Organic−inorganic hybrid halide perovskites (OIHPs) have attracted significant attention due to their remarkable optoelectronic properties, including strong light−matter interaction, high defect tolerance, long carrier-diffusion lengths, high photoluminescence quantum yields (PL QYs) and tunable structure and chemical composition^[1−4]. These unique features make them suitable for developing high-performance optoelectronic devices, such as solar cells^[5−7], light-emitting diodes^[8−12], photodetectors^[13−17] and lasers^{[1, 18, 19]}. In particular, the dimensionality and their associated distinct properties of OIHPs can be engineered through introducing various organic amine cations^{[20, 21]}. For instance, the incorporation of large and chiral ligands into OIHPs facilitate a new material system of 2D chiral halide perovskites (CHPs)^[22−25]. 2D CHPs single crystals have been demonstrated to exhibit intriguing chiroptical^{[26, 27]}, spintronic^{[28, 29]}, ferroelectric^{[30, 31]} and piezoelectric^[32] properties since 2003.

The successful chirality transfer from chiral organic cations to the inorganic perovskite sublattices unlocks a plethora of chiroptical phenomena and promising applications, including circular dichroism (CD)^{[33, 34]}, circularly polarized photoluminescence (CPL) and photodetection^[35−38], chiral induced spin selectivity (CISS)^{[39, 40]}, spin light emitting diodes (spin-LEDs)^{[41, 42]} and chiral second-order nonlinear optical (NLO) effects^[43−45]. Furthermore, 2D CHPs can form van der Waals heterostructures with other 2D semiconductors, such as WS₂, MoS₂, and 2D achiral halide perovskites, and exhibit efficient chiral charge transfer and enhanced CPL^{[6, 37, 46−51]}. However, the study of 2D CHPs is still in its early stages. To synthesize high-quality and stable crystals of 2D CHPs, and explore their chiroptical properties and understand corresponding fundamental mechanisms are highly desired.

Here, we report a new type of 2D CHPs bulk single crystals to the best of our knowledge, namely R/S-3BrMBA₂PbBr₄ [3BrMBA = 1-(3-bromphen-yl)-ethylamine]. These crystals were successfully synthesized through a temperature-gradient crystallization approach. The synthesized R/S-3BrMBA₂PbBr₄ crystals show ultra-broadband white-light emission. Through conducting power-dependent and time-resolved PL spectroscopy, we reveal that such broadband emission is governed by strong exciton−phonon coupling effect and originates from free excitons (FEs) and self-trapped excitons (STEs). Furthermore, we demonstrate novel linear and nonlinear chiroptical properties of R/S-3BrMBA₂PbBr₄. These 2D CHP single crystals have characteristic circular dichroism (CD) absorption and, more importantly, they display chiral second-harmonic generation (SHG) performance with a high polarization ratio and significant NLO CD. Our investigations develop deep insights for future engineering of crystal structures and chiroptical properties of 2D CHPs.

In this study, R/S-3BrMBA was selected as the chiral organic ligand due to its ability to synthesize chiral perovskites with higher lattice distortion and asymmetry, as compared to other chiral organic ligands, such as R/S-MBA and R/S-4BrMBA^[52]. The chiral R/S-3BrMBA₂PbBr₄ crystals were obtained by incorporating bulky R- and S-3BrMBA cations into the inorganic perovskite sublattices, as illustrated in Fig. 1(a). We employed the slow-cooling crystallization process to grow 2D CHPs single crystals (see details in the Supplementary material and Fig. S1). During the growth process, chiral cations were strategically introduced to achieve chirality transfer and spatial inversion symmetry breaking in the 2D CHPs. Single-crystal X-ray diffraction (SCXRD) was performed to explore crystal structures of chiral R/S-3BrMBA₂PbBr₄. As illustrated in Fig. 1(b), their single crystals exhibit 2D layered structure, wherein the inorganic octahedron forms a [PbBr₆]²⁻ octahedral layer through the corner-sharing configuration and is sandwiched between two layers of R- or S-3BrMBA cations. The R- or S-3BrMBA cations are held together by van der Waals forces and further stabilized by π−π stacking interactions in 2D CHPs. The inorganic layers and organic cations are connected via N−H···Br hydrogen bonds, forming a natural quantum well structure. Notably, both perovskite enantiomers crystallize in the chiral space group of P2₁ (see Table S1, Supplementary material). The unit cell parameters of R/S-3BrMBA₂PbBr₄ are a=8.7980(11)/8.7793(7)A, b=7.8893(9)/7.8782(5)A, c=18.020(3)/17.9737(14)A, and β=97.505(4)°/97.480(2)°, respectively. The Pb−Br bond lengths and Br−Pb−Br angles range from 2.976(2)−3.193(2) Å and 80.88(6)°−102.14(7)° for R-3BrMBA₂PbBr₄, and 2.966(2)−3.187(2) Å and 80.82(7)°−101.99(8)° for S-3BrMBA₂PbBr₄, respectively (Table S2−Table S5, Supplementary material). Based on the unit cell parameters, the distortions of the PbBr₆ octahedra are calculated using bond length distortion (Δd) and band angle variance (σ2) using the following equations^{[37, 50, 53]}:

