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Herein, we investigate sulfur substitutional defects in single-walled carbon nanotubes (SWCNTs) and graphene by using first-principles calculations. The estimated formation energies for the (3,3), (5,5), and (10,0) SWCNTs and graphene lie between 0.9 and 3.8 eV, at sulfur concentrations of 1.7-4 atom %. Thus, from a thermodynamic standpoint, sulfur doping is not difficult. Indeed, these values can be compared with that of 0.7 eV obtained for a nitrogen-doped (5,5) SWCNT. We suggest that it may be possible to introduce sulfur into the SWCNT framework by employing sulfur-containing heterocycles. Our simulations indicate that sulfur doping can modify the electronic structure of the SWCNTs and graphene, depending on the sulfur content. In the case of graphene, sulfur doping can induce different effects: the doped sheet can be a small-band-gap semiconductor, or it can have better metallic properties than the pristine sheet. Thus, S-doped graphene may be a smart choice for constructing nanoelectronic devices, since it is possible to modulate the electronic properties of the sheet by adjusting the amount of sulfur introduced. Different synthetic routes to produce sulfur-doped graphene are discussed.

Employing first-principle calculations, we have investigated the interaction between graphene and the thioepoxide and thiol groups. The SH radical cannot be chemisorbed on perfect graphene, although it is physisorbed. The chemisorption energy can be increased to 0.4 eV if multiple SH groups are bonded to the sheet or if they are attached to Stones-Wales defects. However, when free-energy corrections are considered, the addition of SH groups to perfect graphene is not spontaneous. In the case of the SW defects, the addition is favorable if two SH groups are attached to the shortest CC bond and in opposite sites of the sheet. The single vacancy defect site has the highest affinity for the SH radical, which is dissociatively attached. Finally, employing nanoribbons, we have simulated the reactivity of bare and hydrogen-terminated edges of graphene. The SH group is dissociatively bonded to bare edges. However, hydrogen-terminated zigzag edges prefer to bind the SH group. Considering the different reactivities observed, the defect sites and edges of graphene can be labeled by employing SH radicals. The sites containing sulfur can be used to attach gold nanoparticles or create vertical arrays of graphene sheets on Au surfaces. Finally, for thioepoxidated graphene, we have determined that the binding energy per S atom is 0.49 eV, larger than that determined for the thiol group but very small to be achieved experimentally because the free-energy change is expected to be close to 0 for this process. These results confirm the experimental evidence, which indicated that the sulfur-containing groups present in sulfur-graphite nanocomposites are attached to the edges of graphite, although vacancy defect sites must be considered. The electronic properties of the functionalized and defective graphene sheets are discussed. As a byproduct, we have found that the free-energy term may turn the attachment of a single HO group to graphene to be not spontaneous. Thus, the OH groups observed in graphene oxide are present at defect sites or agglomerated, to have their binding energy increased due to cooperative effects, confirming earlier experimental results.

Herein, we investigate the structural, electronic and mechanical properties of zigzag graphene nanoribbons in the presence of stress by applying density functional theory within the GGA-PBE (generalized gradient approximation-Perdew-Burke-Ernzerhof) approximation. The uniaxial stress is applied along the periodic direction, allowing a unitary deformation in the range of ± 0.02%. The mechanical properties show a linear response within that range while a nonlinear dependence is found for higher strain. The most relevant results indicate that Young's modulus is considerable higher than those determined for graphene and carbon nanotubes. The geometrical reconstruction of the C-C bonds at the edges hardens the nanostructure. The features of the electronic structure are not sensitive to strain in this linear elastic regime, suggesting the potential for using carbon nanostructures in nano-electronic devices in the near future.

