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Free Metal Clusters Studied by Photoelectron Spectroscopy
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Surface and Interface Science.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Clusters are aggregates of a finite number of atoms or molecules. In the present work, free clusters out of metallic parent materials have been created and studied by synchrotron radiation-based photoelectron spectroscopy. The clusters have been formed and studied in a beam and the electronic structure of the clusters has been investigated. Conclusions have been drawn about the spatial distribution of atoms of different elements in bi-component clusters, about the development of metallicity in small clusters, and about the excitation of plasmons.

Bi-component alloy clusters of sodium and potassium and of copper and silver have been produced. The site-sensitivity of the photoelectron spectroscopy technique has allowed us to probe the geometric distribution of the atoms of the constituent elements by comparing the responses from the bulk and surface of the clusters. In both cases, we have found evidence for a surface-segregated structure, with the element with the largest atoms and lowest cohesive energy (potassium and silver, correspondingly) dominating the surface and with a mixed bulk.

Small clusters of tin and lead have been probed to investigate the development of metallicity. The difference in screening efficiency between metals and non-metals has been utilized to determine in what size range an aggregate of atoms of these metallic parent materials stops to be metallic. For tin this has been found to occur below ~40 atoms while for lead it happened somewhere below 20-30 atoms.

The excitation of bulk and surface plasmons has been studied in clusters of sodium, potassium, magnesium and aluminium, with radii in the nanometer range. The excitation energies have been found to be close to those of the corresponding macroscopic solids. We have also observed spectral features corresponding to multi-quantum plasmon excitation in clusters of Na and K. Such features have in macroscopic solids been interpreted as due to harmonic plasmon excitation. Our observations of features corresponding to the excitation of one bulk and one surface plasmon however suggest the presence of sequential excitation in clusters.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. , 55 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 992
Keyword [en]
clusters, nanoparticles, electronic structure, photoelectron spectroscopy, synchrotron radiation, surface segregation, nanoalloys, size-dependence, metallicity, plasmons
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-183031ISBN: 978-91-554-8525-2 (print)OAI: oai:DiVA.org:uu-183031DiVA: diva2:563371
Public defence
2012-12-14, 80121, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2012-11-22 Created: 2012-10-21 Last updated: 2012-11-22Bibliographically approved
List of papers
1. Single-component surface in binary self-assembled NaK nanoalloy clusters
Open this publication in new window or tab >>Single-component surface in binary self-assembled NaK nanoalloy clusters
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2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 3, 033405- p.Article in journal (Refereed) Published
Abstract [en]

Free metallic nanoalloy clusters created in a self-assembling process out of sodium and potassium mixed vapor have been studied by synchrotron-based photoelectron spectroscopy. The clusters are shown to consist of an alloy core surrounded by a surface layer containing only K in a range of conditions from K-rich to Na-rich nanoalloys. The size of the clusters as well as the fraction of the elements has been estimated from the spectra using our results on pure clusters. The mechanism behind the observed structure is discussed in terms of the total cohesive-energy minimization.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-128322 (URN)10.1103/PhysRevB.80.033405 (DOI)000268617800026 ()
Available from: 2010-07-22 Created: 2010-07-20 Last updated: 2017-12-12Bibliographically approved
2. Size-dependent transformation of energy structure in free tin clusters studied by photoelectron spectroscopy
Open this publication in new window or tab >>Size-dependent transformation of energy structure in free tin clusters studied by photoelectron spectroscopy
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2010 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 81, no 2, 023203- p.Article in journal (Refereed) Published
Abstract [en]

Free nanoscale tin clusters in the average size range of a few tens up to similar to 500 atoms are studied by means of valence and core-level photoelectron spectroscopy. A self-consistent picture arises from comparison of the work functions derived from the 4d core-level spectra, the experimental ionization energies extracted from the 5p valence spectra, and the expectations from the metallic sphere model for the clusters. The present studies demonstrate that the 4d core-level energy changes are a sensitive probe of the metal-to-semiconductor transition in the size range under investigation.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-137127 (URN)10.1103/PhysRevA.81.023203 (DOI)000275072500115 ()
Available from: 2010-12-15 Created: 2010-12-15 Last updated: 2017-12-11Bibliographically approved
3. Plasmon single- and multi-quantum excitation in free metal clusters as seen by photoelectron spectroscopy
Open this publication in new window or tab >>Plasmon single- and multi-quantum excitation in free metal clusters as seen by photoelectron spectroscopy
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2011 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 134, no 9, 094511- p.Article in journal (Refereed) Published
Abstract [en]

