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  • 1
    Language: English
    In: Journal of the American Chemical Society, 15 July 2015, Vol.137(27), pp.8676-9
    Description: The activation of CO2 and its hydrogenation to methanol are of much interest as a way to utilize captured CO2. Here, we investigate the use of size-selected Cu4 clusters supported on Al2O3 thin films for CO2 reduction in the presence of hydrogen. The catalytic activity was measured under near-atmospheric reaction conditions with a low CO2 partial pressure, and the oxidation state of the clusters was investigated by in situ grazing incidence X-ray absorption spectroscopy. The results indicate that size-selected Cu4 clusters are the most active low-pressure catalyst for catalytic CO2 conversion to CH3OH. Density functional theory calculations reveal that Cu4 clusters have a low activation barrier for conversion of CO2 to CH3OH. This study suggests that small Cu clusters may be excellent and efficient catalysts for the recycling of released CO2.
    Keywords: Inorganic, Organic, Physical, And Analytical Chemistry ; Environmental Sciences ; Environmental Sciences ; Chemistry;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 2
    Language: English
    In: Angewandte Chemie International Edition, 09 February 2015, Vol.54(7), pp.2111-2115
    Description: How many of the several attributes of the bulk metallic state persist in a nanoparticle containing a finite number of atoms of a metallic element? Do all those attributes emerge suddenly at a well‐defined cluster size or do they rather evolve at different rates and in a broad size range? These fundamental questions have been addressed through a conjoint experimental/theoretical investigation of zinc clusters. We report the observation of novel coexistence phenomena involving different electronic phases: for some sizes, metallic and insulating electronic states coexist within a single, Janus‐like, nanoparticle; for the rest of sizes, we report the coexistence of two weakly interacting metallic phases with different dimensionalities, localized at the shell and the core of the nanoparticle. These fascinating features are due to an anomalously long core–shell separation that equips the shell and core regions with largely independent structural, vibrational, and thermal properties. , metallic and insulating electronic states coexist within a single nanoparticle; for others, the coexistence of two weakly interacting metallic phases localized at the shell and the core is found. These features are due to an anomalously long core–shell separation that equips those regions with largely independent structural, vibrational, and thermal properties.
    Keywords: Corrosion‐Protecting Coatings ; Insulating States ; Metallic States ; Zinc Clusters
    ISSN: 1433-7851
    E-ISSN: 1521-3773
    Source: John Wiley & Sons, Inc.
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  • 3
    Language: English
    In: Angewandte Chemie (International ed. in English), 09 February 2015, Vol.54(7), pp.2111-5
    Description: How many of the several attributes of the bulk metallic state persist in a nanoparticle containing a finite number of atoms of a metallic element? Do all those attributes emerge suddenly at a well-defined cluster size or do they rather evolve at different rates and in a broad size range? These fundamental questions have been addressed through a conjoint experimental/theoretical investigation of zinc clusters. We report the observation of novel coexistence phenomena involving different electronic phases: for some sizes, metallic and insulating electronic states coexist within a single, Janus-like, nanoparticle; for the rest of sizes, we report the coexistence of two weakly interacting metallic phases with different dimensionalities, localized at the shell and the core of the nanoparticle. These fascinating features are due to an anomalously long core-shell separation that equips the shell and core regions with largely independent structural, vibrational, and thermal properties.
    Keywords: Corrosion-Protecting Coatings ; Insulating States ; Metallic States ; Zinc Clusters
    ISSN: 14337851
    E-ISSN: 1521-3773
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  • 4
    Language: English
    In: Physical review letters, 05 December 2014, Vol.113(23), pp.233001
    Description: Extending the range of quantum interferometry to a wider class of composite nanoparticles requires new tools to diffract matter waves. Recently, pulsed photoionization light gratings have demonstrated their suitability for high mass matter-wave physics. Here, we extend quantum interference experiments to a new class of particles by introducing photofragmentation beam splitters into time-domain matter-wave interferometry. We present data that demonstrate this coherent beam splitting mechanism with clusters of hexafluorobenzene and we show single-photon depletion gratings based both on fragmentation and ionization for clusters of vanillin. We propose that photofragmentation gratings can act on a large set of van der Waals clusters and biomolecules which are thermally unstable and often resilient to single-photon ionization.
