First-Principles Design Of Silicon-Based Two-Dimensional Quantum Materials

Silicene,the silicon layer in a honeycomb lattice analogous to graphene,is emerging as a two-dimensional quantum material which hosts relativistic Dirac fermions and stronger spin–orbital interactions than its carbon counterpart.Multilayer silicene also draws intensive attentions.Recent experiments show multilayered silicene on Ag(111)can grow up to ～50 layers with a unique √3 surface relaxation.While the atomic structures and electronic properties of multilayer silicene,and the origin of observed Dirac states are still heavily debated.Therefore,it is of crucial importance to clarify these questions in theory.Moreover,silicon is the most important element for current electronic industry.The silicon-based materials achieving quantum spin Hall(QSH)effects and quantum anomalous spin Hall(QAH)effects might give to broad applications in silicon-compatible spintronic and magnetoelectronic devices.In this thesis,we focus on the exploration and design of two dimensional silicon-based nanostructures.The systematic and in-depth studies on silicene aim at clarifying the crucial issues of the atomic structures,growth mechanism and electronic properties in the field of multilayer silicene.Besides,the ensuing researches,such as the long-range ferromagnetism order on silicon surface and large spin-orbital coupling in functionalized silicene structures,extend silicon-based nanoelectronics and electromechanics into a quantum regime.The main contents of this thesis are listed as below:1.Combining first principles investigations,we clarified the atomic structures,growth mechanism and electronic properties of multilayer silicene under the cooperation with experimentalists.First,we predicted a new bilayered silicene with indirect energy gap of 1.16 eV(close to the value for bulk Si)and a metal-semiconductor phase transition when a minimal shear forceis applied on the configuration.Then,we studied the layer-by-layer growth of multilayer silicene fabricated on Ag(111)substrates.We identified that the presumable van der Waals packed multilayered silicene sheets spontaneously transform into a diamond-structure bulk Si film due to strong interlayer couplings.Multilayered silicene prepared by bottom-up epitaxy on Ag(111)exhibits a nearly ideal flat surface with a weak surface relaxation.Without invoking Ag surfactants,√3 × √3 honeycomb patterns emerge thanks to dynamic fluctuation of mirror-symmetric rhombic phases,similar to monolayered silicene.The weak relaxation enables novel surface states with a Dirac linear dispersion.2.Our further research found that the ferromagnetic ordering occurring on Si(111)-√3 surface with spontaneous atomic reconstruction stems from the topologically nontrivial surface states hosting spin-polarized Dirac fermions with half filling.In contrast to conventional routes relying on introduction of alien charge carriers or specially patterned substrates,the spontaneous magnetic order and spin-orbit coupling on the pristine silicon surface together gives rise to QAH effect with a finite Chern number C =-1.More interestingly,the QAH effect can be experimentally accessed on pristine silicon surface due to a nontrivial SOC gap of 15 meV.3.In addition,we studied the halogenated silicon films under biaxial tensile stain.The results predicted these two-dimensional silicon-based materials as excellent candidates for two-dimensional QSH insulators.Especially,the Si-I films have the tunable spin-orbit coupling gaps up to ~0.5 eV under minimal strain of 2.5%.The silicon-based materials are most promising for developing silicon-compatible QSH insulators at ambient conditions.Our researches bring potential candidates for developing next-generation two-dimensional silicon-based magnetic and spintronic devices.The studies make a contribution to extend silicon-based nanoelectronic and electromenchaniscs in to a quantum regime.