1Δd=16∑i=16[di−d0d0]2,

2σ2=111∑i=112(θi−90)2,

where di is the corresponding Pb−Br bond length, d0 represents the average Pb−Br bond length, and θi means Br−Pb−Br bond angle in octahedral structure. The Δd and σ2 of R- and S-3BrMBA₂PbBr₄ were calculated to be ~6.08 × 10⁻⁴ and 45.33 (6.23 × 10⁻⁴ and 44.87). These calculated values are comparable with those reported for low-dimensional hybrid perovskites exhibiting broadband white-light emission^{[53, 54]}. Additionally, we calculated interoctahedral distortion angle β (Pb−Br−Pb bond angle) utilizing the following equation^{[52, 55]}:

3Δβ=βmax−βmin.

The interoctahedral distortion angle Δβ for R/S-3BrMBA₂PbBr₄ were calculated to be 14.14 and 13.87, respectively (Table S6, Supplementary material). This indicates that we have successfully introduced a significant lattice distortion into the inorganic sublattice of the synthesized CHPs. Within the PbBr₆ octahedra, the locally distorted Pb−Br bond and Br−Pb−Br bond angles contribute to the formation of a non-centrosymmetric structure in the CHPs. Furthermore, the increased degree of Pb−Br bond distortion would help to lower the energy barrier between free exciton states and STE states, which would facilitate the generation of STEs and enhances the broadband white-light emission^{[53, 54]}. Indeed, we observed white-light emission of R/S- 3BrMBA₂PbBr₄ and will discuss in detail in the following text.

The morphologies of R/S-3BrMBA₂PbBr₄ were examined through scanning electron microscopy (SEM), revealing their layer stacking characteristics (Fig. S4, Supplementary material). The powder XRD patterns of these 2D CHPs show sharp peaks with narrow full width at half maximum (FWHM), indicating that these samples have high crystallinity (Fig. 1(c)). Both R/S-3BrMBA₂PbBr₄ exhibit a colorless and bulky shape, and they emit white light under ultraviolet (UV) light of 365 nm (Fig. 1(d) and Fig. S7(a), Supplementary material). Additionally, the synthesized CHP single-crystal samples show excellent stability even after being stored in air for over 6 months (Fig. S2, Supplementary material).

We conducted UV−Vis absorbance, circular dichroism (CD) and photoluminescence (PL) spectroscopy measurements to investigate optical properties and chiralities of the synthesized CHP samples. Utilizing the spin-coating technique, thin films of CHPs were prepared for UV−Vis absorbance and CD spectroscopy measurements (see details in the Supplementary material). The XRD patterns of the CHP thin films are shown in Fig. S3 (Supplementary material), exhibit sharp and narrow diffraction peaks, similiar to their single crystals. This confirms that the CHP thin films also possess excellent crystallinity. Fig. 2(a) shows the UV−Vis absorbance spectra of the R/S-3BrMBA₂PbBr₄ thin films at room temperature. We observed an exciton absorption peak at ~385 nm. Their optical bandgap, estimated using the Tauc-plot method, is found to be 3.09 eV^{[13, 50]} (Fig. S5(a), Supplementary material). The CD spectra in Fig. 2(b) confirm the chirality of the CHP samples. The CD spectra of R/S-3BrMBA₂PbBr₄ show peaks at the same positions but with opposite signs, indicating the presence of opposite chiral lattice distortions within each compound. Furthermore, a sign change in the CD signal is observed near the exciton absorption peak (383−390 nm), located on both sides of the absorption peak. This phenomenon is attributed to the Cotton effect^{[23, 24]}, since the chiral perturbations lead to energy level splitting of the band-edge electronic states and thus distinct resonant absorption peaks for left-handed and right-handed circularly polarized light by the inorganic perovskite framework. The exciton splitting energy (ΔE) is estimated to be ~58 meV (see Fig. S6, Supplementary material for detailed explanation). We calculated the absorption dissymmetry factor gCD for the CHPs using the equation given by Refs. [21, 44]:

4gCD=CD(mdeg)32980×Absorbance.

