Egon Gross

Chemical reaction mediated by excited states of Si nanocrystals: Singlet oxygen formation in solution

Formation of singlet oxygen in solution by using Si nanocrystals as photosensitizers has been demonstrated. It has been shown that the absorption band of 1,3-diphenylisobenzofuran (DPBF) in benzene centered at 416 nm decreases by irradiating green (514.5 nm) or red (632.8 nm) light if fresh porous Si powder is dispersed in the solution. The decomposition of DPBF was observed only when fresh porous Si was irradiated by light, i.e., without light irradiation no effects were observed. Furthermore, the effect was drastically suppressed if porous Si powder was annealed and a monolayer of oxide was formed on the surface of nanocrystals. The rate of the decomposition of DPBF was accelerated when the solution was bubbled by oxygen gas. These results indicate that electronic excitation of Si nanocrystals is transferred to molecular oxygen dissolved in solution, resulting in the formation of singlet oxygen. Generated singlet oxygen reacts with DPBF (1,4-cycloaddition reaction), forming endoperoxides, which in turn ...

Photodegradation of porous silicon induced by photogenerated singlet oxygen molecules

We report on the mechanism of photodegradation of porous silicon luminescence in ambient containing molecular oxygen. Energy transfer from excitons confined in silicon nanocrystallites to molecular oxygen results in the generation of highly chemically reactive singlet oxygen molecules. The subsequent interaction of singlet oxygen molecules with the hydrogenated surface of silicon nanocrystals results in its photooxidation and the creation of additional nonradiative defects, i.e., the luminescence fatigue effect.

Dichroic behavior of multilayer structures based on anisotropically nanostructured silicon

Multilayer structures of anisotropically nanostructured (birefringent) silicon have been fabricated and studied by polarization-resolved reflection and transmission measurements. We demonstrate that stacks of birefringent porous silicon layers with alternating refractive indices and thicknesses act as dichroic Bragg reflectors or dichroic microcavities with a transmission/reflection dependent on the polarization direction of the incident light. The possibility of separate fine tuning of two orthogonally polarized transmission/reflection bands and their spectral splitting is demonstrated.

Planar silicon-based light polarizers.

Silicon-based thin-film polarizers operating in the visible and near-infraed spectral range are fabricated by electrochemical etching of bulk silicon wafers. Anisotropically etched (110) porous silicon layers exhibit a strong in-plane anisotropy of the refractive index. Stackes of alternating layers with different mean refractive indices and thicknesses act as dichroic Bragg reflectors or microcavities, respectively. Both structures have two distinct reflection and transmission bands depending on the polarization of the incident linearly polarized light. Planar polarizers are realized through the combination, in one structure, of a dichroic reflector with either a second reflector or a microcavity with different spectral responses.

Capillary condensation monitored in birefringent porous silicon layers

We performed studies on the capillary condensation of various substances in optically anisotropic porous silicon layers. Their strong in-plane form birefringence has been utilized to analyze the polarization state of the transmitted light when molecules penetrate into the pores. The polarization state of the transmitted light is correlated with the filling fraction of the pores by an effective-medium model for anisotropic porous materials. The experimentally obtained adsorption/desorption isotherms show hysteresis typical for capillary condensation in porous materials. We discuss the shape of the hysteresis loops in the framework of the morphology of the layers. Pore-size distributions derived from adsorption/desorption isotherms are presented.

Spatially nanostructured silicon for optical applications

We report on a strong intrinsic optical anisotropy of silicon induced by its dielectric nanopatterning. As a result, an in-plane birefringence for nanostructured (110) Si surfaces is found to be 105 times stronger than that observed in bulk silicon crystals. A difference in the main values of the anisotropic refractive index exceeds that one of any natural birefringent crystals. The anisotropy parameters are found to be strongly dependent on the typical size of the silicon nanowires assembling the layers. The value of birefringence is dependent also on the dielectric surrounding of silicon nanoparticles assembling these layers. We show that stacks of layers having alternative refractive indices act as a distributed Bragg reflectors or optical microcavities. Dichroic reflection/transmission behavior of these structures sensitive to the polarization of the incident linearly polarized light is demonstrated. These findings open the possibility of an application of optical devices based on birefringent silicon layers in a wide spectral range.

Porous Silicon as an Open Dielectric Nanostructure: an Ensemble of Aspheric Silicon Nanocrystals

Reducing the dimensions of semiconductor structures is nowadays a well established field in solid state physics, not at least on demand by the preceding miniaturization in microelectronics. To confine carriers in two dimensions was one of the first steps towards the current nano-science and technology. At present, three dimensional confinement of carriers in quantum dots or nanocrystals (NCs) with typical dimensions of the order of nanometers is a standard technique to modify the physical properties known from the bulk material. One of the most prominent consequences arising from the reduced dimensionality is an altered density of states which is accompanied by a widening of the bandgap with decreasing size due to quantum confinement. Therefore the optical properties, especially the photoluminescence (PL), of such objects differ significantly from that of the bulk material.

Light amplification in a liquid network confined in a porous matrix

We report on a medium exhibiting extremely efficient light scattering properties: a liquid network formed in a porous matrix. Liquid fragments confined in the solid matrix result in a random fluctuation of the dielectric function and act as scattering objects for photons. The optical scattering efficiency is defined by the filling factor of the liquid in the pores and its dielectric constant. The spectral dependence of the scattering length of photons indicates that the phenomenon is governed by a Mie-type scattering mechanism. The degree of the dielectric disorder of the medium, i.e. the level of opacity is tunable by the ambient vapor pressure of the dielectric substance. In the strongest scattering regime the scattering length of photons is found to be in the micrometer range. By incorporation of dye molecules in the voids of the porous layer a system exhibiting optical gain is realized. In the multiple scattering regime the optical path of diffusively propagating photons is enhanced and light amplification through stimulated emission occurs: a strong intensity enhancement of the dye emission accompanied by significant spectral narrowing is observed above the excitation threshold for a layer being in the opalescence state.

Energy exchange between optically excited Silicon Nanocrystals and Molecular Oxygen

We report on the photosensitizing properties of optically excited Silicon (Si) nanocrystal assemblies that are employed for an efficient generation of singlet oxygen. Spin triplet state excitons confined in Si nanocrystals transfer their energy to molecular oxygen (MO) adsorbed on the nanocrystal surface. This process results in a strong suppression of the photoluminescence (PL) from the Si nanocrystal assembly and in the excitation of MO from the triplet ground state to singlet excited states. The high efficiency of the energy transfer if favored by a broad energy spectrum of photoexcited excitons, a long triplet exciton lifetime and a highly developed surface area of the nanocrystal assembly. Due to the specifics of the coupled system Si nanocrystal – oxygen molecule all relevant physical parameters describing the photosensitization process are accessible experimentally. This includes the role of resonant and phonon-assisted energy transfer, the dynamics of energy transfer, and its mechanism.

Optical devices based on anisotropically nanostructured silicon

Anisotropic nanostructuring of bulk silicon (Si) leads to a significant optical anisotropy of single porous silicon (PSi) layers. A variation of the etching current in time allows a controlled modification of the porosity along the growth direction and therefore a three-dimensional variation of the refractive index (in plane an in depth). This technique can be important for photonic applications since it is the basis of a development of a variety of novel, polarization sensitive, silicon-based optical devices: retarders, dichroic Bragg Reflectors, dichroic microcavities and Si based polarizers.