RESUMO
Many values of the observed conductivity percolation exponent t cannot be explained by the classical universal theory or by the existing nonuniversal theories. In particular, the 1.3≤t≤4.0 clustering of t values, in both composite materials and porous media has not been accounted for. In this work we were concerned with a pseudononuniversal percolation behavior that, unlike the genuine nonuniversal behavior, explains the statistics of the experimentally observed percolation conductivity exponents in continuum systems.
RESUMO
We have applied scanning tunneling spectroscopy in studies of the electronic level structure of surface-functionalized colloidal Si nanocrystals (Si-NCs) as a function of their size for various capping ligands. The energy gaps extracted from the tunneling spectra increase with decreasing NC size, manifesting the effect of quantum confinement. This is consistent with the blueshift revealed by photoluminescence (PL) from dodecene functionalized Si-NCs. The tunneling spectra measured on NCs functionalized with NH4Br or allylamine show band-edge shifts toward higher energies, akin to p-type doping. This behavior can be accounted for by the combined contributions of the ligands' dipole moments and charge transfer between a Si-NC and its surface groups. Concomitantly, size-independent PL spectra, which cannot be associated with NC band gap variations, were observed for the latter Si-NCs.
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
We report on Monte Carlo simulations of continuum percolation thresholds, by implementing highly efficient algorithms for very large samples. Our work, which includes percolation of hyperspheres, hypercubes, and boxes, in various dimensions, sizes, and shapes, has confirmed the expected dependence of the threshold on Vex, the total excluded volume, and on Bc, the average number of bonds per site. We have further confirmed that Vex=Bc, and that Bc is dependent on the objects shape, for which we offer a possible explanation. In particular we find that, counterintuitively, one can have Bc<1, as we have found for hyperspheres of dimension > or =12. From our many results for differently sized hyperspheres, we were also able to derive the correlation length exponent v solely from the behavior of the thresholds using finite-size scaling.
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.