Electric-Field- and Hydrogen-Passivation-Induced Band Modulations in Armchair ZnO Nanoribbons Liangzhi Kou, † Chun Li, ‡ Zhuhua Zhang, † and Wanlin Guo* ,† Institute of Nano Science, Nanjing UniVersity of Aeronautics and Astronautics, Nanjing 210016, China, and School of Mechanics, CiVil Engineering and Architecture, Northwestern Polytechnical UniVersity, Xi’an 710072, People’s Republic of China ReceiVed: October 7, 2009; ReVised Manuscript ReceiVed: NoVember 13, 2009 We report on the electric-field- and H chemical-absorption-induced band manipulations of armchair ZnO nanoribbons using first-principles calculations. It is shown that the band gap of a semiconducting armchair nanoribbon can be reduced monotonically with increasing transverse field strength, demonstrating a giant Stark effect. The critical field strength to completely close the band gap decreases with increasing ribbon width, while it is almost independent of the stacking thickness. On the other hand, the nanoribbon with the edges fully passivated shows an enhanced gap but a slightly weaker Stark effect. We also observe hydrogen- termination-induced metallization of the ribbons when only the edge O atoms are passivated, which results from an n-type doping effect. These findings suggest potential ways of band engineering in armchair ZnO nanoribbons. Introduction Since the discovery of nanoribbons of semiconducting oxides in 2001, 1 it has sparked an intense research effort toward the understanding of these novel materials with promising applica- tions in nanoelectronic and spintronic devices. 2,3 The ZnO nanoribbon (ZNNR) is found to be one of the most typical and successful examples of these oxide nanoribbons. Owing to the excellent optical, piezoelectric, and biocompatible properties inherited from its bulk material, 4,5 ZNNRs have been success- fully applied in field effect transistors, 6 ultrasensitive nanosize gas sensors, 7 nanoresonators, 8 nanocantilevers, 9 and so on. As functional building blocks in various nanoscale devices, especially in field effect transistors, a tunable band gap in nanostructures would be highly desirable because it would entail great flexibility in the design and optimization of nanodevices, in particular, if it could be tuned by applying a well-controlled external electric field. In the past decades, an applied external electric field has been extensively proved to efficiently modulate the electronic properties and even enable insulator-metal transitions in numerous low-dimensional nanostructures, such as boron nitride nanoribbons, 10 carbon nanotubes, and boron nitride nanotubes, 11 etc. 12 However, the effects of electric fields on the electronic properties of semiconducting armchair ZnO nanoribbons (A-ZNNRs) remain to be investigated. On the other hand, chemical absorptions at the edges of nanostructures, especially hydrogen absorptions, have been found to be able to affect the electronic structures of nanoscale materials. 13-15 In the fabrication process of ZNNRs, hydrogen is frequently present during the growth using different techniques. 16 Even in high- vacuum systems, H 2 O always exists as a residual gas, which may serve as a source of hydrogen. In addition, the response of ZNNRs to gas atmosphere, especially vapor, is also an important issue that needs to be addressed for applications in functional devices. The knowledge obtained may be used not only to assess precisely the operating performance and life of ZNNR-based devices but also to derive more meaningful applications in gas sensing. 7 Moreover, exposure to hydrogen can strongly affect the electronic properties of other ZnO nanostructures, as revealed in recent investigations. 17-20 Recently, using ab initio calcula- tions, Jia et al. demonstrated that the hydrogen adsorption on ZnO nanowires will convert a semiconducting nanowire 21,22 into metal. 23 In addition, some current calculations have shown that hydrogen absorption to the Zn-terminated edges will result in ferromagnetic half-metal zigzag ZnO nanoribbons (Z-ZNNRs), 24 whereas passivating the edge with sulfur will significant affect the electronic and magnetic properties. 25,26 Therefore, the interaction between hydrogen and A-ZNNRs is fundamentally interesting as well, and a deep understanding of the band modulation in the potential A-ZNNRs by hydrogen absorptions at the edges is thereby necessary for the development of nanoscale optical and electrical devices. In this paper, we present from density functional theory (DFT) calculations strong band modulations by external electric fields and atomic hydrogen chemical absorption in A-ZNNRs. It is shown that the band gap of an A-ZNNR can be decreased by a transversely applied electric field and eventually closed under field strengths beyond a critical value. This electric field effect is more remarkable in wider A-ZNNRs and is robust to the stacking of ribbon layers and full hydrogen termination. Most interesting, partially passivating all the edge O atoms of A-ZNNRs could induce metallization of the systems. Models and Technique Details The A-ZNNRs in our studies are originally constructed by cutting a monolayer along armchair lines, as shown in Figure 1. For the structure with a few number of ZnO layers, it has been recently demonstrated that each layer prefers a planar configuration in which both cations and anions are coplanar after relaxation. 27 The multilayer ribbons are constructed by placing the planar layers on top of each other with AB stacking. Following conventional custom, A-ZNNRs are classified by the * To whom correspondence should be addressed. E-mail: wlguo@ nuaa.edu.cn. † Nanjing University of Aeronautics and Astronautics. ‡ Northwestern Polytechnical University. J. Phys. Chem. C 2010, 114, 1326–1330 1326 10.1021/jp909584j  2010 American Chemical Society Published on Web 12/10/2009