The physical origin of the luminescence bands of porous silicon is dis
cussed using results of calculations of electronic and optical propert
ies of silicon nanostructures. Silicon crystallites, wires or molecule
s are investigated as possible sources of emission of light. We show t
hat the slow visible band in porous silicon is probably due to quantum
confinement in silicon crystallites with dimensions lower than simila
r to 4.5 nm. The long radiative lifetime in this band is due to a smal
l oscillator strength typical of an indirect bandgap semiconductor. We
calculate that the presence of a dangling bond in a crystallite must
kill its luminescence in the visible range but can produce an emission
in the infrared region, in agreement with recent experimental results
showing a correlation between the presence of dangling bonds and the
infrared emission of light. We show that polysilanes can emit visible
light in some cases but with radiative lifetimes in the nanosecond ran
ge. The physical origin of the fast blue band of porous silicon is exa
mined in light of these results.