The Investigation Of Structures And Properties Of Several Multilayer Two-dimensional Materials By First-principle Calculations

The history of human development is accompanied by the continuous discovery of materials processing and device fabricating.With the fabrication of single-crystal silicon and doping technology,humans have entered the information era by fabricating the large-scale integrated circuit.However,as Moore's law nears its physical limits,two-dimensional atomic crystal materials,a promising candidate to replace silicon,has become an intriguing research direction in the field of semiconductors.Two-dimensional materials have received extensive attention because of their excellent electronic properties,such as ultra-high carrier mobility.With the development of two-dimensional material synthesis methods,many types of monolayer two-dimensional materials such as graphene,transition metal disulfide,and other compounds have been synthesized.However,there are still many materials with excellent properties that cannot be synthesized into monolayer two-dimensional materials.At this time,the multilayer two-dimensional material has a wider application space because of its advantages,such as better stability and easier synthesis than the single layer.In this thesis,we investigated the structures and properties of several multilayer two-dimensional materials by density functional theory?DFT?based first principle calculations.The thesis contains three parts as following:1. Bandgap opening of bilayer graphene?BLG?grown on Ru?0001?with monolayer silicene intercalation.It has been reported that an external electric field or the adsorption of atom/molecule on one side of BLG can open a bandgap in BLG.However,using these methods,to open up a large bandgap,BLG needs to be highly doped,which makes it hard to fabricate/control the devices.In this work,taking advantage of the cooperative effect of electron doping and rippling,a?0.2 eV bandgap in the BLG was opened.BLG has epitaxially grown on Ru?0001?rippled due to the lattice mismatch between BLG and Ru?0001?.This rippled structure applied a strain on BLG.While this strain cannot open the bandgap in BLG because of the strong interaction between BLG and the substrate.The subsequently intercalated monolyer silicene decouple the BLG and the substrate and keep a proper amount of electron doping.The cooperative contribution of the doping and rippling/strain opened a bandgap as large as 0.2 eV.This work provides an ideal approach to open up the bandgap that can be used in electronic devices.2. Unusual anisotropic thermal expansion behaviors of multilayer Sn Se and the influence on the Poisson's ratio.By employing DFT calculations,a significant notable thermal-expansion behavior of Sn Se from monolayer to bulk has been demonstrated.It is found that the unusual thermal expansion persists in multilayers Sn Se,while the coefficients of thermal expansion of different numbers of Sn Se layers are almost identical.This behavior results from a delicate interplay between the elastic stiffness coefficient and Gruneisen parameters.Moreover,the Poisson's ratio of multilayer Sn Se,which is positive at 0K,gets smaller with increasing temperature.The Poisson ratio turns negative,signaling a zero Poisson's ratio at a particular temperature.This result provides a possible approach to the design of materials with zero Poisson's ratio.3. Magnetic order of V₅S₈ from bulk form to multilayer form.Different types of exchange-correlation functionals and on-site Coulomb potential have been tested.For the first time,the antiferromagnetic?AFM?order has been successfully produced in DFT calculations.DFT calculations reveal that the magnetic exchange coupling is from superexchange interaction.Calculations of thickness-dependent magnetic order for multilayer V₅S₈ shows that the ferromagnetic?FM?states become more energetic favor with reducing thickness.The critical thickness of the AFM-FM transition ranges from1.4 nm to 2.2 nm depending on different surface terminations.The major magnetism of the thin films originates from the same type of vanadium atoms,the intercalated vanadium,as that in the bulk material.However,the other types of vanadium atoms,those in VS₂ layers,gradually exhibit small magnetic moments with reducing thickness.It is also found that the phase transition can be affected by the strain applied to the multilayer V₅S₈.This work reproduces the experimental reported ground state of bulk V₅S₈ using DFT calculation the first time,predicts the AFM-FM phase transition while thinning down the multilayer V₅S₈,and provides a benchmark for calculations in V₅S₈ system.