Two-Scale Kirchhoff Theory: Comparison of Experimental Observations With Theoretical Prediction

We introduce a non-perturbative two scale Kirchhoff theory, in the context of light scattering by a rough surface. This is a two scale theory which considers the roughness both in the wavelength scale (small scale) and in the scales much larger than the wavelength of the incident light (large scale). The theory can precisely explain the small peaks which appear at certain scattering angles. These peaks can not be explained by one scale theories. The theory was assessed by calculating the light scattering profiles using the Atomic Force Microscope (AFM) images, as well as surface profilometer scans of a rough surface, and comparing the results with experiments. The theory is in good agreement with the experimental results.Comment: 6 pages, 8 figure

High-efficiency CdTe/CdS core/shell nanocrystals in water enabled by photo-induced colloidal hetero-epitaxy of CdS shelling at room temperature

We report high-efficiency CdTe/CdS core/shell nanocrystals synthesized in water by epitaxially growing CdS shells on aqueous CdTe cores at room temperature, enabled by the controlled release of S species under low-intensity ultraviolet (UV) light illumination. The resulting photo-induced dissociation of S2O32− ions conveniently triggers the formation of critical two-dimensional CdS epitaxy on the CdTe surface at room temperature, as opposed to initiating the growth of individual CdS core-only nanocrystals. This controlled colloidal hetero-epitaxy leads to a substantial increase in the photoluminescence (PL) quantum yield (QY) of the shelled nanocrystals in water (reaching 64%). With a systematic set of studies, the maximum PL QY is found to be almost independent of the illuminating UV intensity, while the shell formation kinetics required for reaching the maximum QY linearly depends on the illuminating UV intensity. A stability study of the QD films in air at various temperatures shows highly improved thermal stability of the shelled QDs (up to 120 °C in ambient air). These results indicate that the proposed aqueous CdTe/CdS core/shell nanocrystals hold great promise for applications requiring efficiency and stability. [Figure not available: see fulltext.] © 2015, Tsinghua University Press and Springer-Verlag Berlin Heidelberg

Layer-by-layer self assembly deposition and characterization of TiO

Using low molecular weight polyethylenimine (PEI), transparent thin films of TiO2 nanoparticles were prepared by layer-by-layer self assembly method. UV-visible spectrophotometry was employed in a quantitative manner to monitor the adsorbed mass of TiO2 and PEI after each dip cycle. The adsorption of both TiO2 and PEI showed a saturation dip time of 10 min. The effect of dip time on the growth mode and surface morphology was investigated by scanning electron microscopy (SEM) and non-contact atomic force microscopy (AFM). It was found that growth proceeds in the form of laterally broad islands in case of short dip times, and taller but laterally smaller islands in case of longer dip times. A model was proposed which describes the role of dip time on the lateral growth of TiO2 islands. Low molecular weight PEI resulted in around 25% less adsorption of PEI and TiO2 in comparison with high molecular weight PEI, but because of lower remaining ash, could be promising for dye-sensitized solar cell photoelectrode applications, in which removal of polyelectrolyte after the formation of thin film enhances the electrical properties and therefore the efficiency of solar cell

Europium-doped yttrium silicate nanoparticles embedded in a porous SiO 2 matrix

International audienceEuropium-doped yttrium silicate nanoparticles were grown inside a porous silicon oxide matrix by chemical impregnation of porous silicon layers, followed by heat treatments. The average size of the nanoparticles is 50 nm and they are dispersed almost uniformly within the whole porous layer. Local composition measurements demonstrate that Y and Eu are found only in nanoparticles, indicating a good phase separation efficiency. There is indirect evidence that yttrium silicate nanoparticles are nucleated around Eu ions. The crystalline phase of the particles is pure α-Y2Si2O7, with no trace of Y2O3 or Y2SiO5 or other Y2Si2O7 polymorphs. Structural purity is an advantage for this method, as in the case of powder or sol–gel methods single-phase yttrium silicate formation is hardly possible, even at high firing temperatures

