Study On The Modification Of Two-dimensional Layered Materials Via Advanced Plasma Technology

Since the discovery of graphene,two-dimensional layered materials have been valued for their unique electrical,optical,mechanical and other physical properties in both basic research and technical applications.The control of layer number has a decisive influence on the properties of two-dimensional layered materials.For example,the electrical properties of graphene and molybdenum disulfide vary with thickness.Besides,because of the interlayer van der Waals interaction between each layer,it is easy to integrate highly different two-dimensional layered materials by means of intercalation to make them have a wide range of adjustable electronic and optical properties.However,the current reported thinning process is still uneven,destructive and uncontrollable,and the intercalation process also faces with the challenges of low efficiency,high materials consumption and poor repeatability.In view of these problems,two advanced plasma processes based on a non-parallel plate capacitance-coupled plasma system in our laboratory were developed.Firstly,two plasma processes based on ion bombardment effect and chemical reaction were systematically studied,and utilized to achieve controllable thinning of graphene and rhenium disulfide?ReS₂?,respectively.Secondly,this work developed a mild oxygen plasma intercalation process to realize intercalation of two-dimensional transition metal chalcogenides?2D-TMDCs?,constructed 2D atomic crystal molecule superlattices and analyzed the intercalation mechanism.The main research contents and achievements are summarized as follows:1.Two plasma thinning processes based on ion bombardment effect and chemical reaction etching principle were developed,respectively,to achieve layer by layer thinning of graphene and ReS₂.It is found that only ion bombardment can effectively etch graphene because of the very stable nature of its carbon-carbon bonds.Therefore,the paper adopted the plasma operated in the transition stage between capacitive and inductive discharge?with high plasma density?and employed hydrogen or argon?easily ionized gas?as the precursor gas to achieve layer by layer etching of graphene.Since the SF₆/N₂ plasma can decompose the TMDCs,such as ReS₂,into volatile gases,the paper adopted the plasma operated in capacitive discharge?with extremely low plasma density?and employed the SF₆/N₂ mixed gas as the precursor gas to achieve layer by layer thinning of ReS₂.In this thesis,rapid and slow etching modes were realized by adjusting the precursor gas and regulating the input power.In combination with these two etching modes,uniform graphene and ReS₂ films of any number of layers could be obtained.It is also found that the defects of graphene caused by ion bombardment can be repaired by subsequent thermal annealing.The above experimental results show that the soft plasma thinning process is reliable and compatible with semiconductor manufacturing process,thereby holding great promise for two-dimensional layered material devices.2.A mild oxygen plasma intercalation process was developed,and the effects of the plasma intercalation process on the structure,composition,thickness,optical and electronical properties of two-dimensional TMDCs materials were systematically studied.The mild plasma intercalation process operates in capacitive discharge mode.Therefore,not only is the plasma density extremely low,but also the plasma electric field is parallel to the substrate surface.As such,the parallel electric field can drive the O₂⁺ions generated by the plasma into the interlayer space of TMDCs,and the stable O₂ molecular layers are thus formed there via the van der Waals interaction with the adjacent TMDCs.The resulting widened layer spacing effectively separates each TMDCs monolayer and almost suppresses the interlayer coupling.Compared with the original mutilayered 2D-TMDCs materials,MoS₂[O₂]_x,WS₂[O₂]_x,MoSe₂[O₂]_x and ReS₂[O₂]_x superlattices exhibit similar properties to those of single layer materials such as direct bandgap.In addition,compared with the original heterojunction phototransistor,the superlattice heterojunction obtained by plasma intercalation display a better photoresponse performance.In particular,the photocurrent of the bilayer MoS₂[O₂]_x/WS₂[O₂]_x superlattice lateral heterostructure increases by 100 times,which may be attributed to the fact that such original heterojunction with indirect band gap transformed into the supperlattice heterojunction with direct band gap.These results demonstrate that our interlayer process not only provides a potentially universal approach to tune interlayer stacking and interactions in 2D atomic crystal molecular superlattices but also opens a new way to impart such superlattices with the exotic properties intrinsic to monolayer materials such as the direct optical bandgap needed for future optoelectronic devices.