ARCHAMBAULT AND ROCHEFORT VOL. 7 ’ NO. 6 ’ 5414–5420 ’ 2013 www.acsnano.org 5414 May 15, 2013 C 2013 American Chemical Society States Modulation in Graphene Nanoribbons through Metal Contacts Chloe ´ Archambault and Alain Rochefort * Engineering Physics Department and Regroupement Québécois sur les Matériaux de Pointe (RQMP),  Ecole Polytechnique de Montréal, Montréal, Québec H3C 3A7, Canada E ver since its discovery, the interest in graphene and its possible applications in electronics has grown. Graphene nanoribbons (GNRs), because of their semi- conducting behavior and their atomic thick- ness, appear as very promising candidates for future miniaturized electronic devices. Beyond its exceptional electrical properties, its perfect two-dimensional structure makes graphene particularly appealing since it could be more easily patternable by current micro- fabrication techniques. On the other hand, most of our electronic devices such as transis- tors do not require a semimetal material as graphene but rather a semiconductor. Fortu- nately, the engineering of graphene into nano- ribbons opens a band gap that depends on the chirality. 1À3 This band gap is predicted to be the largest for armchair GNRs, for which we are expecting large On/Off ratios. Following recent experimental progress, GNRs can now be created with a nearly atomic precision. 4 Hence, transistors made of nanoribbons have already been reported. 5À8 According to Schwierz, 9 one major advantage of using GNRs is related to their low dimensionality, which makes them less subject to short channel effects, such as threshold voltage roll-off and drain-induced barrier lowering, responsible for degraded characteristics. 10 This characteristic sounds quite appealing to sustain a constant miniaturization, but this does not consider an additional but crucial component of any working devices, which are the metallic contacts. The influence of these contacts becomes even more critical when scaling down as the chan- nel length becomes comparable to the length over which contact-induced effects span. If not taken into account, the latter can be- come dominant and lead to data misinterpre- tation. 11 In general, the electronic description of the interaction between GNRs and electro- des is often neglected even though this may play a critical role in the final device perfor- mance. For example, the charge doping by long-range charge transfer 12,13 and by metal- induced gap states (MIGS) 14 has already been observed for different graphene/metal sys- tems. MIGS could dramatically affect the On/Off ratio, especially in short devices. As suggested by quantum calculations, similar effects are also anticipated in GNRs. 15À18 MIGS are exponentially decaying states that arise from the complex band structure of a semiconductor connected to a metal. 19 In other words, they are Bloch states derived from the valence and conduction bands with a complex wave vector such that the density decays as e Àx/L . The decay length L is proportional to 1/q, q being the imagi- nary part of the wave vector, which is only * Address correspondence to alain.rochefort@polymtl.ca. Received for review March 18, 2013 and accepted May 15, 2013. Published online 10.1021/nn401357p ABSTRACT We are reporting the results of density functional calculations of the electronic structure of finite graphene nanoribbons adsorbed on Au, Pd, and Ti electrodes. While the interaction of nanoribbons with the Au contact is more characteristic of a physisorbed state, the adsorption of Pd and Ti involves much stronger state mixing as in chemisorption. Metal-induced gap states, which can potentially short-circuit the device, are clearly revealed for the first time, allowing us to evaluate their penetration length. The evanescence of MIGS is primarily governed by the band gap of the nanoribbon, and we can estimate an acceptable minimal length for an effective transport channel to a few nanometers. Different impacts of the presence of metal- induced gap states on the properties of graphene nanoribbons are discussed in terms of charge transfer and electrostatics. KEYWORDS: graphene nanoribbons . metal contacts . metal-induced gap states . charge transfer . density functional theory . electronic structure ARTICLE