**3.2 Native oxide**

The Si NWs tends to form a native oxide at the surface. The chemical stoichiometric can be explored by XPS. It was found that there are two types of Si oxide: (i) interfacial sub-stoichiometric oxides, termed as *transient oxides* including Si2O (n = 1), SiO (n = 2), and Si2O3 (n = 3), and (ii) stoichiometric or full oxide SiO2 (n = 4) as schematically shown in **Figure 6** [33–36].
