INTRODUCTION
Solid-state crystalline materials that enable the switching of nonlinear optical (NLO) properties between two or more states under external stimuli, such as light, heat, and pressure, have been attracting considerable attention for their potential applications in photoswitches, biosensors, environment monitors, and memory storage[1-4]. For liquid systems, the switching of NLO properties has been achieved through chemical modifications, such as redox reactions, protonation, and photochromism[5-7]. However, realizing the high reversibility of NLO switching in liquids remains a challenge because they face serious detuning or instability problems. In contrast, solid-state NLO materials that show the controllable second harmonic generation (SHG) signal between different phases offer a potential platform for achieving excellent NLO switching behavior[8,9]. The SHG-active phase is generally known as the “SHG-on” state, while the SHG-inactive phase corresponds to the “SHG-off” state. In this regard, the thermally-induced structural phase transition from a non-centrosymmetric (NCS) phase to a centrosymmetric (CS) structure might be effective in triggering switchable SHG properties in solid-state crystalline materials[10]. Several typical solid-state materials that exhibit switchable NLO behaviors have so far been reported, such as single-component plastic crystals[11], host-guest inclusion compounds[12], organic-inorganic hybrids[13], covalent organic frameworks[14], and ionic salts[15]. These compounds undergo structural changes with different phase transition temperatures (Tc), such as [NH3(CH2)5NH3]SbCl5 (365.4 K)[16], (C6H14N)2SbCl5 (477 K)[17], [(CH3)3PCH2OH][Cd(SCN)3] (248.5 K)[18] and [(CH3)4P]2Cr2O7] (221 K)[19], in which the SHG responses display remarkable variations. Despite the reasonable development of phase transition materials, near-room-temperature reversible NLO switches have rarely been reported. Hence, there is an urgent need to explore new NLO switching materials that undergo the NCS-to-CS phase transition near room temperature.
Organic-inorganic hybrid perovskites provide a significant opportunity to combine phase transitions with multiple switching responses[20]. Compared with purely inorganic or organic materials, hybrid perovskites combine the advantages of inorganic networks and organic components into a single-phase structure to achieve multifunctional properties[21-23]. Among them, the low-dimensional perovskites derived from the 3D ABX3-type prototype have also attracted significant interest because of their structural flexibility and diversity in triggering dynamic phase transitions[24,25]. Structurally, large-sized organic components can easily be accommodated to form low-dimensional perovskites [e.g., zero-, one-dimensional (1D) and two-dimensional (2D) subclasses], which possess considerable freedom of dynamic motion. Under external stimuli, the flexible organic cations may undergo rotations or reorientations that afford a driving force to structural phase transitions[26,27]. Consequently, low-dimensional hybrid perovskites have been regarded as ideal platforms for constructing solid-state phase transition materials[28-30]. For instance, Ai et al. reported a 1D perovskite-like hybrid of (trimethylchloromethylammonium)CrCl3, in which the ordering of the organic cations leads to a ferroelectric-type phase transition along with notable SHG properties[31]. Considering their structural flexibility and diversity, it is believed that low-dimensional hybrid perovskites provide tremendous opportunities to induce phase transitions and explore new materials for NLO switches.
The pyrrolidine ring is an important building block that has been exploited as a dynamic source to assemble phase transition materials, as exemplified by ferroelectrics and switchable dielectrics[32,33]. Under thermal stimuli, pyrrolidine moieties can spontaneously assemble into a disordered architecture because of their pseudosymmetric nature. In this work, we report a new 1D perovskite-like hybrid (MP)PbBr3 (where MP is 1-methylpyrrolidinium), showing the near-room-temperature reversible switching of NLO properties. This material undergoes a solid-state NCS-to-CS structural phase transition at 316 K upon cooling, as confirmed by differential scanning calorimetry (DSC) and dielectric measurements. At room temperature, strong SHG activities are observed in (MP)PbBr3, which displays an intensity that is ~1.6 times as large as that of potassium dihydrogen phosphate. Strikingly, it can achieve the near-room-temperature reversible switching of SHG characteristics with a contrast of up to ~40 between its “SHG-on” and “SHG-off” states, which is beyond that of most thin films and their liquid counterparts[34]. Further microscopic structural analyses reveal that the dynamic ordering of the organic MP+ cation and the inorganic chain-like skeleton triggers its centrosymmetric (P63/mmc) to acentric (P212121) phase transition at 316 K upon cooling, which contribute to its NLO switching properties. This work shows the potential of (MP)PbBr3 as a solid-state NLO switch and promotes the progress of NLO and NLO switching materials based on low-dimensional hybrid perovskites.
