Pressure Effects on Structure and Optical Properties in Cesium Lead Bromide Perovskite Nanocrystals

Article abstract of DOI:10.1021/jacs.7b05260

Metal halide perovskites (MHPs) are gaining increasing interest because of their extraordinary performance in optoelectronic devices and solar cells. However, developing an effective strategy for achieving the band-gap engineering of MHPs that will satisfy the practical applications remains a great challenge. In this study, high pressure is introduced to tailor the optical and structural properties of MHP-based cesium lead bromide nanocrystals (CsPbBr₃ NCs), which exhibit excellent thermodynamic stability. Both the pressure-dependent steady-state photoluminescence and absorption spectra experience a stark discontinuity at ～1.2 GPa, where an isostructural phase transformation regarding the Pbnm space group occurs. The physical origin points to the repulsive force impact due to the overlap between the valence electron charge clouds of neighboring layers. Simultaneous band-gap narrowing and carrier-lifetime prolongation of CsPbBr₃ trihalide perovskite NCs were also achieved as expected, which facilitates the broader solar spectrum absorption for photovoltaic applications. Note that the values of the phase change interval and band-gap red-shift of CsPbBr₃ nanowires are between those for CsPbBr₃ nanocubes and the corresponding bulk counterparts, which results from the unique geometrical morphology effect. First-principles calculations unravel that the band-gap engineering is governed by orbital interactions within the inorganic Pb-Br frame through structural modification. Changes of band structures are attributed to the synergistic effect of pressure-induced modulations of the Br-Pb bond length and Pb-Br-Pb bond angle for the PbBr₆ octahedral framework. Furthermore, the significant distortion of the lead-bromide octahedron to accommodate the Jahn-Teller effect at much higher pressure would eventually lead to a direct to indirect band-gap electronic transition. This study enables high pressure as a robust tool to control the structure and band gap of CsPbBr₃ NCs, thus providing insight into the microscopic physiochemical mechanism of these compressed MHP nanosystems.

Full text of DOI:10.1021/jacs.7b05260

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Article
Pressure Effects on Structure and Optical Properties
in Cesium Lead Bromide Perovskite Nanocrystals
Guanjun Xiao, Ye Cao, Guangyu Qi, Lingrui Wang, Chuang Liu,
Zhiwei Ma, Xinyi Yang, Yongming Sui, Weitao Zheng, and Bo Zou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05260 • Publication Date (Web): 06 Jul 2017
Downloaded from http://pubs.acs.org on July 6, 2017
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Pressure Effects on Structure and Optical Properties in Cesium Lead
Bromide Perovskite Nanocrystals
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Guanjun Xiao, Ye Cao, Guangyu Qi, Lingrui Wang, Chuang Liu, Zhiwei Ma, Xinyi Yang, Yong-
ming Sui, Weitao Zheng, and Bo Zou*
th
Dedicated to Prof. Guangtian Zou on the occasion of his 80 birthday
State Key Laboratory of Superhard Materials, College of Materials Science and Engineering, Jilin University, Chang-
chun 130012, P. R. China
KEYWORDS. High pressure, Diamond anvil cell, Perovskite, Bandgap engineering.
ABSTRACT: Metal halide perovskites (MHPs) are gaining increasing interest because of their extraordinary performance
in optoelectronic devices and solar cells. However, developing an effective strategy for achieving the bandgap engineering
of MHPs that will satisfy the practical applications remains a great challenge. In this study, high pressure is introduced to
tailor the optical and structural properties of MHP-based cesium lead bromide nanocrystals (CsPbBr NCs), which exhibit
3
excellent thermodynamic stability. Both the pressure-dependent steady-state photoluminescence and absorption spectra
experience a stark discontinuity at ~1.2 GPa, where an isostructural phase transformation regarding Pbnm space group
occurs. The physical origin points to the repulsive force impact due to the overlap between the valence electron charge
clouds of neighboring layers. Simultaneous bandgap narrowing and carrier-lifetime prolongation of CsPbBr trihalide per-
3
ovskite NCs were also achieved as expected, which facilitates the broader solar spectrum absorption for photovoltaic ap-
plications. Note that the values of phase change interval and bandgap redshift of CsPbBr nanowires are between the ones
3
for CsPbBr nanocubes and the corresponding bulk counterparts, which results from the unique geometrical morphology
3
effect. First-principles calculations unravel that the bandgap engineering is governed by orbital interactions within inor-
ganic Pb–Br frame through structural modification. Changes of band structures are attributed to the synergistic effect of
pressure-induced modulations of Br–Pb bond length and Pb–Br–Pb bond angle for PbBr octahedral framework. Further-
6
more, the significant distortion of lead–bromide octahedron to accommodate the Jahn–Teller effect at much higher pres-
sure would eventually lead to a direct to indirect bandgap electronic transition. This study enables high pressure as a ro-
bust tool to control the structure and bandgap of CsPbBr NCs, and thus providing an insight into the microscopic physi-
3
ochemical mechanism of those compressed MHP nanosystems.
unlock spatial confinement effects. This change allows
INTRODUCTION
large spectral tunability, thus facilitating the bandgap
Metal halide perovskites (MHPs) have come into consid-
erable attention for their applications in solar cells with
high power conversion efficiency. The intriguing optical
properties and the prospect for low-cost device fabrica-
tion promote the rapid development of MHPs. In particu-
lar, the recent success of organometallic halide perov-
skites in photovoltaic devices has triggered research activ-
ities on inorganic halide perovskite nanocrystals (NCs)
due to their good stability, ultrahigh photoluminescence
5-10
optimization for devices.
