Electro-Optic Modulation in Hybrid Metal Halide Perovskites

Article abstract of DOI:10.1002/adma.201808336

Rapid and efficient conversion of electrical signals to optical signals is needed in telecommunications and data network interconnection. The linear electro-optic (EO) effect in noncentrosymmetric materials offers a pathway to such conversion. Conventional inorganic EO materials make on-chip integration challenging, while organic nonlinear molecules suffer from thermodynamic molecular disordering that decreases the EO coefficient of the material. It has been posited that hybrid metal halide perovskites could potentially combine the advantages of inorganic materials (stable crystal orientation) with those of organic materials (solution processing). Here, layered metal halide perovskites are reported and investigated for in-plane birefringence and linear electro-optic response. Phenylmethylammonium lead chloride (PMA₂PbCl₄) crystals are grown that exhibit a noncentrosymmetric space group. Birefringence measurements and Raman spectroscopy confirm optical and structural anisotropy in the material. By applying an electric field on the crystal surface, the linear EO effect in PMA₂PbCl₄ is reported and its EO coefficient is determined to be 1.40 pm V^?1. This is the first demonstration of this effect in hybrid metal halide perovskites, materials that feature both highly ordered crystalline structures and solution processability. The in-plane birefringence and electro-optic response reveal that layered perovskite crystals could be further explored for potential applications in polarizing optics and EO modulation.

Full text of DOI:10.1002/adma.201808336

COMMUNICATION
2
D Materials
www.advmat.de
Electro-Optic Modulation in Hybrid Metal Halide
Perovskites
Yuan Gao, Grant Walters, Ying Qin, Bin Chen, Yimeng Min, Ali Seifitokaldani, Bin Sun,
Petar Todorovic, Makhsud I. Saidaminov, Alan Lough, Sefaattin Tongay, Sjoerd Hoogland,
and Edward H. Sargent*
Optical anisotropy enables both linear
and nonlinear optical properties, enabling
Rapid and efficient conversion of electrical signals to optical signals is
needed in telecommunications and data network interconnection. The linear
electro-optic (EO) effect in noncentrosymmetric materials offers a pathway
to such conversion. Conventional inorganic EO materials make on-chip
integration challenging, while organic nonlinear molecules suffer from
thermodynamic molecular disordering that decreases the EO coefficient of
the material. It has been posited that hybrid metal halide perovskites could
potentially combine the advantages of inorganic materials (stable crystal ori-
entation) with those of organic materials (solution processing). Here, layered
metal halide perovskites are reported and investigated for in-plane bire-
fringence and linear electro-optic response. Phenylmethylammonium lead
applications such as polarizing optics, fre-
quency upconversion, and electro-optic
(
EO) modulation. EO modulation relies
on optical anisotropy and the changes
to it that occur under electric fields.
Optical anisotropy originates from a lack
of symmetry in the material’s structure:
when a material lacks inversion sym-
metry, second-order nonlinear optical
effects can occur. The linear EO effect, or
Pockels effect, is of use in optical modu-
lation for network interconnection and
telecommunications.
chloride (PMA PbCl ) crystals are grown that exhibit a noncentrosymmetric
2
4
space group. Birefringence measurements and Raman spectroscopy confirm
optical and structural anisotropy in the material. By applying an electric field
on the crystal surface, the linear EO effect in PMA PbCl is reported and its
EO coefficient is determined to be 1.40 pm V . This is the first demonstra-
tion of this effect in hybrid metal halide perovskites, materials that feature
both highly ordered crystalline structures and solution processability. The
in-plane birefringence and electro-optic response reveal that layered perov-
skite crystals could be further explored for potential applications in polar-
izing optics and EO modulation.
Silicon photonics offers the possibility
of condensing optical interconnects to
intra-/interchip length scales. However,
silicon-based EO modulators that exploit
the plasma dispersion effect suffer from
significant energy losses incurred in
changing the carrier density; and they also
require large device footprints, in light of a
weak index change, that stand in the way
2
4
−
1
[
1]
of high-speed operation.
