Crystal Structure And Optical Properties Of Organic-inorganic Hybrid Halides Synthesized By CH₃NH₃I And HgI₂

HgI₂ has the advantages of a large atomic number,large band gap（2.13 e V）,high energy resolution,high X andγ-ray detection efficiency and excellent spatial resolution.It is an excellent material for room temperature nuclear radiation detectors and medical imaging.It also has a high photoelectric linear absorption coefficient and a typical two-dimensional van der Waals layer material,which has potential applications in the field of optoelectronics and luminescence.However,its poor chemical stability,high chemical activity and temperature dependence of luminescence limit its application.Organic-inorganic hybrid materials can not only maintain the optical,electrical and magnetic properties of the original inorganic raw materials,but also have the flexibility of organic molecules and the adjustability of structural,which is hot research topic at present.In this paper,CH₃NH₃HgI₃and（CH₃NH₃）₂HgI₄organic-inorganic hybrid halide materials were obtained by compounding HgI₂with organic CH₃NH₃I.The electronic energy band structures and optical properties were investigated.The improvement of their performance as detectors and their application value as luminescent materials were discussed.The crystal growth morphology of CH₃NH₃HgI₃in 15 solvent-antisolvent systems and（CH₃NH₃）₂HgI₄in 3 solvent-antisolvent systems were investigated by crystal morphology simulation.It was found that CH₃NH₃HgI₃was mostly crystalline in the form of flakes in ethyl acetate-toluene,ethyl acetate-toluene+benzene,ethyl acetate-toluene+chloroform and ethyl acetate-chlorobenzene,while granular crystals were obtained in the other systems.（CH₃NH₃）₂HgI₄was crystalline in the form of flakes in the solvent-antisolvent system of methanol-chlorobenzene.The morphology prediction preferred a suitable solvent-antisolvent system for the growth of large area/volume crystals.The growth of CH₃NH₃HgI₃and（CH₃NH₃）₂HgI₄crystals by the antisolvent method resulted in CH₃NH₃HgI₃flake crystals of 9mm×7 mm×1 mm and（CH₃NH₃）₂HgI₄flake crystals of 6 mm×5 mm×0.1 mm,respectively.The crystal structures of CH₃NH₃HgI₃and（CH₃NH₃）₂HgI₄were studied.CH₃NH₃HgI₃belongs to the monoclinic structure with space group P121/n1.（CH₃NH₃）₂HgI₄belongs to the orthorhombic structure with space group Pbca.The steric hindrance effect of CH₃NH₃⁺hinders the two-dimensional planar expansion of[HgI₄]^2-,which in turn constitutes the 1D chain structure of CH₃NH₃HgI₃and 0D structure of（CH₃NH₃）₂HgI₄.The single crystal XRD shows that the optimal growth surface for CH₃NH₃HgI₃is（101）and for（CH₃NH₃）₂HgI₄is（002）,which indicates that the habitual growth plane of crystals obtained by the solution method and the results are in good agreement with the morphological simulations.DSC results show that CH₃NH₃HgI₃and（CH₃NH₃）₂HgI₄had higher stability than HgI₂,with the highest sublimation temperatures of 179℃and 223℃,respectively.The determination of this temperature also provides the basis for the determination of the growth temperature of physical vapor deposition large-area thin film imaging detectors.The results show that CH₃NH₃HgI₃is a direct band gap semiconductor（Eg=2.80 e V）;（CH₃NH₃）₂HgI₄is an indirect band gap semiconductor（Eg=3.28 e V）.The UV-Visible spectra show that the optical band gaps were 2.88 e V and 3.18 e V respectively.The optical absorption coefficient of the two substances shows a blue shift in their UV absorption limits compared to HgI₂.The exciton binding energies of the first energy levels of CH₃NH₃HgI₃and（CH₃NH₃）₂HgI₄were calculated to be 166.81 me V and 256.00 me V,respectively.The room temperature fluorescence spectra confirmed the exciton binding energies of 162.22 me V and253.01 me V for the first energy level.The photoluminescence spectra of（CH₃NH₃）₂HgI₄show that there is a wide luminescence band at 540 nm spanning almost the entire visible spectrum with full width at half maximum（FWHM）is 201 nm,which indicates that（CH₃NH₃）₂HgI₄has potential application value as a broadband white light emitter and scintillator.