizations of the NCs after air
exposure confirmed the absence of the CsPbI3 yellow phase, since
no absorption features appeared at ~440nm, which can be ascribed
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salts (i.e. MX2 where M = Pb, Zn, Mg, Cu, Ca and X = Cl, Br, I) and
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We then extended our protocol to the colloidal synthesis of hyꢀ
brid organicꢀinorganic APbX3 (A=MA, FA) perovskite NCs. We
first tested our hotꢀinjection approach by preparing MAPbX3 NCs
which, to date, have been mainly produced using the LARP techꢀ
nique since standard hotꢀinjection techniques result in NCs with
poor optical properties.32 These hybrid organicꢀinorganic NCs
could be synthesized using the protocol we devised for CsPbX3
NCs, with only minor modifications (see Experimental Details).
Bright field TEM images of representative MAPbX3 NCs are shown
in Figure 4aꢀc. In all three cases, a nearly cubic morphology with a
narrow size distribution was observed: the average length of the
NCs was 20.1±2.9nm for MAPbCl3, 15.5±1.8nm for MAPbBr3 and
8.9±2.4nm for MAPbI3 (see Figure 4aꢀc and Figure S8 of the SI). It
is worth mentioning that PbO was used as the lead precursor for
the synthesis of MAPbX3 NCs instead of lead acetate as it enabled a
greater control over the size distribution and shape of the final NCs
(see the Experimental Details and Figure S9 of the SI).
oleylammonium or tetrabutylammonium halides.20ꢀ25 However, a
disadvantage of these halide sources is that they either suffer from
poor solubility in nonꢀpolar solvents or their reactivity is limited at
room temperature. On the contrary, benzoyl halides were observed
to be efficient precursors for anion exchange reactions: the addition
of benzoyl chloride or benzoyl iodide to preꢀsynthesized CsPbBr3
NCs led to a fast blue shift or red shift, respectively, of both the PL
and the absorption spectra of the NCs (see Figure 3, Experimental
Details and Figure S6a of the SI). In both cases, the XRD patterns
of the resulting NCs confirmed the retention of the parent cubic
perovskite structure, and there was a systematic shift of the peaks
induced by the variation of the lattice parameters (see Figure S6b
of the SI). Interestingly, the back exchange reactions, CsPꢀ
bCl3ꢀCsPbBr3 and CsPbI3ꢀCsPbBr3, also worked efficiently
when benzoyl bromide was added to the CsPbCl3 and CsPBI3 NCs,
respectively (see Figure S7 of the SI).
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Figure 4. Bright field TEM images of (a) MAPbCl3, (b) MAPbBr3, and (c) MAPbI3 NCs. Scale bars are 100 nm in all images. XRD patterns
of (d) MAPbCl3, (e) MAPbBr3, and (f) MAPbI3 NCs along with their corresponding bulk cubic reference patterns. In the case of MAPbCl3,
the bulk reflections were calculated using the crystal structure that was reported by Maculan et al.65Absorption and PL spectra of (g) MAPꢀ
bCl3, (h) MAPbBr3, and (i) MAPbI3 NCs dispersed in toluene.
According to XRD analysis, the structure of the MAPbBr3 NCs
matched the cubic perovskite structure (ICSD code 252415), while
that of MAPbI3 NCs exhibited a tetragonal CH3NH3PbI3 crystal
phase (space group I4/mcm, ICSD code 238610). This is in
agreement with a recent report by Zhang et al. on identical systems
(see Figure 4eꢀf).38 Given the absence of any reference pattern for
MAPbCl3 in the ICSD database, we compared the XRD pattern of
our MAPbCl3 NCs with those of bulk crystals that are reported in
literature, and we found a good match with that reported by Macuꢀ
lan et al., which is a cubic perovskite structure (space group Pmꢀ
Remarkably, the MAPbBr3 and MAPbI3 NCs had a PLQY as high
as 92% and 45%, respectively, while MAPbCl3 NCs the PLQY was
around 5%. Similar to the case of CsPbCl3 NCs, we did observe an
enhancement of the PL emission of MAPbCl3 NCs when increasꢀ
ing the amount of the Cl precursor to 1mmol (see Figure S10a of
the SI). Such improvement occurred along with the formation of a
secondary PbCl2 undesired phase (see Figure S10b of the SI).
