We investigate the structural, electronic, and vibrational properties of graphene nanoflakes (GNFs) with a small number of atoms (<250) and distinct shapes (triangular, rectangular, and hexagonal) through classical molecular dynamics (CMD) and density functional theory (DFT) calculations. We show that these graphene nanostructures are able to retain their planarity for simulated temperatures up to 1500 K, starting to degrade into amorphous nanocarbon for temperatures above 3000 K. The shapes and types of border of the GNFs have a strong influence on their electronic properties and spin. The HOMO−LUMO energy gap of the studied nanoflakes spans the full range of the visible spectrum, suggesting potential applications in the fabrication of optical emission nanodevices, which is confirmed by TDDFT calculations to obtain the UV−vis absorption spectra of triangular armchair GNFs. In particular, the UV−vis maximum absorption energies and intensities scale linearly with the linear size of the GNF. In the special case of zigzag-edged triangular nanoflakes, a nonzero net spin which increases linearly with the edge size was found, pointing toward possible spintronic applications by tuning the spin distribution. The DFT calculations of the infrared spectra allowed the identification of shape- and border-related fingerprints.

Magnetic properties of graphenic carbon nanostructures, relevant for future spintronic applications, depend crucially on doping and on the presence of defects. In this paper we study the magnetism of the recently detected substitutional Ni (Ni sub ) impurities. Ni sub defects are non-magnetic in flat graphene and develop a non-zero magnetic moment only in metallic nanotubes. This surprising behavior stems from the peculiar curvature dependence of the electronic structure of Ni sub . A similar magnetic/non-magnetic transition of Ni sub can be expected by applying anisotropic strain to a flat graphene layer.

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We present the structural, electronic, and optical properties of anhydrous crystals of DNA nucleobases (guanine, adenine, cytosine, and thymine) found after DFT (Density Functional Theory) calculations within the local density approximation, as well as experimental measurements of optical absorption for powders of these crystals. Guanine and cytosine (adenine and thymine) anhydrous crystals are predicted from the DFT simulations to be direct (indirect) band gap semiconductors, with values 2.68 eV and 3.30 eV (2.83 eV and 3.22 eV), respectively, while the experimentally estimated band gaps we have measured are 3.83 eV and 3.84 eV (3.89 eV and 4.07 eV), in the same order. The electronic effective masses we have obtained at band extremes show that, at low temperatures, these crystals behave like wide gap semiconductors for electrons moving along the nucleobases stacking direction, while the hole transport are somewhat limited. Lastly, the calculated electronic dielectric functions of DNA nucleobases crystals in the parallel and perpendicular directions to the stacking planes exhibit a high degree of anisotropy (except cytosine), in agreement with published experimental results.

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The electronic band structure, density of states, dielectric function, optical absorption, and infrared spectrum of cubic BaSnO3 were simulated using density functional theory, within both the local density and generalized gradient approximations, LDA and GGA, respectively. Dielectric optical permittivities and polarizabilities at ω=0 and ω=∞ were also estimated. Indirect band gaps E(R→Γ) of 1.01 eV (LDA) and 0.74 eV (GGA) were found, which are smaller than the experimental one (≈3.1 eV). A comparison of the calculated cubic BaSnO3 band gap with those of others stannates ASnO3 (A = Ca, Sr, Cd) already published highlights their dependence on each crystal profile. The cubic BaSnO3 effective masses of electrons and holes were computed by parabolic fittings along different directions at the conduction band minimum and valence band maximum, being anisotropic for both electrons and holes. The experimental band gap and calculated effective masses confirm the semiconductor character of cubic BaSnO3. Finally, the vibrational normal modes and the infrared spectrum of cubic BaSnO3 were obtained and assigned.