1
Pressure, as a unique thermodynamic variable, provides
a powerful means to study the structural and electronic
behaviors of MHPs materials. The loading of pressure by
diamond anvil cell (DAC) can lead to a close packing and
reduce the interatomic distances, which would profound-
11-14
ly modify electronic orbitals and bonding patterns.
In
recent years, high-pressure studies on MHP materials
15-19
have set off a new wave of upsurge.
Significant pro-
(
PL) quantum yield, and composition-dependent lumi-
2
-4
gress in the pressure response of methylammonium/
nescence with wide color gamut. The cesium lead bro-
formamidinium-based lead halide perovskites has recent-
mide (CsPbBr ) perovskites deliver outstanding optoelec-
3
20,21
ly been made by several independent groups.
However,
tronic properties and are significantly more stable than
organic–inorganic hybrid perovskite compounds. Re-
stricting the physical dimensions of the CsPbBr perov-
most high-pressure objects were confined to the organo-
metallic halide perovskites, whose instability and
3
skite crystallites to a nanometer region would further
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Figure 1. (a) TEM image and (b) high-resolution TEM
image of CsPbBr NCs before compression. (c) HAADF-
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STEM image and element mapping (Cs, Pb and Br) of the
Figure 2. Ambient PL spectra of as-prepared CsPbBr NCs
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as-prepared CsPbBr NCs.
3
and the corresponding photograph of CsPbBr NCs under
3
UV light.
sensitivity to the oxygen/water largely limit the actual
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2-28
photovoltaic applications.
EXPERIMENTAL SECTION
In this regard, we devoted the efforts to imposing pres-
sure on the all inorganic halide perovskite nanosystems,
Synthesis and Characterization of CsPbBr NCs. The
3
targeted CsPbBr NCs were synthesized according to the
3
CsPbBr NCs, which exhibit a relatively good thermody-
6
3
modulated colloidal chemistry method. In a typical syn-
namic stability. The pressure-driven structure and optical
properties of this material were characterized by the
combination of in situ time-resolved carrier dynamics,
angle dispersive synchrotron X-ray diffraction, and optical
spectroscopy measurements. All of the high-pressure ex-
perimental results indicated that CsPbBr3 NCs experi-
enced an isostructural phase transformation regarding
Pbnm space group at approximately 1.2 GPa. The physical
origin was dictated by the repulsive force impact, owing
to the overlap between valence electron charge clouds of
neighboring layers. Time-resolved PL decay indicates a
significant prolongation of carrier lifetime under high
pressure. We further investigated the geometrical mor-
phology effects on the values of phase change interval and
thesis, cesium carbonate (0.407 g, Cs CO , Sigma-Aldrich,
2
3
9
9.9%) and oleic acid (1.2 mL, OA, Sigma-Aldrich, 90%)
together with octadecene (20 mL, ODE, Sigma-Aldrich,
0%) were loaded in a 50 ml three-neck flask. After dry-
ing for 1 h at 120 °C, the solvent was heated under N to
9
2
1
50 °C until all CsCO powders were dissolved, indicating
3
the formation of Cs-oleate complex. Then, Cs-oleate was
cooled down to 100 °C before injection. Subsequently, 5
mL of ODE, 0.069 g of PbBr (Sigma-Aldrich, 98%), 0.5
2
mL of oleylamine (OLA, Sigma-Aldrich, 70%), and 0.5 mL
of OA were loaded in a 50 mL three-neck flask. The mix-
ture was heated to 120 °C with strong stirring and re-
mained for 1 h under N . After complete solubilization of
2
PbBr , the temperature was raised to 170 °C and Cs-oleate
2
bandgap redshift under high pressure among CsPbBr3
solution (0.5 mL) was quickly injected. About 20 s later,
the reaction mixture was extracted and quenched by tolu-
ene. The resultant mixture was isolated using toluene and
excess acetone by centrifuging for 5 min at 10,000 rpm.
