Giant Optical Activity and Second Harmonic Generation in 2D Hybrid Copper Halides

Article abstract of DOI:10.1002/anie.202015445

Hybrid organic–inorganic metal halides have emerged as highly promising materials for a wide range of applications in optoelectronics. Incorporating chiral organic molecules into metal halides enables the extension of their unique optical and electronic properties to chiral optics. By using chiral (R)- or (S)-methylbenzylamine (R-/S-MBA) as the organic component, we synthesized chiral hybrid copper halides, (R-/S-MBA)₂CuCl₄, and investigated their optical activity. Thin films of this material showed a record anisotropic g-factor as high as approximately 0.06. We discuss the origin of the giant optical activity observed in (R-/S-MBA)₂CuCl₄ by theoretical modeling based on density functional theory (DFT) and demonstrate highly efficient second harmonic generation (SHG) in these samples. Our study provides insight into the design of chiral materials by structural engineering, creating a new platform for chiral and nonlinear photonic device applications of the chiral hybrid copper halides.

Full text of DOI:10.1002/anie.202015445

Angewandte
Communications
Chemie
How to cite: Angew. Chem. Int. Ed. 2021, 60, 8441–8445
International Edition: doi.org/10.1002/anie.202015445
German Edition: doi.org/10.1002/ange.202015445
Optical Materials
Hot Paper
Giant Optical Activity and Second Harmonic Generation in 2D Hybrid
Copper Halides
Zhihang Guo+, Junzi Li+, Changshun Wang, Rulin Liu, Jiechun Liang, Yang Gao, Jiaji Cheng,
Wenjing Zhang, Xi Zhu,* Ruikun Pan,* and Tingchao He*
Abstract: Hybrid organic–inorganic metal halides have
emerged as highly promising materials for a wide range of
applications in optoelectronics. Incorporating chiral organic
molecules into metal halides enables the extension of their
unique optical and electronic properties to chiral optics. By
using chiral (R)- or (S)-methylbenzylamine (R-/S-MBA) as the
organic component, we synthesized chiral hybrid copper
halides, (R-/S-MBA)2CuCl4, and investigated their optical
activity. Thin films of this material showed a record anisotropic
g-factor as high as approximately 0.06. We discuss the origin of
the giant optical activity observed in (R-/S-MBA)2CuCl4 by
theoretical modeling based on density functional theory (DFT)
and demonstrate highly efficient second harmonic generation
(SHG) in these samples. Our study provides insight into the
design of chiral materials by structural engineering, creating
a new platform for chiral and nonlinear photonic device
applications of the chiral hybrid copper halides.
Hybrid organic–inorganic metal halides, featuring struc-
tural versatility and unique photophysical properties, have
recently emerged as promising semiconducting materials for
high-performance optoelectronic applications.[5] The variable
lattice structure and molecular composition offer tremendous
possibilities for tuning their optoelectronic properties. In
particular, hybrid metal halides with organic sites allow for
the introduction of chiral organic molecules, which further
extends their applications in the field of chiral optoelectron-
ics.[6] Since Moon and co-workers first investigated the chiral
optical properties of two-dimensional (2D) (R-/S-MBA)2PbI4
(MBA = a-methylbenzylamine),[7] chiral hybrid metal halides
have prompted intensive research interest. Subsequently,
great efforts have been devoted to synthesizing various chiral
hybrid metal halides through changing the organic compo-
nent, metal ion or halogen element, investigating their optical
activity, and developing their diverse applications such as
CPL sources,[8] circularly polarized light detectors,[9] and
spintronics.[10] For example, Li and co-authors developed 2D
(R-/S-MBA)2PbI4 and investigated their CPL and circularly
polarized light detection.[8b] Very recently, using one-dimen-
sional (R-/S-NEA)PbI3 (NEA = 1-(1-naphtyl)ethylamine)
with a highly anisotropic factor of 0.04, Ishii and Miyasaka
fabricated a circularly polarized light detector, which ach-
ieved a polarization discrimination ratio as high as approx-
imately 25.4.[9a] Sargent and co-workers demonstrated a 3%
spin-polarized photoluminescence of reduced-dimensional
chiral perovskites even in the absence of an applied external
magnetic field.[10a] Due to their non-centrosymmetric struc-
ture, chiral hybrid metal halides can also been applied for
