Preparation And Properties Of Molybdenum Disulfide Thin Films

As a graphene-like materials,Molybdenum disulfide?MoS₂?have attracted enormous research interest because of its exceptional electronic,optical and mechanical properties.Meanwhile,MoS₂ has the adjustable band gaps with the number of layers,which make them suitable for next generation transistors,nanoelectronics and spintronics devices.In this paper,we have explored the different synthesis method of molybdenum disulfide thin films,inculding a sputtering+CVD and a two-step thermolysis of ammonium molybdate.The morphology structure and growth mechanism of MoS₂ thin layers have been investigated by XRD,SEM,EDS,AFM and four-point probe resistance test method,and discussed the effects of experiment parameter?temperature,atmosphere environment,etc.?on the morphology and structure of the MoS₂ thin layers.On this basis,Raman spectrum,Uv-vis and PPMS,EPR have used to characterize the optical and magnetic properties of the sample.The experimental conclusions are as follows:1.Sputtering+CVD:Sputtering metal Mo in pure Ar environment,crystal particle become increasingly dense and uniform with the increase of sputtering temperature?250°C?.In the sulfuration process,The growth chamber have been heated to 600°C,700°C,750°C and800°C,respectively,and an Ar/H₂?9:1?gas have been introduced.With increasing vulcanization temperature,the crystallinity of the sample have got better and better,the by-products?MoO₃,MoS₃?less and less,especially at 800°C.When sulfur powder quality was 1.0 g,the metal Mo could been completely vulcanized without environment pollution.2.Two-step thermolysis of ammonium molybdate:In this method,the ammonium molybdate have been decomposed at the first thermolysis process,then to improve the sample crystallinity structure by the second thermolysis process.After continuous experiments,the precursor of?NH₄?₂MoS₄ and DMF has been completely decomposed at500°C?1h?+950°C?1h?,and the sample had a high degree of crystallinity.When the concentration of precursor solution was 0.039 mol/L,the spin speed was 500 r/min+6000r/min,the MoS₂ film thickness and surface morphology of have been easily controlled.Adding additional sulfur powder in the thermolysis process,which could increase sulfur saturation vapor pressure in furnace chamber,thereby avoiding sample contamination due to the forming gas mixed with inevitable air.3.Optical properties:Raman spectroscopy and UV-dis have been used characterize the as-synthesis samples,which prepared by two-step thermolysis of ammonium molybdate.Raman spectroscopy accurately identified the layer number of MoS₂ films about 1,2,few-layers.UV-dis had obvious absorption peak in the 615nm?2.02eV?and 675nm?1.73eV?of as-synthesis MoS₂ films.However,the absorption peak has a slight blue shift with decreasing of the number of layers because of defects existed in the samples.4.Magnetism properties:magnetism performance of different thickness MoS₂ films have analyzed by PPMS and EPR.Hysteresis loop have analyzed at different temperature,at room temperature?300k?,sample saturation magnetization intensity increases with decreasing of the thickness of the sample,the maximum value was 0.28 emu/g.At 10k,saturation magnetization of the same sample didn't have significantly change,but the coercive force increased from 123.83Oe to 123.83Oe with decreasing of the thickness of the sample.Which indicated the MoS₂ films have vary weak ferromagnetic with different thickness.By EPR analyzing of different samples,which had obvious signal in g=2.0,And line width become wider as the layers decreaseing,which was proportion to sulphur vacancy concentration.We attribute the observed ferromagnetic properties to the sulphur vacancy?V_S?.