For this, samples were placed between two aluminium holders (d=3mm), both containing a notch with a diameter of 2 mm, inclosing the sample and rapidly frozen in slush nitrogen (SN) at -210 °C. The samples were then transferred into the sputter coater with a Leica EM VCT100 cryo-shuttle at -140°C (Leica Microsystems ACE 400, Wetzlar, Germany). Here, the upper half of the sample was knocked off to create a fresh fractured surface and freeze-etched at -85 °C for 15 minutes under vacuum (< 1•10 -3 mbar). The samples were finally sputtered with 3 nm platinum and transferred with the cryo-shuttle into the SEM chamber. The morphology of the fractured surfaces was imaged at -140 °C, by detecting SE using acceleration voltages of 2 kV or 8 kV.