Relativistic peculiarities at stepped surfaces: Energetics and diffusion patterns obtained fromab initiocalculations

Abstract

We revive intriguing, yet still unexplained, experimental results of Ehrlich and coworkers [Phys. Rev. Lett. 77, 1334 and Phys. Rev. Lett. 67, 2509Lett. 67, (1991] who have observed, that 5d adatoms distributed on (111) surface islands of 5d metals favor the adsorption at the cluster's edge rather than at the cluster's interior, which lies in contrast with the behavior of 4d and 3d elements. Our state of the art ab initio calculations demonstrate that such behavior is a direct consequence of the relativity of 5d metals.PACS numbers: 68.43. Bc, 68.43.Jk, 73.20.At The recent surge of attention to nanoscale noble metal systems, serving as a base for nanostructured materials and paving the road to novel electronic devices and nanocatalytic agents, has reignited the community's interest in numerous peculiar properties of 5d elements. Among the abundance of experiments done over the past two decades on 5d metals some still remain not completely understood despite fascinating and potentially fundamental results that have been obtained.In the present paper we revive the results of Ehrlich and coworkers 1 who have observed, that the energetics and diffusion patterns of Pt on Pt(111) and Ir or W on Ir(111) display quite surprising features. In a classical adsorption picture, known to be true for 3d and 4d metals 2,3 , single adatoms diffusing on stepped or islandcovered surfaces at low temperatures tend to avoid areas close to descending step edges. Such repulsion is usually attributed to the consequences of the electron density smearing at step edges, known as the Smoluchowski effect 4 . A sharp step in the atomic potential causes electrons to redistribute (flow down from the step) so as to smoothen out the electron density profile. The resulting electronic depletion in the vicinity of a descending step edge makes adsorption there unfavorable, thus introducing a barrier (a fewÅ wide) for diffusing adatoms. The depletion is strongest directly at the step so that diffusing atoms, once able to overcome the edge repulsion, tend to jump down onto the lower terrace or incorporate into the step 3,5 . Such diffusion behavior can be described by a potential energy profile schematically presented in Fig. 1a.The first surprising feature found in experiments by Ehrlich and coworkers 1 was that while the step repulsion barrier was still observed for the diffusion of Pt on Pt(111) and Ir or W on Ir(111) its width was significantly increased (up to 10Å). Yet the most surprising finding, was the fact that once the temperature was raised high enough for the atoms to overcome the barrier introduced by the electronic depletion at the cluster's edge, the atoms did neither jump onto the lower terrace nor incorporate into the step but tended to migrate to the step and diffuse along its upper edge without ever coming back to the cluster's interior. A potential energy profile which might result in such a diffusion pattern is sketched in Fig. 1b (e.g. see 1 ). The novelty of the observed behavior has provoked a series of further studies on ...

Effect of mesoscopic misfit on growth, morphology, electronic properties and magnetism of nanostructures at metallic surfaces

“…Fig. 10 shows the adsorption energy profile for a single Pt atom on top of a Pt island on Pt(111) surface for relaxed and unrelaxed geometries [91]. It is apparent that while the relaxation plays an important role in defining the diffusion characteristics, they alone cannot account for the observations of Wang and Ehrlich.…”

“…The "mystery" was later clarified by Brovko and coworkers [91] who have shown that the effect observed by Wang and Ehrlich [88] is the consequence of a delicate balance between mesoscopic relaxations in the system, reduced coordination numbers at the edges of clusters and, most importantly, relativistic effects. Fig.…”

Long jumps contribution to the adatom diffusion process near the step edge: The case of Ag/Cu(110)

“…On the other hand, the heteroepitaxial of metals on the metallic substrates is of great interest from the point of view of applications. While, the bimetallic compound (heterosystem) is still the subject of active research . In such systems, we have to take into account the lattice mismatch between the growing layer and substrate (the bond‐length misfit in Cu and Ag is 12%) and the disparity of binding energy between the two metals.…”