Coupling Effect Study Of Two Dimensional Transition Metal Sulfides

The past decade has witnessed tremendous investment in two-dimensional?2D?materials and remarkable achievements in basic research and technological development.Among them,heterojunctions of vertical stacked 2D materials exhibit diverse properties from single two-dimensional materials due to their different physical parameters such as band gap,electronic affinity,dielectric constant and absorption coefficient,which brings new dawn for the development and application of modern new materials.However,the lattice structure,interface contact and carrier mobility of the heterojunctions are affected by the coupling effects in the structures,which includes interlayer coupling and the coupling of substrates.Therefore,many unknown factors are introduced into the research and development of photodetectors,sensors and light emitting diodes based on 2D materials and heterojunctions.As a consequence,it is of significance to study and analyze the interlayer and substrate coupling effects of vertical stacked 2D heterojunctions.However,there is no systematic study on the coupling of 2D materials.In this paper,a series of studies have been carried out on the coupling effect of 2D transition metal sulfides.These include:substrate coupling effect of 2D materials based on atomic layer thickness,interlayer coupling control based on single-layer MoS₂,WS₂ and WSe₂heterojunctions,and coupling study of transition metal sulfides and 2D insulating material Mg?OH?₂.Phased progress has been made in the following aspects:1.A differential analysis model based on the mismatch effect of thermal expansion coefficient between substrate and two-dimensional atomic crystals is constructed.The coupling coefficient between substrate and atomic crystals is quantified.The substrate-induced lattice distortion,which is ubiquitous in 2D materials synthesized at high temperature,is revealed. The relationship between the coupling coefficient and the lattice defects when the temperature changes is explored.Experiments show that in a certain range,the smaller the coupling coefficient is,the more easily the 2D atomic crystal slides on the substrate surface and produces folds and other lattice defects,which results in defect-bound fluorescence transition at low temperature.The results take WS₂ as an example and discover that,due to the different thermal expansion coefficients between the two-dimensional materials and the substrate,a shrinkage or tensile stress will be applied to the two-dimensional materials during the process of cooling after synthesis.It results in the substrate-induced lattice distortion of the two-dimensional materials synthesized by chemical vapor deposition.With different coupling conditions between the materials and substrates,the residual distortion in two-dimensional films is also diverse.It makes the surface potential and fluorescence distribution of the same material revealed in different studies are generally diverse.For example,the transitions of single layer WS₂ fluctuate from 1.88 to 2.01 eV.In this paper,a different analysis model is proposed based on temperature-dependent Raman experiment.With the model,it is revealed that the coupling coefficient between the substrate and four WS₂ monolayer fluctuates from 5.50 cm^-1/%to 0.75 cm^-1/%.When the sample temperature changes from 350 K to 77 K,lattice disorder happens with the film sliding on the substrate.under the mismatch effect of thermal expansion between the substrate and monolayers.This phenomenon is most evident in the film with small coupling coefficient,which leads to the dominant transition of defect state at low temperature.This research promotes the basic research and practical application of two-dimensional materials from preparation to electronic structure properties.2.Vertically stacked MoS₂-WS₂,MoS₂-WSe₂,and WS₂-WSe₂ heterojunctions were fabricated.The dependence of interlayer coupling on the distance between layers in the heterojunctions are revealed.The effect of coupling on the transition of heterojunctions is studied.The sensitivity of the coupling to the distance between MoS₂-WS₂,MoS₂-WSe₂,and WS₂-WSe₂ is compared.The effects of annealing,laser and temperature on the interlayer coupling are discussed by experiments.The results show that interlayer interaction of vertically stacked heterojunctions is very sensitive to the interlayer spacing,which will affect the coupling between the monolayers and allow band structure modulation.Here,with the aid of density functional theory?DFT?calculations,an interesting phenomenon is found that MoS₂-WS₂,MoS₂-WSe₂,and WS₂-WSe₂heterostructures turn into direct-gap semiconductors from indirect-gap semiconductors with increasing the interlayer space.Morever,the electronic structure changing process with interlayer spacing for MoS₂-WS₂ is most significant.As a result,the transition of MoS₂-WS₂ changes most when lowering the temperature or annealing.The study also reveals that temperature is a very sensitive factor in the interlayer coupling control of two-dimensional heterojunctions compared with annealing and laser.In summary,this study not only compares the tunning mechanism of the band structure control mechanisms for MoS₂-WS₂,MoS₂-WSe₂,and WS₂-WSe₂ heterojunctions,but also opens up a new direction for two-dimensional devices to be modulated by changing external conditions.It also advances the research of MoS₂-WS₂,MoS₂-WSe₂,and WS₂-WSe₂ based device applications.3.The heterojunctions based on WS₂ and insulated 2D Mg?OH?₂ were fabricated.The coupling regulation of insulated 2D Mg?OH?₂ on WS₂ monolayer was revealed.It was proved that the coupling of Mg?OH?₂ could reduce the bandgap and surface potential distribution of WS₂.As a consequence,there is a a p-n junction between WS₂ and WS₂-Mg?OH?₂.The perturbation effect of thin Mg?OH?₂ with different thickness on the band structure and electron transition of single layer WS₂ was investigated.The results show that the the interplanar crystal spacings for?100?facets of WS₂?0.273 nm?and Mg?OH?₂?0.271 nm?are quite close,and the lattice mismatch is greatly reduced when the heterojunction is composed.Moreover,by stacking 3-layer-Mg?OH?₂,the Fermi level of WS₂ is increased by 0.39 eV,thus forming a p-n junction between WS₂ and WS₂-Mg?OH?₂.Moreover,the Fermi level of WS₂-Mg?OH?₂ increases with the thickness of Mg?OH?₂.When Mg?OH?₂ becomes 9 layer and 15 layer,the Fermi levels of heterojunctions are increased by 0.88 eV and 1.63 eV compared with that of single layer WS₂,respectively.Therefore,the built-in potential in the p-n junction can be adjusted by tunning the of layers of Mg?OH?₂.It makes the charge transfer in the photovoltaic devices based on the heterojunction can be regulated by changing the thickness of Mg?OH?₂.On the other hand,DFT calculations and PL experiments reveal that the bandgap of WS₂ decreases with the coupling effect of Mg?OH?₂,so that the carrier mobility of photovoltaic devices based on this heterojunction can be controlled by using Mg?OH?₂ thickness.