These findings suggest that, as in the case of CsPbX3 systems, a lead
halide rich environment could enhance the PL emission of the
resulting MAPbCl3 NCs. Unfortunately, in this case such environꢀ
ment leads also to the formation of PbCl2 which, thus, limits the
effective amount of the Cl precursor that can be employed. Decay
lifetimes, which were acquired by means of TCSPC measurements,
were 5.4ns for MAPbCl3 NCs, 35ns for MAPbBr3 NCs and 35.7ns
for MAPbI3 NCs (see Figure S3b and Table S1 of the SI).
3m) with a
= 5.67 Å (see Figure 4d).65, 66
The UV−visible absorption and PL spectra of MAPbX3 NCs are
shown in Figure 4gꢀi. Similar to what has been previously reported,
all NC samples had a narrow PL emission with a FWHM of 15nm
32, 66
for MAPbCl3, 19nm for MAPbBr3 and 43nm for MAPbI3.15,
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Although the synthesis of MAPbX3 NCs has been optimized
match was found with the cubic structures reported by Zhumekeꢀ
nov et al.70 (see Figure 5dꢀf). On the other hand, no cubic bulk
structure has been reported so far for FAPbCl3 compounds. This
can be explained by its calculated tolerance factor (1.150), which in
principle is too large to allow a 3D phase formation.71 Conversely,
the refinement of the XRD pattern of our FAPbCl3 NCs led to a
cubic structure (space group Pmꢀ3m) with a = 5.67 Å. This repreꢀ
sents the first report of a cubic FAPbCl3 structure. UVꢀvis and PL
spectra of the FAPbX3 NCs are shown in Figure 5gꢀi. Furthermore,
Br and I based compounds exhibited excellent optical properties,
and had a high PLQY (90% for FAPbBr3 and 65% for FAPbI3) and
narrow PL emission (20nm for FAPbBr3 and 48nm for FAPbI3).
The FAPbCl3 NCs were characterized by having a narrow PL
(FWHM = 16nm), but a low PLQY (about 2%). Decay lifetimes,
which were acquired by means of TCSPC measurements, were
14.8ns for FAPbCl3 NCs, 30.3 ns for FAPbBr3 NCs and 75.2ns for
FAPbI3 NCs (see Figure S3c and Table S1 of the SI).
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over the last few years, FAPbX3 NCs with optimal optical properties
as well as a narrow size distribution and phase purity have not yet
been prepared by either hotꢀinjection techniques or by the LARP
approach.18, 40, 44ꢀ46 Lately, these compounds have received considerꢀ
able interest since they have several advantages over their meꢀ
thylammonium counterparts, such as a higher stability due to a
more symmetric and tightly packed crystal structure.67ꢀ69 We could
also synthetize FAPbX3 NCs using our protocol (see Experimental
Details). Typical TEM images of FAPbCl3 and FAPbBr3 NCs
evidenced a narrow size distribution, which became slightly broadꢀ
er in the case of FAPbI3 NCs (see Figure 5aꢀc and Figure S11 of the
SI). The average size of the NCs was 11.2±1.4nm for FAPbCl3,
12.4±1.6nm for FAPbBr3 and 14.2±2.8nm for FAPbI3. Regarding
the structural analysis, given the absence of any FAPbX3 reference
patterns in the ICSD database, we had to compare the XRD pattern
of our FAPbX3 NCs with those of bulk crystals which have been
recently published. In the case of FAPbBr3 and FAPbI3 NCs, a good
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Figure 5. Bright field TEM images of (a) FAPbCl3, (b) FAPbBr3, and (c) FAPbI3 NCs. Scale bars are 100 nm in all images. XRD patterns of
(d) FAPbCl3, (e) FAPbBr3, and (f) FAPbI3 NCs along with the corresponding bulk cubic reference patterns. For FAPbBr3 and FAPbI3 NCs,
the bulk reflections were calculated using the crystal structure reported by Zhumekenov et al.,70 while the reflections of the FAPbCl3 NCs
were generated using a cubic perovskite structure (Pmꢀ3m) with
FAPbI3 NCs dispersed in toluene.
