Experimental evidence for moleculelike absorption and emission of porous silicon using far-field and near-field optical spectroscopy

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Abstract
Far-field photoluminescence (PL) and near-field scanning optical microscopy (NSOM) have been used to provide insight into both the light emission and light absorption mechanisms of porous silicon (PS). Experiments were performed in air and in mercaptoethanol on two types of surface-prepared porous silicon samples. Dynamic studies on a time scale of the order of several minutes suggest that the surface state of the porous silicon is the principal determinant of the PL emission spectrum and the PL efficiency. From the results of experiments performed in various chemical environments, we infer that the PL emission may comprise multiple molecular spectral transitions. Further, variations in the nature of the PL as a function of excitation energy can be explained on the basis of the absorption of moleculelike species on the PS surface but not by the nature of absorption by nanocrystalline quantum structures. Finally, high-spatial-resolution NSOM spectroscopy is employed to determine that the spectral nature of the PL is spatially homogeneous down to a scale approaching 40 nm. Primarily, our experiments suggest that surface complexes are principally involved in the light emission from PS. Second, we also provide evidence that surface complexes, and not quantum confinement, may be responsible for the absorption of the incident excitation radiation.