MATERIALS AND METHODS
Materials preparation
First, 2.53 g of lead acetate trihydrate (4 mmol) were added to a 100 mL round-bottomed flask, followed by 20 mL of a hydrobromic acid solution (40%) and 1.25 g of 1-methylpyrrolidine (3 mmol). They were then heated and stirred at 373 K for 1 h until all the precipitates were dissolved. The solvent was slowly evaporated at room temperature over several weeks and colorless crystals were obtained.
Structural characterization
Variable-temperature X-ray single-crystal diffraction analyses were carried out on a Bruker D8 diffractometer (Mo Kα radiation, λ = 0.71073 Å) at 193 and 350 K, respectively. Powder X-ray diffraction (PXRD) data were collected on a Mini Flex 600 (Rigaku Co., Tokyo, Japan) diffractometer in the 2θ range of 5°-40° with a scan step of 0.02°.
Property measurements
A thermal analysis of the sample was carried out in the temperature range of 300-360 K. In the DSC experiments, the aluminum crucibles with the dry powder of (MP)PbBr3 were put in a DSC instrument (NETZSCH 200 F3). Dielectric experiments were performed using an impedance analyzer (Tong Hui TH2828A). The SHG activities of (MP)PbBr3 were investigated by applying the Kurtz-Perry model (Nd:YAG laser, λ = 1064 nm). Further details are available in the Supplementary Material.
RESULTS AND DISCUSSION
Colorless crystals of (MP)PbBr3 were obtained from the acidic solution through a slow evaporation process at room temperature. The phase purity of (MP)PbBr3 was further verified by PXRD, which agrees reasonably well with the simulated pattern [Supplementary Figure 1]. Moreover, (MP)PbBr3 is thermally stable below 584 K [Supplementary Figure 2 and Supplementary Table 1]. Initially, the phase transition of (MP)PbBr3 was detected by DSC and variable-temperature dielectric permittivity measurements. As depicted in Figure 1A, a pair of endothermic and exothermic peaks are clearly observed at Tc = 339 and 316 K, respectively, in the process of heating and cooling, thereby revealing the reversible phase transition in (MP)PbBr3. These sharp anomalous peaks and the large thermal hysteresis of ~23 K indicate that the phase transition should be of the first-order type. The corresponding enthalpy change (H) and entropy change (S) were calculated to be 12.69 J/g and 19.96 J/(mol·K) in the heating mode, respectively [Supplementary Figure 3]. A relatively small thermal anomaly was observed in the cooling run, probably due to the stepwise ordering of structural moieties. For convenience, the phases above and below Tc are designated as the high-temperature phase (HTP) and low-temperature phase (LTP), respectively. Furthermore, crystalline materials that show a thermally-induced structural phase transition usually display typical temperature-dependent dielectric responses[35].
As shown in Figure 1B, the temperature-dependent dielectric real part (ε′) was measured at various frequencies (300 kHz-1 MHz), with two dielectric plateaus observed. For the LTP, the ε′ values are ~15 and almost remain stable with the temperature variation. In the vicinity of Tc, the measured ε′ rapidly increases to a plateau of ~19, corresponding to the high-dielectric state of (MP)PbBr3. The switching performance of the dielectric properties is reversible and displays a rectangular thermal hysteresis of ~20 K between the heating and cooling runs. Such dielectric bistability coincides well with the DSC traces, behaving as an indicator to the appearance of the phase transition.
For a deeper insight into the structural phase transition behavior, the single-crystal structures of (MP)PbBr3 were collected at 350 K (above Tc, HTP) and 193 K (below Tc, LTP), respectively [Supplementary Table 2]. For the HTP, (MP)PbBr3 crystallizes in a hexagonal crystal system with the CS space group P63/mmc. As shown in Figure 2A, the asymmetric unit of the molecular structure contains an organic 1-methylpyrrolidinium cation and an inorganic [PbBr3]- component, which construct the ABX3-type perovskite-like architecture. The 1D infinite chain-like frameworks consisting of the face-sharing PbBr6 octahedra are almost aligned along the crystallographic c-axis direction [Figure 2B]. The most intuitive characteristic is the disordering of both the organic cations and inorganic scaffolds. The PbBr6 inorganic octahedra adopt a highly-disordered configuration, as verified by the equivalent length of Pb-Br bonds (~3.0328 Å, Supplementary Table 3). Furthermore, organic MP+ cations also have the disordered molecular symmetry with six equivalent sites and closely link to the PbBr6 octahedra skeleton through N-H···Br hydrogen bonding interactions [Figure 2A and Supplementary Figure 4]. From the perspective of crystallography, the structural moieties locate at symmetric positions, thus satisfying the basic requirements of its CS symmetry.
