Tunable fluorescence and optical nonlinearities of all inorganic colloidal cesium lead halide perovskite nanocrystals

Article abstract of DOI:10.1016/j.jallcom.2017.06.034

The formation of cesium lead halide perovskite nanocrystals are very effortless due to its extremely fast reaction kinetics. Herein, we present simple hot-inject approach and anion exchange reaction to fabricate all inorganic colloidal CsPbX₃ perovskite nanocrystals (NCs). The colloidal CsPbX₃ NCs with controlling-size exhibit the characteristics of the high luminescent intensity, superior quantum yields (QY, above 69%) and the narrow-band emission tuning, the bandgap energies and emission spectra are readily tunable over the entire visible spectral region of 425–710 nm. Moreover, the third-order nonlinear optical properties of all inorganic colloidal perovskite nanocrystals (CsPbCl₃, CsPbBr₃, CsPbI₃) were measured using the Z-scan technique, indicating excellent third-order optical nonlinearities. Nonlinear optical susceptibilities reaching to 3.78 × 10^?11 esu for CsPbBr₃ NCs, figure of merit is achieving above 5 for CsPbI₃ NCs. These interesting results demonstrate that the as-prepared all inorganic colloidal CsPbX₃ perovskite nanocrystals are expected to hold great promise for application on nonlinear optoelectronic devices.

Full text of DOI:10.1016/j.jallcom.2017.06.034

Accepted Manuscript
Tunable fluorescence and optical nonlinearities of all inorganic colloidal cesium lead
halide perovskite nanocrystals
Sijin Liu, Guixiang Chen, Yunyun Huang, Sai Lin, Yijun Zhang, Meiling He, Weidong
Xiang, Xiaojuan Liang
PII:
S0925-8388(17)32010-8
10.1016/j.jallcom.2017.06.034
JALCOM 42102
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 24 February 2017
Revised Date: 6 May 2017
Accepted Date: 4 June 2017
Please cite this article as: S. Liu, G. Chen, Y. Huang, S. Lin, Y. Zhang, M. He, W. Xiang, X. Liang,
Tunable fluorescence and optical nonlinearities of all inorganic colloidal cesium lead halide perovskite
nanocrystals, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.034.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Tunable fluorescence and optical nonlinearities of all inorganic colloidal cesium lead
halide perovskite nanocrystals
ꢀ
ꢀ
Sijin Liu, Guixiang Chen, Yunyun Huang, Sai Lin, Yijun Zhang, Meiling He, Weidong Xiang , Xiaojuan Liang
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, People’s Republic of China.
Abstract
The formation of cesium lead halide perovskite nanocrystals are very effortless due to its extremely fast reaction kinetics.
Herein, we present simple hot-inject approach and anion exchange reaction to fabricate all inorganic colloidal CsPbX3
perovskite nanocrystals (NCs). The colloidal CsPbX NCs with controlling-size exhibit the characteristics of the high
3
luminescent intensity, superior quantum yields (QY, above 69%) and the narrow-band emission tuning, the bandgap energies
and emission spectra are readily tunable over the entire visible spectral region of 425-710 nm. Moreover, the third-order
nonlinear optical properties of all inorganic colloidal perovskite nanocrystals (CsPbCl , CsPbBr , CsPbI ) were measured using
3
3
3
the Z-scan technique, indicating excellent third-order optical nonlinearities. Nonlinear optical susceptibilities reaching to 3.78
-11
×
10 esu for CsPbBr NCs, figure of merit is achieving above 5 for CsPbI NCs. These interesting results demonstrate that
3 3
the as-prepared all inorganic colloidal CsPbX perovskite nanocrystals are expected to hold great promise for application on
3
nonlinear optoelectronic devices.
Keywords: Cesium lead halide, nanocrystal, tunable fluorescence, optical nonlinearity, Z-scan technique
1
.
Introduction
The metal halide perovskite NCs have presented superior performance in the fields of fluorescent light [1,2,3], polarized
emission [4], photodetector [5,6], LED [7-10] and laser [11-14]. This arouses the interest in the comprehension of the optical
characters of these materials. Previous works have demonstrated that the photoluminescence (PL) in CsPbX (X being either Cl,
3
Br, I or a combination of both) NCs, which show that the band gap energies and emission spectra of perovskite NCs are readily
tunable over the whole visible spectra region [2,14]. The PL of CsPbX NCs are characterized by strong light absorption,
3
narrow emission line widths, high QY, and short radiative lifetimes [15, 16, 17]. Kovalenko and the co-workers firstly reported