second harmonic generation (SHG).[11] For example, Xu and
co-workers determined second-order nonlinear optical
Chirality, a ubiquitous phenomenon in nature, occurs in
structures that lack Sn symmetry elements; thus, chiral
structures cannot be superimposed on their mirror images.[1]
As a consequence of possessing non-centrosymmetric struc-
tures, chiral materials tend to exhibit unique optical proper-
ties, such as circular dichroism (CD) due to the difference in
absorption of left- and right-hand circularly polarized (LCP
and RCP) light and circularly polarized luminescence (CPL)
due to the difference in LCP and RCP emission. The
development and design of chiral materials for multidiscipli-
nary applications in biometric detection and sensing,[2] chiral
bioimaging,[3] and polarization-based optoelectronic devices
have attracted immense interest.[4]
[*] Z. Guo,[+] J. Li,[+] Prof. Y. Gao, Prof. T. He
Key Laboratory of Optoelectronic Devices and Systems of Ministry of
Education and Guangdong Province, College of Physics and
Optoelectronic Engineering, Shenzhen University
Shenzhen 518060 (China)
Shenzhen University
Shenzhen 518060 (China)
Prof. C. Wang
State Key Laboratory of Advanced Optical Communication Systems
and Networks, School of Physics and Astronomy
Shanghai Jiao Tong University
E-mail: tche@szu.edu.cn
Z. Guo,[+] Prof. J. Cheng, Prof. R. Pan
Shanghai 200240 (China)
Key Laboratory for the Green Preparation and Application of
Functional Materials, Ministry of Education
School of Materials Science and Engineering, Hubei University
Wuhan 430062 (China)
E-mail: rkpan@hubu.edu.cn
J. Li,[+] Prof. W. Zhang
R. Liu, J. Liang, Prof. X. Zhu
School of Science and Engineering
The Chinese University of Hong Kong
Shenzhen, Guangdong 518172 (China)
E-mail: zhuxi@cuhk.edu.cn
[+] These authors contributed equally to this work.
SZU-NUS Collaborative Innovation Center for Optoelectronic
Science and Technology, International Collaborative Laboratory of 2D
Materials for Optoelectronics Science and Technology of Ministry of
Education, College of Physics and Optoelectronic Engineering
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202015445.
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(NLO) coefficient of (R-/S-MPEA)1.5PbBr3.5(DMSO)0.5
of chiral R- or S-MBA molecules. (R-/S-MBA)2CuCl4 crys-
tallizes in the polar and chiral C2 space group (C2 point
group), assigned to the monoclinic crystal system.[14] X-ray
diffraction (XRD) patterns of both (R-/S-MBA)2CuCl4
exhibit sharp diffraction peaks that can be assigned to the
(0 0 l) planes (Figure 1b), suggesting that the orientation of
the [CuCl4]2ꢀ tetrahedral inorganic layers is highly parallel to
the substrate plane. The narrow full width at half maximum of
the XRD patterns further indicates the excellent crystallinity
of the as-prepared films. Atomic force microscopy (AFM)
measurements were used to determine the morphology and
thickness of the two chiral films (Supporting Information,
Figure S1). The low roughness with an Ra (average surface
roughness) of approximately 6 nm indicated the uniform
morphology of the solid films.
UV–vis absorption and CD spectra of the ammonium
halide salts and (R-/S-MBA)2CuCl4 films are shown in
Figure 1c,d, respectively. The ammonium halide salts R- or
S-MBACl (chemical structures shown in the inset of Fig-
ure 1c) show mirrored CD signals below 280 nm, with a peak
at 268 nm. However, much stronger CD signals were
observed in the (R-/S-MBA)2CuCl4 films from the ultraviolet
to visible regions, indicating that the chirality could not be
entirely ascribed to the pure ammonium halide salts. (R-/S-
MBA)2CuCl4 exhibited mirrored CD signals peaking at 378,
266, and 226 nm, which are consistent with their absorption
bands, as a result of the Cotton effect.[15] In addition, the peak
at 378 nm corresponds to the characteristic ligand-to-metal
charge transfer (LMCT) transition band of [CuCl4]2ꢀ,[16] as
a consequence of charge transfer from fully occupied chloride
ion localized orbitals or sCu-Cl and pCu-Cl orbitals to copper ion
semioccupied dxy orbital.[17] These results demonstrate that
the chirality of the organic component was successfully
(MPEA = b-methylphenethylamine)
as
approximately
0.68 pmVꢀ1 at 850 nm.[12] Very recently, a series of chiral
metal halides were manufactured by Heine and co-workers,[13]
and the SHG signals were 5–45 times smaller than the quartz
reference.
