Influence of pulse repetition rate on the growth of cobalt-doped ZnO thin films by pulsed electron beam ablation
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ARTICLE Influence of pulse repetition rate on the growth of cobalt-doped ZnO thin films by pulsed electron beam ablation Asghar Alia) and Redhouane Henda School of Engineering, Laurentian University, Sudbury, Ontario P3E 2C6, Canada (Received 21 May 2018; accepted 9 July 2018)
Cobalt-doped ZnO (CZO) film nanocomposites have been deposited on Si(100) substrates by pulsed electron beam ablation from a single Co0.2Zn0.8O target. The films have been deposited at various electron beam repetition rates (1, 2, 4, and 8 Hz), under a background argon (Ar) pressure of ;3 mtorr, an accelerating voltage of 16 kV, and a deposition temperature of 450 °C. The effect of beam frequency on the structural, chemical, and morphological properties of the films has been assessed. The findings reveal that film thickness, film roughness, and degree of crystallinity of the ZnO wurtzite structure increase with beam frequency, while globule size and density reach maximum and minimum values, respectively, as the beam frequency is increased. The pulse frequency does not appear to affect the average nanoparticulate size. X-ray photoelectron spectroscopy data support the co-existence of metallic cobalt (Co0), CoO, and Co2O3 in CZO films near the surface. Phase analysis by X-ray diffraction also confirms the presence of hexagonal close-packed metallic cobalt whose content in the films is practically unaffected by beam frequency.
I. INTRODUCTION
Oxide-supported metal thin film nanocomposites (NCs), where metal nanoparticles serve as the dopant, constitute a class of functional materials, which have attracted much attention, in recent years, due to their technologically important properties and ensuing potential application in many industrial processes including energy-intensive chemical processes.1,2 In the latter case, zinc oxide doped with the hexagonal close-packed (hcp) Co phase is of particular interest as a model nanocatalyst in many important chemical processes such as Fischer– Tropsch synthesis,1–3 photocatalysis,4 steam reforming,5 and hydrogen production.6 While nanoparticles have an exceptionally large surface area-to-volume ratio relatively to bulk material counterparts, the properties of nanocatalysts depend on size, structure, and the presence of the metal in elemental form.2 Achieving desirable features in a nanocatalyst, viz., selectivity and activity, depends on the ability to control its morphology, surface composition, and crystal structure, and on how the parameters of the fabrication method affect those properties.7 The microstructure of cobalt-doped ZnO (CZO) films, while temperature-dependent, is affected by the doping level of cobalt in ZnO. The properties of CZO are practically similar to those of zinc oxide for a Co doping level of up to 10–12 wt%.8,9 For higher values of Co a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.266 J. Mater. Res., 2018
doping, secondary phases such as elemental cobalt and cobalt oxides have been observed in CZO films.9 Since morphological and structural cha
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