The resulting samples were characterized by transmission
electron microscopy (TEM), high-resolution TEM, high-
angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) images, and elemental map-
ping performed on a JEM-2200FS with an emission gun
operating at 200 kV.
nanocubes, nanowires, and the corresponding bulk coun-
terparts. Theoretical computations predicted that orbital
interactions associated with the variations of Br–Pb bond
length and the shrinkage of Pb–Br–Pb bond angle within
lead–bromide octahedral motif were simultaneously re-
sponsible for the pressure-induced structural modulation
and bandgap shift. Upon further compression, the origi-
nal structure theoretically showed the tendency to an
indirect bandgap configuration, owing to the great distor-
tion of PbBr octahedron resulting from the Jahn-Teller
6
In situ High-Pressure Optical Experiments. All of
the in situ high-pressure experiments shown in this study
were carried out with a symmetric DAC apparatus. IIa-
type ultralow fluorescence diamonds with the culet size of
effect at much higher pressure. Our findings represented
the fundamental relationship between structural varia-
tions and optoelectronic properties of CsPbBr3 NCs,
thereby providing an insight into the microscopic physio-
chemical mechanism of MHP nanosystem at extremes.
3
00 μm were used. A T301 stainless steel gasket was pre-
indented by the diamonds and was drilled to generate a
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Figure 3. Pressure-dependent PL spectra of CsPbBr NCs.
Inset shows the schematic diagram of symmetric dia-
mond anvil cell used in high-pressure experiments. Right
panel represents the microphotographs in sample cham-
ber at selected pressures of 1 atm, 0.49, 0.78, and 1.21 GPa
irradiated by a 355-nm laser.
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1
00-µm-diameter cavity for loading the samples. Typically,
the prepared CsPbBr NCs was enclosed into the gasket
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hole together with a ruby ball for determining the actual
pressure through the standard ruby fluorescent tech-
29
nique. Therein, silicon oil with the viscosity of 10 cst was
utilized as the pressure transmitting medium (PTM),
which was purchased from the Dow Corning Corporation
Figure 4. (a) Lifetime of CsPbBr NCs as a function of
3
(
South Saginaw Road, Midland, MI). The high-pressure
evolution of steady-state PL spectra was collected utiliz-
ing modified spectrophotometer (Ocean Optics,
pressure up to 1.03 GPa by using a high-resolution nitro-
gen and dye laser with an excitation wavelength of 481
nm. (b) Time-resolved PL decay profiles at selected pres-
sures of 1 atm, 1.28 GPa, and decompression, as well as IRF
curve.
a
QE65000). A semiconductor laser with excitation wave-
length of 355 nm was employed for the entire fluores-
cence experiments. Note that all the parameters are fixed
completely over each high-pressure PL experiment to
avoid the effects of different excitation laser intensities
and luminous fluxes on the resulting PL intensity of in-
pressure, and then were integrated on the basis of FIT2D
program, yielding 1D intensity versus diffraction angle 2-
3
0
theta patterns. All the high-pressure experiments were
conducted at room temperature.
trinsic CsPbBr NCs. The pressure-dependent PL decay
3
curves were obtained with a high-resolution nitrogen and
dye laser (Photon Technology International, Inc.) as the
excitation source (λexc=481 nm, 800 ps pulse width, 10 Hz
repetition rate). The goodness of the decay fits exponen-
tial formula was estimated by considering the reduced chi
Computation. First-principles density functional theo-
ry (DFT) computations were performed using the plane-
wave pseudopotential technique with generalized gradi-
ent approximation (GGA), as implemented in the Cam-
2
31
square (χ ) value and the randomness of the weighted
bridge serial total energy package (CASTEP). A plane–
residuals. Meanwhile, the in situ high-pressure absorption
wave basis set with an energy cutoff of 290 eV was used,
and the Brillouin zone sampling for geometry optimiza-
tions was performed using a 5×5×3 Monkhorst-Pack k-
points mesh. The convergence thresholds were set at val-
spectra of CsPbBr NCs were recorded with an optical
3
fiber spectrometer (Ocean Optics, QE65000) using a deu-
terium-halogen light source. In situ high-pressure angle-
dispersive X-ray diffraction (ADXRD) patterns were ob-
tained with a wavelength of 0.6199 Å at beamline 15U1,
Shanghai Synchrotron Radiation Facility (SSRF), China. A
−6
−2
−1
ues of 5.0e eV/atom for energy and 10 eV Å for force.
The partial DOSs of isolated atoms were collected by
adopting the identical pseudopotentials of GGA that is
implemented in the Vienna ab initio simulation package
2
focused beam size of about 4×3.5 μm was adopted for
32
data collection. CeO was used as a standard sample for
(VASP).
2
the calibration of geometric parameters. The Bragg dif-
fraction rings were collected using the Mar-165 CCD de-
tector with an average acquisition time of 10 s for each
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Considering the outstanding optical properties of CsPbBr3
NCs, an in situ high-pressure PL measurement was per-
formed. Selected steady-state PL spectra with increasing
pressure and optical micrographs of the emission in the
sample chamber are shown in Figure 3. Inset shows the
schematic diagram of symmetric DAC used in high-pressure
experiments. Apparently, the PL peak displayed a redshift in
wavelength and gradually became broad with the increase in
pressure. Additionally, the PL intensity decreased sharply
upon compression to 1.21 GPa, after which the fluorescence
happened to an abrupt quenching that may be governed by
the structure-related change. Moreover, we plotted PL inten-
sity vs. pressure and wavelength vs. pressure of the synthe-
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sized CsPbBr NCs as Figure S3 in Supporting Information. It
3
is found that the PL intensity of CsPbBr NCs was decreased
3
linearly with increasing pressure, whereas the PL location
was linearly shifted to high wavelength under pressure. The
optical photographs, with the assistance of a microscope,
clearly reveal the darkening trend of the PL brightness in
Figure 5. Shift of bandgap energy with increasing pres-
sure. The shadow region indicates the phase transition
interval.