The integration of high-performance
nonlinear optical materials within pho-
tonic waveguides is therefore desired. The
inorganic nonlinear crystals such as lithium niobate (LNO)
Dr. Y. Gao, Dr. G. Walters, Dr. B. Chen, Y. Min, Dr. A. Seifitokaldani,
Dr. B. Sun, P. Todorovic, Dr. M. I. Saidaminov, Dr. S. Hoogland,
Prof. E. H. Sargent
Department of Electrical and Computer Engineering
University of Toronto
and barium titanate (BTO) possess high phase transition tem-
[2,3]
peratures (above 120 °C)
and large coercive field strengths
5
−1 [2]
(
greater than 10 V m ); however, the integration of inorganic
nonlinear crystals relies on costly and complicated bonding or
[
4,5]
1
0 King’s College Road, Toronto, Ontario M5S 3G4, Canada
deposition processes (heterogeneous integration).
Organic
E-mail: ted.sargent@utoronto.ca
nonlinear optical molecules can be solution processed, and
therefore offer convenient integration with on-chip photonic
Y. Qin, Prof. S. Tongay
School for Engineering of Matter
Transport and Energy
Arizona State University
Tempe, AZ 85287, USA
[
6–8]
structures.
individually) have impressively high EO coefficients, lose
performance when they depole in light of thermodynamic
However, such organic molecules, which
(
[
9,10]
Dr. A. Lough
Department of Chemistry
University of Toronto
instabilities in ordered films of organic molecules.
The past decade has seen the rapid development of metal
halide perovskite optoelectronics, including solar cells, light
emitting diodes, lasers, and detectors. These materials are
highly crystalline and can be solution processed, enabling inte-
gration with photonic structures. The most widely studied metal
halide perovskites, which features the chemical structure APbX3
8
0 St. George Street, Toronto, Ontario M5S 3H6, Canada
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201808336.
DOI: 10.1002/adma.201808336
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+
+
+
(
A = CH NH , HC(NH ), Cs , and X = Cl, Br, I), have been
effect, the bandgap of the 2D perovskite is increased. As a result,
the perovskite is transparent over a broader spectral range, per-
mitting a wider window for the modulation wavelength.
3
3
2
[11,12]
studied for optical and electronic anisotropies.
Despite
density functional theory (DFT) predictions showing high
polarizations (14.03[ or 38 µC cm for CH NH PbI ) and
13]
[14]
−2
Here we investigate a layered hybrid metal halide perov-
skite, phenylmethylammonium lead chloride (PMA PbCl ),
3
3
3
[13]
noncentrosymmetric crystalline structures, only small polari-
zations (a few nC cm ) have been measured experimentally.
2
4
−2
[15]
which belongs to a noncentrosymmetric space group and has
an inherent polarization. The phase transition temperature of
This small polarization is likely due to dynamic rotation and
antiparallel alignment of the molecular cations.[ The absence
of global structural noncentrosymmetry has then lead to obser-
vations of only very weak second-order nonlinear optical effects:
for example, second harmonic generation (SHG) was seen from
15]
[22]
PMA PbCl crystals is 165 °C, higher than that of the inor-
2
4
[
2]
ganic nonlinear optical crystals BTO (123 °C) and also above
the typical glass-transition temperatures of organic nonlinear
optical molecules. Therefore, the linear EO effect from
PMA PbCl crystals is thermally stable, encouraging on-chip
[
10]
[16]
CH NH PbI to be less than 5% of that of quartz. Third-order
3
3
3
2
4
optical nonlinearities, which do not require noncentrosym-
and two-photon absorption,
have been well studied in hybrid metal halide perovskites. Many
integration. Motivated by the noncentrosymmetric structure
and high phase transition temperature, we studied the linear
and nonlinear optical properties of PMA PbCl . We employed
metry, such as the Kerr effect[
17,18]
[19]
2
4
layered metal chalcogenide perovskites possess noncentrosym-
angle-resolved Raman spectroscopy and the measurement of
the in-plane birefringence of PMA PbCl to confirm its struc-
metric structures;[
20,21]
however, they are usually prepared via
2
4
solid reaction instead of via solution processing. In hybrid perov-
tural anisotropy. We present the first account of linear electro-
optic modulation using a hybrid metal halide perovskite.