Plasmons are investigated in free nanoscale Na, Mg, and K metal clusters using synchrotron radiation-based x-ray photoelectron spectroscopy. The core levels for which the response from bulk and surface atoms can be resolved are probed over an extended binding energy range to include the plasmon loss features. In all species the features due to fundamental plasmons are identified, and in Na and K also those due to either the first order plasmon overtones or sequential plasmon excitation are observed. These features are discussed in view of earlier results for planar macroscopic samples and free clusters of the same materials.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-157480 (URN)10.1063/1.3563751 (DOI)000288085300048 ()
Available from: 2011-08-22 Created: 2011-08-22 Last updated: 2017-12-08Bibliographically approved
4. The electronic structure of free aluminum clusters: Metallicity and plasmons
Open this publication in new window or tab >>The electronic structure of free aluminum clusters: Metallicity and plasmons
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2012 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, no 20, 204504- p.Article in journal (Refereed) Published
Abstract [en]

The electronic structure of free aluminum clusters with similar to 3-4 nm radius has been investigated using synchrotron radiation-based photoelectron and Auger electron spectroscopy. A beam of free clusters has been produced using a gas-aggregation source. The 2p core level and the valence band have been probed. Photoelectron energy-loss features corresponding to both bulk and surface plasmon excitation following photoionization of the 2p level have been observed, and the excitation energies have been derived. In contrast to some expectations, the loss features have been detected at energies very close to those of the macroscopic solid. The results are discussed from the point of view of metallic properties in nanoparticles with a finite number of constituent atoms.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-178867 (URN)10.1063/1.4718362 (DOI)000304818400036 ()
Available from: 2012-08-06 Created: 2012-08-02 Last updated: 2017-12-07Bibliographically approved
5. Valence-band narrowing and metallic-screening disappearance in the metal-to-insulator transition in small Pb clusters—as seen by photoelectron spectroscopy
Open this publication in new window or tab >>Valence-band narrowing and metallic-screening disappearance in the metal-to-insulator transition in small Pb clusters—as seen by photoelectron spectroscopy
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

A metal-to-insulator transition in small Pb clusters within the size range from a few tens of atoms up to ~100 atoms is investigated using synchrotron-based photoelectron spectroscopy. The electronic structure is seen to differ increasingly from that of the solid when the cluster size decreases. With the cluster size decrease the valence band narrows gradually -down to 25% of that of the solid-state at the smallest size of few tens of atoms. Simultaneously the Fermi-edge energy increases. The Pb 5d core level binding energy also increases and the spectral features broaden and change the shape. The changes observed for the valence and for the 5d energies and shapes are consistent with the picture of the loss of metallicity in Pb below the critical size of 20-30 atoms/cluster.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-183029 (URN)
Available from: 2012-10-26 Created: 2012-10-21 Last updated: 2012-10-26
6. Core-shell structure disclosed in self-assembled Cu-Ag nanoalloy particles—explanation for enhanced catalytic activity?
Open this publication in new window or tab >>Core-shell structure disclosed in self-assembled Cu-Ag nanoalloy particles—explanation for enhanced catalytic activity?
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Core-shell segregation of copper and silver in self-assembled, free nanoparticles is established by means of electron spectroscopy using different photon energies. The nanoparticles are formed from metal-atom mixed vapor created by sputtering of a bimetallic sample. Even at dominating copper concentration in the primary vapor the surface of the nanoparticles is composed of only silver. These conclusions are based on the analysis of the photon-energy-dependent changes of the Cu 3d and Ag 4d photoelectron spectra. The role of such segregation is discussed in connection to the suggested enhanced catalytic properties of Cu-Ag nanoparticles.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-183030 (URN)
Available from: 2012-10-26 Created: 2012-10-21 Last updated: 2012-10-26

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