Pablo A. Denis

Is It Possible to Dope Single‐Walled Carbon Nanotubes and Graphene with Sulfur?

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.

Mechanical properties of graphene nanoribbons

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 Youngs 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.

Chemical Reactivity and Band‐Gap Opening of Graphene Doped with Gallium, Germanium, Arsenic, and Selenium Atoms

Herein, the effects of substitutional doping of graphene with Ga, Ge, As, and Se are shown. Ge exhibits the lowest formation energy, whereas Ga has the largest one. Ga- and As-doped graphene display a reactivity that is larger than that corresponding to a double vacancy. They can decompose H2 and O2 easily. Variation of the type and concentration of dopant makes the adjustment of the interlayer interaction possible. In general, doping of monolayer graphene opens a band gap. At some concentrations, Ga doping induces a half metallic behavior. As is the element that offers the widest range of gap tuning. Heyd-Scuseria-Ernzerhof calculations indicate that it can be varied from 1.3 to 0.3 eV. For bilayer graphene, the doped sheet induces charge redistribution in the perfect underneath sheet, which opens a gap in the range of 0.05-0.4 eV. This value is useful for developing graphene-based electronics, as the carrier mobility of the undoped sheet is not expected to alter.

On the Addition of Aryl Radicals to Graphene: The Importance of Nonbonded Interactions

Dispersion-corrected density functional theory is utilized to study the addition of aryl radicals to perfect and defective graphene. Although the perfect sheet shows a low reactivity against aryl diazonium salts, the agglomeration of these groups and the addition onto defect sites improves the feasibility of the reaction by increasing binding energies per aryl group up to 27 kcalu2009mol(-1). It is found that if a single phenyl radical interacts with graphene, the covalent and noncovalent additions have similar binding energies, but in the particular case of the nitrophenyl group, the adsorption is stronger than the chemisorption. The single vacancy shows the largest reactivity, increasing the binding energy per aryl group by about 80 kcalu2009mol(-1). The zigzag edge ranks second, enhancing the reactivity 5.4 times with respect to the perfect sheet. The less reactive defect site is the Stone-Wales type, but even in this case the addition of an isolated aryl radical is exergonic. The arylation process is favored if the groups are attached nearby and on different sublattices. This is particularly true for the ortho and para positions. However, the enhancement of the binding energies decreases quickly if the distance between the two aryl radicals is increased, thereby making the addition on the perfect sheet difficult. A bandgap of 1-2 eV can be opened on functionalization of the graphene sheets with aryl radicals, but for certain configurations the sheet can maintain its semimetallic character even if there is one aryl radical per eight carbon atoms. At the highest level of functionalization achieved, that is, one aryl group per five carbon atoms, the bandgap is 1.9 eV. Regarding the effect of using aryl groups with different substituents, it is found that they all induce the same bandgap and thus the presence of NO(2), H, or Br is not relevant for the alteration of the electronic properties. Finally, it is observed that the presence of tetrafluoroborate can induce metallic character in graphene.

Hydrogenated double wall carbon nanotubes

Herein, we investigate the chemisorption of hydrogen on double wall carbon nanotubes (DWCNT) employing density functional theory and periodic boundary conditions. In agreement with recent investigations based on Lennard-Jones potentials, we found that the (n,m)@(n+9,m) combination is favored for tubes with small diameters. The C-H binding energies determined for the (16,0) single wall carbon nanotubes (SWCNT) are nearly identical to those computed for the (7,0)@(16,0) and (8,0)@(16,0) DWCNTs. For both of the latter we found that interlayer interaction modifies the band structure of the inner tube. In the case of hydrogenated DWCNTs, the electronic structure of the inner tube experiences very small changes at high coverages (50%). However, at lower hydrogen coverages (3%-25%) changes are observed in the electronic structure of the inner tube. In agreement with recent experimental results we conclude that, for heavily functionalized DWCNTs, the electronic properties of the inner tube remain unchanged. For zigzag SWCNTs, the band gap becomes larger upon increase in hydrogen coverage; at 50% of coverage the hydrogenated (16,0) SWCNT has a band gap of 3.38 eV. Finally, based on the fact that high coverages significantly elongate C-H bond distances, we propose that the hydrogenation coverage may be determined measuring the C-H vibrational modes.

Theoretical characterization of the low-lying electronic states of NbC.

We have studied the potential-energy curves and the spectroscopic constants of the ground and low-lying excited states of NbC by employing the complete active space self-consistent field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified 23 low-lying electronic states of NbC with different spin multiplicities and spatial symmetries within 40,000 cm(-1). At the multireference single and double configuration interaction level of theory the 2sigma+ and 2delta states are nearly degenerated, with the 2delta state located 187 cm(-1) lower than the 2sigma+ state. The estimated spin-orbit splitting for the 2delta state results in a 2delta(3/2) ground state and A 2sigma+ which is placed 650 cm(-1) above the ground state, in reasonable agreement with the experimental result, 831 cm(-1). Our computed spectroscopic constants are in good agreement with experimental values although our results differ from those of a previous density-functional investigation of the excited states of NbC, mainly due to the strong multiconfigurational character of NbC. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally.