In order for a material to exhibit second-order nonlinear
skites with a ferroelectric phase, SHG attests to the polarity of
the crystals.[
22–24]
Other second-order nonlinear optical effects,
2
such as the linear EO effect, have yet to be studied.
optical properties (P ∝ E ), a lack of inversion symmetry is
Reducing the dimensionality of the bulk perovskite to layered
structures through the introduction of bulky ammonium-
terminated spacer cations brings additional compelling proper-
ties. The 2D network of corner-sharing octahedra also acts as
a scaffold that anchors and has the potential to align the long
aliphatic- or aromatic-ammonium-terminated cations. The col-
lective alignment of the bulky cations can introduce crystalline
noncentrosymmetry and induce spontaneous polarization in the
required. This then leads to nonvanishing tensor components
[
29]
of the second-order nonlinear optical susceptibility.
PMA PbCl crystal is a type of layered perovskite and has exhib-
ited ferroelectricity and piezoelectricity,
The
2
4
[
22,25]
indicating its non-
centrosymmetric nature. We adopted the temperature-lowering
method to grow PMA PbCl4 crystals (see the Experimental
2
Section). The as-grown crystals exhibit defined hexagonal
shapes with lateral sizes greater than 1 cm (see Figure S1 in
the Supporting Information). The purity of the phase was
confirmed by powder X-ray diffraction (Figure S2, Supporting
Information). Single crystal X-ray diffraction was conducted on
the synthesized crystals as well (Figure 1). The crystal structure
2
D perovskite, thereby allowing for observations of ferroelec-
[22,25–27]
tricity and piezoelectricity.
Moreover, the 2D perovskites
exhibit improved stability against moisture and heat compared
to their 3D counterparts.[ Due to the quantum confinement
28]
Figure 1. Crystal structure of PMA PbCl . The structure viewed along the a) a-axis, b) b-axis, and c) c-axis. A coherent off-centered shift of the Pb atoms
2
4
and alignment of the PMA manifests along the c-axis.
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displays a typical layered perovskite system, where an inorganic
As can be seen from Figure 2b, there are four maxima and four
minima. The transmitted power reaches a minimum when the
first polarizer is parallel and perpendicular to the optical axis
(0°, c-axis) and reaches a maximum when the first polarizer is
45° with respect to the optical axis. The birefringence measure-
ment, in turn, confirms the anisotropy in the crystal. Variation
of such birefringence with the electric field will be discussed in
the latter part of this paper.
PbCl network tethers PMA as the spacer cations.
6
As seen in Figure 1a, the lead atoms sit in the distorted octa-
hedrons formed by the chloride ions with anisotropic PbCl
bonds. The layer formed by the corner sharing PbCl6 octa-
hedra extends throughout the crystallographic bc-plane. Viewed
along the b-axis (Figure 1b), the Pb atoms (central coordinate:
(0.5, 0.5, 0.41))—the centers of negative charge—display
coherent off-centered displacement in the direction antiparallel
to the c-axis. The layer spacing PMA cations and the CN⋅⋅⋅Cl
hydrogen bonds (central coordinate of N: (0.5, 0.5, 0.33)) exist
with a collective tilting along the c-axis. Since there is a sepa-
ration between the centers of positive and negative charge, an
electric dipole moment forms in the unit cells and the addition
of these dipoles (aligned along the c-axis) gives rise to overall
spontaneous polarization in the crystal.[ In contrast, as shown
in Figure 1c, opposing dipole moments along the a- or b-axes
cancel out the global polarization in these directions.
The imaginary part of the refractive index (κ) was deter-
mined by ellipsometry. κ, if it is positive, is the extinction
coefficient, which characterizes the optical absorption of light
in the material. The PMA PbCl crystal is a direct-bandgap
2
4
[
22]
semiconductor with a band gap of 3.65 eV (Figure S5, Sup-
porting Information). The absorption is extremely low in the
PMA PbCl crystals beyond their first excitonic peak (329 nm)
2
4
30]
(Note 2, Supporting information), and hence they are suitable
for polarizing devices that are designed to function across the
visible and near-infrared spectra.
