| 1. |
- Adamovic, Dragan, 1973-
(author)
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Molecular Dynamics Studies of Low-Energy Atom Impact Phenomena on Metal Surfaces during Crystal Growth
- 2006
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Doctoral thesis (other academic/artistic)abstract
- It is a well-known fact in the materials science community that the use of low-energy atom impacts during thin film deposition is an effective tool for altering the growth behavior and for increasing the crystallinity of the films. However, the manner in which the incident atoms affect the growth kinetics and surface morphology is quite complicated and still not fully understood. This provides a strong incentive for further investigations of the interaction among incident atoms and surface atoms on the atomic scale. These impact-induced energetic events are non-equilibrium, transient processes which complete in picoseconds. The only accessible technique today which permits direct observation of these events is molecular dynamics (MD) simulations.This thesis deals with MD simulations of low-energy atom impact phenomena on metal surfaces during crystal growth. Platinum is chosen as a model system given that it has seen extended use as a model surface over the past few decades, both in experiments and simulations. In MD, the classical equations of motion are solved numerically for a set of interacting atoms. The atomic interactions are calculated using the embedded atom method (EAM). The EAM is a semi-empirical, pair-functional interatomic potential based on density functional theory. This potential provides a physical picture that includes many-atom effects while retaining computational efficiency needed for larger systems.Single adatoms residing on a surface constitute the smallest possible clusters and are the fundamental components controlling nucleation kinetics. Small two-dimensional clusters on a surface are the result of nucleation and are present during the early stages of growth. These surface structures are chosen as targets in the simulations (papers I and II) to provide further knowledge of the atomistic processes which occur during deposition, to investigate at which impact energies the different kinetic pathways open up, and how they may affect growth behavior. Some of the events observed are adatom scattering, dimer formation, cluster disruption, formation of three-dimensional clusters, and residual vacancy formation. Given the knowledge obtained, papers III and IV deal with growth of several layers with the aim to study the underlying mechanisms responsible for altering growth behavior and how the overall intra- and interlayer atomic migration can be controlled by low-energy atom impacts.
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| 2. |
- Gerber, Timm, et al.
(author)
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CO-Induced Smoluchowski Ripening of Pt Cluster Arrays on the Graphene/Ir(111) Moire
- 2013
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In: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 7:3, s. 2020-2031
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Journal article (peer-reviewed)abstract
- Regular Pt cluster arrays grown on the moire template formed by graphene on Ir(111) were tested for their stability with respect to CO gas exposure. Cluster stability and adsorption-Induced processes were analyzed as a function of cluster size, with In situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. Small clusters containing fewer than 10 atoms were unstable upon CO adsorption. They sintered through Smoluchowski ripening-cluster diffusion and coalescence rather than the frequently reported Ostwald ripening mediated by metal-adsorbate complexes. Larger dusters remained immobile upon CO adsorption but became more three-dimensional. Careful analysis of the experimental data complemented by ab initio density functional theory calculations provides insight Into the origin of the CO-induced Pt cluster ripening and shape transformations.
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| 3. |
- Grånäs, Elin, et al.
(author)
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CO Intercalation of Graphene on Ir(111) in the Millibar Regime
- 2013
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:32, s. 16438-16447
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Journal article (peer-reviewed)abstract
- Here we show that it is possible to intercalate CO under graphene grown on Ir(111) already at room temperature when CO pressures in the millibar regime are used. From the interplay of X-ray photoelectron spectroscopy and scanning tunneling microscopy we conclude that the intercalated CO adsorption structure is similar to the (3 root 3 X 3 root 3)R30 degrees) adsorption structure that is formed on Ir(111) upon exposure to similar to 1 mbar of CO. Further, density functional theory calculations reveal that the structural and electronic properties of CO-intercalated graphene are similar to p-doped freestanding graphene. Finally we characterize nonintercalated stripes and islands that we always observe in the CO-intercalated graphene. We observe these nonintercalated areas predominately in HCP and FCC areas near step edges and suggest that stress release in graphene is the driving force for their formation, while the weak chemical bonds in HCP and FCC areas are the reason for their area selectivity.
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| 4. |
- Grånäs, Elin, et al.
(author)
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Hydrogen intercalation under graphene on Ir(111)
- 2016
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In: Surface Science. - : Elsevier BV. - 0039-6028. ; 651, s. 57-61
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Journal article (peer-reviewed)abstract
- Using high resolution X-ray photoelectron spectroscopy and scanning tunneling microscopy we study the intercalation of hydrogen under graphene/Ir(111). The hydrogen intercalated graphene is characterized by a component in C 1s that is shifted −0.10 to −0.18 eV with respect to pristine graphene and a component in Ir 4f at 60.54 eV. The position of this Ir 4f component is identical to that of the Ir(111) surface layer with hydrogen atoms adsorbed, indicating that the atomic hydrogen adsorption site on bare Ir(111) and beneath graphene is the same. Based on co-existence of fully- and non-intercalated graphene, and the inability to intercalate a closed graphene film covering the entire Ir(111) surface, we conclude that hydrogen dissociatively adsorbs at bare Ir(111) patches, and subsequently diffuses rapidly under graphene. A likely entry point for the intercalating hydrogen atoms is identified to be where graphene crosses an underlying Ir(111) step.