Triple Doped Monolayer Graphene with Boron, Nitrogen, Aluminum, Silicon, Phosphorus and Sulfur

The structure, stability, electronic properties and chemical reactivity of X/B/N triple-doped graphene (TDG) systems (X=Al, Si, P, S) are investigated by means of periodic density functional calculations. In the studied TDGs the dopant atoms prefer to be bonded to one another instead of separated. In general, the XNB pattern is preferred, with the exception of sulfur, which favors the SBN motif. The introduction of a third dopant results in a negligible decrease of the cohesive energies with respect to the dual-doped graphene (DDG) counterparts. Thus, it is expect that these systems can be prepared soon. For SiNB TDG, the introduction of the B dopant reduces the gap opening at the K point and restores the Dirac cones that are destroyed in SiN DDG. On the contrary, for PNB TDG, the bandgap is increased with respect to PN DDG, probably because the introduction of B weakens the PN bonding, and thus the electronic structure is rather similar to that of P-doped graphene. Finally, with regard to the reactivity of the TDGs, for AlNB, PNB, and SNB the carbon atoms are more reactive than in their AlN, PN, and SN DDG counterparts. On the contrary, the reactivity of SiNB is lower than that of SiN DDG. Therefore, to increase the reactivity of graphene, Al, P, and S should be combined with BN motifs.

Multireference configuration interaction study of the electronic states of ZrC.

The potential energy curves and spectroscopic constants of the ground and 32 low-lying electronic states of ZrC have been studied by employing multireference configuration interaction methods, in conjunction with relativistic effective core potentials and 5s3p3d1f, 3s3p1d basis sets con Zr and C, respectively. We have determined that the ground state is (3)Sigma(+). However there are two low-lying (1)Sigma(+) states (below 5000 cm(-1)) which strongly interact resulting in avoided crossings. The lowest (1)Sigma(+) state corresponds to a combination of 1sigma(2) Xsigma(2) 1pi(4) configurations whereas the second is an open shell singlet 1sigma(2) 2sigma(1) 3sigma(1) 1pi(4). Several avoided crossings were observed, for (1)Pi, (3)Pi, (1)Delta, (3)Sigma(+), and (3)Delta states. We have identified (3)Pi and (1)Pi lying at 4367 and 5797 cm(-1), respectively. The results are in good agreement with the recent experimental findings of Rixon et al. [J. Mol. Spectrosc. 228, 554 (2004)], and indicate that the (3)Pi-(3)Sigma(+), and (1)Pi-(1)Sigma(+), bands located between 16 000-19 000 cm(-1) are extremely complex due to near degeneracy of several (1)Pi and (3)Pi states. We also have identified a (1)Sigma(+) state in the same region that may interfere with the (1)Pi emission bands. The present results not only shed further light into the spectra of ZrC but also predict yet to be observed systems.

Electronic states and potential energy curves of molybdenum carbide and its ions

The potential energy curves and spectroscopic constants of the ground and 29 low-lying excited states of MoC with different spin and spatial symmetries within 48 000 cm(-1) have been investigated. We have used the complete active space multiconfiguration self-consistent field methodology, followed by multireference configuration interaction (MRCI) methods. Relativistic effects were considered with the aid of relativistic effective core potentials in conjunction with these methods. The results are in agreement with previous studies that determined the ground state as X (3)Sigma(-). At the MRCISD+Q level, the transition energies to the 1 (3)Delta and 4 (1)Delta states are 3430 and 8048 cm(-1), respectively, in fair agreement with the results obtained by DaBell et al. [J. Chem. Phy. 114, 2938 (2001)], namely, 4003 and 7834 cm(-1), respectively. The three band systems located at 18 611, 20 700, and 22 520 cm(-1) observed by Brugh et al. [J. Chem. Phy. 109, 7851 (1998)] were attributed to the excited 11 (3)Sigma(-), 14 (3)Pi, and 15 (1)Pi states respectively. At the MRCISD level, these states are 17 560, 20 836, and 20 952 cm(-1) above the ground state respectively. We have also identified a (3)Pi state lying 14 309 cm(-1) above the ground state. The ground states of the molecular ions are predicted to be (4)Sigma(-) and (2)Delta for MoC(-) and MoC(+), respectively.

Mechanical and Electronic Properties of Graphene Nanostructures

Quite recently a new carbon nanostructure, called graphite nanoribbon (GNR), has emerged, taking the attention of the scientific community because of its promising use in spintronics. It is manly attributed to the work of Son et al. (Son et al. 2006 a; Son et al. 2006 b), who predicted that in-plane electric field, perpendicular to the periodic axis, induces a half-metal state in zigzag nanoribbons (ZGNR). This state corresponds to a one spin flavour with metallic behaviour, while the opposite spin flavour experiences an increase in the energy gap. Apart from the interesting dependence of the electronic structure upon an electric field, this is a promising material for future spintronic devices, since it could work as a perfect spin filter. Very recently Campos-Delgado et al. (Campos-Delgado et al. 2008) reported a chemical vapour deposition route (CVD) for the bulk production of long, thin, and highly crystalline graphene ribbons (less than 20-30 μm in length), with widths from 20 to 300 nm and small thicknesses (2 to 40 layers). In addition, the bottom up synthesis of these nanostructures may be feasible as noted by Hoheisel and collaborators (Hoheisel et al. 2010). This experimental advance further increases the expectations for the use of these materials in high-tech devices. In parallel there is an increased interest in the physical properties of carbon nanostructures in general, due to their outstanding mechanical and electronic properties. Recently, Lee et al. (Lee et al. 2008) measured the mechanical properties of a single graphene layer, demonstrating that graphene is the hardest material known, since the elastic modulus reaches a value of 1.0 TPa. Besides, many efforts have been dedicated to study the electronic properties of graphene, because creating a gap could allow the use of graphene in field effect transistors. Many mechanisms have been proposed with that purpose: nano-pattering, creating quantum dots, using multilayer, doping with heteroatoms such as sulphur (Denis et al. 2009), covalent functionalization (Bekyarova et al. 2009) and applying mechanical stress (Pereira et al. 2009; Gui et al. 2008). Recently Gui (Gui et al. 2008) proposed that graphene under a symmetrical strain distribution always leads to a zero band-gap semiconductor, and the pseudogap decreases linearly with the strain strength in the elastic linear regime. However, asymmetrical strain induces an opening of