Top view field emission scanning electron micrographs (a–d) and atomic force microscopy images (e–h) of CuO thin films prepared using various Cu concentrations: a and e, parent glass; b and f, 0.2 mol % Cu; c and g, 0.3 mol % Cu; d and h, 0.4 mol % Cu. The insets (cross-sectional views) show the thickness of the CuO thin films on the glass substrates