Angle-resolved Raman spectroscopy was performed to char-
acterize the in-plane anisotropy of PMA PbCl crystals. It has
Powder second harmonic generation was obtained to verify
the second-order optical nonlinearity of PMA PbCl crystals
2
4
2
4
[
35]
been demonstrated to be effective in observing optical and
(see Figure S6 in the Supporting Information). The crystals
were ground into powders with an average size of 100 µm. The
powders were photoexcited using a 965 nm laser. Signals were
collected after a 900 nm short-pass filter, and a sharp SHG peak
was seen at 483 nm. The SHG from the PMA PbCl crystals
structural anisotropy in various 2D crystal systems.[
31–33]
In the
back-scattering Raman and polarized detection configuration
used here, the incident laser was along the a-axis of the crystal,
and its initial polarization direction was set to coincide with the
c-axis of the crystal (Figure S3a, Supporting Information). By
sweeping the angle (θ) between laser polarization direction and
2
4
attests to their noncentrosymmetric structure and second-order
optical nonlinearity. SHG is mainly due to the polarized elec-
trons in the material. However, the linear EO effect originates
crystal c-axis from 0° to 360°, a polar Raman plot of A sym-
1
[24]
[36]
metric stretching of the [PbCl ] pyramids
was obtained in
from contributions from both electronic and ionic parts.
3
Figure 2a. The strong anisotropic response indicates the optical
Therefore, the linear EO effect needs to be explored separately
to gain a picture of the second-order optical nonlinearity of the
material.
anisotropy in the PMA PbCl4 crystal (Note 1, Supporting
2
Information).
The 2D nature of layered perovskites has prompted investi-
The linear EO effect (Pockels effect) offers a direct route
to modify birefringence in the perovskite crystal. The linear
EO coefficient was determined quantitatively at 1550 nm, the
typical optical communications wavelength, where optical scat-
gations of the out-of-plane birefringence.[34] In PMA PbCl crys-
2
4
tals, spontaneous polarization exists along the c-axis, and an
additional in-plane anisotropy exists. This suggests that in-plane
birefringence could also be observed from PMA PbCl crystals.
[
37]
tering and OH ion absorption is minimized. Figure 3a shows
a schematic of the measurement setup used to characterize
the change of the in-plane birefringence caused by the linear
2
4
A circularly polarized laser beam was generated by using a
quarter waveplate at 1550 nm. A sample of well-defined mor-
phology was placed between two crossed polarizers. By simulta-
neously rotating these two crossed polarizers, we measured the
transmitted optical power (Figure S4, Supporting Information).
[
5,38]
EO effect:
a linearly polarized laser beam was launched
normally to the surface of the sample and passed through two
parallel electrodes, having a separation of 50 µm, touching the
Figure 2. a) Normalized intensity of Raman mode 191.5 cm−1 with different excitation polarizations. b) Birefringence of the PMA PbCl crystal. The
2
4
transmitted power is measured by simultaneously rotating two crossed polarizers that sandwich the perovskite crystal. The data points are fitted with
2
a sin function.
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Figure 3. Electro-optic effect characterization. a) Schematics of the experimental setup used to determine the shift of the in-plane birefringence induced
by the EO effect. 1550 nm laser beam with a polarization angle of 45° is launched on the sample. A modulation AC voltage Vac is applied on the surface
of the crystal with a pair of electrodes. The transmitted laser beam becomes elliptically polarized. It then passes through a 1550 nm quarter waveplate,
and finally, the resulting linearly polarized light is analyzed with a Glan–Taylor polarizer. b) The variation of transmitted optical power (P) as a func-
tion of the polarization analyzer angle. c) The modulation of the transmitted optical power (ΔP) due to the EO effect as a function of the polarization
analyzer angle. d) The linear growth of ΔP increasing with the modulation AC voltage at an analyzer position of 45°.
