ISRN Condensed Matter Physics

Volume 2012, Article ID 368634, 5 pages

http://dx.doi.org/10.5402/2012/368634

## Electronic Structure Calculation of Adsorbate Gas Molecules on an Armchair Graphene Nanoribbon

^{1}Department of Physics, Islamic Azad University, Karaj Branch, Karaj 31485-313, Iran^{2}Department of Physics, Islamic Azad University, Shahryar Shahr-e-Qods Branch, Tehran, Iran^{3}Department of Physics, Tarbiat Modares University, Tehran 14115-175, Iran

Received 30 August 2012; Accepted 5 October 2012

Academic Editors: V. Cataudella, Y. Kopelevich, J. J. Quinn, and A. D. Zaikin

Copyright © 2012 Khadije Imani et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

By using the first-principle methods, we have investigated the adsorption of the CO, CO_{2}, NO, and NH_{3} molecules on an armchair graphene nanoribbon (AGNR). The optimal adsorption
positions and orientations of these molecules on AGNR are determined. The adsorption energies, the charge transfer, and the density of states (DOS) are obtained. The NO, CO, and CO_{2} adsorbed molecules act as an acceptor, and the NH_{3} adsorbed molecule acts as a donor. The NO and CO molecules contributed with localized states in the center of the original band gap. But the system exhibits -type or -type semiconductor after NH_{3} or CO_{2} adsorption.

#### 1. Introduction

Recently the study of the sensing property of carbon nanostructures, such as carbon nanotubes (CNTs), graphene, and graphene nanoribbons (GNRs), has attached a lot of research activity [1–3]. Studies show that the graphene and CNTs and GNRs are good gas sensors. Experimental data have shown that the transport properties of nanotube changes upon exposure to gas molecules such as the O_{2}, NO_{2}, and NH_{3} molecules [2, 4–8]. Experimental and theoretical studies of gas molecule adsorption on the graphene surface have been reported already [9–16] showing that H_{2}O, NO, CO, NO_{2}, and NH_{3} molecules are physically adsorbed on the graphene. The NH_{3}, NO, and CO molecules act as donors while H_{2}O and NO_{2} act as acceptors. The conductance of graphene at the Fermi level decreases with adsorbing NO_{2} molecules while it increases with adsorbing NO molecules [17]. Due to having flat structure and thus larger accessible surface area, GNRs and graphene may act better than any other carbon-based materials as gas sensors. GNRs’ electronic states largely depend on the edge structure (armchair or zigzag). Calculations based on tight bonding approximation predict that zigzag graphene nanoribbons (ZGNRs) are always metallic while armchair graphene nanoribbons (AGNRs) can be either metallic or semiconducting, depending on their width [18]. However DFT calculations show that AGNRs are semiconducting with a finite band gap and the value of the gap depends on their width [19–21]. Theoretical and ab initio studies of GNRs sensing properties have been reported already, showing that absorption of CO_{2} or O_{2} molecules changes the AGNR to -type semiconductor, while NH_{3} adsorption changes it to a -type semiconductor [22–24].

In this work we performed the first-principle calculations for the adsorption of CO, CO_{2}, NO, and NH_{3} molecules on armchair graphene nanoribbon consisting of 6 dimer lines across the ribbon width (6-AGNR), as shown in Figure 1(a). After geometrical optimization, we obtained the positions and orientations of the adsorbed molecules. We then calculate the adsorption energies , the density of states (DOS), and the charge transfers between nanoribbon and adsorbate molecules due to bindings between the molecules and carbon in the nanoribbon. By probing the density of states (DOS) of the systems, one can find out the direction of charge transfer (from the molecules to the nanoribbon or vice versa), and from this, one can determine whether the doped molecule acts as a donor or as an acceptor.

In the previous work [22], the exact orientation and position of the adsorbate molecules on the 6-AGNR surface was not calculated. However, we have used the atomic coordinates obtained from simultaneous relaxation in the positions of the adsorbate molecules, 6-AGNR sheet, and their atomic positions.

The organization of this paper is as follows. In Section 2 the computational details are discussed. In Section 3 we investigate the electronic structure of the 6-AGNR with adsorbate molecules, using density functional theory with the local density approximation.

#### 2. Computational Details

Our calculations are based on the use of DFT and the ab initio pseudo-potential plane-wave method using the PWSCF code of the Quantum ESPRESSO [25]. We performed the calculation with LDA and LSDA ultrasoft Vanderbilt pseudopotentials [26]. The Kohn-Sham orbitals are expanded in a plane-wave basis set. The energy cutoff for the wave function and the charge are, respectively, 45 Ry and 300 Ry. The 6-AGNR was simulated by a relatively orthorhombic supercell of 36 carbon atoms, with lattice parameters Å, Å in the planes and Å. The sampling of the Brillouin zone was done using a grid and tested to give convergent results for the total energy. For calculating the density of states (DOS), we used a grid.

The periodic boundary conditions and a supercell approximation were used. The supercell had 36 carbon atoms for a 6-AGNR in a hexagonal lattice. The structural optimizations were performed using a conjugated gradient procedure. Different configurations of gas molecules’ adsorption to the 6-AGNR were considered. For each molecule we chosen the position and the orientation in which the hole system has minimum energy. For this position and orientation, the structure are full relaxed until all the force components were smaller than , in order to obtain the exact position and orientation for the molecule.

