Two-dimensional Hybrid Perovskite-type Ferroelectric for Highly

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Two-dimensional Hybrid Perovskite-type Ferroelectric for Highly Polarization-Sensitive Shortwave Photodetection Lina Li, Xitao Liu, Yaobin Li, Zhiyun Xu, Zhenyue Wu, Shiguo Han, Kewen Tao, Maochun Hong, Junhua Luo, and Zhihua Sun J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12948 • Publication Date (Web): 20 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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Journal of the American Chemical Society

Two-dimensional Hybrid Perovskite-type Ferroelectric for Highly Polarization-Sensitive Shortwave Photodetection Lina Li,§ Xitao Liu, § Yaobin Li,§,‡ Zhiyun Xu,§,‡ Zhenyue Wu, §,‡ Shiguo Han,§, ‡ Kewen Tao,§,‡ Maochun Hong,§ Junhua Luo § and Zhihua Sun,§,* § State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡University

of Chinese Academy of Sciences, Beijing 100049, China

ABSTRACT: Two-dimensional (2D) materials have been well developed for polarization-sensitive photodetection, while new 2D

members used in shortwave region (> 2.5 eV) still remain scarce. The family of 2D hybrid perovskite ferroelectrics, in which the coupling of spontaneous polarization (Ps) and light benefits dissociation of photo-induced carriers, have shown great potentials in this portfolio. Here, we report a new 2D hybrid perovskite ferroelectric, [CH3(CH2)3NH3]2(CH3NH3)Pb2Br7 (1), which exhibits superior Ps of 3.6 μC/cm2 and a relatively wide bandgap (~2.55 eV). The unique 2D perovskite motif results in an intrinsic anisotropy of optical absorption (the ratio αc/αa ≈ 1.9 at 405 nm), involving with its polarization-sensitive activity. As expected, the strongest photoresponses were observed along the c-axis (i.e., parallel to Ps), along with a large dichroism ratio (Iphc/Ipha ≈ 2.0) and highly-sensitive detectivity up to ~109 Jones. Further, crystal-device of 1 shows a fast responding rate (~20 μs) and excellent anti-fatigued merits. As a pioneering work, 1 is the first polarization-sensitive ferroelectric in the new branch of 2D hybrid perovskites. Such intriguing behaviors make 1 a potential candidate for the shortwave polarized-light detection, which also sheds light on new functionalities for future optoelectronic application of hybrid perovskites.

1. INTRODUCTION Two-dimensional (2D) materials are emerging as an important class of optoelectronic materials for assembling the newly-conceptual electronic and photoelectric devices, including graphene, black phosphorus, MoS2 and ReSe2, etc. 1 Among them, polarization-sensitive photodetectors have been applied to diverse fields from optical communication to military applications, based on the intrinsic structure anisotropy of such 2D materials.2 For instance, the highperformance detectors based on black phosphorus can distinguish polarized-light over a broadband region, depending on the anisotropy of its optical absorption capability.3 Currently, the mainstream research on this topic has long focused on narrow-bandgap materials with the bandgap (Eg) smaller than 2.0 eV,4 such as black phosphorus (~0.3 eV), ReSe2 (~1.5 eV), GeS2(~1.2 eV) and MoS2 (~1.2 eV), and so on. In contrast, for the development of polarized-light detectors in the shortwave region (> 2.5 eV), new 2D candidates with the relatively wide bandgap are rarely explored, which enable the direct detection of shortwave polarized-light without any extra optical accessories. 5 In this context, it is urgently desirable to explore the new members of 2D materials with relatively wide optical bandgap for the highly polarization-sensitive shortwave photodetection. Inspired by the breakthrough of 3D organic-inorganic hybrid perovskites (most notably CH3NH3PbX3),6 the family of 2D layered hybrid perovskites have also received great attentions, due to their intriguing physical and photoelectric characteristics.7 Structurally, such 2D hybrid perovskites exhibit unique compatibility and tunability;