The gCD values at 390 nm for R- and S-3BrMBA₂PbBr₄ are determined to be about −1.08 × 10⁻³ and 0.87 × 10⁻³, respectively (Fig. S5(b), Supplementary material). These values are consistent with the order of magnitude reported for other CHPs^{[24, 50]}.

Fig. 2(c) and Fig. S7(a) (Supplementary material) display the PL spectra under the 325 nm UV excitation. Both R/S-3BrMBA₂PbBr₄ exhibit ultra-broadband PL spectra ranging from 380 to 750 nm. The sharp PL peak centered at ~398 nm is identified as emission from free excitons, and the ultrabroad peak centered at ~523 nm is probably from self-trapped excitons. In addition, the corresponding Commission Internationale de l'Eclairage (CIE) chromaticity coordinates for the R- and S-3BrMBA₂PbBr₄ were calculated to be (0.304, 0.405) and (0.302, 0.407), respectively (Fig. 2(d)). The corresponding correlated color temperatures (CCTs) were 6471 and 6526 K, respectively, both demonstrating "cool" white light emission characteristics.

Fig. 3(a) and Fig. S8(a) (Supplementary material) show the power-dependent PL spectra of R/S-3BrMBA₂PbBr₄ single crystals. Their PL intensity increases with increasing laser power, while the PL peaks remain unchanged. Due to the use of a 400 nm long-pass filter, the PL signals within the high-energy region (≤400 nm) were filtered out. In order to justify the origin of the ultrabroad PL peak, we analyzed three distinct emission regions: the free exciton (FE, 395−425 nm), self-trapped excitons 1 (STE₁, 425−560 nm) and self-trapped excitons 2 (STE₂, 560−700 nm). Their PL intensity IPL as a function of the excitation light power P is fitted with the following equation^[56]:

5IPL=αPk,

where α represents the radiative efficiency and k is a constant. As shown in Fig. 3(b) and Fig. S8(b) (Supplementary material), we found that the k is about 1 for all three emission regions of R/S-3BrMBA₂PbBr₄, suggesting that they are originate from exciton/STE recombination instead of defects^{[57, 58]}. Furthermore, the PLE experiments (Fig. S9, Supplementary material) also confirmed the STE origin of the broadband emission. For the emission ranging from 400 to 600 nm, the PLE spectra exhibit identical characteristics with a resonant peak (~384 nm) that corresponds to the exciton absorption band, indicating that the broadband emission arises from the relaxation of the same excited state^[59].

To further confirm the origin of the broadband emission in these 2D CHPs, we conducted time-resolved PL measurements. As shown in Fig. 3(c) and Fig. S7(b) (Supplementary material), R/S-3BrMBA₂PbBr₄ exhibits distinct decay curves in the three emission regions, which can be well fitted by using bi-exponential decay function. Notably, the decay curve in the high-energy emission region (405 nm) exhibits a fast-decay characteristic lifetime (τ1, >90%) that closely resembles the instrument response function (IRF), and a slow-decay characteristic lifetime (τ2, <10%). Here, τ1 is ~0.28 ns and ~0.42 ns for R/S-3BrMBA₂PbBr₄, respectively, which is consistent with the PL lifetime of FEs observed in other 2D halide perovskites^{[52, 58, 60]}. The short lifetime (τ1) is mainly attributed to decay of FEs as illustrated in Fig. 3(d). Meanwhile, τ2 is mostly attributed to the detrapping process of STEs. FEs can relax to self-trapped energy levels and transform into STEs. However, the small potential barriers between FEs and STEs for R/S-3BrMBA₂PbBr₄ facilitate the detrapping of STEs and thus contribute to a relatively long τ2 of FEs. In contrast, STEs are "protected" by the phonon states and typically exhibit longer lifetimes^{[61, 62]}, and τ2 is found to be several nanoseconds (R: ~5−6 ns, S: ~2−3 ns). The robust exciton−phonon coupling in the synthesized 2D CHPs results in weak circularly polarized luminescence (CPL) in R/S-3BrMBA₂PbBr₄ (Fig. S10(a), Supplementary material) and low luminescence dissymmetry factors (glum, Fig. S10(b)) when the 325 nm excitation laser is used. We believe that the strong electron−phonon coupling is responsible for the quick spin relaxation of injected charge carriers.

Notably, we observed that S-3BrMBA₂PbBr₄ exhibits a higher intensity in the broad emission band within the green-light region when compared to that of R-3BrMBA₂PbBr₄. This difference probably stems from the variations between the two different conformational chiral cations, which can influence the weak interactions present in the crystal structure, such as hydrogen bonding and van der Waals forces^[24]. These interactions could subsequently introduce different effects on the optical I∼Pk properties of R/S-CHPs^[52].

