Structural Phase Transitions Of Typical Transition Metal Dichalcogenides Under High Pressure And Piezo-catalytic Properties

Within this class of transition metal dichalcogenides?TMDs?,such as MoS₂,WS₂and WSe₂,have inspired a new wave of research to access a wealth of phenomena including optoelectronics,valleytronics and spintronics.The properties of TMDs are related to the layered structure.As the number of TMDs layers decreases,TMDs undergoes a band-gap transition from an indirect band-gap semiconductor to a direct band-gap monolayer semiconductor.In addition,the diatomic compositions of layered TMDs possess d-orbital electronic states and the layered distance is 6.3-6.5?.The axial compression or pressure could change interlayered coupling,shorten the distance between atoms and lead to structural transition.This qualifies using high pressure as a desirable approach to explore new structure and find novel properties.Although some studies about multi-layered and monolayered TMDs including structural transition and metallization have been reported,it still lacks of systematic research.So we conducted further study about typical TMDs materials,such as the influence of different pressure conditions to structural transition and metallization;tailoring the structure and band-gap of WSe₂ by pressure;the electronic structure transition of WS₂ quantum dot.Moreover,utilizing the piezoelectric property of odd-layer MoS₂,the 2D double-piezoelectric Mo S₂/RGO hybrid was producted.The main results are as follows:1.We performed Raman,XRD,and IR studies of WSe₂ to evaluate the vibrational,structural,electrical,and optical dependence under pressure.IR absorbance data and theoretical ab initio band structure calculations showed a linear decrease in the band gap from the near-infrared to the far-infrared,which demonstrated the highly tunable transport properties of WSe₂.Pressure tuned the band gap of WSe₂ linearly,at a rate of 25 meV/GPa.The high tunability of WSe₂ was attributed to the larger electron orbitals of W²⁺and Se^2-in WSe₂ compared to the Mo²⁺and S^2-in MoS₂.WSe₂ underwent an isostructural phase transition from a 2D layered structure to a 3D structure at approximately 51.7 GPa,where a conversion from van der Waals?VdW?to covalent-like bonding was observed in the valence electron localization function?ELF?.Our results present an important advance toward controlling the band structure of layered materials and suggest significant implications for energy-variable optoelectronic devices via pressure engineering.2.We investigated the pressure-induced structural phase transition of layered semiconductor molybdenum disulfide?MoS₂?using Raman spectroscopy and studied its metallization using infrared?IR?spectroscopy under both non-hydrostatic and quasi-hydrostatic conditions.The pressure point of the structural transition and metallization depends on whether a PTM is used or not.Under quasi-hydrostatic and non-hydrostatic conditions,we found that the structural phase transition from 2Hc stacking to 2Ha stacking starts at approximately 16 GPa and 21 GPa,and finishes at³5 GPa and⁴1 GPa,respectively.Furthermore,the structural phase transition was followed by a semiconductor-to-metal?S-M?electronic transition.The pressure point of metallization under quasi-hydrostatic conditions is⁵ GPa lower than that under non-hydrostatic conditions.Our combined quasi-hydrostatic and non-hydrostatic experimental results enable a further understanding of the relevance of the PTM for the pressure-induced effects in layered materials,especially the pressure-induced changes of interlayer TMD stacking.3.There have been various efforts to tailor the excitonic properties in monolayer transition metal dichalcogenides?TMDs?for exploring their potential applications in optoelectronic devices.However,the low quantum yields?QYs?,despite their direct band gap nature,has limited the application in much fields.Encouragingly,excitons combined with defects endow WS₂ quantum dots?QDs?with certain desirable properties through strain engineering.We report on a strong exciton photoluminescence?PL?of WS₂ QDs even up to approximately 20 GPa by PL measurements.Their PL reveals that a distinct defect-induced peak D is located below the neutral exciton peak A.This peak D originates from defect-bound excitons and intensifies with increasing pressure as more electron transfer from WS₂ QDs to O₂.In addition,a transition from direct to indirect bandgap above 4.5 GPa was revealed by both experimental measurements and theoretical calculations.The evolution of electronic structure was related to lattice structural distortion.Our results provide a new direction for modulating the optical properties of TMDs QDs through utilizing defects-excitons interactions.The pressure-tuned emission of excitons combined with strong PL from defects sites of WS₂ QDs may have promising applications in optoelectronic devices.4.We propose a new fundamental piezo-catalytic mechanism based on a double-piezoelectric heterostructure,which combines the semiconductor MoS₂ and metal RGO?reduced graphene oxide?to form a MoS₂/RGO hybrid piezo-catalyst.The results show that,under ultrasonic irradiation,the MoS₂/RGO hybrid could provide an ultra-high degradation activity toward methylene blue?MB?in an aqueous solution without light assistance.The polarization potential of MoS₂ resulted in band bending and generated electron/hole pair separation upon the application of strain.The sheet-on-sheet structure,which was formed through a facile hydrothermal method,significantly increased the interfacial electron transfer from MoS₂ to RGO.In this process,the piezoelectric potential in RGO from hydrogen bonds?H-bonds?successfully further separated piezo-induced carriers,which significantly enhanced the piezo-catalytic activity.Coupling the piezoelectric properties of the MoS₂nanosheets and RGO,a new fundamental mechanism,the piezo-catalysis of a double-piezoelectric heterostructure?MoS₂/RGO?toward the degradation of organic pollutants by imposing an ultrasonic wave,has been first demonstrated.