Structural Control of Carbon Nickel Thin Films Without Substrate Heating
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Structural Control of Carbon Nickel Thin Films Without Substrate Heating David Bosworth1, Nadia Stelmashenko1 and Zoe H. Barber1 1 Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, UK ABSTRACT A variety of characterization techniques have been employed to study the growth and structure of nickel carbon thin films fabricated by ionized magnetron sputter deposition. A two target magnetron system is used along with an RF powered coil, to create a secondary plasma, with a DC bias applied at the substrate. By varying the bias it is possible to control the energy of the depositing species as well as a range of other plasma properties and therefore the resulting film microstructure. It has been observed that a change from the metastable hexagonal nickel carbide phase to the stable face centered cubic structure of nickel can be induced. This change in metal crystallinity was accompanied by a change in carbon ordering. The ability to control the film structure and morphology without the need for substrate heating opens up the possibility of depositing a range of structures onto polymers. INTRODUCTION Metallic nanoparticles show a wide range of novel properties in fields such as electrochemistry, hard coatings, magnetism and catalysis. However they suffer from high chemical reactivity, which often reduces their lifetime or renders them unsuitable due to low biocompatibility. This reactivity can be countered by embedding such nanoparticles within a carbon matrix. The additional effect of metal induced graphitization creates a self organising structure of metal nanocrystals surrounded by layers of ordered sp2 carbon forming graphite shells. These core-shell structures retain the functionality of the metal but protect the nanoparticles from environmental degradation. As an additional benefit, the carbon shells can then be easily functionalised to produce further novel properties. The equilibrium phase of pure nickel is cubic, however in the presence of carbon, for a wide range of deposition techniques, a metastable hexagonal phase forms instead [1]. Substrate heating is usually required (in excess of 500 °C) in order to produce the cubic Ni phase. EXPERIMENT C-Ni films were deposited using a twin target ionized magnetron sputter deposition system. Two independent targets, carbon (99.95% purity 1mm thick graphite sourced from Goodfellow Limited) and nickel (99.99% purity 1mm thick Ni foil sourced from Advent Limited), were used in a standard balanced magnetron sputtering system operating at 5.18 Wcm-2 and 0.26 Wcm-2 respectively. An additional copper coil, between the magnetrons and the substrate holder, was powered using a 13.5 MHz RF power supply to generate a secondary inductively coupled plasma. The substrate holder could be electrically floating, or biased using a DC power supply (-300 to +300 V). The substrate holder was thermally floating and, during deposition, heated to approximately 150°C as measured by a K-type thermocouple placed beneath a substrate. Films we
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