CsPbBr NCs, accompanied by the color changes from bright
3
green to dark green.
We further recorded the time-resolved PL decay dy-
namics of CsPbBr NCs to explore the exciton recombina-
3
RESULTS AND DISCUSSION
tion rate at a laser of 481 nm under pressure. The sche-
matic diagram of lifetime measurement is shown in Fig-
ure S4. In principle, the observed lifetime profiles are the
convolutional results of the real decay curves and the in-
strumental response function (IRF) to extract the time
constant. The width of the IRF is generally determined to
be less than 1 ns, thereby greatly facilitating the data fit-
ting. As external pressure approached about 1.03 GPa, the
carrier lifetime fitted with exponential formula under-
went an obvious increase by ~19% from 12.6 ns at ambi-
ence, implying a pressure-driven decrease in oscillator
strength that agrees well with the changes of PL intensity
The morphology and structure of the synthesized CsPbBr3
NCs were investigated using TEM and HRTEM character-
izations. As shown in Figure 1a and b, the samples before
compression exhibit a nanocube morphology with good
crystallinity. Elemental mapping indicates that the ele-
ments Cs, Pb and Br are homogeneously distributed
throughout the whole sample, which suggests high purity
of the final products (Figure 1c). The corresponding size
distribution of obtained CsPbBr NCs with Gauss fitting is
3
depicted in Figure S1. It shows that the CsPbBr NCs have
3
an average diameter of 11.7 nm, with the standard devia-
tion of 1.9 nm. Then, the surface molecular decoration of
(
Figure 4a). Detailed lifetime data and the corresponding
synthesized CsPbBr NCs was further characterized. The
3
errors of CsPbBr nanocrystals under different pressure
3
OLA molecule, CH (CH ) CH=CH(CH ) NH , which has a
3
2
7
2
8
2
points are shown in Table S1. The prolonged lifetime of
buckled chain length of ∼2.05 nm (Scheme S1), often acts
CsPbBr NCs is closely involved in surface-related trap
3
S1
as a surfactant in NC synthesis as reported elsewhere.
states and is crucial to the long carrier diffusion length of
these materials. This finding is highly beneficial to effi-
ciently promote the photon-generated carrier charge sep-
The Fourier transform infrared (FTIR) spectra of CsPbBr3
NCs were recorded with a Perkin-Elmer spectrometer
3
3,34
(
Spectrum One B). The FTIR spectra of CsPbBr NCs
3
aration in photovoltaic applications.
Typical time-
could be well assigned to the vibrational mode of the OLA
resolved PL decay profiles at selected pressures are shown
in Figure 4b. Due to the nearly quenched PL intensity at
molecule (Figure S2). In a word, the synthesized CsPbBr3
NCs are passivated by the organic ligand of OLA on the
surface after the washing treatment. In addition, as one of
star materials utilized as promising optoelectronic mate-
1
.28 GPa, the PL decay data of CsPbBr NCs were present-
3
ed as discrete points so as not to reveal the lifetime profile
by means of fitting. Upon decompression, the transient
PL decay profile was recovered, indicating the reversibil-
ity of the phase transition.
rials for CsPbBr NCs, there is an urgent need to charac-
3
terize the corresponding optical properties. Figure 2 de-
picts the PL spectrum of the as-prepared CsPbBr NCs
3
The pressure-dependent absorption spectra of CsPbBr3
NCs measured in a DAC apparatus are collected in Figure
S5. At ambient condition, the sample exhibited an appar-
ent absorbance at the band edge of 492 nm. Upon further
compression, the absorption experienced a gradual red-
shift below 1.19 GPa, followed by a stark blue jump until
2.25 GPa. The sudden change of the absorption spectra at
1.19 GPa was in accordance with the discontinuity in the
and the optical photograph under UV-light. It is found
that the PL maximum is centered at ~514 nm irradiated by
a 355-nm laser under ambient conditions. The line-width
of emission is as narrow as about 25 nm, which facilitates
the high-resolution display technology. Under the irradia-
tion of UV-light, the CsPbBr NC colloid displays a bright
3
green color.
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Figure 6. (a) Representative in situ ADXRD patterns of CsPbBr NCs with the presence of silicon oil as PTM during the high-
3
pressure experiments. (b) Refinements of the experimental (red fork), simulated (black profile), and difference (black line)
ADXRD patterns at 1 atm, 1.54 GPa, and quenched phase upon completely releasing the pressure to ambient conditions. Therein,
blue vertical markers indicate the corresponding Bragg reflections.