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a new synthetic process to fabricate all inorganic colloidal CsPbX NCs last year, they obtained the samples by reacting
3
Cs-oleate with Pb-halide in a high boiling solvent at 140-200 ꢁ [2]. After that, on account of the fast kinetics of CsPbX3
perovskite nanoparticle growth, different composite methods of CsPbX NCs have been used, such as anion exchange reactions
3
and supersaturated recrystallization at room temperature [18, 19, 20]. Owing to the relative moderation and accuracy, the
hot-injection approach and anion exchange reaction are effective methods to prepare CsPbX NCs.
3
Recently, people found that the lead halide perovskites show great performance in nonlinear optics [21, 22]. The
researches of nonlinear optical materials are mainly concentrating on amorphous glasses [23], organic substances and some
uncommon materials [24], such as inert gases [25], alkali metal atomic vapor and liquid crystal [26,27]. In contrast with the
traditional nonlinear materials, halide anions in metal lead tri-halide perovskite NCs can be easily extracted and exchanged
with another ones [5]. Basanth and the co-workers systematically studied on the nonlinear optical properties of
organic-inorganic (OI) lead halide perovskites [28], which indicated the OI perovskites have large nonlinear refractive index
and saturable absorption properties. This study identified the OI lead halide perovskites have promising applications in the
field of nonlinear optics in addition to the numerous investigation about solar cells and the optical modulator. Wang and the
co-workers compared the nonlinear optical properties of colloidal CH NH PbBr and CsPbBr quantum dots, and they
3
3
3
3
indicated that OI lead bromine presents relative high nonlinear optical susceptibilities than CsPbBr because of photoinduced
3
orientation effects [29]. Whereas, although both OI and all-inorganic materials possess a resemblant direct optical band gap,
CsPbBr demonstrates a higher thermal stability than MAPbBr for OI lead halide perovskite NCs should be kept dispersible in
3
3
a wide range of organic solvents [30,31], enhancing device lifetime in application. In general, these two families of colloidal
perovskites, OI lead halide NCs and all-inorganic CsPbX3 NCs, feature their respective advantages and properties.
Unfortunately, the devices of CsPbX are mainly focus on CsPbBr rather than CsPbCl or CsPbI , stimulating our desire to
3
3
3
3
systematically study the nonlinear characteristic in different halogen of CsPbX3.
In this paper, we present a simple approach to fabricate all-inorganic colloidal CsPbX NCs through the
3

hot-injection combining anion exchange reaction, and systematically investigate he component dependence of the halogen
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on the PL properties in all-inorganic colloidal CsPbX NCs. Moreover, the third-order nonlinear optical characteristics of
3
CsPbX3 NCs (CsPbCl , CsPbBr , CsPbI3) were measured via using the Z-scan technique, and the results exhibited that
3
3
CsPbBr NCs possess strong third-order nonlinearity susceptibility and high value of figure of merit. Accordingly, CsPbX
3
3
NCs materials are shown to be appropriate for all-optical switches and optical limiting devices.
2
2
2
. Experimental section
.1 Synthesis of CsPbX3.
.1.1 Preparation of Cs-oleate. The preparation process is similar to the previous report by Protesescu with slight revision here
[
2]. Cs CO (0.814 g, Aladdin, 99%), oleic acid (OA 2.5 mL, Aladdin, 85%) and octadecene (ODE 40 mL, Aladdin, 90%) were
2 3
added into a 100 mL 3-neck flask, dried for 1 h at 120 °C and then stored under N . Since Cs-oleate precipitates out of ODE at
2
room-temperature, it must be preheated to 100 °C before injection into reaction flask.
2
.1.2 Synthesis of CsPbX NCs. Dried ODE (5 mL) and different amounts of PbX (0.069 g, PbBr , ABCR, 98%; PbI , 0.087 g,
3 2 2 2
ABCR, 99.999%) were loaded into a 25 mL 3-neck flask and degassed at 120 °C for 1 h. Dried OA (0.5 mL) and oleylamine
(
OAm 0.5 mL, Aladdin, 85%) were injected at 120 °C under N flow. After complete solubilization of the PbBr or PbI salt,
2 2 2
the temperature was raised to 160 °C and Cs-oleate solution (0.4 mL of stock solution prepared as described above) was swiftly
injected and then (about 5 seconds) the reaction mixture was cooled down by the water bath. Importantly, it has been reported
that CsPbCl can be fabricated by CsPbBr using anion exchange reaction, neither did CsPbI [5, 20]. Chlorine anion exchange
3
3
3
reaction was carried out at room temperature using lithium chloride to exchange bromine.

