ARPES Studies of the Electronic Structures of Layered Metal Chalcogenides and Their Modulation

Author:Mu Ke Jun

Supervisor:sun zhang guo bin


Degree Year:2018





Layered metal chalcogenides have been widely concerned after the discovery of graphene,and have been the frontier in the fields of physics,chemistry,and materials.The layered metal chalcogenides possess the layered crystal structure with van der Waals intercations between layers,and the two-dimensional planar structure along the in-plane direction.On the one hand,it can be easily exfoliated into N(N≥1)atom layer thicknesses with different physical and chemical propertis,which makes it to be widely used in the fields of electronics,optoelectronics,catalysis and other fileds;On the other hand,this kind of material has rich chemical composition and many unique physical properties,such as charge density wave(CDW),superconductivity,magnetism and excitontic insulator transition.The properties of layered metal chalcogenides superior to the graphene are that it overcomes the defect of zero bandgap in graphene.Moreover,some materials also exhibit a unique transition from the indirect bandgap semiconductor in bulk to the direct bandgap semiconductor in monolayer,such as MoS2.More importantly,we can easily tune the physical and chemical properties of layered metal chalcogenides through various modulation methods,including defects,intercalation,substitution,layer thickness,and interface interaction(just like heterojunctions).By these modulation methods,we can precisely modulate its physical features,and also could discover other unknown fancy features.Adjustable thickness,two-dimensional in-plane planarity,limited bandgap,suitable carrier mobility,and especially controlled physical and chemical properties,make layered metal chalcogenides hot material for various application fields.The physical and chemical properties of layered metal chalcogenides are often closely related to its internal electronic structures,and the Angle-resolved photoelectron spectroscopy(ARPES)technique is a crucial tool for studying the electronic structures of single crystal materials.It plays an important role in understanding the interaction mechanism between electrons and electrons or other quantum degrees of freedom.In this paper,we performed angle-resolved photoelectron spectroscopy to study the electronic structures of layered metal chalcogenides and their modulation,the main findings are as follows:1.Using ARPES,we study the electronic structures of T-TiSe1.8 prepared by Dual-Native-Defects Engineering,and find that the introduction of Se-anion defects leads to the opening of the band gap between the valence band and conduction band in this system.At the same time,we find that the electrons of conduction band have different responses to RCP and LCP lights,and we need to further explore its physical reasons.2.We investigate the influence of Cu intercalation on the electronic structures of ZrSe2 system(pristine ZrSe2 and Cu0.07ZrSe2).We find that more electrons fill the bottom of conduction band after the intercalation of Cu atoms,which leads to a semiconducting to metallic phase transition.Meanwhile,the intercalation of Cu only slightly increases the distance between layers,and plays a negligible effect on its lattice structure and overall band dispersions.From the obtained ARPES data,we determine that the ZrSe2 has an indirect bandgap of 0.77 eV,which is in good agreement with theoretical calculations.3.Using the combination of the ARPES and other related characterization methods,we study the electronic structures of SnSe2 before and after the intercalation of Co(Cp)2 in detail.Firstly,we find that the intercalated Co(Cp)2 possesses the strong electron-donating ability,which increases the density of states of the conduction band near the Fermi level EF,and the system undergoes a transition from the semiconductor to the superconductor.Then,the intercalation of Co(Cp)2 leads to a significant increasing of the distance between layers in SnSe2.The band structures change greatly and the band dispersions along the interlayer direction show stronger two-dimensionality.4.We detect and analyze the electronic structures of one-dimensional ternary material Ta2NiS5,and find that it has strong in-plane anisotropic band structures,which are similar to the band structures of the excitonic insulator material Ta2NiSe5.However,the top of valence band does not show flat-band but obvious parabolic features.Compared to Ta2NiSe5,Ta2NiS5 has a larger bandgap after replacing Se with S which exceeds the binding energy of excitons,preventing the spontaneous formation of excitons.This is the first time to demonstrate from our ARPES experimental data that Ta2NiS5 is not an excitonic insulator.