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applscilettersa>vol-01>issue>04>Stacking-Mediated Diffusion of Ruthenium Nanoclusters in Graphite

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Research

Stacking-Mediated Diffusion of Ruthenium Nanoclusters in Graphite

James G. McHugh
Pavlos Mouratidisy
Abstract


The diffusion, penetration, and intercalation of metallic atomic dopants is an important question for various graphite applications in engineering and nanotechnology. We have performed systematic first-principles calculations of the behaviour of ruthenium nanoclusters on a graphene monolayer and intercalated them into a bilayer. Our computational results show that at a scientifically high density of single Ru atom interstitials, intercalated atoms can shear the surrounding lattice to an AA stacking confi duration, an effect which weakens with increasing cluster size. Moreover, the interlayer stacking con figuration, in turn, has a sign cant affect cluster diffusion. We therefore nd different trends in diffusivity as a function of cluster size and interlayer stacking. For monolayer graphene and an AA graphene bilayer, the formation of small clusters generally lowers diffusion barriers, while the opposite behaviour is found for the preferred AB stacking con duration. These results demonstrate that conditions of local impurity concentration and interlayer disregistry are able to regulate the diffusivity of metallic impurities in graphite.

Introduction

The energetic and dynamical properties of transition metal impurities adsorbed on top of graphene and intercalated between the layers of graphite is a recurring topic of considerable interest in materials science. Foreign elemental impurities are one of the most promising ways to modify the physical properties of pristine graphene [1], and they hold considerable promise in engineering desirable electronic phases [2]. For example, transition-metal doping of monolayer graphene (MLG) and bilayer graphene (BLG) can be exploited to increase the weak intrinsic spin-orbit effects of the native carbon atoms [3{5], allowing the engineering of novel quantum states with advantageous transport properties and prospective applications as topological insulators or in quantum computing [6{8]. Intercalated metallic species are also of interest in their own right, and the layered structure of van der Waals materials, such as graphite, provides an excellent platform to grow quasi-two-dimensional sheets of selected transition metals [9{12]. These two-dimensional transition metal sheets have many desirable properties, which are greatly enhanced by their lower-dimensional topology. Due to the higher (2D) bulk to surface ratio and the associated change in coordination, layered metals can completely change electronic properties such as band gaps and transport properties [13, 14]. Two-dimensional metallic layers have been grown underneath the top monolayer of highly-oriented pyrolytic graphite (HOPG) [11, 15], a process which is known to proceed via the diffusion of impurities through lattice defect "entry portals" (monovacancy and multi-vacancy complexes) [12, 16], and vacancy sites are also known to promote the intercalation of other elemental species such as Cs and Dy [17, 18]. Transition metals are also important from the perspective of graphite applications. Some of the most important nuclear fusion products are transition metals [19], and the penetration of these impurities into the bulk and subsequent diffusion through the graphite lattice is a pressing problem in the design of new reactors and in the assessment of the safety and decommissioning of retired reactors [20{22].

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