RESUMO
For quite a few systems in the continuum, such as carbon nanotube polymer composites and segregated composites, percolation electrical conductivity exponents that are much smaller than the universal value have been reported. This is unexpected in view of the classical lattice percolation theory. Here we provide a simple general phenomenological model that accounts for such observations within the framework of universality. We suggest that these small value exponents are due to the interplay between the connectivity and the structural variations that follow the increase of the fractional volume content of the conducting phase.
RESUMO
Modification of films containing Si nanocrystallites embedded in SiO2 by irradiation with high-energy ions was found to induce peaks in their low-frequency capacitance-voltage characteristics. Considering the nanocrystallite spatial distribution that follows the ion tracks we interpret these peaks as due to the charge transfer along these tracks, similar to the process that was reported previously for two-dimensional arrays of such crystallites. The ion irradiation of the above three-dimensional system appears to be useful then for the fabrication of nanostructures, which have also the properties of low-dimensional arrays.
Assuntos
Modelos Químicos , Nanotecnologia/métodos , Pontos Quânticos , Silício/química , Simulação por Computador , Íons Pesados , Substâncias Macromoleculares/química , Substâncias Macromoleculares/efeitos da radiação , Teste de Materiais , Conformação Molecular/efeitos da radiação , Tamanho da Partícula , Doses de Radiação , Propriedades de Superfície/efeitos da radiaçãoRESUMO
We have followed the effects of heavy ion irradiation on the structural, electrical, and photoluminescence properties of ensembles of silicon nanocrystallites embedded in a dielectric (SiO(2)) matrix. This was done as a function of the irradiation dose and the content of the Si phase. The results obtained can be accounted for self-consistently assuming that a relatively small dose of the irradiation enhances the crystallization while for higher doses the irradiation enhances the amorphization. The corresponding processes suggest that tuning of the above properties can be achieved by swift heavy ion irradiation.
Assuntos
Cristalização/métodos , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Nanotecnologia/métodos , Dióxido de Silício/química , Íons Pesados , Medições Luminescentes , Substâncias Macromoleculares/química , Substâncias Macromoleculares/efeitos da radiação , Teste de Materiais , Conformação Molecular/efeitos da radiação , Nanoestruturas/efeitos da radiação , Tamanho da Partícula , Doses de Radiação , Dióxido de Silício/efeitos da radiação , Propriedades de Superfície/efeitos da radiaçãoRESUMO
A striking correlation between infrared photoinduced absorption spectra and the photoluminescence from silicon nanocrystals indicates that quantized electronic sublevels of the nanocrystals are resonantly coupled to surface vibrational modes via a polarization field produced by coherent longitudinal polar vibrations. Our experimental results and model support the assumption that the mechanism responsible for the efficient photoluminescence from silicon nanocrystals should be assigned to inhibition of nonradiative channels rather than enhancement of radiative channels.
