Preparation And Photoelectric Performance Of Novel Ternary Transition Metal Chalcogenides

The discovery of graphene has completely overturned the traditional mindset of physics community,allowing us to glimpse the wonders of two-dimensional（2D）world.2D materials exhibit physical,and chemical properties that are completely different from those of bulk materials,arousing the researchers’enthusiasm for further exploration and preparation of novel 2D materials.2D materials possess unique characteristics of ultra-thin channel,high mobility,and adjustable energy band.They can effectively suppress the short-channel effect below the 5 nm process node,and are expected to bring significant technological changes for the microelectronic devices in the‘post-Moore era’.Transition metal chalcogenides（TMCs）as one of the typical 2D materials with diverse chemical compositions and crystal structures,exhibit various electronic properties and are considered to be the most potential 2D materials after graphene.Ternary transition metal chalcogenides（TTMCs）produced by intercalating or reducing TMCs with transition metals,present adjustable composition,controllable and variable structure,and rich electronic structure,received extensive attention over the past few years.TTMCs exhibit many interesting physical phenomena and great application in the field of broadband photodetection by adjusting their symmetries and electronic structures,and are considered to be an excellent material system for exploring novel physical properties and studying optoelectronic properties.However,the harsh preparation conditions slow the current experimental research.In this dissertation,several TTMCs with novel electronic structures are successfully prepared by flux or chemical vapor transport methods.The crystal size is on the order of millimeters,showing good crystallinity and purity.Subsequently,their physical properties and optoelectronic properties are systematically measured and analysized.The research contents mainly include the following aspects:（1）The prepared Fe_0.75Ta_0.5S₂ single crystal by KCl/Na Cl flux method exhibits the coexistence og spin glass and antiferromagnetism.Because the uncompensated magnetic moment of the spin glass state is pinned by the moment of the antiferromagnetic state in the as-prepared single crystal,a large exchange bias field of～1.98 T is found at 2 K when cooled down at a field of 7 T.Due to the contribution of orbital effect caused by the interference of the hopping path between the two regions with hopping length R,the as-prepared samples show a large negative magnetoresistance（n MR）at low temperatures.The field dependence of n MRs displays a similar trend up to 50 K,which is likely to originate from the significant dependence of the localization length on the magnetic field.In addition,the two-electrode photodetector prepared using Fe_0.75Ta_0.5S₂ flakes exhibits a fast response time（121.7 ms）,good stability,high responsivity of 26.1 A W^-1,and broadband photodetection,displaying great potential for advanced multifunctional spin-optoelectronic devices application.（2）The prepared Ta Fe Te₂ single crystal exhibits a typical spin glass behavior with a transition temperature of～20 K.Owing to the weak localization effect and orbital effect,it exhibits intrinsic unsaturated n MRs up to 9 T.The phonon-induced noncentrosymmetric in Ta Fe Te₂ flakes may provide a polarization electric field to promote the separation of photogenerated electron-hole paors,thus the prepared two-electrode photodetector exhibites a fast photocurrent response,high responsivity（76 m A W-1）,and a robust photocurrent in the inner regions away from the junction region under the self-powered mode.In conventional mode,the prepared detector exhibits a fast response time（～2.51ms）,high responsivity(0.18 A W^-1),and broadband spectral response up to 10.6mm,showing great application potentials in spin-optoelectronics.（3）Because of the thermal activation of impurities in the prepared Ta₂Ni₃Te₅ single crystal,its resistivity exhibits unusual changes with temperature,and a rare negative MR effect under a weak magnetic field around the transition temperature region.Interestingly,the MR of Ta₂Ni₃Te₅ single crystal exhibits a linear dependence on the external magnetic field at low temperatures similar to the superquantun effect.Possibly due to its second-order topology and Luttinger liquid behavior,the prepared two-electrode Ta₂Ni₃Te₅photodetector exhibits a fast response time（～21.25ms）,high responsivity(110mA W^-1),and a non-localized photocurrent response under the self-powered mode.In traditional mode,the prepared Ta₂Ni₃Te₅ photodetector also exhibits a fast response time（～0.605ms）,high responsivity(0.26 A W^-1),and broadband spectral response up to 10.6mm under the conventional mode,showing great application potential in novel electronic and optoelectronic fields.（4）The prepared high-quality Ta Co₂Te₂ single crystal exhibits a large,un-saturating,anisotropic positive MR up to～2682%at 2.5 K and 9 T.The curves of MR significantly deviate from the expected H² dependence probably owing to the incompletely compensated carrier concentration and multiple scattering mechanisms.The prepared two-electrode photodetector by using Ta Co₂Te₂ flakes under the self-powered mode demonstrates the extreme photocurrent responses at the metal/Ta Co₂Te₂ junctions and robust edge photocurrent response when the laser is far away from the junction regions probably owing to crystalline-symmetry breaking.Under the conventional mode,the photodetector exhibits excellent negative photocurrent with a high responsivity of～3.13A W^-1,fast response time of～800ms,and broadband photo-response from 405 nm to10.6mm.The detection range of the prepared Ta Co₂Te₂ photodetector is expected to contain the far-infrared and even the Terahertz region,owing to the gapless electronic structure.In summary,several novel TTMCs synthesized in this dissertation have different electronic structures and symmetries,present rich and diverse physical phenomena and optoelectric characteristics,and exhibit great potential in broadband photodetection.Thus,it provides a good materials platform for the study of novel physical phenomena and photoelectric properties,effectively promoting the development of condensed matter physics and broadband photodetection.