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Fig. 1. The synthesis schematic of CsPbX NCs (X = Cl, Br, I).
3
2
.1.3 Isolation and purification of CsPbX NCs. The crude solution cooled down with water bath and the aggregated NCs were
3
separated by centrifuging, and the addition of tert-butanol (t-BuOH, Aladdin, 99%) to the crude solution (ODE:t-BuOH = 1:1
by volume) were found to be helpful for a complete precipitation. After centrifugation, the supernatant was discarded and the
precipitates were re-dispersed in hexane to form long-term stable colloidal solutions for the following measurements.
2
.2 Instrumentation.
To explore the crystalline structure of the colloidal perovskites had formed, X-ray diffraction (XRD) analysis was
performed on a Germany Bruker X-ray diffractometer (operation voltage and current were 40 kV and 40 mA) with Cu Kα
−1
radiation (λ = 1.5418 Å). The radiation at a scan was at the rate of 0.02° s in the range of 10−60°. Transmission electron
microscopy (TEM) images were carried out on a FEI Tecnai G2 F20 S-TWIN transmission electron microscope operating at an
acceleration voltage of 200 kV. The linear optical absorption of the NCs in the wavelength range of 200-800 nm was obtained
at room temperature by a PerkinElmer Lambda 750 UV-vis Spectrometer. Fluorescence spectra and PL QY were recorded with
a Horiba Jobin Yvon Fluromax-4P spectrophotometer equipped with absolute QY measurement apparatus. Time-resolved PL
lifetime measurements were carried out using a time-correlated single-photon counting (TCSPC) lifetime spectroscopy system
with a nanosecond-pulsed diode laser (nanoLED-370) as the single wavelength excitation light source. X-ray photoelectron
spectroscopy (XPS) measurement was obtained using AXI ULTRA DLD spectrometer with monochrome Al Ka as the

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excitation source. For nonlinear study, we used a femtosecond Q-switched laser (Spectra Physics Explorer), operating at a
wavelength of 787 nm in TEM00 operation mode. The repetition frequency was fixed at 1 kHz, pulse width of 396 fs and the
Z-scan with the operating power levels have been typically greater than 3 mW.
3
. Results and discussion
CsPbBr and CsPbI NCs are formed by direct hot-injection synthesis at 160 ꢁ, while CsPbCl are compounded through
3
3
3
exchanging bromine anion with chlorine anion in our work. Perovskite CsPbX NCs are recognized to crystallize in
3
orthorhombic, tetragonal, and cubic crystal, and colloidal CsPbX NCs crystallize in cubic crystal [2, 32] (Fig. 2a). Fig. 2b
3
reveals the XRD patterns of CsPbX NCs, which keep the similar profile in the range of 10-60° (2θ = 14.6° and 30.1°
3
correspond to diffractions formed at (100) and (200) planes for CsPbBr , 2θ = 15.4° and 30.9° for CsPbCl , 2θ = 14.1° and
3
3
2
1
8.6° for CsPbI ) owing to their same crystal structure. The XRD pattern of CsPbI brings out the PbI diffraction peaks (2θ =
3 3 2
2.8° and 26.7° correspond to diffractions formed at (001) and (011) planes) in Fig. 2b, but no extra phase is found in CsPbBr3
and CsPbCl NCs, attributing to that the lead triiodide perovskite has a higher reaction enthalpy [33]. This shows CsPbBr and
3
3
CsPbCl NCs possess more stable structure than CsPbI NCs, and they are to be preferred as a practical device into application.
3
3
Notably, it is also found that the diffraction peak at (100) and (200) planes slightly shift to the smaller 2θ angle from CsPbCl3
-
I
-
-
to CsPbI , which is attributed to the lattice expansion by the substitution of larger ion for the smaller ion (r > r Br > r Cl ),
3
indicating that halides ions redistributed congeneric throughout the NCs [11], according to Braggs equation.
nλ = 2dsinθ
(1)
Remarkably, some refraction peaks in the XRD patterns disappear in comparison with the corresponding standard phase
(
CsPbBr PDF#54-0752,), which is ascribed to the preferential orientation assembly of perovskite CsPbX causing to the
3, 3
appearance of some stronger peaks with the (100), (110) and (200) lattice plans [34].