both organic and inorganic components can be tailored to modulate electronic, optical and photoelectric activities.8, 9 Emphatically, the dynamic organic cation moieties enable very large freedom of motions, which behaves as the driving force to induce ferroelectricity. 10 Consequently, the coupling of spontaneous polarization (Ps) and light benefits dissociation of photo-induced carriers, and makes 2D hybrid perovskites the competitive candidates for the newgeneration photonic devices.11, 12 For instance, a wide optical bandgap range of 3.05~2.74 eV was achieved in a series of 2D perovskite ferroelectrics, (cyclohexylaminium)2PbBr4−4xI4x,13 and the x = 0 compound displays strong anisotropy of photoelectric activities.14 More recently, high-performance detector of MoS2/ poly(vinylidene fluoridetrifluoroethylene) was assembled by using ferroelectric films as gate material to depress dark current and enhance the device sensitivity.15 Despite extensive studies on hybrid perovskites, however, 2D layrered hybrid perovskite ferroelectrics have never explored for the polarization-sensitive photodetection.16 In this aspect, we proposed that the class of 2D hybrid perovskite ferroelectrics provides great opportunities to achieve candidates for polarized-light detection in the shortwave region, if combining strong anisotropy of their structural architecture and the facile bandgap engineering. Here, we report the first polarization-sensitive ferroelectric in the family of 2D hybrid perovskites, (BA)2(MA)Pb2Br7 (1, where BA is n-butylammonium and MA is CH3NH3+), which exhibits superior ferroelectricity with Ps value of ~3.6 μC/cm2 and a relatively wide optical bandgap of ~2.55 eV. Strikingly, 1 shows polarization-sensitive behavior with a dichroic ratio Iphc/Ipha ≈ 2.0 in the shortwave region, and the strongest photo-responses were obtained parallel to Ps direction.

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Theoretical analyses reveal this performance of 1 involves with its strong anisotropy optical absorption (the ratio is αc/αa ≈ 1.9 at 405 nm), stemmed from the unique 2D perovskite motif. Moreover, crystal-devices of 1 enable a very fast photo-responding rate of ~20 μs, as well as excellent anti-fatigued merits under ambient conditions. All these features make 1 a potential candidate for shortwave polarized-light detection, and highlights new functionalities in the future optoelectronic device application of organicinorganic hybrid perovskites.

2. RESULTS AND DISSCUSSION 2.1 Structural assessment. Basic structure motif of 1 adopts a typical 2D Ruddlesden-Popper perovskite framework of (Aʹ)2(A)n-1MnBr3n+1, where Aʹ denotes the bulky spacer cation, A is the small cation that templates inorganic perovskite framework (termed “perovskitizer”), and n is the number of thickness for inorganic layers.17 This fascinating branch of 2D hybrid perovskites usually features the structural motif like that of inorganic oxides, such as Sr3Ti2O7 18 and Sr4Ti3O10.19 Such dimensionally-reduced hybrid perovskites demonstrate a strong structural anisotropy and compatibility. As depicted in Figure 1, the bilayered 2D perovskite motif of 1 is obtained by alloying BA cation into the cubic prototype of MAPbBr3. Distinct from that of MAPbBr3, the alternate arrangements of organic cations and inorganic bilayers result in a strong intrinsic anisotropy of structure for 1. Figure 1 displays that its inorganic perovskite bilayers distribute in the bc plane, featuring a tilted network of corner-sharing PbBr6 octahedra due to the presence of stereochemically active electron lone pairs of Pb2+ cation. This 2D extended inorganic framework determines the electronic structures of 1, resembling that of 3D prototypes,20 which contribute to the high mobilities of photoexcited carriers. Meanwhile, along the out-of-plane direction (i.e. the crystallographic a-axis), its organic layers behave as the “spacer” that split inorganic perovskite networks. Dynamically, the freedom for molecular motions of bulky organic cations affords the driving force to generate electric polarization (as shown in Figure 1), as exemplified by some ferroelectrics, such as (BA)2(formamidinium)2Pb3Br1016c and (BA)2CsPb2Br7,21 etc. Hence, strong intrinsic anisotropy of structural, electric and optical features is formed in 1, which will favor its potential polarized-light detection.

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dynamic reorientation of bulky organic cations, corresponding to the emergence of Ps along the crystallographic c-axis.

At room temperature (ferroelectric phase, FEP), 1 adopts a polar crystal structure of Cmc21 (the point group mm2). The smaller MA+ cation locates in the perovskite cavities, acting as the “perovskitizer”. In contrast, the organic BA+ cations reside between two inorganic bilayers, forming the cleavage of 3D cubic prototype along (100) crystallographic direction. Hence, a bilayer-thick sheet is formed in 1 by incorporating bulky cation into the cubic prototype of MAPbBr3. The most evident difference lies in the MA+ cation, which adopts a dynamic disordered state with C3v symmetry in MAPbBr3;22 for 1, the MA+ cations are fully ordered (Figure 2a), of which the ammonium groups are tightly fixed by strong N-H∙∙∙Br hydrogen bonds and methyl groups almost direct along its caxis (Figure S3). It is believed that this alignment will be favorable to electric polarization of 1. Moreover, inorganic PbBr6 octahedra exhibit a sterically distorted configuration, as revealed by the equatorial Pb-Br lengths and Br-Pb-Br angles (Figure S3). For instance, the Br-Pb-Br angles deviate away from the standard value of 90º (from 87.8 to 92.2º); the quite small-angle tilting of bridging Br(1)-Pb(1)-Br(3) angle (~176.2º) further confirms the distortion of PbBr6 octahedra. From the packing views, the tilting of such PbBr6 octahedra leads to the displacements of negatively-charged centers along the c-axis. Meanwhile, the reorientation of organic BA+ and MA+ cations favors the shift of positively-charged centers in the [001] direction. Due to the coupling of such dynamic motions, the positive and negative charge centers are separated, as shown in Figure 2a, which give rise to the molecular dipole moments and electric polarization along the c-axis direction.

