Silicon on a graphene nanosheet with triangle- and dot-shape: Electronic structure, specific heat, and thermal conductivity from first-principle calculations

Publisher's version (útgefin grein).The electronic structure, specific heat, and thermal conductivity of silicon embedded in a monolayer graphene nanosheet are studied using Density Functional Theory. Two different shapes of the substitutional Si doping in the graphene are studied, a triangular and a dot shape. The silicon doping of a graphene nanosheet, with the silicon atoms arranged in a triangular configuration in ortho- and para-positions, opens up a band gap transforming the sheet to a semiconducting material. The opening of the band gap is caused by the presence of the repulsion force between the silicon and carbon atoms decreasing the density of states around the Fermi energy. Consequently, the specific heat and the thermal conductivity of the system are suppressed. For graphene nanosheet doped with a dot-like configuration of silicon atoms, at the ortho-, meta-, and para-positions, the valence band crosses the Fermi level. This doping configuration increases the density of state at the Fermi level, but mobile charge are delocalized and diminished around the silicon atoms. As a result, the specific heat and the thermal conductivity are enhanced. Silicon substitutionally doped graphene nanosheets may be beneficial for photovoltaics and can further improve solar cell devices by controlling the geometrical configuration of the underlying atomic systems.This work was financially supported by the University of Sulaimani and the Research Center of Komar University of Science and Technology . The computations were performed on resources provided by the Division of Computational Nanoscience at the University of Sulaimani.Peer Reviewe

Buckling effects in AlN monolayers: Shifting and enhancing optical characteristics from the UV to the near visible light range

The structural, electronic, and optical properties of flat and buckled AlN monolayers are investigated using first-principles approaches. The band gap of a flat AlN monolayer is changed from an indirect one to a direct one, when the planar buckling increases, primarily due to diminishing sp$^2$ overlapping and bond symmetry breaking in the conversion to sp$^3$ bonds. The sp$^3$ hybridization thus results in a stronger $\sigma\text{-}\pi$ bond rather than a $\sigma\text{-}\sigma$ covalent bond. The calculations of the phonon band structure indicates that the buckled AlN monolayers are structurally and dynamically stable. The optical properties, such as the dielectric function, the refractive index, and the optical conductivity of an AlN monolayer are evaluated for both flat systems and those impacted with planar buckling. The flat AlN monolayer has outstanding optical characteristics in the Deep-UV and absorbs more effectively in the UV spectrum due to its large band gap. The results reveal that optical aspects are enhanced along different directions of light polarization, with a considerable shift in the optical spectrum from Deep-UV into the visible range. Additionally, depending on the polarization direction of the incoming light, increased planar buckling enhances the optical conductivity in both the visible and the Deep-UV domains. The ability to modify the optical and electronic properties of these essential 2D materials using planar buckling technique opens up new technological possibilities, particularly for optoelectronic devices.Comment: RevTeX - pdfLaTeX, 9 pages with 7 included pdf figure

Role of planar buckling on the electronic, thermal, and optical properties of Germagraphene nanosheets

We report the electronic, the thermal, and the optical properties of a Germagraphene (GeC) monolayer taking into account buckling effects. The relatively wide direct band gap of a flat GeC nanosheet can be changed by tuning the planar buckling. A GeC monolayer has an sp$^2$ hybridization in which the contribution of an $s$-orbital is half of the contribution of a $p$-orbital leading to stronger $\sigma\text{-}\sigma$ bonds compared to the $\sigma\text{-}\pi$ bonds. Increasing the planar buckling, the contribution of an $s$-orbital is decreased while the contribution of a $p$-orbital is increased resulting in a sp$^3$-hybridization in which the $\sigma\text{-}\pi$ bond becomes stronger than the $\sigma\text{-}\sigma$ bond. As a result, the band gap of a buckled GeC is reduced and thus the thermal and the optical properties are significantly modified. We find that the heat capacity of the buckled GeC is decreased at low values of planar buckling, which is caused by the anticrossing of the optical and the acoustic phonon modes affecting phonon scattering processes. The resulting optical properties, such as the dielectric function, the refractive index, the electron energy loss spectra, the absorption, and the optical conductivity show that a buckled GeC nanosheet has increased optical activities in the visible light region compared to a flat GeC. The optical conductivity is red shifted from the near ultraviolet to the visible light region, when the planar buckling is increased. We can thus confirm that the buckling can be seen as another parameter to improve GeC monolayers for optoelectronic devices.Comment: RevTeX - pdfLaTeX, 10 pages with 12 included pdf figure

Effect of BN dimers on the stability, electronic, and thermal properties of monolayer graphene

Publisher's version (útgefin grein)We theoretically investigate structural stability, electronic and thermal characteristic of boron and nitrogen codoped monolayer graphene using density functional theory and Boltzmann transport equation. Three types of BN dimers, ortho, meta, and para dimers, are identified at different concentration ratios of B and N atoms. Our DFT calculations suggest that the BN ortho dimers are structurally favorable configurations due to the lowest required formation energy. At low doping ratio, large bandgap for BN para dimer is predicted leading to high Seebeck coefficient and figure of merit. In addition, a large deviation in the Wiedemann–Franz ratio is also seen, and a maximum value of the Lorenz number is thus found. In contrast, at high doping ratio, high Seebeck coefficient and figure of merit are found for BN ortho dimer and a low Seebeck coefficient for BN para dimer is noticed. Furthermore, a small deviation in Lorenz number is found for high doping ratio where the distance between BN pair is large.This work was financially supported by the University of Sulaimani and the Research center of Komar University of Science and Technology. The computations were performed on resources provided by the Division of Computational Nanoscience at the University of Sulaimani.Peer Reviewe