The Study Of Controllable Preparation And Optoelectronic Properties Of Two-dimensional Ternary W_xMo_1-xS₂ Alloy And Heterostructure

Two-dimensional materials（2D materials）have attracted extensive attention due to their unique optical and electrical properties,such as strong light-matter interactions,broad spectrum absorption,high carrier mobility et al.The band structure of 2D materials determines the absorption and emission processes of light,and the continuously tunable band structure is of great significance for the application of two-dimensional materials in optoelectronic devices.The band structure of the ternary alloy gradually transitions from one end material to another end material as the composition changes.This feature makes 2D materials achieve the offsets of energy band and band gap on a single sample.Up to now,the composition gradient span of alloys synthesized by reported methods is fixed,namely,the span of the band offset and band gap gradient are also fixed,so that it is impossible to further control the size of the built-in electric field induced by band offset.Additionally,the heterostructures of 2D materials exhibit distinct properties from single 2D material,such as electron transport,interlayer excitons,etc.,and is an important means to realize high-performance and novel optoelectronic devices.Band structure is one of the most important factors affecting charge transport and exciton properties in 2D heterojunctions.How to realize the regulation of the charge transport process through the control of the energy band offset in the 2D heterostructure is also a key concern in the field.In order to solve the above problems,we further understood the growth mechanism of W_xMo_1-xS₂ alloy synthesized by the two-step method,and further precisely controlled the W composition x.In addition,we also established a WS₂/W_xMo_1-xS₂heterostructure model to study the changes in charge transfer efficiency under different energy band offset and hybridization strength.The specific contents are as follows:（1）The principle of the optical contrast method to characterize the number of layers in 2D materials is clarified and two commonly used measurement methods for the number of layers are given:reflection spectroscopy and optical imaging methods.Then,the influence of various parameters such as gamma value,numerical aperture,thickness of the silicon dioxide layer and light source brightness on the optical imaging method was studied.Finally,the optical image method is standardized based on the reflectance spectroscopy method which is less affected by the test conditions.（2）A high-quality W_xMo_1-xS₂ alloy was synthesized by a two-step CVD growth method and its growth mechanism was shown.Then the W/Mo steam ratio is controlled by the flow rate of the carrier gas Ar,thereby regulating the span of the composition gradient.That is,the band offset and the span of band gap gradient are regulated.Then W_xMo_1-xS₂ alloy detectors with different spans were prepared,indicating the formation of a built-in electric field and its magnitude is controlled by the span of band offset.It exhibits a fast response time of 31 ms.At the same time,under 20 V reverse bias,the responsivity and detectivity reach 33.7 A/W and 2.33×10¹¹ Jones,which are 3 to 6orders of magnitude higher than pure Mo S₂,WS₂,and Mo S₂-WS₂.（3）By establishing a 2D WS₂/W_xMo_1-xS₂ heterostructure,it is proved that the strong energy level hybridization can actuate fast and effective charge transfer under the minimum energy band offset.Theoretical calculation results show that the low band offset results in strong hybridization between the energy levels of WS₂ and W_xMo_1-xS₂.This is due to the fact that the energy levels involved in charge transfer are closer together and the energy level vibrations are stronger.A charge transfer time of 2.7 ps is achieved at x=0.78,which is faster at 0.78<x<1.These results are in good agreement with the experimental results of complete quenching of Photoluminescence（PL）of WS₂ in the WS₂/W_xMo_1-xS₂ heterostructure region（x=0.78）.