The amount of charge transfer from 6-AGNR to the molecules can be estimated by projecting the charge density onto the atomic orbitals. We calculated the charge transfer as the difference between the Löwdin charges for pristine 6-AGNR and 6-AGNR with the adsorbate molecules. From this result, one can determine whether the adsorbate molecule acts as an acceptor or as a donor. It is important to note that the size of the charge transfer slightly depends on the method chosen for calculations. The adsorption energy () is defined as the difference between the energy of the fully relaxed 6-AGNR and the adsorbed molecule on it and the sum energy of the pristine 6-AGNR and the energy of the isolate molecule :

#### 3. Molecular Adsorption on 6-AGNR

In this section, after full relaxation, the optimized geometry was obtained, then the charge transfer and the adsorption energy were determined.

As Figure 2 shows, the pristine 6-AGNR is a semiconductor with 1.1 eV band gap. Gas adsorption changes the DOS of 6-AGNR, so it could change 6-AGNR properties.

The CO molecules are adsorbed to the 6-AGNR in the 6-AGNR plane, via its carbon atom as shown in Figure 1(b). The bond length obtained for C–C is 1.35 Å and the angle C–C–O is while the bond length of the adsorbed CO is 1.17 Å. The calculated adsorption energy of CO molecules on the 6-AGNR is −2.34 eV. As Figure 2 shows, the lowest unoccupied molecular orbital (LUMO), which appeared as a peak, is located at the center of original band gap slightly above the Fermi level of the pristine 6-AGNR so a charge transfer from the 6-AGNR, to the CO molecule is expected. There is a total charge transfer of 0.02 e from the 6-AGNR to CO molecules; therefore it can be deduced that the CO molecule acts as an acceptor.

The results of CO_{2} molecule adsorbed on the ribbon show that in the CO_{2} molecule, the C atom is closer to the ribbon surface than the O atoms (Figure 1(c)) and the C–C bond length is 1.50 Å. The bond length and the angle of the adsorbed CO_{2} molecule are 1.25 Å and , respectively. The angle of the C–O bond with the ribbon plan . The calculated adsorption energy of the CO_{2} molecule on the 6-AGNR is −5.23 eV. As can be seen in Figure 3, the LUMO is above Fermi level; it is the orbital that can have an overlap with the 6-AGNR orbitals and thus can cause charge transfer from the ribbon to the molecule. There is a total charge transfer of 0.09 e from the 6-AGNR to the CO_{2} molecule. Therefore it can be deduced that the CO_{2} molecule acts as an acceptor. After adsorbing the CO_{2} molecule, the Fermi level shifts toward the valance band (Figure 3), so the system exhibits -type semiconducting behavior.

The results of calculations show that the NO molecule sits in the ribbon plane. NO molecules attached to the ribbon via the N atom, with their N–C bond length 1.43 Å and the O–N–C angle (Figure 1(d)). The calculated adsorption energy of the NO molecule on the 6-AGNR is −1.15 eV. As Figure 4 shows, the highest occupied molecular orbital (HOMO), which has appeared as a peak near the Fermi level of pristine 6-AGNR, is half filled and is associated with both and orbitals. Due to these two facts, it can also be ascribed as the lowest unoccupied molecular orbital (LUMO). It causes charge transfer in both directions due to the mixing of the HOMO or the LUMO of NO with the 6-AGNR orbitals below or above the Fermi level. There is a total charge transfer of 0.15 e from the 6-AGNR to the NO molecule. Therefore it can be deduced that the NO molecule acts as an acceptor.

NH_{3} molecules attach to the ribbon via the N atom (Figure 1(e)). The N atom sits 0.9 Å away from the edge of the ribbon, the length of N–C bond is 1.48 Å, and the H–N–H angle is . The angle that the N–C bond makes with the ribbon plane is . The calculated adsorption energy of the NH_{3} molecule on the 6-AGNR is −3.103 eV. There is a total charge transfer of 0.16 e from the NH_{3} molecule to the 6-AGNR, so the NH_{3} molecule acts as a donor. As Figure 5 shows the band gap decreases after adsorbing the NH_{3} molecule and the Fermi level shifts toward the conduction band, so the system exhibits -type semiconducting behavior.

#### 4. Conclusions

In this work the adsorption of CO, NO, and NH_{3} molecules on 6-AGNR was investigated. We optimized the position of the adsorbate molecules on 6-AGNR. After full relaxation, the adsorption energy and the charge transfer properties between the molecules and the 6-AGNR are determined. We showed that the NO, CO_{2}, and CO molecules act as an acceptor and that the NH_{3} molecule acts as a donor which has also been verified in experiment [9]. The NO and CO adsorbed molecules contributed with localized states in the center of original band gap. AGNR semiconductor became -type semiconductor after NH_{3} molecule adsorption. The adsorption of NH_{3} molecule converts the electronic structure of 6-AGNR as a degenerate semiconductor. These results are consistent with those obtained by Huang et al. [22] and Saffarzadeh [24].

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