Growth Mechanism And Physical Properties Of Lowdimensional Atomic Crystal Materials On Metal Substrates-First Principles Calculations

The developments of human society,science and technology are always accompanied with the development of material.Human beings gain a lot from advancement of the materials engineering,from natural to the artificial materials,from the traditional to novel materials.Since the first exfoliation of graphene from graphite,low-dimensional materials have attracted great attention due to their plenty of peculiar properties,such as high mobility,excellent mechanical,optical properties,quantum Hall effect at room temperature,catalytic effect and so on.The first key question before application is how to get these low-dimensional materials with high quality and large area and how to effectively tune their properties.In this context,we investigated the growth mechanism of graphene and two dimensional silicon structures on Ru(0001)substrate,and band-gap engineering of S-doped graphene nanoribbons(GNRs)using first principles calculations based on density functional theory(DFT)combined with scanning tunneling microscopy(STM).The main results of this study are as follows:Growth mechanism of graphene growth on Ru(0001).Carbon precursor that forms by the dissociation of feedstock gas plays an important role in the controllable growth of graphene on metal substrates.However,the configuration about the precursor has so far remained elusive.Here,using first principles calculations based on density functional theory,the growth precursor for graphene growth was investigated.The calculation results unveil the formation of non-planar CH2 and CH2 dimer chains as the growth precursors at the nucleation stage of graphene growing on a Ru(0001)substrate,when C2H4 molecules are used as carbon source.The nonplanar CH2 and CH2 dimer chains locate at the hexagonal-close-packed hollow sites in the substrate,along the three highest symmetry directions,which is confirmed by the direct observation of uniformly structured precursor units in a scanning tunneling microscopy experiment.These findings reveal that nonplanar CH2 and CH2 dimer chains act as precursors in graphene growth and are helpful to improve the quality and the domain size of desired graphene by precursor or feedstock controlSequence of Silicon Monolayer Structures Grown on a Ru Surface: from a Herringbone Structure to Silicene.Silicon-based two-dimensional(2D)materials are uniquely suited for integration in Si-based electronics.Silicene was recently fabricated on several substrates,such as Ag(111),Ir(111)and ZrB2(0001).Recently,the first field-effect transistor based on silicene was successfully made,which inspires people's great interest in this filed.In this work,the calculation results reveal that when Ru(0001)is used as a substrate,a range of distinct monolayer silicon structure forms,evolving toward silicene with increasing Si coverage.A 50% Si coverage produces a herringbone structure,a hitherto undiscovered 2D phase of silicon.With increasing Si coverage to 85.7%,silicene forms in registry with the substrate.By simulating the transformation process from herringbones to silicene,it is confirmed that silicene grows from the elbow sites on herringbones.The distinct Si 2D structures were observed by using scanning tunneling microscopy.This work paves the way for further investigations of monolayer Si structures.The corresponding growth mechanism,and possible functionalization by impurities.Bandgap engineering of GNRs by doping heteroatoms.Theoretical works suggest that GNRs can possess semiconducting band gap characteristics that can be tuned by controlling the GNR width,edge structures,and/or impurity doping.However,achieving such control has been a major challenge in the fabrication of GNRs.N doping only shift the valence band maximum and conduction band minimum positions,without changing the size of the band gap.Chevron-type GNRs were recently achieved by surface-assisted polymerization of pristine or N-substituted oligophenylene monomers.In this work,the first-principles calculations show that the HOMO-LUMO gaps of the oligophenylene monomers can be tailored by different S configurations.In addition,the results indicate that chevron-type GNRs composed of the sulfur-substituted oligophenylene monomers have various band gaps in which the S substitution at different positions.Furthermore,these predictions have been confirmed by Scanning Tunneling Microscopy(STM)and Scanning Tunneling Spectroscopy(STS).S-doping gives new directions to achieve GNRs band engineering.