Construction Of Two-Dimensional Heterostructure Under Ultrahigh Vacuum And Spectroscopic Investigation Of Emerging Layered Semiconductors

Since the discovery of two-dimensional（2D）materials represented by graphene,the unique optical,electrical,magnetic and other physical properties of lowdimensional systems have attracted wide attention.For layered semiconductor materials,there exist abundant topological states,excitonic states or phonon states,which provide an excellent platform for the research of fundamental physics and the construction of new quantum information devices.The preparation of high-quality 2D samples is a prerequisite for the physical property investigation,especially for improving the surface and interface quality of the sample,or developing new heterostructures.Focusing on the above two basic issues,in this thesis,we actively explored the construction of 2D heterostructures under ultrahigh vacuum and the spectroscopy of layered semiconductors.In addition to the research and development of new preparation techniques,with the help of conventional spectroscopic techniques,such as angleresolved photoelectron spectroscopy,fluorescence and Raman spectroscopy,we have studied the topological flat bands in a semiconductor with new lattice geometry,revealed the bound excitonic state and bandgap engineering in a layered semiconductor,and discussed the characteristics of the lattice vibration spectra in a 2D topological insulator.The main content of this article is as follows:1.In view of the shortcomings of the existing 2D material exfoliation and stacking techniques,such as surface and interface contamination and degradation of air-sensitive materials,we directly exfoliate and transfer 2D materials in an ultrahigh vacuum.It is worth noting that we can not only repeat the large-area 2D material exfoliation by conventional methods,but also construct heterostructures between various 2D materials and substrates conveniently,including directly transferring few-to monolayer samples onto atomically flat single-crystal surfaces.After proving the universality of our technique,we turned to investigate two systems that cannot be achieved previously,including the electronic structure of monolayer phosphorene and optical responses of transition metal dichalcogenides on different metal substrates.Our proposed method paves a new way to study rich interface-induced phenomena,such as the proximity effect.2.The destructive interference of wavefunctions in a kagome lattice can give rise to topological flat bands（TFBs）.There have been many experimental reports on the observation of TFBs in kagome metals in previous studies.Nonetheless,kagome materials that are both exfoliable and semiconducting are lacking,which seriously hinders their device applications.In view of this,we show that in Nb₃Cl₈,a material with a breathing kagome lattice,the inversion symmetry breaking opens up the bandgap,while the TFBs survive due to the preservation of the mirror reflection symmetry.By angle-resolved photoemission spectroscopy measurements and first-principles calculations,we pointed out the location of this flat band.Its semiconducting ground state can be further proved by optical absorption spectroscopy and device measurements.Nb₃Cl₈ can be thinned to a single layer,which still hosts a good environmental stability.In addition,theoretical calculations show that monolayer Nb₃Cl₈ has a magnetic ground state,and the topological flat band will spin-split,thus providing opportunities to study the interplay among geometry,topology,and magnetism.3.We systematically studied the optical properties of a stable single-element 2D material—violet phosphorus by means of second harmonic generation,photoluminescence,and optical absorption.We observed strong bound exciton emission that is 0.48 eV away from the free exciton emission at low temperature,which is among the largest in 2D materials.In addition,the optical bandgaps of violet phosphorus are highly sensitive to the number of layers and external strain,which provides convenient approaches for bandgap engineering: when the layered violet phosphorus is thinned from bulk to bilayer,its bandgap value will increase from 1.78 eV to 2.23 eV;with the increase of surface tensile strain,its bandgap value will decrease up to 0.1 eV.The strong bound exciton emission and tunable bandgaps makeVP a promising material in optoelectronic devices.4.Recently,2D topological edge state and one-dimensional Luttinger liquid boundary state have been discovered in a semiconducting layered material,Ta₂Pd₃Te₅.However,controlled preparation of few-to monolayer samples experimentally and realization of quantum spin Hall devices still face great challenges.In view of this,we successfully obtained large-area few-to monolayer Ta₂Pd₃Te₅ nanosheets by mechanical exfoliation,and carried out detailed layer-and temperature-dependent Raman spectroscopy measurements.Our results demonstrate that Raman spectra can provide valuable information to determine the layer number of ultrathin Ta₂Pd₃Te₅.In addition,angle-resolved polarized Raman spectroscopy measurements revealed that the two types of Raman peaks are strong anisotropic due to the quasi-one-dimensional lattice structure,which can be used to directly determine its crystal orientation.Our findings may stimulate further efforts to realize quantum devices based on few or monolayer Ta₂Pd₃Te₅.