Structural Control And Optical Properties Of MX₂ Van Der Waals Heterostructure

Van der Waals（vdW） heterostructure based on two-dimensional materials have shown excellent performance in field-effect transistors（FETs）,light-emitting diodes（LEDs）,photodetectors,logic circuits,and flexible devices due to their abundant and tunable energy band structure,ultra-thin thickness,and atomically smooth surface interfaces.Among the many van der Waals heterostructure materials,tin disulfide/molybdenum disulfide（SnS₂/MoS₂）heterostructure have attracted extensive research because of their large energy band shifts and type II energy band ordering.However,the controlled growth ofSnS₂ on monolayer MoS₂ films remains a challenge,and there is a lack of systematic studies on the electron-phonon coupling betweenSnS₂/MoS₂ heterostructure.In this paper,controlled thicknessSnS₂ thin films were grown on monolayer MoS₂ andSi O₂/Si substrates by adjusting the carrier gas and source temperature of the chemical vapour deposition（CVD）process,and then the electro-acoustic coupling of the heterostructure was investigated using Raman spectroscopy.The details of the study are as follows.SnS₂ flakes with a thickness of 2 nm were obtained by introducing hydrogen（H₂）as a carrier gas under a suitable high temperature environment.It was shown that the edges of the flakes in the hydrogen environment have a selective melt state and precursor adsorption,thus favouring in-plane epitaxial growth and inhibiting vertical growth.The Raman shift was blue-shifted from 312.cm^-1 to 315.2 cm^-1 when the thickness of theSnS₂ flakes was increased from 3 layers to a block.SnS₂/MoS₂ vertical stacks were obtained by a two-step CVD process using monolayer MoS₂ nanosheet films as substrates and their layer thicknesses were tuned.The photoluminescence（PL）spectroscopy showed that the PL luminescence of the MoS₂films was quenched after the formation ofSnS₂/MoS₂ heterostructure,indicating a significant photogenerated charge separation and transfer between the heterostructure.Furthermore,the local phonon spectra ofSnS₂/MoS₂heterostructure are studied.Local Raman tests of heterostructure with lowSnS₂ coverage revealed a maximum displacement of1.6 cm^-1 for the A_1gvibrational mode and up to 0.6 cm^-1 for the E¹_2g vibrational mode of MoS₂ compared to the separate MoS₂ layer,while local Raman tests of heterostructure with highSnS₂ coverage revealed a maximum displacement of 1.3 cm^-1 for the A_1g vibrational mode and up to 0.6 cm^-1 for the E¹_2g vibrational mode of MoS₂.The maximum displacement of the A_1g vibrational mode of MoS₂ is 1.3 cm^-1and the maximum displacement of the E¹_2gvibrational mode is up to 1.9 cm^-1.The phonon mode displacements occur at the interface boundary of these heterostructure and become more pronounced as theSnS₂ coverage increases.The study of the mechanism suggests that the photoexcited charge separation causes electron-phonon coupling in the heterostructure,which results in phonon mode shifts.In summary,the thickness of theSnS₂ flakes was reduced by the introduction of hydrogen,and then a vertical stack ofSnS₂/MoS₂ with controlled thickness was synthesised using a two-step CVD method,and PL quenching of the heterostructure was observed.Further Raman phonon spectroscopy studies revealed that photoexcited charges induce electron-phonon interactions between the heterostructure,leading to phonon mode shifts.A new idea for the synthesis of high quality vertical heterostructure and the study of optical properties is provided.