Growth Of MoS₂ By Inorganic Sodium/ammonium Salts-assisted CVD And Its Transistor Performance

As one of the two-dimensional transition metal disulfides(TMDs)materials,MoS₂ has become a hot spot of scientific research because of its unique physical and chemical properties,showing excellent practical application prospects in the fields of electricity and catalysis.Especially in the semiconductor industry,MoS₂ thin films with atomic thickness are usually regarded as the future substitutes for silicon.Therefore,it is necessary to realize the controllable synthesis of high quality MoS₂ thin films,especially large-scale monolayers.In this regard,chemical vapor deposition(CVD)is a method that can achieve large-scale growth of MoS₂ at a reasonable cost.However,it is still inefficient to synthesize high-quality and large-area MoS₂ by conventional CVD method.It is necessary to introduce appropriate synergistic additives in the reaction process to improve the reaction act ivity,so as to promote the growth of large-area uniform MoS₂ film and novel MoS₂ structure.In this work,high-quality MoS₂ films with centimeter-scale and large-area MoS₂ with fractal structure were successfully synthesized by introducing two synergistic additives Na₂SO₄ and ammonium salts into conventional CVD system.(1)Growth of high quality MoS₂ monolayers and fractal structure catalyzed by Na₂SO₄and its transistor performanceContinuous centimeter-scale MoS₂ thin films with the largest composite domain edge of632?m were directly grown on SiO₂/Si substrates by the synergistic effect of highly active MoO_x(x=2-3)precursor and Na₂SO₄ catalysis.The synthesized MoS₂ exhibits excellent micro-uniformity without any lattice defects,macro-uniformity in centimeter-scale.The results of X-ray photoelectron spectroscopy and density functional theory show that Na tends to react with SiO₂ rather than MoS₂.High concentration of sodium salt can also promote the homoepitaxial growth of hierarchical MoS₂ fractal patterns on MoS₂ monolayers,which provides a method for the growth of MoS₂ mixed monolayer-bilayer-multilayer vertical structure.MoS₂ FETs array fabricated by standard lithography technology has good electrical uniformity and high electrical performance.The electron mobility is up to 5.9 cm²V^-1s^-1,and the switching current ratio is about 10⁵-10⁶,which opens up an opportunity for large-scale electrical applications of MoS₂.However,alkali metal ions have high reactivity with SiO₂(FET dielectric layer)substrate,which affects the direct application of MoS₂ FET on as-growth substrate.Therefore,the types of salts that promote the growth of two-dimensional materials need to be expanded.(2)Ammonium salts-assisted controllable growth of high quality large-area MoS₂ and its itstransistor and hydrogen evolution catalysis performanceBy introducing(NH₄)₂SO₄ and NH₄Cl into the CVD process,MoS₂ domains with different thicknesses and shapes,and centimeter-scale MoS₂ films can be obtained by controlling the concentration of(NH₄)₂SO₄ and NH₄Cl.Transmission electron microscope and atomic force microscope images show that the triangular domains and centimeter-scale MoS₂ single-crystal films synthesized by NH₄Cl catalysis have standard single-crystal characteristics and low surface roughness.The growth mechanism of adsorption-deposition-growth(ADG)model can explain the process of MoO_x precursors being adsorbed and deposited on the substrate by ammonium salts,and the nucleation,growth of MoS₂.The MoS₂ FET can be fabricated directly on the substrate.The introduction of ammonium salts,especially(NH₄)₂SO₄,into CVD process can greatly increase the amount of MoS₂ grown on carbon paper,and further improve the HER performance of MoS₂,which provides an effective method to promote the direct growth of MoS₂ on conductive substrate.