Preparation And Characterization Of TMDCs And Their Heterostructures Based On All Inkjet Printed Water-based Precursors

Two-dimensional layered semiconductors such as transition metal dichalcogenides（TMDCs）and their heterostructures are an emerging family of materials that show remarkable applications in both electronic and optical fields,and thus the require of large-area and high-quality synthesis of TMDCs is highly desirable to open the way for device applications.However,it still remains a challenge to prepare high-quality TMDCs in a controlled manner,with a low cost and environmentally friendly growth strategy at a large scale.In addition,2D lateral heterostructures have attracted much attention.Although the diodelike responses across the 2D lateral heterostructure have been widely reported,the essential electrical properties,such as the Fermi levels and the lateral built-in potential,have been barely studied,especially with the applied gate voltage.The main research contents of this work are listed as follows:Firstly,based on the detailed analysis of sulfur powder,sodium sulfate and ammonium persulfate,a comprehensive comparison in four aspects,including the water solubility,the thermogravimetry,the air pressure as well as the safety has been conducted.It has been revealed that ammonium sulfate has the highest water solubility as a safety precursor,and the volatilization process of its decomposition by heat was basically consistent with those of other molybdenum sources.Thus,the ammonium persulfate material has been chosen as the precursor of sulfur source without further contamination,and the foundation for the preparation of water-based precursor ink in the next step was laid.Secondly,based on the previous experiments,by adding triton and propylene glycol,the rheological parameters of the solution have been adjusted.The ink concentrations of ammonium molybdate and ammonium persulfate have been determined by analyzing the solubility to make sure that the ammonia persulfate solution could be inkjet-printed.After that,the printing parameters have been carefully tuned to realize the printing of two kinds of water-based precursors.Then,single layer Mo S₂ films of several hundred micrometers in size could be grown in situ by CVD method,and Raman,PL and AFM characterizations were carried out on these samples,as well as the preparation of the corresponding field effect transistor for electrical performance testing.The mobility of the device was calculated as 1.6 cm²·V^-1s^-1.Then,molybdenum and tungsten sources were prepared by inkjet printing of water-based precursors,and Mo S₂-WS₂ single-layer lateral heterostructure was synthesized by chemical vapor deposition.After that,Raman,PL,AFM and TEM characterization were carried out to prove that it is a single layer material with high crystallinity.Moreover,the atomic diffusion and lattice at the interface of Mo S₂-WS₂ single-layer lateral heterostructure are proved by TEM,which reveals the single-crystal properties of the heterostructure.Finally,MoO₃ microribbons were used to develop a photolithographic process,and devices with clean Mo S₂-WS₂ single-layer lateral heterostructure were successfully fabricated.The quantitative determination between the gate voltage and the built-in potential was revealed by the combined use of Kelvin probe force microscopy and I-V characteristics,in good agreement with the theoretical calculation results.A built-in potential up to 262 me V was observed at V_g=40 V,which was three times larger than that at V_g=0 V.A trap density mediated mechanism has been proposed to explain the highly gate-tunable built-in potential.Based on the band-discontinuity mode,the energy-band diagram of the monolayer Mo S₂-WS₂lateral heterostructures was presented,which belongs to the 2D type II n-n heterojunction.Our findings have demonstrated the strongly gate voltage-dependent and highly tunable built-in potential within the 2D type II heterojunction,which will become the novel building blocks for2D optoelectronic,photodetection,and photovoltaic devices.