Blue- and red-emitting phosphor nanoparticles embedded in a porous matrix

International audienceEu3+- and Ce3+-doped yttrium silicate, as well as Eu2+-doped zinc silicate nanoparticles, were grown in a porous SiO2 matrix using an impregnation method. For Y2Si2O7:Eu3+, particles of about 50 nm size were obtained that exhibited several photoluminescence (PL) peaks in red. Different peaks showed slightly different decay times; however, their excitation mechanism was found the same. Increasing the Eu concentration increased the PL intensity while reducing the decay time. Y2Si2O7:Ce3+ nanoparticles in the porous matrix showed bright blue emission, consisting of two peaks at 358 nm and 378 nm. Re-impregnation process was found effective in changing the relative intensity of the two peaks. Zn2SiO4:Eu2+ nanoparticles in porous glass consisted of amorphous particles of about 20 nm size inside the porous matrix. The luminescence was a broad peak centered at 418 nm. These phosphor systems, together with our previously reported Zn2SiO4:Mn2+ in porous SiO2 structure, comprise a red–green–blue system that can be used in display applications

Interstitial sulfur photoluminescence in thermochemically synthesized CdS nanocrystals (NCs)

We have synthesized CdS NCs (NCs) by a thermochemical approach. CdSO4 and Na2S2O3 were used as the precursors and thioglycolic acid (TGA) was used as capping agent molecule. The structure and optical property of the NCs were characterized by means of XRD, TEM, UV-visible optical spectroscopy and photoluminescence (PL). XRD and TEM analyses demonstrated hexagonal phase CdS NCs with an average size of around 2 nm. Synthesized NCs exhibited a band gap of about 3.21 eV and showed a broad band emission from 400 to 750 nm centered at 503 nm. This broad band emission is related to surface states of CdS and our results showed that the emission peak can be attributed to the interstitial sulfur. The best attained photoluminescence quantum yield of the NCs was about 11%. At the same conditions the PL quantum yield of TGA capped NCs was about 20 times higher than that of TG capped NCs

Charge transport properties in nanocomposite photoanodes of DSSCs: crucial role of electronic structure

TiO2 nanorods, TiO2 nanorod/TiO2 nanoparticle and TiO2 nanorod/ZnO nanoparticle composite structures were integrated as photoanodes in backside illuminated dye-sensitized solar cells (DSSCs). Incorporation of TiO2 nanoparticles into the bare nanorods increased the dye loading and improved the short-circuit current density (Jsc) from 2.22 mA/cm2 to 3.57 mA/cm2. ZnO nanoparticles electrochemically grown into the TiO2 nanorod layer could increase the surface area. Nevertheless, this considerably reduced the Jsc to 0.57 mA/cm2 and consequently cell efficiency. Electrochemical impedance spectroscopy (EIS) results showed that ZnO incorporated samples have better effective diffusion coefficient of electrons in comparison with bare TiO2 nanorods while the recombination rate of injected electrons to photoanode with electrolyte is near eight times faster than bare TiO2 nanorods. ZnO incorporated samples showed lower electron density at steady state in the conduction band also. The worse performance of ZnO incorporated samples was attributed to lower electron injection efficiency from excited dye molecules. Monitoring electron transport properties of the cells measured by EIS pointed out the crucial role of electronic structure of composite film components on the performance of cells. Our results showed that EIS technique could be used as an efficient characterization method for precise monitoring of charge transport in nanocomposite photoanodes for DSSCs

TiO

In this study TiO2 nanotubular fibers were prepared and subsequently loaded with CdS nanoparticles to obtain visible light activate nanofibers with modified structure. Preparation of TiO2 fibers was based on templating method and Liquid phase deposition technique (LPD) with cellulose fibers as templates. Using LPD, thickness of the TiO2 layer could be controlled precisely by adjusting the reaction conditions, therefore after removal of the template, the resulting material has a fibrous structure, mimicking the cellulose fibers shape. CdS nanoparticles were synthesized by thermochemical growth method and attached to TiO2 fibers through impregnation method. The pure composite nanofibers were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV-visible spectroscopic techniques. To investigate the photoactivity of CdS/TiO2 fibers, using MB decomposition test under visible light irradiation was studied and compared with that of pure TiO2 nanofibers and CdS nanoparticles. A possible mechanism of sensitization of TiO2 with CdS nanoparticles and visible light MB decomposition was also discussed. Photocatalytic decomposition test under visible light irradiation shows that these novel structures are appropriate for visible photocatalysis applications