PL spectra. The consistent variations of both the absorp-
tion and PL spectra should be correlated with the onset of
in this study. The pressure-dependent PL spectra of CsP-
bBr bulk materials underwent an initial redshift and then
3
phase transformation in CsPbBr NCs under high pressure.
a blueshift until the PL intensity was vanished with fur-
ther compression. This trend in PL spectra is in accord-
ance with the variations of absorption as a function of
pressure. However, the blueshift of PL spectra in present
3
Upon the complete release of pressure, the absorption
recovered to the initial position, suggesting the reversibil-
ity of the pressure-driven phase transition. Detailed evo-
lution of bandgap energy for CsPbBr NCs with increasing
CsPbBr nanocubes was not observed, which should be
3
3
pressure is displayed in Figure 5. At relatively low pres-
sure, the electronic band exhibited a linear narrowing by
mainly attributed to the short phase change interval of
CsPbBr nanocubes with high surface energy.
3
0
.11 eV from the initial 2.52 eV. With further compression,
Likewise, we further investigated the morphology effect
of the compressed CsPbBr3 NCs (nanocubes vs. nan-
owires). In terms of the recent report by Yang et al., we
the rate of the photon energy presented a sharp increase
between 1.19 and 1.45 GPa, where the pressure range of
0
8
.26 GPa should be ascribed to the phase change interval.
successfully synthesized high-crystallinity CsPbBr nan-
3
Thereafter, the bandgap energy underwent a successive
increase up to 2.98 eV. Moreover, the bandgaps at select-
ed pressures are given through the extrapolation of the
linear region to the energy axis intercept in terms of Tauc
plot (Figure S6).
owires, as shown in Figure S9. The obtained CsPbBr nan-
3
owires possess an average diameter of 12 nm, which is
close to the dimension of the aforementioned CsPbBr3
nanocubes. To make a comparison, we conducted the
pressure-dependent UV absorption and PL spectra in the
Thus far, high-pressure modulation of energy gap in
obtained CsPbBr nanowires (Figure S10 and S11). It is
3
CsPbBr bulk materials has not yet been reported. We
here carried out the pressure response of UV absorption
found that the absorption and PL spectra underwent a
similar trend with bulk materials as pressure increased.
3
and PL spectra in CsPbBr bulk materials in order to make
The absorption spectrum of CsPbBr nanowires was ini-
3
3
a comparison. As shown in Figure S7 and S8, the absorp-
tially redshifted upon compression to 1.22 GPa. Thereafter,
a blueshift occurred until 1.80 GPa, at which a new phase
peak appeared denoted with asterisk. The phase change
interval of 0.58 GPa ranging from 1.22 GPa to 1.80 GPa of
tion spectrum of CsPbBr bulk materials was initially red-
3
shifted upon compression to 1.21 GPa. Thereafter, it exhib-
ited a blueshift to 2.03 GPa, at which a new phase peak
appeared denoted with asterisk. The phase change inter-
val of 0.82 GPa ranging from 1.21 GPa to 2.03 GPa is signif-
CsPbBr nanowires is larger than that (0.26 GPa) of CsP-
3
bBr NCs, but smaller than that (0.82 GPa) of bulk mate-
3
icantly larger than that (0.26 GPa) of CsPbBr NCs. On the
rials. Whereas, the bandgap redshift of 0.05 eV in CsPbBr3
3
contrary, the bandgap redshift of 0.02 eV is far less than
nanowires is less than the CsPbBr nanocubes
3
the corresponding CsPbBr nanocubes (0.11 eV) presented
3
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ergy level. In this study, more comprehensive techniques,
including in situ high-pressure-absorption, -PL, -lifetime,
and -ADXRD measurements, were used to investigate the
pressure-induced bandgap and structure evolutions in
CsPbBr nanocrystals. Therein, silicon oil with the viscosi-
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ty of 10 cst was utilized as the PTM to provide the perfect
hydrostaticity.
In order to clarify the correlation between the optical
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properties and structure of CsPbBr NCs in this study, in
3
situ ADXRD experiments were carried out to provide
more information about the crystal structure. Although
the changes of PL and absorption spectra were so obvious,
the ADXRD patterns over pressures were hardly altered,
except for a normal shift to higher diffraction angles.
When the pressure reached roughly 2.09 GPa, only a few
broad diffraction peaks were detected, indicating that
crystallinity is becoming worse and CsPbBr NCs have the
3
tendency to be amorphous at higher pressures. The over-
all observation of the ADXRD experiments indicated that
CsPbBr NCs displayed a regular reduction of the lattice
3
parameters and did not experience a distinct structural
variation during the whole compression up to 2.09 GPa.
The refinements of the selected ADXRD patterns were
depicted in Figure 6. It is observed that the refinement of
the ADXRD pattern at 1.54 GPa matches the experimental
Figure 7. Pressure dependence of the unit cell volume of
CsPbBr3 NCs fitted by third-order Birch−Murnaghan
equation.
