Preparation,Characterization And Properties Of MoS₂/MoO₂ Conformal Heterostructures

In recent years,with the in-depth study of two-dimensional（2D）materials,2D material heterostructures have begun to attract extensive attention.2D materials are connected by covalent bond or van der Waals force to form heterostructures.Because the external field and local field are easy to couple at the heterostructure interface,new physical properties are induced,which makes up for the shortcomings of a single 2D material and expands its application prospects.MoS₂/MoO₂ heterostructure is semiconductor/metal heterostructure with broad application prospects in catalysis,lithium ion batteries and other fields.The main methods for preparating 2D materials and their heterostructures include chemical vapor deposition（CVD）,hydrothermal synthesis,etc.Among which the CVD method has the advantages of simple operation,strong controllability,obtaining high-quality and large-area2D materials and their heterostructures.At present,there are few studies on the synthesis of MoS₂/MoO₂ conformal heterostructures through CVD method,and the synthesis mechanism still needs further investigation.Furthermore,due to the lack of step at the MoS₂/MoO₂conformal heterostructure interface,accurately characterizing the laye rnumber of MoS₂ is a challenge.The multi-physical field coupling at the interface of MoS₂/MoO₂ heterostructure needs further study.This article systematically studies the preparation and characterization of MoS₂/MoO₂ conformal heterostructures（including traditional and optical characterization methods）,as well as the optical and electrical properties of MoS₂ in heterostructures.The main research contents of the paper are as follows:（1）Firstly,MoO₂ nanosheets were prepared by CVD method,and the effects of growth parameters on the growth of MoO₂ nanosheets were investigated,and the optimal growth parameters were obtained.Subsequently,sulfur vapor was used as the precursor to sulfurize MoO₂ nanosheets to obtain MoS₂/MoO₂ conformal heterostructures.The effects of sulfurization temperature and sulfurization time on MoS₂/MoO₂ conformal heterostructures were investigated,and the relationships between the layer number of MoS₂ and sulfurization time and sulfurization temperature were obtained.Atomic force microscope（AFM）was used to characterize the thickness of MoO₂ nanosheets before and after sulfurization,and it was found that the thickness difference showed a trend of first decreasing and then increasing.The cross section of MoS₂/MoO₂ conformal heterostructure was characterized by high-angle annular dark field scanning transmission electron microscope（HAADF-STEM）,and the lattice structure and atomic distribution maps of MoS₂ and MoO₂ were obtained.Finally,the sulfurization mechanism of MoO₂ nanosheets was revealed.（2）Based on Fresnel’s law,a four-layer film reflection model was established to calculate the reflection spectrum of MoS₂/MoO₂ conformal heterostructures on Si O₂/Si substrate.By combining the calculated reflectance spectrum with the color space of the International Commission on Illumination（CIE）,a simulation diagram of the color change of MoS₂/MoO₂conformal heterostructures with the layer number of MoS₂ was obtained.The color bar were in good agreement with the OM images of MoS₂/MoO₂ conformal heterostructures.In addition,the effects of the thickness of Si O₂ and the wavelength of incident light on the optical contrast of MoS₂/MoO₂ conformal heterostructures were investigated.Based on the above research,an optical method for quickly identifying the upper 2D material in a 2D material conformal heterostructure has been developed.By comparing the optical image of the heterstructure with the calculated color bar,the layer number of the upper-layer 2D material is quickly deduced.（3）The PL spectra of MoS₂ in MoS₂/MoO₂ conformal heterostructures were investigated and the PL spectra intensity was found to be one order of magnitude higher than that of mechanically stripped MoS₂.It was attributed to the enhancement of the local electric field caused by surface plasmon resonance of the MoO₂ substrate.Through the HAADF-STEM characterization of MoS₂/MoO₂ conformal heterostructure cross section,it was found that there was a 1.1 nm gap between the MoS₂ and MoO₂ substrate,which is much larger than the van der Waals gap,effectively preventing the charge transfer between MoS₂ and MoO₂substrate,and reducing the fluorescence quenching effect caused by the metallic MoO₂substrate.In addition,the contribution of residual strain and intrinsic defects of MoS₂ to PL enhancement is excluded.The current density of MoS₂/MoO₂ conformal heterostructures device（～10² m A/mm²）is one order of magnitude higher than that of the separated MoS₂device（～10¹ m A/mm²）,and the resistance of both devices decreased with the increased of MoS₂ layers and the incident light intensity.In this paper,MoS₂/MoO₂ conformal heterostructures was prepared by CVD method,and the sulfurization mechanism of MoO₂ nanosheets was revealed.Based on the four-layer reflection model,the relationship between the color of MoS₂/MoO₂ conformal heterostructures and the layer number of MoS₂ was given.The mechanism of enhancing the PL quantum efficiency of multilayer MoS₂ by surface plasmon polaritons on the metallic MoO₂ substrate was revealed by HAADF-STEM characterization.The research content of this paper provides a reference for the preparation and characterization of 2D material heterostructures,and makes it possible for the design of multilayer MoS₂ in optoelectronic devices with more potential application value.