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Fig. 2. Colloidal CsPbX NCs dispersed in hexane and their structural characterization. (a) Schematic of the cubic perovskite
3
lattice of CsPbBr . (b) XRD patterns for typical ternary and mixed-halide CsPbX NCs.
3
3
Fig. 3 shows the TEM and HR-TEM images of colloidal CsPbX NCs as well as the size distribution. Fig. 3a, 3c, and 3e
3
clearly show that the CsPbCl NCs are obtained via anion exchange reaction retain the parental construction CsPbBr and
3
3
CsPbI NCs. As TEM images depict, the growth durations yield cubic phase of CsPbX NCs, and the average side length are
3
3
2
8.7 nm, 21.4 nm, and 15.2 nm of CsPbI , CsPbBr , and CsPbCl NCs, respectively. The insets in Fig. 3a, 3c, and 3e highlight
3 3 3
the selected area electron diffraction (SAED), providing the diffractive images which correspond to the cubic crystal form of
CsPbX . To characterize the NCs microstructure from the crude reaction, the distinct lattice fringes are observed in the
3
HR-TEM images of Fig. 3b, 3d, and 3f, including (101) spaced by 4.0 Å, (002) spaced by 2.8 Å for CsPbCl , (110) spaced by
3
4
.3 Å, (200) spaced by 2.9 Å for CsPbBr , (120) spaced by 3.6 Å and (111) spaced by 4.4 Å for CsPbI . XRD pattern and TEM
3 3
images indicate that we have prepared the CsPbX NCs (X = Cl, Br, and I) successfully.
3

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Fig. 3. TEM images of the parent. (a) CsPbI NCs and (c) CsPbBr NCs and halide exchange synthesized (e) CsPbCl NCs.
3
3
3
Inset: Selected area electron diffraction pattern obtained from part a, c, e, respectively. HR-TEM images of (b) CsPbI NCs, (d)
3
CsPbBr , and (f) CsPbCl NCs.
3
3
In addition, XPS was carried out to further confirm the valence states of cesium, lead, and halogen of colloidal CsPbX3
NCs in hexane. Fig. 4 represents the survey scan XPS spectra of cesium, lead and halogen, respectively. No significant
differences are observed among those obtained spectra for the Cs 3d and Pb 4f. Fig. 4a shows the Cs 3d peak of CsPbX at
3
7
38.8 eV and 724.9 eV, corresponding to the Cs 3d5/2 and Cs 3d3/2 electron energy levels, respectively. The Pb 4f XPS peaks

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(
shown in Fig. 4b) appear as the doublet because of spin-orbit splitting for each chemical species present [35]. Interestingly,
the Pb 4f spectrum obtains for the as-received sample showed two main peaks at 137.4 eV and 142.2 eV (Pb 4f7/2 and Pb 4f5/2),
while the Pb 4f spectrum of CsPbCl presents two peaks (about 138.5 eV and 143.3 eV) locating at the nearby of the main
3
peaks. Fig. 4c, 4d, and 4e show the I 3d peaks of CsPbI at 619.9 eV and 631.4 eV, Br 3d peaks of CsPbBr at 67.1 eV and 67.5
3
3
eV, and Cl 2p peaks of CsPbCl at 197.4 eV and 198.9 eV, which correspond to the I 3d and I 3d3/2 electron energy levels, the
3
5/2
Br 3d5/2 and Br 3d3/2 electron energy levels, Cl 2p3/2 and Cl 2p1/2 electron energy levels, respectively. In general, XPS results
indicate that cesium, lead, and halogen maintain their valence states in CsPbX NCs. To sum up, on the analysis of the structure
3
and composition of colloidal NCs with the utility of XRD, TEM, and XPS, the main phases of the samples with cubic crystal
and valence state were confirmed to be CsPbX , and these results contribute to further optical properties of the CsPbX NCs.
3
3
Fig. 4. High-resolution cesium, lead, halogen XPS spectra: (a), (b) Cs 3d and Pb 4f high-resolution spectrum of CsPbCl3,
CsPbBr and CsPbI ; (c) Cl 2p, (d) Br 3d, and (e) I 3d high-resolution spectrum of CsPbCl , CsPbBr and CsPbI .
3
3
3
3
3
In order to demonstrate the excellent luminescent properties of CsPbX NCs, optical images of the CsPbX NCs samples
3
3
under ambient light and a 365 nm UV lamp are showed in Fig.5a, 5b, and PL spectra are presented in Fig. 5c. For the highly
luminescent colloidal NCs, CsPbX perovskite NCs present superior QY (above 69%) and narrow emission line widths of
3
1
3-30 nm with superior size tuning of the emission peaks. The PL emission spectra of the hybrid CsPbX (X = Cl, Br, I or a
3

