Fig. 4. (a) Spatial map of the maximum positive current measured by the I–V spectroscopy over the 30 × 30 points. The map size is 500 × 500 nm2. (b) The bias waveform used for the I–V spectroscopy with a frequency of 1 Hz and an amplitude of ±2 V. The I–V curves at (c) the grain boundary and (d) grain core regions marked by open squares in (a).