    Keywords: Coherence ; Beam Splitters ; Clusters ; Diffraction ; Interferometry ; Nanoparticles ; Ionization ; Matter Waves ; Plasma Physics (General) (So) ; Physics (General) (Ah);
    ISSN: 00319007
    E-ISSN: 1079-7114
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  • 5
    Language: English
    In: Journal of Physics: Condensed Matter, 2018, Vol.30(35), p.354002 (10pp)
    Description: Size-selected anionic silicon clusters, ( n   =  14–20), have been investigated by photoelectron spectroscopy and density functional theory (DFT) calculations. Low-energy structures of the clusters are globally searched for by using a genetic algorithm based on DFT calculations. The electronic density of states and vertical detachment energies have been simulated by using ten DFT functionals and compared to the experimental results. We systematically evaluated the DFT functionals for the calculation of the energetics of silicon clusters. CCSD(T) single-point energies based on MP2 optimized geometries for selected isomers of are also used as benchmark for the energy sequence. The HSE06 functional with aug-cc-pVDZ basis set is found to show the best performance. Our global minimum search corroborates that most of the lowest-energy structures of ( n   =  14–20) clusters can be derived from assembling tricapped trigonal prisms in various ways. For most sizes previous structures are confirmed, whereas for a new structure has been found.
    Keywords: Physics - Atomic And Molecular Clusters ; Condensed Matter - Other Condensed Matter ; Physics - Chemical Physics;
    ISSN: 0953-8984
    E-ISSN: 1361-648X
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  • 6
    Language: English
    In: Angewandte Chemie International Edition, 07 April 2015, Vol.54(15), pp.4498-4501
    Description: X‐ray magnetic circular dichroism spectroscopy has been used to characterize the electronic structure and magnetic moment of Cr. Our results indicate that the removal of a single electron from the 4sσ bonding orbital of Cr drastically changes the preferred coupling of the 3d electronic spins. While the neutral molecule has a zero‐spin ground state with a very short bond length, the molecular cation exhibits a ferromagnetically coupled ground state with the highest possible spin of S=11/2, and almost twice the bond length of the neutral molecule. This spin configuration can be interpreted as a result of indirect exchange coupling between the 3d electrons of the two atoms that is mediated by the single 4s electron through a strong intraatomic 3d‐4s exchange interaction. Our finding allows an estimate of the relative energies of two states that are often discussed as ground‐state candidates, the ferromagnetically coupled Σ and the low‐spin Σ state. from the 4sσ bonding orbital of Cr fully localizes all the 3d electrons and drastically changes the preferred coupling of their spins. The molecular cation exhibits a ferromagnetically coupled ground state with the highest possible spin of =11/2, and almost twice the bond length of the neutral molecule. This spin configuration can be interpreted as a result of indirect exchange coupling.
    Keywords: Chromium ; Exchange Interactions ; Ion Trap ; Magnetic Properties ; X‐Ray Absorption Spectroscopy
    ISSN: 1433-7851
    E-ISSN: 1521-3773
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  • 7
    In: Nature, 1998, Vol.393(6682), p.238
    Description: Small particles have a lower melting point than bulk material. The physical cause lies in the fact that small particles have a higher proportion of surface atoms than larger particles - surface atoms have fewer nearest neighbours and are thus more weakly bound and less constrained in their thermal motion than atoms in the body of a material. The reduction in the melting point has been studied extensively for small particles or clusters on supporting surfaces. One typically observes a linear reduction of the melting point as a function of the inverse cluster radius. Recently, the melting point of a very small cluster, containing exactly 139 atoms, has been measured in a vacuum using a technique in which the cluster acts as its own nanometre-scale calorimeter. Here the authors use the same technique to study ionized sodium clusters containing 70 to 200 atoms. The melting points of these clusters are on average 33 % (120 K) lower than the bulk material; furthermore, the authors observe surprisingly large variations in the melting point (of +- 30 K) with changing cluster size, rather than any gradual trend. These variations cannot yet be fully explained theoretically.
