Tuning the morphology and composition of ultrathin cobalt oxide films via atomic layer deposition

Abstract

Ultrathin cobalt oxide (CoO_x) films (<10 nm) have been prepared on both planar and three dimensional substrates by atomic layer deposition (ALD) using Co(Cp)₂ and O₃ as precursors. The optimal temperature window was 150–250 °C with a saturated growth rate of ∼0.37 Å per cycle. While the main composition of the thicker film consists of Co₃O₄ as verified by Raman spectroscopy, the initial few layers grown by ALD show a mixture of Co₃O₄ and CoO as revealed by X-ray photoelectron spectrum (XPS). The surface morphology of the film is greatly influenced by the deposition temperature as atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HR-TEM) characterization show the formation of crystalline islands and uneven coating on porous structures at high temperatures, while a smooth and uniform coating can be obtained at lower temperatures. The role of nucleation sites and crystallization speed on film morphology is discussed. Furthermore, we show that tuning of deposition temperatures could lead to improved catalytic activities as demonstrated in the CO oxidation light off test.