The effect of chalcogens (O, S) on coarsening of nanoislands on metal surfaces

Journal of Vacuum Science &Amp; Technology A: Vacuum, Surfaces, and Films

et al. 2010

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Coarsening (i.e., ripening) of single-atom-high, metal homoepitaxial islands provides a useful window on the mechanism and kinetics of mass transport at metal surfaces. This article focuses on this type of coarsening on the surfaces of coinage metals (Cu, Ag, Au), both clean and with an adsorbed chalcogen (O, S) present. For the clean surfaces, three aspects are summarized: (1) the balance between the two major mechanisms-Ostwald ripening (the most commonly anticipated mechanism) and Smoluchowski ripening-and how that balance depends on island size; (2) the nature of the mass transport agents, which are metal adatoms in almost all known cases; and (3) the dependence of the ripening kinetics on surface crystallography. Ripening rates are in the order (110)>(111)>(100), a feature that can be rationalized in terms of the energetics of key processes. This discussion of behavior on the clean surfaces establishes a background for understanding why coarsening can be accelerated by adsorbates. Evidence that O and S accelerate mass transport on Ag, Cu, and Au surfaces is then reviewed. The most detailed information is available for two specific systems, S/Ag (111) and S/Cu(111). Here, metal-chalcogen clusters are clearly responsible for accelerated coarsening. This conclusion rests partly on deductive reasoning, partly on calculations of key energetic quantities for the clusters (compared with quantities for the clean surfaces), and partly on direct experimental observations. In these two systems, it appears that the adsorbate, S, must first decorate-and, in fact, saturate-the edges of metal islands and steps, and then build up at least slightly in coverage on the terraces before acceleration begins. Acceleration can occur at coverages as low as a few thousandths to a few hundredths of a monolayer. Despite the significant recent advances in our understanding of these systems, many open questions remain. Among them is the identification of the agents of mass transport on crystallographically different surfaces e.g., 111, 110, and 100. KeywordsAmes Laboratory, Materials Science and Engineering, Mathematics, Physics and Astronomy Disciplines Biological and Chemical Physics | Materials Science and Engineering | Mathematics | Physical Chemistry CommentsThe following article appeared in Journal of Vacuum Science and Technology A 28, no. 6 (2010) Coarsening ͑i.e., ripening͒ of single-atom-high, metal homoepitaxial islands provides a useful window on the mechanism and kinetics of mass transport at metal surfaces. This article focuses on this type of coarsening on the surfaces of coinage metals ͑Cu, Ag, Au͒, both clean and with an adsorbed chalcogen ͑O, S͒ present. For the clean surfaces, three aspects are summarized: ͑1͒ the balance between the two major mechanisms-Ostwald ripening ͑the most commonly anticipated mechanism͒ and Smoluchowski ripening-and how that balance depends on island size; ͑2͒ the nature of the mass transport agents, which are metal adatoms in almost all known cases; and ͑3͒ the dependence of the ripe...

Section: Sulfur-enhanced Coarsening Of Ag/ag"111…mentioning

confidence: 99%

Section: A Deductive Reasoning: Process Of Eliminationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Adsorbate-enhanced transport of metals on metal surfaces: Oxygen and sulfur on coinage metals

Thiel

Shen

Journal of Vacuum Science &Amp; Technology A: Vacuum, Surfaces, and Films

et al. 2010

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“…However, the electronegative adsorbate sulfur can accelerate coarsening of homoepitaxial islands on (111) surfaces of these metals by several orders of magnitude. [4][5][6][7][8] It has been proposed that the acceleration of coarsening is due to a change in the nature of the carrier, from single metal atoms to small clusters that contain both metal (M) and sulfur atoms. Two different clusters have been identified as potential carriers on the (111) surfaces: MS 2 and M 3 S 3 .…”

