SEM imaging was performed using a Zeiss FE-SEM field-emission microscope with variable pressure, operating at 1.5 kV, as previously reported (34 (link), 35 , 38 (link)). Shortly, all the specimens were sputtered with a 5- to 10-nm platinum-palladium layer before imaging. ImageJ Fiji (version 1.47d) software package was used for the visualization and analysis of the micrographs.