Phase coexistence of clusters and islands: europium on graphene

Abstract

The adsorption and equilibrium surface phases of europium (Eu) on graphene on Ir(111) are investigated in the temperature range from 35 to 400 K and for coverages ranging from a small fraction of a saturated monolayer to the second layer by scanning tunnelling microscopy. Using density functional theory including 4f-shell Coulomb interactions and modelling of electronic interactions, excellent agreement with the experimental results for the equilibrium adsorbate phase, adsorbate diffusion and work function is obtained. Most remarkably, at 300 K in an intermediate coverage range a phase of uniformly distributed Eu clusters (size 10-20 atoms) coexists in two-dimensional equilibrium with large Eu-islands in a ( √ 3 × √ 3)R30 • structure. We argue that the formation of the cluster phase is driven by the interplay of three effects. Firstly, the metallic Eu-Eu binding leads to the local stability of ( √ 3 × √ 3)R30 • structures. Secondly, electrons lower their kinetic energy by leaving the Eu clusters, thereby doping graphene. Thirdly, the Coulomb energy penalty associated with the charge transfer from Eu to graphene is strongly reduced for smaller clusters.

Metals on Graphene: Interactions, Growth Morphology, and Thermal Stability

“…Most of the metal adatoms favor the H-site, while Pb, Cu, Au adatoms prefer the T-site and Cr, Pd, Pt, and Ag adatoms take the B-site. The adsorption energies (E a ) and diffusion barrier (ΔE) for various metal adatom adsorbed on graphene obtained from DFT calculations of Liu et al are shown in Figure 8a,b, respectively, in comparison with other works [29,31,38,40,45,[48][49][50][51]55]. The metals in the horizontal axis are arranged in the order of group I-IV metals, 3d-transition metals, group 10 transition metals, noble metals, as well as rare earth metals.…”

“…Recently, Ir, Pt, W, Re, Co, Eu, Au, AuIr, and FeIr nanoislands have been grown on the graphene moiré structure on Ir(111) substrate as shown by Michely et al [16,[27][28][29][79][80][81]. Noble metal clusters on graphene prepared by chemical methods have also been shown to enhance the electrocatalytic activity [65][66][67][68][69][70].…”

“…We also note that in the literature, growth of various metals on graphene with different substrate (e.g., on Si-terminated SiC, SiO 2 , or on transition metal surfaces) have been reported, but with the growth outcome depending on the type of graphene and the substrate is grown on. For example, for graphene grown on metals, the moiré pattern formed between graphene and the metal substrate on Cu(111) [86], Ni (111) [85,87], Au (111) [88], Ru(0001) [22,25,30], and Ir (111) [27][28][29][79][80][81] have been considered as a good substrate for patterned growth of metal clusters networks. The reason for this is because the graphene substrate interaction is strong so the moiré supercell provides the nucleation sites for the deposited atoms to aggregate regular island distributions.…”

Direct capture and electrostatic repulsion in the self-assembly of rare-earth atom superlattices on graphene

“…Only for higher temperatures, dimers form by lateral attachment of adatoms diffusing toward each other from far distances. This is indicative of a repulsion between the atoms that can only be overcome for T > 50 K. This repulsion is very likely electrostatic, since rare-earth atoms, similar to alkali atoms [24,31], are expected to transfer electrons to graphene [25,31,32], resulting in positively charged adatoms.…”

“…We simplify the potential landscape induced by the moiré by attributing a global minimum in the atop stacking area and the same height to hollow and bridge stacked regions. The repulsion is described by a screened Coulomb potential [34,35] since a pure Coulomb potential turns out to be too long-ranged, and dipolar interactions are too weak at the relevant distances, as estimated from dipole moments calculated for rare-earth atoms [31,32] and alkali metals [31,34] on freestanding graphene. The screening effect is due to the redistribution of the electrons in graphene around the charged adatoms, yielding a lower effective charge for the repulsion and a faster decay.…”