PHYSICAL REVIEW B 83, 165417 (2011) Effect of edge reconstruction and passivation on zero-energy states and magnetism in triangular graphene quantum dots with zigzag edges O. Voznyy, 1 A. D. G ¨ uc ¸l¨ u, 1 P. Potasz, 1,2 and P. Hawrylak 1 1 Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Canada 2 Institute of Physics, Wroclaw University of Technology, Wroclaw, Poland (Received 13 October 2010; revised manuscript received 2 March 2011; published 14 April 2011) We present the results of ab initio calculations of the effect of reconstruction and passivation of zigzag edges on the electronic and magnetic properties of triangular graphene quantum dots. We find that, similarly to nanoribbons, hydrogen-passivated ideal zigzag edges are energetically favored over the pentagon-heptagon zigzag. However, the reconstructed edge is more stable in the absence of hydrogen, thus, delayed passivation with H may lock the dot in such an unfavorable configuration. Both hydrogen-passivated edge morphologies lead to a band of states at the Fermi level. Unlike in nanoribbons, this quasidegenerate band results in net spin polarization for structures with zigzag edge of all sizes studied here. For triangular dots with pentagon-heptagon zigzag edge, a larger width of the zero-energy band is predicted, leading to the loss of net magnetization. DOI: 10.1103/PhysRevB.83.165417 PACS number(s): 73.22.Pr, 68.65.Pq, 75.75.−c Graphene, an atomically thick honeycomb lattice of carbon atoms, exhibits fascinating properties due to the relativisti- clike nature of quasiparticle dispersion close to the Fermi level. 1–5 Graphene’s potential for nanoelectronics applica- tions, particularly magnetization of edges, and its use for spintronics motivated considerable amount of research in graphene nanoribbons 6–10 and, more recently, graphene quan- tum dots. 11–25 Large optical absorptivity, tunable electronic levels, high charge mobility, and nontoxicity make graphene nanostructures attractive also for photovoltaic applications. 26 In low-dimensional graphene structures, the overall shape and the character of the edges drastically affect the electronic properties near the Fermi level. 25,27–29 In particular, theoretical models predict that, in triangular graphene quantum dots (TGQDs) with exclusively zigzag edges, the energy spectrum near the Fermi level collapses to a band of degenerate states, isolated from the rest of the spectrum by a well-defined gap, with states predominantly localized on the edges. 16–21,23–25 It was shown that, unlike in graphene nanoribbons that have no net magnetic moment, in this band of degenerate states, strong electron-electron interactions lead to ferromagnetism and peculiar magnetic 19,21,24,30 and optical 22,25 properties, e.g., magnetic moment proportional to the dot size and controlled by an external gate. TGQDs might also offer additional advantages for third-generation solar cells utilizing the presence of the intermediate band in the gap 31 and multi- exciton generation (MEG). 32 MEG was already demonstrated in carbon nanotubes, 33,34 while a recent theoretical study 35 suggests that it is also possible in graphene QDs, and that localization of states on the edges increases MEG efficiency. Despite the fact that TGQDs with zigzag edges have not been demonstrated yet, recent experimental works sug- gest that they are conceivable in the future. Typically, the solution-derived organic chemistry methods produce graphene nanostructures with H-passivated edges with a predominantly armchair structure. 10,36,37 Other techniques, such as etching lithography or mechanical exfoliation, result in nanostruc- tures with a mixture of zigzag and armchair edges, and their reconstructed counterparts. The edges’ morphology is found to be highly dynamic under nonequilibrium preparation conditions, and interconversion between different types of edge reconstructions is often observed. 29,38–42 Techniques for preparation of graphene nanostructures with controlled- edge morphology are constantly emerging, e.g., nanotube unzipping 43 and Joule heating, 40,44 and some of them, such as anisotropic etching using Ni (Ref. 46) or Co (Ref. 47) nanoparticles and carbothermal decomposition of SiO 2 , 45 can already produce exclusively zigzag edges and triangular shapes. Other alternatives based on patterned hydrogenation rather than etching were also proposed. 48 Theoretical predictions for infinite edges suggest that, with hydrogen passivation, armchair configuration is the most favorable one, with only a slight energy advantage over zigzag (ZZ). Without H, a reconstructed edge terminated by pentagon-heptagon pairs (ZZ 57 ) is predicted to be the lowest in energy, followed by armchair. 8,39,49,50 For finite-size structures, however, confinement effects and the presence of corners would affect the stability of the overall structure and the preferred edge morphology. For example, ZZ edges are suggested to be dominating for small carbon clusters, both with and without H passivation, while injection of pentagon and heptagon defects may lower the total energy for some structures. 51 Determining the degree of edge passivation (one, two, or no H atoms attached to edge carbon 8 ) in experimental structures is still complicated. 42,45 Under the damaging high flux of electrons in transmission electron microscopy (TEM) there is, likely, no H. A consequent hydrogenation of such unpassivated edges is possible. 8,27 Interconversion between reconstructions requires overcoming high-energy barriers, even for nonpassivated carbon edges. 44 Thus, locking in a nonoptimal configuration remains quite possible. So far, only TGQDs with ideal H-passivated ZZ and armchair edges were studied. Effects of pentagons and heptagons injection, leading to mixing of the sublattices, has not been addressed yet. In this paper, using ab initio methods, we investigate the robustness of TGQD properties of interest (zero-energy states and magnetism) versus edge reconstructions and passivation as a function of size. We use the most feasible with current man- ufacturing technique reconstructions (ZZ and ZZ 57 ), 38,39,41 165417-1 1098-0121/2011/83(16)/165417(5) ©2011 American Physical Society