Vapor-Phase Synthesis And Electric/Optoelectronic Properties Of Two-Dimensional CuCrS₂ And CuScS₂ Nanosheets

Two-dimensional materials possess a range of distinctive characteristics,such as ultra-thin thickness,strong light-matter interaction,and adjustable band structure,which offer extensive prospects for applications in electronics,optoelectronics,and other fields.CuCrS₂and Cu ScS₂ are two emerging semiconductor materials,whose crystal structures consist of alternating stacking of Cu ion layers and[Cr S₂]or[Sc S₂]layers in the c-axis direction,with Cu ions confined within the two-dimensional layers.Moreover,the diversity of Cu ion hybrid orbitals for bonding results in various lattice sites in the two-dimensional layers,along with low migration energy barriers,which favor controllable ion migration and significant ion-electron coupling effects,and thereby hold crucial research values in phase transitions,memristors,optoelectronics,and artificial neural networks.Nevertheless,due to their non-van der Waals structures,which possess strong chemical bonds in the three-dimensional direction,and complex element compositions,the synthesis of two-dimensional samples of CuCrS₂ and Cu ScS₂ is particularly challenging,thereby severely limiting their research and applications in electronics and optoelectronics.Thus,this thesis focuses on the synthesis of high-quality two-dimensional CuCrS₂ and Cu ScS₂ nanosheets,investigating the properties of Cu ions confined in the two-dimensional layers,and exploring the room-temperature ferroelectricity resulting from the polarized Cu ion site in two-dimensional CuCrS₂,as well as the photoelectric response characteristics of two-dimensional Cu ScS₂based on the coupled ionic-electronic effects.The research content is as follows:（1）Ultra-thin CuCrS₂ nanosheets（as thin as 3 nm）were synthesized by constructing a quartz U-shaped groove-limited growth space.Discrimination of the material’s Raman peaks vibration types was achieved through polarized Raman spectroscopy.The results indicate that the frequency of the A₁ vibration mode associated with Cu ion vibrations along the c-axis is 322 cm^-1.Temperature-dependent and laser power-dependent Raman testing showed that the harmonic vibrations of Cu ions in CuCrS₂ nanosheets decreased as temperature or laser power increased,revealing the feature that interlayer Cu ions are easy to be disturbed by external fields.Furthermore,optical second harmonic generation（SHG）testing revealed that two-dimensional CuCrS₂ nanosheets have a non-centrosymmetric structure.（2）CuCrS₂ nanosheets exhibit controllable interlayer Cu ion displacement-induced stable room-temperature ferroelectricity and high Cu rie temperature（up to 700 K）.Under ambient temperature,the phase and amplitude curves of the piezoresponse force microscopy test on CuCrS₂ nanosheets exhibit significant hysteresis characteristics,and controlled reading and writing of ferroelectric domains have been achieved,revealing their two-dimensional ferroelectricity at room temperature.Additionally,CuCrS₂ nanosheets display the characteristic of polarization locking between the in-plane and out-of-plane directions.The high Cu rie temperature of up to 700 K for two-dimensional CuCrS₂ has been revealed by detecting the high-temperature SHG signals.Combining first-principles theoretical calculations and crystal structure analysis has revealed the source of the high Cu rie temperature:the ferroelectricity of two-dimensional CuCrS₂ comes from the Cu ion displacement in the adjacent enantiomorphic tetrahedral sites.The Cu ions are confined to the two-dimensional[Cr S₂]layer and form a stable Cu S₄ tetrahedral bond with S.Due to the ABC-type stacking feature of CuCrS₂,there are additional Cu -Cr bonds（bond length:2.77（?））,which lead to a potential barrier of 0.38 e V for a single Cu ion to transition from an ordered state to a disordered state,enabling its ordered ferroelectric polarization to be maintained even at high temperatures.（3）Using a quartz sleeve to construct a confinement-space-assisted gas-phase synthesis method has enabled the preparation of ultra-thin Cu ScS₂ nanosheets（as thin as 10 nm）.The AA stacking of Cu ScS₂ increases the Cu -Sc distance to 3.31（?）,weakening the metal-metal interaction in the material,and thus reducing the stability of interlayer Cu ions.This results in the Cu ions in the two-dimensional Cu ScS₂ exhibiting long-range migration properties at room temperature.High-temperature SHG testing results show that the SHG signal of the material decreases with increasing temperature and disappears above 370 K.This indicates that the Cu ion bond stability is weak,and a slight temperature increase can cause the Cu ions to deviate from the non-centrosymmetric lattice sites.Furthermore,theoretical calculations show that the migration barrier for Cu ions between layers is about 0.24 e V.The low migration barrier promotes the ion migration characteristics of the two-terminal electrode devices of the Cu ScS₂ nanosheets,which can be observed at room temperature.（4）In two-dimensional Cu ScS₂ nanosheets,the controllable long-range migration of Cu ions induces tunable positive and negative photovoltaic responses.The I-V sweep curves of Cu ScS₂ two-terminal devices under limited current exhibit reversible switching behavior between high-resistance and low-resistance states,with a current switch ratio of 10².Furthermore,by controlling the migration of Cu ions in Cu ScS₂ nanosheets through electrical pulses,electrically modulated switching between high and low resistance states,as well as polarization direction tunable photovoltaic responses,have been achieved in the two-terminal devices.