The Study Of Manipulation Of Charge Density Wave Of Monolayer TiSe₂ And TaSe₂ By Strain And Carrier Doping

Transition metal dichalcogenides?TMDCs?1T-TiSe₂,1T-TaSe₂ and 2H-TaSe₂ have abundant charge density wave?CDW?phase transition behavior and superconductivity.Strain and carrier doping can effectively manipulate the intrinsic physical properties of materials without introducing impurities,defects,etc.They have become important means for studying the physical properties of materials at present.Based on the first-principles calculations,this thesis investigates the effects of strain and carrier doping on CDW in these three monolayer materials.The main content is:Experimental studies have shown that low dimensionality can enhance the CDW transition temperature of 1T-TiSe₂ from?200 K in the bulk to?230 K in the monolayer,becoming the highest CDW transition temperature in monolayer TMDCs materials.However,for practical applications,it is very necessary to further increase the transition temperature.We investigate the evolution of CDW in monolayer 1T-TiSe₂ under biaxial biaxial strain and carrier doping by first-principles calculations.The compressive strain can suppress the CDW,and the tensile strain enhances the CDW,indicating that the tensile strain is expected to further increase the CDW transition temperature.Both electron and hole doping can suppress the CDW instability.When the CDW was completely suppressed,we have studied its superconductivity.The 6%compressive strain and carrier doping can induce superconductivity of?7 K and 0.3-7.3 K,respectively.Controllable electronic phase transition from semiconductor state to metal state or even superconducting state can be realized in monolayer 1T-TiSe₂ through strain and carrier doping,which makes monolayer 1T-TiSe₂ possess a promising application in electronic devices.Low dimensionality can increase the CDW transition temperature of 1T-TiSe₂ and can also lower the CDW transition temperature of other materials.Recent experimental studies have shown that the CDW transition temperature of 1 T-TaSe₂ decreases as the thickness of the material decreases.When the material is thinned to 18 nm,the CDW transition temperature decreases from?473 K in the bulk to?353 K.According to the above experimental rule,if the material is thinned into the monolayer,the CDW transition temperature of 1 T-TaSe₂ may be further reduced,even below room temperature.We investigate the evolution of the CDW in monolayer 1T-TaSe₂ under strain and carrier doping.The compressive strain slightly enhanced the CDW,while the tensile strain slightly weakened the CDW.Electron doping has a weak effect on the CDW,and hole doping can suppress the CDW.When the hole doping completely suppresses the CDW,the superconductivity of?6 K can be induced in the monolayer 1T-TaSe₂.2H-TaSe₂ is also a typical CDW material with a CDW transition temperature of 90 K,which is the highest CDW transition temperature in the 2H phase TMDCs.Recent experimental studies have shown that low dimensionality can increase its CDW transition temperature.We investigate the effect of strain and carrier doping on the CDW in monlayer 1H-TaSe₂.Compressive strain can enhance the CDW,while tensile strain can suppress the CDW.Electron doping has a weak influence on the CDW,and hole doping can suppress the CDW.In summary,strain and carrier doping can effectively manipulate the CDW order in monolayer TMDCs.By summarizing the regulation laws and conducting in-depth researchs,it is expected to find more low-dimensional materials with higher CDW transition temperatures,and create new electronic devices based on CDW.In addition,related researches can also provide a certain reference for further revealing the physical origin of CDW and its relationship with superconductivity.