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(a) Schematics diagrams showing the local negative biasing, followed by KPFM. For positive biasing, the dipoles are of reverse sign. (b) Topography and KPFM images of ZnO:Cu (8 at. %) film after the engagement of a negative or positive biased tips, respectively (scale bar: 500 nm). (c) Dependence of the peak values of the surface potential with the applied biases. (d) Schematic flat band diagram of undoped ZnO and ZnO:Cu under negative/positive biasing.
(a) Surface potential for the undoped ZnO, ZnO:Cu (2 at. %), ZnO:Cu (8 at. %) films, and their corresponding KPFM images after applying a series of dc biases in the middle of the images, ranging from −8 V (top) to (bottom) the values of the surface potential are obtained based on the line scans shown. (b) Exponential fits to the time evolution of the surface potential on the ZnO:Cu (8 at. %), with and without bottom electrode, under a biased tip of dc voltages of −10 and for 1 min. The corresponding KPFM images immediately and 4 h after biased are shown (scale bars: ).
(a) Surface potential of the annealed ZnO:Cu (8 at. %) film after applying the same line writing procedure as in Fig. 2(a). (b) Raman spectra of ZnO:Cu (8 at. %) before and after being annealed to in open air for 12 h.
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