(
0.11 eV), but greater than the corresponding bulk coun-
terparts (0.02 eV). The pressure-dependent PL spectra of
CsPbBr nanowires experienced a similar change as com-
3
data points very well (Rwp = 0.44%, R = 0.21%). This ena-
p
pared with the CsPbBr bulk materials under high pres-
3
bles us to unambiguously determine the high-pressure
phase to be orthorhombic with Pbnm space group, pos-
sessing the same crystallographic system at 1 atm. Moreo-
ver, the corresponding lattice constants at 1.54 GPa are a =
sure. Note that the values of phase change interval and
bandgap redshift of CsPbBr nanowires are between the
3
ones for CsPbBr nanocubes and the corresponding bulk
3
counterparts, respectively. Obviously, the anisotropic
7.96 Å, b = 8.09 Å, and c = 11.52 Å. Note that the ortho-
wire-like morphology of CsPbBr nanowires has a great
3
rhombic crystal structure could be persisted when the
pressure was released to the ambient conditions.
impact on the pressure-induced variations. The unique
geometrical shape of nanowires with anisotropic dimen-
sions should be responsible for the phenomena, owing to
the nanosized diameter and the length up to micrometers
analogous to bulk size.
To further analyze the structural evolution, the normal-
ized lattice constants of CsPbBr NCs as a function of
3
pressure up to 2.25 GPa were elucidated (Figure S12). It
was found that the pressure response of b and c axes for
As compared with the recent reports on pressure-
CsPbBr NCs was similar, but the compressibility of a axis
3
induced fusion of CsPbBr superlattices by Nagaoka et
3
was more sensitive to the pressure compared with b and c
axes. Meanwhile, the compressed rate happened to be a
turning point for b axis at above 1.34 GPa. Furthermore,
the experimental pressure–volume (P–V) data (Figure 7)
ranging from ambience to 2.25 GPa were fitted utilizing
the following third-order Birch–Murnaghan equation of
state:
1
9
al., a completely distinct result was observed. Nagaoka et
al. reported a blueshift in PL wavelength with pressure in
CsPbBr superlattices, followed by a shift to longer wave-
3
lengths. Whereas, we observed an opposite trend with
increasing pressure in our high-pressure experiments.
The inflection point of PL peak at 0.4 GPa, as described by
Nagaoka et al., was attributed to the pure orthorhombic
phase transition from the initial mixture including cubic
and orthorhombic phases. Our initial structure was pure
orthorhombic phase, which is recently corroborated by
Brutchey et al. entitled “On the crystal structure of colloi-
ꢊ
ꢋ
ꢍ
ꢋ
ꢓ
ꢋ
3ꢅꢆ ꢂꢆ
ꢂꢆ
3
ꢂꢆ
′
ꢀ
ꢁꢂꢃ ꢄ
ꢇꢈ ꢉ ꢌ ꢈ ꢉ ꢎ ꢏ1 ꢐ ꢑꢅꢆ ꢌ 4ꢒ ꢇꢈ ꢉ ꢌ 1ꢎꢔ
2
ꢂ
ꢂ
4
ꢂ
where V is the zero-pressure volume, B is the bulk mod-
0
0
3
5
ulus at ambient pressure, and B ′ is a parameter for
dally prepared CsPbBr quantum dots” . The work by
0
3
pressure derivative. No further discontinuities were ap-
parent in the P–V profile over the whole compression.
Nevertheless, a significant change could be observed in
compressibility between 1.0 and 1.3 GPa, which is well
agreed with the saltation of the absorption and PL spectra.
Since the isostructural phase transitions are usually con-
sidered to be originating from the electronic structural
Nagaoka et al. was mainly focused on the pressure-
sintered large two-dimensional nanoplatelets from the
initial CsPbBr superlattices by the non-hydrostatic pres-
3
sure. Thus, the deviatoric stress effect resulting from the
anisotropic pressure gradient should be considered.
Therein, only pressure-dependent PL spectra were given
without deep understanding from the view of atomic en-
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Figure 8. Total and partial density of states (DOSs) of isolated atoms for Cs, Pb, and Br atoms at 1 atm (a, b) 2 GPa (c, d), and 5
GPa (e. f). The transverse dashed line represents the Fermi level E . Right panels of (b, d, and f) represent the corresponding
F
enlarged views in the CBM region.
change in the matter, thus the as-prepared CsPbBr NCs
ical energy band structures and density of states of CsP-
3
16,36
do undergo an isostructural phase transition.
