CdS quantum dots (QDs) decorated ZnO nanorod (NR) arrays were fabricated by a two-step method. The first step consisted in electrochemical growth of single-crystalline ZnO NR arrays, followed by the novel spin-coating assisted SILAR method for decorating the ZnO NRs with CdS QDs. Structural, morphological and optical characterization of CdS QDs/ZnO NR arrays were done. ZnO NRs had a single crystal wurtzite structure growing along the c-axis. The decorated CdS QDs had a quasi-spherical shape with a mean diameter of about 5 nm. The increase of CdS content produces an increase in the visible part of the absorption spectrum. Bandgap energy values for ZnO between 3.26–3.29 eV were obtained. For CdS the measured absorption edge values are between 2.35–2.65 eV (decreasing with the number of coating cycles). Numerical simulations based on effective medium approximation were done to verify these features. The Urbach tail parameter in CdS absorption edge is between 44–52 meV. The photovoltaic performance of ZnO and CdS QDs/ZnO NRs have been evaluated in a photoelectrochemical solar cell configuration with a polysulfide electrolyte under white illumination. The decoration of ZnO NRs with CdS QDs leads to a cell performance of JSC = 2.67 mA/cm2, VOC = 0.74 V, FF = 0.30 and η = 1.48%.

The optical properties of bare ZnO nanorods and sensitized nanostructures, with Cu 2 O and CdS, are comparatively studied. These nanostructures may show improved photovoltaic performance compared to planar ones. ZnO nanorod arrays were grown by electrochemical deposition. In a second step, Cu 2 O was also deposited electrochemically, while for CdS successive ion layer adsorption and reaction techniques were used. The experimental results are interpreted using numerical simulation based on an effective medium theory. Bare nanorod samples reveal mainly the direct ultraviolet absorption edge of ZnO (between 3.25 and 3.30 eV) and a monotonically increasing transmittance from the ultraviolet into the red. This increase is originated in light scattering, probably by the nanometric structure of the samples. For the sensitized samples reduced transmittance in the solar spectrum region is observed and several well-defined absorption edges appear. Spectral absorption edge shifts are interpreted comparing with numerical simulations. For CdS the measured shifts are larger than the ones obtained from numerical simulations. The difference may be due to the combined influence of sub-bandgap absorption, light scattering in the nanorod array and quantum confinement in the nanocrystalline structure of sensitizer layers. For Cu 2 O its more complex electronic structure gives larger dispersion in the results although major absorption edges are clearly observed.

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We have developed a method to measure the characteristic interstice size of transparent porous dielectric media. The method is based upon the observation of the transient decay of alkali atoms contained in the medium pores and excited to a sufficiently long-lived state. We present a simple model describing the dependence of the decay transients on the distance L traveled by the atoms between interstice walls. Fitting of the observed transient data to the model predictions allows the determination of L. The method is tested with two well-characterized micrometric thin cells and used to measure the characteristic interstice size of several porous glass samples.

We study the two-photon laser excitation to the 5D$_{5/2}$ energy level of $^{85}$Rb atoms contained in the interstices of a porous material made from sintered ground glass with typical pore dimensions in the 10 - 100 $\mu$m range. The excitation spectra show unusual flat-top lineshapes which are shown to be the consequence of wave-vector randomization of the laser light in the porous material. For large atomic densities, the spectra are affected by radiation trapping around the D2 transitions. The effect of the transient atomic response limited by time of flight between pores walls appears to have a minor influence in the excitation spectra. It is however revealed by the shortening of the temporal evolution of the emitted blue light following a sudden switch-off of the laser excitation.Comment: 7 pages, 4 figures, regular articl