As a result of spatial inversion symmetry breaking, 2D CHPs typically exhibit nonlinear optical (NLO) effects, such as second-harmonic generation (SHG). By using a home-built experimental setup (see the Experimental Section for details), we explore the NLO properties of R/S-3BrMBA₂PbBr₄ as a function of excitation wavelength, power and polarization. First of all, as shown in Fig. 4(a) and Fig. S13(a) (Supplementary material), when the laser wavelength is varied in steps of 20 nm (with constant power) from 800 to 1000 nm, both R/S-3BrMBA₂PbBr₄ exhibited robust SHG. Nevertheless, a broad PL peak was also observed when the excitation wavelength was 800 nm, which is attributed to resonant two-photon absorption (TPA) process^[63]. Secondly, Figs. 4(b) and 4(c), and Fig. S11 and Fig. S12 in Supplementary material demonstrate the power-dependent SHG of R/S-3BrMBA₂PbBr₄ crystals with different excitation wavelengths. The SHG intensity shows a power-law dependence against the excitation power with the coefficient fitted to be 1.92/2.03, which are in close agreement with the theoretical value of 2 derived from the electric dipole approximation^[64]. Additionally, R/S-3BrMBA₂PbBr₄ thin films also show prominent SHG signals as shown in Fig. S14.

Thirdly, polarization-dependent SHG of R/S-CHPs exhibits a distinct "8"-shaped dipole contour in a polar coordinate system as shown in Fig. 4(d) and Fig. S13(b) (Supplementary material). The SHG intensity can be fitted by the function I=I0cos(2θ), where θ represents the angle between the polarization of the incident laser and the crystal axis, and I and I0 represent the detected SHG intensity and its maximum intensity, respectively. The maximum SHG signals for R/S-3BrMBA₂PbBr₄ crystals are observed at 90°/280° and 50°/230°, respectively, which correspond to their optical axis. This observation confirms the anisotropic SHG response of the synthesized R/S-CHPs. To quantify this polarization-dependent phenomenon, the polarization ratio is defined as ρ=(Imax−Imin)/(Imax+Imin). The polarization ratios of R/S-3BrMBA₂PbBr₄ crystals are ~90.8% and ~98.7% (under the 880 nm excitation), respectively, indicating a high sensitivity of the second-harmonic generation regarding to the crystal symmetry.

Lastly, we investigate nonlinear chiroptical properties of R/S-3BrMBA₂PbBr₄ crystals. During our measurements, the polarization of the excitation laser was varied between left- and right-handed (σ− and σ+) by rotating a quarter-wave plate (λ/4 waveplate). As shown in Figs. 4(e), 4(f) and Figs. S13(c), S13(d) (Supplementary material), the SHG intensity of R/S-CHPs change with the rotation of the λ/4 waveplate. In particular, the SHG signal of S-3BrMBA₂PbBr₄ crystals is significantly stronger under σ− excitation compared to that under σ+excitation, while the opposite trend is observed for R-3BrMBA₂PbBr₄ crystals. To quantify the second-order NLO chirality of these CHPs, the second-order non-linear anisotropy factor (gSHG−CD) is defined as^{[43, 44]}:

6gSHG−CD=2×|Iσ−−Iσ+|Iσ−+Iσ+,

where Iσ− and Iσ+ refer to the SHG intensity for the σ− and σ+ excitation, respectively. The obtained values of gSHG−CD under 840 nm excitation are ~0.337 and ~0.334 for R/S-3BrMBA₂PbBr₄ crystals, respectively. The structural asymmetry induced by chirality within 2D CHPs results in the corresponding asymmetry of their electronic ground states, and therefore contributes to the observed gSHG−CD^{[65, 66]}. This anisotropic response further demonstrates that our 2D CHPs have the capability to directly distinguish left-handed and right-handed circularly polarized light.

In summary, a new type of 2D CHPs, R/S-3BrMBA₂PbBr₄, has been synthesized and shows excellent optoelectronic and chiroptical properties. These 2D CHPs exhibit prominent CD properties at room temperature and show ultra-broadband "cool" white-light emission under UV light irradiation. Furthermore, the spatial inversion symmetry breaking in these CHPs leads to a strong NLO response. We observed that R/S-3BrMBA₂PbBr₄ exhibits high-efficiency SHG performance and non-linear chiroptical properties with high polarization ratios. Our study not only provides a novel design of chiral halide perovskites, but also paves the way for the development of high-performance nonlinear optical and chiroptical devices.