When the temperature decreases below Tc, the symmetry of the crystal structure is broken. In the LTP, (MP)PbBr3 transitions to the orthorhombic crystal system with the NCS space group P212121[Supplementary Table 2]. Furthermore, the basic molecular structure of (MP)PbBr3 still contains one MP+ cation and one inorganic [PbBr3]- anion, which locate in completely ordered states. The 1D chain-like frameworks are composed of the face-sharing PbBr6 octahedra extended along the a-axis direction [Figure 2C and Supplementary Figure 4]. However, the geometry of these 1D chain-like scaffolds is distorted during the structural phase transition, as verified by the unequal Pb-Br bond lengths of 2.8593-3.2076 Å [Supplementary Table 3]. Comparative studies of the variable-temperature crystal structures of (MP)PbBr3 show that the dynamic ordering of the organic cations and inorganic frameworks affords the driving force to trigger its structural symmetry breaking at 316 K [Figure 2C]. The rotations and reorientations of such structural components make a dominant contribution to the generation of molecular dipole moments, as well as the macroscopic NLO properties. Therefore, it is expected that the NCS-to-CS phase transition of (MP)PbBr3 will lead to NLO switching behavior.
The above structural analyses demonstrate that the phase transition of (MP)PbBr3 takes place from the centrosymmetric HTP (P63/mmc) to the non-centrosymmetric LTP (P212121), leading to the notable breaking of crystallographic symmetry. Therefore, we analyzed the variations in the symmetry operation elements in (MP)PbBr3 during its phase transition. As shown in Figure 3, there are 24 symmetric elements (E, 2C6, 2C3, C2, 3C′2, 3C″2, I, 2S3, 2S6, σh, 3σv and 3σd) for the HTP, which dramatically decrease to 4 (E, C2, σv and σ′v) for the LTP[36]. From the perspective of structural changes, it is the dynamic ordering of both the organic cations and inorganic chain-like scaffolds that give rise to the phase transition, along with the disappearance of the mirror planes and inversion center. Considering this structural change of (MP)PbBr3, second-order NLO properties (e.g., SHG) would be expected in (MP)PbBr3 for the LTP, as well as the solid-state switching of its SHG effects during the phase transition.
Consequently, we measured the SHG signals of (MP)PbBr3 using the Kurtz-Perry method at room temperature. As shown in Figure 4A, the SHG signal intensities of (MP)PbBr3 display a gradual enhancement with increasing particle size and become almost saturated, indicating the phase matching attribute of its NLO property. Compared with that of the KH2PO4 (KDP) reference in the same range of particle size, the SHG intensity of (MP)PbBr3 is ~1.6 times greater (inset in Figure 4A and Supplementary Table 4), in good agreement with its NC structure in the LTP. Most intriguingly, the temperature-dependent reversible switching of the SHG intensities of (MP)PbBr3 is observed, as depicted in Figure 4B. The SHG signals keep almost stable below ~335 K upon heating, corresponding to the “SHG-on” state. However, with the temperature continuing to increase, the SHG signals present a dramatic step-like decrease and then completely disappear above Tc, except for weak noise, which is termed the “SHG-off” state. This variation in SHG properties coincides with its structure changes during the phase transition. Taking the noise level as the reference, the “on/off” contrast for the switching quadratic NLO effects of (MP)PbBr3 could be estimated to be at least 40, which is larger than those of the reported solid-state NLO-switching materials, such as (C6H14N)2SbCl5 (~13)[37], NH4[(CH3)4N]SO4·H2O (~24)[38], (Hdabco+)(CF3COO-) (~35)[8] and (C4H16N3)BiBr6 (~35)[39]. One of the most attractive attributes of (MP)PbBr3 is the near-room-temperature phase transition point, in the vicinity of which its SHG signals demonstrate the most significant variation with respect to the temperature change. Compared with other solid NLO-switching counterparts, e.g., [(DPA)(18-crown-6)]ClO4 (214 K)[12], [C4H12NO]MnCl3 (270 and 382 K)[40] and [C6H11NH3]2CdCl4 (215 and 367 K)[41], this merit should be indicative of its application potential at room temperature.
