Its clear that the surface of porous increased and energy gap depends on type of porous Silicon (Nonporous, Mesoporous and Macroporous)
I think, this increase in energy gap comes from SiO2 bond because the porous surface Si-O bond is increased, especially in macroporous.
You're both right. Yes, the main factor is the quantum confinement effect. But the surface termination of the nanostructured Si also affects the band structure and hence the energy gap. This is explained in the following paper for Si nanocrystals:
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.78.035339
In porous silicon, the pores are bound by nanoscale thin walls of silicon crystallites. These nano crystallites of silicon are responsible for quantum confinement of electrons. As we know that the energy gap is inversely proportional to square of length, thinner nanocrystallites sizes mean higher energy gap. The energy gap depends on the silicon crystallite size and cannot be inferred directly from the pore size. However, we can say that as the silicon is getting electrochemically etched or polished, the pore size increases and in macroporous the pore size is large and the silicon crystallites wall is very thin, resulting in increased band gap energy.
You're both right. Yes, the main factor is the quantum confinement effect. But the surface termination of the nanostructured Si also affects the band structure and hence the energy gap. This is explained in the following paper for Si nanocrystals:
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.78.035339
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