Samples were mounted on an aluminium stub using carbon tape and sputter coated with gold to avoid sample charging. The samples were coated using a current of 40 mA for one minute. SEM analysis was performed at 10 keV using a FEI Nova Nano SEM 450, calibrated with a copper metal standard. Secondary electron (SE) and backscattered electron (BSE) imaging modes were used, details are in the figure captions. EDS mapping was performed at 15 keV.