Although great progress in the investigation on the optical
activity of hybrid metal halides has been made, there is still
extensive room for improving their optical activity. Obviously,
advancements in exploring new chiral hybrid metal halides
are in high demand. Towards this end, the design, synthesis,
and characterization of hybrid metal halides with large optical
activity are urgently required for the realization of relevant
applications.
Herein, we report the synthesis of thin films of the chiral
2D hybrid metal halides, (R)- or (S)-methylbenzylammonium
copper chloride ((R-/S-MBA)2CuCl4). The as-prepared films
exhibit strong mirrored CD signals with a giant anisotropic g-
factor of approximately 0.06, which is even greater than the
record value (ca. 0.04) recently reported for chiral (R-/S-
NEA)PbI3.[9a] We discuss the origin of giant optical activity in
these samples through theoretical modeling based on density
functional theory (DFT). Significantly, the chiral (R-/S-
MBA)2CuCl4 films exhibited high second-order NLO coef-
ficients that are estimated to be approximately 50 times larger
than the Y-cut quartz reference.
(R-/S-MBA)2CuCl4 thin films were prepared by a spin-
coating approach using chiral R- or S-MBA as the organic
component (see details in the Supporting Information). The
lattice structures of (R-MBA)2CuCl4 and (S-MBA)2CuCl4 are
characteristic of 2D layered materials (Figure 1a), where each
central Cu atom coordinates with four Cl atoms to form
[CuCl4]2ꢀ tetrahedra that are sandwiched between two layers
Figure 1. a) Lattice structure of (R-/S-MBA)2CuCl4. H atoms are omitted for clarity. b) XRD patterns of (R-/S-MBA)2CuCl4 films. c) CD and
absorption spectra of the ammonium halide salts dissolved in dimethylformamide (10 mgmLꢀ1). Inset shows the chemical structures of R-/S-
MBACl. d) CD and absorption spectra of (R-/S-MBA)2CuCl4 films. e) Corresponding g-factor spectra.
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transferred to the internal structural framework. Notably, the
anisotropic g-factor, which is defined as j gCDꢀ ꢀ gCD+ j /2,[18]
was calculated to be approximately 0.06 at the LMCT band of
[CuCl4]2ꢀ (Figure 1e), which is one order of magnitude larger
than that of the chiral (R-/S-MBA)2PbI4 hybrid lead halides
(ca. 0.006),[7] and is even larger than the record value of 0.04
recently reported for the chiral one-dimensional (R-/S-
NEA)PbI3.[9a] Additionally, the CD spectra of (R-/S-
MBA)2CuCl4 did not change significantly after being stored
under ambient conditions for one month, demonstrating the
excellent long-term stability of the as-prepared films (Sup-
porting Information, Figure S2).
In order to reveal the origin of giant g-factors in chiral (R-/
S-MBA)2CuCl4, theoretical calculation on their optical activ-
ity was performed. The CD intensity can be calculated by
Equation (1):[19]
CD ¼ wðLÞ ꢀ wðRÞ
ð1Þ
Figure 2. Density of states of a) (R-/S-MBA)2CuCl4 and b) (R-/S-
where w denotes the transition probability density per unit
time, L and R refer to LCP and RCP light, respectively. The
transition was shown to be proportional to the probability of
the transition,[20] that is, the joint density of states (JDOS). As
the JDOS of transition (JðhꢀwÞ) with energy ꢀhw can be
expressed as[21]
MBA)2PbI4 according GW approximation calculation, and spatial dis-
tribution of the relevant orbitals. The arrows point to the initial and
final states of the transitions. By multiplying the respective states
shown in the figure, it can be observed that the JDOS of the copper-
containing material is larger than that of the lead-containing material,
thus accounting for the higher CD intensity observed in the former.
