Structural And Physical Properties Of Graphene-like Materials Studied From First Principles

With the rapid development of nanoscience and nanotechnology, the graphene and some other graphene-like nanomaterials such as MoS2 and phosphorene have been a hot topic in this field. Their distinctive physical properties not found in their bulk counterparts stimulate great interests of researchers working in this field,In this paper, we have studied the structural and physical properties of graphene-like nanomaterials from the first principles method, including the strain-induced ferromagnetism in the zigzag edge graphene nanoribbon with a topological line defect, the (2×1) dimerized structure of monolayer 1T-molybdenum disulfide, electronic and magnetic properties of armchair MoS2 nanoribbons under both external strain and electric field, the vacancies in phosphorene and the mechanical and electronic properties of phosphorene under applied strains.In chapter 1, we review the history of nanotechnology briefly, and previous theoretical and experimental works on these graphene-like nanomaterials.In chapter 2, we present the theory and method used in this paper, especially the density functional theory. We firstly present a brief introduction of the density functional theory. In the second section, we show how the Kohn-Sham equation is derived, and how to solve it in practice by different methods. At the last section, we introduce some popular softwares based on the density functional theory.In chapter 3, we have studied the strain-induced ferromagnetism in zigzag edge graphene nanoribbon with a topological line defect (LD-ZGNR). The applied strain along ribbon axis induces the local magnetic moments on the line defect, whose coupling with those on the edges leads to a turnover of the spin polarization on one edge, making the LD-ZGNR become a ferromagnetic metal at a large enough strain. A detailed calculation on the variation of the local magnetic moments and the spin polarized electronic structures of the LD-ZGNR with the applied strain reveals a possibility to tune the electronic and magnetic properties of the LD-ZGNR by an applied strain, which would cause the LD-ZGNR to be used in future spintronics devices and electromechanical ones.In chapter 4, we have investigated the geometric structures of monolayer 1T-molybdenum disulfide (1T-MoS2), finding a new (2 x 1) dimerized structure of it, called as 1T-MoS2, which is semi-conducting and more stable than the previously found ones. Its dimerized structure could be still kept under both the tensile and compressive strains, but a semiconductor-metal (S-M) transition is predicted at a tensile strain of about 4% or a compressive strain of about 5%。Moreover, the vibrational properties of three kinds of monolayer MoS2, i.e., the 1H-MoS2, 1T-MoS2, as well as the 1T’-MoS2, have been studied, based upon which the different isomer structures of monolayer MoS2 can be easily distinguishable in experiments by analyzing their characteristic Raman spectra.In chapter 5, the electronic and magnetic properties of armchair edge MoS2 nanoribbons (MoS2-ANRs) under both the external strain and transverse electric field (Et) have been systematically investigated. It is found that:(1) If no electric field is applied, an interesting structural phase transition would appear under a large tensile strain, leading to a new phase MoS2-A’NR, in which the Mo atoms at the edges move towards the hexagon centers, leading to an edge reconstruction from pristine armchair edges into S-terminated zigzag-like edges, making one edge hexagon transform into two tetragons. And a big jump peak of the band gap is induced in the transition region. But, the band gap response to compressive strains is much different from that to tensile strain, showing no the structural phase transition. (2) Under the small tensile strains, the combined Et and tensile strain give rise to a positive superposition effect on the band gap reduction at low Et, and oppositely a negative superposition one at high Et. On the other hand, the external compressive strains have always presented the resonant effect on the band gap reduction, induced by the electric field. (3) After the structural phase transition, an external large tensile strain could greatly reduce the critical field Etc causing the band gap closure, and make the system become a ferromagnetic (FM) metal at a relative low Et, which is very helpful for its promising applications in nano-mechanical spintronics devices. (4) At high Et, the magnetic moments of both the MoS2-ANR and MoS2-A’NR in their FM states could be enhanced greatly by a tensile strain.In chapter 6, we present our numerical works about the geometric structures, stabilities of both the monovacancy (MV) and divacancy (DV) in two-dimensional phosphorene, and MV’s diffusions, as well as their influences on the vibrational and electronic properties of phosphorene. Two possible MVs and 14 possible DVs have been found in phosphorene, in which the MV-(5|9) with a pair of pentagon-nonagon is the ground state of MVs, and the DV-(5|8|5) with a pentagon-octagon-pentagon structure is the most stable DV. All 14 DVs could be divided into four basic types based upon their topological structures and transform between different configurations via bond rotations. The diffusion of MV-(5|9) is found to exhibit an anisotropic character, preferring to migrate along the zigzag direction in the same half-layer. The introduction of MV and DV in phosphorene influences its vibrational properties, inducing the localized defect modes, which could be used to distinguish different vacancy structures in future experiments by using the Raman spectra. The MVs and DVs also have a significant influence on the electronic properties of phosphorene. Our calculation results are helpful for understanding the basic properties of MV and DV in phosphorene, which are useful for the promising application of the phosphorene in the nanoelectronics.In the last chapter, we systematically investigated the properties of phosphorene under external strain. In section 7.3, we have studied the strain-modulated mechanical and electronic properties of both monolayer and bilayer phosphorenes under either isotropic or uniaxial strain, finding the direct-indirect band gap transition, as well as the semiconductor-metal transition. In section 7.4, the effects of normal compressive strain on the structures of phosphorene have been investigated. It is quite intriguing to find that a controlled introduction of mechanical deformation could induce the structural phase transitions in phosphorene. By the phase transitions, the pristine Z-phosphorene with its puckered structure alternated by the zigzag lines could transform into a new A-phosphorene with its specific puckered structure alternated by the armchair lines under a normal compressive strain e= 48% or an anisotropic biaxial strain of εx=-16% and sy= 54%. In the extreme case, where the pucker structure is flattened into a plane, the phosphorene structure is quite unstable at finite temperatures, transforming into a new H-phosphorene phase. The anisotropic structure of A-phosphorene gives rise to its anisotropic mechanical properties with the Young’s modulus in the zigzag direction about 1.25 times larger than their counterparts in the armchair direction. While the H-phosphorene exhibits isotropic mechanical properties with the effective Young’s modulus to be 77.64 N/m. Both the A-phosphorene and H-phosphorene are semiconductors with indirect band gap of about 0.42 eV and 1.94 eV, respectively. The electronic properties of the two new phases are found to be sensitive to the magnitude and direction of the applied strains.