Synthesis And Characterzation Of NiS And NiS₂ Nanoplates

Recently,two-dimensional transition metal dichalcogenide(2D-TMDCs)have received extensive attentions for their unique electrical,optical,thermal and mechanical properties that are absent in bulk form.One of the most important fields is the reliable synthesis of 2D atomic thin layers.So far,there are two main ways to obtain 2D-TMDCs,one is chemical vapor deposition(CVD),and the other is mechanical or liquid exfoliation of bulk materials.The mechanical exfoliation method can obtain high-quality micron-level monolayer 2D-TMDCs with a clean surface,which is suitable for basic research and manufacturing of electronic devices.However,mechanical exfoliation is not suitable for large-scale production with less controllability.On contrary,CVD method shows great potential in the preparation of high-quality TMDCs with tunable size,controllable thickness,and excellent electronic properties.In the past few years,huge progresses have been achieved in the preparation of 2D materials using the CVD method.But there are still many issues to be solved.For example,when the metallic element have multiple valence states in TMDCs,the controllable synthesis of pure 2D-TMDCs remain a challenge,because it can form compounds with different stoichiometric ratios between sulfur,selenium,and tellurium.Due to the complexity of the valence state,it is still difficult to obtain certain a single-phase pure substance.This is unfavorable for exploring the performance of its pure substance.Preparation of pure 2D-TMDCs with multiple valence state metallic element is conductive to explore their intrinsic physical,chemical properties and potential applications.Therefore,developing the controllable synthesis methods of 2D-TMDCS with tunable pure stoichiometry is highly desirable.In view of this goal,this paper mainly carried out the following work:1)Selective growth of Ni S and Ni S₂nanoplates achieved by CVD method.In this paper,Ni Cl₂powders and S powders are used as the reaction source.By controlling the growth temperature and the amount of S powdesr,Ni S and Ni S₂nanoplates can be selectively prepared on the Si O₂/Si substrate.The effects of changing the growth conditions on the morphology,size and thickness of Ni S and Ni S₂nanoplates were discussed separately.By using thermodynamic calculations,the change of the Gibbs free energy of the reaction was discussed to explain our experimental results.Materials characterization shows that we have obtained two different stoichiometric nickel sulfides with high crystal quality.The two kinds of nickel sulfides are hexagonal Ni S and cubic Ni S₂,respectively.The thickness of the non-layered Ni S and Ni S₂obtained by our method can be tailored down to 2.0 nm and7.0 nm,respectively.2)The electrical characteristics of Ni S and Ni S₂nanoplates.Using the electron beam lithography(EBL)and thermal evaporation,the Ni S and Ni S₂nanoplates were fabricated into a FET-like electronic devices.The I_DS-V_DSoutput transfer curve is linearly symmetrical,indicating that a good ohmic contact is formed between the electrode and the two kinds of nickel sulfides nanoplates.And it is not sensitive to the change of back gate voltage(±60 V),also proving that the two nanoplates we obtained are metallic.The electrical conductivity of the Ni S and Ni S₂nanoplates with the thickness of 10 nm can be calculated to be 4.6×10⁵S m^-1and 6.3×10⁵S m^-1.It proves that the Ni S and Ni S₂nanoplates we obtained have very high electrical conductivity.