| Hybrid organic-inorganic halide perovskites with the chemical formula AMX3(A=CH3NH3+,HC(NH2)2+,or Cs+;M=Pb2+,Ge2+,Sn2+;X=I-,Br-,Cl-)has attracted extensive attention in the last ten years.They show broad applications in the fields of photoconductivity,conductivity,ionic conductivity and luminescence because of its suitable band gap,high absorption coefficient,excellent carrier transport performance and high defect tolerance.However,the commercialization of AMX3 perovskites suffers from instability problem caused by external factors such as humidity,light illumination and heating treatment.The dimensionality of perovskites structure is reduced by replacing A-cation with larger organic cations,which provides an effective way to synthesize perovskite with higher chemical stability.The diversified organic cations and inorganic molecules provide the feasibility to realize the customizable structure and adjustable photoelectric properties of low-dimensional perovskites.The research of low dimensional perovskite is mostly based on polycrystalline films in recent years,and the intrinsic photophysical properties of low-dimensional perovskites are still controversial.On the other hand,the wide band gap of most low dimensional perovskite hinders their photovoltaic applications.How to design and synthesize organic-inorganic perovskites with low band gap and high stability is the development direction in the future.The relationship between material structure and properties is the fundamental issue of material science.Seeking new type of perovskite is always one of the main directions to obtain original innovation and promote application development.Based on the above problems,we select the bifunctional organic molecules of aminocarboxylic acid and introduces them into the perovskite lattice.We design and synthesize a series of low dimensional perovskites with high stability,and systematically study their structure,thermal properties,and photoelectric properties.The structure and properties of low dimensional perovskite is regulated and the band gap of low dimensional perovskites is tuned.The main results of this paper are as follows:1、We adopted a series of bifunctional molecules of amino acids(glycine,γaminobutyric acid,6-aminocaproic acid,8-aminooctanoic acid,10-aminodecanoic acid)with different number of carbon atoms to cross link[PbX6]4-octahedral to form a crosslinked framework structure.The five single crystals have been grown for the first time:(COOH(CH2)nNH3)mPbIm+2(n=1,3,5,7,9),(COOH(CH2)NH2)PbI2(C2),(COOH(CH2)3NH3)2PbI4(C4),(COOH(CH2)5NH3)PbI3(C6),(COOH(CH2)7NH3)2PbI4(C8),(COOH(CH2)9NH3)2PbI4(C10).We find that these crystals exhibit structural diversity and properties controllability.Bifunctional ligand amino acid organic molecules of perovskite structure directly influence[PbI6]4+octahedral connection mode,and result in the different degree of distortion,which influence the properties of the crystals.With the increase of C chain length,the five crystals employ different structure types vary from edge-sharing structures to face-and corner-sharing Pb/I structures.C4,C8 and C10 belong to corner-sharing RuddlesdenPopper(RP)two-dimensional perovskite structure.The distortion degree of inorganic layer is calculated by the Pb-I-Pb bond angles of adjacent[PbI6]4-octahedral structure,and the distortion degree of C4 structure is the smallest.For all structures,the extension length of Pb-I bond(λ)and the deviation degree of Pb2+ from the octahedral center(σ2)are used to calculate the degree of individual[PbI6]4-octahedral distortion,C6 structure shows larger individual[PbI6]4-octahedral distortion.2、The bifunctional organic molecules of aminocarboxylic acid bring additional interlayer hydrogen bonding force to the perovskite structure,and the long hydrophobic alkane chain can separate water from the octahedral structure framework.C6,C8 and C10 showed high stability and did not decompose after being stored with humidity of 75%RH,40℃ for nearly one year.The stability of C2 and C4 is poor due to the short C chain.The band gap of the series of perovskites increased from 2.19 eV to 2.6 eV.The band gap is mainly related to the connection mode of[PbI6]4-octahedron and the degree of structural distortion,which follows the corner-sharing<edge-sharing<facesharing.Therefore,C6 crystal with face-sharing structure has the largest band gap of 2.6 eV.The density functional theory(DFT)calculation shows a consistent trend with our experimental results.The larger band gap of face-sharing[PbI6]4-is attributed to the relatively flat electronic energy band.Steady state fluorescence spectra show that C8 and C10 crystals with corner-sharing[PbI6]4-octahedron structure have strong fluorescence,which is mainly related to the lower distortion degree of individual[PbI6]4-octahedron.3、We report the single-crystal to single-crystal(SCSC)phase transition induced by high temperature of RP perovskites C4.Single-crystal X-ray diffraction indicated that the crystal structure changes from layered RP at 302 K to "X" network composed of face-sharing and corner-sharing[PbX6]4-octahedron at 425 K,and the COOH(CH2)3NH3+cations exhibit a distinct disorder because of the thermal effect.The phase transition mechanism was analyzed by a variety of in-situ testing methods.The phase transition process was visualized by in-situ hot stage microscope.The new phase was generated from both sides of the crystal,and the two-phase interface gradually moved to the center of the crystal until the whole phase transition process was completed.DSC data show that the phase transition requires a large enthalpy of 49.08 kJ/mol,and it takes a long time to change to room temperature structure during cooling.Combined with the analysis of single crystal structure,it can be inferred that the phase transition type belongs to reconstructive transformation.Due to the change of octahedral connection and the disorder of organic cations,the dielectric and absorption spectra of the materials change significantly at the phase transition temperature.Considering the thermochromism of the material,we apply it for anti-counterfeiting and information encryption.4、The mechanism of photoluminescence of RP perovskites C4 was explored.C4 crystal shows double emission under the excitation of 420 nm,and the emission peaks are at 528 nm and 548 nm at 215 K,respectively.The peak at 528 nm near to the band edge absorption can be attributed to free exciton(FE)emission.The intensity of the broad peak at 548 nm increases linearly with the increase of excitation power density and does not reach saturation,which can be attributed to self-trapped exciton(STE).The material exhibits excitation wavelength dependent emission(EDE)properties with fluorescence color changing from green to red upon 420 nm and 546 nm excitation,respectively.Time-dependent density functional theory(TD-DFT)indicated that this phenomenon is mainly related to the[PbI6]4-octahedra structural deformation.The results show that the material has potential application for multifunctional devices.5、In order to expand the photoelectric application of low dimensional perovskite,MA+ ion exchange and Sn2+doping are designed to narrow the wide bandgap of low dimensional perovskite.The C6 single crystal is subjected to MA+ion exchange to form a heterostructure.The rate of ion exchange is mainly related to the crystal plane and crystal direction.The conversion mechanism is analyzed by AFM.Quasi two dimensional perovskite single crystal(COOH(CH2)3NH3)2MAPb2I7 was synthesized and grown,and its band gap was further reduced by doping Sn2+. |
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