Figure 2 (a) Packing diagram of 1 viewed along the b-axis at FEP. Red arrowhead denotes the possible direction of electric polarization. (b) Packing diagram of 1 at PEP. Right: the building block of its perovskite framework containing the disordered MA+ cation.

Figure 1. Schematic diagram for the design of highly anisotropic 2D perovskite motif of 1, i.e. alloying the organic BA+ cation into the cubic prototype MAPbBr3. Red arrowheads denote the possible

Upon heating to 355 K (paraelectric phase, PEP), 1 transforms to the centrosymmetric structure with space group of Cmca (the point group mmm). Although its cell parameters are still approximate to those at FEP (see Table S1), both the organic MA+ and BA+ cations become highly disordered with two equivalent positions, which adopt a symmetric mirror geometry normal to its c-axis, well corresponding to their orientational disorder (Figure 2b). Meanwhile, the inorganic frameworks also feature a highlysymmetric configuration, as verified by the standard value of 90º for Br-Pb-Br angles. For 1, this variation satisfies the


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Journal of the American Chemical Society crystallographic requirements of mirror symmetry at PEP. Due to disordered characteristics of organic cations, the packing diagram in Figure 2b shows that the neighboring molecular dipole moments cancel each other out, and ultimately eliminate bulk electric polarization. As a result, its phase transition mechanism can be classified as an orderdisorder type, and the frozen reorientation of organic cations affords a driving force to the generation of electric polarization of 1. Moreover, during its phase transition, the number of symmetry operations decrease by half from 8 (Cmca, FEP) to 4 (Cmc21, PEP); this obeys the symmetry breaking principle with an Aizu notation mmmFmm2 (Figure S4),23 being reminiscent of its possible ferroelectricity.

Figure 3. (a) Variable-temperature NLO signals collected using the polycrystalline sample of 1, indicating its symmetry breaking in the vicinity of Tc. Inset: comparison of NLO intensity for 1 and KDP. (b) Ferroelectric P-E hysteresis loops recorded by using the double wave method along c-axis at different temperatures.

2.2 Ferroelectric and related properties. The emergence of Ps in 1 was verified by variable-temperature quadratic NLO, thermal and dielectric measurements. DSC and the peak-like dielectric anomalies suggest the characteristics for a proper ferroelectric transition of 1 at Tc = 352 K (Figures S5 - S7). In particular, temperature dependence of NLO signal confirms the symmetry breaking for 1. As shown in Figure 3a, its NLO signal is basically zero above Tc, well coinciding with its centrosymmetric structure of Cmca (Table S1). With temperature decreasing below Tc, the detectable NLO signals show a gradual enhancement and become stable at room temperature (~0.4*KDP, Figure S8), which is reminiscent of electric polarization for 1 below Tc. As a direct evidence, P-E hysteresis loops were measured along its c-axis (Figure 3b). At 318 K, the rectangular ferroelectric loops were recorded, affording Ps value of ~3.6 μC/cm2 and the coercive electric field of ~26 kV/cm. The Ps value of 1 falls in the range of some other molecule-based ferroelectrics,24 such as (Nmethylpyrrolidinium)3Sb2Br9, trimethylchloromethyl ammonium trichloromanganese(II), and (3pyrrolinium)CdCl3, but smaller than recently-reported organic perovskites.25 If used for photodevice, this electric

polarization may create a very high electrostatic field (~107 V/cm, estimated from Εi = σ/εrε0, where σ is the charge density at electrode surface, εr is dielectric constant and ε0 is the vacuum permittivity), which might greatly depress the dark current of the detector and thus improve the device efficiencies.26 At 355 K, the linear P-E

relationship reveals its paraelectric feature (i.e. Ps = 0). This temperature dependence of Ps definitely confirms bulk ferroelectricity of 1. Considering that its inorganic perovskite framework enables the photosensitive activities, 1 is expected

to be a potential ferroelectric candidate for polarized-light detection.