Switching reversibility of the dielectric constant and NLO properties are also important to evaluate the application potential of solid-state switch materials[42]. As shown in Figure 5A, the SHG effects of (MP)PbBr3 can recover rapidly after six cycles without any attenuation during the repeated heating and cooling runs. A similar anti-fatigue merit is also observed in the dielectric measurements. With the temperature changing periodically, the corresponding ε′ values demonstrate evident reversibility and remain quite stable after eight switching cycles [Figure 5B]. The superior reversibility figures of merit for (MP)PbBr3 make it exhibit higher sensitivity and lower dissipation, which are essential for near-room-temperature solid-state NLO switching materials.
To illuminate the optical range for the potential NLO applications of (MP)PbBr3, we measured its UV-Vis absorption spectrum at room temperature. As shown in Figure 6A, the optical absorption edge is located at 362 nm, corresponding to the bandgap of ~3.52 eV calculated from the Tauc equation (inset in Figure 6A). This bandgap value falls in the range of some values recently reported for 1D perovskite hybrids, such as [(CH3)3PCH2F]PbBr3 (3.215 eV)[43] and [C6H14N]PbBr3 (3.30 eV)[44]. Since the physical properties of solid-state materials are closely related to their crystal and electronic structures[45,46], we thus calculated the partial density of states of (MP)PbBr3 to analyze its composition of electronic band structures [Supplementary Material].Figure 6B shows that the H-1s states overlap with those of the C-2s/2p and N-2p orbitals, revealing that the C-H and N-H bonds of the organic moiety have strong covalent linkages. However, the upper part of the valence bands is dominated by the Br-4p orbitals near the Fermi level and the bottom of the conduction bands involves the Pb-6p and Br-4s/4p orbitals. Obviously, both the maximum of the valence band and the minimum of the conduction band primarily originate from the Pb and Br atoms, and it is, therefore, the inorganic frameworks that determine the optical bandgap of (MP)PbBr3. Considering the structural diversity of this organic-inorganic family, further optimization of the optical properties to a wide range could be achieved by modifying the inorganic components.
CONCLUSION
In summary, we have successfully obtained a one-dimensional perovskite-like hybrid, (MP)PbBr3, which enables the near-room-temperature switching behavior of SHG properties. Microstructural analyses disclose that the dynamic ordering of the organic cations and inorganic networks leads to its CS-to-NCS symmetry breaking at 316 K. Strong quadratic NLO activities with an intensity ~1.6 times greater than that of KH2PO4 are verified in (MP)PbBr3 in the LTP (termed the “SHG-on” state), while the SHG signals fully disappear above the Tc (“SHG-off” state). This NLO switching behavior demonstrates a large contrast of ~40 between the two different states, as well as notable switching repeatability. This work illustrates the excellent potential of (MP)PbBr3 as an NLO material and also sheds light on the exploration of new NLO switching candidates in the family of hybrid perovskites.
DECLARATIONS
AcknowledgmentsThe authors thank Aurang Zeb for help with experimentation.
Authors’ contributionsSynthesized and characterized the perovskite materials: Fan Q
Determined the single-crystal structures: Ma Y
Performed calculation and analysis: Liu Y
Implemented the characterization of the NLO material: Xu H, Song Y
Designed and directed the studies: Sun Z
Wrote the manuscript: Fan Q, Sun Z
Discussed the results and reviewed the manuscript: Fan Q, Ma Y, Xu H, Song Y, Liu Y, Luo J, Sun Z
Availability of data and materials Not applicable.
Financial support and sponsorshipThis work has been supported by NSFC (22125110, 21875251, 21833010, 22075285, and 21921001), the Key Research Program of Frontier Sciences of CAS (ZDBS-LY-SLH024), Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2021ZR126), the NSF of Fujian Province (2020J01112), the Strategic Priority Research Program of CAS (XDB20010200), and the National Postdoctoral Program for Innovative Talents (BX2021315).
Conflicts of interestAll authors declared that there are no conflicts of interest.
Ethical approval and consent to participateNot applicable.
Consent for publicationNot applicable.
Copyright© The Author(s) 2022.
Supplementary Materials
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