PMA PbCl crystal. An electric field was applied parallel to the
proportional to the derivative of the total transmitted power P
2
4
crystal surface along the c-axis and perpendicular to the inci-
dent light. The following expression was used to evaluate the
shift in the electric field induced birefringence (see Note 3 in
the Supporting Information)
(see Note 3 in the Supporting Information)
dP
∆P =δ
(2)
dθa
1 c
2
3
Figure 3c presents the measured evolution of ΔP that can be
∆n =
r
n E
(1)
fit with Equation (2). The magnitude of the modulated trans-
mitted power ΔP increases linearly as a function of the applied
AC voltage (Figure 3d). These observations confirm the EO
effect from the PMA PbCl crystal. The linear increase of ΔP
The polarization of the incident beam was rotated by an angle
of 45° with respect to the applied electric field. In order to char-
acterize accurately the birefringence resulting from the applied
electric field, we applied an AC voltage with a sinusoidal wave-
form and analyzed the optical signal using a lock-in amplifier.
The frequency of the modulation voltage used was 6 kHz, and
the peak-to-peak voltage was 10 V, which generated an electric
field much smaller than the coercive force of the PMA PbCl
2
4
with applied voltage rules out the Kerr effect, which would
manifest a quadratic relation.
The change of the polarization angle δ can be determined
by fitting ΔP as a function of analyzer angle θ . The AC field
a
induced phase change Γ is equal to 2 × δ and the EO coefficient
[
38]
[39]
along the c-axis, rc, can be written as
2
4
crystal.[ As the laser beam passed through the crystal, the
linearly polarized light became elliptically polarized due to the
birefringence of the sample. A quarter waveplate (λ/4 plate)
was used to compensate the phase change and return the beam
to linear polarization. The polarization was therefore changed
by a small angle, δ.[ Finally, before the signal was collected by
a Ge near infrared photodetector, a Glan laser calcite polarizer
was placed to analyze the polarization of the resultant beam.
22]
Γλ
πn tEac
r
c
=
(3)
3
where t is the effective thickness of the perovskite crystal, and
Eac is the electric field along the c-axis. Therefore, we quantify
5]
−
1
the linear EO coefficient along the c-axis to be 1.40 pm V .
While the linear EO coefficient is not high in comparison to
that of conventional nonlinear crystals (Table S1, Supporting
Information), it is hoped that future studies will be motivated
When we vary the polarization analyzer angle, θ , the modu-
lation AC field induced a power variation, ΔP, that should be
a
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Table 1. Optical linear and nonlinear electronic susceptibilities.
Obtained from DFPT calculations.
birefringence through the linear EO effect. We determined
the effective linear EO coefficient. We demonstrated that the
2
D metal halide perovskite network can act as a scaffold to
Optical dielectric constant,
ε (ω)
ij
Nonlinear LEO susceptibility,
anchor and align aromatic spacer cations to manifest an ani-
sotropic structure over a large scale and produce an overall
polarization in the crystal. The ability to align bulky spacer
cations gives a degree of freedom for engineering the non-
linear properties of the layered perovskite. The flexibility of
these materials and their solution-processing offers the poten-
tial for noncentrosymmetric layered metal halide perovskites
to herald in new and compelling opportunities of low-cost
high-performance EO modulation in telecommunications
and interconnections.
∞
(2)
−1
χ
(−ω;ω,0)[pm V
]
ijk
ε11
ε22
ε33
2.989
2.978
2.819
(2)
χ1
0.21
−0.05
1.49
13
(2)
χ2
23
(2)
χ3
33
to increase further the linear EO coefficients of this class of
materials. We tried to pole the PMA PbCl crystal by applying
2
4
a DC voltage (100 V) that is greater than its coercive force. No
significant enhancement of the EO response was observed
Experimental Section
Crystal Growth: Phenylmethylammonium chloride (PMACl) was
prepared by dissolving phenylmethylammonium in methanol and drop
wise adding HCl solution (37%). The reaction was carried out in a flask
in an ice bath, and the solution was stirred vigorously. The resultant
white precipitation was washed with diethyl ether and methanol three
times and dried in vacuum. The large PMA2PbCl4 crystal was prepared
(
Figure S7, Supporting Information). We conclude that the
domain size of the parallelly aligned dipoles on the PMA PbCl4
2
crystal is at least larger than the gap between the electrodes
(
50 µm).
To complement the experimental investigations of the EO
by adopting the temperature lowering method. PMACl (431 mg), PbCl2
effect, we used DFT calculations on the nonlinear optical
response of the layered perovskite. Through calculations done
under electric field perturbations, we determined the optical
(417 mg), and 15 mL HCl aqueous solution (37%) were mixed in a
2
0 mL vial. The mixture was fully dissolved after it was heated to 100 °C.
