Formation of different micro-morphologies from VO2 and ZnO crystallization using macro-porous silicon substrates

Square-shaped macropores produced by electrochemical anodization of n- and p-type Si wafers have been used as centers of nucleation to crystallize VO2 and ZnO. Substrate roughness dependent formation of different morphologies is revealed in the form of squared particles, spheres, bars and ribbons in the case of VO2 and hexagonal piles and spheres in the case of ZnO, have been observed.The presence of nano-/micro-metric crystals was studied through field emission scanning electron microscopy and energy dispersive X-ray spectroscopy mapping. Crystal structure of metal oxides was confirmed by micro-Raman spectroscopy. The growth of the different morphologies has been explained in terms of the surface free energy of a bare Si/SiO2 substrate and its modification originated from the roughness of the surface and of the walls of the porous substrates. This energy plays a crucial role on the minimization of the required energy to induce heterogeneous nucleation and crystal growth. Present work strengthens and provides an experimental evidence of roughness dependent metal oxide crystal growth with well-defined habits from pore corners and rough sides of the pore walls, similar to already reported protein crystals.