Potential role of silanones in the photoluminescence-excitation, visible-photoluminescence-emission, and infrared spectra of porous silicon

Ab initio molecular electronic structure calculations on select silanones, silylenes, and tricoordinated silicon compounds $\mathrm{Si}\mathrm{XYZ}$ with a dangling electron are presented. The calculations are used to evaluate the nature of the electronic spectra, which are to be associated with surface-bound Si/O/H compounds. In concert, they are used to suggest an explanation for the nature of the photoluminescence-excitation spectrum (PLE) and the subsequent visible luminescence (PL) from porous silicon (PS) based on the optical properties of the silanone-based oxyhydrides. In order to make this selection, we treat a set of compounds that includes the silanones ${\mathrm{Si}(\mathrm{O})\mathrm{H}}_{2},$ Si(O)H(OH), ${\mathrm{Si}(\mathrm{O})(\mathrm{OH})}_{2},$ ${\mathrm{Si}(\mathrm{O})\mathrm{H}(\mathrm{O}\mathrm{S}\mathrm{i}\mathrm{H}}_{3})$, ${\mathrm{Si}(\mathrm{O})\mathrm{H}(\mathrm{S}\mathrm{i}\mathrm{H}}_{3}),$ ${\mathrm{Si}(\mathrm{O})(\mathrm{OH})(\mathrm{S}\mathrm{i}\mathrm{H}}_{3}),$ ${\mathrm{Si}(\mathrm{O})(\mathrm{S}\mathrm{i}\mathrm{H}}_{3}{)}_{2},$ and ${\mathrm{Si}(\mathrm{O})(\mathrm{S}\mathrm{i}\mathrm{H}}_{3}{)(\mathrm{O}\mathrm{S}\mathrm{i}\mathrm{H}}_{3}),$ the silylenes HSiOH, HOSiOH, and ${\mathrm{HOSiOSiH}}_{3},$ and the tricoordinated silicon compounds ${\mathrm{SiH}}_{3},$ ${\mathrm{Si}(\mathrm{OH})\mathrm{H}}_{2},$ ${\mathrm{Si}(\mathrm{OH})}_{2}\mathrm{H},$ and ${\mathrm{Si}(\mathrm{OH})}_{3}.$ The silanone-based oxyhydride structures containing either an OH or ${\mathrm{OSiH}}_{3}$ group all display adiabatic ground-state singlet-excited-state triplet exciton separations in the range close to 400 nm. This adiabatic energy is consistent with the vertical transition energies associated with the PS excitation spectrum (PLE) as a large change in the $\mathrm{Si}\mathrm{=}\mathrm{O}$ bond distance $(\ensuremath{\sim}0.17\AA{})$ accompanies the transition from the silanone ground electronic singlet state to the low-lying triplet exciton (or its closely lying singlet coupled configuration). The maximum in the PLE spectrum, obtained through optical pumping from the lowest vibrational levels of the ground electronic state to considerably higher levels of the triplet exciton electronic state, should therefore be shifted to considerably shorter wavelength consistent with an absorption spectrum peaking at 350 nm as observed by several researchers. A shift to larger internuclear distance in the excited-state triplet exciton will also produce a considerable redshift in the PL emission spectrum relative to the absorption-excitation wavelengths, again consistent with experimental observation. The calculated IR spectra for the silanone-based oxyhydrides are also consistent with the observed Fourier transform IR spectra of porous silicon. In clear contrast, neither the silylenes nor the tricoordinated silicon compounds with dangling electrons can account for the excitation or emission features that are associated with PS as their lowest-lying transitions result in minimal changes in bonding and/or occur at much higher energy (shorter wavelength). The results obtained in this study also suggest that surface passivation or the saturation of valency is incommensurate with the formation of the fluorophors that will produce the emission from PS.