a= 5.76 Å. Absorption and PL spectra of (g) FAPbCl3, (h) FAPbBr3, and (i)
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Figure 6. ASE dynamics for a) CsPbBr3, b) MAPbBr3 and c) FAPbBr3 together with the ASE threshold calculations (insets).
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(2) Li, X.; Wu, Y.; Zhang, S.; Cai, B.; Gu, Y.; Song, J.; Zeng, H., Adv.
Funct. Mater. 2016, 26, 2435ꢀ2445.
Finally, we investigated the amplified spontaneous emission
(ASE) which occurred in the APbBr3 NC films. These particular
NCs had the highest PLQYs (more than 90% in all three of cases),
much higher than those of their Cl and I counterparts (APbCl3 and
APbI3). Figure 6aꢀc reports the emission spectra showing ASE of
the three APbBr3 NC samples together with the ASE thresholds
that were calculated by the ratio of the ASE to the PL peaks. All
three systems manifested very low ASE thresholds, ranging from
2.2 to 8.1 μJ/cm2, which are either comparable to or lower than the
lowest values that have been reported in the literature (see Table
S2).72 Moreover, all three of the systems had very narrow ASE
FWHMs due to the very narrow gain in bandwidth (See Figure S12
of the SI).73
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CONCLUSIONS
Here, we have demonstrated a new colloidal route for the prepaꢀ
ration of both allꢀinorganic and hybrid organicꢀinorganic APbX3
NCs (A= Cs, MA, FA and X= Cl, Br, I). Our approach is based on
the injection of benzoyl halides (as halide precursors) into a soluꢀ
tion of desired cations and proper ligands (oleylamine and oleic
acid) at a desired temperature. After the injection, a fast release of
halide ions occurs, which is followed by the nucleation and growth
of metal halide NCs. In all the cases, the resulting APbX3 NCs show
a high phase stability, a very good size distribution and excellent
optical properties. They exhibit a narrow PL emission and high
PLQYs, which are around 90% in the case of APbBr3 systems, 55%
in the case of APbI3 materials, and a record value of 65% in the case
if CsPbCl3 NCs. The optical quality of our materials was also reꢀ
flected by the low values of their ASE thresholds. The origin of such
improvements with regards to the stability and optical properties of
CsPbX3 NCs was tentatively ascribed to the formation of leadꢀ
halide terminated surfaces in which Cs ions are partially replaced
by oleylammonium ions. Indeed, the formation of such surfaces is
promoted by our synthetic conditions. To conclude, we believe
that the versatility of our synthetic approach could allow for the
future development of inorganic and organic metal halide NC
systems.
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ASSOCIATED CONTENT
Supporting Information. NMR characterization of benzoyl iodide, size
distribution of all the APbX3 NCs, Time Resolved Spectroscopic Charꢀ
acterization, control experiments on MAPbCl3 NCs, XPS analysis,
CsPb(Br/I)3 and CsPb(Cl/Br)3 NCs, anion exchange results, MAPꢀ
bBr3 NCs synthesized with lead acetate trihydrate, Overview of reportꢀ
ed ASE thresholds.
AUTHOR INFORMATION
Corresponding Author
luca.detrizio@iit.it, liberato.manna@iit.it
ACKNOWLEDGMENT
We acknowledge funding from the European Union under grant
agreements n. 614897 (ERC Grant TRANSꢀNANO).
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