title: In Situ Observations of Phase Transitions in Metastable Nickel (Carbide)/Carbon Nanocomposites.
abstract: Nanocomposite thin films comprised of metastable metal carbides in a carbon matrix have a wide variety of applications ranging from hard coatings to magnetics and energy storage and conversion. While their deposition using nonequilibrium techniques is established, the understanding of the dynamic evolution of such metastable nanocomposites under thermal equilibrium conditions at elevated temperatures during processing and during device operation remains limited. Here, we investigate sputter-deposited nanocomposites of metastable nickel carbide (Ni3C) nanocrystals in an amorphous carbon (a-C) matrixduringthermal postdeposition processing via complementary in situ X-ray diffractometry, in situ Raman spectroscopy, and in situ X-ray photoelectron spectroscopy. At low annealing temperatures (300 °C) we observe isothermal Ni3C decomposition into face-centered-cubic Ni and amorphous carbon, however, without changes to the initial finely structured nanocomposite morphology. Only for higher temperatures (400-800 °C) Ni-catalyzed isothermal graphitization of the amorphous carbon matrix sets in, which we link to bulk-diffusion-mediated phase separation of the nanocomposite into coarser Ni and graphite grains. Upon natural cooling, only minimal precipitation of additional carbon from the Ni is observed, showing that even for highly carbon saturated systems precipitation upon cooling can be kinetically quenched. Our findings demonstrate that phase transformations of the filler and morphology modifications of the nanocomposite can be decoupled, which is advantageous from a manufacturing perspective. Our in situ study also identifies the high carbon content of the Ni filler crystallites at all stages of processing as the key hallmark feature of such metal-carbon nanocomposites that governs their entire thermal evolution. In a wider context, we also discuss our findings with regard to the much debated potential role of metastable Ni3C as a catalyst phase in graphene and carbon nanotube growth.
authors.affiliation: Department of Engineering, Department of Materials Science and Metallurgy, and Cavendish Laboratory, University of Cambridge, Cambridge CB2 1TN, United Kingdom; Faculty of Physics and Department of Lithospheric Research, University of Vienna, 1010 Vienna, Austria.
authors.forename: Bernhard C
authors.initials: BC
authors.lastname: Bayer
authors.affiliation: Department of Engineering, Department of Materials Science and Metallurgy, and Cavendish Laboratory, University of Cambridge , Cambridge CB2 1TN, United Kingdom.
authors.forename: David A
authors.initials: DA
authors.lastname: Bosworth
authors.affiliation: Department of Engineering, Department of Materials Science and Metallurgy, and Cavendish Laboratory, University of Cambridge , Cambridge CB2 1TN, United Kingdom.
authors.forename: F Benjamin
authors.initials: FB
authors.lastname: Michaelis
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie , 14109 Berlin, Germany.
authors.forename: Raoul
authors.initials: R
authors.lastname: Blume
authors.affiliation: Faculty of Physics and Department of Lithospheric Research, University of Vienna , 1010 Vienna, Austria.
authors.forename: Gerlinde
authors.initials: G
authors.lastname: Habler
authors.affiliation: Faculty of Physics and Department of Lithospheric Research, University of Vienna , 1010 Vienna, Austria.
authors.forename: Rainer
authors.initials: R
authors.lastname: Abart
authors.affiliation: Department of Engineering, Department of Materials Science and Metallurgy, and Cavendish Laboratory, University of Cambridge , Cambridge CB2 1TN, United Kingdom.
authors.forename: Robert S
authors.initials: RS
authors.lastname: Weatherup
authors.affiliation: Department of Engineering, Department of Materials Science and Metallurgy, and Cavendish Laboratory, University of Cambridge , Cambridge CB2 1TN, United Kingdom.
authors.forename: Piran R
authors.initials: PR
authors.lastname: Kidambi
authors.affiliation: Department of Engineering, Department of Materials Science and Metallurgy, and Cavendish Laboratory, University of Cambridge , Cambridge CB2 1TN, United Kingdom.
authors.forename: Jeremy J
authors.initials: JJ
authors.lastname: Baumberg
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft , 14195 Berlin, Germany.
authors.forename: Axel
authors.initials: A
authors.lastname: Knop-Gericke
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft , 14195 Berlin, Germany.
authors.forename: Robert
authors.initials: R
authors.lastname: Schloegl
authors.affiliation: Institute of Radiation Physics, Helmholtz-Zentrum Dresden-Rossendorf , 01314 Dresden, Germany.
authors.forename: Carsten
authors.initials: C
authors.lastname: Baehtz
authors.affiliation: Department of Engineering, Department of Materials Science and Metallurgy, and Cavendish Laboratory, University of Cambridge , Cambridge CB2 1TN, United Kingdom.
authors.forename: Zoe H
authors.initials: ZH
authors.lastname: Barber
authors.affiliation: Faculty of Physics and Department of Lithospheric Research, University of Vienna , 1010 Vienna, Austria.
authors.forename: Jannik C
authors.initials: JC
authors.lastname: Meyer
authors.affiliation: Department of Engineering, Department of Materials Science and Metallurgy, and Cavendish Laboratory, University of Cambridge , Cambridge CB2 1TN, United Kingdom.
authors.forename: Stephan
authors.initials: S
authors.lastname: Hofmann