Z
þ1
JðꢀhwÞ ¼
NvðEÞNcðE þ ꢀhwÞdE
ð2Þ
ꢀ1
than (R-/S-MBA)2CuCl4, as a consequence of larger JDOS in
the latter.
where Nv;c refers to the JDOS on the valence and conduction
band. Thus, the g-factor from the JDOS of a given material
can be estimated.
According to Fukuiꢀs method,[24] the optical selectivity of
circularly polarized light versus the band gap of the chiral (R-/
S-MBA)2CuCl4 films was also calculated (Supporting Infor-
mation, Figure S3), which can serve as a reference to identify
the source of chirality. As a result of the optical activity of the
hybrid copper halides, inverse optical selectivity to circularly
polarized light was observed, with peaks at 2.96 and 3.18 eV
for (R-MBA)2CuCl4, and 2.93 and 3.17 eV for (S-
MBA)2CuCl4, respectively; this was in agreement with their
measured absorption peak positions in the visible region.
Importantly, our calculations indicated that the orbitals of the
chiral carbons could not be coupled with those of copper or
In order to unveil the mechanism that causes much larger
optical activity in (R-/S-MBA)2CuCl4 compared with (R-/S-
MBA)2PbI4, their density of states was calculated by GW
approximation. The self-consistent PWSCF is used, which can
well establish LDA and GGA functionals with spin-polar-
ization.[22] As shown in Figure 2, the arrows indicate the
transitions from the initial to final states. Different from
regular DFT calculation, the GW method includes DFT and
focuses on the quasiparticle situation, and the screening
effects contribute more to the final result. In addition, the
GW method can generate a wave function as a precalculation.
It is noteworthy that the band gap value calculated by GW
method is always larger than the experimentally measured
one. The orbital distribution relevant to the chiral excitation
was plotted (insets in Figure 2). Importantly, it was found that
the orbitals were mainly distributed around the Cu/Pb atoms.
This means that the JDOS for the chiral transition could be
altered by the metal atoms. Compared with (R-/S-
MBA)2PbI4, (R-/S-MBA)2CuCl4 possesses a higher JDOS,
thus accounting for the enhanced g-factors by one order of
magnitude in the latter. Actually, Kotov, Moon, and co-
workers also investigated the effect of chemical-composition
modification on the optical activity of (R-/S-
MBA)2PbI4(1ꢀx)Br4x, whose g-factors can vary by one order
of magnitude through the change of mixing ratio of bromide
and iodide anions.[23] However, the g-factors of (R-/S-
MBA)2PbI4(1ꢀx)Br4x were still an order of magnitude smaller
ꢀ
ꢀ
chlorine due to the long N H···Cl and C H···p hydrogen
bonding. From the perspective of quantum theory, the
chirality of the hybrid copper halides was a consequence of
their chiral crystal structure (C2 space group), which can be
described as[25]
CD / R ¼ Imfmmg
ð3Þ
where R is the theoretical rotational strength, Im is the
imaginary part of the parameter, m is the electric dipole
transition moment for the transition from the final to the
initial state, and m is the magnetic dipole transition for the
reverse transition. The condition for the occurrence of
chirality is that the electric and magnetic dipole moment
operators must be converted into the same irreducible
representation. Under such circumstances, both (R-/S-
MBA)2CuCl4 crystallize in the C2 point group, which makes
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m and m non-zero and induces optical chirality (Supporting
Information, Table S1).
