Conformational analysis and electronic structure of chiral carbon and carbon nitride nanotubes

© 2011 Materials Research. 2011; 14(4): 461-465 DDOI: 10.1590/S1516-14392011005000067 Conformational Analysis and Electronic Structure of Chiral Carbon and Carbon Nitride Nanotubes Cristiano Geraldo de Faria, Mariza Grassi, Ana Claudia Monteiro Carvalho* Grupo de Desenvolvimento de Estruturas Nanométricas e Materiais Biocompatíveis – GDENB, Departamento de Física e Química, Instituto de Ciências Exatas – ICE, Universidade Federal de Itajubá – UNIFEI, CEP 37.500-930, Itajubá, MG, Brazil Received: September 15, 2010; Revised: August 2, 2011 Geometry and electronic structure of chiral carbon and carbon nitride (CNx) nanotubes were investigated through quantum chemical methods. Finite nanotubes with diameters ranging from 5 to 10 Å and containing up to 500 atoms were considered. CNx structures were built through random substitution of carbon atoms by nitrogen. The molecules were fully optimized by semi-empirical quantum chemical method (PM3). Our results show that the energy associated with nitrogen incorporation depends strongly upon the tube helicity and diameter. The doping of nanotubes with nitrogen contributes to reduce the stress caused by the small diameter of the studied systems. Density of States (DOS) results for pure carbon and CNx nanostructures, obtained through DFT and Hartree-Fock calculations, were analyzed. The introduction of nitrogen in the tube produce states in the gap region which characterizes the metallic behavior, as expected for these systems after N-doping. Keywords: chiral nanotubes, nitrogen, PM3, DFT 1. Introduction Since their discovery by Sumio Iijima1 in 1991, carbon nanotubes (CNT) have been intensively investigated, both theoretically and experimentally, due to their outstanding electronic and mechanical properties. These systems are usually treated as one-dimensional semiconductors or metals, depending on the geometry of the tubes2. The classification of single walled carbon nanotubes (SWNTs) is based on two chiral indices (n,m) which give the geometry of the basic graphene ribbon that is rolled to form a nanotube. According to the usual nomenclature, nanotubes are said to be achiral when one of the indices is zero (zig-zag) or when n = m (armchair) and all the others are chiral. It has been proposed that CNTs behave as 1-D conductors when the difference between the chiral indices is a multiple of 3: n – m = 3q, where q is an integer2. Semiconducting nanotubes are of interest in the fabrication of electronic devices as they combine the outstanding mechanical properties of small band gap semiconductors, altogether in systems of nanoscopic dimensions. Metallic nanotubes are interesting as well since they are prototypes of mechanically robust molecular wires. The development of experimental techniques that precisely synthesize carbon nanotubes with uniform helicity and electronic properties is still a challenge. This fact might impose great limitations on the technological applications of these nanostructures. Theoretical3 and experimental4-6 studies have shown that is possible to modify the electronic properties of the nanotubes by replacing some of carbon atoms with heteroatoms7. Futhermore the incorporation of these heteroatoms also changes the nanotube estructure8,9, chemical reactivity10 and mechanical properties11, presenting the possibility of controlling nanotube properties. In recent years, various synthesis methods to produce CNx nanotubes have been reported, including chemical vapor deposition (CVD)12-14, arc discharging15,16, laser *e-mail: vaporization 17, catalytic pyrolysis 18-21, ion implantation 22, and others. Quantum chemical calculations of the structural stability and electronic properties carbon-nitride systems have been reported by several authors4,23-27. In our previous work15,28, simulations of a random doping of CNTs showed that substitutional nitrogen on the hexagonal carbon network produces localized distortions on the tube walls. Analyzing nitrogen incorporation energy results for molecules of similar diameter, we conclude that carbon atoms are more easily substituted by nitrogen atoms in zigzag than in armchair nanotubes. In the present work we report a quantum chemical study on chiral nanotubes with diameters varying from 5 to 10 Å. We analyze the role played by nitrogen doping in the stabilization of these molecular systems. In the case of chiral nanotubes our theoretical results showed that the energies of nitrogen incorporation are close to the calculated for armchair nanotubes regardless of its helicity. The modifications in the electronic structure due to random nitrogen substitution are also analyzed. A band associated with donor states emerges below the bottom of the conduction band after doping. 2. Computational Details The geometry of tubular structures composed by carbon and nitrogen containing form 100 to 500 atoms were fully optimized through the semi-empirical quantum chemical method Parametric Method 3 (PM3)29. PM3 is a semi-empirical method derived from the Hartree-Fock theory. The advantages of semiempirical calculations are that they are much faster than ab initio calculations, and can be used for large organic molecules. The disadvantage of semiempirical calculations is that some properties cannot be predicted reliably. In the case of the properties analyzed in this study, PM3 semiempirical 462 Faria et al. Materials Research method is very reliable to predict molecular geometries and heats of formation of carbon materials. PM3 error in heats of formation is about 8.0 kcal.mol–1[30], with respect to the experimental values. Average error in bond length is 0.05 Å[30]. Terminal bonds at tube ends were saturated with hydrogen atoms. Chiral nanotubes were then nitrogen-doped and the geometries were re-optimized. Nitrogen atoms were randomly placed substituting carbons at a given concentrations. For these substitutions, we adopted the following criteria: (i) adjacent atoms should not be substituted; (ii) the substitution of even number of atoms is preferable because a closed shell system is formed. The energy associated to nitrogen incorporation was calculated as the difference in formation enthalpy of N-doped and pure carbon systems divided by the number of nitrogens. These calculations were performed within the quantum chemical package GAMESS31. The electronic structure of the optimized tubular molecules was obtained through Hartree-Fock theory adopting CEP-4G basis set and the functional BLYP in the 6-31G basis set as implemented in GAUSSIAN03 package32. Becke’s33 exchange functional along with the correlation functional of Lee, Yang, and Parr34 (BLYP) has been used successful in the electronic properties calculations of carbon nanotubes. However, this functional has been shown to be not allowed to large systems as some model molecules analyzed in this work. The Compact Effective Potencial (CEP) proposed by Stevens and co-workers in 1984[35] is a pseudo-orbital basis set which con (...truncated)