| Graphene, a two-dimensional (2D) honeycomb structure of single layer of sp2 hybridized carbon atoms, has attracted tremendous attention and research interest since its discovery in 2004. Graphene is a zero-gap semiconductor and has shown many unique properties, such as excellent thermal conductivity, high-speed carrier mobilities, great mechanical strength, high surface area and good optical transparency. Due to its exceptional structure and electrical properties, graphene has been widely applied in hydrogen storage, gas sensors, Lithium-ion battery, transistor and catalyst et al. Therefore, in this paper, by using the first-principles based on density-functional theory (DFT), the structure and hydrogen storage behavior of Pd-decorated graphene with nitrogen or boron dopants and various vacancy defects have been systematically investigated. Then, the sensitivity of Pd-doped graphene toward a series of small gas molecules (CO, NH3, O2 and NO2) has been studied. Lastly, the stabilities, electronic and magnetic properties of silicene nanoribbons (SiNRs) substitutionally doped with a single and double N or B atoms have been systematically investigated. The main conclusions can be summarized as follows.(1)We systematically investigated the structure and hydrogen storage behavior of Pd-decorated nitrogen-doped graphene. Among the three types of defective structures, it is found that Pd-decorated graphene with pyridinic and pyrrolic N-doped defects are more stable and exhibit hydrogen uptake ability up to three H2per Pd atom. A single H2 or two H2 are molecularly chemisorbed on the Pd atom, where the streched H-H bond is relaxed but not dissociated. The binding mechanism of H2 molecule is attributed to hybridization of the 4d orbitals of Pd with the σ orbitals of H2 (so-called Kubas interaction). Out of two adsorbed H2, the first and second H2 are still chemisorbed molecularly, the nature of bonding is very weak physisorption for the third adsorbed H2. Double-side Pd-decorated graphene with pyridinic and pyrrolic N defects can theoretically reach a gravimetric capacity of 1.99 wt% hydrogen.(2) The geometric stability and hydrogen capacity of Pd-decorated graphene with experimentally realizable boron dopants and various vacancy defects (including single carbon vacancy (SV), "585"-type double carbon vacancy (585 DCV) and "555-777"-type double carbon vacancy (555-777 DCV)) were systematically investigated. It is found that among the four types of defective structures, Pd’s binding energies on SV and 585 DCV defect graphene sheets exceed the cohesive energy of the Pd metal bulk, thus Pd atoms are well dispersed above defective graphene sheets and effectively prevent Pd clustering. Up to three H2 molecules can bind to Pd atom on graphene with B dopants, SV and 555-777 DCV defects. For the cases of Pd-decorated graphene with B dopants and 555-777 DCV defect, the hydrogen storage behaviors are similar to Pd-decorated graphene with pyridinic and pyrrolic N defects. Different from above two cases, three H2 are all molecularly chemisorbed to Pd atom with stretched H-H bond for Pd-decorated S V defect graphene, the hybridization of the Pd-4d orbitals with the H2-σ orbitals and the electrostatic interaction between the Pd cation and the induced H2 dipole both contribute to the H2 molecules binding, and the binding energies of 0.25～0.41 eV/H2 is in the range that can permit H2 molecules recycling at ambient conditions.(3)We systematically investigated sensitivity of pristine graphene and Pd-doped graphene toward a series of small gas molecules (CO, NH3, NO2 and O2). It is found that four gas molecules are weakly adsorbed on pristine graphene with low adsorption energy of 0.08-0.24 eV, and the electronic properties of pristine graphene are only sensitive to the presence of O2 and NO2 molecules. In contrast, doping graphene with Pd dopants significantly enhances the strength of interaction between adsorbed molecules and the modified substrate. The dramatically increased adsorption energy and charge transfer of these systems are expected to induce significant changes in the electrical conductivity of the Pd-doped graphene sheet. The results reveals that the sensitivity of graphene-based chemical gas sensors could be drastically improved by introducing the Pd dopants, so Pd-doped graphene is more suitable for gas molecules detection compared with pristine graphene.(4)The structures, formation energies and electronic properties of armchair silicene nanoribbons (ASiNRs) substitutionally doped with a single and double N or B atoms located at various sites ranging from the edge to the center of the ribbon, have been investigated by the first-principles calculations. From minimization of the formation energy, we find that the substitutional doping is favorable at the edge of ribbon. A single N or B atom and double B atoms substitution in ASiNRs lead to a transition from semiconductor to metal, while double N atoms substitution remains semiconducting. N and B co-doping at the opposite edges also does not affect the semiconducting character of ASiNRs.(5) We performed a spin polarized density-function theory study of the stabilities, electronic and magnetic properties of zigzag silicene nanoribbons (ZSiNRs) substitutionally doped with a single N or B atom located at various sites ranging from edge to center of the ribbon. From minimization of the formation energy, it is found that the substitutional doping is favorable at edge of the ribbon. A single N or B atom substitution one edge Si atom of ZSiNRs can greatly suppress the spin-polarizations of the impurity atom site and its vicinity region, and leads to a transition from antiferromagnetic (AFM) state to ferromagnetic (FM) state, which is attributed to the splitting of the original spin degenerate edge bands. A single N atom doped ZSiNRs still keep semiconductor property but a single B atom doped ZSiNRs exhibit a half-metallic character. The double N or B doped, N and B co-doped ZSiNRs at the opposite edge turn into nomagnetic (NM) state. In addition, the double N and N-B co-doped ZSiNRs still remain semiconductor but band gap decreasing. The double B doped ZSiNRs exhibit metallic. Our results reveal that substitution doped SiNRs have potential applications in Si-based nanoelectronics, such as field effect transisitors (FETs), negative differential resistance (NDR) and spin filter (SF) devices.
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