By using the first-principle methods, we have investigated the adsorption of the CO, CO2, NO, and NH3 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 CO2 adsorbed molecules act as an acceptor, and the NH3 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 NH3 or CO2 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 O2, NO2, and NH3 molecules [2, 4–8]. Experimental and theoretical studies of gas molecule adsorption on the graphene surface have been reported already [9–16] showing that H2O, NO, CO, NO2, and NH3 molecules are physically adsorbed on the graphene. The NH3, NO, and CO molecules act as donors while H2O and NO2 act as acceptors. The conductance of graphene at the Fermi level decreases with adsorbing NO2 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 CO2 or O2 molecules changes the AGNR to -type semiconductor, while NH3 adsorption changes it to a -type semiconductor [22–24]. In this work we performed the first-principle calculations for the adsorption of CO, CO2, NO, and NH3 molecules on armchair graphene nanoribbon consisting of 6 dimer lines across the ribbon width (6-AGNR), as shown in Figure 1(a). After
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