Section: Introductionmentioning

confidence: 99%

Destabilization of Ag nanoislands on Ag(100) by adsorbed sulfur

Shen

Russell

The Journal of Chemical Physics

et al. 2011

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Sulfur accelerates coarsening of Ag nanoislands on Ag(100) at 300 K, and this effect is enhanced with increasing sulfur coverage over a range spanning a few hundredths of a monolayer, to nearly 0.25 monolayers. We propose that acceleration of coarsening in this system is tied to the formation of AgS 2 clusters primarily at step edges. These clusters can transport Ag more efficiently than can Ag adatoms (due to a lower diffusion barrier and comparable formation energy). The mobility of isolated sulfur on Ag(100) is very low so that formation of the complex is kinetically limited at low sulfur coverages, and thus enhancement is minimal. However, higher sulfur coverages force the population of sites adjacent to step edges, so that formation of the cluster is no longer limited by diffusion of sulfur across terraces. Sulfur exerts a much weaker effect on the rate of coarsening on Ag(100) than it does on Ag(111). This is consistent with theory, which shows that the difference between the total energy barrier for coarsening with and without sulfur is also much smaller on Ag(100) than on Ag(111). KeywordsAmes Laboratory, Materials Science and Engineering, adsorption, diffusion barriers, island structure, monolayers, nanostructured materials, silver, sulphur Disciplines Biological and Chemical Physics | Life Sciences | Materials Science and Engineering | Physical Chemistry CommentsReprinted with permission from The Journal of Physical Chemistry C 135, no. 15 (2011) Sulfur accelerates coarsening of Ag nanoislands on Ag(100) at 300 K, and this effect is enhanced with increasing sulfur coverage over a range spanning a few hundredths of a monolayer, to nearly 0.25 monolayers. We propose that acceleration of coarsening in this system is tied to the formation of AgS 2 clusters primarily at step edges. These clusters can transport Ag more efficiently than can Ag adatoms (due to a lower diffusion barrier and comparable formation energy). The mobility of isolated sulfur on Ag(100) is very low so that formation of the complex is kinetically limited at low sulfur coverages, and thus enhancement is minimal. However, higher sulfur coverages force the population of sites adjacent to step edges, so that formation of the cluster is no longer limited by diffusion of sulfur across terraces. Sulfur exerts a much weaker effect on the rate of coarsening on Ag(100) than it does on Ag(111). This is consistent with theory, which shows that the difference between the total energy barrier for coarsening with and without sulfur is also much smaller on Ag(100) than on Ag(111).

“…Recently, for instance, Parkinson et al have shown that CO interacts with Pd atoms adsorbed on a Fe 3 O 4 surface, forming a highly-mobile Pd-CO complex [3]. Other adsorbates that form mobile surface complexes with metals include hydrogen [4,5], oxygen [6,7], alkylsulfides [8], and-the subject of this study-sulfur [9][10][11][12][13][14]. The soft metals Cu, Ag, and Au, which are of great interest because of their catalytic and plasmonic properties, are expected to be particularly susceptible to this effect.…”

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Cu2S3complex on Cu(111) as a candidate for mass transport enhancement

Walen

et al. 2015

Phys. Rev. B

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Sulfur-metal complexes, containing only a few atoms, can open new, highly efficient pathways for transport of metal atoms on surfaces. For example, they can accelerate changes in the shape and size of morphological features, such as nanoparticles, over time. In this study, we perform STM under conditions that are designed to specifically isolate such complexes. We find a new, unexpected S-Cu complex on the Cu(111) surface, which we identify as Cu 2 S 3 . We propose that Cu 2 S 3 enhances mass transport in this system, which contradicts a previous proposal based on Cu 3 S 3 . We analyze bonding within these Cu-S complexes, identifying a new principle for stabilization of sulfur complexes on coinage metal surfaces. Introduction.It has been proposed that metal-adsorbate complexes can greatly accelerate rearrangements of metal nanostructures and surfaces. This issue is of importance for stability of catalysts or nanostructures, and has been the subject of prolonged speculation given that the complexity of such systems typically precludes definitive analysis [1,2]. Nonetheless, evidence continues to accumulate supporting the presence of mobile complexes on surfaces and, by implication, their role in metal transport. Recently, for instance, Parkinson et al. have shown that CO interacts with Pd atoms adsorbed on a Fe 3 O 4 surface, forming a highly-mobile Pd-CO complex [3]. Other adsorbates that form mobile surface complexes with metals include hydrogen [4,5], oxygen [6,7], alkylsulfides [8], and-the subject of this study-sulfur [9][10][11][12][13][14]. The soft metals Cu, Ag, and Au, which are of great interest because of their catalytic and plasmonic properties, are expected to be particularly susceptible to this effect.The challenge in identifying such complexes is their high mobility, plus their potential condensation into extended ordered structures at moderate to high coverage. Together, these considerations mean that conditions of low temperature and low coverage offer the best chance for isolating and observing such species. The present work is a search for S-Cu complexes under these conditions. Previously, Feibelman [9] proposed that a Cu 3 S 3 complex can enhance metal transport on Cu(111), not because of high mobility (relative to metal adatoms), but rather because of high population (reflecting high stability), combined with moderate mobility (cf. Ref. [1]). The stability of the cluster was attributed to the fact that S atoms could adsorb at pseudo-4-fold-