Controllable Growth And Charaterization Of Polymorphic Layered In₂Se₃ And Its Heterostructure

In₂Se₃is a semiconductor material in III-VI group,which has shown wide application prospects in phase change memory,photodetector and non-volatile ferroelectric memory.Unfortunately,the crystal structure of In₂Se₃is extremely complicated,and there are at least five stable phases of it under ambient pressure,namelyα,β,γ,δ,andκ,among whichα-In₂Se₃andβ-In₂Se₃are both van der Waals layered materials.In addition,In₂Se₃exhibits complex phase transitions under high temperature and high pressure conditions.Correspondingly,this dissertation has thoroughly studied the controllable growth of various polymorphs of In₂Se₃by chemical vapor deposition（CVD）.Its crystal structures and the phase transitions fromβ-In₂Se₃toβ’-In₂Se₃at low temperatures have been characterized in detail.In addition,β-In₂Se₃/monolayer WS₂heterostructure was synthesized,and the Layer-dependent structural transformation in epitaxially grown heterostructure of multilayerβ-In₂Se₃on monolayer WS₂was studied.Furthermore,the photoluminescence（PL）and Raman spectrum oscillation behaviors in bubbles of monolayer WS₂andβ-In₂Se₃/monolayer WS₂heterostructure were investigated,and the origin of oscillations was explored.The main research contents include:（1）Synthesis of polymorphic In₂Se₃and characterization of corresponding crystal structure.The controversy on the structures of In₂Se₃strongly hinders the exploitation of its properties and potentially gives rise to misdirection of its applications.In this paper,a variety of crystal forms of In₂Se₃were synthesized via chemical vapor deposition by adjusting the growth conditions.The morphology of the grown polymorphic In₂Se₃was characterized by optical scanning electron microscopy.In addition,taking advantage of state-of-the-art aberration-corrected scanning transmission electron microscopy,we demonstrate the crystal structures of all the lab-created and purchased In₂Se₃compounds.Six phases in three polymorphs at room temperature have been observed among all the samples,which include 2H and 3Rα-In₂Se₃,1T,2H,and 3Rβ-In₂Se₃,and none-layeredγ-In₂Se₃.Raman spectra are directly correlated to individual In₂Se₃phases,providing fingerprints for identifying various phases of In₂Se₃.（2）Phase transition behavior of layeredβ-In₂Se₃at low-temperature.Previous theoretical studies have predicted thatβ-In₂Se₃will undergo phase transition at low temperatures.However,there is still lack direct experimental evidence on the phase behaviors ofβ-In₂Se₃.Here,in-situ electron microscopy and variable temperature Raman were employed to investigate the phase transition behaviors of 2Hβ-In₂Se₃grown on a highly oriented pyrolytic graphite（HOPG）substrate,showing a reversible thermally driven phase transition fromβ-In₂Se₃toβ’-In₂Se₃.The phase transition temperature was measured to be～180 K.Then,two-step chemical vapor deposition method was used to build a multilayerβ-In₂Se₃/monolayer WS₂van der Waals heterostructure,and the Layer-dependent structural transformation in epitaxially grown heterostructure of multilayerβ-In₂Se₃on monolayer WS₂was studied.The result shows that as the number ofβ-In₂Se₃layers on the monolayer WS₂decreases,the phase transition temperature gradually decreases.It is worth noting that the phase transition of the epitaxialβ-In₂Se₃can only be observed when the thickness of the epitaxialβ-In₂Se₃is greater than 4 layers.These results indicate that the strong coupling effect exists in the van der Waals epitaxyβ-In₂Se₃/WS₂heterostructure,being able to effectively affect a few adjacent layers,providing important information for understanding the interlayer interaction of the two-dimensional van der Waals epitaxial heterostructure.（3）Photoluminescence and Raman spectra oscillations in bubbles of monolayer WS₂andβ-In₂Se₃/WS₂heterobilayer.The bubbles have been widely used to measure the adhesion energies,friction coefficient,stiffness,and Young’s modulus of 2D materials,demonstrating great potential applications in electronics and photonics.Here,by controlling the growth conditions,spherical capshaped bubbles were successfully grown on monolayer WS₂andβ-In₂Se₃/monolayer WS₂heterostructure.Optical and fluorescence images reveal obvious interference rings on the grown bubbles.In mapping images of PL and Raman,oscillatory behaviors have been observed not only for peak intensity but also for peak position and width.The origin of the PL and Raman spectrum oscillation of the bubble is explored through changing the external pressure and adjusting the change of the interference ring on the bubble.It is demonstrated the oscillatory PL and Raman peak intensities originate from constructive and destructive interference on the bubble’s surface,while the oscillatory peak positions and widths are induced by the corresponding enhanced and lowered local heating effects owing to the constructive and destructive interference.It will provide basic research support for the development of monolayer WS₂andβ-In₂Se₃/WS₂heterobilayer bubbles-based photodetectors and pressure sensors in the future.