Mechanical Properties Of Several Families Of Hybrid Organic-Inorganic Perovskites

Hybrid organic-inorganic perovskites(HOIPs)have attracted significant attention due to their outstanding physical properties and promising application potential in the fields of optoelectronics.For future commercialization,mechanical properties of hybrid perovskites are vital for their manufacturing,processing and durability.In this context,mechanical properties of HOIPs need to be well understood.By focusing on the three dimensional MDABCO-BI₃ system(B=K⁺or NH₄⁺,MDABCO=N-methyl-N'-diazabicy-clo[2.2.2]o--ctonium),the two dimensional(PMA)₂Pb X₄ system(X=Cl,Br or I,PMA=Phenylme--thylammonium)and the[TPr A][Cu(NO₃)(dca)₂](TPr A=Tetrapropylammonium)system,we studied the relationship between their crystal/electronic structures and the corresponding mechanical properties.In addition,we explored the microscopic pathways to tune mechanical properties.The main discoveries of this thesis are listed below.(1)The mechanical properties of three dimensional MDABCO-BI₃ system.We extensively studied the mechanical properties of MDABCO-BI₃(B=K⁺or NH₄⁺)family compounds with the ABX₃ perovskite architecture,and elucidated the influences of the B-site cations.The two HOIPs are isostructural which crystallize in the trigonal system with the same R3 space group and similar cell parameters.The calculations based on the density functional theory provided the elastic constants and corresponding elastic properties of the two HOIPs,respectively.The Young's modulus of MDABCO-KI₃(E?71.43 GPa)is about 3.75 times as that of MDABCO-NH₄I₃(E?19.04 GPa).Similarly,the maximum value of shear modulus of the former(G?31.60 GPa)is about 4.56 times of the latter(G?6.93 GPa),and the bulk modulus of the former(K?22.7 GPa)is about 2.03 times of the latter.Moreover,the range of Poisson's ratio of the former(0.178?0.002)decreases by 78.3%compared with that of the latter(0.732?-0.097).Furthermore,high-pressure synchrotron X-ray powder diffraction experiments reveal that the amorphization of MDABCO-KI₃ and MDABCO-NH₄I₃ occur respectively at 4.50 and3.23 GPa,and the crystallinity of MDABCO-KI₃ upon decompression is significantly lower compared with that of MDABCO-NH₄I₃.The distinct difference in mechanical properties of the two analogues are rationalized by examining their underlying crystal structures.The framework of MDABCO-KI₃ is constructed by strong K-I ionic bonding which is substantially more robust than the N–H···I hydrogen bonding in MDABCO-NH₄I₃framework.Therefore,the distinct bonding strength difference results in the remarkably different mechanical properties.(2)The mechanical properties of two dimensional(PMA)₂Pb X₄ system.We systematically studied the mechanical properties of two-dimensional(PMA)₂Pb X₄(X=Cl,Br or I)family with the A₂BX₄ perovskite architecture,and investigated the influences of the X-site halides on mechanical behaviors.The microscopic mechanisms in tuning mechanical properties were also discussed in detail at the atomic level.Both(PMA)₂Pb Cl₄ and(PMA)₂Pb Br₄ crystallize in the orthorhombic system with non-centrosymmetrical space group Cmc2₁,while(PMA)₂Pb I₄ crystallizes in the centrosymmetrical space group Pbca.We calculated the elastic constants and obtained full elastic properties of each HOIP based on density functional theory.The results show that the minimum values of Poisson's ratios of(PMA)₂Pb Cl₄,(PMA)₂Pb Br₄ and(PMA)₂Pb I₄ are 0.127,0.009 and-0.201,respectively,demonstrating a change of Poisson's ratio from positive to zero then negative.However,the range of Poisson's ratio of(PMA)₂Pb X₄ is increased from 0.127?0.469 to0.009?0.872 then to-0.201?0.736 with increasing halide radius.Moreover,the bulk modulus of(PMA)₂Pb Cl₄,(PMA)₂Pb Br₄ and(PMA)₂Pb I₄ are 15.846,13.616 and 12.288GPa,respectively,which are consistent with the results from the high-pressure synchrotron X-ray powder diffraction experiments.By examining the crystal and electronic structures of(PMA)₂Pb X₄ family compounds,we found that the bond distances of Pb-X are 2.877–2.886(Cl),2.979–2.996(Br)and 3.205–3.194(I)that increase with the decreasing of electronegativity of halide ions(Cl>Br>I).In this regard,the decrease of the Pb-X bonding strength gives rise to the enhancement of framework flexibility,the decrease of elastic and bulk modulus.Most importantly,the increment of structural degree of freedom of the inorganic layers in this family of perovskites leads to the occurrence of the positive to zero and then negative Poisson's ratio.(3)The mechanical properties of two dimensional[TPr A][Cu(NO₃)(dca)₂]and the mechanism of its mechanical exfoliation.We synthesized a new two-dimensional hybrid perovskite,[TPr A][Cu(NO₃)(dca)₂],and found its effective exfoliation could not be achieved by the facile mechanical cleavage method(i.e.scotch taping)but more intense exfoliating(liquid phase exfoliation).This phenomenon could be attributed to the corrugation of the Cu(NO₃)(dca)₂^-layer in the structure,the strong interlayer electrostatic interactions,and the dense packing structural fashion.As these bonding strengths can be directly reflected by elastic properties of materials,we performed nanoindentation measurements along in-plane and out-of-plane orientations respectively to obtain corresponding mechanical properties.The Young's moduli of in-plane and out-of-plane in[TPr A][Cu(NO₃)(dca)₂]are 14.4 and 10.0 GPa,respectively,giving an anisotropy ratio of 1.44.The anisotropic compressibility ratio between the in-plane and out-of-plane,extracted from the high-pressure synchrotron X-ray powder diffraction experiments,is 1.06 which also indicates the low modulus anisotropy.In this regard,the mechanical exfoliation processed of two-dimensional materials may be closely related to their mechanical anisotropies.To prove our hypothesis,we selected other 7representative two-dimensional materials with diverse inter-/intra-layer bonding interactions,including graphite,h-BN,Mo S₂,WS₂,black phosphorus and metal-organic framework Mn(C₆H₈O₄)(H₂O)in which the adjacent layers are tied via weak van der Waals forces,and Cu_1.5(H₂O)[O₃PCH₂CO₂]in which the neighboring layers are adhered via hydrogen bonding.By exfoliating these 8 materials in same experimental conditions,their corresponding nanosheets were obtained and characterized via AFM and TME to quantify lateral lengths(L)and thicknesses(N).Clearly,large ratio of L/N indicates a 2D material can be exfoliated to nanosheets more easily,and vice versa.Taking the mechanical properties of materials into consideration,we found that the ratio of in-plane/out-of-plane Young's moduli of a given two-dimensional material is proportional to the L/N ratio of its corresponding nanosheets.Thus,it is reasonable to define the ratio of A_In/Out=E_In-plane/E_Out-of-plane to quantify the exfoliation ease levels of two-dimensional materials.This anisotropy index not only provides a convenient way of quantifying the mechanical exfoliation ease level of two-dimensional materials,but also avoid to use the time-consuming theoretical calculations which can only indirectly inflect the in-plane and out-of-plane bonding strengths.