The Exploration On Chemical Vapor Synthesis And Ferroelectric Properties Of Two-Dimensional Cd₇Te₇Cl₈O₁₇ Flakes

Two-dimensional(2D)ferroelectrics combine the excellent properties of classic ferroelectrics and ultrathin 2D materials in performance and structure,and meet the requirements of integration,miniaturization and multifunction of devices,which have attracted increasing interest.However,the exploration of intrinsic 2D ferroelectric materials is still in the early stage,and it is urgent to introduce new members,which will provide more opportunities and objects for the basic research and practical ferroelectric devices.Cd₇Te₇Cl₈O₁₇ crystals belong to the Pca2₁ space group,owning non-centrosymmetric polar structures and layers with easily broken bonds,which mean potential 2D ferroelectric materials.Therefore,this thesis mainly carried out research work on the synthesis of high-quality 2D Cd₇Te₇Cl₈O₁₇ nanoflakes by chemical vapor deposition and the exploration of their ferroelectric properties.The main research contents are as follows:(1)The selenium-assisted chemical vapor deposition method was developed to achieve the controllable preparation of high quality ultrathin 2D Cd₇Te₇Cl₈O₁₇ nanoflakes on mica substrates which thickness can be thinned down to?3.8 nm.Combining Raman spectroscopy and micro-zone absorption spectroscopy,the optical anisotropy was revealed.The growth mechanism was confirmed by density functional theory(DFT)calculation and experiments.The introduction of Se and H₂O in Cd Cl₂·5/2H₂O made the reaction produce strong oxidants,resulting in the complete oxidation of Cd and Te,and finally realized the controllable preparation of Cd₇Te₇Cl₈O₁₇ nanoflakes with the preferential growth of(100)crystal plane.The selenium-assisted strategy may provide a new approach for synthesizing other high quality 2D materials.(2)The nonlinear optical and ferroelectric properties of synthetic 2D Cd₇Te₇Cl₈O₁₇nanoflakes were further studied.Second-order nonlinear optical characterization of Cd₇Te₇Cl₈O₁₇ flakes revealed its inversion symmetry breaking and domain structure with different lattice orientations.Piezoresponse force microscopy confirmed that there are spontaneous microscopic ferroelectric domains,controlled polarization direction under an applied electric field and non-volatile ferroelectric polarization consists in 2D Cd₇Te₇Cl₈O₁₇flakes.Moreover,ferroelectricity can be maintained to the thickness of 6 nm,which demonstrated that Cd₇Te₇Cl₈O₁₇is a novel and robust and intrinsic 2D ferroelectricity with out-of-plane spontaneous polarization.The temperature-induced phase transition in 2D Cd₇Te₇Cl₈O₁₇ flakes around 380 K was revealed by temperature-dependent second harmonic spectrum.Robust out-of-plane ferroelectricity in room temperature of 2D Cd₇Te₇Cl₈O₁₇flakes predicts attractive applications in memory,sensing and piezoelectricity.