The
bBr at selected pressures of 1 atm, 2 GPa, and 5 GPa are
3
physical origin points to the repulsive force impact be-
shown in Figure S17. The initial direct bandgap energy of
2.14 eV decreased to 2.04 eV as the pressure approached
to 2 GPa. After which bandgap energy then exhibited an
increase to 2.31 eV until 5 GPa. As the theoretical calcula-
tions were based on the corresponding bulk materials at
zero Kelvin and vacuum. Hence, different pressure points
are normally observed between the experiments and
computations (Figure S14-S15). As is known, the bandgap
was exclusively distinguished by the separation of con-
duction band minimum (CBM) and valence band maxi-
mum (VBM). Partial density of states, as shown in Figure
cause of the overlap between the valence electron charge
37
clouds of the neighboring layers. With B ′ fixed as 4,
0
the isothermal bulk modulus B for phase II of CsPbBr
0
3
NCs was estimated as 42.8(8) GPa, which is much larger
than that of phase I (17.7(4) GPa). This finding indicates
the highly difficult compressibility toward phase II.
Moreover, the morphology and crystallinity of CsPbBr3
NCs did not exhibit a distinct change before and after
compressions, which could be readily deduced from the
TEM and high-resolution TEM images, as shown in Figure
S13.
8, suggested that the CsPbBr perovskites exhibited an
3
inverted band structure, since the VBM was identified as
the antibonding hybridization between the Pb 6s and Br
First-principles calculations on CsPbBr were employed
3
to assess the response of the electronic structure to the
external pressure. Figure S16 depicts the simulated
bandgap energy with the increase of pressure. The calcu-
lated results in bandgap energy matched well with the
experiments. Energies for both of them have the tendency
to decrease in the low-pressure region, followed by the
increase during the high-pressure range. Detailed theoret-
4
p orbitals, whereas the CBM state was governed by the
orbital interactions between Pb 6p and Br 4s orbitals in
4–
the [PbBr6] octahedral network. Note that the VBM state
has almost the Br 4p character, owing to
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quence, the optical properties are inevitably influenced by
the redefinition of boundary conditions for the electronic
wave functions owing to pressure-driven structure chang-
4
1
es. The reduction of bond lengths played the dominant
role in the decrease of bandgap energy. At above 2 GPa,
the decreasing rate with pressure for all the Pb–Br bond
lengths became slow, even presenting an apparent rising
tendency for length 3 (Figure S20b). Meanwhile, an initial
rise in the bond angle 2 was observed, then a decline in
this angle occurred afterwards. In stark contrast, the bond
angle 1 throughout exhibited a significant drop, which
contributed to the widened electronic band (Figure S20c).
Upon further compression, the compound eventually
evolved into an indirect bandgap structure as a result of
the considerable distortion of the more compact octahe-
dral skeleton to accommodate the Jahn–Teller effect
(stage III in Figure 9). The results highly corroborate the
strong relations between the band conformation and the
0
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0
1
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0
Figure 9. Schematic illustrations with respect to polyhe-
dral views of crystal structures of CsPbBr perovskites
3
under high pressure.
the higher energy level of the Br 4p orbital compared with
that of the Pb 6s state. While the CBM was almost con-
tributed from the Pb 6p state, because the Pb 6p orbital
possessed a much greater energy level than Br 4s state.
crystal structure of CsPbBr
perovskites.
3
When the pressure was applied to CsPbBr perovskites,
3
CONCLUSIONS
the CBM was mostly a nonbonding localized state of Pb
6p orbitals, which is less sensitive to external pressure
stimulus. Therefore, the bandgap evolution of CsPbBr3
perovskites was initially determined by the change of
The pressure-engineered structure and optical properties
of CsPbBr NCs were systematically investigated by means
3
of DAC apparatus. Combining the ADXRD patterns with
in situ absorption, PL, and carrier lifetime against pres-
3
8
VBM when pressure is applied. With the increase of
pressure, the enhanced orbital coupling between the Pb
6s and Br 4p states pushed up the VBM due to its anti-
bonding character, which explains the redshift of the
sures, the CsPbBr NCs indeed experienced an isostruc-
3
tural phase transition. Simultaneous bandgap narrowing
and carrier-lifetime prolongation of CsPbBr3 trihalide
perovskites NCs were also achieved as expected. The
unique geometrical morphology effect was responsible for
the different values of phase change interval and bandgap
bandgap with pressure. As the CsPbBr trihalide perov-
3
skite was subjected to pressure beyond 2 GPa, the CBM
happened to be dominated by the strong coupling of Pb
6p and Br 4p orbitals. Accordingly, the blue jump can be
interpreted from the significant shift to higher energy
level of the CBM by the exertion of pressure, thereby lead-
ing to the widened bandgap. When much higher pressure
of 7 GPa was applied, an electronic transition to indirect
bandgap was observed resulting from the VBM movement
from the G point to the X point (Figure S18), which should
be greatly correlated with the amorphous state of the
redshift among CsPbBr nanocubes, nanowires and the
3
corresponding bulk counterparts. First-principles calcula-
tions demonstrated that the variations of bandgaps were
invoked to orbital interactions associated with the syner-
gistic effects of the pressure-induced variations of Br–Pb
bond length and Pb–Br–Pb bond angle within the PbBr6
octahedral network. Furthermore, the significant defor-
mation of lead–bromide octahedron resulting from the
Jahn–Teller effect at much higher pressure would ulti-
mately give rise to a direct to indirect bandgap electronic
transition. These findings provide an insight into the mi-
croscopic physiochemical mechanism of those com-
pressed MHP nanosystems, based on the tuning of struc-
CsPbBr that appeared in the ADXRD patterns. Note that
3
the indirect bandgap features of solar absorbers are
known to be beneficial for low carrier recombination rates
3
9,40
and thus long minority carrier diffusion lengths.
