Study On Preparation And Ferroelectricity Of Two-dimensional 2H α-In₂Se₃ Nanosheets

Two-dimensional（2D）layered ferroelectric materials have attracted widely attention for their stable ferroelectricity that even remains in atomically thin layers.They are not only important for exploring the interaction between dimensionality and ferroelectric order,but also can well solve the problems faced by traditional ferroelectric materials,and enabling ultra-high-density information storage and other novel functions applications.Among them,α-In₂Se₃is a recently discovered semiconductor material with unique in-plane（IP）and out-of-plane（OOP）intercorrelated ferroelectricity,which still exhibits room-temperature stable ferroelectricity when the thickness is even reduced to a monolayer.It has two stacking configurations:rhombohedral（3R）and hexagonal（2H）.Unlike the 3R structure,the 2H stackedα-In₂Se₃has a special interlayer antiparallel arrangement of polarization configuration,which has more flexible and diverse tunability.It has both ferroelectric and semiconductor properties,and has very good potential for applications in novel functional nano-information devices.However,the study of 2Hα-In₂Se₃is still in its infancy.Further studies on its high-quality preparation,ferroelectricity,and new functional applications of ferroelectric correlations are needed.Based on these,the main research and results of this thesis are as follows.（1）Preparation of high-quality large-size 2Hα-In₂Se₃nanosheets and study of ferroelectricity.2Hα-In₂Se₃nanosheets with different thicknesses were successfully prepared by physical vapor deposition（PVD）technique after optimizing the growth process.The nanosheets with different thicknesses have different transmittance and absorbance in the visible wavelength band,and the transmittance of 4.3 nm-thick nanosheets is up to 72%.The structure and ferroelectricity of the nanosheets were characterized.The results showed that the nanosheets have an asymmetric structure and room-temperature intercorrelated ferroelectricity.These results provide a basis for further in-depth study of layer-dependent ferroelectricity and ferroelectric correlation for functional device applications.（2）Layer-dependent ferroelectricity and polarization-dependent electrical transport studies in 2Hα-In₂Se₃.The IP electric dipole moment of 2Hα-In₂Se₃with 1-4 layers were calculated by density functional theory,and it was found that there is a significant odd-even oscillation in IP polarization,and this odd-even oscillation phenomenon could also be switched to OOP polarization by applying an IP electric field.The experimental characterization of microscopic ferroelectric domains with different thicknesses demonstrated that the existence of intrinsic IP and switchable OOP polarization odd-even effects within the thickness range of 10 nm for 2Hα-In₂Se₃.In addition,a significant negative differential effect was found in the fabricated 2Hα-In₂Se₃ferroelectric field-effect transistor（Fe FETs）,and the peak current density and peak-to-valley current ratio could be tuned by white light and positive gate voltage,respectively.（3）The resistive transformation characteristics of 2Hα-In₂Se₃ferroelectric correlations were investigated.Both 2Hα-In₂Se₃-based Fe FETs and vertical devices undergoing electrical transport tests exhibited significant ferroelectric hysteresis as well as a switchable dual-resistance state with good durability,and the resistance of Fe FETs could also be regulated by the gate voltage.In addition,2Hα-In₂Se₃-based planar device was fabricated.Three different resistive states were successfully achieved in the device by applying different IP polarization voltages to precisely control the polar order configuration of multilayer 2Hα-In₂Se₃.These resistive states have millisecond fast switching speed,excellent durability and retention,especially the reversible switching between the medium and low resistive states by～5.7×10²V·cm^-1ultra-low electric field.The device also exhibits distinctive photoresponse at different resistive states,allowing more resistive states to be obtained by light modulation of the high resistance state.These results indicate that 2Hα-In₂Se₃has a very high application potential for high-density,low-energy information storage.（4）Study on the application of 2Hα-In₂Se₃in bioreceptor simulation for function expansion.The monolayer 2Hα-In₂Se₃-based planar two-terminal device was successfully fabricated on flexible mica substrate for the first time.Using light-regulated polarization reversal,the monolayer device successfully simulated the basic behaviors of nociceptor,namely threshold,no adaptation,relaxation,allodynia and hyperalgesia under white light pulses.In addition,switching the polarization direction by electric field,the few-layer 2Hα-In₂Se₃bases planar devices equally well presented the basic characteristics of nociceptor under pulsed voltages.