The Regulation Of Structure And Electronic States Of Borophene And Two-dimensional Van Der Waals Materials

As the old saying goes,“Great truths are always simple”.Reducing the dimensions explores new orientation for the development of materials.The discovery of graphene has invoked a popular research interest in two-dimensional（2D）materials.In recent years,a large number of 2D materials have been successfully prepared.The monolayer of 2D materials still exhibit excellent properties such as mechanical,thermal,electrical,magnetic and optical properties.On the one hand,due to the reduction in size of 2D materials and the influence of quantum confinement effect,many novel quantum states will be generated,such as quantum Hall effect,topological edge states,superconducting states,valley polarization electronics,etc.The studies of these are very important for the properties and the final application trend of materials.On the other hand,many functional 2D materials,such as ferroelectric and ferromagnetic materials,maintain ferroelectricity and ferromagneticity in one layer or few layers and easily form heterostructure to improve compatibility with other materials,providing opportunities for 2D materials as candidates for information storage and spintronics.Molecular beam epitaxy（MBE）technology is an accurate material growth method and plays an important role in the preparation of high-quality 2D materials with controllable monolayer and chemical composition.Scanning tunneling microscopy（STM）enables people to observe the topography and lattice structure of material surfaces without damage,which provides strong evidence for confirming the quality and lattice structures of material surfaces.Furthermore,scanning tunneling spectroscopy（STS）can realize the direct characterization of density of states on the material surfaces in real space.The combination of the above two technologies is helpful for us to better fabricate and study the quantum states of 2D materials.In this dissertation,we explore the high-quality growth of borophene and monolayer Ni I₂by using MBE technology,and characterize the lattice structures and electron states of single-element materials borophene,binary 2D semiconductors MoS₂ and InSe,and 2D magnetic materials Ni I₂ using STM/STS combined with first-principles calculations.The main contents of this dissertation are as follows:1.The regular-mixed borophene phases were synthesized.Because of the polymorphism of borophene,different phases were obtained on different substrates in experimentally.In particular,various pure borophene phases consisting of quasi-one-dimensional boron chains with different width were usually obtained on Ag substrates.In our work,borophene was synthesized on Ag（100）substrate by MBE and characterized by STM.The results shown that regular-mixed phase with long-range order composed of different quasi-one-dimensional borophene chains.There are two main types of mixed borophene phases consisting of regular mixed arrangements of the（2,3）and（2,2）chains in ratios of 2:1 and 1:2,respectively.Theoretical calculations of charge transfer and formation energies shown that all mixed chains phases without bucking interact weakly with Ag（100）substrate,and were more stable than pure phases.The mixed-chain phases with different proportions of chains can be well separated based on the crystal direction of the substrate.Detailed structural analysis shown that the matching of lattice constants and orientation between borophene and Ag（100）played a crucial role in the formation of the mixed chains phases.2.The detection and manipulation of quantum well states in few-layer MoS₂ were realized in real space.Quantum confinement has a significant effect on the band structures and photoelectrical properties of semiconductor materials.Compared with artificial quantum wells,the electron confinement formed along the van der Waals（vd W）layers in TMDs has unique advantages.The local density of states（LDOS）was detected and directly observe the quantized states in few-layer MoS₂ by using STM/STS.The number and energy positions of LDOS peaks in STS are closely related to the number of layers.The theoretical calculations indicated that these peaks originate from the QWSs of MoS₂ and reproduce the evolution process of the QWSs with increasing layers.In addition,we adopted two strategies:changing the interlayer distance and oxygen substitution atoms to adjust the QWSs.Our study shown that there is strong interlayer hybridization in the traditional weakly coupled vd W interface,leading to the formation of QWSs in few-layer MoS₂.3.The ideal two-dimensional electron gas（2DEG）system hosted by multiple quantum well states（MQWSs）in the conduction bands of few-layer InSe,the quantum well states and the novel electron states in the valence bands were characterized systematically.On the one hand,we implemented MQWSs with ideal 2DEG in few-layers InSe compared to conventional 2D free electron systems on semiconductor heterojunctions and noble metal surfaces.The number of MQWSs is strictly equal to the number of layers.These QWSs all exhibit stair-like DOS and parabolic dispersion,which are typical 2DEG characteristics.DFT calculations and numerical simulations based on quasi-bounded square potential wells from the Kronig-Penney model provided strong explanation of 2DEG in MQWSs.On the other hand,we observe the quantum well states in the valence bands of few-layer InSe similar to those in MoS₂ system.Besides,novel electron states in the valence bands were detected.The number and energy positions of new states are strongly layer-dependent and can be tuned by changing the electric field intensity,strain and moiréperiodic potential.Hence,we speculated that the novel electron states are related to the structural aberration,ferroelectric polarization and interlayer interaction of InSe layers.Finally,the overall bands and bandgap of few-layer InSe were characterized,and the layer dependence of DOS and bandgap were confirmed.4.The growth and electron states of monolayer（ML）magnetic material were studied.Monolayer Ni I₂were synthesized by MBE on iodine-modified Au（111）and HOPG substrates.Before the synthesis of Ni I₂,the iodine monolayer deposited on Au（111）surface with several reconstructed phases due to different coverage and desorbed with increasing substrate temperature.The formation of ML iodine weakens the interaction between the metal substrates and adsorbates,making the adsorbates keep its intrinsic properties better epitaxially.Subsequently,the ML Ni I₂ islands were synthesized on the iodine-modified Au（111）surface and would decompose gradually with the increase of annealing temperature.The Ni I₂ islands exhibited two opposite orientations,and two kinds of moirépatterns formed due to stacking with different reconstructed iodine phases.The STS results shown that orientation-selective edge states in the edges of the islands with long penetration depths.Combined with the first-principles calculations,it is further confirmed that the edge states were along the Ni-terminated edges rather than the I-terminated edges,and the calculated edge states are pure spin-polarized.In addition,the large area Ni I₂ monolayer was synthesized on HOPG substrates,and the polaron-like charge phenomena on the ML Ni I₂ surface was observed.At the edge of the step,the number of polarons was the largest,which can be connected into one-dimensional charge lines.The polarons on the terrace exhibited obvious triple symmetry within a certain range of bias.The observations of edge states and polarons indicate unique electron behavior in Ni I₂.