Controllable Preparation And Photoelectric Properties Of Anisotropic Two-dimensional ReX₂ (X=S,Se) Materials

In recent years,two-dimensional transition metal dichalcogenides?TMDs?have been attracting wide attention in the applications of nano-electronic and optoelectronic devices due to their excellent optical and electrical properties.At present,many excellent properties of high lattice symmetric 2H structure two-dimensional materials represented by MoS₂ and WS₂ have been discovered by researchers,such as ultra-high carrier mobility,excellent mechanical property,high quantum efficiency,Spin-orbit coupling effect,and low electrocatalytic overpotential.Compared with the high-lattice symmetric two-dimensional materials,the group VII-TMDs,such as Re-based dichalcogenides ReX₂?X=Se,S?,in addition to the above excellent properties,its greatest feature is the IT' structure with lower lattice symmetry.The reduced in-planecrystal symmetry of ReX₂ endows it unusual 2D in-plane anisotropic optical and electrical properties.These excellent properties make ReX₂ materials have great potential applications in field effect transistors,polarization photodetectors,logic inverters,and new concept devices.Although two-dimensional ReX₂ material has many excellent properties and great application prospects,its controllable preparation has been the main bottleneck restricting the research and application of the material.At present,the materials are mainly prepared by micro-mechanical exfoliation method and chemical vapor transport?CVT?method,but the materials obtained by these two methods have many problems,such as low yield and uncontrollable number of layers,and the prepared samples can not meet the needs of large scale electronic devices.Chemical vapor deposition?CVD?growth is considered to be an effective method for the controllable preparation of large-area,high quality semiconductor materials and has been widely used for the preparation of numerous two-dimensional materials.However,due to the particularity of the structure and properties of the ReX₂ material,there are many problems and challenges in the controllable synthesis of the material by the CVD method.In allusion to the problems of CVD-grown ReX₂ material,such as the variable valence state of rhenium source is uncontrollable volatilization,the anisotropic growth and out-of-plane growth occur easily,yielding irregular dendritic structure and the non-2D thick flower-like structure.In this thesis,we demonstrate a eutectic-assisted method and aspace-confinement epitaxial growth method,solving the problem of uncontrollable morphology and number of layers caused by ReX₂ structure,and realizing the controllable preparation of large-area,high-quality monolayer ReS₂ and ReSe₂.By studying the effects of substrate surface energy and growth temperature during the growth and morphology of ReX₂,we revealed the new patterns of anti-symmetric growth and out-of-plane growth of these materials.In addition,on the basis of controllable preparation of ReS₂ and ReSe₂,a series of monolayer 1T'ReS₂,Se_2?1-x?semiconductor alloys with different compositions were successfully prepared,realizing the tunable of optical band gap and electrical properties of such materials.By means of high resolution transmission electron microscopy imaging analysis,for the first time,we found the phenomenon of "sub-nanometer-scale local atomic distribution" in anisotropic alloy structure.Finally,we designed and constructed the field effect transistor devices based on ReS₂,ReSe₂ and ReS_2xSe_2?1-x?materials,and deeply explored their electrical and optoelectronic properties.The specific contents of this thesis are as follows:?1?Tellurium-asisted epitaxial growth of large-area,highly crystalline ReS₂ atomic layersFor the problem of low growth efficiency caused by the low volatilization of Re metal?melting point 3180 ??source in the preparation of ReS₂ materials,we developed a low eutectic-assisted CVD growth method.The binary eutectic formed by Re and Te can effectively reduce the melting point of metal Re?as low as 430 ??and increase the vapor pressure of Re source in the reaction system,thus increasing the growth efficiency of ReS₂.On this basis,fluorophlogopite with low surface energy was selected as the growth substrate,then successfully realized the van der Waals epitaxy growth of large-area,high-quality monolayer ReS₂.By systematically studying the effect of the substrate surface energy and growth temperature on the growth and morphology of ReS₂,the unique anti-symmetric growth pattern and mechanism of the material were revealed.?2?Epitaxial growth of ReSe₂ atomic layers by space-confinement methodIn the preparation of ReSe₂ with ReO3 as the rhenium source,the nucleation density and the number of layers of ReSe₂ in the growth system are not controlled,because ReO3 is prone to produce highly volatile rhenium source by the disproportionation reaction.In order to achieve controllable growth of ReSe₂ materials,we creatively proposed a space-limited epitaxial growth method.In CVD system,a micro-reaction chamber was ingeniously constructed by stacking two mica substrates together,this micro-reaction chamber plays a space-confinement role to reduce the nucleation density and the growth rate of ReSe₂.With this method,large-area,high-quality ReSe₂ film with uniform monolayer thickness were successfully synthesized.In addition,we systematically studied the effect regulation and mechanism ofthe substrate type,growth temperature and hydrogen content in carrier gas on the growth mode,morphology,nucleation density of ReSe₂.?3?Controllable synthesis of large-size 1T' ReS_2xSe_2?1-x?alloy monolayer with tunable bandgap and carrier typeOn the basis of ReS₂ and ReSe₂ with similar lattice structure,different electrical properties?ReS₂:n type;ReSe₂:p type?and bandgap?ReS₂:1.60 eV;ReSe₂:1.30 eV?,'for the first time,we successfully prepared a series of different components single-layer 1T' semiconductor alloy ReS_2xSe_2?1-x?by chemical vapor deposition growth.By adjusting the composition of the alloy,the optical bandgap of alloy materials can be continuously modulated from 1.32 eV to 1.62 eV,meanwhile,the carrier type can be modulated from n-type to bipolar and p-type.By means of high resolution transmissionelectron microscopy imaging analysis,for the first time,we found the phenomenon of"sub-nanometer-scale local atomic distribution" in anisotropic alloy structure.Finally,we designed and constructed the field effect transistor devices based on ReS_2xSe_2?1-x?materials,and systematically explored their anisotropic electrical and optoelectronic properties.