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combination of both) NCs present emission peaks at the range of 425-710 nm, hence CsPbX NCs can produce saturated colors
3
by controlling the composition of halogen [2]. Moreover, the overall PL intensity and optical density of the NCs do not rise
anymore after 5 seconds, indicating that changing time cannot control the size in conventional batch syntheses [16]. In addition,
absorption spectra of CsPbX (X = Cl, Br, I) are showed in Fig. 6, consistent with the PL spectra.
3
Fig. 5. The samples of Color-tunable CsPbX NCs and wide PL spectra. (a),(b) Optical images of CsPbX NCs under ambient
3
3
light and a 365 nm UV lamp. (c) PL spectra of CsPbX NCs spanning the whole visible region with narrow emission
3
linewidths.
Fig. 6. Uv-vis absorption spectra of CsPbCl , CsPbBr , CsPbI .
3
3
3
The PL decay curves were measured by analyzing the PL dynamics from CsPbX NCs solution of hexane. Fig. 7 shows
3
the time-resolved PL spectrum of CsPbBr3−xCl and CsPbBr I NCs at the peak of emission wavelength, respectively. The
x
3−x x
decays are double-exponential components with time constants (T ) and respective amplitudes (K ) [15]. The
i
i
double-exponential decay function is calculated using equation (2), fitting accurately to the sum of double-exponential.

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2
(
K T
)
ꢂ
ꢂ
i
i
i
i
(2)
τ=
(
KiT
i
)
The CsPbBr NCs decay curve exhibits a decay time τ = 68.4 ns. For the CsPbBr I series, the decay time monotonically
3
3−x x
increases to a τ = 252 ns with the increasing of emission wavelength. Besides, for the CsPbBr3−xCl series, the decay time
x
decrease to τ = 4.67 ns with decreasing of emission wavelength when the PL emission peak at 496 nm, and the decay time
increases to τ = 111.9 ns when the PL emission peak at 456 nm. The results show that the decay time is sensitive to the sample
fabrication process. This phenomenon may be related to localized defects in the crystals [37], thus decay time may markedly
vary from sample to sample for the control of their imperfect fabrication of NCs [15].
Fig. 7. PL decay curves of the perovskite CsPbI Br series NCs and CsPbI Br series NCs as a typical sample in hexane
3
-x
x
3-x
x
measured using 370 nm excitation.
The energy band diagram of CsPbX was illustrated by Kondo et al [14]. Since they elucidated the one-electron energy
3
2+
−
states of CsPbX based on the molecular orbital (MO) theory applied to the octahedral Pb (X ) quasi-complex of O
h
3
6
symmetry, the prototype of the research on perovskite materials has gained great achievement. Aimed to evaluate the optical
energy gap, the optical absorption coefficient α(hν) was obtained using the Davis and Mott equation, as shown in equation (3)
[
36].
B(hυ-Eopt )m
hυ
(3)
αhυ =
( )