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| 5. |
- Grånäs, Elin, et al.
(author)
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Oxygen Intercalation under Graphene on Ir(111): Energetics, Kinetics, and the Role of Graphene Edges.
- 2012
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In: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851.
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Journal article (peer-reviewed)abstract
- Using X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM) we resolve the temperature-, time-, and flake size-dependent intercalation phases of oxygen underneath graphene on Ir(111) formed upon exposure to molecular oxygen. Through the applied pressure of molecular oxygen the atomic oxygen created on the bare Ir terraces is driven underneath graphene flakes. The importance of substrate steps and of the unbinding of graphene flake edges from the substrate for the intercalation is identified. With the use of CO titration to selectively remove oxygen from the bare Ir terraces the energetics of intercalation is uncovered. Cluster decoration techniques are used as an efficient tool to visualize intercalation processes in real space.
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| 6. |
- Grånäs, Elin, et al.
(author)
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Water Chemistry beneath Graphene : Condensation of a Dense OH-H2O Phase under Graphene
- 2022
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:9, s. 4347-4354
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Journal article (peer-reviewed)abstract
- Room temperature oxygen hydrogenation below graphene flakes supported by Ir(111) is investigated through a combination of X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory calculations using an evolutionary search algorithm. We demonstrate how the graphene cover and its doping level can be used to trap and characterize dense mixed O-OH-H2O phases that otherwise would not exist. Our study of these graphene-stabilized phases and their response to oxygen or hydrogen exposure reveals that additional oxygen can be dissolved into them at room temperature creating mixed O-OH-H2O phases with an increased areal coverage underneath graphene. In contrast, additional hydrogen exposure converts the mixed O-OH-H2O phases back to pure OH-H2O with a reduced areal coverage underneath graphene.
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| 7. |
- Hartl, Tobias, et al.
(author)
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Carbon Embedding of Pt Cluster Superlattices Templated by Hexagonal Boron Nitride on Ir(111)
- 2021
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:42, s. 23435-23444
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Journal article (peer-reviewed)abstract
- With the goal to delevop the fabrication of a new type of Pt-nanoparticle carbon-support electrocatalyst, we investigate the carbon embedding of Pt cluster superlattices grown on the moiré of a monolayer of hexagonal boron nitride (h-BN) on Ir(111). Our combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) study establishes conformal C embedding of the Pt clusters on h-BN/Ir(111) without deterioration of superlattice order, preferential and strong binding of the embedding carbon to the Pt clusters, and upon annealing the formation of a homogeneous amorphous carbon (a-C) matrix. There are indications that while the a-C matrix and the Pt clusters bind strongly to each other, upon annealing both weaken their binding to h-BN.
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| 8. |
- Hartl, Tobias, et al.
(author)
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Cluster Superlattice Membranes
- 2020
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In: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 14:10, s. 13629-13637
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Journal article (peer-reviewed)abstract
- Cluster superlattice membranes consist of a two-dimensional hexagonal lattice of similar-sized nanoclusters sandwiched between single-crystal graphene and an amorphous carbon matrix. The fabrication process involves three main steps, the templated self-organization of a metal cluster superlattice on epitaxial graphene on Ir(111), conformal embedding in an amorphous carbon matrix, and subsequent lift-off from the Ir(111) substrate. The mechanical stability provided by the carbon-graphene matrix makes the membrane stable as a free-standing material and enables transfer to other substrates. The fabrication procedure can be applied to a wide variety of cluster materials and cluster sizes from the single-atom limit to clusters of a few hundred atoms, as well as other two-dimensional layer/host matrix combinations. The versatility of the membrane composition, its mechanical stability, and the simplicity of the transfer procedure make cluster superlattice membranes a promising material in catalysis, magnetism, energy conversion, and optoelectronics.
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| 9. |
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| 10. |
- Herbig, Charlotte, et al.
(author)
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From Permeation to Cluster Arrays : Graphene on Ir(111) Exposed to Carbon Vapor
- 2017
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In: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 17:5, s. 3105-3112
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Journal article (peer-reviewed)abstract
- Our scanning tunneling microscopy and X-ray photoelectron spectroscopy experiments along with first-principles calculations uncover the rich phenomenology and enable a coherent understanding of carbon vapor interaction with graphene on Ir(111). At high temperatures, carbon vapor not only permeates to the metal surface but also densifies the graphene cover. Thereby, in addition to underlayer graphene growth, upon cool down also severe wrinkling of the densified graphene cover is observed. In contrast, at low temperatures the adsorbed carbon largely remains on top and self-organizes into a regular array of fullerene-like, thermally highly stable clusters that are covalently bonded to the underlying graphene sheet. Thus, a new type of predominantly sp2-hybridized nanostructured and ultrathin carbon material emerges, which may be useful to encage or stably bind metal in finely dispersed form.
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