Theoretical Study on the Oxidation Mechanism and Dynamics of the Zigzag Graphene Nanoribbon Edge by Oxygen and Ozone

Graphene nanoribbons (GNRs), as an emerging class of material, hold great potential for the future high speed and low power electronic and spintronic devices. The fabrication of GNRs is of the utmost interest in terms of graphene based device research. Chemical narrowing of GNRs by oxidation is a promising technique in producing nanoribbons of desired widths. In this article, we hope to elucidate the etching mechanism of zigzag GNR (ZGNR) edge by oxidation through theoretical investigations. The oxidation mechanisms and dynamics of the ZGNR edge by O2 and O3 are fully revealed by density functional theory and statistical theory. The relationship between the reaction time and pressure as well as temperature is estimated dynamically. These theoretical results successfully interpret the recent experimental results and can be further used to predict the appropriate oxidation conditions for the precision etching of ZGNRs. I t is well-known that graphene has become the center of attention in the field of material research over the past decade. The major milestones in graphene research led to the development of large area graphene sheets, transparent electrodes, and graphene based field effect transistors (FETs). The high electron mobility of >15 000 cm V−1 s−1 has been reported in ambient conditions. Along with long spin diffusion length and relaxation times, graphene is recognized as one of the most promising candidates for future electronic and spintronic devices. However, graphene is semimetallic without a bandgap. In order to overcome such a hurdle, attempts to open a bandgap have been made. Quantum confinement by narrowing graphene into nanoribbons allows the creation of a bandgap in the orders of the tenth of an eV. Experimental results demonstrated the graphene nanoribbon (GNR) energy bandgap increases with reduced width. It is crucial to finetune the techniques for the fabrication of GNRs to achieve the desired nanoribbon widths for specific bandgap openings. For the two possible edge structures (zigzag and armchair), zigzag graphene nanoribbons (ZGNRs) possess localized nonbonding edge states, in which every vertex carbon atom along the edge acts as a radical. Therefore, the edges of ZGNRs are prone to intense chemical activity. In recent experimental investigation with the electron spin resonance (ESR) measurements, Rao et al. reported that the triplet state O2( Σg) can be adsorbed at the ZGNR edges easily at T = 300 K and PO2 = 1420 Torr. However, ZGNRs do not have the ability to adsorb many other gaseous molecules, such as H2, N2, He, and Ar. It is obvious that there is a particular interaction between ZGNRs and oxygen. Furthermore, Wang and Dai. apply this particular property to narrow ZGNRs without damaging the basal plane of grapheme at high temperature, which can make the ZGNRs width to less than 10 nm. It becomes one of the important techniques to obtain the ZGNRs with desired band gaps. Nevertheless, despite several theoretical papers discussing this oxidation reaction published in recent years, the oxidation reaction mechanism remains largely unclear. As this process occurs exclusively at the edge, we have conducted a systematical investigation aimed at unravelling the oxidation mechanisms of the edge carbon atom with oxygen and ozone and their dynamic features. We attempt to predict the etching rate ZGNRs by oxygen and ozone with high level of accuracy. It is hopeful that this study would enable much higher control for experimentalists who design fabrication processes to tailor the dimensions of ZGNRs. ■ COMPUTATIONAL METHODS The geometric parameters of the stationary points on the oxidation reaction potential-energy surfaces of the ZGNR edge with O2/O3 were optimized by Gaussian 09 program 26 utilizing B3LYP functional with 6-311G(d,p) and 3-21G(d) basis sets. For these solid−gas reaction systems, the critical parts concerning direct reaction atoms (High level) was calculated by the 6-311G(d,p) basis set and the other atoms (Low level) by the 3-21G(d) basis sets, which was performed by ONIOM method implemented in Gaussian. The vibrational frequencies were calculated at all stationary points to determine the energy minimum structures and transition states. There is only Received: January 19, 2014 Revised: April 23, 2014 Published: April 23, 2014 Article