Alt-
hough it was unable to determine the indirect bandgap
nature from the present absorption spectrum at 7 GPa
(Figure S19), our calculation indeed provided a possibility
ture and bandgaps of CsPbBr NCs under high pressure.
3
about the electronic structure evolution of CsPbBr perov-
3
ASSOCIATED CONTENT
skites at considerably higher pressure.
Supporting Information. In situ high-pressure absorption
spectra, PL lifetime profiles, Pressure-dependent lattice con-
stants, TEM and high-resolution TEM images before and
after compression, Simulated band structures with increasing
pressure, Bond lengths and angles within lead–bromide oc-
tahedron as a function of pressure. This material is available
free of charge via the Internet at http://pubs.acs.org.
For the inorganic trihalide perovskite of CsPbBr , the
3
divalent metal Pb cation is octahedrally coordinated by
halide Br anion (Figure 9). There are three non-equivalent
Pb–Br bonds and two non-equivalent Pb–Br–Pb angles
within the PbBr octahedral framework for orthorhombic
6
CsPbBr crystal, as shown in Figure S20a. Before 2 GPa, all
3
of the bond lengths and angles were shortened under
high pressure, thereby resulting in a condensed lead–
bromide octahedron (stage II in Figure 9). As a conse-
AUTHOR INFORMATION
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Corresponding Author
E-mail: zoubo@jlu.edu.cn.
(15) Jaffe, A.; Lin, Y.; Beavers, C. M.; Voss, J.; Mao, W. L.;
Karunadasa, H. I. ACS Cent. Sci. 2016, 2, 201.
*
(16) Jaffe, A.; Lin, Y.; Mao, W. L.; Karunadasa, H. I. J. Am.
Chem. Soc. 2015, 137, 1673.
Notes
Any additional relevant notes should be placed here.
(17) Ou, T.; Yan, J.; Xiao, C.; Shen, W.; Liu, C.; Liu, X.; Han,
Y.; Ma, Y.; Gao, C. Nanoscale 2016, 8, 11426.
ACKNOWLEDGMENT
(18) Lü, X.; Wang, Y.; Stoumpos, C. C.; Hu, Q.; Guo, X.;
Authors gratefully acknowledge funding supports from the
National Natural Science Foundation of China (Nos.
Chen, H.; Yang, L.; Smith, J. S.; Yang, W.; Zhao, Y.; Xu, H.;
Kanatzidis, M. G.; Jia, Q. Adv. Mater. 2016, 28, 8663.
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21673100, 11404135, 11504126 and 91227202), Chang Jiang
Scholars Program (No. T2016051), the China Postdoctoral
Science Foundation (Nos. 2015T80295 and 2014M550171),
Changbai Mountain Scholars Program (No. 2013007), and
Jilin Provincial Science & Technology Development Program
(No. 20150520087JH). The work concerning in situ ADXRD
measurements was performed at beamline 15U1, Shanghai
Synchrotron Radiation Facility (SSRF).
(19) Nagaoka, Y.; Hills-Kimball, K.; Tan, R.; Li, R.; Wang, Z.;
Chen, O. Adv. Mater. 2017, 29, 1606666.
(20) Wang, Y.; Lü, X.; Yang, W.; Wen, T.; Yang, L.; Ren, X.;
Wang, L.; Lin, Z.; Zhao, Y. J. Am. Chem. Soc. 2015, 137, 11144.
(21) Wang, L.; Wang, K.; Zou, B. J. Phys. Chem. Lett. 2016, 7,
2556.
(22) Wang, L.; Wang, K.; Xiao, G.; Zeng, Q.; Zou, B. J. Phys.
Chem. Lett. 2016, 7, 5273.
23) Li, Q.; Li, S.; Wang, K.; Quan, Z.; Meng, Y.; Zou, B. J.
Phys. Chem. Lett. 2017, 8, 500.
24) Swainson, I. P.; Tucker, M. G.; Wilson, D. J.; Winkler,
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Journal of the American Chemical Society
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ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 21
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3x49mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 13 of 21
Journal of the American Chemical Society
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2x64mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
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57x39mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 15 of 21
Journal of the American Chemical Society
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38x231mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
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8x56mm (300 x 300 DPI)
ACS Paragon Plus Environment

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Journal of the American Chemical Society
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0x50mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
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3x65mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 19 of 21
Journal of the American Chemical Society
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35x120mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
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5x41mm (300 x 300 DPI)
ACS Paragon Plus Environment

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37x14mm (300 x 300 DPI)
ACS Paragon Plus Environment