The cross-sectional microstructure of the samples was investigated on a Nova NanoSEM 450 field emission scanning electron microscope (FEI, Japan) equipped with an ETD detector, set to an accelerating voltage of 5 kV. Prior to analysis, the samples were freeze fractured using liquid nitrogen (N2) and placed onto an aluminum pin stub. A thin layer of gold (10 nm) was deposited on the surfaces of the samples by an SC7620 sputtering device (Quorum Technologies, UK) to avoid the surface charging effect.