We have applied dispersion corrected density functional theory to gauge the reactivity of the most common defects found in graphene. Specifically, we investigated single vacancies, 585 double vacancies, 555–777 reconstructed double vacancies, Stone–Wales defects, and hydrogenated zigzag and armchair edges. We found that the extent to which defects increase reactivity is strongly dependent on the (a) functional group to be attached and (b) number of functional groups attached. For the addition of one H, F, and phenyl groups to defective graphene, we found the following decreasing order of reactivity: single vacancy > hydrogenated zigzag edge > 585 double vacancy > 555–777 reconstructed double vacancy > Stone–Wales > hydrogenated armchair edge > perfect graphene. However, when two phenyl groups are attached, the Stone–Wales defect becomes more reactive than the 585 double vacancy and 555–777 reconstructed double vacancy. The largest increase of reactivity is observed for the functional groups whose binding energy onto perfect graphene is small. In contrast with recent experimental results, we determined that the reactivity of edges in comparison with perfect graphene is much higher than the reported value. When two groups are attached onto a 585, 555–777, or Stones–Wales defect, they prefer to be paired on the same CC bond on opposite sides of the sheet. However, for the single vacancy, this is not the observed behavior as the preferred addition sites are those carbon atoms that were previously bonded to the missing carbon.

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We have investigated the polyoxides HOOH, HOOOH, HOOOOH, and HOOO employing the CCSD(T) methodology, and the correlation consistent basis sets. For all molecules, we have computed fundamental vibrational frequencies, structural parameters, rotational constants, and rotation-vibration corrections. For HOOOH, we have obtained a good agreement between our results and microwave and infrared spectra measurements, although for the symmetric OO stretch some important differences were found. Heats of formation were computed using atomization energies, and our recommendation is as follows: DeltaH(o)(f,298)(HOOOH) = -21.50 kcal/mol and DeltaH(o)(f,298)(HOOOOH) = -10.61 kcal/mol. In the case of HOOO, to estimate the heat of formation, we have constructed three isodesmic reactions to cancel high order correlation effects. The results obtained confirmed that the latter effects are very important for HOOO. The new DeltaH(o)(f,298)(HOOO) obtained is 5.5 kcal/mol. We have also calculated the zero-point energies of DO and DOOO to correct the experimental lower limit determined for the DeltaH(o)(f,298)(HOOO). The Delta(DeltaZPE) decreases the binding energy of HOOO by 0.56 kcal/mol. Employing the latter value, the new experimental lower limit for DeltaH(o)(f,298)(HOOO) is 3.07 kcal/mol, just 2.4 kcal/mol lower than our determination. We expect that the fundamental vibrational frequencies and rotational constants determined for HOOOOH and DOOOOD contribute to its identification in the gas phase. The vibrational spectrum of HOOOOH shows some overlapping with that of HOOOH thus indicating that one may encounter some difficulties in its characterization. We discuss the consequences of the thermochemical properties determined in this work, and suggest that the amount of HOOO present in the atmosphere is smaller than that proposed recently in this journal ( J. Phys. Chem A 2007, 111, 4727).

We present an ab initio density functional theory study of the magnetic moments that arise in graphite by creating single carbon vacancies in a three-dimensional ͑3D͒ graphite network using full potential, all electron, spin polarized electronic structure calculations. In previous reports, the appearance of magnetic moments was explained in a two-dimensional graphene sheet just through the existence of the vacancies itself ͓Carbon-Based Magnetism, edited by F. Palacio and T. Makarova ͑Elsevier, Amsterdam, 2005͒; D. C. Mattis, Phys. Rev. B 71, 144424 ͑2005͒; Y. Kobayashi et al., ibid. 73, 125415 ͑2006͒; R. Yoshikawa Oeiras et al., ibid. ͑to be pub-lished͒; P. O. Lehtinen et al., Phys. Rev. Lett. 93, 187202 ͑2004͔͒.The dependence of the arising magnetic moment on the nature and geometry of the vacancies for different supercells is reported. We found that the highest value of magnetic moment is obtained for a 3 ϫ 3 ϫ 1 supercell and that the highly diluted 5 ϫ 5 ϫ 1 supercell shows no magnetic ordering. The results obtained in this paper are indicative of the importance of interlayer interactions present in a 3D stacking. We conclude that this should not be underestimated when vacancy-based studies on magnetism in graphitic systems are carried out.