Investigation Of Electronic And Magnetic Properties Of Embedded Graphene Quantum Dots And Graphene Nanoribbons With Grain Boundary

Graphene, a two-dimensional (2D) monolayer of C atoms tightly packed into ahoneycomb lattice, has attracted considerable attention due to its extraordinary physicalproperties and promising applications. However, the metallic character of graphene limits itspotential applications in nanoelectronic devices. Quantum confinement is one of the generalmethod to open the band gaps (Eg) of graphene, such as one dimensional (1D) graphenenanoribbons (GNRs) and zero dimensional (0D) graphene quantum dots (GQDs)。Owing tothe different bonding characteristics of edge C atoms from the inner C atoms, they possessunique electronic and magnetic behaviors that differ from2D graphene sheet. For example,the triangular GQDs (TGQDs) have been increasingly considered since the diversity ofelectronic and magnetic properties can render them as potential components for nanodevices,such as spin memory, transistors and optical emission. However, there remain severalobstacles to overcome before TGQDs can be utilized. Firstly, it is challenging to achievesmooth and defect-free edges in real samples, which unavoidably involve structural defectsand impurities of foreign atoms. This will lead to spin suppression of TGQDs and hencebrings practical difficulties for spintronic applications. In additon, the integration of TGQDsinto devices is very hard, and even the manipulation must require extreme dexterity.Previous studies have demonstrated that the GNRs embedded in bulk graphane possesssimilar electronic and magnetic behaviors to the isolated ones. If the properties of TGQDsare kept when they are embedded in bulk graphane, a simple way can be provided to realizesmooth egdes for TGQDs without cutting. Moreover, the issues that occur during theintegration into devices can also be solved.On the other hand, the electronic properties of graphene and GNRs originate from theirspecific structures. During the growth process or post processing, the GBs are inevitablyintroduced into the host graphene lattice. They can disturb the honeycomb lattice symmetrythrough introducing non-hexagonal rings and hence engineer the local properties of grapheneto achieve new functionalities. The most common GBs that experimentally synthesized arepredominately composed of pentagon-heptagon pairs. Recently, Lahiri et al. havedemonstrated that a new GBs can be realized on the Ni(111) substrate, which consists ofalternating pairs of pentagons and octagons (558GB). The theoretical and experimental works have demonstrated that the bulk graphene embedded with558GB display uniqueelectronic and magnetic behaviors compared to the cases of GBs that composed ofpentagon-heptagon pairs. However, there has been hardly any systematic work studying theinfluences of558GB on GNRs in detail.Based on the above considerations, by using first principles density functional theory(DFT) calculations, we have investigated the stability, electronic and magnetic properties ofTGQDs embedded in graphane. Furthermore, the effects of558GB on the electronic andmagnetic properties of GNRs are systematically studied. The main results are divided intotwo parts as following:Firstly, since the diffusion of H atoms from the graphane region to the embeddedTGQD is energetically unfavorable with high energy barriers, the graphane/embeddedTGQD interface is rather stable. The electronic and magnetic properties of the wholesystems depend on the embedded TGQDs. The Egof graphane-embedded ATGQDs(armchair-edged TGQDs) arise due to the quantum confinement, while the nonbondingstates of graphane-embedded ZTGQDs (zigzag-edged TGQDs) play an important role intheir electronic and magnetic properties. Furthermore, graphane-embedded ZTGQDs exhibita ferrimagnetic ground state in which their size-dependent total spin are consistent withLieb’s theorem. The results have indicated that our work provides a possible way to realizeTGQDs without physical cutting.Secondly,(1) When558GB are periodically repeated in zigzag-edged GNR with widthof12(12-ZGNR), the comprehensive studies about the locations of this defect on theelectronic and magnetic properties of12-ZGNR are performed. It is found that558GBpreferably forms near the edge. As558GB shifts from the center to the ribbon edges, thesystems experience transitions from antiferromagnetic (AFM) semiconductors to AFMhalf-metal and then to ferromagnetic (FM) metal. Under the tensile strain (), the Egof theAFM semiconductors decrease and then AFM semiconductors change into AFM half-metals.Finally, all the AFM systems turn into FM metals.(2) When558GB with finite length is embedded in the armchair-edged GNRs(AGNRs), the edge configurations of558GB are introduced. The impurity states thatcontributed by558GB appear near the Fermi level, which have a detrimental effects on theelectronic and magnetic properties of AGNRs. For the cases of the AGNRs with the sameedge configurations of558GB, three groups can be divided. The variations of electronic andmagnetic properties with two adjustable parameters W (ribbon widths) and NZ(the distances between neighboring558GBs) are systematically investigated for each group. The resultsshow that different transitions of the electronic and magnetic states with W and NZarepresented by varying edge configurations of558GB, including nonmagnetic (NM) metals,FM metals and NM semiconductors. The AGNRs with different edge configurations of558GB also display similar electronic and magnetic behaviors. Furthermore, a ferromagneticordering along the two zigzag chains of558GB occurs for the FM systems due to the spinsplitting energy bands.These intriguing electronic and magnetic properties of Z(A)GNRs introduced by558GB would enrich their potential applications in electronic and spintronic devices.