Atomic Characterization Of The Interface Between 2D-TMD Semiconductor And Metal

Since the discovery and successful preparation of graphene,two-dimensional layered materials with atomic layer thickness have attracted unprecedented attention and interest.Among them,due to the unique semiconductor properties,two-dimensional transition metal chalcogenides(2D-TMDs)have great potential for future applications in the fields where traditional semiconductor materials are applicable.When 2D-TMDs are used in electronic devices such as field effect transistors,their contact with metal electrodes becomes a bottleneck restricting the performance of related devices.At the physical level,when ideal interface is formed between 2D-TMDs and metals,the electrical properties of the interface only depend on the metal work function and the semiconductor electron affinity.At the chemical level,however,it is difficult for 2D-TMDs and metals to form the ideal interface contacts.The deposition process of metal electrodes onto 2D-TMDs often damages their interfaces.Therefore,it is currently a hot research topic on making a perfect Van der Waals(vd W)contact between 2D-TMDs and metals,which could also avoid the loss of interface transmission performance caused by the Fermi level pinning(FLP).On the other hand,the research and development of 2D-TMDs semiconductors are very rapid that a lot of meaningful results have been achieved in various links such as material preparation,structural characterization and performance analysis.Among them,chemical vapor deposition(CVD)is the main method to prepare large-scale monolayer 2D-TMDs films.If a metal,e.g.,Au is used as the CVD substrate,2D-TMDs can be grown directly on the gold surface.However,there are few studies up to now on the interface between CVD-grown 2D-TMDs and their substrates,especially the atomic-scale analysis of their interface structures.In this research,continuous monolayer layer MoS₂/WS₂ film was successfully prepared on the surface of nanoporous gold(NPG)via CVD method.The interface structure and related changes between 2D-TMDs and Au were carefully characterized and analyzed at the atomic scale by Cs-corrected transmission electron microscope(TEM).The main research contents and findings are as follows:1.The CVD system with a three-temperature-zone tube furnace was built,and the preparation of 2D-TMDs by CVD was explored.(1)By controlling the reaction conditions,high-quality monolayer MoS₂ films and nanosheets were prepared on glass substrates.(2)Using the same experimental conditions,monolayer MoS₂ films/nanosheets were grown successfully on the surface of NPG.(3)XPS results show the difference between MoS₂grown on glass and NPG substrates.There is strong charge transfer in the latter one,as well as a special Au-S phase.2.The atomic structure of the interface between 2D-TMDs and Au was characterized by STEM,where S was found to be participated into the reconstructed interface.(1)The monolyer MoS₂ film covers across the NPG with abundant surfaces,and there is no visible chemical interaction between MoS₂ and gold.(2)Viewing from[100]_Au and[110]_Audirections,the reconstructed layer at MoS₂-Au(001)interface reveals an unexpected periodic structure.Owing to the experimental and simulation results,it is composed of Au₄S₄ monomer.(3)For the MoS₂-Au(110)and MoS₂-Au(111)interface,S-induced interface reconstruction could also be found confirmed by EELS results.3.The reconstructed MoS₂-Au interface was further researched on how they formed and their effect to this system.(1)By shortening the CVD reaction time,a discontinuous monolayer MoS₂ film was grown on the NPG surface,and it was found that the MoS₂coating and protection are the key point for maintaining the Au-S reconstructed structures.(2)By controlling the CVD conditions,e.g.,the source materials,Mo/S ratios and temperatures,it was found that excessive S source is one of the most important one to help form the Au-S phase at the interface.(3)The Au-S reconstruction layer could in turn change affect the structural symmetry of MoS₂ by dragging the S layer a little bit away from Mo layer in MoS₂.(4)The interface reconstruction of MoS₂-Au also changes the electronic structure and carrier transport properties of their system.4.The stability of the Au-S reconstructed phase between the MoS₂-Au interface was studied and explored preliminarily.(1)After the outside exposure for 18 months,the reconstructed Au₄S₄ layer at MoS₂-Au(001)interface kept well while parts of MoS₂ has even been oxidated.(2)An extra Au thin film was deposited onto the MoS₂-Au sample,during which the reconstructed Au₄S₄ layer also kept well without damage.(3)The reconstructed Au₄S₄ phase is sensitive under the irradiation of foced electron beam in TEM.It could be changed back to original Au(001)surface by“knocking on”the S atoms in Au-S bonds.In conclusion,a continuous and stable vd W interface was constructed between 2D-TMDs and Au by CVD method,and the reconstructed interface was well-studied,wiich paved a new way to design and develop new TMD-based device.