2.3 Optical and photo-responsive properties. The energy bandgap of 1 was determined using optical absorption spectroscopy. As depicted in Figure 4a, it shows an absorption cutoff around ~490 nm, which affords the bandgap (Eg) of ~2.55 eV using the Tauc equation of (αhv)2 = A(hv-Eg), where A is a constant, hv is photon energy, α is the absorption coefficient (Figure S9). This figure is close to some other lead-based hybrids,27 and comparable to that of BiFeO3 (~2.7 eV).28 Further calculation of electronic structure and energy gap suggests direct bandgap feature for 1 (Figure S10); the partial density of states (pDOS) analysis reveals that its valence band maximum mainly results from Br-4p state and the conduction band minimum originates from Pb-5p states. Hence, it is proposed that inorganic frameworks determinate optical bandgap and energy structure of 1, as revealed by HOMO and LUMO orbitals (Figure 4b).29 It is notable that 1 also displays a sharp photoluminescence (PL) peak at ~485 nm (Figure 4a), which involves with the direct radiative recombination of excitons in the lead halide perovskite layers,7a,30 and the biexponential fitting of the decay trace affords fast and slow components of PL lifetime as τ1 = 22.3 ns and τ2 = 150.2 ns, respectively. Such large components of lifetime probably relate to the ferroelectric alignment of polar moieties, which weakens electron-phonon coupling and increases the excited-state lifetime of 1.31 As far as we are aware, the suppressed electron-hole recombination and improved carrier lifetime of 1 would be favorable to its potential polarized-light detection.

Figure 4. (a) Optical absorption and PL spectra of 1. Insert: the timeresolved PL spectrum and fitted data using double exponentials. (b) Total density of states (DOS) and pDOS for 1. Total DOS, red; pDOS: Pb, blue; I, green; organic part, pink. Inset: the calculated charge density isosurfaces for HOMO and LUMO orbitals. (c) Dark current and photocurrent of 1 under unpolarized light with different incident power (λ=405 nm). Inset: the logarithmic I–V traces. (d) Dependence of photocurrent for 1 under different wavelength illumination.



Before studying the polarization-sensitive performance of 1, we first investigated its photo-responsive properties under the unpolarized light illumination, using two-terminal detector with lateral architecture fabricated on the cleaved crystal-wafer. Figure 4c depicts that photocurrent increases greatly under the light irradiation of 405 nm, showing a notable dependence of incident power. For instance, the current increases from ~8.1 pA (Idark) to 9.8 nA (Vbias = 2 V; the incident light power of 2.1 mW). As a key merit to device reliability, this extremely low dark current is likely due to the low intrinsic carrier concentration inside high-quality singlecrystal wafers of 1, which will greatly favor to the high photodetectivity. From the logarithmic I-V curve (the inset in Figure 4c), the light “on/off” ratio can be estimated to be ~1.2×103, which is of the same magnitude order to that of MAPbI3 crystals,32 but much larger than those observed in many oxide ferroelectric crystals and thin films.33 Emphatically, under the different incident light illumination, the photocurrent of 1 displays an obvious dependence of the incident wavelength (see Figure 4d). The variation tendency coincides fairly well with its absorption spectrum, which suggests the potential of 1 for the shortwave photodetection (>2.5 eV). As a consequence, such excellent photoresponses to the normal light and wide bandgap of 1, promote us to further study its polarization-sensitive performance in the shortwave region.

Figure 5. (a) Obvious anistropy of absorbance properties for 1 along different crystallographic axes. (b) Polarization dependence of the angle-resolved photocurrent under incident polarized light. At 0° polarization angle, electric field induced by polarized-light is parallel to the direction of its a-axis. (c) Long-time repeative switching cycles of photoresponse for 1. (d) Temporal measurements of photocurrent. The values of trise/tfall are estimated to be ~20/28 μs, respectively.

