| Ferroelectrics are a kind of special polar dielectric materials.The ordered arrangement of dipole elements in ferroelectrics presents spontaneous polarization,and the direction of spontaneous polarization can be reversed or reoriented under the external electric field.In recent years,metal halide perovskite has gradually developed into an important class of optoelectronic functional materials.Among them,the two-dimensional Ruddlesden-Popper perovskite with the general formula（R-NH3）2An-1BnX3n+1has strong structural compatibility and adjustability,which is beneficial to the realization of polar structure design and optoelectronic performance regulation at the molecular level.In particular,the built-in electric field formed by spontaneous polarization interacts with photo-generated carriers,which generates novel photophysical phenomena,and becomes the candidate for constructing novel photoelectric devices.In this paper,the preparation of two-dimensional perovskite ferroelectrics is the main objective.A series of compounds with two-dimensional perovskite structure were synthesized by using three-dimensional metal coordinated perovskite skeleton ABX3as template and long-chain organic amines such as isobutylamine,n-hexylamine and n-heptylamine as spacer unit R-NH3+of inorganic skeleton layer.The crystal structure,optical and electrical properties of the compound were systematically studied,and the mechanism of ferroelectric action was explained.The specific research contents include:（1）The long-chain organic amine is used as the spacer unit R-NH3+of the inorganic framework layer,and the guanidine（C（NH2）3+）with larger size is used as an A-position unit to be introduced into the framework cavity formed by the metal coordination octahedron BX6,thereby constructing the two-dimensional perovskite.Three examples of compounds with two-dimensional multilayer perovskite structure were obtained:[（CH3）2CHCH2NH3]2GPb2I7（1）,[CH3（CH2）5NH3]2GPb2I7（2）,and[CH3（CH2）6NH3]2GPb2I7（3）.The crystal structure analysis results show that all three compounds are crystallized in C2/c space group,and the guanidine ions are stabilized in the perovskite cavity formed by Pb I6octahedron through hydrogen bonds（N-H···I）,forming a two-dimensional inorganic skeleton layer{C（NH2）3Pb2I7}.The inorganic skeleton layer and the organic spacer layer R-NH3+are alternately arranged to form a unique quantum well structure.Photo-generated carriers generated during illumination are transported in the inorganic layer,which endows the material with excellent semiconductor photoelectric performance.The planar-array photodetector based on the single crystal of 1 exhibits good detection performance with the switch ratio of～1,000and the detectivity of 9.5×1011Jones（0.14 mW/cm2）.In this work,long-chain organic amines are used as the design strategy of interlayer units,which provides a useful reference for the subsequent synthesis of two-dimensional perovskite ferroelectrics.（2）By using long-chain alkyl amines（n-hexylamine and n-heptanamine）as the spacer units and methylamine and ethylamine as the A-cation,two-dimensional bilayer perovskite ferroelectric compounds have been successfully constructed.Both of them have two-dimensional double-layer perovskite structures.The order-disorder change of long-chain alkyl amines induces the structural phase transformation.Taking 5 as an example,DSC and dielectric results showed that a reversible structural phase transition occurred near 373 K（Tc）.The ferroelectric phase belonged to the polar space group Ima2,while the paraelectric phase was converted to the central symmetric space group I4/mmm,which conformed to the symmetry breaking of 4/mmm Fmm2.This process completely coincided with the experimental results of SHG.The saturated spontaneous polarization（Ps）of 5 is about 1.6μC/cm2at room temperature,and the spontaneous polarization reversal is achieved under the electric field of 34 kV/cm.Along the pole axis of 5,the built-in electric field formed by the ferroelectric spontaneous polarization can rapidly separate the photo-generated carriers and generate a stable photo-generated voltage（0.83V）.The relevant work has confirmed that the method for constructing the two-dimensional perovskite ferroelectric with photoelectric response by the long-chain alkyl amine is feasible,and provides an important reference for realizing the ferroelectric-photoelectric functional recombination in the future.（3）Based on the long-chain alkylamine（n-heptylamine）,we selected Cs+ion as the A-cationtoobtainthetwo-dimensionalperovskiteferroelectric[CH3（CH2）6NH3)]2CsPb2Br7（6）,and realized the self-driven photoelectric detection application through the ferroelectric-photothermal discharge effect.The absorption cutoff wavelength of 6 is near 436 nm and the band gap value is about 2.84 eV.DSC and temperature-dependent dielectric showed that the 6 had reversible structural phase transition at 333 K and 373 K.In the low temperature phase,6 crystallizes in the polar space group Pmc21.As for the intermediate phase,6 is converted into the space group Fmm2.When the temperature is raised to the high temperature phase,6 crystallizes in the I4/mmmspacegroup.Therefore,6hasacontinuous ferroelectric-ferroelectric-paraelectric structural phase transition.The long-chain alkylamine undergo an order-disorder transition,while Cs+generates atomic displacement,which results in ferroelectricity.At 340 K,the saturated spontaneous polarization（Ps）of 6 is about 1.9μC/cm2,and the coercive electric field（Ec）is 17.6kV/cm.Under the irradiation of the 405 nm laser,the crystal exhibits obvious ferro-pyro-phototronic effect along the polar axis direction,in which the ratio of the current generated by transient photothermal discharge to the steady-state bulk photovoltaic current is 1459%,the open-circuit voltage is as high as 0.83 V.This work indicates that the materials have potential application value in self-driven photoelectric detection,and provides a new perspective for the design and synthesis of ferroelectric materials with photoelectric function orientation. |
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