Chemical Vapor Deposition Preparation And Properties Investigation Of Two-dimensional Chromium-based Chalcogenide

The advent of graphene intrigued the research interest in two dimensional?2D?materials.The special 2D scale brings a series of novel physical properties and good compatibility to the plane processing technology,which makes the 2D functional materials as one of the most preferred candidate for future electronic devices.Among them,2D magnetic materials are the foundation for the development of future low-dimensional spintronic devices.However,most 2D materials do not possess intrinsic magnetism and the early study on 2D magnetic materials mainly focused on the magnetic modification of non-magnetic 2D materials.Until the past two years,with the successful preparation of 2D magnetic materials,the research on intrinsic 2D magnetism began to rise and develop.Currently reported 2D magnetic materials,such as CrI₃,Cr₂Ge₂Te₆,Fe₃GeTe₂ etc.,are all typical van der Waals crystals,which can be obtained by simple micromechanical cleavage method,but their yield is greatly limited.CrI₃ and other materilas cannot be stable in the atomosphere,so the acquisition of materials,construction of devices and even the later performance test should be carried out in the glove box.The low Curie transition temperature also hindered the practical application of 2D magnetic materials.Therefore,it is one of the most urgent problems to obtain high yield and high stability room-temperature intrinsic 2D magnetic materials.In this work,non-layered chromium chalcogenides with room-temperature stability and intrinsic magnetism were used as the research objects.2D thin layer samples were successfully synthesized by improved chemical vapor deposition?CVD?method,and their basic structure,optical,electrical,magnetic properties were characterized and analyzed.The work are listed as follows:?1?Combining van der Waals epitaxial growth and construction of the confined growth space,ultra-thin triangular or hexagonal 2D samples were successfully synthesized.The sample is as large as 40?m,with the thickness down to 2.5 nm?about two structural unit?.XRD,XPS,EDS,TEM and other means were used to characterize and analyze the phase,structure and compositions of the materials,confirming that the samples we obtained is rhombohedral Cr₂S₃.With the aid of confocal Raman microscope,the Raman variation spectra of Cr₂S₃ were collected and analyzed systematically,and the influence of thickness and temperature on the Raman shift was studied,and the Raman vibration modes were assigned for the first time.The low temperature electrical transport performance of the sample showed that the materials have good conductivity and exhibit typical semiconductor characteristics.?2?2D ultra-thin T?-phase Cr_0.68Se nanoflakes were successfully prepared by introducing a quartz U-shaped groove to build a limited growth space,so as to regular the rate and concentration of the gas flow transmission,and combining the van der Waals epitaxial growth mechanism.The phase,composition and structure were characterized and analyzed.The Raman vibrations of Cr_0.68Se nanoflakes were collected and assigned for the first time.The influence of thickness on the Raman shift was well investigated.The variations of Raman position in low-temperature zone and high temperature zone were studied to explore the thermal conductivity and thermal stability of the material.Cr_0.68Se nanoflakes show good conductivity and insensitive to the applied back gate,and exhibit weak p-type semiconductor characteristics.?3?By van der Waals epitaxial growth,hexagonal monoclinic Cr₅Te₈ nanoflakes were successfully synthesized on mica substrate.By means of XRD,XPS and TEM,the phase,composition and structure of the material were characterized and analyzed.We also investigated the optical performance of Cr₅Te₈ nanoflakes with the aid of confocal Raman spectroscope and second harmonic measurement system.At the end of the thesis,we summarized our work and put forward the exsiting problems and listed plans for furture research.