2.4 Polarized-light detection behaviors. Generally, the anisotropy of optical properties is an essential basis for the polarized-light detection. Figure 5a depicts the absorbance of 1 along the different crystallographic axes by calculating the imaginary part of dielectric functions.34 Along the direction of c- and a-axes, an approximate ratio of αc/αa ≈ 1.9 was estimated for absorbance intensity at ~405 nm, along with the absorption coefficient of ~104 cm-1. This anisotropy affords a feasible method to achieve the optimized linearly polarization dichroism; that is, light can be incident along b-

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axis and linearly polarized along a- and c- axes directions, which correspond to the absorbance minimum and maximum, respectively. Subsequently, we controlled the incident polarized-light using a polarizer and half-wave plate, and the incident light direction was linearly polarized inside the ac-plane (Figure S10). Figure 5b displays the angleresolved photocurrent of 1 in the polar coordinate, which exhibits dramatic changes as a function of polarization angle. In detail, photocurrent reaches the minimum with initial incident light polarized parallel to its a-axis (0º polarization), while the maximum photocurrent emerges with polarizedlight along its c-axis (90º polarization). Further rotation of polarization angle leads to a reversible cycle at 180° polarization. Based on this result, a polarization-dependent dichroism ratio Imax:Imin, where Imax and Imin are photocurrent maximum and minimum at 90º and 0º polarization, was estimated to be ~2.0; this figure-of-merit is almost comparable with those of some classic 2D materials, such as black phosphorus (~3.5),3 GeSe2 (~1.09 at 532 nm) 35 and ReS2 (~3.5 at 520 nm),36 etc. From a structural viewpoint f 1, the strong anisotropy of its 2D electronic structure induces variations in the real and imaginary parts of the complex refractive index along different crystallographic axes. As a result, the anisotropy of optical absorbance is created in 1, with the absorption ratio (αc/αa ≈ 1.9) for the directions along c-axis and a-axis, respectively (Figure 5a). This figure coincides with above-mentioned linear dichroic behavior of 1 for the polarization-sensitive photodetection. Photodetectivity (D*) is an essential merit to evaluate the ability of photodetector to weak optical signals. For 1, the detectivity to the polarized-light is estimated by using D* = IphA1/2/[Pi×(2eIdark)1/2], if dark current is the major factor to the noise.37 Hence, the detectivity is calculated to be ~1.1 × 109 Jones, while the incident light is polarized along the c-axis (90º polarization). This figure almost catches up with some integrated devices based on other 2D materials, 38 such as photogate heterostructure of black phosphorus-WSe2, MoS2Si, and WSe2-SnS2, etc. Further, the temporal measurements were performed to study photoresponse dynamics of 1. Figure 5c displays that photocurrent shows no obvious fatigue after a long-term repetition of switching “on/off” the incident polarized-light (~104 cycles). This suggests that crystal-device of 1 has remarkable anti-fatigued merits under ambient conditions. Although 2D materials (e.g. black phosphorus) have attracted great attention due to their preeminent electronic and optoelectronic properties, the air instability remains an issue that hampers the development of their practical devices. In contrast, crystal-devices of 1 have overcome the possible stability problem in the ambient air. Another striking virtue for 1 is the very fast photo-responding time to incident polarized-light. The times were measured with photocurrent increasing from 10% to 90% for rise, and decreasing to 10% for the decay. From a complete cycle of “on/off” switching cycle (Figure 5d), the rise (trise) and decay (tfall) photoresponding times were measured to be ~20 and ~28 μs, respectively. As far as we are aware, these responding times are even shorter than some integrated detectors using classical 2D materials,39 such as MoS2/MAPbI3 and graphene/MAPbI3, and comparable to inorganic counterparts.40 Such a fast responding feature might be attributed to the high crystallinity of 1, which demonstrates


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Journal of the American Chemical Society great potentials of 1 for the future high-speed detection of polarized-light.

3. Conclusions In summary, we have explored a new 2D hybrid perovskite ferroelectric for polarization-sensitive detection in the short wave region (> 2.5 eV), affording an important complement to traditional 2D materials with narrow bandgap (< 2.0 eV). Besides high electrostatic field stemmed from ferroelectricity, its structural and optical anisotropy account for highly polarization-sensitive behaviors with a dichroism ratio of ~2.0, corresponding to the maximum and minimum photoresponses along crystallographic c- and a- axes, respectively. Moreover, the exfoliated crystal-device of 1 exhibits a high photodetectivity (~109 Jones) and fast photo-responding rate (~20 μs) to the polarized-light, along with an excellent antifatigue merit under ambient condition. All the characteristics make 1 a promising candidate for the shortwave polarizedlight detection, and explore new branch of 2D members in the class of organic-inorganic hybrid perovskites for the future optoelectronic device applications.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Experimental details; bulk crystals and growth morphology; DSC trace; temperature dependence of the real part (ε’) of complex dielectric permittivity; SHG intensity of 1 and KDP measured at room temperature; Experimental and calculated bandgap; CIF files for crystal structures of 1 at 200 and 355 K.

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was financially supported by the National Natural Science Foundation of China (21622108, 21875251, 21833010, 21525104, 21601188, 21571178 and 51502290), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20010200), and Youth Innovation Promotion of CAS (2015240).

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