    Keywords: Schmelztemperatur ; Schmelzverhalten ; Mikropartikel ; Versuchsergebnis ; Wärmekapazität ; Teilchengröße ; Oberflächenenergie ; Temperaturgang ; Nanotechnologie ; Atom-Cluster ; Anomalie ; Nanoteilchen ; Sciences (General) ; Physics;
    ISSN: 0028-0836
    E-ISSN: 1476-4687
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  • 8
    Language: English
    In: Chemistry – A European Journal, 28 May 2018, Vol.24(30), pp.7631-7636
    Description: Preservation of protein conformation upon transfer into the gas phase is key for structure determination of free single molecules, for example using X‐ray free‐electron lasers. In the gas phase, the helicity of melittin decreases strongly as the protein's protonation state increases. We demonstrate the sensitivity of soft X‐ray spectroscopy to the gas‐phase structure of melittin cations ([melittin+H], =2–4) in a cryogenic linear radiofrequency ion trap. With increasing helicity, we observe a decrease of the dominating carbon 1 s–π* transition in the amide C=O bonds for non‐dissociative single ionization and an increase for non‐dissociative double ionization. As the underlying mechanism we identify inelastic electron scattering. Using an independent atom model, we show that the more compact nature of the helical protein conformation substantially increases the probability for off‐site intramolecular ionization by inelastic Auger electron scattering. : Soft X‐ray absorption in gas‐phase protein cations leads to Auger‐electron emission. Emitted Auger electrons can induce electron‐impact ionization elsewhere in the same molecule. The cross‐section of this process is sensitive to protein secondary structure.
    Keywords: Auger Electrons ; Gas-Phase Biomolecules ; Mass Spectrometry ; Protein Conformation ; Soft X-Ray Spectroscopy
    ISSN: 0947-6539
    E-ISSN: 1521-3765
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  • 9
    Language: English
    In: The Journal of Physical Chemistry C, 05/21/2015, Vol.119(20), pp.11148-11152
    ISSN: 1932-7447
    E-ISSN: 1932-7455
    Source: American Chemical Society (via CrossRef)
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  • 10
    In: Nanoscale, 2018, Vol.10(40), pp.19162-19181
    Description: We report the results of a conjoint experimental/theoretical effort to assess the structures of free-standing zinc clusters with up to 73 atoms. Experiment provides photoemission spectra for Zn N cluster anions, to be used as fingerprints in structural assessment, as well as mass spectra for both anion and cation clusters. Theory provides both a detailed description of a novel protocol to locate global minimum structures of clusters in an efficient and reliable way, and its specific application to neutral and charged zinc clusters. Our methodology is based on the well-known hybrid EP-DFT (empirical potential-density functional theory) approach, in which the approximate potential energy surface generated by an empirical Gupta potential is first sampled with unbiased basin hopping simulations, and then a selection of the isomers so identified is re-optimized at a first-principles DFT level. The novelty introduced in our paper is a simple but efficient new recipe to obtain the best possible EP parameters for a given cluster system, with which the first step of the EP-DFT method is to be performed. Our method is able to reproduce experimental measurements at an excellent level for most cluster sizes, implying its ability to locate the true global minimum structures; meanwhile, if exactly the same method is applied based on the existing Gupta potential (fitted to bulk properties), it leads to wrong predicted structures with energies between 1 and 2 eV above the correct ones. Opposite to what was claimed in the past, our work unequivocally demonstrates that Zn clusters are not amorphous, and they rather adopt high symmetry structures for most sizes. We show that Zn clusters have a number of exotic, unprecedented structural and electronic properties which are not expected for clusters of a metallic element, and describe them in detail.
    Keywords: Cluster-Struktur ; Anion ; Zink ; Kation ; Dichtefunktionaltheorie ; Ortung ; Photoemission ; Isomer ; Massenspektrum ; Potenzielle Energie ; Fingerabdruck ; Metallisches Element ; Engineering;
    ISSN: 2040-3364
    E-ISSN: 2040-3372
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