Cathodic Behavior and Redox Couples - Electrochemistry of Silicon and Its Oxide - page 267

of Kooij 969 on porous silicon show: (1) Chemiluminescence, with wavelength varying
between 650 and 720 nm, occurs on p- Si at the OCP in containing strong
oxidizing agents with potentials considerably more positive than the flatband potential.
Species such as and can inject holes into the valence band during reduction
reaction. On the other hand, and which have very positive potentials, do
not inject holes into the valence band but the reduction proceeds via the conduction
band. (2) The luminescence intensity on p- Si increases with the concentration of the
oxidants and is independent of the cathodic potential. (3) In contrast to p- Si the emis-
sion intensity on n -Si depends on potential; it decreases from a relatively high value at
the OCP to a lower value in the cathodic potential region. (4) The luminescence on
n- Si is quenched at more negative potentials at which hydrogen is evolved. The lumi-
p nescence on -Si is not affected by cathodic potential because hydrogen does not evolve

on p -Si due to the lack of conduction band electrons. (5) In contrast, for which
does not inject holes during reduction, luminescence occurs at large cathodic currents
where hydrogen evolution occurs. (6) Luminescence does not occur on samples that
are not covered with a porous silicon layer.
According to Kooij et al ., 629,969 luminescence is due to hole injection from the
reducing species in the solution into the valence band. The holes are captured in Si-Si
surface bonds which leads to the formation of an oxidation intermediate. Subsequent
electron injection from the intermediate into the conduction band and the radiative
recombination of the electrons with holes in the valence band results in the emission
of photons. Such a reaction mechanism is postulated to be responsible for the chemi-
luminescence of porous silicon in these solutions at the OCP.
Kooij et al. described the individual steps involved as follows:

According to them, strong oxidizing agents like or give very limited lumi-
nescence at OCP 969 because the reduction of these two-electron redox couples requires