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B is a constant, hν presents the photon energy, and E is referred as the optical band gap. m is a constant, which represents the
opt
type of the optical transition. Fig. 8a illustrates the result as a function of the photon energy by measuring the curve of the
absorption coefficient. Furthermore, the straight lines are fitting curve to estimate the energy gap E . The discreet values of E
g
g
are 1.73 eV, 2.39 eV, and 2.78 eV for CsPbI , CsPbBr , and CsPbCl , respectively. The VBMs and CBMs of CsPbX (X = Cl,
3
3
3
3
Br, I) are estimated, and their energy levels with respect to vacuum are depicted in Fig. 8b. Moving from Cl to Br to I, the
VBM shifted prominently toward higher energies, while a smaller shift is observed for the CBM toward lower energies [37].
Obviously, the band alignment diagram that the change in the band gap E of the CsPbX compounds is mainly owing to the
g
3
conduction band move to higher energy, while the valence band energy remaining practically unchanged [38]. The energy gaps
demonstrate that the absorption of these perovskites are in the range of visible light.
2
Fig. 8. (a) The spectrum plotted as (αhν) -(hν), the straight line is fitting curve to estimate the energy gap Eg. (b) A schematic
energy-level diagram for CsPbX (X = Cl, Br, I).
3

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Fig. 9 represents the transmitted laser beam schematic divided by means of a beam splitter, providing the two different
experimental configurations. The results of Z-scan measurements shown in Fig. 10 are carried out to determine the nonlinear
refraction and absorption of the CsPbX perovskites NCs samples, corresponding to open aperture (OA) curves and closed
3
aperture (CA) curves. Ultraviolet-visible absorption spectra can be turned over the entire visible spectral region through
adjusting their composition, which are shown in Fig. 6. The Z-scan technique is a single excitation beam to survey the
nonlinear refractive index (NLR) and nonlinear absorption coefficient (NLA) of the materials [39]. The technique provides
sensitive and straightforward measurement of the sign and the values of the nonlinear refractive index (n ), the two-photon
0
absorption (TPA) coefficient (β), and the free carrier absorption coefficient (σ) [40].
Fig. 9. Experimental Z-scan setup: L1, L2, and L3, lenses; BS, beam splitter; S, sample; D1 and D2, photodetectors; A,
aperture.
To quantitatively measure the NLA and NLR of the CsPbX (X = Cl, Br, I) NCs, Fig. 10 exhibits the Z-scan response of
3
the solution of CsPbCl , CsPbBr , CsPbI NCs, corresponding the black dots displayed the experimental results and the red,
3
3
3
green, and blue solid lines fitted curves in the figure fit the theoretical values of the normalized transmittance. Fig. 10a, 10c,
and 10e demonstrate the OA of CsPbCl , CsPbBr , CsPbI NCs, and they all show symmetric relative to the focus (Z = 0)
3
3
3
where it had minimum transmittance, illustrating reverse saturable absorption would emerge in the three perovskite NCs.
Fig. 10b, 10d, and 10f display the CA of CsPbX NCs. The CA Z-scan curves with prefocal (Z < 0) minimum and the
3
postfocal (Z > 0) maximum indicated a self-focusing process and a positive sign of NLR index (γ > 0). To depict the intensity
of NLA and NLR, both β and γ of the perovskites NCs were calculated from the OA and CA Z-scan curves derived from the

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(3)
Z-scan theory [41]. The real and imaginary parts of the third-order nonlinear susceptibility χ are deduced according to the
parameters β and γ, as shown in equations (4) and (5) [42], and the results are shown in Table 1.
2
2
λεc n
0
(4)
(5)
lmχ(3) (esu) =
0
β(m/W)
4
π
ε
c2n 2
0 0
Reχ(3) (esu) =
γ(m /W )
π
λ, ε , c, and n are the wavelength of the laser light, permittivity of free space, speed of light, and linear refractive index of the
0
0
(3)
(3)
perovskite NCs, respectively. For CsPbCl and CsPbI , the value of Re χ is higher than Im χ , which imply nonlinear
3
3
refraction is the main third-order optical nonlinearity response in these two kinds of NCs, and they have potential application in
(3)
(3)
optical switches. But for CsPbBr , the value of Im χ is higher than Re χ , which implies nonlinear absorption is the main
3
nonlinear behavior of CsPbBr NCs, and it has potential application in optical limiting devices.
3
Fig. 10. Nonlinear absorption of CsPbCl , CsPbBr and CsPbI samples under irradiation of a 787 nm pulsed laser. OA (a)
3
3
3

