The hysteresis and kinetics of capillary condensation of N2 and Ar in linear mesopores, produced by etching of Si wafers, have been studied for different pore shapes, including the ink bottle geometry. Pore blocking has been observed in the solid state of the pore fillings, but not in the liquid state. We conclude that individual local geometries such as the pore mouth, a blind end, or a single constriction have no effect on the shape of sorption isotherms, that the pore space should be regarded as a statistical ensemble of pore segments with a lot of quenched disorder.

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We show that light emission from different systems of silicon nanocrystals does behave as expected for indirect-band-gap quantum dots. Photoluminescence excited on the low energy part of the distribution of Si nanocrystals exhibits a set of narrow peaks associated with Si TA and TO momentumconserving phonon-assisted optical transitions. These spectra allow us to determine the ratio of nophonon transitions to TA and TO phonon-assisted processes over a wide range of confinement energies. The ratio between these recombination channels changes by 2 orders of magnitude with increasing confinement energy. For confinement energies above 0.7 eV the radiative transitions are governed by no-phonon quasidirect processes. [S0031-9007 (98)07199-3] PACS numbers: 78.55.Ap, 78.66.Li

Molecular oxygen plays an important role in many of the chemical reactions involved in the synthesis of biological life. In this review, we explore the interaction between O2 and silicon nanocrystals, which can be employed in the photosynthesis of singlet oxygen. We demonstrate that nanoscale Si has entirely new properties owing to morphological and quantum size effects, i.e., large accessible surface areas and excitons of variable energies and with well‐defined spin structures. These features result in new emerging functionality for nanoscale silicon: it is a very efficient spin‐flip activator of O2, and therefore, a chemically and biologically active material. This whole effect is based on energy transfer from long‐lived electronic excitations confined in Si nanocrystals to surrounding O2 via the exchange of single electrons of opposite spin, thus enabling the spin‐flip activation of O2. Further, we discuss the implications of these findings for physics, chemistry, biology, and medicine.

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