First-Principles Studies Of Growth Theory And Functionality Of Two-Dimensional Materials

It is the successful exfoliation of single layer graphene in 2004 that opens the door to the new world of two-dimensional materials for scientists.The researches on two-dimensional(2D)materials have attracted enormous attention in areas of physics,chem-istry,and materials science.A lot of materials were found to be stable in layered or 2D structures.Their diverse structures and unique properties provide new platforms for the development of basic science and may have many applications in modern technologies.In exploiting the properties of 2D materials for applications,there are some fundamen-tal aspects we would usually care about,among which are:materials fabrication,the growth mechanism,half-metallicity,electronic band gap engineering,topological prop-erties,and the design of functional heterostructures.The development in these aspects is important for the design of functional materials and their applications.In this thesis,we present several projects on the properties and functionalities of certain 2D systems based on first-principle calculations and theoretical methods.The contents have seven major parts:In Chapter 1,we make a brief introduction about the 2D materials,their commonal-ity,unique properties that diff from three-dimensional materials,materials fabrication,and the growth mechanism.There are abundant of 2D materials,which have diverse structures and high tunability in their properties due to the special dimensionality,in-cluding the magnetic properties,physical properties,and chemical properties.We also briefly introduce the recent progresses in researches of 2D materials and the related methods.In Chapter 2,we first introduce the theoretical background,including the frame-work of density functional theory(DFT),commonly used exchange-correlation func-tionals,and some DFT based simulation packages.Vienna Ab initio Simulation Package(VASP)is used in this thesis.Secondly,we briefly introduce some methods or packages beyond DFT,which are embedded in or based on the VASP or other DFT packages.In Chapter 3,we first show that Black Phosphorene(BlackP)and Blue Phospho-rene(BlueP)monolayers can be readily stabilized on Cu(111),Au(111),and GaN(0001)based on the comparative total energy calculations,with the semiconducting GaN(0001)inherently highly preferred among the three for various device applications.Further-more,our systematic energetic and kinetic studies of phosphorus adatoms,clusters,and monolayers reveal a novel Half Layer By Half Layer(HLBHL)growth mechanism for fabricating monolayered BlueP on Ga-terminated GaN(0001).Once formed,such a BlueP monolayer is thermodynamically stable,as tested using ab initio molecular dy-namics simulations at 400 K.The HLBHL growth mechanism discovered in this chapter may also be applicable to epitaxial growth of BlackP and other 2D materials.In Chapter 4,we first demonstrate that 2D crystals composed of fullerene molecules C28 in honeycomb lattices can now be added as new members of the topological materi?als family.When compared with the 2D atomic crystals such as graphene,the structural,electronic,and topological properties of such molecular crystals can be more readily tuned by encapsulating different functional atoms into the fullerene molecules and/or by rotating the molecules on the sublattice sites of the honeycomb structure.In such molecular crystals,the C28 molecules can act as superatoms to harbor Dirac-like band structures.As prototypical examples,we show that the Bi@C28-HL-C3v and In@C28-HL-C3 are respectively Z2 topological insulator and valley-based topological insulator.We may propose appealing new platforms for realizing quantum spin Hall and quantum valley Hall effects in 2D molecular crystals.In Chapter 5,we design functional materials in lateral heterostructures composed of graphene(G)and hexagonal boron nitride(/h-BN)as examples.We first introduce a new way to form heterostructures of G ribbons and chiral h-BN ribbons by properly tai-loring the orientational misalignment between the two.The strain energy accumulation in such misaligned heterostructures is essentially eliminated,while the half-metallicity is found to be drastically enhanced from the orientationally aligned cases,back to be comparable in magnitude to that of freestanding G nanoribbons.The pronounced half-metallicity is further attributed to the restored strength of the superexchange interaction between the electrons located at the two opposite interfaces of the G ribbon.Overall,these revelations are valuable for potential practical realization of the intriguing emer-gent electronic and spintronic properties of G/h-BN heterostructures.In Chapter 6,we focus on the interactions between 2D materials and the substrates in three different vertical heterostuctures.(1)We compare the different mechanisms of band gap opening in a graphene on topological materials and investigate the competi-tion between these mechanisms.(2)The graphene overlayers on metal surfaces tune the properties of the metals.The graphene covered Ni surface is predicted to be compara-ble with Pt in the catalytic performance due to the confinement effects in heterogeneous catalysis.(3)Interactions between the ZnSe overlayers and the layered topological ma-terials are investigated to tune the catalytic properties of the ZnSe overlayers.The ZnSe monolayer on a Bi2Se3 substrate is predicted to be a good catalyst for the hydrogen evolution reaction.The studies on vertical heterostructures may benefit the design of functional materials.