Biomedical subjects

W L Vos

Publications and source records attributed to W L Vos.

5 recordsLinked to original sources

PubMed2005

Frequency-dependent spontaneous emission rate from CdSe and CdTe nanocrystals: influence of dark states.

We studied the rate of spontaneous emission from colloidal CdSe and CdTe nanocrystals at room temperature. The decay rate, obtained from luminescence decay curves, increases with the emission frequency in a supralinear way. This dependence is explained by the thermal occupation of dark exciton states at room temperature, giving rise to a strong attenuation of the rate of emission. The supralinear dependence is in agreement with the results of tight-binding calculations.

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PubMed2003

Near-field optical investigation of three-dimensional photonic crystals.

We show that the coupling of light from an external pointlike light source into a three-dimensional photonic crystal depends on the relative launching position with respect to the crystal lattice as well as on the frequency of light. The results are obtained with a near-field technique which is used to acquire optical information beyond the diffraction limit and to access optical details within the unit cell of the crystal. The experiments are performed at frequencies near the second-order L-gap. As a result, the changes in the shape of the near-field pattern are explained by the photonic properties of the crystal.

Journal Article↗

PubMed2002

Macroporous germanium by electrochemical deposition.

Germanium is electrodeposited in a template formed from a dried suspension of silica spheres. The germanium completely fills the pores of the silica matrix. The semiconductor, as deposited, is amorphous but can be crystallized by annealing. Selective dissolution of the silica template gives a macroporous germanium-air sphere matrix, which offers interesting possibilities for photonic applications.

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PubMed2001

Particle excursions in colloidal crystals.

We have performed synchrotron small-angle x-ray diffraction experiments on charge-stabilized colloidal crystals. The mean squared displacements, obtained from Debye-Waller factors, decrease with increasing density beyond freezing. Irrespective of the range of the repulsion, our data and previous results lie on a common curve, linearly scaled between the melting and close-packed densities. Surprisingly, the excursions are smaller than predicted by theory.

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PubMed2000

Effective screening of hydrodynamic interactions in charged colloidal suspensions.

We investigate the hydrodynamic interaction in suspensions of charged colloidal silica spheres. The volume fraction as well as the range of the electrostatic repulsion between the spheres is varied. Using a combination of dynamic x-ray scattering, cross-correlated dynamic light scattering, and small angle x-ray scattering, the hydrodynamic function H(q) is determined experimentally. The effective hydrodynamic interactions are found to be screened, if the range of the direct interaction is relatively long and the static density correlations are strong. This observation of effective hydrodynamic screening is in marked contrast to hard-sphere-like systems.

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