Researches On Energy Band Tuning And Interfacial Coupling Of 2D Semiconductor MSe₂（M=Mo,W）

As the most promising candidates to extend Moore’s Law,two-dimensional（2D）semiconductors have excellent potential in applications such as electronics,optoelectronics and valleytronics.By precisely control the chemical composition,phase structure,physical size,and electro-optical properties of two-dimensional semiconductors,the application scenarios of 2D semiconductors can be greatly enriched.In order to meet the needs of different optoelectronic devices,the energy band tuning of 2D semiconductors is particularly important.Precisely designing the band gap of 2D semiconductors and adjusting the doping type of 2D semiconductors not only provide the possibility to customize optoelectronic devices with diverse spectral responses,but also lay the foundation for different types of energy band matching in heterostructures.In addition,for the application of high-performance optoelectronic devices,high-quality interfacial contact between metal and 2D semiconductor is critical important.However,due to the limitation of the interface scale,the modulation of the interface structure and electronic properties is still rare.In this dissertation,by optimizing growth conditions,high-quality weakly coupled2D semiconductors molybdenum selenide（MoSe₂）and tungsten selenide（WSe₂）were prepared on Au（100）.Scanning tunneling microscopy/spectroscopy（STM/STS）was employed to studied their local structure and electronic properties of 2D semiconductors.The energy band control of the 2D semiconductor MSe₂（M=Mo,W）is realized by adjusting its defect and interface coupling properties.The main research contents of this dissertation are as follows:1.Research on the growth of high-quality weakly coupled 2D semiconductor MSe₂（M=Mo,W）.By optimizing the annealing conditions of gold foil,single-oriented Au（100）substrate was obtained.High-quality single-layer 2D semiconductors MoSe₂and WSe₂ were prepared by chemical vapor deposition.For monolayer MoSe₂,Raman spectroscopy shows the unique 2ZA double resonance vibration mode at the Brillouin zone M point.Furthermore,the atomic structure and localized electronic states were studied by STM/STS.The results showed that MoSe₂ could form a large-area monolayer film crossing the steps.STS showed that its quasiparticle band gap was 1.96e V and has good robustness at the edge.The origin of the weak coupling between Au（100）and MoSe₂ is analyzed based on first-principles calculations.The oscillation of the top gold atoms of the hexagonal reconstruction on Au（100）perpendicular to the surface increases the interfacial distance with MoSe₂,resulting in a weak interaction.Therefore,it can be transferred to other substrates by electrochemical bubbling method,and the intrinsic physical properties keep unchanged before and after transfer,showing rich Raman vibration modes.The high-quality,weakly coupled and large-area MoSe₂prepared on gold foil provides a basis for the subsequent development of the physical property.2.Research on the surface defect structures and electronic properties of the prepared MSe₂（M=Mo,W）semiconductor.Single Se atom vacancy defects were generated on the surface of WSe₂ upon ultra-high vacuum annealing.STS shows that Se vacancies results in N-type（electron）doping,with two deep defect states appearing near the conduction band.Combined with first-principles theoretical calculations,it is well elucidated that the defect state of Se vacancies originates from spin-orbit splitting,with value reaching 288 me V.Further,the Se vacancies on the surface are occupied by introducing germanium atoms,resulting in the disappearance of spin-orbit splitting near the conduction band of Se vacancies,and the introduction of new interstitial states in the valence band,and then the generation of P-type（hole）doping in WSe₂.Corresponding theoretical calculations demonstrate that the defect states at the valence band originate from the exchange of splitting.Upon replacement of Se vacancies by germanium,there are unpaired electrons,resulting in a 1.5 u B spin magnetic moment at the defects.3.Research on the twist controlling van der Waals interfacial coupling of WSe₂/Au（100）,and then the doping type of WSe₂.Combined with STM/STS,the structure and local physical properties of WSe₂ were studied.Thermal induced changing the relative lattice angle between WSe₂ and Au（100）terrace,resulting in the doping type of WSe₂ tuning from P-type to N-type.Differential conductance spectroscopy,Raman spectroscopy and X-ray photoelectron spectroscopy（XPS）reveal the regulation of the electronic properties by interfacial angle.At the edge of the Au step an NPN-type homojunction of WSe₂ was constructed with the built-in electric field highly adjustable,reaching 2.4×10⁶ V.cm^-1.Corresponding difference in average charge density indicates that the interfacial twist angle induces different charge concentrations and transfer direction,for example,for 0°twist angle,the band structure of WSe₂changes from p-type to n-type due to the high interfacial transfer electron concentration.4、Research on the germanium intercalation tuning band structure of WSe₂.Under high coverage and low annealing temperature,a single-layer Dirac germanene was successfully grown on WSe₂and form germanene/WSe₂ heterojunction.STS shows that the germanene/WSe₂ heterojunction has an asymmetric linear energy dispersion.Corresponding first-principles calculations reveals that this asymmetric band structure mainly results from the contribution of the germaneneσorbital.The interlayer coupling between germanene and WSe₂can be further enhanced by higher temperature annealing,and the band gap of germanene was successfully opened to 0.17 e V.First-principles calculations indicate that the disappearance of sublattice symmetry in germanene,and charge transfer from one triangular sublattice to another triangular sublattice of the germanene honeycomb lattice as well as the appearance of a net electric field induce its bandgap open.Under low coverage and high annealing temperature,germanium intercalation happens and effectively weakens the coupling between WSe₂ and the underlying Au（100）substrate.The surface of WSe₂ had atomic-level flatness,and its energy band structure was re-regulated to P-type.Combined with first-principles calculations,it was revealed that germanium intercalation acts as a buffer layer at the interface between WSe₂ and Au（100）,which weakens the interfacial coupling,shields electron scattering from the gold substrate,and the band structure of WSe₂ is retuned to intrinsic P type.