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CsPbCl , (c) CsPbBr , (e) CsPbI NCs, and CA (b) CsPbCl , (d) CsPbBr , (f) CsPbI NCs. (Theoretical data in black squares
3
3
3
3
3
3
and fitting curves in solid line).
(3)
(3)
The total optical third-order nonlinear susceptibility χ is given by equation (6) [41], all the values of χ are listed in
Table 1. Obviously, the perovskites of colloidal CsPbCl , CsPbBr , CsPbI NCs possess similar third-order nonlinear properties.
3
3
3
(3 )
The third-order nonlinear absorption of the three kinds of NCs exhibit reverse saturable absorption and the values of χ reach
-11
-11
-11
2
.85 × 10 , 3.78 × 10 , and 3.55 × 10 esu in CsPbCl , CsPbBr , and CsPbI NCs, respectively. In addition, for most of
3 3 3
third-order nonlinear-based optical devices, nonlinear refraction is an essential parameter [43]. In our case, owing to the
ultrafast pulse of 396 fs (shorter than 500 fs), accumulative thermal effects would be limited, indicating that the optical
response is distinctly sensitive to the electronic ground state.
1
/ 2
2
2
χ(3)


Reχ(3)
lmχ(3)

(6)
(
esu
)
=
[
]
+
[
]



Moreover, figure of merit (F) is defined to evaluate the third-order nonlinear performance of materials at a certain
wavelength [44]. For the different composition of CsPbX , shown in table 1, the values of F reach 4.95, 1.85, 5.59,
3
demonstrating that CsPbX NCs have favorable nonlinear optical properties (the values of F are over 2 for CsPbCl and
3
3
CsPbI ). Importantly, CsPbCl presents a more stable structure than CsPbI , signifying CsPbCl may possess a broader prospect
3
3
3
3
for application.
Table 1. Third-order nonlinear optical parameters of the Three CsPbX at 767 nm with operating power levels of 3 mW.
3
Wavelength
2
(3)
sample
β (m/W)
γ (m /W)
Im χ (esu)
Re χ (esu)
χ (esu)
F
(
nm)
-13
-13
-13
-19
-11
-11
-11
CsPbCl3
CsPbBr3
CsPbI3
787
787
787
1.36 ×10
3.22 ×10
1.54 ×10
5.30 ×10
4.69 ×10
6.75 ×10
1.29×10
2.54×10
2.85 ×10
3.78 ×10
3.55 ×10
4.95
1.85
5.59
-19
-19
-11
-11
-11
-11
3.04×10
2.25×10
-11
-11
1.45×10
3.24×10
In conclusion, we have investigated the fluorescent characteristic and the nonlinear optical response of the all-inorganic

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perovskites CsPbX . Colloidal CsPbX nanocrystals present highly luminescent (X = Cl, Br, I, and mixed Cl/Br and Br/I
3
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systems) with bright (QY is above 69%), stable, spectrally narrow, and broadly tunable photoluminescence (425-710 nm with a
narrowing of FWHM to 13 nm-30 nm). On the basis of XRD and TEM analysis, CsPbX nanocrystals exhibit the regular shift
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of the peak in XRD and lattice distance differences in TEM, which attributed to the same crystal structure of CsPbCl , CsPbBr ,
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CsPbI and their anion-exchange compounds. In addition, the nonlinear optical properties of the colloidal CsPbX NCs were
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(3)
determined using Z-scan and the results indicated that the NCs had excellent third-order optical nonlinearities. The χ of the
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−11
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CsPbCl , CsPbBr , CsPbI were measured to be 2.85 × 10 , 3.78 × 10 , and 3.55 × 10 esu, and figure of merit reaching
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.95, 1.85, 5.59, respectively. Therefore, hybrid all-inorganic semiconductor perovskites CsPbX have favorable nonlinear
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optical properties for potential use in applications such as optical switching. Future studies with these novel NCs will
concentrate on increasing stability by changing tolerance factor (t) in order to apply CsPbX to optoelectronic applications such
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as light-emitting diodes, lasers, all optical switches and photodetectors.
Acknowledgment
The work was financially supported by National Nature Science Foundation of China (51672192, and 51472183)

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Highlights
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•
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CsPbX NCs were fabricated via hot-injection combining anion exchange reaction.
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The PL properties depended on halogen in CsPbX NCs on have been investigated.
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CsPbX NCs possess high value of figure of merit.
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CsPbX NCs is a candidate of nonlinear optoelectronic devices.
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