After that, the temperature was decreased at a rate of 20 °C per day.
Large transparent crystals were obtained at the end.
dielectric constant (Table 1). PMA PbCl shows biaxial bire-
2
4
fringence with a considerable difference in refractive index
along the c-axis and so the direction of polarization. Along
with these quantities, the second-order linear EO suscepti-
bilities can are determined (Table 2). The electronic contri-
butions to the electro-optic coefficients are then estimated by
combining these calculated quantities (Table 2). The linear EO
coefficient along the c-axis is then r33 − r23 = −0.39 pm V .
This indicates that since the measured coefficient is much
larger, the EO response derives mainly from ionic contribu-
tions, likely relating to the separation of the centers of the pos-
itive and negative ions. Note, the quantities calculated from
DFT do not account for dispersion and represent the zero-
frequency limits.
X-Ray Crystallographic Analysis: A small fraction of the crystal was
cleaved from the as-grown PMA2PbCl4 crystal. Single crystal X-ray
diffraction data were collected at room temperature on a Bruker Kappa
APEX-DUO diffractometer equipped with a rotating anode with graphite-
monochromated Mo-Kα radiation (Burker Triumph, λ = 0.71073Å).
Both ϕ scans and ω scans were conducted. SADABS was adopted for
[40]
absorption corrections.
Fitting and refinement (full-matrix least-
−
1
2
squares refinement based on F ) of single-crystal structures were
performed with SHELXT and SHELXL-2016/6, respectively.[
41a,b]
All the
hydrogen atoms were included in the position calculation, which allowed
the refinement with the riding-motion approximation.
Angle-Resolved Raman Spectroscopy: Raman spectra were recorded on
a home-made Raman spectrometer with a 532 nm excitation laser in the
−1
backscattering configuration with a 1200 mm grating and Andor 750
spectrometer. The spot size of the focused laser was ≈1 µm, and the
laser power was 47 µW. Angle-resolved Raman spectra were recorded in
the condition that the incident light and the scattered light were aligned
parallel to each other. The angle θ was defined as the angle between the
c-axis and excitation laser polarization and was swept from 0° to 360°
with a 15° step size by rotating a half-wave plate.
In sum, the off-centering of the metal cation (Pb) in the
layered hybrid metal halide perovskite PMA PbCl , a trait of
2
4
many inorganic perovskite nonlinear optical crystals, leads
to inherent polarization. We show the in-plane birefringence
of the PMA PbCl crystals and present, for the first time for
2
4
Density Functional Theory: DFT calculations were implemented using
a hybrid metal halide perovskite, the modulation of such
[42]
the Quantum Espresso software. Norm-conserving pseudopotentials
produced with the Troullier–Martins method were used.[ Calculations
43]
were done within the local density approximation using Perdew–
Table 2. Electronic contributions to the linear electro-optic coefficients.
Obtained from DFPT calculations.
[44]
Wang exchange-correlation functionals.
Lattice constants were
kept at the values obtained from single crystal X-ray diffraction, while
atomic positions were relaxed so that internal forces were less than
pm V−
1
1 × 10 Ry Bohr . An energy cut-off of 60 Ry and Monkhorst–Pack
k-point grid of 6 × 6 × 6 was used. Density functional perturbation theory
calculations were done to determine the linear and nonlinear electronic
susceptibilities.[ The nonlinear susceptibilities were determined within
the 2n + 1 theorem.
−4
−1
Electro-optic tensor element
el
−0.046
r1
3
45]
el
0.012
−0.38
r
2
3
el
r
r
r
33
el
−0.013
−0.049
Supporting Information
42
el
Supporting Information is available from the Wiley Online Library or
from the author.
51
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Y.G. and G.W. contributed equally to this work. This work was financially
supported by Natural Sciences and Engineering Research Council
(NSERC). G.W. acknowledges funding from NSERC. M.I.S. acknowledges
2
018, 6, 1800400.
the support of Banting Postdoctoral Fellowship Program.
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Conflict of Interest
The authors declare no conflict of interest.
[
[
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