polarized pump light at 800 nm. Because the accurate
determination of their second-order NLO coefficients
depends on the actual experimental parameters and the
reabsorption effect must be considered, the deff value of the
chiral (R-MBA)2CuCl4 film is an estimation on the order of
magnitude. Importantly, this value is several orders of
magnitude larger than the previously reported chiral (R-/S-
MPEA)1.5PbBr3.5(DMSO)0.5 (0.68 pmVꢀ1, 850 nm),[12] and
comparable to that for CH3C(NH2)2GeI3 (57.2 pmVꢀ1,
1064 nm) single crystals.[29]
In summary, we synthesized chiral 2D hybrid metal
halides, that is, (R-/S-MBA)2CuCl4, and they exhibit
a record anisotropic g-factor of approximately 0.06 at the
LMCT band of [CuCl4]2ꢀ. Theoretical calculations confirm
that the larger optical activity of (R-/S-MBA)2CuCl4 com-
pared with other chiral hybrid metal halides is induced by the
higher JDOS in the former. In addition, it is also theoretically
proven that the chirality in the hybrid copper halides is
induced by the chiral crystal structure (chiral space group).
Impressively, the second-order NLO coefficients of the chiral
(R-/S-MBA)2CuCl4 films are about 50 times larger than that
of the Y-cut quartz reference at 800 nm. This work demon-
strates the tremendous potential of the chiral hybrid copper
halides for chiral and nonlinear photonic applications.
Furthermore, the SHG signals of the (R-MBA)2CuCl4
film were investigated to elucidate the feasibility in nonlinear
optics. SHG measurements were carried out using a combined
polarized light optical microscope in reflection geometry
using perpendicular incident excitation (Supporting Informa-
tion, Figure S4). The wavelength-dependent SHG spectra of
the (R-MBA)2CuCl4 film was investigated at different
excitation wavelengths with the same excitation power and
strong SHG signals were observed in the chiral film with
excitation wavelengths in the broad wavelength range of 780–
920 nm (Figure 3a). Analysis of the pumping fluence-depen-
dent SHG intensities under 800 nm excitation (inset in
Figure 3a) revealed square relationship that confirmed the
two-photon nature of this second-order NLO response.
Additionally, the nonlinear efficiency of the (R-MBA)2CuCl4
film was evaluated by comparing the SHG signals with that of
Y-cut quartz reference. The SHG signals of the (R-
MBA)2CuCl4 and Y-cut quartz films were measured under
the same conditions (including the incident laser power
intensity). The second-order NLO coefficient of the chiral
film is determined by using Y-cut quartz as a reference, and is
given by[26]
sffiffiffiffiffiffi
rffiffiffiffiffiffiffi
dseff
dreff
Is2w Irw Lr nsw ns2w
Ir2w Isw Ls nrw nr2w
ð4Þ
ꢁ
Acknowledgements
We acknowledge financial support by the Guangdong Basic
and Applied Basic Research Foundation (2019A1515012094),
the Project of Department of Education of Guangdong
Province (2018KTSCX19), the Science and Technology
where Iw is the pump intensity, deff is the effective second-
order NLO coefficients (the superscripts “s” and “r” refer to
the hybrid copper halide film and Y-cut quartz, respectively;
dreff = 0.6 pmVꢀ1 [27]
), L is the effective thickness, and nw and
,
Planning
Project
of
Shenzhen
Municipality
n2w are the refractive indices at the frequency w and 2w,
respectively. We used laser pumping under the same con-
ditions (20 mW, 800 nm) and the SHG signals were detected
at 400 nm for both films. The refractive indices of the Y-cut
quartz at 800 and 400 nm are 1.54 and 1.56,[28] respectively,
while relevant values for the chiral film are 1.55 and 2.8 at 800
and 400 nm, respectively. Using the Y-cut quartz as a refer-
ence, deff of the chiral (R-MBA)2CuCl4 film was determined
to be approximately 28.75 pmVꢀ1 with the optimized linearly
(JCYJ20190808121211510), the National Natural Science
Foundation of China (92050116, 21805234, 22075240), the
Shenzhen Institute of Artificial Intelligence and Robotics for
Society (AIRS; 2019-INT018,2020-IND002) and the Guang-
dong Introducing Innovative and Entrepreneurial Teams
(2019ZT08L101).
Conflict of interest
The authors declare no conflict of interest.
Keywords: chirality · circular dichroism · hybrid copper halides ·
nonlinear optics · second harmonic generation
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Manuscript received: November 19, 2020
Accepted manuscript online: January 22, 2021
Version of record online: March 1, 2021
Angew. Chem. Int. Ed. 2021, 60, 8441 –8445
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8445